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Pack boronizing of AISI H11 tool steel: Role of surface mechanical attrition treatment
Vacuum 97 (2013) 36e43
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Pack boronizing of AISI H11 tool steel: Role of surface mechanical attrition treatment T. Balusamy a, b, T.S.N. Sankara Narayanan a, c, *, K. Ravichandran b, Il Song Park c, Min Ho Lee c, * a b c
CSIR-National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India Department of Analytical Chemistry, University of Madras, Maraimalai (Guindy) Campus, Chennai 600 025, India Department of Dental Biomaterials, School of Dentistry, Chonbuk National University, Jeonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2012 Received in revised form 3 April 2013 Accepted 4 April 2013
The role of surface mechanical attrition treatment (SMAT) on pack boronizing of AISI H11 type tool steel is addressed. SMAT induced plastic deformation, enabled nanocrystallization at the surface, reduced the grain size and increased the volume fraction of non-equilibrium gain boundaries, increased the accumulation of defects and dislocations at the grain boundaries and within the grains. These features helped to promote the diffusion of boron during boronizing and increased the case depth and hardness of the borided layer. Duplex treatment on SMATed H11 steel samples helps to achieve a higher case depth when compared to the single stage treatment. The findings of the study suggest that SMAT can be used as a pretreatment for boronizing of H11 tool steel. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Surface mechanical attrition treatment (SMAT) Boronizing Iron borides Alloy borides Diffusion Duplex treatment
1. Introduction The surface properties of materials such as wear resistance, friction, corrosion resistance and oxidation resistance play a key role in determining the service life of many engineering components, which lead to the development of numerous surface modification methods. Boronizing is a thermo-chemical surface treatment process that involves diffusion of boron atoms into the surface of metal/alloy to produce a layer of borides of the corresponding metal/alloying elements [1,2]. The major advantages of surface treatment by boronizing are its ability to impart high hardness, excellent abrasion and wear resistance and, oxidation resistance compared with other similar treatments [3e9]. Tool steels, particularly the H type are commonly used in many industrial applications. Being an alloy steel, improving the surface reactivity and accelerating the diffusion of boron is a challenging task.
* Corresponding authors. Department of Dental Biomaterials, School of Dentistry, Chonbuk National University, Jeonju 561-756, South Korea. Tel./fax: þ82 063 270 4040. E-mail addresses: [email protected], [email protected] (T.S.N. Sankara Narayanan), [email protected] (M.H. Lee). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.04.006
It is well known that in ultra-fine grained (UFG) and nanocrystalline materials, the presence of a large number of grain boundaries and triple junctions could act as fast atomic diffusion channels [10,11]. In the past decade, diffusion behaviour in UFG or nanostructured materials produced by severe plastic deformation (SPD) has become an attractive topic, since SPD is a promising route for producing bulk nanostructured materials with enhanced properties [12,13]. Surface mechanical attrition treatment (SMAT) is a surface severe plastic deformation method that enables nanocrystallization at the surface of various metallic materials [14e21]. SMAT enhanced the kinetics of diffusion of nitrogen, aluminium, chromium and zinc during nitriding, aluminizing, chromizing and diffusion zinc coatings, respectively [22e26]. These processes, however, are relatively low temperature processes when compared to boronizing. Hence, materials subjected to SMAT could maintain their nanostructured surface layer. In addition, the increase in grain size during these processes is rather limited to cause any significant influence on the kinetics of diffusion of the corresponding elements. However, the temperature employed for boronizing of most of the ferrous alloys is of the order of 1173e1273 K for a few hours [3e9,27] and, hence it is very difficult to realize the benefits of surface nanostructuring at such conditions. This has also been addressed in our earlier paper on the effect of SMAT on boronizing
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Fig. 1. Optical micrographs taken at the surface (a, b) and cross section (c, d) of AISI H11 tool steel: (a, c) untreated; and (b, d) after SMAT using 8 mm B 316L SS balls for 3600 s (c, d e dark field images).
100
40 20 SMATed
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Fe (200)
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Fe (211)
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AISI H11 steel square blocks (70 mm 70 mm and 8 mm thick), having a chemical composition (in wt. %) C: 0.40; Si: 0.84; Mn: 0.34; P: 0.012; S: 0.016; Cr: 4.80; Mo: 1.19; Ni: 0.26; Cu: 0.07; Ti: 0.005; V: 0.34; Fe: balance; were used as substrate materials. These samples were procured in heat-treated (at 1020 C for 2 h followed by quenching in air) condition. Generally, hot work tool steel that contains about 0.4% C and 5% Cr is likely to form carbides of M23C6
Fe (211)
60
Fe (200)
Fe (110)
80
Fe (110)
2. Experimental details
and/or M7C3-type (ferrite and spheroidized carbides). In addition, it is alloyed with small amounts of vanadium and molybdenum. Hence, to avoid the precipitation of secondary carbides at the grain boundaries, heat treatment of tool steels is performed at w1020 C to achieve proper functionality of the tools. The H11 steel samples were degreased using acetone and subjected to SMAT using 8 mm Ø 316L stainless steel (SS) balls for 3600 s. All experiments were performed at a fixed frequency of 50 Hz and under vacuum. The
Intensity / (cps)
of EN8 steel [28]. Surface nanocrystallization by SMAT offers the benefit of formation of borided layers at relatively lower temperatures. Xu et al. [27] have shown that boronizing of SMATed H13 steel could be achieved at 973 K for 28,800 s. In our earlier study, we have shown that it is possible to get a reasonably thick boronized layer on SMATed EN8 steel at 923 K for 25,200 s [28]. However, such a long processing time is not practicable for many industrial components. Boronizing of various types of tool steels such as H11 type [29], H13 type [27,30], D type [31e34], M type [35,36], W type [37,38] and hot work tool steel X38CrMoV5-1 [39] has been studied earlier. Studies on boronizing of H type hot work tool steels are rather limited. The effect of SMAT on boronizing of AISI H11 tool steel is not studied earlier. Since boronizing of alloy steels requires a higher temperature of the order of 1223 K, it would be interesting to know to what extent the surface nanocrystallization generated by SMAT will be useful in promoting the kinetics of diffusion of boron. In addition, it is worthwhile to verify the effectiveness of a duplex treatment similar to the one employed in our earlier study [28]. The present paper aims to address the above issues.
20 Untreated
0 20
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Diffraction angle (2θ) Fig. 2. XRD patterns of untreated AISI H11 tool steel and those subjected to SMAT using 8 mm Ø 316L SS balls for 3600 s.
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schematic and details of SMAT has already been described elsewhere [15,20]. Optical microscopy (Leica DMLM metallurgical microscope) was used to assess the microstructure and extent of deformation. X-ray diffraction (XRD) measurements were performed (system 3003TT GE Technologies) using Cu-Ka radiation at a step-scanning rate with a 2q step of 0.00033 /s to determine the crystallite size and microstrain. The crystallite size was calculated using the full width at half maximum (FWHM) of the Fe (110) plane by fitting it using a pseudo-Voigt function. The instrumental broadening was estimated using a standard silicon sample and its FWHM was subtracted from that of the Fe (110) plane to calculate the actual FWHM. The broadening due to both crystallite size and microstrain were taken care of by considering the constituent integral breadths of pseudo-Voigt function [40]. The crystallite size was calculated using the Scherrer’s formula (D ¼ (0.9l)/bc cos q)
where D, l, q and bc represent the crystallite size, wavelength of the incident X-ray beam, diffraction angle and integral breadth of the Cauchy component of the structurally broadened profile, respectively. Untreated and SMATed H11 steel samples having a dimension of 17 8 4 mm3 were packed along with a powder mixture of boron carbide (5%), potassium tetrafluoborate (5%) and silicon carbide (90%) in an Inconel box and subjected to boronizing at 1223 K for 3600, 10,800 and 18,000 s in a muffle furnace. After treatment, the boronized samples were quenched in air up to room temperature. Pack boronizing was also performed by a duplex treatment. In the first stage, the temperature of the samples was increased to 973 K and they were soaked at 973 K for 3600 s. Subsequently, in the second stage, the temperature was increased from 973 to 1223 K and the samples were soaked at 1223 K for another 3600 s followed
Fig. 3. Optical micrographs taken at the cross section of untreated and SMATed AISI H11 tool steel pack boronized at 1223 K for 3600 s (a, b), 10,800 s (c, d) and 18,000 s (e, f): (a, c, e) untreated; and (b, d, f) SMATed using 8 mm B 316L SS balls for 3600 s.
