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Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding
Accepted Manuscript Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding
J.Z. Lu, J. Cao, H.F. Lu, L.Y. Zhang, K.Y. Luo PII: DOI: Reference:
S0257-8972(19)30434-7 https://doi.org/10.1016/j.surfcoat.2019.04.063 SCT 24558
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
16 January 2019 14 April 2019 21 April 2019
Please cite this article as: J.Z. Lu, J. Cao, H.F. Lu, et al., Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding, Surface & Coatings Technology, https://doi.org/10.1016/ j.surfcoat.2019.04.063
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ACCEPTED MANUSCRIPT Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding J.Z. Lua,*, J. Caoa, H.F. Lua, L.Y. Zhangb, K.Y. Luoa a
School of Civil Engineering, Xuzhou University of Technology, Xuzhou 221000, PR China
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b
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China
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Abstract: Microhardness, wear properties and microstructure of Fe-based coatings with various
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WC contents on H13 hot-working die steel by laser cladding were investigated. Special attention
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was paid to the effects of various WC contents on the microstructural characterization and wear properties of different coatings by means of optical microscopy (OM) and scanning electron
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microscopy (SEM). Results indicated that only a small part of the periphery of the WC in the WC-added coatings was melted, and the un-melted WC particles in the coatings acted as a hard
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reinforcement. Besides, the microhardness in the cladding layer increased with the increment of
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the mass fraction of WC particles, and presented a gradient distribution along in-depth direction of the coatings. Furthermore, the Fe-based clad coating with WC particles exhibited a higher wear
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resistance and a lower friction coefficient compared with the substrate and Fe-based clad coating.
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In addition, the influence mechanism of WC particles on microstructural evolution and wear resistance was also discussed. Keywords: Laser cladding, Fe-based coatings, WC particles, Wear property, Microstructure, Microhardness
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Corresponding author: Xuefu Road 301, Jingkou District, Zhenjiang 212013, PR China
Email address: [email protected] (J.Z. Lu) Tel.: +86-511-88797198; Fax: +86-511-88780219 1
ACCEPTED MANUSCRIPT 1. Introduction Laser cladding is an efficient surface material deposition technique, which uses a high energy density laser beam to completely melt the metallic powders and form melt pool on the surface of the substrate to produce high quality metal layer with fast solidification characteristics [1-3]. Compared with other surface techniques, laser cladding exhibits unique advantages, such as
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low rate of dilution, low substrate deformation, narrow heat affected zone, dense metallurgical
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bond between cladding layer and substrate, and relatively fine microstructure. Owing to the high
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hot strength and outstanding toughness, H13 hot-working die steel is widely applied in casting dies, extrusion dies, hot forming dies and other plastic deformation fields. Particularly, when it is
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used as a mold material, it will be subjected to intense friction caused by hot and cold cycles, and metal flow accompanied by wear, erosion and thermal fatigue cracking. Accordingly, the strength
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in the surface layer of the mold is one of important factors influencing its fatigue life in engineering research. Fabricating cladding layer on the surface of steel components is a good
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choice to improve the properties and service life of H13 steel dies [4-5].
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Some typical cladding powders, such as Fe-, Ni-, and Co-based alloy powders [6] are relatively common in laser cladding. Among them, Fe-based powder is widely used because of its
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low cost and high performance. Telasang et al. prepared H13 powder cladding layer on the surface of H13 hot-working die steel, significantly improving its surface microhardness.
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Furthermore, the use of alloy powders with the same or similar chemical composition to the substrate material would ensure better metallurgical bonding interface and similar properties [7]. Recently, much more attentions have paid to fabricate composite coatings with various content of added particles, for example, WC [8], VC [9], TiC [10], and TiB [11], leading to the improvement in the microhardness and wear resistance. Wang et al. investigated the effects of TiC particles on the mechanical properties of Fe-based coatings, and it was found that Fe-based coatings with TiC particles obtained lower friction coefficient and higher wear resistance [12]. Among steel-based 2
ACCEPTED MANUSCRIPT reinforcements, WC particles is recognized to be one of the most commonly used hard-phase which combines lows thermal expansion coefficient, high hardness and good wettability. Yuan et al. investigated WC-Co composites coatings for enhancing the microhardness and sliding wear resistance [13]. Pre-machined trapezoidal groove had been repaired by the hybrid technology combining laser cladding with laser surface alloying, and 316L stainless steel powders and WC
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powders were used in mixture [14]. However, when the thickness of the functional cladding layer
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formed on the surface of the metal substrate ranges from 0.5 mm to 2 mm, it exhibited a tendency
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to crack initiation [15]. Therefore, to better improve the surface properties, when adding WC particles, it is necessary to control the mass fraction of the added WC particles to obtain a
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crack-free, void-free high-quality cladding layer, and to control the thickness of the cladding layer within a reasonable range to meet the actual machining needs [16].
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The aim of this paper is to investigate the micro-hardness, wear properties and microstructures of Fe-based coatings with various WC contents on H13 hot-working die steel
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prepared by laser cladding. Special attentions were paid to the effects of various WC contents on
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the microstructural characterization and wear properties of different coatings. In addition, the influence mechanism of WC particles on microstructural evolution and wear resistance was also
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discussed.
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2. Experimental procedure
2.1 Experimental material and laser cladding parameters The substrate material in the present work was H13 hot-working die steel plates with dimensions of 150 mm × 60 mm × 12 mm in hardened (austenitized to 1030 °C for 10 min followed by oil quenching) and tempered (at 620 °C for 2 h) conditions. The chemical composition of the substrate was 0.4 wt.% C, 5.2 wt.% Cr, 0.4 wt.% Mn, 1.0 wt.% Mo, 1.0 wt.% Si, and balanced Fe, five substrate specimens (one of them is substrate) were cut from a same 3
ACCEPTED MANUSCRIPT plate. Before laser cladding, all substrate specimens were mechanically gritted using different-graded (from 400# to 2000#) SiC sandpapers, polished and rinsed with absolute ethyl alcohol. The cladding material was Fe104 steel alloy powder with a particle size of 100-140 μm, and the normal composition of Fe104 steel powder was 0.24 wt.% C, 15.3 wt.% Cr, 1.2 wt.% B,
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0.55 wt.% Mn, 1.45 wt.% Mo, 1.9 wt.% Ni, 1.23 wt.% Si, and balanced Fe. The combination of
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pure WC powder and Fe104 powder formed the WC-Fe mixed powder for the present experiment.
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After sufficient drying treatment and stirring, the WC contents of the final prepared composite powder were set to 3%, 6%, and 9%, respectively.
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Laser cladding experiments were carried out using an IPG 4kW ytterbium fiber system
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equipped with a KUKA type 6-axis robot at 980 nm wavelength. Through a range of optimization experiments, the laser power was determined to be 2,000 W, and the laser spot diameter was set as
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4 mm with a 60% overlapping rate. Meanwhile, the scanning speed was kept to 240 mm/min with
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a powder feeding rate of 15 g/min. Four types of Fe-based coatings were prepared, in which the cladding powder mixed with 0 wt.%, 3 wt.%, 6 wt.% and 9 wt.% were defined as Specimen 0,
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Specimen 1, Specimen 2, and Specimen 3, respectively.
