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Microstructure and Mechanical Properties of As-cast 42CrMo Ring Blank During Hot Rolling and Subsequent Quenching and Tempering Fangcheng Qin, Yongtang Li, Huiping Qi, and Xiaojian Wei (Submitted May 26, 2016; in revised form December 10, 2016; published online January 26, 2017) The hot rolling of as-cast 42CrMo ring blank and its subsequent quenching and tempering were conducted based on the casting-rolling compound forming technique. The effects of feed rate and tempering tem- perature on the microstructure were studied by optical microscopy and scanning electron microscopy. The mechanical properties of the rolled rings were examined. The results show that when the feed rate of the idle roll increases, the degree of grain refi nement becomes slightly smaller and the average grain size is approximately 44 lm through the whole thickness of the rolled ring. The microstructure is inhomogeneous near the center-layer and minimum spread region, which is characterized by a small amount of irregular and coarse grain. The strength and hardness of the hot-rolled rings are high, and the plasticity and toughness are relatively low. The depth and diameter of the dimples in the fracture of the ring fabricated with a low feed rate are larger than those of the ring fabricated with a high feed rate. The carbide particles cannot be observed in the rolled rings after the rings are quenched and tempered at 803 K, but the fi ne and dispersed particles are precipitated by tempering at 863 K. As a result, the mechanical properties are signifi cantly improved and satisfy the technical demands after quenching and tempering. The fractures of both tensile and impact specimens are characterized by regular and fi ne dimples at a higher tempering temperature, which indicates that a dimple fracture and an excellent combination of strength, plasticity and toughness are obtained. Keywordsas-cast 42CrMo ring blank, feed rate, fracture, hot ring rolling, mechanical properties, microstructure 1. Introduction 42CrMo (American grade: AISI 4140) is one of the representative medium carbon and low alloy steels (Ref 1-3). Due to its good balance of strength, toughness and wear resistance, 42CrMo bearing rings are widely used in many industries, especially as critical components in high-speed rail, aerospace, automobile and wind power applications. The 42CrMo bearing rings are commonly fabricated by using the hot ring rolling (HRR) technique. In HRR, the rotational motion of the driven roll and the feed motion of the idle roll act directly on the ring blank to reduce the wall thickness of the ring blank, expand the diameter and shape the cross sec- tion. Over the last several decades, the HRR process based on as-forged state materials (Ref 4-8) and the deformation behavior of as-forged state 42CrMo alloy steel (Ref 9, 10) have been extensively investigated. Lin et al. (Ref 1, 11) proposed the constitutive model of as-forged 42CrMo alloy by compensating for strain using isothermal compression tests, and its processing maps were constructed to identify dynamic recrystallization and fl ow instability domains. The recrystal- lization models were also established to reveal the response of grain refi nement to deformation parameters (Ref 12, 13). The existing process for producing 42CrMo bearing ring is mainly composed of pouring ingot, cogging, sawing, upsetting, punching, HRR and heat treatment (Ref 14). Before HRR, the starting material began in an as-forged state. However, this process still has some disadvantages, such as multi-pass heating, wasting material and energy, and a high cost due to the huge forming machines required in cogging, upsetting and punching. To solve the problems, a novel casting-rolling compound forming (CRCF) process is proposed that includes casting a ring blank, HRR and heat treatment, as shown in Fig. 1 (Ref 14-16). Compared with the current process, the CRCF has become a high-end technique with many advantages, including shorter heating times, considerable savings in both material and energy, productivity improvement and lower costs. Thus, the geometrical dimensions forming and microstructure modifi cation of as-cast 42CrMo ring blank should be completed in HRR. The effects of temperature, strain rate and interval time on the fl ow stress of as-cast 42CrMo alloy were studied by Qi (Ref 16) using interrupted compression tests. Li et al. (Ref 14) analyzed the deformation