Microstructure and wear properties of high-speed steel with high molybdenum content under rolling-sliding wear

Microstructure and wear properties of high-speed steel with high molybdenum content under rolling-sliding wear

Accepted Manuscript Microstructure and wear properties of high-speed steel with high molybdenum content under rolling-sliding wear Liujie Xu, Xiaoman ...

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Accepted Manuscript Microstructure and wear properties of high-speed steel with high molybdenum content under rolling-sliding wear Liujie Xu, Xiaoman Fan, Shizhong Wei, Dongdong Liu, He Zhou, Guoshang Zhang, Yucheng Zhou PII:

S0301-679X(17)30334-1

DOI:

10.1016/j.triboint.2017.07.002

Reference:

JTRI 4807

To appear in:

Tribology International

Received Date: 20 April 2017 Revised Date:

28 June 2017

Accepted Date: 2 July 2017

Please cite this article as: Xu L, Fan X, Wei S, Liu D, Zhou H, Zhang G, Zhou Y, Microstructure and wear properties of high-speed steel with high molybdenum content under rolling-sliding wear, Tribology International (2017), doi: 10.1016/j.triboint.2017.07.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Microstructure and wear properties of high-speed steel with high molybdenum content under rolling-sliding wear

Liujie Xu a,* , Xiaoman Fanb, Shizhong Wei a, Dongdong Liu b, He Zhou b, Guoshang Zhang b,

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Yucheng Zhouc,*

a. Engineering Research Center of Tribology and Materials Protection, Ministry of Education, Henan University of Science and Technology, Luoyang 471003, China

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b. School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China

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c. Institute for Advanced Materials, Henan University of Science and Technology, Luoyang 471003, China

*

Corresponding author. Tel.: +86 379 64270020; fax: +86 379 64231801.

E-mail address: [email protected] (L. Xu), [email protected] (Y. Zhou)

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Abstract: This paper focuses on the microstructures and frictional wear behaviors of high-speed steel (HSS) with high molybdenum content under different rolling-sliding conditions using self-made wear tester. Results showed that the molybdenum element in HSS mainly formed M2C-type carbide ((Fe27.42Mo48.26Cr24.32)2C). M2C (21-1) is coherent with α-Fe (110). The sliding

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ratio has a significant influence on frictional wear behaviors. As sliding ratio increases from approximately 1% to 10%, the frictional coefficient rises and then decreases, and the wear weight

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loss rises obviously because the wear mode varies from fatigue to sliding wear. The high stress rolling-sliding contact can cause not only fracture and desquamating of M2C, but also martensitic transformation in subsurface. The martensitic transformation contributes in improving hardness and wear resistance.

Key words: High-speed steel, Molybdenum carbide, Rolling-sliding, Wear failure 1. Introduction The materials used for work rolls are continually innovated with the development of steel-rolling technology. Forged steel that contains approximately 1.8% Cr and 1% C is the initial basic

ACCEPTED MANUSCRIPT composition [1]. The content of alloy elements uch as Cr and Ni are increased to obtain a thick hardened depth at the surface of roll. Over the past 20 years, a strong demand for higher wear resistance to improve the economic benefits is observed. Furthermore, this is considered an effective method for improving the wear property of work rolls to add high hardness carbides in

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materials. High chromium cast iron rolls are developed and applied in mill rolling [2, 3]. In addition, they presented excellent wear resistance for the said applications given the hard M7C3-type chromium carbides [4, 5]. Compared with high chromium cast irons, high-speed steels (HSS) have higher hardness carbides, such as M6C, M2C, and MC. Therefore, they exhibit excellent wear

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resistance under extreme wear conditions [6—10]. In recent years, HSS have been applied in rolling materials. Previous studies have indicated that HSS rolls exhibit better wear resistance compared

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with high-chromium cast iron rolls because of their excellent red hardness [4, 7, 11–13]. Among them, cast HSS with high tungsten content are the main materials for rolls with excellent wearability because of the high hardness of M6C carbide. However, few researchers have paid attention to HSS with high molybdenum content even though the molybdenum element can form high hardness carbides.

