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Enhanced boronizing kinetics and high temperature wear resistance of H13 steel with boriding treatment assisted by air blast shot peening
Enhanced boronizing kinetics and high temperature wear resistance of H13 steel with boriding treatment assisted by air blast shot peening Haopeng Yang, Xiaochun Wu, Guanghui Cao, Zhe Yang PII: DOI: Reference:
S0257-8972(16)30900-8 doi: 10.1016/j.surfcoat.2016.09.029 SCT 21575
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
21 June 2016 6 September 2016 15 September 2016
Please cite this article as: Haopeng Yang, Xiaochun Wu, Guanghui Cao, Zhe Yang, Enhanced boronizing kinetics and high temperature wear resistance of H13 steel with boriding treatment assisted by air blast shot peening, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.09.029
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ACCEPTED MANUSCRIPT Enhanced boronizing kinetics and high temperature wear resistance of H13 steel with boriding treatment assisted by air blast shot peening
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Haopeng Yanga,b,c*, Xiaochun Wua,b,c, Guanghui Caoa,b,c, Zhe Yanga,b,c a
School of Materials Science and Engineering, Shanghai University, Shanghai 200072, People’s Republic of China
State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, People’s Republic of China
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Shanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai 200072, People’s Republic of China
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Abstract
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A nanostructured surface layer was fabricated on H13 steel by means of air blast shot
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peening (ABSP). A much thicker borided layer on the ABSP sample can be synthesized by a duplex boronizing treatment (DBT) at 600℃ for 2h, which is followed by at a higher
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temperature for a certain time. The borided layer was composed with monophase of Fe2B and
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the growth of it exhibited a (002) preferred orientation. Moreover, the activation energy of
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boron diffusion for the ABSP sample is 227.4kJ/mol, which is lower than 260.4kJ/mol for the coarse-grained counterpart. The results indicate that the boronizing kinetics can be effectively enhanced in the ABSP sample with DBT. The high temperature wear resistance of H13 steel
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with DBT can be improved significantly. Furthermore, the H13 steel with DBT assisted by ABSP possesses more superior wear resistance property at elevated temperatures than that of coarse-grained sample with DBT, which can be attributed to the fact that the thickness and microhardness of the borided layer can be increased with the help of ABSP. Meanwhile, the fatigue crack initiation and propagation in borided layer during the wear test can be impeded by the compressive residual stress and the refined grains in the borides of ABSP sample with DBT. *
Corresponding author. Tel.: +86 21 56331153; fax: +86 21 56331461. E-mail addresses: [email protected] (H.P.Yang) 1
ACCEPTED MANUSCRIPT Keywords: H13 steel; Surface nanocrystallization; Air blast shot peening; Boronizing kinetics;
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Wear
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ACCEPTED MANUSCRIPT 1. Introduction The AISI H13 hot work die steels are used extensively for extrusion dies as well as for
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die cast of aluminium alloy. They are usually characterized with high strength and toughness.
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However, this kind of die steels is commonly used in aggressive environments. It is necessary to adopt thermochemical treatment technique to improve their surface properties, such as wear resistance, thermal fatigue resistance and corrosion resistance. One of the effective
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surface treatments is boriding technique. Owning to the nature of the diffusion process, the
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borided layers possess excellent adhesion to the substrate when compared to prevalent physical coating process. It also has the advantage of high hardness when compared with
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conventional surface treatments, such as carburizing, nitriding and carbonitriding [1, 2], due
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to the superior hardness of borides (1500~2000HV).
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The boriding technique can be carried out in solid, liquid or gaseous medium [3]. Genel et al. [4] prepared both phases of FeB and Fe2B on the surface of H13 in the solid medium
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consisting of Ekabor-I powders at 800, 900, 1000℃ for periods of 1–5h. Taktak et al. [5] reported the FeB and Fe2B can form on the surface of H13 steels in slurry salt bath consisting of borax, boric acid and ferrosilicon at temperature range of 800–950℃ for 3, 5 and 7h. In fact, many previous literatures have reported the boriding techniques which are used to prepare borided layers on the surface of steels. Among the various boronizing processes, solid-state pack boriding treatment is the most frequently used. Yet most of them have the disadvantages of requiring relative high processing temperature or time consuming. Studies have been carried out to improve the efficiency of pack boriding treatment over the past decades. 3
ACCEPTED MANUSCRIPT It is well known that borided layers generated in thermochemical treatment depend on boriding condition and on the properties of the materials itself. Both factors are strongly
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affected by grain boundaries and defect densities in the surface layer. The combination of
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pack boriding treatment with surface mechanical attrition treatment (SMAT) could improve the efficiency of boronizing kinetics. After SMAT, various metals, such as pure Fe [6], 38CrMoAl [7], AISI 321 austenitic stainless [8] and other alloys [9] possess a nanostructured
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surface layer. Due to the significant enhanced diffusion and chemical reaction kinetics in the
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formed nanostructured surface layer, the hardened diffusion layer on the substrate has been fabricated on several ferrous alloys after the subsequent gas nitriding [6, 7] or packed powder
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chromizing treatment [10, 11]. However, the method of SMAT has the disadvantage of its
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restriction on the flat shape of workpiece, which impedes its application in industrial
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production. Fortunately, air blast shot peening (ABSP) can also be used to refine grains and produce high density of defects in the surface layer, which could be a promising method that
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can be used in thermochemical treatment for many kinds of steels. More importantly, ABSP has been applied extensively in material processing. Here, we report that pack boriding treatment was carried out for the hot work die steel H13 assisted by ABSP. The boronizing kinetics in the nanostructured surface layer fabricated by ABSP was studied. Furthermore, the high temperature wear resistances of the borided H13 steels with and without ABSP pretreatment were investigated by reciprocating sliding wear tests at 700°C under the applied loads of 20N.
2. Experimental 2.1 Test materials and ABSP treatment 4
ACCEPTED MANUSCRIPT The chemical compositions of AISI H13 steel used in the experiments contain (wt.%) 0.42C, 4.93Cr, 1.40Mo, 0.98Si, 0.87V, 0.30Mn, 0.018P, 0.005S and balance Fe. The
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spheroidizing annealing process was used for the original H13 steel with the shape of
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60×60×4mm, which was annealed at 840°C for 2h. Before ABSP, the annealed steel was mirror polished. Then, the sample was processed by a flow of cast steel balls with diameter of 0.8mm at 0.5MPa for six cyclic deformation, and the time of each cycle was 5min.The angle
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between the shot jet and the sample surface is in the range of 70~90°. The ABSP samples
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with dimensions of 15×15×4mm were machined from the bulk sample mentioned above and ultrasonically cleaned in acetone.
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2.2 Boriding process
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Two kinds of samples were used in the duplex boronizing treatment(DBT), which
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include the annealed coarse-grained(CG) sample and the annealed ABSP sample. The pack boriding treatment was as follows: the constituents of the boriding media were B4C(10wt.%),
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KBF4(5wt.%), charcoal(5wt.%) with the balance of SiC. The ABSP sample and the CG sample were packed together with a suitable distance from each other in the powder mix and sealed in a stainless steel container. In order to stabilize the nanostructures in the surface layer fabricated by ABSP, a duplex boriding treatment at T1=600°C followed by at a higher temperatures (T2) for a certain time was carried out in an electrical resistance furnace. In the first stage, the samples were heated to T1 = 600°C for 2 h. In the second stage, the temperature was increased to T2 = 750°C, 800°C and 850°C for 2, 4 and 8h, respectively. After the boriding treatment, the container was removed from the furnace and cooled in air. At last, all the borided samples and the annealed CG sample without boriding treatment were 5
ACCEPTED MANUSCRIPT quenched at 1030°C and subsequently tempered twice at 580°C in a vacuum oven. 2.3 High temperature friction and wear testing procedure
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A UMT-3 tribometer (Fig.1) equipped with a reciprocating configuration placed in a
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heating furnace was used to carry out the friction and wear tests at elevated temperatures. A ball holder is connected to both a vertical and lateral linear motion system. The samples are pinned to a reciprocating stage in a high temperature chamber that allows testing
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temperatures up to 1000°C. Three kinds of samples were used in the wear tests at high
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temperature, which include the quenched and tempered sample, the CG sample with DBT and the ABSP sample with DBT. The size and geometry of a high temperature friction and
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wear test specimen are shown in Fig.2. The surface (10×35.6mm) is used for friction and
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wear test. The dry sliding wear tests were operated by the parameters which are shown in
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Table 1. Silicon carbide balls with a diameter of 9.5mm and hardness of 2800HV were chosen as the counterpart in order to evaluate the wear properties of the borided layers at high
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temperatures. The wear rate is calculated by the equation as follows:
Ws V ( p d)
(1)
where V is the wear volume. P is the loading force and d is the total sliding distance. 2.4. Characterization The X-ray diffraction (XRD) measurements were obtained with a RigakuD/MaxRBX-ray diffractometer by using CuKa (40kV, 40mA) radiation and a secondary beam graphite monochromator. A Zeiss Supra 40 scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) system was used to characterize cross-sectional of the microstructures of borided samples and wear morphologies of the 6
ACCEPTED MANUSCRIPT borided samples and sample without boriding treatment. Microstructural features in the surface layer treated with ABSP were characterized by using a field emission transmission
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electron microscopy (TEM, JEOLJEM-2010F) equipped with an energy dispersive spectral
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(EDS) analyzer. Thin foil samples for TEM observations were prepared by cutting, grinding and dimpling with a final ion thinning at low temperatures. The hardness distributions of the cross-sectioned borided layers were measured by a Vickers microhardness tester with a load
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of 100g force. A LECO GDS850A glow-discharge optical emission spectrometer (GD-OES)
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was used to measure the concentrations of boron element in the surface layers of the ABSP sample and the CG sample with boriding treatment at 600°C for 2h. The wear volumes of the
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samples were measured by a Bruker ContourGT-K 3D optical profiler. The method for
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measuring the residual stress after air blast shot peening and duplex boronizing treatment is
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as follows: By using the two-angle sin2ψ method with the ψ angles fixed at 0° and 45° [12], the residual stresses were measured by X-350A X-ray stress analyzer with Cr-Kα radiation.