T. Balusamy et al. / Vacuum 97 (2013) 36e43
by air quenching up to room temperature. During duplex treatment, the rate of increase in temperature was kept at 0.083 K/s. After boronizing, the samples were thoroughly cleaned in acetone using an ultrasonic cleaner for 900 s to remove the boronizing mixture adhered on their surface. The phase content of the untreated and SMATed H11 steel samples after boronizing was determined by XRD measurement taken directly from their surface. For microstructural analysis and microhardness measurement, the boronized H11 steel samples were embedded in a phenolic resin mould, ground using successive grades of SiC coated abrasive papers, polished using 3 mm diamond paste, rinsed with deionized water and dried. The microstructure and case depth of the untreated and SMATed H11 steel samples after boronizing was assessed using Leica DMLM metallurgical microscope. The thickness of each boride phase is calculated using the optical microscope and image analyzer software after drawing a horizontal line on the acquired micrograph across each region. The microhardness of the boronized layer was measured using Leica VMHT (MOT) microhardness tester with 50 gf load applied for 15 s.
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Fig. 4. Variation in case depth (average layer thickness) of untreated and SMATed AISI H11 tool steel pack boronized at 1223 K as a function of treatment time.
3. Results and discussion 3.1. Characteristics of SMAT treated AISI H11 steel During SMAT, impingement of the 316L SS balls on the surface of H11 steel induced plastic deformation with a high strain rate during each impact. Comparison of the surface microstructure of untreated and SMATed H11 steel samples reveals the presence of needle type martensite phase on the untreated one (Fig. 1(a)) and the formation of some fine particle dispersion along the martensite phase for the SMATed sample (Fig. 1(b)). The presence of the needle type martensite phase on untreated H11 steel has also been observed by Jurci et al. [29] and it is due to the effect of heat treatment of the steel at 1020 C for 2 h followed by quenching in air. The formation of some fine particle dispersion along the martensite phase observed for the SMATed sample (Fig. 1(b)) indicates that the initial condition of H11 steel did not affect the SMAT. The dark field optical micrographs of untreated (Fig. 1(c)) and SMATed H11 steel (Fig. 1(d)) samples taken at the cross section indicate that the thickness of the deformed layer after SMAT is limited to w15 mm. This may be due to the hardness of the substrate (56 HRC), which is higher than that of the 316L SS balls (15 HRC) used for the treatment. Xu et al. [27] have observed a deformation layer thickness of w20 mm on H13 steel after SMAT treatment using 8 mm B SS balls for 2400 s. However, the hardness of H13 hot work tool steel (51e53 HRC) is relatively lower than that of H11 steel (56 HRC). SMAT increased the average surface roughness (Ra) of H11 steel from 0.22 0.03 mm to 1.39 0.02 mm. The increase in surface roughness is due to the formation of craters and dimples following the impingement of the SS balls on the surface of H11 steel. The XRD patterns of untreated and SMATed H11 steel are shown in Fig. 2. Broadening of the Bragg diffraction peaks exhibited by SMATed samples is attributed to the reduction in crystallite size and increases in microstrain. The crystallite size of the untreated and SMATed sample is 35 nm and 17 nm, respectively. The decrease in crystallite size following SMAT is due to the progressive refinement of coarse grains to finer grains induced by the mechanical impact.
(Region II); and (iii) the bare matrix that is not affected by boronizing (Region III). The presence of the alloying elements such as Cr and V though likely to cause an inhibiting effect, the boronizing process is not completely inhibited. The borided layers formed on the surface of untreated and SMATed H11 steel samples are not much smooth when compared to that formed on stainless steels [41e43] but they are relatively smooth and compact when compared to those formed on low-alloyed steels, which exhibit a typical saw-tooth pattern [3e5,28]. This is due to the lower amount of alloying elements present in the H11 steel when compared to 304 and 316 SS. During the beginning of boronizing process, the borides nucleate and grow in a columnar nature due to the anisotropic diffusion of boron atoms whereas a transformation into smooth and compact structure occurs only if the uneven growth of the boride layer is inhibited. In case of alloy steels and stainless steels, the alloying elements that concentrate at the tips of the boride columns by a substitutional procedure decrease the active boron flux in these zones and inhibit the uneven growth of the borided layers. The formation of saw-tooth morphology is observed during boronizing of H11 steel [29]. However, in spite of almost a similar composition, H13 steel exhibits a smooth and compact
3.2. Role of SMAT on boronizing of AISI H11 steel The optical micrographs of untreated and SMATed H11 steel samples after pack boronizing at 1223 K for 3600, 10,800 and 18,000 s, are shown in Fig. 3. Three distinct regions could be identified: (i) boronized layer (Region I); (ii) the region below the boride layer, where boron makes solid solution (diffusion zone)
Fig. 5. XRD patterns of untreated and SMATed AISI H11 tool steel after pack boronizing at 1223 K for 3600 s.
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morphology due to the presence of Cr [30]. Another feature that is evident in Fig. 3 is the formation of excess carbides beneath the boronized layer. It is well known that carbon is insoluble in iron borides and it segregates beneath the Fe2B layer, forming additional portions of carbides [29,44]. The diffusion region also contains some of the insoluble carbides that are randomly distributed (white colour regions) while some of it is diffused into the core material. This is clearly visible in the microstructure of samples subjected for treatment for 18,000 s (Fig. 3(e) and (f)). The variation in case depth of untreated and SMATed H11 steel samples measured as a function of treatment time is shown in Fig. 4. It is evident from Fig. 3 (parallel lines indicate the layer thickness) and Fig. 4 that SMAT enables
an increase in case depth of the borided layer. A similar phenomenon was also observed in our earlier study [28]. The extent of increase in case depth of the borided layer following SMAT observed in the present study is comparable with the results of Xu et al. [27]. Jauhari et al. [43] have also observed an increase in case depth of boronized layer of duplex stainless steel subjected to superplastic deformation. The XRD patterns of untreated and SMATed H11 steel samples pack boronized at 1223 K for 3600 s are shown in Fig. 5. The formation of FeB, Fe2B, CrB, Cr2B phases are observed in both samples. However, the intensity of the peaks pertaining to these phases is relatively higher for SMATed sample. This observation suggests the
Fig. 6. Optical micrographs taken at the cross section showing the indentation marks (aed) and hardness profile (e) of AISI H11 tool steel pack boronized at 1223 K for 10,800 s: (a, b) untreated; and (c, d) SMATed using 8 mm Ø 316L SS balls for 3600 s.