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In the working process of steel rolling equipment, work rolls suffer from high stress rolling/sliding contact condition, with a sliding ratio of approximately 3%–10% [14, 15]. Thus, the roll failure is mainly caused by rolling-sliding wear. In previous studies, the sliding or rolling wear

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has been often adopted to investigate the wear of roll [16−18]. However, the test condition is different from the rolling/sliding operating conditions of a roll as it may cause different wear failure mode. This work studied the microstructure and wear property of HSS with high molybdenum

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content under different rolling/sliding conditions and expounded on the relation between microstructure transformation and wear failure behavior in the rolling/sliding wear process of HSS. 2. Experimental methods

2.1. Chemical compositions

The chemical composition of test HSS was fabricated according to conventional HSS. In order to obtain M2C, about 10 wt.% molybdenum element was added into the HSS. In addition, approximately 4 wt.% chromium was mixed in the tested alloy to ensure the realization of a high-hardness HSS. The matched rolled material was 40Cr steel. The actual chemical compositions of the materials are listed in Table 1.

ACCEPTED MANUSCRIPT 2.2. Preparation of samples The alloy ingots were produced by melting the raw materials in a 50 kg intermediate frequency induction melting furnace. The deoxidation was conducted by adding 0.1% pure aluminum. The melting alloys were tapped from the furnace at approximately 1500 °C and casted at 1450 °C.

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The samples were austenized at 1050 °C for 2 hours, air quenched, and then tempered at 550 °C for 2 hours. An SKZ-8-13 silicon-kryptol resistance furnace was used for the quenching furnace in this study and it was controlled with a microcomputer. An SKZ-8-10 resistance furnace was used for the tempering furnace in this study.

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2.3. Mechanical aptitude test

The hardness of the specimens was measured using an HR-150A Rockwell tester. Five points

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were measured for each sample, and the last value was the average of the five values. The toughness of a 20 mm × 20 mm × 110 mm smooth specimen was tested on a JB-300B pendulum-type impact testing machine, and the gauge length was 70 mm. 2.4. Wear performance test

The rolling-sliding wear test was conducted using a self-made ring-ring wear testing machine.

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The rolling load was 5000±20N. The tested HSS sample rotated at a speed of 290rev min-1, and the rotational speed of the matched samples was adjusted according to the requirement of the sliding ratio. For each sample, the total wear time was 4h and the wear weight loss was measured hourly.

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The wear loss was the average result of three repetitions. Five sliding ratios, 1.0%, 3.0%, 5.5%, 8.0% and 10.0%, were designed in this study, which was calculated using Equation (1).

πDR − πdr × 100% πdr

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Rs =

(1)

where Rs is the sliding ratio, d and D are diameters of the tested and matched samples, respectively, and r and R are the rotating speeds of the tested and matched samples, respectively. The frictional coefficient was calculated using Equation (2).

f =

M PR

(2)

where f and M are the frictional coefficient and torsion, respectively, and P and R are the pressure and radius of the torsion, respectively.

ACCEPTED MANUSCRIPT 2.5. Microstructure analysis and worn surface observation The microstructure of the HSS was analyzed using JSM-5160LV type scanning electron microscope (SEM) and JEOL 2010F type high resolution electron microscope (HREM). EDS was employed to analyze the element content of the different phases in HSS. In addition, the worn

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surface and worn cross-section were observed and analyzed using SEM and X-ray diffraction (XRD). The retained austenite contents in HSSs were determined according to the analysis results of XRD.