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The locations of corresponding diffraction angles 2θ0° and 2θ45° were determined by cross-correlation method[13], and the stresses were calculated by using the formula as follows[12]:
2K (2 )
(2)
The residual stresses measurements of samples without pack boriding were made employing the diffraction of the (211) crystallographic plane of α-Fe. The stress constant K of H13 steel is -318MPa/º, which is commonly used for iron-based alloys. The residual stresses in borided samples were determined by using the diffraction of the (330)
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ACCEPTED MANUSCRIPT crystallographic plane of Fe2B, and the stress constant K of Fe2B can be calculated by the
E cot 0 2(1 v) 180
(3)
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following formula[14]:
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where E is Young's modulus, ν is Poisson's ratio, θ0 is the Bragg angle of (330) crystallographic plane of Fe2B in the free stress state. Substituting these material constants of
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Fe2B (E= 290GPa, ν = 0.2 and θ0 =71.9º) into the formula, the stress constant K of Fe2B was -689.3MPa/ º. Residual stress tests were repeated three times for each sample and the mean
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value was calculated.
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3. Results and discussion
3.1 Microstructure of the ABSP sample
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The SEM image of the cross section of the sample treated by ABSP is shown in Fig. 3(a). It can be seen that a plastic deformation layer (~10µm) is present in the treated surface layer,
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which is indicated by a white-white line, where the microstructural morphology differs from that in the matrix. In subsurface zone, it is work-hardened region, which is characterized with the elongated grain boundaries. Fig.3(b) displays a typical bright-field TEM image of the top layer. The statistical distribution of grain size is shown by Fig.3(c), which indicates the size of vast majority of grains is at 12~22nm. The selected-area electron diffraction pattern is shown in Fig.3(d). It reveals that the microstructure is characterized by ultrafine equiaxed grains with random crystallographic orientation. The strong diffraction rings are assigned to polycrystalline -Fe. There is also one reflection that could be interpreted as austenite reflection(311). It may reveal that to transformation in the nanostructured surface layer at 8
ACCEPTED MANUSCRIPT about room temperature during severe plastic deformation with very large strain and strain rate, due to the formation of supersaturated nanocrystalline ferrite and the compression stress
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at the boundaries of nano-size grains [15]. When comparing to the conventional grain
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boundaries(GBs) in the annealed coarse-grained(CG) samples, the GBs formed by the severe plastic deformation are usually in non-equilibrium states which are associated with a high density of defects or a high stored energy [16]. GBs diffusivity in the deformation layer
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produced by ABSP will be considerably enhanced when compared to that along high angle
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GBs in the CG counterpart. It should be noted that the thermal stability of the nanostructured surface layer can be preserved up to 600℃ according to the previous study results [17].
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3.2 Diffusion of boron in the nanostructured surface layer
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The concentration of boron along the depth from the treated surface of the ABSP sample
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and the CG counterpart with pack boriding treatment at 600℃ for 2h were determined by GD-OES tests, as shown in Fig.4. The peak value of concentration in the top surface layer for
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the ABSP sample is about 1.5 wt.%, while the peak value for the CG sample is only about 0.8 wt.%. Furthermore, the depth of boron diffusion in ABSP sample is about 12µm, which is much deeper than that in the CG sample. Based on the experimental results mentioned above, it can be concluded that the thermal diffusion of boron in the nanostructured surface layer fabricated by ABSP can be evidently enhanced when compared to that in the CG sample with the same experimental parameter. According to the diffusion equation for semi-infinite solid diffusion [18]:
X Dmt
(4)
where is a constant (about 1 in the present case), t is the duration of the pack boriding 9
ACCEPTED MANUSCRIPT treatment, and Dm is the mean diffusion coefficient. The measured diffusion depth into the ABSP sample is larger than that into the CG counterpart, indicating the change of diffusion
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mechanism in the ABSP surface layer. Various non-equilibrium GBs dominate the much
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enhanced boronizing kinetics in the ABSP surface layer at 600℃. The mean GBs diffusion coefficient calculated from the measured boron diffusion depth in the ABSP sample according to Eq. (4) is 2.0×10-14 m2/s, which is much higher than 5.0×10-15 m2/s for the CG sample.
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3.3 Microstructure and hardness of the borided layer
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The pack boriding treatment at 600 ℃ was found to facilitate stabilizing the nanostructures in the ABSP surface layer, which was useful to form a much thicker borided
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layer with a subsequent boronizing process at a higher temperature. Fig. 5(a) and (b) show
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the cross-sectional SEM observations for the CG sample and the ABSP sample after a duplex
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boronizing treatment(DBT) at T1=600 ℃ for 2h followed by at T2=850 ℃ for 8h, respectively. It can be seen that the continuous borided layers are on the two sample surfaces.
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The thicknesses of borided layers on the ABSP sample and the CG counterpart are 40µm and 30µm, respectively. Neither FeB(orthorhombic) nor Fe2B(tetragonal) has cubic crystal symmetry [19]. Although H13 steel possesses a certain amount of alloying elements, boron diffusion in the borided layer is of anisotropic nature at high temperatures. Therefore, the morphologies of borided layers exhibit tooth-like growth into substrate. The microhardness gradients for the two samples are showed in Fig. 5(c). The microhardness of borides formed on the surface of ABSP sample is about 1700HV0.1 and it is roughly 3.5 times harder than that of the substrate. In comparison, the microhardness of borides is about 1600HV0.1 for the CG counterpart. The detailed microstructures of the two kinds of boried samples are shown in 10
ACCEPTED MANUSCRIPT Fig.6. For both samples, the surface layer includes three distinct regions, which are borided layer, transition zone and matrix that is not affected by boron diffusion. The EDS point
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microanalysis results for the points A, B, C, D, E and F in these three regions of both samples
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are shown in Table 2. Because the boron is ultralight element, it could not be detected by means of EDS method. Therefore, the boron element does not appear in the EDS point microanalysis results. It should be noted that the mass percent of silicon in transition zones of
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CG sample and ABSP sample with DBT are 2.81wt.% and 2.93wt.%, respectively. The
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concentration of silicon in transition zone of both samples is much higher than that in H13 steel, as silicon was not soluble in borided layer and diffused into transition zone. It is well
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known that silicon is a ferrite-forming element. The high concentration of silicon in the
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transition zone below the borided layer suggests that this region could not completely
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austenitizing at the conventional austenitizing temperature of H13 steel and there were some undissolved ferrite that existed in transition zone, which induce that the microhardness of
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transition zone is a little softer than that of the substrate. Similar results were reported for H11, H12 and H13 steels [20, 21]. Meanwhile, during the process of DBT, a certain amount of boron atoms could diffuse and dissolve in cementite that existed in transition zone, where the Fe3(C,B) phase with relative large particle size could form. Therefore, the phase composition of transition zone included tempered martensite, undissolved ferrite and Fe3(C, B) phase. In addition, in the process of fabricating the borided layer on H13 steels, the carbon atoms can diffuse from surface layer to internal substrate, which can be ascribed that carbon is not soluble in borided layer. The concentration of carbon atoms in the transition zone below borided layer of the ABSP and CG samples with DBT could be much higher than that 11
ACCEPTED MANUSCRIPT in the internal substrate, as shown in Table 2. Therefore, the chemical composition of the transition zone is close to that of high carbon alloy steel. After the borided samples were
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quenched at 1030℃, the concentration of the carbon atoms in quenching martensite in the
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transition zone is much higher than that in the internal substrate. When the borided samples with microscopic structure of quenching martensite were tempered twice at 580℃ in vacuum oven, the quantity of alloy carbide which precipitated from quenching martensite in the
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transition zone could be significantly higher than that in the internal substrate, which can be
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confirmed by the detailed microstructures of the ABSP sample and CG sample with DBT in Fig.6. As shown in Fig. 7, based on the pack boriding treatment at T1=600℃ for 2h, a much
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thicker borided layer was formed on the ABSP sample than that on the CG counterpart after
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the boronizing treatment at any T2 from 750-850℃ for 8h. The thicknesses of borided layers
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in both samples increase with an increasing T2. It can be seen that the low temperature treatment at 600℃ will enhance the boronizing kinetics of the ABSP sample in a DBT with
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any T2 employed.