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2.2 Microstructural observation and component testing Cross-sectional morphologies of all coatings perpendicular to the cladded surface were cut, and then subjected to grinding and polishing for metallographic investigation according to standard procedures. After cleaned by deionized water with ultrasonic wave, all polished cross-sections were corroded with a solution consisting of 3 ml HCL and 1 ml HNO3 for 10s, and subsequently washed with absolute ethyl alcohol. The microstructure and chemical compositions of the coatings were investigated using ZESSI optical metallographic microscope (OM) and 4
ACCEPTED MANUSCRIPT HITACHI S 3400 scanning-electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The phase identifications of all specimens were carried out using D/max2500Pc X-ray diffraction (XRD) at a scanning speed of 5 °/min, ranging from 20° to 100°. 2.3 Microhardness measurement
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In-depth microhardness tests of substrate and four types of coatings were carried out using a
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HVS-1000 Vickers hardness tester along the depth direction from the top surface to the bonding line
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at intervals of 0.2 mm under a load of 5N and duration for 10 s. One average value in microhardness was determined according to the measured data from five indentations at each depth.
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2.4 Dry sliding wear tests
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Dry sliding wear tests were carried out on a block-on-ring wear test machine (Model M-2000, China) under the load of 150 N for 2 h at 20 °C, and the size of each specimen cut into was 15
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mm (length) × 10 mm (width) × 12 mm (thickness). The grinding ring with a radius of 40 mm
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was made of GCr15 steel (690-710 HV) and the width is10 mm. The machine speed was set to 200 rpm. The friction coefficient was constantly recorded by a computer and the weight losses of
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specimens are weighed by an electronic balance with an accuracy of 0.1 mg, and all specimens
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were washed and dried by absolute ethanol before weighing. 3. Results and Discussion 3.1 Phase composition analysis of Fe-based coatings with different WC contents XRD analyses are conducted on the coatings to identify the phase composition of clad coatings. The XRD results are presented in Fig. 1, showing that the coating of Specimen 0 mainly consists of α-Fe, γ-Fe, M23C6 (M=Cr, Fe) and M7C3 phases. After adding WC particles, the main phases are the same as that without WC particles in the clad coatings. However, it is noted that the 5
ACCEPTED MANUSCRIPT amount of diffraction peaks of phases increase as increasing WC content. When the mass fraction of WC reaches 6%, Fe3W3C appears in the clad coating, indicating the W and C elements have begun to inter-diffuse with elements in the iron matrix. It can be found that there are melted and re-precipitated WC particles during the cladding process. Furthermore, with increasing WC
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content, Fe6W6C and WC diffraction peaks are also found, revealing a fact that a few Fe3W3C has
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transformed into a more stable and hard phase Fe6W6C. The presence and dispersion
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strengthening of these hard phases contributes to high microhardness and good wear resistance of the clad coatings. Some small characteristic peaks in Fig. 1 reveal that there may be some other
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3.2 Typical microstructure of Fe-based coatings
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carbon and boron metal compounds in the clad coatings.
The microstructural features in the bonding zone and upper zone in the cladding layer of
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Specimen 0 are shown in Fig. 2. It can be seen from Fig. 2a that the solidification process begins
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at the junction interface between the cladding layer and the substrate. At first, the temperature (G) is extremely high and the solidification rate (R) is relatively low, so there a large value of G/R. At
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this moment, the crystal nucleation speed is much faster than the growth rate, leading to the
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growth of crystals on the substrate in a planer crystal state. At the liquid-solid interface, the actual temperature in the liquid phase gradually decreases before crystallization, and the solidification speed increases. At this time, the G/R ratio decreases, and planar crystals are turn into columnar crystals, showing a rapid solidification characteristics. The grain growth at the bottom of the cladding layer presents obvious epitaxial properties, resulting in many dendrites growing at the surface of Specimen 0. In the cladding region slightly away from the junction interface, the solidification rate increases, and the G/R ratio decreases. There are coarse dendrites and cell 6
ACCEPTED MANUSCRIPT crystals inside the grain [17]. As shown in Fig. 2b, in the upper region of the cladding, the temperature (G) reaches a maximum and the solidification rate (R) increases to a peak value. Under this circumstances, the growth rate is much larger than the nucleation speed, thus there are uniform equiax crystals and fine dendrites are important microstructural features during the grain
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growth [18].
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To observe the details of the constituent phases and microscopic morphologies of the coating
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layers, cross-sectional morphologies as a function of WC contents are shown in Fig. 3. There is a good bonding state between the cladding layer and the substrate, and no obvious defects, such as
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voids and cracks, are observed at the junction interface. When WC content is 3%, WC particles
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distribute relatively uniform in the clad coating, as shown in Fig. 3b. When WC content reaches 6% and 9%, the agglomeration phenomenon emerges, and the agglomeration effect of WC particles
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tends to be serious with increasing WC content, as shown in Figs. 3c and 3d. In addition, it can be
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found that the WCparticles have a wide distribution in the upper and middle part of the cladding layer, which lays solid foundation for further strengthening the coatings [19].
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The bond zone of clad coating is mainly composed of dendrites and inter-dendritic netlike
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eutectic structures, which is in accordance with OM observation, as shown in Fig. 4a and Fig. 2. The eutectic structure is mainly generated due to the fact that the dendrites are firstly precipitated from the molten pool and gradually developed into dendritic matrix. Immediately the inter-crystalline metal solution is cooled to a solid state, which undergoes a eutectic reaction. As shown in Fig. 4b, after adding WC particles, the particle plays as significant role as a nucleation which can be recognized as a second substrate which is smaller and tougher. Therefore, partial dissolution occurs at the edge of the WC particles. At this moment, a crystal grows in a planar 7
ACCEPTED MANUSCRIPT state and a dendritic matrix grows in the radial direction of the particle, and this phenomenon can be explained by the fact that the temperature of the WC particles in the liquid state is relatively low and they possess small size. Compared with Specimen 0, Specimen 2 with 6 wt. % WC shows smaller grains and more
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secondary dendrites in the zone around WC particle, as shown in Fig. 4c. Interestingly, the grain
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refinement is more obvious with increasing WC content, and there are much more WC particles in
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the combined zone, as shown Fig. 4d. With increasing WC content, more nucleation sites contribute further to grain refinement and strengthening of Fe-based coating.