3. Results

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3.1. Microstructure of HSS

Fig. 1 illustrates the microstructure of HSS with high molybdenum content. The main needle-like

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eutectic carbides precipitate in the solidification process of HSS. An evident transition zone between eutectic carbide and matrix in the as-cast HSS is present (Fig. 1a). Spectrum analysis results show that the element contents, such as Cr and Mo, are higher in the transition area when compared with the matrix (Figs. 1c and d). After heat treatment, the transition zone disappears (Fig. 1b). The matrix microstructure changes from ferrite to compose structure of martensite and residual

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austenite. Few second carbides precipitate in tempered HSS in the heat treatment process. Fig. 2 illustrates the XRD analysis results. The carbide in HSS with high molybdenum is the M2C-type carbide with an orthorhombic structure. Matrixes are composed of matensite and residual

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austenite after heat treatment.

Fig. 3 is HREM photos and a diffraction pattern of interface between the eutectic M2C and matrix

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in HSS with high molybdenum content. The energy spectrum analysis indicates that the alloy element in M2C mainly includes Mo, Fe, and Cr. Furthermore, the molybdenum content acquires 62.35 wt% of total alloys in M2C (Fig. 3d). According to the energy spectrum results, the eutectic M2C carbide formula can be expressed as (Fe27.42Mo48.26Cr24.32)2C. A crystallographic relationship between eutectic M2C and α-Fe is present. M2C (21-1) is coherent with α-Fe (110) (Fig. 3b). The interplanar spacing of M2C (21-1) and α-Fe (110) is 0.20294 and 0.20268 nm, respectively, thus, the lattice mismatch is 0.13%.

3.2. Mechanical property Table 2 shows the hardness and impact toughness of experimental materials. After heat treatment, the hardness of the HSS becomes 64.6 HRC, with high hardness M2C carbide (1520 HV). The

ACCEPTED MANUSCRIPT hardness of the matched 40 Cr reaches 55 HRC. The impact toughness of HSS is approximately 5J/ cm2.

3.3. Friction coefficient Fig. 4 illustrates the relation of friction coefficient and sliding ratio. Friction coefficient rapidly

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increased when sliding ratio increased from 1.0% to 5.5% but decreased when sliding ratio further increased to 10.0%.

3.4. Wear property

Fig. 5 shows the wear property of HSS. The relationship of wear weight loss and wear time under

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different sliding ratio conditions is presented in Fig. 5(a). The wear weight loss is linearly increased with prolonged wear time. The relation of wear weight loss and wear time can be specified using

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Equation (3), and the fitting equations according to the least-squares fitting and mean-square errors are listed in Table 3. The wear ratio is obtained according to Equation (4), the value of which is equal to coefficient A in Equation (3). Fig. 5(b) illustrates the effect of sliding ratio on wear ratio. As sliding ratio increases, wear ratio rises rapidly at first, and then rises slowly as sliding ratio further increased. Furthermore, wear ratio showed a significantly quadratic increase as sliding ratio

W = At + B I = d W / dt

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rose.

(3) (4),

coefficients.

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where W is the wear weight loss, t is the wear time, I is the wear ratio, and A and B are the

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3.5. Effect of sliding ratio on retained austenite Fig. 6 shows the XRD analysis results of HSS before and after wear under different sliding ratio conditions. The matrix microstructure of HSS is composed of martensite and retained austenite. In the process of wear, a number of retained austenite transformed to martensite. The sliding ratio has evident effects on the transformation of retained austenite, as indicated by the change of diffraction peak intensity in Fig. 6. According to the diffraction intensity of the matrix, the retained austenite amounts are calculated, and the results are shown in Fig. 7. As sliding ratio increases, the retained austenite amount evidently decreases. The transformation ratio of retained austenite to martensite (rA) is defined by Equation (5).

ACCEPTED MANUSCRIPT rA =

V0 − V × 100% V0

(5),

where V0 is the original volume fraction of retained austenite in the unworn sample, and V is the volume fraction of retained austenite in the worn sample.