behavior of sand casting 42CrMo by considering continuous compression tests and the fi nite element method, and they then clarifi ed the relationship between the microstructure evolution and key rolling parameters, including the feed rate of the idle roll, rolling ratio and rotation speed of the driven roll. The industry tests indicate that the 42CrMo alloy rings with precise geometrical dimensions and sound microstructures hot-rolled directly from as-cast ring blank are obtained and that the mechanical properties satisfy the standard requirements (Ref 14, 17). However, currently, research on the HRR process of as-cast 42CrMo alloy and subsequent heat Fangcheng Qin, Yongtang Li, Huiping Qi, and Xiaojian Wei, School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China. Contact e-mails: qfcqfc1988 and liyongtang. JMEPEG (2017) 26:13001310?ASM International DOI: 10.1007/s11665-016-2497-21059-9495/$19.00 1300Volume 26(3) March 2017Journal of Materials Engineering and Performance treatment is rarely seen. Further, it is widely reported that the cleavage fracture behavior is signifi cantly impaired by the coarse carbides and other brittle particles (Ref 18). The fi ne and dispersed distribution carbide particles are precipitated by appropriate quenching and tempering; thus, the mechanical properties, particularly the low-temperature impact toughness, are also improved (Ref 19-21). Therefore, to eliminate the unfavorable rolled residual stress, homogenize the through- thickness microstructure and obtain a proper combination between strength and plasticity-toughness of the HRR-fabri- cated ring, it is essential to conduct an in-depth study of subsequent the quenching and tempering process (Ref 18, 22). In this study, the HRR of as-cast 42CrMo ring blank and its subsequent quenching and tempering (Q if it does not, SRX will occur (Ref 15). In the severe deformation bands and triangular grain boundary regions, the fi ne grains are observed as a consequence of the DRX. The meta-dynamic recrystallization is also generated because the outside defor- mation area materials are still under high temperature during rotation stages. Thus, it is generally recognized that the HRR process is characterized by accumulative and multi-pass deformation. 3.2 Microstructure of the Rolled Ring After Q&T The loose and low density characteristics are the main microstructure characteristic in as-cast 42CrMo ring blank compared to the as-forged ring blank. The as-cast ring blank undergoes the complicated stress and strain states under the actions of friction and pressure from the driven roll and the idle roll, which results in the large residual stress in the rolled ring (Ref 21). Thus, the appropriate Q&T process is essential to eliminate unfavorable stress and improve microstructure sta- bility through the whole thickness of the rolled ring (Ref 28, 29). Figure 7 shows SEM micrographs of carbide morphology in different regions of the A1 rolled ring after being quenched and tempered at 863 K. The dispersed carbide particles are precipitated in the martensite matrix, which display different sizes, morphologies and distributions in different regions. In the surface-layer, the carbides are characterized by uniform and refi ned particles with a size of 0.1-0.15 lm in diameter. However, fragmentized carbide morphologies,which are precipitated within martensite laths and at boundaries, are found near the center-layer and minimum spread region. The carbide particles display a continuous distribution in the A1 rolled ring after being tempered at 863 K, as shown in Fig. 7(c) Fig. 5Through-thickness microstructures of the A1 rolled ring: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread region Journal of Materials Engineering and PerformanceVolume 26(3) March 20171303 and (d). Additionally, the microstructure is inhomogeneous because a small amount of small carbide is exhibited on the bottom of the matrix. During impact plastic deformation, the partially connected carbides can result in a large stress concentration and further develop into micro-cracks. The Charpy impact toughness can also be greatly reduced. Fig. 6Through-thickness microstructures of the A2 rolled ring: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread region Fig. 7SEM micrographs of carbide morphology of the A1 rolled ring after being tempered at 863 K: (a) outer-layer, (b) center-layer, (c) inner- layer, (d) minimum spread region 1304Volume 26(3) March 