3.4 XRD measurements The XRD patterns of surface layer of the ABSP sample and the CG sample after a DBT at T1=600℃ for 2h followed by at T2=850℃ for 8h are presented in Fig. 8. For both samples, it is obvious that monophase of Fe2B are detected, which is beneficial for industrial application, owing to its much better toughness when compared to that of FeB phase. The strongest peak can be indexed to the (121) diffraction peak of Fe2B. The intensity of (002) diffraction peak of Fe2B is the second strongest peak, which shows that the growth of Fe2B phase exhibits a (002) preferred orientation. Moreover, from the characteristic of diffraction 12
ACCEPTED MANUSCRIPT peaks, it can be concluded that the Fe2B phase possesses good crystallinity. Additionally, some weak diffraction peaks can be indexed to the phase of Fe2O3, indicating that the slight
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oxidation appeared on the sample surface during the tempering process.
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3.5 Kinetics study of borided layer growth
The growth of borided layer occurs as a consequence of the boron diffusion perpendicular to the specimen surface. According to Eq. (4), the growth of Fe2B layer obeys
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the parabolic law. The square of borided layer thickness synthesized by a DBT as a function
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of boronizing time can be described as follows:
d2 Kt
(5)
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where d is the thickness of borided layer that is measured by the method described in a
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previous paper [22], K is the growth rate constant depending on boronizing temperature and
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is calculated from the slope of the d2 versus treatment time graph, which are presented in Fig.9 and Table 3.
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The relationship between growth rate constant K and boronizing temperature can be expressed by an Arrhenius-type equation as follows [23]:
K K0 exp(
Q ) RT
(6)
where K0 is called the collision factor and is a measurement of the effectiveness of collisions between reactive species, Q is an activation factor which indicates the amount of energy(J/mol) required to make the reaction occur. T is the absolute temperature and R is the gas constant(J/(molK)). Taking the natural logarithm of each side of Eq. (6) gives Eq. (7) as follows:
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ACCEPTED MANUSCRIPT InK InK0
Q RT
(7)
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The plot of InK versus the reciprocal absolute temperature (1/T) is a linear relationship.
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The activation energy Q and the collision factor K0 can be calculated from the slopes and the
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intercepts of the graphs in Fig.10, respectively, which are presented in Table 3. The calculated activation energy value for ABSP sample is lower than that for CG sample, which confirms
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the enhancement of the boronizing kinetics of the ABSP sample with a DBT. Based on the results mentioned above, the following empirical equations were derived to estimate the
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sample:
dCG =1.9110-1 exp(
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For
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borided layer thicknesses in a given treatment time and temperature.
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(8)
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For ABSP sample: dABSP =3.6510-2 exp(
27351 )t T
1023 T 1123
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time.
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where d is the thickness of borided layer; T is boronizing temperature(K) and t is boronizing
3.6 High temperature wear property of borided sample The high temperature wear property is very important for hot work die steels in the industrial application process. The wear rates of the quenched and tempered sample (A) without boriding treatment, CG sample (B) with DBT and ABSP sample(C) with DBT are shown in Fig.11. The wear rates of A sample is 3.42×10-13m3(N·m)-1. In contrast, the wear rates of B sample and C sample are 2.57×10-13m3(N·m)-1 and 2.02×10-13m3(N·m)-1, respectively. When compared to that of A sample, the wear rates of B sample and C sample are decreased by 25% and 41%, respectively. The results showed that the high temperature 14
ACCEPTED MANUSCRIPT wear resistance of H13 steel with DBT can be improved significantly, owing to the high hardness and high oxidation resistance at elevated temperatures of borided layer. Furthermore,
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the H13 steel with DBT assisted by ABSP possesses more superior wear resistance property
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at elevated temperatures than that of CG sample with DBT，which could be attributed that the thickness and microhardness of the borided layer can be increased significantly with the help of ABSP. At the same time, owing to the significant refined grains in the nanostructured
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surface layer of ABSP sample, the refined grains of borides on ABSP sample could be
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synthesized with a DBT. Therefore, the borided layer on H13 steel with DBT assisted by ABSP possesses good toughness and the fatigue wear can be relieved in the process of wear
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tests at elevated temperatures.
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Fig. 12 shows the XRD patterns of the worn surfaces of the quenched and tempered
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sample (a) without boriding treatment, CG sample (b) with DBT and ABSP sample(c) with DBT. The phase composition of worn surface of sample without DBT includes α-Fe and
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Fe2O3. While the worn surfaces of CG sample with DBT and ABSP sample with DBT contain α-Fe, Fe2O3, Fe2B, B2O and α-Fe, Fe2B, Fe2O3, respectively. The Fe2B can be detected on the worn surfaces of the samples with DBT, owing to the borided layer at the edge of wear scars. It is clear that the oxidative wear occurred when the high temperature friction and wear tests were carried out under the atmospheric conditions. The diffraction intensity of Fe2O3 on worn surface of sample without DBT is stronger than those on worn surfaces of CG sample and ABSP sample with DBT, which shows there are less oxide that exists on the worn surfaces of CG sample and ABSP sample with DBT, due to the high oxidation resistance of borided layer on them. The types of iron oxides include FeO, Fe3O4 and Fe2O3. The forming temperature of 15
ACCEPTED MANUSCRIPT FeO is above 570℃.While the forming temperature of Fe2O3 and Fe3O4 are below 200℃and 200-570℃, respectively [24]. However, there is only Fe2O3 on the worn surfaces of all
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samples at the temperature of wear test. The reason is as follows: the crystal defects in the
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surface layers of all samples could increase during the process of wear test, which is beneficial to the diffusion of oxygen atoms or oxygen ion in the worn surfaces [25]. Meanwhile, on the basis of thermodynamics, FeO or Fe3O4 can be easily reacted with O2,
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which could produce Fe2O3 at 700℃.
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The morphologies of the worn surfaces of the samples are shown in Fig.13. Fig.13(a) displays that local plastic deformation occurs on the wear track of sample without boriding
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treatment, which can be attributed to massive plastic shearing in the surface layer during the
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dry sliding wear test. As showed in Fig.13(b), the signs of adhesive wear were observed and
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some flaking areas exist in the wear track. Local delamination in the wear track is due to the propagation and coalescence of microcracks during the durative dry sliding. Fig.13(c)-(f)
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shows the worn surface morphologies of the borided samples with and without ABSP. The worn morphologies confirm that wear resistance could be significantly improved by means of the boriding treatment. There are no signs of severe plastic deformation on the wear track of CG sample with DBT, as showed by Fig.13(c) and (d). However, some fatigue cracks can be found along the edge of wear scar and some flaking areas can be seen on the worn surface. When the SiC ball slides against the sample surface, local plastic deformation occurs on the wear track and microcracks initiation and propagation on the wear surface, which is the sign of fatigue wear. The worn morphology of ABSP sample with DBT is showed in Fig.13(e) and (f). There are no obvious fatigue cracks on the worn surface. A relatively smooth 16
ACCEPTED MANUSCRIPT appearance with some flaking areas can be seen on the worn surface of ABSP sample with DBT, which indicates that its surface possesses better wear resistance when compared to that
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of CG sample with DBT. The EDS analysis of point A, B and C show that the mass percent
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of oxygen element are 40.6%，35.3% and 27.5%, respectively. It demonstrates that the worn surfaces of three samples had been oxidized and oxidative wear had occurred during wear tests at high temperature for samples with and without boriding treatment.
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In order to further explore the wear mechanism, the section morphologies of the worn
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surfaces for three kinds of samples were observed. In Fig.14(a), the obvious plastic deformation appeared in the subsurface and the double layers of tribo-oxides (indicated by
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red arrows) can be seen on the wear surface of the sample without boriding treatment. The
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wear mechanism and wear loss are closely related with thermal softening extent of the matrix.
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During the wear test at elevated temperatures, the friction force and normal pressure resulted in the thermal softening of matrix, which incorporated with plastic deformation, dynamic
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recovery and recrystallization [26]. When the large softening region appeared in the subsurface, more plastic deformation occurred and it could not support tribo-oxides, which results in accelerating delamination of the tribo-oxides. At the plastic deformation region, the density of crystal defects increased and oxygen atoms or ions could easily diffuse into subsurface of matrix to form oxide or oxide layer. Hence, oxidation could occur not only on the worn surfaces, but also inside the matrix by the direct diffusion of oxygen through the cracks in delaminated regions [27, 28]. Therefore, the double layers of tribo-oxides could form on the worn surfaces. However, before the borided layer was worn out, the deformation and oxidation of the matrix were prevented due to the high hardness, high wear resistance and 17
ACCEPTED MANUSCRIPT oxidation resistance of the borides on the borided samples with and without ABSP. Moreover, although the sample with DBT was finally worn out in the middle of the wear scar, the
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remained borided layers on both sides played a role of supporting the SiC ball. Therefore, the
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plastic deformation was not found in the subsurface layer and single layer of oxides on the worn surface of samples with DBT, as shown in Fig.14(b) and (c). The EDS analysis of point A, B and C show that the mass percent of oxygen element are 37.5%，30.8% and 21.9%,
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respectively. As can be seen from Fig.14(d) and (e), the borided layers on samples with DBT
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were finally worn out in the middle of the wear scar, but the borided layer along the edge of wear scar still adhered to the substrate. However, there are some cracks in the borided layer
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on CG sample with DBT.