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The sliding ratio affects the transformation ratio of retained austenite to martensite, as shown in Fig. 8. As the sliding ratio increases to 5.5%, the rA value rises rapidly, and then a slow increase of the rA value as the sliding ratio further increases from 5.5% to 10%.

3.6. Change of hardness in the process of wear

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The high stress rolling-sliding wear not only transforms the microstructure, but also changes mechanical property in the wear-affected zone. Fig. 9 illustrates the micro-hardness of the matrix on

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the worn cross-section of HSS at a sliding ratio of 5.5%. Furthermore, Fig. 9 shows that as the distance from the worn surface nears, the hardness increases. Micro-hardness gradually decreases from the worn surface to the interior of the specimen. Under the test condition, the depth affected by wear is approximately 300 µm. When the depth is beyond 300 µm, the hardness becomes constant as the distance from the worn surface increases, and the hardness is approximately equal to

3.7. Wear failure analysis

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the original microhardness of the matrix.

Fig. 10 shows the worn surfaces under different sliding ratio conditions. The sliding ratio has an

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evident effect on the wear failure mode. For the small sliding ratio of approximately 1.0%, the worn surfaces of the samples are characterized by fatigue desquamating flakes. The fatigue-induced flakes on the worn surface decreased and the slide-induced furrows significantly increased as the

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sliding ratio increases. When the sliding ratio reached 10%, the fatigue-induced flakes disappeared, and the worn surface took on several furrows. Furthermore, a number of big spalled pits and micro-voids on the worn surface are observed. The big spalled pits are caused by fatigue flake. However the micro-voids relates to the desquamating of carbides [19]. Fig. 11 illustrates the worn cross-section of the samples under different sliding ratio conditions. At a small sliding ratio of approximately 1.0%, the worn subsurface exhibited shallow layer cracks caused by fatigue (Fig. 11a). The cracks propagation resulted in the fatigue desquamating of flakes on the worn surface. As the sliding ratio increases to 5.5%, the plastic deformation layer occurs on the subsurface, and the carbides beneath the worn surface exhibited severe bending and fracture

ACCEPTED MANUSCRIPT (Fig. 11b). Furthermore, a number of cracks propagate along M2C (Fig. 11c). Wear failure is caused by the combined actions of rolling fatigue and sliding furrow. As the sliding ratio further increased to 10.0%, the plastic deformation layers deepened, and an angle became present at approximately 20 degrees, between the cracks and the worn surface (Fig. 11d). The angle is caused by the

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combination of normal pressure and friction. Based on the abovementioned observation and analysis, it could be concluded that the wear failure mode transformed from fatigue to main sliding wear as the sliding ratio increased.

Fig. 12 illustrates the TEM micrographs of HSS at approximately 25 µm beneath the wear

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surface when compared with the original microstructure. Furthermore, the martensite took on evident fragmentation after the HSS suffers wear (Fig. 12b) when compared with the original

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microstructure (Fig. 12a). Several microcracks in M2C (Fig. 12c) are present. The propagations of microcracks in M2C resulted in M2C fracture.

4. Discussion

4.1. Effect of sliding ratio on wear failure mechanism

The wear failure mechanism is related to the microstructure and properties of materials and wear

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conditions. In this study, the same HSS suffered different wear conditions, i.e., different sliding ratios. Thus, the wear failure mechanism of HSS depends on the sliding ratio. The change in the sliding ratio causes the change in wear contact mode. When the sliding ratio is about 0.5%, the

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work condition is near rolling contact. The wear failure mechanism is main fatigue [20,21].As the sliding ratio increases from approximately 0.5% to 10%, the wear contact mode varies from rolling contact to rolling-sliding and sliding contacts, thereby changing the wear failure mechanism from

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fatigue to furrow wear. The wear failure mechanism is consistent with previous research results under a rolling-sliding condition [22]. Some models of rolling-sliding contacts have been built [23,24].

