Effect of annealing cooling rate on microstructure and mechanical property of 100Cr6 steel ring manufactured by cold ring rolling process

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1 J. Cent. South Univ. (2014) 21: DOI: /s Effect of annealing cooling rate on microstructure and mechanical property of 100Cr6 steel ring manufactured by cold ring rolling process WEI Wen ting( 魏文婷 ) 1, 2, WU Min( 吴敏 ) 3 1. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan , China; 2. Hubei Key Laboratory of Advanced Technology of Automotive Parts (Wuhan University of Technology), Wuhan , China; 3. Technical Center of Dongfeng Motor Corporation, Wuhan , China Central South University Press and Springer Verlag Berlin Heidelberg 2014 Abstract: Pre heat treatment is a vital step before cold ring rolling and it has significant effect on the microstructure and mechanical properties of rolled rings. The 100Cr6 steel rings were subjected to pre heat treatment and subsequent cold rolling process. Scanning electron microscopy and tensile tests were applied to investigate microstructure characteristic and mechanical property variations of 100Cr6 steel rings undergoing different pre heat treatings. The results indicate that the average diameter of carbide particles, the tensile strength and hardness increase, while the elongation decreases with the decrease of cooling rate. The cooling rate has minor effect on the yield strength of sample. After cold ring rolling, the ferrite matrix shows a clear direction along the rolling direction. The distribution of cementite is more homogeneous and the cementite particles are finer. Meanwhile, the hardness of the rolled ring is higher than that before rolling. Key words: ring rolling; annealing; cooling rate; microstructure; mechanical property 1 Introduction Cold ring rolling is an advanced incremental plastic forming technology, which has been widely applied for manufacturing seamless ring parts with a variety of cross sectional shape like bearings and gears. During this process, a pre treated blank (ring which may be unshaped or has special profiles) is rolled between the mandrel and the main roll at room temperature. It is quite a complicated physical deformation process, as shown in Fig. 1(a)). The driven roll rotates actively around its axis meanwhile the idle roll feeds linearly towards the ring. Under the action of the two rolls, the ring produces the incremental deformation of thickness reduction and the diameter expansion [1 2]. Published researches during the past few decades on ring rolling process have been mainly carried out by using experimental [3 4], analytical [5 7] and simulation [8 12] methods, majority of which have concentrated on the macroscopic plastic deformation and geometric accuracy control of the cold rolled rings. But very few have focused on the microstructural development during cold ring rolling or on the properties of the final products. Due to the complexity of cold ring rolling, it is quite necessary to do a further research on the microstructure evolution during cold ring rolling. WANG et al [13] has studied the effect of rolling parameters on the microstructure and mechanical properties in an acicular ferrite pipeline steel. RYTTBERG et al [14] has explored the effect of cold ring rolling on the evolution of microstructure and texture. SHAO et al [15] has done the numerical and experimental investigations into strain distribution and metal flow of low carbon steel in cold ring rolling. Nevertheless, the pre heat treatment of blanks is critical during the cold ring rolling process which could exert a profound effect on the microstructure and properties of the subsequently rolled rings [16 17]. A reasonable pre heat treatment is quite important which could improve the manufacture efficiency and products properties. However, the pre heat treatment usually costs a significant amount of time and energy [18]. Rapid annealing is necessary to accelerate the pre heat treatment. The purpose of this work is to investigate the effect of rapid annealing rate on the microstructure characteristics and mechanical properties in cold ring Foundation item: Project(2011CB706605) supported by the National Basic Research Program of China; Project(2011CDA12) supported by the Innovative Research Groups of the Natural Science Foundation of Hubei Province, China; Projects(2012 Ia 017, 2013 IV 014) supported by the Fundamental Research Funds for the Central Universities, China Received date: ; Accepted date: Corresponding author: WEI Wen ting, PhD Candidate; Tel/Fax: ; E mail: zitongacat@163.com