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Fig. 5 presents the cross-sectional mapping scanning data in the cladding layer of Specimen
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0, and there are inhomogeneous element distributions due to the severe elemental segregation that occurs during the solidification. As can be seen from Figs. 5b, c, and e, Fe, Cr, Mo, and Si
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elements in the cladding layer distribute relatively uniform. Furthermore, Cr element exists
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mainly inside the grain to form a compound of Cr7C3 and M23C6, and a large amount of Cr is also found in the dendrite structures due to the alloy powder containing a massive amount of Cr
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element. Figs. 6-8 show the corresponding cross-sectional element distributions (Fe, Cr, Mo, Si
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and C) of Specimen 1, Specimen 2, and Specimen 3, respectively, and detailed EDS data of Points 1-5 (Figs. 4a and 4b) are presented in Table 1. It can be seen that there are Mo, Si and C elements with relatively high contents in the inter-dendritic netlike eutectics while Fe element uniformly distributes at Points 1 and 2 in the coating layer. W elements are detected at Point 3 and 4, showing that the WC particles dissolve limitedly and phases around WC particles are Fe-W-C compounds which improve the mechanical properties of Fe-based coatings [20]. In addition, W element does not appear at Point 5, and there 8
ACCEPTED MANUSCRIPT is nearly no precipitation of W element in mapping scanning images, indicating that the partially dissolution of WC leads to a limited scope influenced by a single WC particle. 3.3 Microhardness distribution and wear resistance In-depth microhardness distributions in four types of Fe-based coatings are measured and
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presented Fig. 9. It can be seen that all specimens exhibit higher microhardness in the coating
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layer than H13 substrate, which is attributed to the formation of hard phases such as M23C6, M7C3
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and other type phases during laser cladding. Besides, it is obvious that microhardness of all four coatings holds stable first and then decrease steeply along the depth direction. It is interesting to
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note that there is no obvious difference in the in-depth microhardness distribution of Fe-based
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coatings when the mass fraction of WC particles does not exceed 6%, and the microhardness value keeps stable in a span of 640 HV to 680 HV. In addition, when the mass fraction of WC
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particles comes to 9%, there is a significant improvement in microhardness due to the appearance
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of a great amount of Fe3W3C and Fe6W6C hard phases. As the WC content increases further, some of the W elements precipitate during laser cladding to form a high hardness FWC compound, and
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WC particles act as dispersion strengthening in the cladding layer because of its own
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characteristics. Furthermore, It is noted that WC particles tend to segregate in the bottom region because of its higher density (15.63 g/cm3), causing an improvement in microhardness. Fig. 10 shows friction coefficient curves of substrate and four types of Fe-based coatings. It can be found that the average friction coefficient of substrate is 0.351, and Specimens 1-4 are 0.349, 0.342, 0.336 and 0.313, respectively. It is well known that the friction coefficient is inversely proportional to the corresponding microhardness. As discussed above, M23C6, M7C3 are important factors to improve the microhardness of Fe-based coatings, which also result in the 9
ACCEPTED MANUSCRIPT improvement of their wear resistance. However, it should be noted that the friction coefficient of Specimen 3 is a little smaller than those of other three coatings. Fig. 11 shows the wear rate comparison of H13 substrate and Specimens 0-3. Compared with H13 substrate, all four types of Fe-based coatings present a small wear rate, indicating that the
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wear resistance of cladding layer with WC particles has been significantly improved. Furthermore,
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the wear rate of WC-Fe coatings shows a gradually decreasing trend with increasing WC particles.
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Particularly, when the mass fraction of WC reaches 9%, the wear rate of Specimen 3 is reduced by 35.79% compared with Specimen 1, indicating that the addition of WC particles plays an
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important role in further improving the wear resistance of Fe-based coating.
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Combined with these experimental data above, it can be concluded that during laser cladding, WC particles act as a hard phase to strengthen the Fe-based coating, and the increase of WC
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content and its uniform distribution are two important factors to the improvement in
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microhardness and wear resistance of Fe-based coatings. 3.4 Surface damage behavior
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Figs. 12a-e illustrate the typical SEM images of the worn surfaces of the H13 substrate and
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four types of Fe-based coatings. It is obvious that Fe-based coatings significantly prevent the surface damage. Specially, plastic deformation and sever spalling are dominating on the surface of substrate whose surface condition is rougher than that of cladding layers. Fig. 12a shows that there are deep, long and uniform grooves on the worn surface of substrate, indicating that the main mechanism of sliding wear is micro-cutting[21-22]. Besides, during sliding wear, the grinding ring pushes the substrate to cause plastic flow and slight peeling, illustrating that peeling wear occurs on the worn surface. It is show in Fig 12b that there is much smoother worn profile in 10
ACCEPTED MANUSCRIPT the surface layer of Specimen 0, and at this stage intermittent and short grooves dominates, indicating a severe adhesive wear with scale-like feature. In addition, it is obvious in Figs. 12c, d, and e that Specimens 1-3 further relieve the surface damage compared with the substrate and Specimen 0. The worn surfaces of these specimens are mainly characterized by shallow and fine
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grooves, however, the amount of peeling pits increase with increasing WC content. Because of
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the cutting action during sliding wear, the WC particles in the Fe-based coatings are gradually
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exposed. Combined with Fig. 12f, the scratching is interrupted or the path is converted due to the barriers effect of un-melted WC particles. The dispersed WC particles in the Fe-based coating act
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as a skeleton, reducing the ploughing effect on the coating by the grinding ring and lubricating the
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friction pair to slow down adhesion. Hence, the wear resistance of these Fe-based coatings has been improved [23]. It is noted that, as the WC particles content increases, the main wear
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mechanism of the worn surface is abrasive wear presented by shallower and slender grooves, but
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the degree of adhesive wear is gradually decreased. In addition, Fig 13 shows some typical EDS diffraction patterns of Points 1-4 in Fig. 12, indicating the varying degrees of oxygen in white
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bands. These data show that the oxidative wear occurs during sliding wear. Hence, laser cladded
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Fe-based coatings have the ability to improve the wear resistance, and WC particles are beneficial to the improvement in wear properties of Fe-based coatings. 3.5 Influence mechanism of WC particles on microstructural evolution and wear resistance The influence mechanism of the WC particles on the microstructural evolution of Fe-based coatings is presented in Fig. 14. When there is no WC particle in the cladding layer, dendritic matrix and inter-crystalline reticular eutectic structure exist in the cladding layer. After the addition of the WC particles, as shown in Fig. 14a, WC particles partially melt during laser 11
ACCEPTED MANUSCRIPT cladding, and the decomposed W and C elements with the Fe, Cr and other elements of the matrix powder easily form carbide structures with high hardness. Thus, the re-precipitated W and C elements contribute to solution strengthening. In addition, the WC particles that are not completely melted maintain a good integrity of the particle, as shown in Fig. 14b, and act as the
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nucleation during laser cladding. Fine dendrites grow radially around these WC particles, so the
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grains around the WC particles are refined, plays a significant role in fine-grain strengthening.
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Furthermore, the WC particles which are not dissolved in the cladding layer can also act as the dispersion strengthening. In the present work, with increasing of WC contents, more and more W
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and C elements are solidified into the matrix, and more Fe-W-C compounds are produced, leading
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to the obvious strengthening effect. In addition, with increasing of WC contents, WC particles distribute more uniformly, and the better dispersion strengthening will be obtained.