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Shot peening is usually applied as surface modification technology to increase fatigue
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resistance. This beneficial effect is associated with compressive residual stress induced in the surface layer. The residual stresses of CG samples and ABSP samples before and after the
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DBT are listed in Table 4. The result shows that compressive residual stresses were detected on the surfaces of all samples. After the balls impacted during the ABSP treatment, a uniform layer of compressive residual stress was generated on the surface of ABSP sample without a DBT through overlapping dimples [29]. In addition, polishing could also induce a compressive stress on the surface layer of CG sample without a DBT. For the CG sample with a DBT, a compressive stress was also obtained. This can be explained by the formation of borides with the higher specific volume in the surface layer. Since the compressive residual stress originated from ABSP, the compressive stress in the surface layer of ABSP sample with DBT is much higher than that of CG sample with DBT. It is well known that 18
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compressive residual stress and the refined grains of borides in the borided layer of ABSP
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sample with DBT can help to impede the fatigue crack initiation and propagation in the borides during the wear test. The fatigue wear could not easily occur on the surface of ABSP sample with DBT. Meanwhile, the thickness of the borided layer can be increased
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significantly with the help of ABSP, which is beneficial for improving the high temperature
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wear resistance of H13 steel. Therefore, the wear rate of borided sample with ABSP is smaller than that of borided sample without ABSP. The H13 steel with DBT assisted by
4. Conclusions
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CG sample with DBT.
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ABSP possesses more superior wear resistance property at elevated temperatures than that of
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The nanostructured surface layer can be fabricated on H13 steel assisted by ABSP. There are various non-equilibrium GBs in it, along which the diffusion rate of boron is much higher than that along the relaxed GBs in the coarse-grained counterpart. By means of a DBT at T1 =600℃ for 2h followed by at T2=850℃ for 8h，the thicknesses of Fe2B phase on the ABSP sample and the coarse-grained counterpart are 40µm and 30µm, respectively. The calculated activation energy value for ABSP sample (227.4 kJ/mol) is lower than that for coarse-grained sample(260.4 kJ/mol). It can be concluded that the boronizing kinetics of the ABSP sample after a DBT at T1=600℃ for 2h followed by at a higher temperature for a certain time can be enhanced effectively. The high temperature wear resistance of H13 steel with DBT can be 19
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layer can be increased with the help of ABSP. Meanwhile, the compressive residual stress and
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the refined grains in the borides of ABSP sample with DBT can impede the fatigue crack initiation and propagation in borided layer during the wear test. Therefore, the H13 steel with DBT assisted by ABSP possesses more superior wear resistance property at elevated
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temperatures than that of coarse-grained sample with DBT.
Acknowledgments
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The authors acknowledge the “11th Five” National Science and Technology Support
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Project of China (Project Number: 2007BAE51B04) and Shanghai Leading Academic
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Discipline Project for the financial support (Project Number: S30107). The authors would like to thank X.J.He from School of Materials Science and Engineering of Shanghai
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University for the help with the SEM measurements.
References
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ACCEPTED MANUSCRIPT [5] S.Taktak, Mater. Des. 28 (2007) 1836–1843. [6] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Science 299 (2003) 686-688.
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[27] M.X.Wei, S.Q.Wang, L.Wang, X.H.Cui, K.M.Chen, Tribol. Int. 44 (2011) 898–905. [28] M.X.Wei, F.Wang, S.Q.Wang, X.H.Cui, Mater. Des. 30 (2009) 3608–3614. [29] O. Higounenc, Correlation of shot peening parameters to surface characteristic, in:
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[30] K. Zhan, C.H. Jiang, V. Ji, Mater. Lett. 99 (2013) 61-64. [31]C.M.Suh, G.H.Song, M.S.Suh,Y.S.Pyoun, Mater. Sci. Eng. A 443 (2007) 101-106.
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Table 1
The experimental parameters of friction and wear tests. Normal load
Reciprocating frequency
Sliding stroke
Temperature
Time
20N
5Hz
10mm
700°C
1200s
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Mn K
Cr K
Mo L
VK
Fe K
Totals
A(wt.%)
1.42
B(wt.%)
4.75
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4.12
0.76
0.42
93.28
100
2.81
0.21
5.35
1.67
0.92
84.29
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C(wt.%)
3.78
1.14
0.37
5.20
1.46
0.76
87.29
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D(wt.%)
2.03
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4.93
0.48
0.32
92.24
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E(wt.%)
4.18
2.93
0.40
5.22
1.75
0.82
84.70
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F(wt.%)
3.28
1.25
0.45
5.09
1.28
1.03
87.62
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Growth rate constant, calculated activation energy and collision factor for test materials as a
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ABSP samples
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Treatment temperatures T(℃) 750 800 850 750 800 850
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Growth rate constant K(m2/s) 4.48E-15 1.50E-14 4.84E-14 2.43E-15 1.04E-14 3.72E-14
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Activation energy Q(kJ/mol) 227.4
Collision factor K0 (m2/s)
260.4
3.63E-2
1.33E-3
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The residual stresses (MPa) of different samples.
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Fig.1. Schematic representation of the UMT-3 high temperature friction and wear test system.
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Fig.2. The size and geometry of a high temperature friction and wear test specimen (unit: mm).
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Fig.3. (a) Typical SEM image of the cross section of the sample treated by ABSP, (b)bright field TEM image showing the microstructure of the top surface layer of the sample treated by ABSP, (c)the graph of statistical distribution of grain size and (d)corresponding selected area electron diffraction pattern.
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Fig.4. The boron concentration gradients of the CG specimen and the ABSP specimen with pack boriding treatment at 600℃ for 2h.
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Fig.5. Typical SEM images of the cross section of (a) CG specimen, (b) ABSP specimen with a duplex pack boriding treatment and (c) the corresponding microhardness gradients of them.
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Fig.7. The graph of thicknesses of borided layers formed on the CG specimen and the ABSP specimen with pack boriding treatments at 600℃for 2h followed by heating at 750℃、800℃ and 850℃ for 8h.
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Fig.8. XRD patterns of the CG specimen and the ABSP specimen with a duplex pack boriding treatment at T1 =600℃ for 2h followed by at T2 =850℃ for 8h. Fig.9. Square of borided layers thicknesses of (a) the CG specimen and (b) the ABSP specimen with a duplex pack boriding treatment as a function of boronizing time at different temperatures. Fig.10. Natural logarithm of growth rate constants of the CG specimen and the ABSP specimen as a function of reciprocal of boronizing temperatures. Fig.11. The wear rates at an elevated temperature of (A) the quenched and tempered specimen without boriding treatment, (B) the CG specimen and (C) the ABSP specimen with a duplex pack boriding treatment. Fig.12. XRD patterns of the wear surfaces at an elevated temperature of (a) the quenched and tempered specimen without boriding treatment, (b) the CG specimen and (c) the ABSP specimen with a duplex pack boriding treatment. Fig.13. Typical SEM images of the wear surface at an elevated temperature of (a) and (b) the 27
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quenched and tempered specimen without boriding treatment, (c) and (d) the CG specimen and (e) and (f) the ABSP specimen with a duplex pack boriding treatment. Fig.14. The cross section morphologies of the worn surfaces of (a) the quenched and tempered sample without boriding treatment, (b) borided sample without ABSP and (c) borided sample with ABSP; the cross section morphologies of borided layers at the edge of wear scars of (d) borided sample without ABSP and (e) borided sample with ABSP.
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The experimental parameters of friction and wear tests.
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Reciprocating frequency
Sliding stroke
Temperature
Time
20N
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10mm
700°C
1200s
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CK
Si K
Mn K
Cr K
Mo L
VK
Fe K
Totals
A(wt.%)
1.42
-
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4.12
0.76
0.42
93.28
100
B(wt.%)
4.75
2.81
0.21
5.35
1.67
0.92
84.29
100
C(wt.%)
3.78
1.14
0.37
5.20
1.46
0.76
87.29
100
D(wt.%)
2.03
-
-
4.93
0.48
0.32
92.24
100
E(wt.%)
4.18
2.93
0.40
5.22
1.75
0.82
84.70
100
F(wt.%)
3.28
1.25
0.45
5.09
1.28
1.03
87.62
100
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Growth rate constant, calculated activation energy and collision factor for test materials as a function of treatment temperatures.
Growth rate constant K(m2/s) 4.48E-15 1.50E-14 4.84E-14 2.43E-15 1.04E-14 3.72E-14
Activation energy Q(kJ/mol) 227.4
Collision factor K0 (m2/s)
260.4
3.63E-2
1.33E-3
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Treatment temperatures T(℃) 750 800 850 750 800 850
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ACCEPTED MANUSCRIPT Dear Editors of Surface & Coatings Technology: The main highlights of this rearch are as follows:
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1. The boronizing kinetics of ABSP sample can be enhanced effectively.
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2. The fatigue spall of borides at the edge of wear scar was prevented by ABSP.
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3. The oxidative wear was relieved for borided samples with wear tests at 700°C.