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formation of a higher volume fraction of metal borides for SMATed H11 steel when compared to the untreated one. It is difficult to quantify the alloy boride phases because both FeB and CrB phases possess a similar type of structure [45]. It is obvious to expect an increase in the volume fraction of metal borides with an increase in temperature and treatment time [9,46]. In addition, it is also very clear that considerable amount of Cr can dissolve in the FeB phase. Generally, diffusion of Cr from the bulk to the surface is likely to occur with increase in temperature. The FeB and Fe2B phases dissolve considerable amounts of Cr [47,48] and the extent of solubility is much higher in FeB than Fe2B [48]. This enables substitution of Cr for Fe that results in the formation of CrB and Cr2B. In the present study, untreated and SMATed H11 steel samples are boronized under similar conditions. Hence, the observed increase in volume fraction of the metal borides of the SMATed sample (Fig. 5) is mainly attributed to the increase in boron diffusion kinetics. The decrease in grain size, formation of nanocrystalline structure with a high free energy state near the surface, increase in volume fraction of non-equilibrium grain boundaries, accumulation of high density of dislocations at the grain boundaries as well as within the grains are considered responsible for promoting the diffusion of boron and accelerating the nucleation and growth of metal borides, that results in a higher case depth of SMATed H11 sample. The optical micrographs showing the indentation marks on untreated and SMATed H11 steel samples after pack boronizing at 1223 K for 10,800 s and their hardness profile are shown in Fig. 6. The size of the indentation marks formed on SMATed sample (Fig. 6(b)) is relatively small when compared to those formed on the untreated one (Fig. 6(a)). The micro hardness of the untreated H11 steel after pack boronizing at 1223 K for 10,800 s is w1750 VHN50gf, while the hardness of SMATed sample after boronizing under similar conditions reaches w1850 VHN50gf. The difference in hardness between the untreated and SMATed sample at the top layer is w100 VHN50gf (Fig. 6(e)). It is evident from Fig. 6(e) that the hardness of the transition zone is low when compared to the substrate and the boronized layer. The decrease in hardness at this region can be ascribed to concentration of silicon atom between the borided layer and substrate. The presence of ferrite-forming alloying elements that are not soluble in the boride layer, particularly silicon, is responsible for the decrease in hardness [2,30,49]. The hardness of borided layers is a function of the type of phases, the volume fraction of these phases and the pores and voids in the boronized layer [46,50]. The formation of pores in the boride layer is mainly attributed to the heterogeneous distribution of the boron and they are detrimental to its mechanical properties [1,2,50]. In the present study, the microstructures of the boronized layer formed on untreated and SMATed H11 steel did not reveal much difference in the porosity. The formation of a relatively higher volume fraction of chromium borides along with iron borides is considered responsible for the increase in hardness of the boronized layer formed on SMATed H11 steel. In spite of the increase in hardness of the boronized layers on SMATed H11 steel, the increase in case depth is rather limited. This is due to the temperature employed (1223 K) for boronizing, which would promote grain growth and the beneficial effect of surface nanocrystallization by SMAT could not be fully realized. A similar inference was also made in our earlier study on pack boronizing of SMATed EN8 steel at 1173 K [28]. These inferences suggest that further modification of the process conditions is warranted. In recent years, a duplex treatment during the formation of diffusion coatings is suggested as one of the possible modifications to improve the materials properties and performance [28,51]. In our earlier study, a duplex treatment is found to be an effective in increasing the boron diffusion kinetics and the formation of a dense
41
and compact boronized layer with a relatively higher case depth on EN8 steels [28]. Wang et al. [51] have also addressed the benefits of duplex treatment during chromizing of SMATed low carbon steel. The optical micrographs of untreated and SMATed H11 steel samples after pack boronizing by duplex treatment (973 K for 3600 s and 1223 K for 3600 s followed by air quenching to room temperature) is shown in Fig. 7. Comparison of the thickness of the borided layer formed by single (Figs. 3 and 4) and two stage (Fig. 7) treatments indicates the effectiveness of these treatments. For untreated H11 steel, the total thickness of the boride layer is 24 and 27 mm for single and duplex treatments, respectively whereas for SMATed H11 steel it is 30 and 35 mm for single and duplex treatments, respectively. It is evident that the duplex treatment is effective in increasing the case depth on SMATed H11 steel when compared to untreated one. During the first stage treatment (973 K for 3600 s), diffusion of boron enables the formation of iron borides along with other alloy borides since no significant grain growth is expected at this condition [25]. Xu et al. [27] have also reported that there is no significant grain growth of H13 steel and it can maintain
Fig. 7. Optical micrographs taken at the cross section of untreated and SMATed AISI H11 tool steel pack boronized by duplex treatment (973 K for 3600 s and 1223 K for 3600 s followed by air quenching to room temperature): (a) untreated; and (b) after SMAT using 8 mm Ø 316L SS balls for 3600 s.
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beneficial to achieve a higher case depth when compared to the single stage treatment. The study concludes that SMAT can be used as an effective pre-treatment for boronizing of H11 steel. Acknowledgements TB and TSNSN thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for their financial support in the form a SUPRA Institutional Project and Dr. S. Srikanth, Director, CSIRNational Metallurgical Laboratory, Jamshedpur, for his constant encouragement and support. References
Fig. 8. X-ray diffraction patterns of untreated and SMATed AISI H11 tool steel after pack boronized by duplex treatment (973 K for 3600 s and 1223 K for 3600 s followed by air quenching to room temperature).
the nanostructure at 973 K for 1800 s. During the second stage treatment (1223 K for 3600 s), the kinetics of formation of iron borides and other alloy borides are increased. The formation of a nanostructured surface layer is the key factor in increasing the diffusion of boron and the kinetics of formation of iron and other alloy borides for SMATed H11 steel sample. A similar trend was observed earlier by Wang et al. [51] during chromizing of low carbon steel. Yang et al. [52] have reported that formation of large number of grain boundaries with various kinds of non-equilibrium defects induced during SMAT enabled an increase in kinetics of plasma boronizing of H13 steel performed at 853 K for 14,400 s. In the present study, the increase in case depth observed during the duplex treatment of H11 steel is due to ability of SMAT to increase the rate of nucleation of boron at relatively lower temperature following a reduction in activation energy for the reaction during the first stage. In addition, the rate of diffusion of boron is increased further with increase in temperature at the second stage. The XRD patterns of untreated and SMATed H11 steel after duplex treatment of pack boronizing is shown in Fig. 8. The types of phases are quite similar to those formed in single stage boronizing. The marginal increase in volume fraction of metal borides could be due to the lower temperature during the first stage treatment. 4. Conclusions SMAT of H11 steel using 8 mm Ø 316L SS balls for 3600 s induced plastic deformation with a high strain rate and increased the surface roughness from 0.22 0.03 mm to 1.39 0.02 mm. After SMAT, the deformation depth is w15 mm and the crystallite size of the top surface is w17 nm. The case depth of the borided layers formed on untreated and SMATed H11 steel confirms the ability of SMAT to increase the kinetics of boron diffusion. The decrease in grain size, formation of nanocrystalline structure with high free energy state near the surface, increase in the volume fraction of non-equilibrium grain boundaries, accumulation of high density of dislocations at the grain boundaries as well as within the grains are considered responsible for the increase in case depth of SMATed H11 steel sample. The formation of a higher volume fraction of metal borides on SMATed H11 steel enables an increase in hardness at the top layer by w100 VHN50gf. The duplex treatment during boronizing is