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The influence mechanism of WC particles on the wear resistance of Fe-based coatings is
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presented in Fig. 15. When there is no WC particle in the cladding layer, dendritic matrix and inter-crystalline reticular eutectic structures in the cladding layer are composed of high-strength
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and high-hardness hard phases which is rich in Fe, Cr, C, and Si elements. Therefore, compared
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with substrate, there is an obvious improvement in microhardness and mechanical performance of the cladding layer. As shown in Fig. 15a, during sliding wear, a large amount of hard phases reduce molecular adhesion and shearing action during wear test, and effectively enhance the adhesion strength of Fe-based coatings. Hence, the wear resistance of Fe-based coatings is significantly improved [24]. After the addition of WC particles in the cladding layer, the cladding layer obtains more obvious solution strengthening and dispersion strengthening as the WC content increases, effectively reducing the corresponding wear rates. In addition, when the surface 12
ACCEPTED MANUSCRIPT of the cladding layer is severely worn, the WC particles will gradually expose in the surface layer, and directly contact the grinding ring after debris continuously generating, avoiding the occurrence of sever wear in the substrate immediately. Therefore, during sliding wear, the dispersed WC particles can not only serve as a
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high-hardness skeleton to prevent the wear of the abrasive particles on the matrix or change the
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moving direction of the abrasive particles, but also effectively prevent the ploughing effect during
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sliding wear, and therefore protect the substrate to reduce wear and even interdict wear, forming a shadow area, as shown in Fig. 15b [25]. However, the accumulation of wear debris will produce
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repeated relative movement in the friction pair, which may aggravate the oxidative wear on the
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top surface of Fe-based coatings. Hence, when the mass fraction of WC particles is 3%, the wear resistance of Fe-based coating is comparable to that of the coating without WC particles. But
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when the mass fraction of WC particles reaches 6% or 9%, the agminated effect of WC particles
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is reflected, and the wear resistance of Fe-based coatings increases with increasing WC content. 4. Conclusions
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(1) Fe-based coatings with different WC contents were prepared using laser cladding.
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Results showed that hard phases appear in coatings were M7C3 and M23C6, contributing to the initial microhardness of Fe-based coatings. Furthermore, the increment of WC content lead to the appearance of FeWC-like compounds in Fe-based coatings, leading to an obvious improvement in microhardness of Fe-based coatings. (2) In the WC-added coatings, because only a small part of the periphery of the WC is melted, the un-melted WC particles in the coatings act as a hard reinforcement. At the same time, coarse grains around WC particles are refined by the nucleation effect. Furthermore, the 13
ACCEPTED MANUSCRIPT microhardness of the cladding layer increases with the increment of WC content. (3) Compared with the substrate, there is an improvement in the wear resistance of Fe-based coating without WC particles to a greater extent, and increasing WC content further enhance the wear resistance of Fe-based coatings. The wear mechanism of WC particle on the Fe-based
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coatings is also presented, and abrasive wear accompanied slighter spalling wear is the dominate
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wear form during sliding wear.
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Acknowledgement
This work was supported by the National Key R&D Program of China (No. 2017YFB1103603),
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the National Natural Science Foundation of China (grant numbers 51775250, 51875262 and
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51575242), the Jiangsu Provincial Science and Technology Projects (grant numbers BE2016148, and BE2017142), and the Priority Academic Program Development of Jiangsu Higher Education
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Institutions.
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model in the cross-section of laser cladded NiCrBSi + WC coatings, for different wt% WC, Surf. Coat. Technol. 324 (2017) 298-306.
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[20] X. Tong, F.H. Li, M. Kuang, W.Y. Ma, X.C. Chen, M. Liu, Effects of WC particle size on the wear resistance of laser surface alloyed medium carbon steel, Appl. Surf. Sci.58(7) (2012)
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3214-3220.
[21] Y. Zhou, S. Q. Wang, K. Z. Huang, B. Zhang, G.H. Wen, X.H. Cui, Improvement of tribological performance of TC11 alloy via formation of a double-layer tribo-layer containing graphene/Fe2O3 nanocomposite, Tribol. Int. 109 (2017) 485-495. [22] B.B. Chen, S. C, J. Y, Tribological properties of Cu-based composites with S-doped NbSe2, Rare Met. 34(6) (2015) 407-412. [23] M. K. Zhang, G. F. Sun, Y. K. Zhang, Laser Surface Alloying of Cr-CrB2 on a SUS 304 16
ACCEPTED MANUSCRIPT Stainless Steel Substrate, Lasers Eng.2014, 27 (2014) 231-245. [24] X.P. Tao, S. Zhang, C.H. Zhang, C.L. Wu, J. Chen, A.O. Abdullah, Effect of Fe and Ni contents on microstructure and wear resistance of aluminum bronze coatings on 316 stainless steel by laser cladding, Surf. Coat. Technol. 342 (2018) 76-84. [25] S.A. Alidokht, S. Yue, R.R. Chromik, Effect of WC morphology on dry sliding wear behavior of
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cold-sprayed Ni-WC composite coatings, Surf. Coat. Technol. 357 (2019) 849-863.
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Table 1 EDS analyses of the cladding layer at different positions.
Cr
C
Mo
Si
W
Fe
1
20.55
9.06
2.67
0.93
-
69.51
2
13.15
6.07
0.89
1.09
-
78.01
3
16.14
5.32
2.37
3.10
9.45
4
17.15
7.58
3.09
2.60
7.18
5
10.69
5.19
0.92
1.34
-
59.05 60.86 80.86
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Point
ACCEPTED MANUSCRIPT List of Figure Captions Fig. 1 XRD spectra for four types of Fe-based specimens. Fig. 2 Optical micrographs of the cladding layer of Specimen 0. (a) The bottom region, and (b) the upper region.
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Fig. 3 Cross-sectional morphology in four types of Fe-based coatings. (a) Specimen 0, (b) Specimen
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1, (c) Specimen 2, and (d) Specimen 3.
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Fig. 4 Typical SEM images of different Fe-coatings. (a) The bottom area of Specimen 1, (b) the high-magnification image of zone A, (c) the middle area of Specimen 2, and (d) the middle area of
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Specimen 3.
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Fig. 5 Cross-sectional map scanning images of elements in the cladding layer of Specimen 0. (a) Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C.
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Fig. 6 Cross-sectional map scanning images of elements in the cladding layer of Specimen 1. (a)
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Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C. Fig. 7 Cross-sectional map scanning images of elements in the cladding layer of Specimen 2. (a)
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Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C.
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Fig. 8 Cross-sectional map scanning images of elements in the cladding layer of Specimen 3. (a) Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C. Fig. 9 In-depth microhardness distributions in four types of specimens. Fig. 10 Friction coefficient curves of substrate and four types of specimens. Fig. 11 Wear rate comparison of substrate and four types of specimens. Fig. 12 Typical SEM images of worn surfaces. (a) H13 substrate, (b) Specimen 0, (c) Specimen 1, (d) Specimen 2, (e) Specimen 3, and (f) high-magnitude surface contains WC particle. 19
ACCEPTED MANUSCRIPT Fig. 13 Typical EDS diffraction patterns of Points 1-4 in Fig. 12. (a) Point 1, (b) Point 2, (c) Point 3, and (4) Point 4. Fig. 14 Influence mechanism of WC particles on the microstructural evolution. (a) Distribution of WC particles in the cladding layer, and (b) the magnified image of Circle B.