2 J. Cent. South Univ. (2014) 21: Table 1 Treatment technology of ring blanks Number of ring blank Treatment 1 Initial ring 2 Normalising 3 Normalising + Annealing process A 4 Normalising + Annealing process B 5 Normalising + Annealing process C Normalising + Annealing process A + Cold ring rolling Normalising + Annealing process B + Cold ring rolling Normalising + Annealing process C + Cold ring rolling Fig. 1 Schematic diagram of cold ring rolling (Unit: mm) rolling. The reported results hopefully shed some light on the cooling rate chosen to improve the precision and properties of cold rolled rings. 2 Experimental 2.1 Pre heat treatment experiments 100Cr6 steel was used in the study and its composition was Fe 1.05C 1.61Cr 0.34Mn 0.33Si 0.02P 0.023S (mass fraction, %). The as received 100Cr6 steel was hot upset forged, and then it was machined to eight ring blanks with rectangular cross sections, as shown in Fig. 1(b). The eight ring blanks numbered from 1 to 8 were subjected to pre heat treatment according to Table 1. Seven ring blanks were super heated to 925 C and preserved for 1 h, followed by air cooling (see Fig. 2). Among them, six normalised ring blanks were divided into three groups (ring blanks 3 and 6, 4 and 7, 5 and 8) for different spheroidizing annealing treatments, as illustrated in Fig. 3. The normalised ring blanks were pre heated to 790 C for 15 min, then cooled in the hearth to 690 C and preserved for 180 min. When the ring blank was cooled in the air from 690 C to 500 C, the annealing cooling rate was 7.6 C/min, 4.3 C /min and 2.1 C /min, which was named annealing process A, B and C, respectively (see Fig. 3). Three 100Cr6 steel bars with a dimension of d20 mm 200 mm were annealed by process A C (shown in Table 2) and then machined to standard specimens for mechanical property testing. A Zwick/Roell Z100 material testing machine was employed to measure the tensile strength of the specimens. Fig. 2 Normalising diagram Fig. 3 Annealing diagrams for three annealing process Table 2 Treatment technology of 100Cr6 steel bars Number of steel bar Treatment 1 Normalising 2 Normalising + Annealing process A 3 Normalising + Annealing process B 4 Normalising + Annealing process C 2.2 Cold ring rolling experiment Ring blanks 6 8 were subjected to cold ring rolling

3 16 J. Cent. South Univ. (2014) 21: tests on a D56G90 cold ring rolling mill. In the experiment, the drive roll diameter was 217 mm, the idle roll diameter was 36 mm and the rotating speed was 2.43 rad/s. The whole rolling process lasted for 12.2 s. Other processing parameters are listed in Table 3. Fig. 4 Schematic diagram of wire electrode cutting Table 3 Cold ring rolling processing parameters Parameter Value Diameter of driven roll/mm 217 Rotation speed of driven roll/(rad s ) 2.43 Diameter of idle roll/mm 25 Highest feeding speed/(mm s ) 0.7 Second high feeding speed/(mm s ) 0.6 First middle feeding speed/(mm s ) 0.5 Second middle feeding speed/(mm s ) 0.35 First low feeding speed/(mm s ) Lowest feeding speed/(mm s ) Ring blanks 1 8 were wire electrode cut into small samples for scanning electron microscopic (SEM) observations (see Fig. 4). All the samples were first coarse ground, fine ground, then polished and eroded with a 4% Nital solution before observation. SEM observation was carried out using a filed emission microscope (Philips Quanta 200). 3 Results and discussion 3.1 Microstructure after pre heat treatment Figure 5 shows the material microstructure before and after pre heat treatment. As shown in Fig. 5(a), the initial microstructure of the received 100Gr6 steel used in this work is composed of lamellar pearlite. The lamellar spacing of pearlite is significantly smaller and the pearlite slice layer is denser after normalizing, as shown in Fig. 5(b). The micrographs of the spheroidizing annealed ring blanks are shown in Figs. 5(c) (e). As can be seen, the lamellar pearlite is transformed into granular pearlite. The gray black grain is ferrite and the bright Fig. 5 Microstructures before and after pre heat treatment: (a) Initial microstructure (b) Normalising (c) Annealing process A (d) Annealing process B (e) Annealing process C

