Effect of Rolling Process on Microstructure and Wear Properties of High Carbon Equivalent Gray Cast Iron

ZHAO Yihong, CHEN Qianyu, ZHENG Zhiwei, CAO Pei, GONG Ziyu, GENG Haoran, CHEN Rongfa

(1. School of Mechanical Engineering, Yangzhou University, Yangzhou 225127, China; 2. School of Engineering, University of Birmingham, UK; 3. Tongfu (Kunshan) Heat Treatment Co. Ltd, Suzhou 215300, China)

Abstract: Rolling process based on the plastic deformation as a surface strengthening treatment was employed, aiming to improve the wear resistance ability and functional performance of the high carbon equivalent gray cast iron (HCEGCI). The microstructures and tribological performance of the untreated and rolled samples were characterized. In addition, the wear mechanism of HCEGCI samples was also studied via pin-on-disc tests. The experimental results show that the as-rolled samples possess the structure-refined layer of 15 μm and work-hardened layer of 0.13 mm. In comparison with the surface hardness of untreated samples, the surface hardness of as-rolled samples increases by 84.6% (from 240HV0.1 to 443HV0.1) and the residual compressive stresses existed within the range of 0.2 mm. The wear rates of as-rolled samples were decreased by 38.4%, 37.5%, and 44.4% under different loads of 5 N, 10 N, and 15 N, respectively. The wear characteristics of the untreated samples mainly exhibit the peeling wear coupled with partial adhesive and abrasive wear. However, as for the as-rolled samples, the adhesive wear was limited by the structure-refined layer and the micro-crack propagation was controlled by the work-hardened layer. Therefore, the wear resistance of as-rolled samples can be improved significantly due to the low wearing degree of the friction contact zone.

Key words: high carbon equivalent alloy gray cast iron; rolling process; microstructure; wear resistance

1 Introduction

High carbon equivalent gray cast iron (HCEGCI) contained with over 3.6% carbon equivalent has been widely used to fabricate brake components in auto industry owing to its high wear resistance ability and excellent thermal conductivity[1-3]. However, the mechanical properties of HCEGCI materials are severely affected due to the existence of lamellar graphite particles[4]. Numerous published studies reported that the mechanical properties of HCEGCI materials could be further improved by employing various alloy elements[5-7]. For example, the addition of Cr elements could inhibit the nucleation of ferrite and then refine the pearlite, resulting in improving the strength and hardness of matrix. The similar strengthening mechanism could be found by applying Cu elements, which could prevent the spread of C elements in austenite and then refine the pearlite. In addition, Nb element additives could react with C elements to quickly form NbC particles possessed with high strength, fine and uniform, and then the wear-resistant skeleton phases were formed. Along this line of consideration, the desired mechanical properties of HCEGCI materials can be achieved by refining the grain structure.

In recent years, the rolling process has been attracted an increasing attention to improve the wear resistance, fatigue strength, corrosion resistance and bearing capacity of parts by refining the grain structure[8-10]. Maet al[11]and Mekichaet al[12]reported that the grains of metallic composites could be refined significantly via rolling process. When the refined grains were formed by stress, the dislocation regions at the grain boundaries would be gradually accumulated. Moreover, the original microcracks formed around the grain boundaries and then the refined grains have no conducive to the crack propagation due to the crack turns or bridges around the grain boundary[13-17]. In this case, the wear resistance and mechanical properties could be improved via the surface rolling process. Therefore, in order to improve the wear resistance and mechanical properties of HCEGCI materials, the wear mechanism of as-rolled materials should be discussed. However, compared with the numerous reported literatures on the wear mechanism of gray cast irons, less attention paid to the wear mechanism of HCEGCI materials, especially as-rolled HCEGCI materials.