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Fig. 15 Schematic illustration showing the influence mechanism of WC particles on wear resistance.
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ACCEPTED MANUSCRIPT Research highlights
WC particles reinforced Fe104 alloy coatings were fabricated by laser cladding. Various-content unmelted WC particles in coatings act as a hard reinforcement. In-depth micro-hardness of the coatings presents a gradient variation. Fe104 coatings with different WC contents are mainly subjected to abrasive wear.
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Influence mechanism of WC particles on wear performance was schematically revealed.
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Figure 2
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J.Z. Lu, J. Cao, H.F. Lu, L.Y. Zhang, K.Y. Luo PII: DOI: Reference:
S0257-8972(19)30434-7 https://doi.org/10.1016/j.surfcoat.2019.04.063 SCT 24558
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
16 January 2019 14 April 2019 21 April 2019
Please cite this article as: J.Z. Lu, J. Cao, H.F. Lu, et al., Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding, Surface & Coatings Technology, https://doi.org/10.1016/ j.surfcoat.2019.04.063
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ACCEPTED MANUSCRIPT Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding J.Z. Lua,*, J. Caoa, H.F. Lua, L.Y. Zhangb, K.Y. Luoa a
School of Civil Engineering, Xuzhou University of Technology, Xuzhou 221000, PR China
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b
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China
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Abstract: Microhardness, wear properties and microstructure of Fe-based coatings with various
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WC contents on H13 hot-working die steel by laser cladding were investigated. Special attention
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was paid to the effects of various WC contents on the microstructural characterization and wear properties of different coatings by means of optical microscopy (OM) and scanning electron
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microscopy (SEM). Results indicated that only a small part of the periphery of the WC in the WC-added coatings was melted, and the un-melted WC particles in the coatings acted as a hard
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reinforcement. Besides, the microhardness in the cladding layer increased with the increment of
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the mass fraction of WC particles, and presented a gradient distribution along in-depth direction of the coatings. Furthermore, the Fe-based clad coating with WC particles exhibited a higher wear
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resistance and a lower friction coefficient compared with the substrate and Fe-based clad coating.
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In addition, the influence mechanism of WC particles on microstructural evolution and wear resistance was also discussed. Keywords: Laser cladding, Fe-based coatings, WC particles, Wear property, Microstructure, Microhardness
*
Corresponding author: Xuefu Road 301, Jingkou District, Zhenjiang 212013, PR China
Email address: [email protected] (J.Z. Lu) Tel.: +86-511-88797198; Fax: +86-511-88780219 1
ACCEPTED MANUSCRIPT 1. Introduction Laser cladding is an efficient surface material deposition technique, which uses a high energy density laser beam to completely melt the metallic powders and form melt pool on the surface of the substrate to produce high quality metal layer with fast solidification characteristics [1-3]. Compared with other surface techniques, laser cladding exhibits unique advantages, such as
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low rate of dilution, low substrate deformation, narrow heat affected zone, dense metallurgical
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bond between cladding layer and substrate, and relatively fine microstructure. Owing to the high
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hot strength and outstanding toughness, H13 hot-working die steel is widely applied in casting dies, extrusion dies, hot forming dies and other plastic deformation fields. Particularly, when it is
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used as a mold material, it will be subjected to intense friction caused by hot and cold cycles, and metal flow accompanied by wear, erosion and thermal fatigue cracking. Accordingly, the strength
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in the surface layer of the mold is one of important factors influencing its fatigue life in engineering research. Fabricating cladding layer on the surface of steel components is a good
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choice to improve the properties and service life of H13 steel dies [4-5].
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Some typical cladding powders, such as Fe-, Ni-, and Co-based alloy powders [6] are relatively common in laser cladding. Among them, Fe-based powder is widely used because of its
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low cost and high performance. Telasang et al. prepared H13 powder cladding layer on the surface of H13 hot-working die steel, significantly improving its surface microhardness.
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Furthermore, the use of alloy powders with the same or similar chemical composition to the substrate material would ensure better metallurgical bonding interface and similar properties [7]. Recently, much more attentions have paid to fabricate composite coatings with various content of added particles, for example, WC [8], VC [9], TiC [10], and TiB [11], leading to the improvement in the microhardness and wear resistance. Wang et al. investigated the effects of TiC particles on the mechanical properties of Fe-based coatings, and it was found that Fe-based coatings with TiC particles obtained lower friction coefficient and higher wear resistance [12]. Among steel-based 2
ACCEPTED MANUSCRIPT reinforcements, WC particles is recognized to be one of the most commonly used hard-phase which combines lows thermal expansion coefficient, high hardness and good wettability. Yuan et al. investigated WC-Co composites coatings for enhancing the microhardness and sliding wear resistance [13]. Pre-machined trapezoidal groove had been repaired by the hybrid technology combining laser cladding with laser surface alloying, and 316L stainless steel powders and WC
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powders were used in mixture [14]. However, when the thickness of the functional cladding layer
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formed on the surface of the metal substrate ranges from 0.5 mm to 2 mm, it exhibited a tendency
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to crack initiation [15]. Therefore, to better improve the surface properties, when adding WC particles, it is necessary to control the mass fraction of the added WC particles to obtain a
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crack-free, void-free high-quality cladding layer, and to control the thickness of the cladding layer within a reasonable range to meet the actual machining needs [16].
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The aim of this paper is to investigate the micro-hardness, wear properties and microstructures of Fe-based coatings with various WC contents on H13 hot-working die steel
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prepared by laser cladding. Special attentions were paid to the effects of various WC contents on
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the microstructural characterization and wear properties of different coatings. In addition, the influence mechanism of WC particles on microstructural evolution and wear resistance was also
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discussed.
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2. Experimental procedure
2.1 Experimental material and laser cladding parameters The substrate material in the present work was H13 hot-working die steel plates with dimensions of 150 mm × 60 mm × 12 mm in hardened (austenitized to 1030 °C for 10 min followed by oil quenching) and tempered (at 620 °C for 2 h) conditions. The chemical composition of the substrate was 0.4 wt.% C, 5.2 wt.% Cr, 0.4 wt.% Mn, 1.0 wt.% Mo, 1.0 wt.% Si, and balanced Fe, five substrate specimens (one of them is substrate) were cut from a same 3
ACCEPTED MANUSCRIPT plate. Before laser cladding, all substrate specimens were mechanically gritted using different-graded (from 400# to 2000#) SiC sandpapers, polished and rinsed with absolute ethyl alcohol. The cladding material was Fe104 steel alloy powder with a particle size of 100-140 μm, and the normal composition of Fe104 steel powder was 0.24 wt.% C, 15.3 wt.% Cr, 1.2 wt.% B,
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0.55 wt.% Mn, 1.45 wt.% Mo, 1.9 wt.% Ni, 1.23 wt.% Si, and balanced Fe. The combination of
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pure WC powder and Fe104 powder formed the WC-Fe mixed powder for the present experiment.
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After sufficient drying treatment and stirring, the WC contents of the final prepared composite powder were set to 3%, 6%, and 9%, respectively.