[1] Chatterjee-Fischer R. Boriding and diffusion metallizing. In: Sudarshan TS, editor. Surface modification technologies. Marcel Dekker, Inc.; 1989. p. 567e609. [2] Sinha AK. Boriding (boronizing) of steels, heat treating, vol. 4. Ohio: ASM Handbook/ASM International; 1991437e47. [3] Bindal C, Ucisik AH. Characterization of boriding of 0.3% C, 0.02% P plain carbon steel. Vacuum 2007;82:90e4. [4] S¸ahin S. Effects of boronizing process on the surface roughness and dimensions of AISI 1020, AISI 1040 and AISI 2714. J Mater Process Technol 2009;209:1736e41. [5] Campos I, Farah M, López N, Bermúdez G, Rodríguez G, VillaVelázquez C. Evaluation of the tool life and fracture toughness of cutting tools boronized by the paste boriding process. Appl Surf Sci 2008;254:2967e74. [6] Tabur M, Izciler M, Gul F, Karacan I. Abrasive wear behavior of boronized AISI 8620 steel. Wear 2009;266:1106e12. [7] Campos I, Palomar-Pardavé M, Amador A, VillaVelázquez C, Hadad J. Corrosion behavior of boride layers evaluated by the EIS technique. Appl Surf Sci 2007;253:9061e6. [8] Tavakoli H, Mousavi Khoie SM. An electrochemical study of the corrosion resistance of boride coating obtained by thermo-reactive diffusion. Mater Chem Phys 2010;124:1134e8. [9] Jiang J, Wang Y, Zhong Q, Zhou Q, Zhang L. Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance. Surf Coat Technol 2011;206:473e8. [10] Gleiter H. Nanocrystalline materials. Prog Mater Sci 1989;33:223e315. [11] Lu K. Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure and properties. Mater Sci Eng R 1996;16:161e221. [12] Valiev RZ, Langdon TG. Principles of equal-channel angular pressing as a processing tool for grain refinement. Prog Mater Sci 2006;51:881e981. [13] Valiev RZ, Zehetbauer MJ, Estrin Y, Hoppel HW, Ivanisenko Y, Hahn H, et al. The innovation potential of bulk nanostructured materials. Adv Eng Mater 2007;9:527e33. [14] Lu K, Lu J. Surface nanocrystallization of metallic meterial Presentation of the concept behind a new approach. J Mater Sci Technol 1999;15:193e7. [15] Lu K, Lu J. Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment. Mater Sci Eng A 2004;375e377: 38e45. [16] Tao NR, Wang ZB, Tong WP, Sui ML, Lu J, Lu K. An investigation of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment. Acta Mater 2002;504:603e16. [17] Zhang HW, Hei ZK, Liu G, Lu J, Lu K. Formation of nanostructured surface layer on AISI 304 stainless steel by means of surface mechanical attrition treatment. Acta Mater 2003;51:1871e81. [18] Zhou L, Liu G, Ma XL, Lu K. Strain-induced refinement in a steel with spheroidal cementite subjected to surface mechanical attrition treatment. Acta Mater 2008;56:78e87. [19] Li WL, Tao NR, Lu K. Fabrication of a gradient nano-micro-structured surface layer on bulk copper by means of a surface mechanical grinding treatment. Scripta Mater 2008;59:546e9. [20] Balusamy T, Kumar Satendra, Sankara Narayanan TSN. Effect of surface nanocrystallization on the corrosion behaviour of AISI 409 stainless steel. Corros Sci 2010;52:3826e34. [21] Anand Kumar S, Ganesh Sundara Raman S, Sankara Narayanan TSN, Gnanamoorthy R. Fretting wear behaviour of surface mechanical attrition treated alloy 718. Surf Coat Technol 2012;206:4425e32. [22] Tong WP, Tao NR, Wang ZB, Lu J, Lu K. Nitriding iron at lower temperatures. Science 2003;299:686e8. [23] Tong WP, Liu CZ, Wang W, Tao NR, Wang ZB, Zuo L, et al. Gaseous nitriding of iron with a nanostructured surface layer. Scripta Mater 2007;57:533e6. [24] Si X, Lu B, Wang Z. Aluminizing low carbon steel at lower temperatures. J Mater Sci Technol 2009;25:433e6. [25] Lu SD, Wang ZB, Lu K. Enhanced chromizing kinetics of tool steel by means of surface mechanical attrition treatment. Mater Sci Eng A 2010;527:995e1002. [26] Wang HL, Wang ZB, Lu K. Enhanced reactive diffusion of Zn in a nanostructured Fe produced by means of surface mechanical attrition treatment. Acta Mater 2012;60:1762e70.
T. Balusamy et al. / Vacuum 97 (2013) 36e43 [27] Xu L, Wu X, Wang H. Influence of surface nano-structured treatment on pack boriding of H13 Steel. J Mater Sci Technol 2007;23:525e8. [28] Balusamy T, Sankara Narayanan TSN, Ravichandran K. Effect of surface mechanical attrition treatment (SMAT) on boronizing of EN8 steel. Surf Coat Technol 2012;213:221e8. [29] Jur ci P, Hudáková M. Diffusion boronizing of H11 hot work tool steel. J Mater Eng Perform 2011;20:1180e7. [30] Genel K. Boriding kinetics of H13 steel. Vacuum 2006;80:451e7. [31] Oliveira CKN, Casteletti LC, Lombardi Neto A, Totten GE, Heck SC. Production and characterization of boride layers on AISI D2 tool steel. Vacuum 2010;84: 792e6. [32] Sista V, Kahvecioglu O, Eryilmaz OL, Erdemir A, Timur S. Electrochemical boriding and characterization of AISI D2 tool steel. Thin Solid Films 2011;520: 1582e8. [33] Vourlias G, Chaliampalias D, Zorba TT, Pavlidou E, Psyllaki P, Paraskevopoulos KM, et al. A combined study of the oxidation mechanism and resistance of AISI D6 steel exposed at high temperature environments. Appl Surf Sci 2011;257:6687e98. [34] Rodríguez-Castro G, Campos-Silva I, Chávez-Gutiérrez E, Martínez-Trinidad J, Hernández-Sánchez E, Torres-Hernández A. Mechanical properties of FeB and Fe2B layers estimated by Berkovich nanoindentation on tool borided steel. Surf Coat Technol 2013;215:291e9. [35] Ozbek I, Bindal C. Kinetics of borided AISI M2 high speed steel. Vacuum 2011;86:391e7. [36] Uglov VV, Anishchik VM, Vetushka AM, Rumyanceva IN, Richter E. Structure and phase transformations of AISI M2 high-speed tool steel treated by PIII and subsequent compression plasma flows of nitrogen. Surf Coat Technol 2004;183:35e44. [37] Ozbek I, Bindal C. Mechanical properties of boronized AISI W4 steel. Surf Coat Technol 2002;154:14e20. [38] Genel K, Ozbek I, Kurt A, Bindal C. Boriding response of AISI W1 steel and use of artificial neural network for prediction of borided layer properties. Surf Coat Technol 2002;160:38e43.
43
[39] Lerchbacher C, Zinner S, Leitner H. Atom probe study of the carbon distribution in a hardened martensitic hot-work tool steel X38CrMoV5-1. Micron 2012;43:818e26. [40] Vives S, Gaffet E, Meunier C. X-ray diffraction line profile analysis of iron ball milled powders. Mater Sci Eng A 2004;366:229e38. [41] Ozdemir O, Omar MA, Usta M, Zeytin S, Bindal C, Ucisik AH. An investigation on boriding kinetics of AISI 316 stainless steel. Vacuum 2008;83:175e9. [42] Campos-Silva I, Ortiz-Domínguez M, Bravo-Bárcenas O, Doñu-Ruiz MA, BravoBárcenas D, Tapia-Quintero C, et al. Formation and kinetics of FeB/Fe2B layers and diffusion zone at the surface of AISI 316 borided steels. Surf Coat Technol 2010;205:403e12. [43] Jauhari I, Yusof HAM, Saidan R. Superplastic boronizing of duplex stainless steel under dual compression method. Mater Sci Eng A 2011;528:8106e10. [44] Jur ci P, Hudáková M, Kusý M. Nature of phases in boronized H11 hot work tool steel. Kovove Mater 2012;50:177e84. [45] Dybkov VI, Lengauer W, Gas P. Formation of boride layers at the Fee25% Cr alloyeboron interface. J Mater Sci 2006;41:4948e60. [46] Gunes I, Ulker S, Taktak S. Plasma paste boronizing of AISI 8620, 52100 and 440C steels. Mater Des. 2011;32:2380e6. [47] Ozbek I, Sen S, Ipek M, Bindal C, Zeytin S, Ucisik AH. A mechanical aspect of borides formed on the AISI 440C stainless-steel. Vacuum 2004;73:643e8. [48] Carbucicchio M, Grazzi C, Palombarini G, Rateo M, Sambogna G. On the phase transformations in CreFeB and FeeCrB systems at high temperature. Hyperfine Interact 2002;139e140:393e8. [49] Chicco B, Borbidge WE, Summerville E. Engineering the subsurface of borided AISI H13 steel. Surf Eng 1998;14:25e30. [50] Yoon JH, Jee YK, Lee SY. Plasma paste boronizing treatment of the stainless steel AISI 304. Surf Coat Technol 1999;112:71e5. [51] Wang ZB, Lu J, Lu K. Chromizing behaviors of a low carbon steel processed by means of surface mechanical attrition treatment. Acta Mater 2005;53:2081e9. [52] Haopeng Yong, Xiaochun Wu, Tianrong Wu. The study on plasma boriding of H13 steel at low temperature assisted by surface nanocrystallization technology. Rare Metal Mater Eng 2012;41(S1):309e14.