The wear failure mode decides the roughness of worn surface, thereby playing an important role in affecting friction coefficient. As the sliding ratio reaches approximately 5.5%, the worn surface exhibits large roughness given the combination of fatigue and furrow failure modes, resulting in high friction coefficient. Nevertheless, the worn surface has a relatively small roughness when the sliding ratio approximately reaches 10%, which slightly decreases the friction coefficient.

4.2. Changes in microstructure and property in the wear process

ACCEPTED MANUSCRIPT The HSS suffered high stress cyclic loading during the rolling-sliding wear, which changes the microstructure and property on the worn surface. A number of retained austenite were transformed into martensite on the worn surface, as verified by the XRD analysis results (Fig. 6). The transformation of austenite to martensite could be explained through theory of deformation that

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induces martensite. According to the kinetics model of the strain-induced α-martensites transformation developed by Olson and Cohen [25], the volume fraction of αˊ-martensites is related to plastic strain (ε) as follows: fαˊ = 1-exp{-β[1-exp(αε)]n}

(4),

intersection that forms αˊ-martensite embryo.

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where α represents the rate of shear band formation, and β represents the probability of an

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The increase of sliding ratio heightens plastic strain (ε), which increases transformation ratio of retained austenite to martensite (Fig. 8). Furthermore, the martensite in worn subsurface took on obvious fragmentation after the HSS suffers wear (Fig. 12b).

The rolling-sliding wear caused not only microstructure transformation but changed the mechanical property on the worn surface. In the present study, the hardness of the worn surface has

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increased evidently. The increase of hardness on worn surface is related to two factors. The first factor is the transformation of austenite to martensite. Martensite has a relatively higher hardness when compared with austenite. Thus, the increase of martensite content increases hardness. Fig. 13

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shows the quantitative relation between microhardness of matrix and retained austenite content on the worn surface. As the retained austenite content decreases, the martensite content increases, and the microhardness of matrix evidently rises. The samples undergo great elastic-plastic deformation

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in the wear process. Several grains are stretched and broken in the process. Deformation sharply increases dislocation density in the grains given that a large amount of carbides in HSS can hinder dislocation slip, resulting in the dislocation tangle under high stress rolling-sliding contact condition. All the above reasons will result in increased hardness called work hardening. The samples especially suffer many times cyclic deformations, which exacerbates work hardening. The increase of surface hardness is conducive to heightened wear resistance. Nevertheless, when Fig. 5 is compared with Fig. 8, the large sliding ratio increases wear weight loss although the transformation ratio of the retained austenite to martensite (rA) increases, which may be contradictory. However, wear weight loss is affected by the composed factors of wear

ACCEPTED MANUSCRIPT condition and microstructure. The “sliding ratio” belongs to the wear condition factor, whereas the “phase transformation of austenite to martensite” belongs to the microstructure factor. Although the phase transformation of austenite to martensite contributes in improving wear resistance, the increase of sliding ratio can lead to rapid wear given that the wear failure mode varies from fatigue

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to sliding. The combined action of the abovementioned two factors increases wear weight loss as the sliding ratio increases.

Based on the aforementioned reasons, the matrix microstructure of HSS for roll should be designed as a duplex structure for martensite and austenite. Furthermore, the duplex matrix

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structures for HSS rolls have been adopted by a number of researchers in previous references [4–8].

4.3. Role of molybdenum in affecting the wear property of HSS

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Molybdenum element in HSS is mainly formed by M2C type carbide with high hardness. In the present work, 10 wt% molybdenum elements was added in HSS, thereby forming large amounts of M2C and distributed evenly in the matrix composed of martensite and retained austenite. In the process of high stress rolling-sliding wear, these high hardness M2C standing out from worn surfaces can efficiently resist scratch to protect the HSS matrix. Therefore, M2C plays the role of a

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wear-resistance skeleton that slows the wear failure of HSS. Moreover, as the sliding ratio increases, M2C can play a more important role given that HSS has mainly suffered sliding wear failure.