This study evaluated the microstructure and mechanical properties of as-rolled HCEGCI materials. In addition, the wear mechanism of as-rolled HCEGCI materials was investigated via pin-on-disc tests. The rest of the paper is divided as follows. Section 2 showed the as-rolled HCEGCI materials and characterization techniques. Section 3 detailed the microstructure, mechanical properties, wear morphologies, and wear mechanism of the untreated and as-rolled HCEGCI samples, respectively. Section 4 summarized the conclusions of the present paper.

2 Experimental

2.1 Sample preparation

The HCEGCI samples were fabricated with the medium-frequency induction technique, annealing treatment, and CNC machining process. At first, the HCEGCI materials were melted in a medium-frequency induction furnace and formed with a dimension ofΦ70 mm×60 mm. Prior to the machining process, the samples should be conducted with annealing treatments to reduce the hardness of samples. Subsequently, the second annealing treatment was performed to eliminate the residual stress produced in the machining process. Finally, the top surface of HCEGCI samples was rolled using a horizontal CNC lathe under the machining conditions of the rotating speed of 660 r/min, the feeding speed of 0.12 mm/r, the rolling pressure of 500 N, and the diameter of cemented carbide rolling ball of 10 mm. Fig.1 shows the horizontal CNC lathe in the study. After performing the rolling process, the as-rolled HCEGCI samples were machined with a dimension of 15 mm×15 mm×10 mm by the high-speed wire electrical discharge machining method for the further detection. Table 1 lists the chemical composition of HCEGCI materials with high carbon equivalent of 3.82%.

Fig.1 Optical photographs of horizontal CNC lathe during rolling process

Table 1 Chemical compositions of HCEGCI materials/at%

2.2 Characterization

The HCEGCI samples were polished with different sizes of the metallographic sandpaper (successively 120, 240, 600, and 1 200 mesh size) and the polishing cloth. After the ultrasonic cleaning in acetone for 5 min, the 4% nitric acid alcohol solution was applied to corrode the detected surface. And then, the metallographic morphologies of samples were observed using a DM3000m metallographic microscope. In addition, the microhardness and residual stress of as-rolled samples were detected by an HV-1000A Vickers hardness tester (load 0.1 kg, loading time 10 s) and an X-350A X-ray stress tester, respectively. Here, the main parameters of this X-ray stress tester are as follows: CrKα-ray, voltage at 22 kV and current at 6 mA of X-ray tube, collimating tube diameter ofΦ2 mm, scanning angle of diffraction peak at 150°-161° with a scanning interval of 0.1°. In order to evaluate the wear mechanism of asrolled samples, the pin-on-disc tests were conducted using a Brooke UMT-2 reciprocating friction and wear machine under various loads (i e, 5 N, 10 N, and 15 N). The cemented carbide ball as the grinding ball with a diameter of 4 mm and a hardness of 1600HV. Here, the sliding distance was fixed at 10 mm and the frequency was 4 Hz. Then, the X-ray scanning electron microscopy (SEM) and a Brooke Contour GT-K 3D surface profile-meter were utilized to detect the wear morphologies and three-dimensional morphologies to obtain the worn volume of samples.

3 Results and discussion

3.1 Microstructure

Fig.2 shows the SEM images of the refined layer of as-rolled HCEGCI samples. The boundary between the refined layer and original matrix material can be clearly observed and the average thickness of this refined layer is about 15 μm (see Fig.2(a)). In addition, seen from Fig.2(b), the near-surface graphite particles in the refined layer are crushed and pulled out. Moreover, the graphite particles embedded in the subsurface layer are obviously refined. This phenomenon can be explained that the grains on the outer surface of HCEGCI samples are fractured and refined by the significant contact stress during the rolling process. Then, the dislocation at the grain boundary is produced and gradually accumulated, which contributes to the improvement of the mechanical properties of HCEGCI samples. Meanwhile, the fracture and refinement of graphite particles prevent the matrix separating by the lamellar-structured graphite particles. Therefore, the probability of the generation of cracks can be reduced due to the stress concentration around graphite particles.