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Pack boronizing of AISI H11 tool steel: Role of surface mechanical attrition treatment T. Balusamy a, b, T.S.N. Sankara Narayanan a, c, *, K. Ravichandran b, Il Song Park c, Min Ho Lee c, * a b c
CSIR-National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India Department of Analytical Chemistry, University of Madras, Maraimalai (Guindy) Campus, Chennai 600 025, India Department of Dental Biomaterials, School of Dentistry, Chonbuk National University, Jeonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2012 Received in revised form 3 April 2013 Accepted 4 April 2013
The role of surface mechanical attrition treatment (SMAT) on pack boronizing of AISI H11 type tool steel is addressed. SMAT induced plastic deformation, enabled nanocrystallization at the surface, reduced the grain size and increased the volume fraction of non-equilibrium gain boundaries, increased the accumulation of defects and dislocations at the grain boundaries and within the grains. These features helped to promote the diffusion of boron during boronizing and increased the case depth and hardness of the borided layer. Duplex treatment on SMATed H11 steel samples helps to achieve a higher case depth when compared to the single stage treatment. The findings of the study suggest that SMAT can be used as a pretreatment for boronizing of H11 tool steel. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Surface mechanical attrition treatment (SMAT) Boronizing Iron borides Alloy borides Diffusion Duplex treatment
1. Introduction The surface properties of materials such as wear resistance, friction, corrosion resistance and oxidation resistance play a key role in determining the service life of many engineering components, which lead to the development of numerous surface modification methods. Boronizing is a thermo-chemical surface treatment process that involves diffusion of boron atoms into the surface of metal/alloy to produce a layer of borides of the corresponding metal/alloying elements [1,2]. The major advantages of surface treatment by boronizing are its ability to impart high hardness, excellent abrasion and wear resistance and, oxidation resistance compared with other similar treatments [3e9]. Tool steels, particularly the H type are commonly used in many industrial applications. Being an alloy steel, improving the surface reactivity and accelerating the diffusion of boron is a challenging task.
* Corresponding authors. Department of Dental Biomaterials, School of Dentistry, Chonbuk National University, Jeonju 561-756, South Korea. Tel./fax: þ82 063 270 4040. E-mail addresses: [email protected], [email protected] (T.S.N. Sankara Narayanan), [email protected] (M.H. Lee). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.04.006
It is well known that in ultra-fine grained (UFG) and nanocrystalline materials, the presence of a large number of grain boundaries and triple junctions could act as fast atomic diffusion channels [10,11]. In the past decade, diffusion behaviour in UFG or nanostructured materials produced by severe plastic deformation (SPD) has become an attractive topic, since SPD is a promising route for producing bulk nanostructured materials with enhanced properties [12,13]. Surface mechanical attrition treatment (SMAT) is a surface severe plastic deformation method that enables nanocrystallization at the surface of various metallic materials [14e21]. SMAT enhanced the kinetics of diffusion of nitrogen, aluminium, chromium and zinc during nitriding, aluminizing, chromizing and diffusion zinc coatings, respectively [22e26]. These processes, however, are relatively low temperature processes when compared to boronizing. Hence, materials subjected to SMAT could maintain their nanostructured surface layer. In addition, the increase in grain size during these processes is rather limited to cause any significant influence on the kinetics of diffusion of the corresponding elements. However, the temperature employed for boronizing of most of the ferrous alloys is of the order of 1173e1273 K for a few hours [3e9,27] and, hence it is very difficult to realize the benefits of surface nanostructuring at such conditions. This has also been addressed in our earlier paper on the effect of SMAT on boronizing
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37
Fig. 1. Optical micrographs taken at the surface (a, b) and cross section (c, d) of AISI H11 tool steel: (a, c) untreated; and (b, d) after SMAT using 8 mm B 316L SS balls for 3600 s (c, d e dark field images).
100
40 20 SMATed
100
Fe (200)
60
Fe (211)
80
40
AISI H11 steel square blocks (70 mm 70 mm and 8 mm thick), having a chemical composition (in wt. %) C: 0.40; Si: 0.84; Mn: 0.34; P: 0.012; S: 0.016; Cr: 4.80; Mo: 1.19; Ni: 0.26; Cu: 0.07; Ti: 0.005; V: 0.34; Fe: balance; were used as substrate materials. These samples were procured in heat-treated (at 1020 C for 2 h followed by quenching in air) condition. Generally, hot work tool steel that contains about 0.4% C and 5% Cr is likely to form carbides of M23C6
Fe (211)
60
Fe (200)
Fe (110)
80
Fe (110)
2. Experimental details
and/or M7C3-type (ferrite and spheroidized carbides). In addition, it is alloyed with small amounts of vanadium and molybdenum. Hence, to avoid the precipitation of secondary carbides at the grain boundaries, heat treatment of tool steels is performed at w1020 C to achieve proper functionality of the tools. The H11 steel samples were degreased using acetone and subjected to SMAT using 8 mm Ø 316L stainless steel (SS) balls for 3600 s. All experiments were performed at a fixed frequency of 50 Hz and under vacuum. The
Intensity / (cps)
of EN8 steel [28]. Surface nanocrystallization by SMAT offers the benefit of formation of borided layers at relatively lower temperatures. Xu et al. [27] have shown that boronizing of SMATed H13 steel could be achieved at 973 K for 28,800 s. In our earlier study, we have shown that it is possible to get a reasonably thick boronized layer on SMATed EN8 steel at 923 K for 25,200 s [28]. However, such a long processing time is not practicable for many industrial components. Boronizing of various types of tool steels such as H11 type [29], H13 type [27,30], D type [31e34], M type [35,36], W type [37,38] and hot work tool steel X38CrMoV5-1 [39] has been studied earlier. Studies on boronizing of H type hot work tool steels are rather limited. The effect of SMAT on boronizing of AISI H11 tool steel is not studied earlier. Since boronizing of alloy steels requires a higher temperature of the order of 1223 K, it would be interesting to know to what extent the surface nanocrystallization generated by SMAT will be useful in promoting the kinetics of diffusion of boron. In addition, it is worthwhile to verify the effectiveness of a duplex treatment similar to the one employed in our earlier study [28]. The present paper aims to address the above issues.
20 Untreated
0 20
30
40
50
60
70
80
90
Diffraction angle (2θ) Fig. 2. XRD patterns of untreated AISI H11 tool steel and those subjected to SMAT using 8 mm Ø 316L SS balls for 3600 s.
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schematic and details of SMAT has already been described elsewhere [15,20]. Optical microscopy (Leica DMLM metallurgical microscope) was used to assess the microstructure and extent of deformation. X-ray diffraction (XRD) measurements were performed (system 3003TT GE Technologies) using Cu-Ka radiation at a step-scanning rate with a 2q step of 0.00033 /s to determine the crystallite size and microstrain. The crystallite size was calculated using the full width at half maximum (FWHM) of the Fe (110) plane by fitting it using a pseudo-Voigt function. The instrumental broadening was estimated using a standard silicon sample and its FWHM was subtracted from that of the Fe (110) plane to calculate the actual FWHM. The broadening due to both crystallite size and microstrain were taken care of by considering the constituent integral breadths of pseudo-Voigt function [40]. The crystallite size was calculated using the Scherrer’s formula (D ¼ (0.9l)/bc cos q)
where D, l, q and bc represent the crystallite size, wavelength of the incident X-ray beam, diffraction angle and integral breadth of the Cauchy component of the structurally broadened profile, respectively. Untreated and SMATed H11 steel samples having a dimension of 17 8 4 mm3 were packed along with a powder mixture of boron carbide (5%), potassium tetrafluoborate (5%) and silicon carbide (90%) in an Inconel box and subjected to boronizing at 1223 K for 3600, 10,800 and 18,000 s in a muffle furnace. After treatment, the boronized samples were quenched in air up to room temperature. Pack boronizing was also performed by a duplex treatment. In the first stage, the temperature of the samples was increased to 973 K and they were soaked at 973 K for 3600 s. Subsequently, in the second stage, the temperature was increased from 973 to 1223 K and the samples were soaked at 1223 K for another 3600 s followed
Fig. 3. Optical micrographs taken at the cross section of untreated and SMATed AISI H11 tool steel pack boronized at 1223 K for 3600 s (a, b), 10,800 s (c, d) and 18,000 s (e, f): (a, c, e) untreated; and (b, d, f) SMATed using 8 mm B 316L SS balls for 3600 s.