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S0257-8972(16)30900-8 doi: 10.1016/j.surfcoat.2016.09.029 SCT 21575
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
21 June 2016 6 September 2016 15 September 2016
Please cite this article as: Haopeng Yang, Xiaochun Wu, Guanghui Cao, Zhe Yang, Enhanced boronizing kinetics and high temperature wear resistance of H13 steel with boriding treatment assisted by air blast shot peening, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.09.029
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ACCEPTED MANUSCRIPT Enhanced boronizing kinetics and high temperature wear resistance of H13 steel with boriding treatment assisted by air blast shot peening
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Haopeng Yanga,b,c*, Xiaochun Wua,b,c, Guanghui Caoa,b,c, Zhe Yanga,b,c a
School of Materials Science and Engineering, Shanghai University, Shanghai 200072, People’s Republic of China
State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, People’s Republic of China
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Shanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai 200072, People’s Republic of China
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Abstract
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A nanostructured surface layer was fabricated on H13 steel by means of air blast shot
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peening (ABSP). A much thicker borided layer on the ABSP sample can be synthesized by a duplex boronizing treatment (DBT) at 600℃ for 2h, which is followed by at a higher
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temperature for a certain time. The borided layer was composed with monophase of Fe2B and
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the growth of it exhibited a (002) preferred orientation. Moreover, the activation energy of
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boron diffusion for the ABSP sample is 227.4kJ/mol, which is lower than 260.4kJ/mol for the coarse-grained counterpart. The results indicate that the boronizing kinetics can be effectively enhanced in the ABSP sample with DBT. The high temperature wear resistance of H13 steel
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with DBT can be improved significantly. Furthermore, the H13 steel with DBT assisted by ABSP possesses more superior wear resistance property at elevated temperatures than that of coarse-grained sample with DBT, which can be attributed to the fact that the thickness and microhardness of the borided layer can be increased with the help of ABSP. Meanwhile, the fatigue crack initiation and propagation in borided layer during the wear test can be impeded by the compressive residual stress and the refined grains in the borides of ABSP sample with DBT. *
Corresponding author. Tel.: +86 21 56331153; fax: +86 21 56331461. E-mail addresses: [email protected] (H.P.Yang) 1
ACCEPTED MANUSCRIPT Keywords: H13 steel; Surface nanocrystallization; Air blast shot peening; Boronizing kinetics;
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Wear
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ACCEPTED MANUSCRIPT 1. Introduction The AISI H13 hot work die steels are used extensively for extrusion dies as well as for
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die cast of aluminium alloy. They are usually characterized with high strength and toughness.
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However, this kind of die steels is commonly used in aggressive environments. It is necessary to adopt thermochemical treatment technique to improve their surface properties, such as wear resistance, thermal fatigue resistance and corrosion resistance. One of the effective
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surface treatments is boriding technique. Owning to the nature of the diffusion process, the
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borided layers possess excellent adhesion to the substrate when compared to prevalent physical coating process. It also has the advantage of high hardness when compared with
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conventional surface treatments, such as carburizing, nitriding and carbonitriding [1, 2], due
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to the superior hardness of borides (1500~2000HV).
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The boriding technique can be carried out in solid, liquid or gaseous medium [3]. Genel et al. [4] prepared both phases of FeB and Fe2B on the surface of H13 in the solid medium
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consisting of Ekabor-I powders at 800, 900, 1000℃ for periods of 1–5h. Taktak et al. [5] reported the FeB and Fe2B can form on the surface of H13 steels in slurry salt bath consisting of borax, boric acid and ferrosilicon at temperature range of 800–950℃ for 3, 5 and 7h. In fact, many previous literatures have reported the boriding techniques which are used to prepare borided layers on the surface of steels. Among the various boronizing processes, solid-state pack boriding treatment is the most frequently used. Yet most of them have the disadvantages of requiring relative high processing temperature or time consuming. Studies have been carried out to improve the efficiency of pack boriding treatment over the past decades. 3
ACCEPTED MANUSCRIPT It is well known that borided layers generated in thermochemical treatment depend on boriding condition and on the properties of the materials itself. Both factors are strongly
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affected by grain boundaries and defect densities in the surface layer. The combination of
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pack boriding treatment with surface mechanical attrition treatment (SMAT) could improve the efficiency of boronizing kinetics. After SMAT, various metals, such as pure Fe [6], 38CrMoAl [7], AISI 321 austenitic stainless [8] and other alloys [9] possess a nanostructured
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surface layer. Due to the significant enhanced diffusion and chemical reaction kinetics in the
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formed nanostructured surface layer, the hardened diffusion layer on the substrate has been fabricated on several ferrous alloys after the subsequent gas nitriding [6, 7] or packed powder
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chromizing treatment [10, 11]. However, the method of SMAT has the disadvantage of its
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restriction on the flat shape of workpiece, which impedes its application in industrial
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production. Fortunately, air blast shot peening (ABSP) can also be used to refine grains and produce high density of defects in the surface layer, which could be a promising method that
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can be used in thermochemical treatment for many kinds of steels. More importantly, ABSP has been applied extensively in material processing. Here, we report that pack boriding treatment was carried out for the hot work die steel H13 assisted by ABSP. The boronizing kinetics in the nanostructured surface layer fabricated by ABSP was studied. Furthermore, the high temperature wear resistances of the borided H13 steels with and without ABSP pretreatment were investigated by reciprocating sliding wear tests at 700°C under the applied loads of 20N.
2. Experimental 2.1 Test materials and ABSP treatment 4
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spheroidizing annealing process was used for the original H13 steel with the shape of
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60×60×4mm, which was annealed at 840°C for 2h. Before ABSP, the annealed steel was mirror polished. Then, the sample was processed by a flow of cast steel balls with diameter of 0.8mm at 0.5MPa for six cyclic deformation, and the time of each cycle was 5min.The angle
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between the shot jet and the sample surface is in the range of 70~90°. The ABSP samples
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with dimensions of 15×15×4mm were machined from the bulk sample mentioned above and ultrasonically cleaned in acetone.
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2.2 Boriding process
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Two kinds of samples were used in the duplex boronizing treatment(DBT), which
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include the annealed coarse-grained(CG) sample and the annealed ABSP sample. The pack boriding treatment was as follows: the constituents of the boriding media were B4C(10wt.%),
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KBF4(5wt.%), charcoal(5wt.%) with the balance of SiC. The ABSP sample and the CG sample were packed together with a suitable distance from each other in the powder mix and sealed in a stainless steel container. In order to stabilize the nanostructures in the surface layer fabricated by ABSP, a duplex boriding treatment at T1=600°C followed by at a higher temperatures (T2) for a certain time was carried out in an electrical resistance furnace. In the first stage, the samples were heated to T1 = 600°C for 2 h. In the second stage, the temperature was increased to T2 = 750°C, 800°C and 850°C for 2, 4 and 8h, respectively. After the boriding treatment, the container was removed from the furnace and cooled in air. At last, all the borided samples and the annealed CG sample without boriding treatment were 5
ACCEPTED MANUSCRIPT quenched at 1030°C and subsequently tempered twice at 580°C in a vacuum oven. 2.3 High temperature friction and wear testing procedure
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A UMT-3 tribometer (Fig.1) equipped with a reciprocating configuration placed in a
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heating furnace was used to carry out the friction and wear tests at elevated temperatures. A ball holder is connected to both a vertical and lateral linear motion system. The samples are pinned to a reciprocating stage in a high temperature chamber that allows testing
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temperatures up to 1000°C. Three kinds of samples were used in the wear tests at high
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temperature, which include the quenched and tempered sample, the CG sample with DBT and the ABSP sample with DBT. The size and geometry of a high temperature friction and
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wear test specimen are shown in Fig.2. The surface (10×35.6mm) is used for friction and
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wear test. The dry sliding wear tests were operated by the parameters which are shown in
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Table 1. Silicon carbide balls with a diameter of 9.5mm and hardness of 2800HV were chosen as the counterpart in order to evaluate the wear properties of the borided layers at high
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temperatures. The wear rate is calculated by the equation as follows:
Ws V ( p d)
(1)
where V is the wear volume. P is the loading force and d is the total sliding distance. 2.4. Characterization The X-ray diffraction (XRD) measurements were obtained with a RigakuD/MaxRBX-ray diffractometer by using CuKa (40kV, 40mA) radiation and a secondary beam graphite monochromator. A Zeiss Supra 40 scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) system was used to characterize cross-sectional of the microstructures of borided samples and wear morphologies of the 6
ACCEPTED MANUSCRIPT borided samples and sample without boriding treatment. Microstructural features in the surface layer treated with ABSP were characterized by using a field emission transmission
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electron microscopy (TEM, JEOLJEM-2010F) equipped with an energy dispersive spectral
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(EDS) analyzer. Thin foil samples for TEM observations were prepared by cutting, grinding and dimpling with a final ion thinning at low temperatures. The hardness distributions of the cross-sectioned borided layers were measured by a Vickers microhardness tester with a load
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of 100g force. A LECO GDS850A glow-discharge optical emission spectrometer (GD-OES)
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was used to measure the concentrations of boron element in the surface layers of the ABSP sample and the CG sample with boriding treatment at 600°C for 2h. The wear volumes of the
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samples were measured by a Bruker ContourGT-K 3D optical profiler. The method for
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measuring the residual stress after air blast shot peening and duplex boronizing treatment is
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as follows: By using the two-angle sin2ψ method with the ψ angles fixed at 0° and 45° [12], the residual stresses were measured by X-350A X-ray stress analyzer with Cr-Kα radiation.