5. Conclusions

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(a) The molybdenum element in HSS mainly formed eutectic M2C-type carbide with flake-like shape. The formula of eutectic M2C can be expressed as (Fe27.42Mo48.26Cr24.32)2C. A crystallographic relationship between eutectic M2C and α-Fe M2C (21-1) is present and is coherent with α-Fe (110),

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with a lattice mismatch of 0.13%. (b) The sliding ratio has a significant influence on the frictional wear properties and failure behaviors of HSS. As the sliding ratio increases, the frictional coefficient rises and then decreases. Furthermore, the wear weight loss evidently rises given that the wear failure mode varies from fatigue to sliding wear. (c) The high stress rolling-sliding contact causes the microstructure evolution of high speed in the worn subsurface. Furthermore, it causes eutectic M2C fracture and the phase transformation of austenite into martensite. The sliding ratio evidently affects the transformation ratio of retained austenite into martensite. As the sliding ratio increases, the amount of strain-induced martensite

ACCEPTED MANUSCRIPT from residual austenite significantly increases. (d) The phase transformation of austenite to martensite results in the increasing of hardness in the worn subsurface, and therefore contributes in improving wear resistance. The matrix microstructure of HSSs rolls is recommended to be designed as a compound of martensite and austenite.

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Acknowledgments The authors greatly acknowledge the Production-study-research Cooperation Project of Henan province, China (No. 162107000062), Plan for Scientific Innovation Talent of Henan province, China (No. 174100510012) and National Natural Science Foundation of China (No. 51171060) for

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financial support.

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[25] Olson GB, Cohen M. Kinetic of strain-induced martensitic nucleation. Metall Trans A 1975;

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ACCEPTED MANUSCRIPT Table 1 Chemical compositions of samples (wt.%) C

Mo

Cr

Mn

Si

S, P

RE

Mo10

1.43

10.01

4.20

0.5-1.0

0.4-0.8

≤0.03

0.4

40Cr

0.40

1.10

0.5-0.8

0.2-0.4

≤0.03

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Alloys

Table 2 Hardness and impact toughness of experimental materials Hardness (HRC)

Main carbide microhardness (HV)

Microhardness of matrix(HV)

Impact toughness (J/cm2)

Mo10

64.6

613.2

4.8

40Cr

55.2

M2C: 1520 —

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Materials

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558.5



Table 3 Equations modeling correlation between wear weight loss and wear time Sliding ratio, Rs /%

Equations between wear weight loss and wear time

Mean square error ( 1 ∑ δ 2 )

` Correlation coefficient, R2

m

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W =0.317 t - 0.175 W =0.799t + 1.163 W =1.218 t + 1.866 W = 1.298t + 2.236 W =1.27 t + 3.292

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1.0 3.0 5.5 8.0 10.0

6.300E-4 0.0012 9.675E-4 6.300E-4 6.750E-5

0.98367 0.97995 0.98794 0.99247 0.99925

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(b)

(a) A

(d)

(e)

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(c)

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C

B

Fig. 1 SEM microscopy and EDAX analysis of HSS with high molybdenum content: (a) as–cast; (b) after heat treatment; (c) EDAX analysis in Area A; (d) EDAX analysis in Area B; (e) EDAX analysis in Area C

•α-Fe



A----as-cast B----after heat treatment

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250

150

♦γ-Fe ♠ M2C

♦ ♠♦ ♠ ♠

100



50

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Intensity/counts

200

0



• B

A

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10 20 30 40 50 60 70 80 90 100

2Theta/deg

Fig. 2 XRD analysis of HSS

ACCEPTED MANUSCRIPT (a)

(b)

α-Fe(002)

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α-Fe(112)

α-Fe(110)

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(000) M2C(21-1) M2C(110)