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by air quenching up to room temperature. During duplex treatment, the rate of increase in temperature was kept at 0.083 K/s. After boronizing, the samples were thoroughly cleaned in acetone using an ultrasonic cleaner for 900 s to remove the boronizing mixture adhered on their surface. The phase content of the untreated and SMATed H11 steel samples after boronizing was determined by XRD measurement taken directly from their surface. For microstructural analysis and microhardness measurement, the boronized H11 steel samples were embedded in a phenolic resin mould, ground using successive grades of SiC coated abrasive papers, polished using 3 mm diamond paste, rinsed with deionized water and dried. The microstructure and case depth of the untreated and SMATed H11 steel samples after boronizing was assessed using Leica DMLM metallurgical microscope. The thickness of each boride phase is calculated using the optical microscope and image analyzer software after drawing a horizontal line on the acquired micrograph across each region. The microhardness of the boronized layer was measured using Leica VMHT (MOT) microhardness tester with 50 gf load applied for 15 s.
39
Fig. 4. Variation in case depth (average layer thickness) of untreated and SMATed AISI H11 tool steel pack boronized at 1223 K as a function of treatment time.
3. Results and discussion 3.1. Characteristics of SMAT treated AISI H11 steel During SMAT, impingement of the 316L SS balls on the surface of H11 steel induced plastic deformation with a high strain rate during each impact. Comparison of the surface microstructure of untreated and SMATed H11 steel samples reveals the presence of needle type martensite phase on the untreated one (Fig. 1(a)) and the formation of some fine particle dispersion along the martensite phase for the SMATed sample (Fig. 1(b)). The presence of the needle type martensite phase on untreated H11 steel has also been observed by Jurci et al. [29] and it is due to the effect of heat treatment of the steel at 1020 C for 2 h followed by quenching in air. The formation of some fine particle dispersion along the martensite phase observed for the SMATed sample (Fig. 1(b)) indicates that the initial condition of H11 steel did not affect the SMAT. The dark field optical micrographs of untreated (Fig. 1(c)) and SMATed H11 steel (Fig. 1(d)) samples taken at the cross section indicate that the thickness of the deformed layer after SMAT is limited to w15 mm. This may be due to the hardness of the substrate (56 HRC), which is higher than that of the 316L SS balls (15 HRC) used for the treatment. Xu et al. [27] have observed a deformation layer thickness of w20 mm on H13 steel after SMAT treatment using 8 mm B SS balls for 2400 s. However, the hardness of H13 hot work tool steel (51e53 HRC) is relatively lower than that of H11 steel (56 HRC). SMAT increased the average surface roughness (Ra) of H11 steel from 0.22 0.03 mm to 1.39 0.02 mm. The increase in surface roughness is due to the formation of craters and dimples following the impingement of the SS balls on the surface of H11 steel. The XRD patterns of untreated and SMATed H11 steel are shown in Fig. 2. Broadening of the Bragg diffraction peaks exhibited by SMATed samples is attributed to the reduction in crystallite size and increases in microstrain. The crystallite size of the untreated and SMATed sample is 35 nm and 17 nm, respectively. The decrease in crystallite size following SMAT is due to the progressive refinement of coarse grains to finer grains induced by the mechanical impact.
(Region II); and (iii) the bare matrix that is not affected by boronizing (Region III). The presence of the alloying elements such as Cr and V though likely to cause an inhibiting effect, the boronizing process is not completely inhibited. The borided layers formed on the surface of untreated and SMATed H11 steel samples are not much smooth when compared to that formed on stainless steels [41e43] but they are relatively smooth and compact when compared to those formed on low-alloyed steels, which exhibit a typical saw-tooth pattern [3e5,28]. This is due to the lower amount of alloying elements present in the H11 steel when compared to 304 and 316 SS. During the beginning of boronizing process, the borides nucleate and grow in a columnar nature due to the anisotropic diffusion of boron atoms whereas a transformation into smooth and compact structure occurs only if the uneven growth of the boride layer is inhibited. In case of alloy steels and stainless steels, the alloying elements that concentrate at the tips of the boride columns by a substitutional procedure decrease the active boron flux in these zones and inhibit the uneven growth of the borided layers. The formation of saw-tooth morphology is observed during boronizing of H11 steel [29]. However, in spite of almost a similar composition, H13 steel exhibits a smooth and compact
3.2. Role of SMAT on boronizing of AISI H11 steel The optical micrographs of untreated and SMATed H11 steel samples after pack boronizing at 1223 K for 3600, 10,800 and 18,000 s, are shown in Fig. 3. Three distinct regions could be identified: (i) boronized layer (Region I); (ii) the region below the boride layer, where boron makes solid solution (diffusion zone)
Fig. 5. XRD patterns of untreated and SMATed AISI H11 tool steel after pack boronizing at 1223 K for 3600 s.
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morphology due to the presence of Cr [30]. Another feature that is evident in Fig. 3 is the formation of excess carbides beneath the boronized layer. It is well known that carbon is insoluble in iron borides and it segregates beneath the Fe2B layer, forming additional portions of carbides [29,44]. The diffusion region also contains some of the insoluble carbides that are randomly distributed (white colour regions) while some of it is diffused into the core material. This is clearly visible in the microstructure of samples subjected for treatment for 18,000 s (Fig. 3(e) and (f)). The variation in case depth of untreated and SMATed H11 steel samples measured as a function of treatment time is shown in Fig. 4. It is evident from Fig. 3 (parallel lines indicate the layer thickness) and Fig. 4 that SMAT enables
an increase in case depth of the borided layer. A similar phenomenon was also observed in our earlier study [28]. The extent of increase in case depth of the borided layer following SMAT observed in the present study is comparable with the results of Xu et al. [27]. Jauhari et al. [43] have also observed an increase in case depth of boronized layer of duplex stainless steel subjected to superplastic deformation. The XRD patterns of untreated and SMATed H11 steel samples pack boronized at 1223 K for 3600 s are shown in Fig. 5. The formation of FeB, Fe2B, CrB, Cr2B phases are observed in both samples. However, the intensity of the peaks pertaining to these phases is relatively higher for SMATed sample. This observation suggests the
Fig. 6. Optical micrographs taken at the cross section showing the indentation marks (aed) and hardness profile (e) of AISI H11 tool steel pack boronized at 1223 K for 10,800 s: (a, b) untreated; and (c, d) SMATed using 8 mm Ø 316L SS balls for 3600 s.