2017Journal of Materials Engineering and Performance Figure 8 presents SEM micrographs of carbide morphology in different regions of the A2 rolled ring after being tempered at 863 K. The refi ned carbides with the size of approximately 0.08 lm in diameter are distributed along the martensite lath boundary and extended inside the lath. Dispersed and homo- geneous particles are exhibited across the whole thickness, Fig. 8SEM micrographs of carbide morphology of the A2 rolled ring after being tempered at 863 K: (a) outer-layer, (b) center-layer, (c) inner- layer, (d) minimum spread region Fig. 9SEM micrographs of carbide morphology of the A1 rolled ring after being tempered at 803 K: (a) outer-layer, (b) center-layer, (c) inner- layer, (d) minimum spread region Journal of Materials Engineering and PerformanceVolume 26(3) March 20171305 particularly in the minimum spread region (Fig. 8d). Thus, the isotropic properties can be obtained in the rolled ring. This is primarily attributed to the plastic deformation per unit area through the whole thickness increasing and the refi ned recrystallization grains occurring at a higher feed rate (4-3- 2 mm/s). Additionally, the solid solubilities of alloying ele- ments increase with the dissolution of carbides in the matrix during subsequent tempering. With an increase in feed rate, the preferential precipitation location for carbide particles trans- forms from the martensite lath boundary to inside the lath by tempering at 863 K, thus leading to inconspicuous martensite orientation and deterioration in toughness (Ref 30, 31). Figure 9 and 10 show SEM micrographs of carbide morphology in different regions of the A1 and A2 rolled rings after being tempered at 803 K, respectively. The carbide particles cannot be observed in different regions of both the A1 and A2 tempered rings, which remain in the orientation of the quenched lath martensite. The martensite undergoes different changes depending on the tempering conditions Fig. 10SEM micrographs of carbide morphology of the A2 rolled ring after being tempered at 803 K: (a) outer-layer, (b) center-layer, (c) in- ner-layer, (d) minimum spread region Table 3Mechanical properties of the as-cast 42CrMo ring blank after HRR and Q&T SampleTempering temperature/KYS, MPaUTS, MPaTEL, %RA, %Ak, JHardness, HB Standard valuesJB/T5000.6-2007510740-8801227200-250 JB/T6396-2006550800135035241-302 Q/LYCC(B)0014550800135027240-280 Rolled ringA1963117413.131.912.7391 A295911535.73.310395 After Q&TA18636447762154106283 803825966175472308 A2863633772225488282 803856996144743331 Fig. 11Engineering stress-strain curves of hot-rolled rings and sub- sequent Q&T process 1306Volume 26(3) March 2017Journal of Materials Engineering and Performance Fig. 12Tensile fracture morphologies of the rolled ring: (a) A1, (b) A2 Fig. 13Impact fracture morphologies of the rolled ring: (a) A1, (b) A2 Fig. 14Tensile fracture morphologies of the rolled ring after Q&T: (a) A1 tempered at 863 K, (b) A2 tempered at 863 K, (c) A1 tempered at 803 K, (d) A2 tempered at 803 K Journal of Materials Engineering and PerformanceVolume 26(3) March 20171307 because it is a highly unstable structure (Ref 32). All of these changes can affect the strength and toughness of the HRR- fabricated 42CrMo ring (Ref 33-35). The activation energy for the transformation and dissolution of martensite and retained austenite decreases at a low tempering temperature of 803 K and explains why the quantity of carbide particles is signifi - cantly reduced. Therefore, the softening in matrix is insuffi - cient, which implies that the toughness is also decreased as the tempering temperature decreases. 3.3 Mechanical Properties and Fracture Morphology Changes in microstructure affect the strength, plasticity and toughness, among other factors. To evaluate the properties of a 42CrMo ring that is hot-rolled directly from an as-cast blank, the ultimate tensile strength (UTS), yield strength (YS), total elongation (TEL) and reduction of area (RA) of tensile specimens, and Charpy absorbed energy (Ak) of Charpy impact specimens after HRR and Q&T are summarized in Table 3. Compared to the Chinese standards (Ref 36, 37), the testing results indicate that the UTS, YS and hardness of the hot-rolled rings are much higher but the plasticity and toughness are relatively low. As shown in Table 3, the YS and UTS of as-cast 42CrMo ring blank after HRR at the feed rate of 3-2-1.5 mm/s are 963 MPa and 1174 MPa, and the TEL, RA