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The locations of corresponding diffraction angles 2θ0° and 2θ45° were determined by cross-correlation method[13], and the stresses were calculated by using the formula as follows[12]:
2K (2 )
(2)
The residual stresses measurements of samples without pack boriding were made employing the diffraction of the (211) crystallographic plane of α-Fe. The stress constant K of H13 steel is -318MPa/º, which is commonly used for iron-based alloys. The residual stresses in borided samples were determined by using the diffraction of the (330)
7
ACCEPTED MANUSCRIPT crystallographic plane of Fe2B, and the stress constant K of Fe2B can be calculated by the
E cot 0 2(1 v) 180
(3)
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K
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following formula[14]:
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where E is Young's modulus, ν is Poisson's ratio, θ0 is the Bragg angle of (330) crystallographic plane of Fe2B in the free stress state. Substituting these material constants of
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Fe2B (E= 290GPa, ν = 0.2 and θ0 =71.9º) into the formula, the stress constant K of Fe2B was -689.3MPa/ º. Residual stress tests were repeated three times for each sample and the mean
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value was calculated.
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3. Results and discussion
3.1 Microstructure of the ABSP sample
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The SEM image of the cross section of the sample treated by ABSP is shown in Fig. 3(a). It can be seen that a plastic deformation layer (~10µm) is present in the treated surface layer,
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which is indicated by a white-white line, where the microstructural morphology differs from that in the matrix. In subsurface zone, it is work-hardened region, which is characterized with the elongated grain boundaries. Fig.3(b) displays a typical bright-field TEM image of the top layer. The statistical distribution of grain size is shown by Fig.3(c), which indicates the size of vast majority of grains is at 12~22nm. The selected-area electron diffraction pattern is shown in Fig.3(d). It reveals that the microstructure is characterized by ultrafine equiaxed grains with random crystallographic orientation. The strong diffraction rings are assigned to polycrystalline -Fe. There is also one reflection that could be interpreted as austenite reflection(311). It may reveal that to transformation in the nanostructured surface layer at 8
ACCEPTED MANUSCRIPT about room temperature during severe plastic deformation with very large strain and strain rate, due to the formation of supersaturated nanocrystalline ferrite and the compression stress
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at the boundaries of nano-size grains [15]. When comparing to the conventional grain
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boundaries(GBs) in the annealed coarse-grained(CG) samples, the GBs formed by the severe plastic deformation are usually in non-equilibrium states which are associated with a high density of defects or a high stored energy [16]. GBs diffusivity in the deformation layer
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produced by ABSP will be considerably enhanced when compared to that along high angle
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GBs in the CG counterpart. It should be noted that the thermal stability of the nanostructured surface layer can be preserved up to 600℃ according to the previous study results [17].
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3.2 Diffusion of boron in the nanostructured surface layer
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The concentration of boron along the depth from the treated surface of the ABSP sample
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and the CG counterpart with pack boriding treatment at 600℃ for 2h were determined by GD-OES tests, as shown in Fig.4. The peak value of concentration in the top surface layer for
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the ABSP sample is about 1.5 wt.%, while the peak value for the CG sample is only about 0.8 wt.%. Furthermore, the depth of boron diffusion in ABSP sample is about 12µm, which is much deeper than that in the CG sample. Based on the experimental results mentioned above, it can be concluded that the thermal diffusion of boron in the nanostructured surface layer fabricated by ABSP can be evidently enhanced when compared to that in the CG sample with the same experimental parameter. According to the diffusion equation for semi-infinite solid diffusion [18]:
X Dmt
(4)
where is a constant (about 1 in the present case), t is the duration of the pack boriding 9
ACCEPTED MANUSCRIPT treatment, and Dm is the mean diffusion coefficient. The measured diffusion depth into the ABSP sample is larger than that into the CG counterpart, indicating the change of diffusion
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mechanism in the ABSP surface layer. Various non-equilibrium GBs dominate the much
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enhanced boronizing kinetics in the ABSP surface layer at 600℃. The mean GBs diffusion coefficient calculated from the measured boron diffusion depth in the ABSP sample according to Eq. (4) is 2.0×10-14 m2/s, which is much higher than 5.0×10-15 m2/s for the CG sample.
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3.3 Microstructure and hardness of the borided layer
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The pack boriding treatment at 600 ℃ was found to facilitate stabilizing the nanostructures in the ABSP surface layer, which was useful to form a much thicker borided
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layer with a subsequent boronizing process at a higher temperature. Fig. 5(a) and (b) show
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the cross-sectional SEM observations for the CG sample and the ABSP sample after a duplex
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boronizing treatment(DBT) at T1=600 ℃ for 2h followed by at T2=850 ℃ for 8h, respectively. It can be seen that the continuous borided layers are on the two sample surfaces.
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The thicknesses of borided layers on the ABSP sample and the CG counterpart are 40µm and 30µm, respectively. Neither FeB(orthorhombic) nor Fe2B(tetragonal) has cubic crystal symmetry [19]. Although H13 steel possesses a certain amount of alloying elements, boron diffusion in the borided layer is of anisotropic nature at high temperatures. Therefore, the morphologies of borided layers exhibit tooth-like growth into substrate. The microhardness gradients for the two samples are showed in Fig. 5(c). The microhardness of borides formed on the surface of ABSP sample is about 1700HV0.1 and it is roughly 3.5 times harder than that of the substrate. In comparison, the microhardness of borides is about 1600HV0.1 for the CG counterpart. The detailed microstructures of the two kinds of boried samples are shown in 10
ACCEPTED MANUSCRIPT Fig.6. For both samples, the surface layer includes three distinct regions, which are borided layer, transition zone and matrix that is not affected by boron diffusion. The EDS point
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microanalysis results for the points A, B, C, D, E and F in these three regions of both samples
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are shown in Table 2. Because the boron is ultralight element, it could not be detected by means of EDS method. Therefore, the boron element does not appear in the EDS point microanalysis results. It should be noted that the mass percent of silicon in transition zones of
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CG sample and ABSP sample with DBT are 2.81wt.% and 2.93wt.%, respectively. The
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concentration of silicon in transition zone of both samples is much higher than that in H13 steel, as silicon was not soluble in borided layer and diffused into transition zone. It is well
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known that silicon is a ferrite-forming element. The high concentration of silicon in the
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transition zone below the borided layer suggests that this region could not completely
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austenitizing at the conventional austenitizing temperature of H13 steel and there were some undissolved ferrite that existed in transition zone, which induce that the microhardness of
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transition zone is a little softer than that of the substrate. Similar results were reported for H11, H12 and H13 steels [20, 21]. Meanwhile, during the process of DBT, a certain amount of boron atoms could diffuse and dissolve in cementite that existed in transition zone, where the Fe3(C,B) phase with relative large particle size could form. Therefore, the phase composition of transition zone included tempered martensite, undissolved ferrite and Fe3(C, B) phase. In addition, in the process of fabricating the borided layer on H13 steels, the carbon atoms can diffuse from surface layer to internal substrate, which can be ascribed that carbon is not soluble in borided layer. The concentration of carbon atoms in the transition zone below borided layer of the ABSP and CG samples with DBT could be much higher than that 11
ACCEPTED MANUSCRIPT in the internal substrate, as shown in Table 2. Therefore, the chemical composition of the transition zone is close to that of high carbon alloy steel. After the borided samples were
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quenched at 1030℃, the concentration of the carbon atoms in quenching martensite in the
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transition zone is much higher than that in the internal substrate. When the borided samples with microscopic structure of quenching martensite were tempered twice at 580℃ in vacuum oven, the quantity of alloy carbide which precipitated from quenching martensite in the
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transition zone could be significantly higher than that in the internal substrate, which can be
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confirmed by the detailed microstructures of the ABSP sample and CG sample with DBT in Fig.6. As shown in Fig. 7, based on the pack boriding treatment at T1=600℃ for 2h, a much
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thicker borided layer was formed on the ABSP sample than that on the CG counterpart after
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the boronizing treatment at any T2 from 750-850℃ for 8h. The thicknesses of borided layers
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in both samples increase with an increasing T2. It can be seen that the low temperature treatment at 600℃ will enhance the boronizing kinetics of the ABSP sample in a DBT with
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any T2 employed.
3.4 XRD measurements The XRD patterns of surface layer of the ABSP sample and the CG sample after a DBT at T1=600℃ for 2h followed by at T2=850℃ for 8h are presented in Fig. 8. For both samples, it is obvious that monophase of Fe2B are detected, which is beneficial for industrial application, owing to its much better toughness when compared to that of FeB phase. The strongest peak can be indexed to the (121) diffraction peak of Fe2B. The intensity of (002) diffraction peak of Fe2B is the second strongest peak, which shows that the growth of Fe2B phase exhibits a (002) preferred orientation. Moreover, from the characteristic of diffraction 12
ACCEPTED MANUSCRIPT peaks, it can be concluded that the Fe2B phase possesses good crystallinity. Additionally, some weak diffraction peaks can be indexed to the phase of Fe2O3, indicating that the slight
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oxidation appeared on the sample surface during the tempering process.