Fig.3 depicts the metallographic of as-rolled HCEGCI samples. The straight flake graphite particles can be clearly observed from the uncorroded as-rolled HCEGCI samples. There are relatively large amounts of graphite particles with a small size (Fig.3(a)). After grinding and polishing, 4% nitric acid alcohol is used for corrosion, and the matrix materials are detected as the pearlite and ferrite distributed around graphite particles (Fig.3(b)). In addition, there is no chain-like phosphorus eutectic structure in the metallographic structure of the corroded samples due to the lack of phosphorus elements of as-rolled HCEGCI samples.

Fig.3 Metallographic structure of as-rolled HCEGCI samples under the (a) uncorroded and (b) corroded treatments

3.2 Microhardness and residual stress

Fig.4 shows the average microhardness and residual stress versus the depth below the surface of as-rolled HCEGCI samples. Compared with the microhardness of untreated samples of 240HV0.1, the microhardness on the surface of as-rolled samples is rapidly increased to 443HV0.1, by 84.6%. The microhardness tends to remarkably decline and then stabilises as the depth below the surface of as-rolled HCEGCI samples increases from 0 to 250 μm. The thickness of hardening layer caused by plastic deformation under rolling process is 130 μm, as shown in Fig.4(a). Seen from Fig.4(b), the compressive residual stress on the near-surface of as-rolled HCEGCI samples appears. Obviously, when the depth below the surface is 200 μm, the residual stress value rapidly decreases from -650 to -50 MPa. Once the depth below the surface exceeds 200 μm, the residual stress value keeps stable, which indicates that the plastic deformation produced during rolling process contributes to refine the grain structures on the surface of as-rolled samples, produce the work hardening layer, and form the compressive residual stress. In this case, the surface roughness and hardness of as-rolled samples can be efficiently improved, which can effectively improve the bearing capacity, wear resistance, fatigue strength, and corrosion resistance of components.

Fig.4 (a) Microhardness and (b) residual stress as a function of depth below the surface of as-rolled HCEGCI samples

3.3 Wear morphologies

Fig.5 shows the wear morphologies of the untreated and as-rolled samples under different loads. The depth of wear profiles increases rapidly as the load increases in a range of 5-15 N regardless of whether samples are rolled. When the applied load ranges from 5 to 15 N, the wear profile depth of untreated samples tends to rapidly increase from 7.2 to 22.7 μm and these curves possess several irregular corners. The fluctuations existed on these curves can be explained that the adhesion fractures happen quite easily due to the soft matrix of untreated samples. The fractured chips at the contact interface melt to form a kind of metal oxidation particles under the effect of friction heats. Meanwhile, the matrix materials tend to soft caused by a quantity of friction heat. In this case, the irregular corners on the surface of grooves appear due to the ploughing of the metal oxidation particles on the soft matrix materials. Moreover, plenty of metal oxidation particles are difficult to remove and thus gathered together as the load increases, resulting in the deeper grooves, as shown in Fig.5(a). Compared to the fluctuating wear profiles of untreated samples, the as-rolled samples possess a smoother curve and less irregular corners due to the excellent surface roughness and mechanical properties (see Fig.5(b)). Obviously, when the load is 15 N, the depth of wear profiles of as-rolled samples is roughly equal to that of untreated samples with the load of 5 N. In addition, the as-rolled samples have significantly less value of the wear profile width in comparison of the untreated samples regardless of the applied load. Fig.6 shows the 3D morphologies of wear grooves of untreated and rolled samples under different loads. As the load increases, the width and depth of wear grooves increase significantly regardless of whether the samples are rolled (Figs.6(a)-6(b)). The grooves of untreated samples on the bottom surface of wear morphologies can be clearly observed. However, the surface of the rolled sample is remarkably smoother than that of untreated samples. This indicates that the wear resistance ability of the matrix materials can be improved via the rolling process.