(c)

d M2C(21-1)=0.20294nm

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dα-Fe(110)=0.20268nm

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(d)

element Wt%

At%

Cr K

17.03 24.32

Fe K

20.62 27.42

Mo K

62.35 48.26

total

100.00 100.00

Fig. 3 Interface between M2C and matrix in HSS: (a) TEM microscopy of eutectic carbide; (b) diffraction pattern of interface between M2C and matrix; (c) HRTEM photo of interface between M2C and matrix; (d) EDAX analysis of eutectic M2C

0.6

0.4

0.2

0.0

0

2

4 6 8 Sliding ratio, Rs/%)

RI PT

Mean friction coefficient

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10

1.2 1.0 0.8 0.6 0.4 0.2 0.0

(b)

0.4

0.3

0.2

0.1

1.0

1.5

2.0

2.5

3.0

3.5

Wear time/h

SC

Rs=1.0% Rs=3.0% Rs=5.5% Rs=8.0% Rs=10.0%

-1

1.4

Wear ratio/gh

1.6

M AN U

(a)

wear weight loss/g

Fig. 4 Mean friction coefficient versus sliding ratio

4.0

0

2

4

6

8

10

TE D

Sliding ratio, Rs/%

Fig. 5 Wear property of HSS: (a) wear weight loss versus wear time and sliding ratio; (b) wear ratio versus sliding ratio

AC C

Intensity

EP



■ M ▲ A ■ ■ ▲ ▲

RS=10.0% RS=5.5% Rs=1.0% Original 30 40 50 60 70 80 90 100 110

2 Theta/deg Fig. 6 X-ray diffraction patterns of wear surface

25 20 15 10 5 0

original

1

3 5.5 8 Sliding ratio,Rs/%

RI PT

Retained austenite amount/Vol.%

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10

SC M AN U

60 50 40 30 20 10 0

TE D

Transformation ratio of retained austenite to martensite, rA /%)

Fig. 7 Effect of sliding ratio on retained austenite

0

2

4 6 8 Slinding ratio, Rs/%

10

Fig. 8 Transformation ratio of retained austenite to martensite versus sliding ratio

AC C

Hardness/HV

EP

750

700

650

600

550

0

100

200

300

400

500

600

Distance from worn surface /µm

Fig. 9 Micro-hardness distribution on the wear cross-section of samples at the sliding ratio of 5.5%

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(a)

(c)

Spalled pits

Furrows

Fatigue flake Micro-voids 20 µm

20 µm

20 µm

(a)

RI PT

Fig. 10 Worn surface of Mo10 samples: (a) Rs = 1.0%; (b) Rs = 5.5%; and (c) Rs = 10.0%

(b)

SC

Plastic deformation layer

M AN U

Fatigue desquamatin

M2C fracture

10 µm

5 µm

(d)

crack

20° crack

AC C

EP

M2 C

TE D

(c)

10 µm

10 µm

Fig. 11 Worn cross-section of Mo10 samples: (a) Rs = 1.0%; (b) Rs = 5.5%; (c) Rs = 5.5%; and (d) Rs = 10.0%

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(b)

Fragmentation of martensite

RI PT

Lath martensite M(200) M(011)

M AN U

SC

M(211)

TE D

M2C fracture

Microcracks in M2C

Microhardness /HV

AC C

EP

Fig. 12 TEM micrographs of HSS at about 25 µm beneath wear surface: (a) lath martensite in original microstructure; (b) fragmentation of martensite after wear; (c) microcracks in M2C after wear 800 700 600 500 400 300 10

12 14 16 18 20 22 24 Retained austenite content /Vol.%

Fig. 13 Effect of retained austenite content on microhardness

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Highlights

► Frictional wear behavior of HSS with M2C was studied under different sliding ratio. ► M2C with formula of (Fe27.42Mo48.26Cr24.32)2C (21-1) is coherent with α-Fe(110).

RI PT

►Sliding ratio evidently affects wear property because of the change of wear mode.

AC C

EP

TE D

M AN U

SC

►Martensitic transformation in wear process contributes in improving properties of HSS.