and Akare 13.1%, 31.9% and 12.7 J, respectively. As the feed rate increases to 4-3-2 mm/s, the YS and UTS do not show any obvious changes, with values of 959 MPa and 1153 MPa. However, the corresponding TEL, RA and Akdecrease to 5.7%, 3.3% and 10 J, respectively. In addition, the values of UTS and YS in this study are superior to those of bearing rings manufactured by the HRR process in (Ref 17). In Table 3, the UTS, YS and hardness are decreased, and the TEL, RA and Ak are signifi cantly improved by the Q&T process. After being tempered at 803 K, the UTS, YS and hardness values are still relatively high, but the plasticity and toughness values are lower than those of the rolled ring tempered at 863 K. The reduced degree of toughness is obvious because the carbide particle does not appear in both A1 and A2 rolled rings after being tempered at 803 K. However, all of the testing mechanical properties nearly satisfy the standardized technical demands of bearing rings. Figure 11 shows the representative engineering stress-strain curves of the hot-rolled rings and subsequent Q&T process. The characteristics are almost the same as the strength and ductility values summa- rized in Table 3. The obvious yield points upon deformation for both the A1 and A2 rolled rings after being quenched and tempered at 863 K are presented. Although the UTS values of both the A1 and A2 rolled rings after Q&T are slightly lower than the standard value in Q/LYCC(B)0014, an excellent combination of strength, plasticity and toughness is obtained. Figure 12 presents the tensile fracture morphologies of the A1 and A2 rolled rings. The mix of quasi-cleavage and dimple is exhibited on the fracture of the A1 rolled ring. The fracture is characterized by a fl uctuant river pattern and equiaxed dimples with small diameters. Although a small amount of dimpling is also exhibited on the fracture of the A2 rolled ring, the quasi- cleavage fracture with river pattern, shear lip and tear ridge plays a dominant role in the A2 rolled ring. In addition, the fracture surface is smooth and clean, thus implying a typical brittle fracture. It seems that the depth and diameter of dimples in the fracture of the A1 rolled ring are larger than those of the A2 rolled ring, as shown in Fig. 12. This is in accordance with the results of TEL and RA illustrated in Table 3. It also can be Fig. 15Impact fracture morphologies of the rolled ring after Q&T: (a) A1 tempered at 863 K, (b) A2 tempered at 863 K, (c) A1 tempered at 803 K, (d) A2 tempered at 803 K 1308Volume 26(3) March 2017Journal of Materials Engineering and Performance seen from impact fracture morphologies in Fig. 13, the fracture of the A2 rolled ring is determined by an irregular shear lip, which penetrates the whole region (see the arrow in Fig. 13b). The micro-cracks may generate initially in these regions and merge into cracks during the impact deformation process (Ref 15). Finally, the cracks spread around shear lip into the surface- layer, thus leading to trans-granular fracture, and the toughness can be dramatically reduced. Figure 14 and 15 present the tensile and impact fracture morphologies of the rolled rings after Q&T, respectively. At a higher tempering temperature, the fractures of both tensile and impact specimens are characterized by regular and fi ne dimples. Further, the quasi-cleavage and irregular fracture are observed in the specimens tempered at 803 K, particularly in some of the steps exhibited on the A2 rolled ring, as shown in Fig. 15(d). It can be concluded that the fracture mode is mainly governed by mixed brittle and ductile fracture for tensile specimens and by brittle fracture for impact specimens. The mixed fracture mode has previously been reported (Ref 38, 39). Compared to the impact fracture of the A2 rolled ring, the depth and diameter of dimples of the A1 rolled ring are also slightly larger, which imply that the toughness of A1 is superior to that of A2. Therefore, a typical ductile fracture plays a dominant role in both tensile and impact fracture surfaces for the A1 rolled ring. Additionally, the variation in fracture morphologies is in accordance with the mechanical properties summarized in Table 3. 4. Conclusions The microstructures and mechanical properties of as-cast 42CrMo ring blank after hot rolling and subsequent quenching and tempering were studied. The fracture morphologies and mechanisms of tensile and impact specimens were revealed by SEM. The fol