Fig.5 Wear profile depth of the (a) untreated and (b) rolled samples under different loads

Fig.6 3D morphologies of the (a), (b), (c) untreated and (d), (e), (f) rolled samples under different loads

3.4 Wear rate and friction coefficient

Fig.7 shows the wear rate and weight of untreated and rolled samples under different loads. The wear rate value tends to gradual decrease and then stabilize with the increase of loads for the untreated and rolled samples (Fig.7(a)). Compared with the wear rate of untreated samples, the wear rate of rolled samples is reduced by 38.4% (from 6.3×10-4to 3.9×10-4mm3·N-1·m-1), 37.5% (from 4.7×10-4to 3.0×10-4mm3·N-1·m-1), and 44.4% (from 4.5×10-4to 2.5×10-4mm3·N-1·m-1) relative to the applied load, respectively. Compared with the rolled samples, the wear weight of untreated samples is increased by 90.9% (from 2.4 to 4.4 mg), 65.5% (from 2.9 to 4.8 mg), and 61.5% (from 3.9 to 6.3 mg) under the applied loads of 5 N, 10 N, and 15 N, respectively (Fig.7(b)). In addition, a slight increase of the wear weight ranging from 2.4 to 3.9 mg can be observed for rolled samples as the applied load increases. However, in comparison with the rolled samples, the untreated samples possess a higher wear weight from 4.4 to 6.3 mg. This indicates that when the load is fixed at 5N, the effective graphite films on the friction interface can hardly form within a short time and thus the extra running in period for the pin-on-disc system is necessary. In this case, the wear resistance of matrix materials is limited. With the increase of load, the running in period is shortened and then the wear rate become stabilize. In addition, when the load is a constant value, the wear resistance of the rolled samples is higher than that of the untreated samples. Therefore, the wear resistance of HCEGCI materials can be effectively improved owing to the surface strengthening under the rolling process.

Fig.7 (a) Wear rate and (b) weight of different samples under different loads

In order to investigate the wear behavior of asrolled samples, the tribological performance is studied under different loads. Fig.8 shows the friction coefficient as a function of the applied load for untreated and rolled samples. At the beginning of wear stages, the friction coefficient fluctuates markedly. When the test time is over 500 s, the friction coefficient tends to stabilize for different samples. However, the friction coefficient of rolled samples is obvious higher than that of untreated samples regardless of the applied loads. This phenomenon can be explained that the rolled surface is covered with a continuous layer of work-hardening alloys, which can improve the strength and plastic deformation ability on the surface of the metal-matrix. Meanwhile, the anti-friction performance of the metal-matrix can be reduced due to the reduction of the amount of fractured and extruded graphite particles under pin-on-disc tests. However, although the surface strength is improved due to the work-hardening layer, the surface toughness is reduced, resulting in the high frictional resistance on the contact interfacial between the metal-matrix and friction component. In this case, the friction coefficient of rolled samples is higher than that of untreated samples.

Fig.8 Friction coefficient curves of the (a) untreated and (b) rolled samples under different loads