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Laser cladding experiments were carried out using an IPG 4kW ytterbium fiber system
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equipped with a KUKA type 6-axis robot at 980 nm wavelength. Through a range of optimization experiments, the laser power was determined to be 2,000 W, and the laser spot diameter was set as
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4 mm with a 60% overlapping rate. Meanwhile, the scanning speed was kept to 240 mm/min with
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a powder feeding rate of 15 g/min. Four types of Fe-based coatings were prepared, in which the cladding powder mixed with 0 wt.%, 3 wt.%, 6 wt.% and 9 wt.% were defined as Specimen 0,
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Specimen 1, Specimen 2, and Specimen 3, respectively.
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2.2 Microstructural observation and component testing Cross-sectional morphologies of all coatings perpendicular to the cladded surface were cut, and then subjected to grinding and polishing for metallographic investigation according to standard procedures. After cleaned by deionized water with ultrasonic wave, all polished cross-sections were corroded with a solution consisting of 3 ml HCL and 1 ml HNO3 for 10s, and subsequently washed with absolute ethyl alcohol. The microstructure and chemical compositions of the coatings were investigated using ZESSI optical metallographic microscope (OM) and 4
ACCEPTED MANUSCRIPT HITACHI S 3400 scanning-electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The phase identifications of all specimens were carried out using D/max2500Pc X-ray diffraction (XRD) at a scanning speed of 5 °/min, ranging from 20° to 100°. 2.3 Microhardness measurement
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In-depth microhardness tests of substrate and four types of coatings were carried out using a
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HVS-1000 Vickers hardness tester along the depth direction from the top surface to the bonding line
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at intervals of 0.2 mm under a load of 5N and duration for 10 s. One average value in microhardness was determined according to the measured data from five indentations at each depth.
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2.4 Dry sliding wear tests
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Dry sliding wear tests were carried out on a block-on-ring wear test machine (Model M-2000, China) under the load of 150 N for 2 h at 20 °C, and the size of each specimen cut into was 15
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mm (length) × 10 mm (width) × 12 mm (thickness). The grinding ring with a radius of 40 mm
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was made of GCr15 steel (690-710 HV) and the width is10 mm. The machine speed was set to 200 rpm. The friction coefficient was constantly recorded by a computer and the weight losses of
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specimens are weighed by an electronic balance with an accuracy of 0.1 mg, and all specimens
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were washed and dried by absolute ethanol before weighing. 3. Results and Discussion 3.1 Phase composition analysis of Fe-based coatings with different WC contents XRD analyses are conducted on the coatings to identify the phase composition of clad coatings. The XRD results are presented in Fig. 1, showing that the coating of Specimen 0 mainly consists of α-Fe, γ-Fe, M23C6 (M=Cr, Fe) and M7C3 phases. After adding WC particles, the main phases are the same as that without WC particles in the clad coatings. However, it is noted that the 5
ACCEPTED MANUSCRIPT amount of diffraction peaks of phases increase as increasing WC content. When the mass fraction of WC reaches 6%, Fe3W3C appears in the clad coating, indicating the W and C elements have begun to inter-diffuse with elements in the iron matrix. It can be found that there are melted and re-precipitated WC particles during the cladding process. Furthermore, with increasing WC
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content, Fe6W6C and WC diffraction peaks are also found, revealing a fact that a few Fe3W3C has
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transformed into a more stable and hard phase Fe6W6C. The presence and dispersion
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strengthening of these hard phases contributes to high microhardness and good wear resistance of the clad coatings. Some small characteristic peaks in Fig. 1 reveal that there may be some other
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3.2 Typical microstructure of Fe-based coatings
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carbon and boron metal compounds in the clad coatings.
The microstructural features in the bonding zone and upper zone in the cladding layer of
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Specimen 0 are shown in Fig. 2. It can be seen from Fig. 2a that the solidification process begins
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at the junction interface between the cladding layer and the substrate. At first, the temperature (G) is extremely high and the solidification rate (R) is relatively low, so there a large value of G/R. At
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this moment, the crystal nucleation speed is much faster than the growth rate, leading to the
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growth of crystals on the substrate in a planer crystal state. At the liquid-solid interface, the actual temperature in the liquid phase gradually decreases before crystallization, and the solidification speed increases. At this time, the G/R ratio decreases, and planar crystals are turn into columnar crystals, showing a rapid solidification characteristics. The grain growth at the bottom of the cladding layer presents obvious epitaxial properties, resulting in many dendrites growing at the surface of Specimen 0. In the cladding region slightly away from the junction interface, the solidification rate increases, and the G/R ratio decreases. There are coarse dendrites and cell 6
ACCEPTED MANUSCRIPT crystals inside the grain [17]. As shown in Fig. 2b, in the upper region of the cladding, the temperature (G) reaches a maximum and the solidification rate (R) increases to a peak value. Under this circumstances, the growth rate is much larger than the nucleation speed, thus there are uniform equiax crystals and fine dendrites are important microstructural features during the grain
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growth [18].
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To observe the details of the constituent phases and microscopic morphologies of the coating
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layers, cross-sectional morphologies as a function of WC contents are shown in Fig. 3. There is a good bonding state between the cladding layer and the substrate, and no obvious defects, such as
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voids and cracks, are observed at the junction interface. When WC content is 3%, WC particles
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distribute relatively uniform in the clad coating, as shown in Fig. 3b. When WC content reaches 6% and 9%, the agglomeration phenomenon emerges, and the agglomeration effect of WC particles
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tends to be serious with increasing WC content, as shown in Figs. 3c and 3d. In addition, it can be
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found that the WCparticles have a wide distribution in the upper and middle part of the cladding layer, which lays solid foundation for further strengthening the coatings [19].
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The bond zone of clad coating is mainly composed of dendrites and inter-dendritic netlike
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eutectic structures, which is in accordance with OM observation, as shown in Fig. 4a and Fig. 2. The eutectic structure is mainly generated due to the fact that the dendrites are firstly precipitated from the molten pool and gradually developed into dendritic matrix. Immediately the inter-crystalline metal solution is cooled to a solid state, which undergoes a eutectic reaction. As shown in Fig. 4b, after adding WC particles, the particle plays as significant role as a nucleation which can be recognized as a second substrate which is smaller and tougher. Therefore, partial dissolution occurs at the edge of the WC particles. At this moment, a crystal grows in a planar 7
ACCEPTED MANUSCRIPT state and a dendritic matrix grows in the radial direction of the particle, and this phenomenon can be explained by the fact that the temperature of the WC particles in the liquid state is relatively low and they possess small size. Compared with Specimen 0, Specimen 2 with 6 wt. % WC shows smaller grains and more
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secondary dendrites in the zone around WC particle, as shown in Fig. 4c. Interestingly, the grain
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refinement is more obvious with increasing WC content, and there are much more WC particles in
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the combined zone, as shown Fig. 4d. With increasing WC content, more nucleation sites contribute further to grain refinement and strengthening of Fe-based coating.