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3.5 Kinetics study of borided layer growth
The growth of borided layer occurs as a consequence of the boron diffusion perpendicular to the specimen surface. According to Eq. (4), the growth of Fe2B layer obeys
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the parabolic law. The square of borided layer thickness synthesized by a DBT as a function
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of boronizing time can be described as follows:
d2 Kt
(5)
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where d is the thickness of borided layer that is measured by the method described in a
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previous paper [22], K is the growth rate constant depending on boronizing temperature and
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is calculated from the slope of the d2 versus treatment time graph, which are presented in Fig.9 and Table 3.
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The relationship between growth rate constant K and boronizing temperature can be expressed by an Arrhenius-type equation as follows [23]:
K K0 exp(
Q ) RT
(6)
where K0 is called the collision factor and is a measurement of the effectiveness of collisions between reactive species, Q is an activation factor which indicates the amount of energy(J/mol) required to make the reaction occur. T is the absolute temperature and R is the gas constant(J/(molK)). Taking the natural logarithm of each side of Eq. (6) gives Eq. (7) as follows:
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ACCEPTED MANUSCRIPT InK InK0
Q RT
(7)
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The plot of InK versus the reciprocal absolute temperature (1/T) is a linear relationship.
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The activation energy Q and the collision factor K0 can be calculated from the slopes and the
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intercepts of the graphs in Fig.10, respectively, which are presented in Table 3. The calculated activation energy value for ABSP sample is lower than that for CG sample, which confirms
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the enhancement of the boronizing kinetics of the ABSP sample with a DBT. Based on the results mentioned above, the following empirical equations were derived to estimate the
CG
sample:
dCG =1.9110-1 exp(
31321 )t T
1023 T 1123
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For
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borided layer thicknesses in a given treatment time and temperature.
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(8)
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For ABSP sample: dABSP =3.6510-2 exp(
27351 )t T
1023 T 1123
(9)
time.
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where d is the thickness of borided layer; T is boronizing temperature(K) and t is boronizing
3.6 High temperature wear property of borided sample The high temperature wear property is very important for hot work die steels in the industrial application process. The wear rates of the quenched and tempered sample (A) without boriding treatment, CG sample (B) with DBT and ABSP sample(C) with DBT are shown in Fig.11. The wear rates of A sample is 3.42×10-13m3(N·m)-1. In contrast, the wear rates of B sample and C sample are 2.57×10-13m3(N·m)-1 and 2.02×10-13m3(N·m)-1, respectively. When compared to that of A sample, the wear rates of B sample and C sample are decreased by 25% and 41%, respectively. The results showed that the high temperature 14
ACCEPTED MANUSCRIPT wear resistance of H13 steel with DBT can be improved significantly, owing to the high hardness and high oxidation resistance at elevated temperatures of borided layer. Furthermore,
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the H13 steel with DBT assisted by ABSP possesses more superior wear resistance property
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at elevated temperatures than that of CG sample with DBT，which could be attributed that the thickness and microhardness of the borided layer can be increased significantly with the help of ABSP. At the same time, owing to the significant refined grains in the nanostructured
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surface layer of ABSP sample, the refined grains of borides on ABSP sample could be
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synthesized with a DBT. Therefore, the borided layer on H13 steel with DBT assisted by ABSP possesses good toughness and the fatigue wear can be relieved in the process of wear
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tests at elevated temperatures.
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Fig. 12 shows the XRD patterns of the worn surfaces of the quenched and tempered
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sample (a) without boriding treatment, CG sample (b) with DBT and ABSP sample(c) with DBT. The phase composition of worn surface of sample without DBT includes α-Fe and
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Fe2O3. While the worn surfaces of CG sample with DBT and ABSP sample with DBT contain α-Fe, Fe2O3, Fe2B, B2O and α-Fe, Fe2B, Fe2O3, respectively. The Fe2B can be detected on the worn surfaces of the samples with DBT, owing to the borided layer at the edge of wear scars. It is clear that the oxidative wear occurred when the high temperature friction and wear tests were carried out under the atmospheric conditions. The diffraction intensity of Fe2O3 on worn surface of sample without DBT is stronger than those on worn surfaces of CG sample and ABSP sample with DBT, which shows there are less oxide that exists on the worn surfaces of CG sample and ABSP sample with DBT, due to the high oxidation resistance of borided layer on them. The types of iron oxides include FeO, Fe3O4 and Fe2O3. The forming temperature of 15
ACCEPTED MANUSCRIPT FeO is above 570℃.While the forming temperature of Fe2O3 and Fe3O4 are below 200℃and 200-570℃, respectively [24]. However, there is only Fe2O3 on the worn surfaces of all
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samples at the temperature of wear test. The reason is as follows: the crystal defects in the
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surface layers of all samples could increase during the process of wear test, which is beneficial to the diffusion of oxygen atoms or oxygen ion in the worn surfaces [25]. Meanwhile, on the basis of thermodynamics, FeO or Fe3O4 can be easily reacted with O2,
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which could produce Fe2O3 at 700℃.
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The morphologies of the worn surfaces of the samples are shown in Fig.13. Fig.13(a) displays that local plastic deformation occurs on the wear track of sample without boriding
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treatment, which can be attributed to massive plastic shearing in the surface layer during the
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dry sliding wear test. As showed in Fig.13(b), the signs of adhesive wear were observed and
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some flaking areas exist in the wear track. Local delamination in the wear track is due to the propagation and coalescence of microcracks during the durative dry sliding. Fig.13(c)-(f)
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shows the worn surface morphologies of the borided samples with and without ABSP. The worn morphologies confirm that wear resistance could be significantly improved by means of the boriding treatment. There are no signs of severe plastic deformation on the wear track of CG sample with DBT, as showed by Fig.13(c) and (d). However, some fatigue cracks can be found along the edge of wear scar and some flaking areas can be seen on the worn surface. When the SiC ball slides against the sample surface, local plastic deformation occurs on the wear track and microcracks initiation and propagation on the wear surface, which is the sign of fatigue wear. The worn morphology of ABSP sample with DBT is showed in Fig.13(e) and (f). There are no obvious fatigue cracks on the worn surface. A relatively smooth 16
ACCEPTED MANUSCRIPT appearance with some flaking areas can be seen on the worn surface of ABSP sample with DBT, which indicates that its surface possesses better wear resistance when compared to that
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of CG sample with DBT. The EDS analysis of point A, B and C show that the mass percent
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of oxygen element are 40.6%，35.3% and 27.5%, respectively. It demonstrates that the worn surfaces of three samples had been oxidized and oxidative wear had occurred during wear tests at high temperature for samples with and without boriding treatment.
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In order to further explore the wear mechanism, the section morphologies of the worn
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surfaces for three kinds of samples were observed. In Fig.14(a), the obvious plastic deformation appeared in the subsurface and the double layers of tribo-oxides (indicated by
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red arrows) can be seen on the wear surface of the sample without boriding treatment. The
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wear mechanism and wear loss are closely related with thermal softening extent of the matrix.
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During the wear test at elevated temperatures, the friction force and normal pressure resulted in the thermal softening of matrix, which incorporated with plastic deformation, dynamic
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recovery and recrystallization [26]. When the large softening region appeared in the subsurface, more plastic deformation occurred and it could not support tribo-oxides, which results in accelerating delamination of the tribo-oxides. At the plastic deformation region, the density of crystal defects increased and oxygen atoms or ions could easily diffuse into subsurface of matrix to form oxide or oxide layer. Hence, oxidation could occur not only on the worn surfaces, but also inside the matrix by the direct diffusion of oxygen through the cracks in delaminated regions [27, 28]. Therefore, the double layers of tribo-oxides could form on the worn surfaces. However, before the borided layer was worn out, the deformation and oxidation of the matrix were prevented due to the high hardness, high wear resistance and 17
ACCEPTED MANUSCRIPT oxidation resistance of the borides on the borided samples with and without ABSP. Moreover, although the sample with DBT was finally worn out in the middle of the wear scar, the
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remained borided layers on both sides played a role of supporting the SiC ball. Therefore, the
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plastic deformation was not found in the subsurface layer and single layer of oxides on the worn surface of samples with DBT, as shown in Fig.14(b) and (c). The EDS analysis of point A, B and C show that the mass percent of oxygen element are 37.5%，30.8% and 21.9%,
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respectively. As can be seen from Fig.14(d) and (e), the borided layers on samples with DBT
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were finally worn out in the middle of the wear scar, but the borided layer along the edge of wear scar still adhered to the substrate. However, there are some cracks in the borided layer
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on CG sample with DBT.