Meanwhile, the friction coefficients of untreated and rolled samples as a function of the testing time are investigated under different friction temperatures (i e, 175 and 215 ℃), as plotted in Fig.9. Along with the influence of friction temperatures on friction coefficients, different fluctuation tendencies can be observed regardless of whether the samples are rolled with the corresponding increase in testing time. The untreated samples tend to rapidly increase and then fluctuate for its friction coefficient distribution along the time axis for both friction temperatures. When the friction temperature is 175 ℃, the friction coefficient reaches the peak value of 0.37 at the time of 280 s and tends to gradually decrease to 0.31 at the end of the test (Fig.9(a)). Compared with the rapid increase of friction coefficient of untreated samples, the rolled samples possess the lower wear rate at the initial friction stage regardless of the friction temperatures. After the initial friction stage, the rolled samples are the steady friction stage and the friction coefficient reaches 0.36 at the time of 280 s. However, the friction coefficient decreases to 0.34 at the end of the test. This indicates that the thermal decay can be efficiently decreased compared to the untreated samples. When the friction temperature increases to 215 ℃, the friction coefficient of rolled samples reaches 0.41 at the time of 200 s and then tends to the slight increase to 0.43 at the end of tests, which indicates that the thermal decay of rolled samples can be eliminated (Fig.9(b)). However, the maximum value of friction coefficient reaches 0.46 at the time of 200 s and then decreases to 0.37 at the time of 400 s. In this case, the thermal decay appears. This can be explained in the view of chemical reactions on the friction interface between the metal-matrix and friction component, as stated by Vicket al[18], Liuet al[19], and Váradiet al[20]. The formed oxidation film on the metal surface contributes to the limitation of the adhesion between the friction contact interface under the high friction heat. Here, the oxidation film consists of Fe3O4and Fe2O3and the film tends to fractures under the friction force, which leads to the declination of friction coefficient and thus the thermal decay appears. However, the metal-matrix fracture caused by graphite particles can be limited due to the high strength and toughness of the rolled samples. Under the influence of cyclic stresses, the cracks on the surface of rolled samples are unable to grow and thus the graphite particles cannot be fractured, which limits the formation of lubricating films. In this case, compared to the untreated samples, the rolled samples possess a higher friction coefficient and more stable friction behavior, leading to improve the resistance of thermal decay and thus improve the mechanical performance of rolled samples.

3.5 Wear mechanisms

Fig.9 shows the wear morphologies of untreated samples under different loads for 3 600 s. The plastic deformation on the wear surface can be observed due to the thermal softening caused by lots of friction heat regardless of the applied load. When the load increases to 10 N or 15 N, the adhesion fractures and ploughing grooves appear on the wear surface because of the tearing phenomenon on the metal-matrix caused by the generated grinding chips. Moreover, the long strips of voids and oxide particles can also be found on the wear surface. However, the fatigue cracks on the wear interface are clearly detected under the effect of cyclic stresses as the sliding time increases. Furthermore, the matrix materials surrounded graphite particles are peeled and then the long strips of voids are formed as the cracks propagates and wear increases. In this case, the surface quantity of untreated samples is severely affected by these defects. Compared with the numerous defects on the wear surface of untreated samples, the rolled samples possess no obvious tearing and ploughing defects due to the work hardening layer on the surface, as shown in Fig.10. This indicates that the plastic deformation on the surface of HCEGCI samples appears after the rolling process and then the materials on the peak flow into the valley parts, resulting in the enhancement and low surface roughness. In this case, the wear resistance can be effectively improved due to the existences of the work hardening layer, surface strength and residual stress.

Fig.9 Friction coefficients versus friction temperatures of untreated and rolled samples: (a) 175 ℃ and (b) 215 ℃

Fig.10 Wear morphologies of untreated specimens after 3 600 s under different loads: (a) 5 N; (b) 10 N; (c) 15 N

Fig.11 Wear morphologies of rolled specimens after 3 600 s under different loads: (a) 5 N; (b) 10 N; (c) 15 N

4 Conclusions

a) Compared with the untreated samples, the rolled samples possess a higher surface hardness due to the existence of the work hardening layer of 0.13 mm and residual compressive stress within the range of 0.2 mm.

b) The wear rate firstly decreases and then stabilises as the load increases regardless of whether the samples are rolled. In addition, the wear rate, wear weight, and the depth and width of grooves of rolled samples are smaller than that of untreated samples.

c) The rolled samples possess more stable friction behavior and prevent the friction thermal decay phenomenon in comparison with the untreated samples.

d) The main peeling wear and the partial adhesion and abrasive wear of untreated samples can be clearly observed due to the declination of surface strength caused by the severe friction heat. However, only the abrasive wear pattern occurs on the surface of rolled samples, which is produced by the melted chips on the contact interface.

e) The as-rolled samples coupled with the work hardening layer and residual compressive stress possess excellent wear resistance ability due to the limitation of the crack propagation and adhesion wear.