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Fig. 5 presents the cross-sectional mapping scanning data in the cladding layer of Specimen
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0, and there are inhomogeneous element distributions due to the severe elemental segregation that occurs during the solidification. As can be seen from Figs. 5b, c, and e, Fe, Cr, Mo, and Si
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elements in the cladding layer distribute relatively uniform. Furthermore, Cr element exists
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mainly inside the grain to form a compound of Cr7C3 and M23C6, and a large amount of Cr is also found in the dendrite structures due to the alloy powder containing a massive amount of Cr
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element. Figs. 6-8 show the corresponding cross-sectional element distributions (Fe, Cr, Mo, Si
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and C) of Specimen 1, Specimen 2, and Specimen 3, respectively, and detailed EDS data of Points 1-5 (Figs. 4a and 4b) are presented in Table 1. It can be seen that there are Mo, Si and C elements with relatively high contents in the inter-dendritic netlike eutectics while Fe element uniformly distributes at Points 1 and 2 in the coating layer. W elements are detected at Point 3 and 4, showing that the WC particles dissolve limitedly and phases around WC particles are Fe-W-C compounds which improve the mechanical properties of Fe-based coatings [20]. In addition, W element does not appear at Point 5, and there 8
ACCEPTED MANUSCRIPT is nearly no precipitation of W element in mapping scanning images, indicating that the partially dissolution of WC leads to a limited scope influenced by a single WC particle. 3.3 Microhardness distribution and wear resistance In-depth microhardness distributions in four types of Fe-based coatings are measured and
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presented Fig. 9. It can be seen that all specimens exhibit higher microhardness in the coating
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layer than H13 substrate, which is attributed to the formation of hard phases such as M23C6, M7C3
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and other type phases during laser cladding. Besides, it is obvious that microhardness of all four coatings holds stable first and then decrease steeply along the depth direction. It is interesting to
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note that there is no obvious difference in the in-depth microhardness distribution of Fe-based
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coatings when the mass fraction of WC particles does not exceed 6%, and the microhardness value keeps stable in a span of 640 HV to 680 HV. In addition, when the mass fraction of WC
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particles comes to 9%, there is a significant improvement in microhardness due to the appearance
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of a great amount of Fe3W3C and Fe6W6C hard phases. As the WC content increases further, some of the W elements precipitate during laser cladding to form a high hardness FWC compound, and
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WC particles act as dispersion strengthening in the cladding layer because of its own
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characteristics. Furthermore, It is noted that WC particles tend to segregate in the bottom region because of its higher density (15.63 g/cm3), causing an improvement in microhardness. Fig. 10 shows friction coefficient curves of substrate and four types of Fe-based coatings. It can be found that the average friction coefficient of substrate is 0.351, and Specimens 1-4 are 0.349, 0.342, 0.336 and 0.313, respectively. It is well known that the friction coefficient is inversely proportional to the corresponding microhardness. As discussed above, M23C6, M7C3 are important factors to improve the microhardness of Fe-based coatings, which also result in the 9
ACCEPTED MANUSCRIPT improvement of their wear resistance. However, it should be noted that the friction coefficient of Specimen 3 is a little smaller than those of other three coatings. Fig. 11 shows the wear rate comparison of H13 substrate and Specimens 0-3. Compared with H13 substrate, all four types of Fe-based coatings present a small wear rate, indicating that the
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wear resistance of cladding layer with WC particles has been significantly improved. Furthermore,
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the wear rate of WC-Fe coatings shows a gradually decreasing trend with increasing WC particles.
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Particularly, when the mass fraction of WC reaches 9%, the wear rate of Specimen 3 is reduced by 35.79% compared with Specimen 1, indicating that the addition of WC particles plays an
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important role in further improving the wear resistance of Fe-based coating.
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Combined with these experimental data above, it can be concluded that during laser cladding, WC particles act as a hard phase to strengthen the Fe-based coating, and the increase of WC
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content and its uniform distribution are two important factors to the improvement in
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microhardness and wear resistance of Fe-based coatings. 3.4 Surface damage behavior
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Figs. 12a-e illustrate the typical SEM images of the worn surfaces of the H13 substrate and
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four types of Fe-based coatings. It is obvious that Fe-based coatings significantly prevent the surface damage. Specially, plastic deformation and sever spalling are dominating on the surface of substrate whose surface condition is rougher than that of cladding layers. Fig. 12a shows that there are deep, long and uniform grooves on the worn surface of substrate, indicating that the main mechanism of sliding wear is micro-cutting[21-22]. Besides, during sliding wear, the grinding ring pushes the substrate to cause plastic flow and slight peeling, illustrating that peeling wear occurs on the worn surface. It is show in Fig 12b that there is much smoother worn profile in 10
ACCEPTED MANUSCRIPT the surface layer of Specimen 0, and at this stage intermittent and short grooves dominates, indicating a severe adhesive wear with scale-like feature. In addition, it is obvious in Figs. 12c, d, and e that Specimens 1-3 further relieve the surface damage compared with the substrate and Specimen 0. The worn surfaces of these specimens are mainly characterized by shallow and fine
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grooves, however, the amount of peeling pits increase with increasing WC content. Because of
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the cutting action during sliding wear, the WC particles in the Fe-based coatings are gradually
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exposed. Combined with Fig. 12f, the scratching is interrupted or the path is converted due to the barriers effect of un-melted WC particles. The dispersed WC particles in the Fe-based coating act
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as a skeleton, reducing the ploughing effect on the coating by the grinding ring and lubricating the
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friction pair to slow down adhesion. Hence, the wear resistance of these Fe-based coatings has been improved [23]. It is noted that, as the WC particles content increases, the main wear
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mechanism of the worn surface is abrasive wear presented by shallower and slender grooves, but
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the degree of adhesive wear is gradually decreased. In addition, Fig 13 shows some typical EDS diffraction patterns of Points 1-4 in Fig. 12, indicating the varying degrees of oxygen in white
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bands. These data show that the oxidative wear occurs during sliding wear. Hence, laser cladded
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Fe-based coatings have the ability to improve the wear resistance, and WC particles are beneficial to the improvement in wear properties of Fe-based coatings. 3.5 Influence mechanism of WC particles on microstructural evolution and wear resistance The influence mechanism of the WC particles on the microstructural evolution of Fe-based coatings is presented in Fig. 14. When there is no WC particle in the cladding layer, dendritic matrix and inter-crystalline reticular eutectic structure exist in the cladding layer. After the addition of the WC particles, as shown in Fig. 14a, WC particles partially melt during laser 11
ACCEPTED MANUSCRIPT cladding, and the decomposed W and C elements with the Fe, Cr and other elements of the matrix powder easily form carbide structures with high hardness. Thus, the re-precipitated W and C elements contribute to solution strengthening. In addition, the WC particles that are not completely melted maintain a good integrity of the particle, as shown in Fig. 14b, and act as the
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nucleation during laser cladding. Fine dendrites grow radially around these WC particles, so the
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grains around the WC particles are refined, plays a significant role in fine-grain strengthening.