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Shot peening is usually applied as surface modification technology to increase fatigue
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resistance. This beneficial effect is associated with compressive residual stress induced in the surface layer. The residual stresses of CG samples and ABSP samples before and after the
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DBT are listed in Table 4. The result shows that compressive residual stresses were detected on the surfaces of all samples. After the balls impacted during the ABSP treatment, a uniform layer of compressive residual stress was generated on the surface of ABSP sample without a DBT through overlapping dimples [29]. In addition, polishing could also induce a compressive stress on the surface layer of CG sample without a DBT. For the CG sample with a DBT, a compressive stress was also obtained. This can be explained by the formation of borides with the higher specific volume in the surface layer. Since the compressive residual stress originated from ABSP, the compressive stress in the surface layer of ABSP sample with DBT is much higher than that of CG sample with DBT. It is well known that 18
ACCEPTED MANUSCRIPT compressive residual stress and work hardening induced by shot peening could effectively delay crack initiation and propagation, as reported by other reports [30-32]. Therefore, the
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compressive residual stress and the refined grains of borides in the borided layer of ABSP
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sample with DBT can help to impede the fatigue crack initiation and propagation in the borides during the wear test. The fatigue wear could not easily occur on the surface of ABSP sample with DBT. Meanwhile, the thickness of the borided layer can be increased
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significantly with the help of ABSP, which is beneficial for improving the high temperature
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wear resistance of H13 steel. Therefore, the wear rate of borided sample with ABSP is smaller than that of borided sample without ABSP. The H13 steel with DBT assisted by
4. Conclusions
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CG sample with DBT.
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ABSP possesses more superior wear resistance property at elevated temperatures than that of
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The nanostructured surface layer can be fabricated on H13 steel assisted by ABSP. There are various non-equilibrium GBs in it, along which the diffusion rate of boron is much higher than that along the relaxed GBs in the coarse-grained counterpart. By means of a DBT at T1 =600℃ for 2h followed by at T2=850℃ for 8h，the thicknesses of Fe2B phase on the ABSP sample and the coarse-grained counterpart are 40µm and 30µm, respectively. The calculated activation energy value for ABSP sample (227.4 kJ/mol) is lower than that for coarse-grained sample(260.4 kJ/mol). It can be concluded that the boronizing kinetics of the ABSP sample after a DBT at T1=600℃ for 2h followed by at a higher temperature for a certain time can be enhanced effectively. The high temperature wear resistance of H13 steel with DBT can be 19
ACCEPTED MANUSCRIPT improved significantly, owing to the high hardness and high oxidation resistance at elevated temperatures of borided layer. Furthermore, the thickness and microhardness of the borided
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layer can be increased with the help of ABSP. Meanwhile, the compressive residual stress and
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the refined grains in the borides of ABSP sample with DBT can impede the fatigue crack initiation and propagation in borided layer during the wear test. Therefore, the H13 steel with DBT assisted by ABSP possesses more superior wear resistance property at elevated
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temperatures than that of coarse-grained sample with DBT.
Acknowledgments
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The authors acknowledge the “11th Five” National Science and Technology Support
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Project of China (Project Number: 2007BAE51B04) and Shanghai Leading Academic
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Discipline Project for the financial support (Project Number: S30107). The authors would like to thank X.J.He from School of Materials Science and Engineering of Shanghai
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University for the help with the SEM measurements.
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[26] S.Q.Wang, M.X.Wei, F.Wang, Y.T.Zhao, Tribol. Int. 43 (2010) 577–584.
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[27] M.X.Wei, S.Q.Wang, L.Wang, X.H.Cui, K.M.Chen, Tribol. Int. 44 (2011) 898–905. [28] M.X.Wei, F.Wang, S.Q.Wang, X.H.Cui, Mater. Des. 30 (2009) 3608–3614. [29] O. Higounenc, Correlation of shot peening parameters to surface characteristic, in:
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International conference on shot peening -ICSP-9, Paris, France, 2005, p. 28-35.
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[30] K. Zhan, C.H. Jiang, V. Ji, Mater. Lett. 99 (2013) 61-64. [31]C.M.Suh, G.H.Song, M.S.Suh,Y.S.Pyoun, Mater. Sci. Eng. A 443 (2007) 101-106.
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[32] L. Trško, O. Bokůvka, F. Nový, M. Guagliano, Mater. Des. 57 (2014) 103-113.
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The experimental parameters of friction and wear tests. Normal load
Reciprocating frequency
Sliding stroke
Temperature
Time
20N
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10mm
700°C
1200s
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The EDS analysis results for the points A, B, C, D, E and F in Fig.6. CK
Mn K
Cr K
Mo L
VK
Fe K
Totals
A(wt.%)
1.42
B(wt.%)
4.75
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4.12
0.76
0.42
93.28
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2.81
0.21
5.35
1.67
0.92
84.29
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C(wt.%)
3.78
1.14
0.37
5.20
1.46
0.76
87.29
100
D(wt.%)
2.03
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4.93
0.48
0.32
92.24
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E(wt.%)
4.18
2.93
0.40
5.22
1.75
0.82
84.70
100
F(wt.%)
3.28
1.25
0.45
5.09
1.28
1.03
87.62
100
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Growth rate constant, calculated activation energy and collision factor for test materials as a
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ABSP samples
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Treatment temperatures T(℃) 750 800 850 750 800 850
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Growth rate constant K(m2/s) 4.48E-15 1.50E-14 4.84E-14 2.43E-15 1.04E-14 3.72E-14
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Activation energy Q(kJ/mol) 227.4
Collision factor K0 (m2/s)
260.4
3.63E-2
1.33E-3
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Samples without boriding treatment
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Borided samples
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Fig.1. Schematic representation of the UMT-3 high temperature friction and wear test system.
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Fig.2. The size and geometry of a high temperature friction and wear test specimen (unit: mm).
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Fig.3. (a) Typical SEM image of the cross section of the sample treated by ABSP, (b)bright field TEM image showing the microstructure of the top surface layer of the sample treated by ABSP, (c)the graph of statistical distribution of grain size and (d)corresponding selected area electron diffraction pattern.
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Fig.4. The boron concentration gradients of the CG specimen and the ABSP specimen with pack boriding treatment at 600℃ for 2h.
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Fig.5. Typical SEM images of the cross section of (a) CG specimen, (b) ABSP specimen with a duplex pack boriding treatment and (c) the corresponding microhardness gradients of them.
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Fig.6. Typical SEM images of the cross section of (a) CG specimen and (b) ABSP specimen with a duplex pack boriding treatment at T1 =600℃ for 2h followed by at T2 =850℃ for 8h.
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Fig.7. The graph of thicknesses of borided layers formed on the CG specimen and the ABSP specimen with pack boriding treatments at 600℃for 2h followed by heating at 750℃、800℃ and 850℃ for 8h.
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Fig.8. XRD patterns of the CG specimen and the ABSP specimen with a duplex pack boriding treatment at T1 =600℃ for 2h followed by at T2 =850℃ for 8h. Fig.9. Square of borided layers thicknesses of (a) the CG specimen and (b) the ABSP specimen with a duplex pack boriding treatment as a function of boronizing time at different temperatures. Fig.10. Natural logarithm of growth rate constants of the CG specimen and the ABSP specimen as a function of reciprocal of boronizing temperatures. Fig.11. The wear rates at an elevated temperature of (A) the quenched and tempered specimen without boriding treatment, (B) the CG specimen and (C) the ABSP specimen with a duplex pack boriding treatment. Fig.12. XRD patterns of the wear surfaces at an elevated temperature of (a) the quenched and tempered specimen without boriding treatment, (b) the CG specimen and (c) the ABSP specimen with a duplex pack boriding treatment. Fig.13. Typical SEM images of the wear surface at an elevated temperature of (a) and (b) the 27
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quenched and tempered specimen without boriding treatment, (c) and (d) the CG specimen and (e) and (f) the ABSP specimen with a duplex pack boriding treatment. Fig.14. The cross section morphologies of the worn surfaces of (a) the quenched and tempered sample without boriding treatment, (b) borided sample without ABSP and (c) borided sample with ABSP; the cross section morphologies of borided layers at the edge of wear scars of (d) borided sample without ABSP and (e) borided sample with ABSP.
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Fig. 1.
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Fig. 2.
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Fig. 3.
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Fig. 4.
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Fig. 5.
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The experimental parameters of friction and wear tests.
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Table 2
CK
Si K
Mn K
Cr K
Mo L
VK
Fe K
Totals
A(wt.%)
1.42
-
-
4.12
0.76
0.42
93.28
100
B(wt.%)
4.75
2.81
0.21
5.35
1.67
0.92
84.29
100
C(wt.%)
3.78
1.14
0.37
5.20
1.46
0.76
87.29
100
D(wt.%)
2.03
-
-
4.93
0.48
0.32
92.24
100
E(wt.%)
4.18
2.93
0.40
5.22
1.75
0.82
84.70
100
F(wt.%)
3.28
1.25
0.45
5.09
1.28
1.03
87.62
100
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Table 3
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Growth rate constant, calculated activation energy and collision factor for test materials as a function of treatment temperatures.
Growth rate constant K(m2/s) 4.48E-15 1.50E-14 4.84E-14 2.43E-15 1.04E-14 3.72E-14
Activation energy Q(kJ/mol) 227.4
Collision factor K0 (m2/s)
260.4
3.63E-2
1.33E-3
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Treatment temperatures T(℃) 750 800 850 750 800 850
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ACCEPTED MANUSCRIPT Dear Editors of Surface & Coatings Technology: The main highlights of this rearch are as follows:
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1. The boronizing kinetics of ABSP sample can be enhanced effectively.
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2. The fatigue spall of borides at the edge of wear scar was prevented by ABSP.
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3. The oxidative wear was relieved for borided samples with wear tests at 700°C.
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