T. Balusamy et al. / Vacuum 97 (2013) 36e43
formation of a higher volume fraction of metal borides for SMATed H11 steel when compared to the untreated one. It is difficult to quantify the alloy boride phases because both FeB and CrB phases possess a similar type of structure [45]. It is obvious to expect an increase in the volume fraction of metal borides with an increase in temperature and treatment time [9,46]. In addition, it is also very clear that considerable amount of Cr can dissolve in the FeB phase. Generally, diffusion of Cr from the bulk to the surface is likely to occur with increase in temperature. The FeB and Fe2B phases dissolve considerable amounts of Cr [47,48] and the extent of solubility is much higher in FeB than Fe2B [48]. This enables substitution of Cr for Fe that results in the formation of CrB and Cr2B. In the present study, untreated and SMATed H11 steel samples are boronized under similar conditions. Hence, the observed increase in volume fraction of the metal borides of the SMATed sample (Fig. 5) is mainly attributed to the increase in boron diffusion kinetics. The decrease in grain size, formation of nanocrystalline structure with a high free energy state near the surface, increase in volume fraction of non-equilibrium grain boundaries, accumulation of high density of dislocations at the grain boundaries as well as within the grains are considered responsible for promoting the diffusion of boron and accelerating the nucleation and growth of metal borides, that results in a higher case depth of SMATed H11 sample. The optical micrographs showing the indentation marks on untreated and SMATed H11 steel samples after pack boronizing at 1223 K for 10,800 s and their hardness profile are shown in Fig. 6. The size of the indentation marks formed on SMATed sample (Fig. 6(b)) is relatively small when compared to those formed on the untreated one (Fig. 6(a)). The micro hardness of the untreated H11 steel after pack boronizing at 1223 K for 10,800 s is w1750 VHN50gf, while the hardness of SMATed sample after boronizing under similar conditions reaches w1850 VHN50gf. The difference in hardness between the untreated and SMATed sample at the top layer is w100 VHN50gf (Fig. 6(e)). It is evident from Fig. 6(e) that the hardness of the transition zone is low when compared to the substrate and the boronized layer. The decrease in hardness at this region can be ascribed to concentration of silicon atom between the borided layer and substrate. The presence of ferrite-forming alloying elements that are not soluble in the boride layer, particularly silicon, is responsible for the decrease in hardness [2,30,49]. The hardness of borided layers is a function of the type of phases, the volume fraction of these phases and the pores and voids in the boronized layer [46,50]. The formation of pores in the boride layer is mainly attributed to the heterogeneous distribution of the boron and they are detrimental to its mechanical properties [1,2,50]. In the present study, the microstructures of the boronized layer formed on untreated and SMATed H11 steel did not reveal much difference in the porosity. The formation of a relatively higher volume fraction of chromium borides along with iron borides is considered responsible for the increase in hardness of the boronized layer formed on SMATed H11 steel. In spite of the increase in hardness of the boronized layers on SMATed H11 steel, the increase in case depth is rather limited. This is due to the temperature employed (1223 K) for boronizing, which would promote grain growth and the beneficial effect of surface nanocrystallization by SMAT could not be fully realized. A similar inference was also made in our earlier study on pack boronizing of SMATed EN8 steel at 1173 K [28]. These inferences suggest that further modification of the process conditions is warranted. In recent years, a duplex treatment during the formation of diffusion coatings is suggested as one of the possible modifications to improve the materials properties and performance [28,51]. In our earlier study, a duplex treatment is found to be an effective in increasing the boron diffusion kinetics and the formation of a dense
41
and compact boronized layer with a relatively higher case depth on EN8 steels [28]. Wang et al. [51] have also addressed the benefits of duplex treatment during chromizing of SMATed low carbon steel. The optical micrographs of untreated and SMATed H11 steel samples after pack boronizing by duplex treatment (973 K for 3600 s and 1223 K for 3600 s followed by air quenching to room temperature) is shown in Fig. 7. Comparison of the thickness of the borided layer formed by single (Figs. 3 and 4) and two stage (Fig. 7) treatments indicates the effectiveness of these treatments. For untreated H11 steel, the total thickness of the boride layer is 24 and 27 mm for single and duplex treatments, respectively whereas for SMATed H11 steel it is 30 and 35 mm for single and duplex treatments, respectively. It is evident that the duplex treatment is effective in increasing the case depth on SMATed H11 steel when compared to untreated one. During the first stage treatment (973 K for 3600 s), diffusion of boron enables the formation of iron borides along with other alloy borides since no significant grain growth is expected at this condition [25]. Xu et al. [27] have also reported that there is no significant grain growth of H13 steel and it can maintain
Fig. 7. Optical micrographs taken at the cross section of untreated and SMATed AISI H11 tool steel pack boronized by duplex treatment (973 K for 3600 s and 1223 K for 3600 s followed by air quenching to room temperature): (a) untreated; and (b) after SMAT using 8 mm Ø 316L SS balls for 3600 s.
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beneficial to achieve a higher case depth when compared to the single stage treatment. The study concludes that SMAT can be used as an effective pre-treatment for boronizing of H11 steel. Acknowledgements TB and TSNSN thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for their financial support in the form a SUPRA Institutional Project and Dr. S. Srikanth, Director, CSIRNational Metallurgical Laboratory, Jamshedpur, for his constant encouragement and support. References
Fig. 8. X-ray diffraction patterns of untreated and SMATed AISI H11 tool steel after pack boronized by duplex treatment (973 K for 3600 s and 1223 K for 3600 s followed by air quenching to room temperature).
the nanostructure at 973 K for 1800 s. During the second stage treatment (1223 K for 3600 s), the kinetics of formation of iron borides and other alloy borides are increased. The formation of a nanostructured surface layer is the key factor in increasing the diffusion of boron and the kinetics of formation of iron and other alloy borides for SMATed H11 steel sample. A similar trend was observed earlier by Wang et al. [51] during chromizing of low carbon steel. Yang et al. [52] have reported that formation of large number of grain boundaries with various kinds of non-equilibrium defects induced during SMAT enabled an increase in kinetics of plasma boronizing of H13 steel performed at 853 K for 14,400 s. In the present study, the increase in case depth observed during the duplex treatment of H11 steel is due to ability of SMAT to increase the rate of nucleation of boron at relatively lower temperature following a reduction in activation energy for the reaction during the first stage. In addition, the rate of diffusion of boron is increased further with increase in temperature at the second stage. The XRD patterns of untreated and SMATed H11 steel after duplex treatment of pack boronizing is shown in Fig. 8. The types of phases are quite similar to those formed in single stage boronizing. The marginal increase in volume fraction of metal borides could be due to the lower temperature during the first stage treatment. 4. Conclusions SMAT of H11 steel using 8 mm Ø 316L SS balls for 3600 s induced plastic deformation with a high strain rate and increased the surface roughness from 0.22 0.03 mm to 1.39 0.02 mm. After SMAT, the deformation depth is w15 mm and the crystallite size of the top surface is w17 nm. The case depth of the borided layers formed on untreated and SMATed H11 steel confirms the ability of SMAT to increase the kinetics of boron diffusion. The decrease in grain size, formation of nanocrystalline structure with high free energy state near the surface, increase in the volume fraction of non-equilibrium grain boundaries, accumulation of high density of dislocations at the grain boundaries as well as within the grains are considered responsible for the increase in case depth of SMATed H11 steel sample. The formation of a higher volume fraction of metal borides on SMATed H11 steel enables an increase in hardness at the top layer by w100 VHN50gf. The duplex treatment during boronizing is