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Furthermore, the WC particles which are not dissolved in the cladding layer can also act as the dispersion strengthening. In the present work, with increasing of WC contents, more and more W
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and C elements are solidified into the matrix, and more Fe-W-C compounds are produced, leading
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to the obvious strengthening effect. In addition, with increasing of WC contents, WC particles distribute more uniformly, and the better dispersion strengthening will be obtained.
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The influence mechanism of WC particles on the wear resistance of Fe-based coatings is
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presented in Fig. 15. When there is no WC particle in the cladding layer, dendritic matrix and inter-crystalline reticular eutectic structures in the cladding layer are composed of high-strength
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and high-hardness hard phases which is rich in Fe, Cr, C, and Si elements. Therefore, compared
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with substrate, there is an obvious improvement in microhardness and mechanical performance of the cladding layer. As shown in Fig. 15a, during sliding wear, a large amount of hard phases reduce molecular adhesion and shearing action during wear test, and effectively enhance the adhesion strength of Fe-based coatings. Hence, the wear resistance of Fe-based coatings is significantly improved [24]. After the addition of WC particles in the cladding layer, the cladding layer obtains more obvious solution strengthening and dispersion strengthening as the WC content increases, effectively reducing the corresponding wear rates. In addition, when the surface 12
ACCEPTED MANUSCRIPT of the cladding layer is severely worn, the WC particles will gradually expose in the surface layer, and directly contact the grinding ring after debris continuously generating, avoiding the occurrence of sever wear in the substrate immediately. Therefore, during sliding wear, the dispersed WC particles can not only serve as a
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high-hardness skeleton to prevent the wear of the abrasive particles on the matrix or change the
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moving direction of the abrasive particles, but also effectively prevent the ploughing effect during
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sliding wear, and therefore protect the substrate to reduce wear and even interdict wear, forming a shadow area, as shown in Fig. 15b [25]. However, the accumulation of wear debris will produce
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repeated relative movement in the friction pair, which may aggravate the oxidative wear on the
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top surface of Fe-based coatings. Hence, when the mass fraction of WC particles is 3%, the wear resistance of Fe-based coating is comparable to that of the coating without WC particles. But
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when the mass fraction of WC particles reaches 6% or 9%, the agminated effect of WC particles
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is reflected, and the wear resistance of Fe-based coatings increases with increasing WC content. 4. Conclusions
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(1) Fe-based coatings with different WC contents were prepared using laser cladding.
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Results showed that hard phases appear in coatings were M7C3 and M23C6, contributing to the initial microhardness of Fe-based coatings. Furthermore, the increment of WC content lead to the appearance of FeWC-like compounds in Fe-based coatings, leading to an obvious improvement in microhardness of Fe-based coatings. (2) In the WC-added coatings, because only a small part of the periphery of the WC is melted, the un-melted WC particles in the coatings act as a hard reinforcement. At the same time, coarse grains around WC particles are refined by the nucleation effect. Furthermore, the 13
ACCEPTED MANUSCRIPT microhardness of the cladding layer increases with the increment of WC content. (3) Compared with the substrate, there is an improvement in the wear resistance of Fe-based coating without WC particles to a greater extent, and increasing WC content further enhance the wear resistance of Fe-based coatings. The wear mechanism of WC particle on the Fe-based
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coatings is also presented, and abrasive wear accompanied slighter spalling wear is the dominate
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wear form during sliding wear.
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Acknowledgement
This work was supported by the National Key R&D Program of China (No. 2017YFB1103603),
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the National Natural Science Foundation of China (grant numbers 51775250, 51875262 and
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51575242), the Jiangsu Provincial Science and Technology Projects (grant numbers BE2016148, and BE2017142), and the Priority Academic Program Development of Jiangsu Higher Education
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Institutions.
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ACCEPTED MANUSCRIPT Stainless Steel Substrate, Lasers Eng.2014, 27 (2014) 231-245. [24] X.P. Tao, S. Zhang, C.H. Zhang, C.L. Wu, J. Chen, A.O. Abdullah, Effect of Fe and Ni contents on microstructure and wear resistance of aluminum bronze coatings on 316 stainless steel by laser cladding, Surf. Coat. Technol. 342 (2018) 76-84. [25] S.A. Alidokht, S. Yue, R.R. Chromik, Effect of WC morphology on dry sliding wear behavior of
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Table 1 EDS analyses of the cladding layer at different positions.
Cr
C
Mo
Si
W
Fe
1
20.55
9.06
2.67
0.93
-
69.51
2
13.15
6.07
0.89
1.09
-
78.01
3
16.14
5.32
2.37
3.10
9.45
4
17.15
7.58
3.09
2.60
7.18
5
10.69
5.19
0.92
1.34
-
59.05 60.86 80.86
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ACCEPTED MANUSCRIPT List of Figure Captions Fig. 1 XRD spectra for four types of Fe-based specimens. Fig. 2 Optical micrographs of the cladding layer of Specimen 0. (a) The bottom region, and (b) the upper region.
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Fig. 3 Cross-sectional morphology in four types of Fe-based coatings. (a) Specimen 0, (b) Specimen
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1, (c) Specimen 2, and (d) Specimen 3.
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Fig. 4 Typical SEM images of different Fe-coatings. (a) The bottom area of Specimen 1, (b) the high-magnification image of zone A, (c) the middle area of Specimen 2, and (d) the middle area of
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Specimen 3.
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Fig. 5 Cross-sectional map scanning images of elements in the cladding layer of Specimen 0. (a) Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C.
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Fig. 6 Cross-sectional map scanning images of elements in the cladding layer of Specimen 1. (a)
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Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C. Fig. 7 Cross-sectional map scanning images of elements in the cladding layer of Specimen 2. (a)
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Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C.
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Fig. 8 Cross-sectional map scanning images of elements in the cladding layer of Specimen 3. (a) Microstructure, (b) Fe, (c) Cr, (d) Mo, (e) Si, and (f) C. Fig. 9 In-depth microhardness distributions in four types of specimens. Fig. 10 Friction coefficient curves of substrate and four types of specimens. Fig. 11 Wear rate comparison of substrate and four types of specimens. Fig. 12 Typical SEM images of worn surfaces. (a) H13 substrate, (b) Specimen 0, (c) Specimen 1, (d) Specimen 2, (e) Specimen 3, and (f) high-magnitude surface contains WC particle. 19
ACCEPTED MANUSCRIPT Fig. 13 Typical EDS diffraction patterns of Points 1-4 in Fig. 12. (a) Point 1, (b) Point 2, (c) Point 3, and (4) Point 4. Fig. 14 Influence mechanism of WC particles on the microstructural evolution. (a) Distribution of WC particles in the cladding layer, and (b) the magnified image of Circle B.
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Fig. 15 Schematic illustration showing the influence mechanism of WC particles on wear resistance.
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ACCEPTED MANUSCRIPT Research highlights
WC particles reinforced Fe104 alloy coatings were fabricated by laser cladding. Various-content unmelted WC particles in coatings act as a hard reinforcement. In-depth micro-hardness of the coatings presents a gradient variation. Fe104 coatings with different WC contents are mainly subjected to abrasive wear.
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Influence mechanism of WC particles on wear performance was schematically revealed.
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