[1] Chatterjee-Fischer R. Boriding and diffusion metallizing. In: Sudarshan TS, editor. Surface modification technologies. Marcel Dekker, Inc.; 1989. p. 567e609. [2] Sinha AK. Boriding (boronizing) of steels, heat treating, vol. 4. Ohio: ASM Handbook/ASM International; 1991437e47. [3] Bindal C, Ucisik AH. Characterization of boriding of 0.3% C, 0.02% P plain carbon steel. Vacuum 2007;82:90e4. [4] S¸ahin S. Effects of boronizing process on the surface roughness and dimensions of AISI 1020, AISI 1040 and AISI 2714. J Mater Process Technol 2009;209:1736e41. [5] Campos I, Farah M, López N, Bermúdez G, Rodríguez G, VillaVelázquez C. Evaluation of the tool life and fracture toughness of cutting tools boronized by the paste boriding process. Appl Surf Sci 2008;254:2967e74. [6] Tabur M, Izciler M, Gul F, Karacan I. Abrasive wear behavior of boronized AISI 8620 steel. Wear 2009;266:1106e12. [7] Campos I, Palomar-Pardavé M, Amador A, VillaVelázquez C, Hadad J. Corrosion behavior of boride layers evaluated by the EIS technique. Appl Surf Sci 2007;253:9061e6. [8] Tavakoli H, Mousavi Khoie SM. An electrochemical study of the corrosion resistance of boride coating obtained by thermo-reactive diffusion. Mater Chem Phys 2010;124:1134e8. [9] Jiang J, Wang Y, Zhong Q, Zhou Q, Zhang L. Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance. Surf Coat Technol 2011;206:473e8. [10] Gleiter H. Nanocrystalline materials. Prog Mater Sci 1989;33:223e315. [11] Lu K. Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure and properties. Mater Sci Eng R 1996;16:161e221. [12] Valiev RZ, Langdon TG. Principles of equal-channel angular pressing as a processing tool for grain refinement. Prog Mater Sci 2006;51:881e981. [13] Valiev RZ, Zehetbauer MJ, Estrin Y, Hoppel HW, Ivanisenko Y, Hahn H, et al. The innovation potential of bulk nanostructured materials. Adv Eng Mater 2007;9:527e33. [14] Lu K, Lu J. Surface nanocrystallization of metallic meterial Presentation of the concept behind a new approach. J Mater Sci Technol 1999;15:193e7. [15] Lu K, Lu J. Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment. Mater Sci Eng A 2004;375e377: 38e45. [16] Tao NR, Wang ZB, Tong WP, Sui ML, Lu J, Lu K. An investigation of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment. Acta Mater 2002;504:603e16. [17] Zhang HW, Hei ZK, Liu G, Lu J, Lu K. Formation of nanostructured surface layer on AISI 304 stainless steel by means of surface mechanical attrition treatment. Acta Mater 2003;51:1871e81. [18] Zhou L, Liu G, Ma XL, Lu K. Strain-induced refinement in a steel with spheroidal cementite subjected to surface mechanical attrition treatment. Acta Mater 2008;56:78e87. [19] Li WL, Tao NR, Lu K. Fabrication of a gradient nano-micro-structured surface layer on bulk copper by means of a surface mechanical grinding treatment. Scripta Mater 2008;59:546e9. [20] Balusamy T, Kumar Satendra, Sankara Narayanan TSN. Effect of surface nanocrystallization on the corrosion behaviour of AISI 409 stainless steel. Corros Sci 2010;52:3826e34. [21] Anand Kumar S, Ganesh Sundara Raman S, Sankara Narayanan TSN, Gnanamoorthy R. Fretting wear behaviour of surface mechanical attrition treated alloy 718. Surf Coat Technol 2012;206:4425e32. [22] Tong WP, Tao NR, Wang ZB, Lu J, Lu K. Nitriding iron at lower temperatures. Science 2003;299:686e8. [23] Tong WP, Liu CZ, Wang W, Tao NR, Wang ZB, Zuo L, et al. Gaseous nitriding of iron with a nanostructured surface layer. Scripta Mater 2007;57:533e6. [24] Si X, Lu B, Wang Z. Aluminizing low carbon steel at lower temperatures. J Mater Sci Technol 2009;25:433e6. [25] Lu SD, Wang ZB, Lu K. Enhanced chromizing kinetics of tool steel by means of surface mechanical attrition treatment. Mater Sci Eng A 2010;527:995e1002. [26] Wang HL, Wang ZB, Lu K. Enhanced reactive diffusion of Zn in a nanostructured Fe produced by means of surface mechanical attrition treatment. Acta Mater 2012;60:1762e70.
T. Balusamy et al. / Vacuum 97 (2013) 36e43 [27] Xu L, Wu X, Wang H. Influence of surface nano-structured treatment on pack boriding of H13 Steel. J Mater Sci Technol 2007;23:525e8. [28] Balusamy T, Sankara Narayanan TSN, Ravichandran K. Effect of surface mechanical attrition treatment (SMAT) on boronizing of EN8 steel. Surf Coat Technol 2012;213:221e8. [29] Jur ci P, Hudáková M. Diffusion boronizing of H11 hot work tool steel. J Mater Eng Perform 2011;20:1180e7. [30] Genel K. Boriding kinetics of H13 steel. Vacuum 2006;80:451e7. [31] Oliveira CKN, Casteletti LC, Lombardi Neto A, Totten GE, Heck SC. Production and characterization of boride layers on AISI D2 tool steel. Vacuum 2010;84: 792e6. [32] Sista V, Kahvecioglu O, Eryilmaz OL, Erdemir A, Timur S. Electrochemical boriding and characterization of AISI D2 tool steel. Thin Solid Films 2011;520: 1582e8. [33] Vourlias G, Chaliampalias D, Zorba TT, Pavlidou E, Psyllaki P, Paraskevopoulos KM, et al. A combined study of the oxidation mechanism and resistance of AISI D6 steel exposed at high temperature environments. Appl Surf Sci 2011;257:6687e98. [34] Rodríguez-Castro G, Campos-Silva I, Chávez-Gutiérrez E, Martínez-Trinidad J, Hernández-Sánchez E, Torres-Hernández A. Mechanical properties of FeB and Fe2B layers estimated by Berkovich nanoindentation on tool borided steel. Surf Coat Technol 2013;215:291e9. [35] Ozbek I, Bindal C. Kinetics of borided AISI M2 high speed steel. Vacuum 2011;86:391e7. [36] Uglov VV, Anishchik VM, Vetushka AM, Rumyanceva IN, Richter E. Structure and phase transformations of AISI M2 high-speed tool steel treated by PIII and subsequent compression plasma flows of nitrogen. Surf Coat Technol 2004;183:35e44. [37] Ozbek I, Bindal C. Mechanical properties of boronized AISI W4 steel. Surf Coat Technol 2002;154:14e20. [38] Genel K, Ozbek I, Kurt A, Bindal C. Boriding response of AISI W1 steel and use of artificial neural network for prediction of borided layer properties. Surf Coat Technol 2002;160:38e43.
43
[39] Lerchbacher C, Zinner S, Leitner H. Atom probe study of the carbon distribution in a hardened martensitic hot-work tool steel X38CrMoV5-1. Micron 2012;43:818e26. [40] Vives S, Gaffet E, Meunier C. X-ray diffraction line profile analysis of iron ball milled powders. Mater Sci Eng A 2004;366:229e38. [41] Ozdemir O, Omar MA, Usta M, Zeytin S, Bindal C, Ucisik AH. An investigation on boriding kinetics of AISI 316 stainless steel. Vacuum 2008;83:175e9. [42] Campos-Silva I, Ortiz-Domínguez M, Bravo-Bárcenas O, Doñu-Ruiz MA, BravoBárcenas D, Tapia-Quintero C, et al. Formation and kinetics of FeB/Fe2B layers and diffusion zone at the surface of AISI 316 borided steels. Surf Coat Technol 2010;205:403e12. [43] Jauhari I, Yusof HAM, Saidan R. Superplastic boronizing of duplex stainless steel under dual compression method. Mater Sci Eng A 2011;528:8106e10. [44] Jur ci P, Hudáková M, Kusý M. Nature of phases in boronized H11 hot work tool steel. Kovove Mater 2012;50:177e84. [45] Dybkov VI, Lengauer W, Gas P. Formation of boride layers at the Fee25% Cr alloyeboron interface. J Mater Sci 2006;41:4948e60. [46] Gunes I, Ulker S, Taktak S. Plasma paste boronizing of AISI 8620, 52100 and 440C steels. Mater Des. 2011;32:2380e6. [47] Ozbek I, Sen S, Ipek M, Bindal C, Zeytin S, Ucisik AH. A mechanical aspect of borides formed on the AISI 440C stainless-steel. Vacuum 2004;73:643e8. [48] Carbucicchio M, Grazzi C, Palombarini G, Rateo M, Sambogna G. On the phase transformations in CreFeB and FeeCrB systems at high temperature. Hyperfine Interact 2002;139e140:393e8. [49] Chicco B, Borbidge WE, Summerville E. Engineering the subsurface of borided AISI H13 steel. Surf Eng 1998;14:25e30. [50] Yoon JH, Jee YK, Lee SY. Plasma paste boronizing treatment of the stainless steel AISI 304. Surf Coat Technol 1999;112:71e5. [51] Wang ZB, Lu J, Lu K. Chromizing behaviors of a low carbon steel processed by means of surface mechanical attrition treatment. Acta Mater 2005;53:2081e9. [52] Haopeng Yong, Xiaochun Wu, Tianrong Wu. The study on plasma boriding of H13 steel at low temperature assisted by surface nanocrystallization technology. Rare Metal Mater Eng 2012;41(S1):309e14.