4 J. Cent. South Univ. (2014) 21: white granular grain is cementite which is dispersed in the ferrite matrix. The average diameter of cementite particles is increased from 0.5 μm to 0.8 and 1.2 μm, when the annealing cooling rate decreases from 7.6 to 4.3 and 2.1 C/min, respectively. 3.2 Mechanical properties after pre heat treatment The mechanical properties of the annealed 100Gr6 steels with different cooling rates are shown in Table 4. The tensile strength increases and the elongation decreases with the decrease of cooling rate. The cooling rate has minor effect on the yield strength of ring. Figure 6 shows the stress strain curves of 100Gr6 steel after three different kinds of annealing. Generally, the materials with good cold working performance should have the properties like small carbide grains, low tensile and yield strength and high elongation [19]. Thus, annealing process A is more conductive to cold working in this work. It can be illustrated as follows: compared to lamellar pearlite, granular pearlite has smaller phase interface, which makes the total energy of the system reduce in annealed condition. Moreover, the resistance of dislocation motion from spherical carbide is small and the ferrite distribution is continuous. Therefore, granular pearlite has a lower hardness and strength, but higher plasticity. The plasticity of granular pearlite is better than that of lamellar pearlite because the separated effect of cementite on ferrite matrix dramatically weakens. The amount of proeutectoid ferrite decreases and that of pearlite increases with the decrease of annealing cooling Table 4 Mechanical properties of 100Gr6 steel samples after annealing treatment Annealing process Yield Ultimate tensile strength/mpa strength/mpa Elongation/% A B C Fig. 6 Stress strain curves of spheroidizing annealed 100Gr6 steel rate, which results in the increase of tensile strength and decrease of elongation. Moreover, the particle size of ferrite grain increases with the decrease of cooling rate, which also contributes to the decrease of plasticity. 3.3 Microstructure characterization of cold rolled ring Figure 7 shows the micrographs before and after rolling of normalised ring and three different kinds of pre heat treated rings. The microstructure of normalised ring before rolling is shown in Fig. 7(a). It can be seen from Fig. 7(a) that the grain distribution is nondirectional. However, the grain distribution shows a clear direction after rolling, as shown in Fig. 7(b). There is also some fragmented lamellar pearlites in the normalised ring after cold rolling. Its inhomogeneous microstructure is detrimental to the mechanical properties of ring products. Specimens must undergo a long spheroidizing annealing treatment before rolling to obtain better performance. Figures 7(c) (h) show the microstructures of rings before (left panel) and after (right panel) cold rolling with different cooling rates [Figs. 7(c) (d) for annealing process A (fast cooling rate), and Figs.7(e) (f) for annealing process B (slower cooling rate), and Figs. 7(g) (h) for annealing process C (lower cooling rate)]. After fast cooling, the microstructure contains fine cementite with a network of proeutectoid ferrite, and cementite particles coarsen along with the cooling rate decreasing [Figs. 7(e) and 7(g)]. After cold rolling process, the distribution of cementite is more homogeneous and the cementite particles are finer. Some cementite particles have a prolonged form or a faulted form, as shown in Fig. 8. The ferrite is elongated and deformed along the rolling direction. 3.4 Mechanical properties after cold ring rolling Figure 9 shows hardness profiles for respective rings in each operation process showing the effect of various pre heat treatments. It can be seen that the hardness of three samples is almost the same after normalizing with the decrease of the lamellar pearlite thickness and the distance between two lamellar pearlites. However, the hardness is sharply reduced because of the transformation from lamellar pearlite to granular pearlite when suffering spheroidizing annealing. Moreover, the hardness increases with the decrease of the annealing cooling rate. The hardness of ring blank after annealing process C has the highest hardness of 247.9, and that of the ring blank after annealing process B and A is and 212.5, respectively. The annealing hardness is mainly determined by the solid solution element in the ferrite type, amount and the interparticle distance of carbide [20]. The cementite particle size increases and

5 18 J. Cent. South Univ. (2014) 21: Fig. 7 Microstructures in middle layers of rings before ((a), (c), (e), (g)) and after ((b), (d), (f), (h)) rolling: (a), (b) Treated with normalizing (c), (d) Treated with annealing process A (e), (f) Treated with annealing process B (g), (h) Treated with annealing process C Fig. 8 Morphology of prolonged cementite particles the distance between cementite particles decreases with the decrease of annealing rate. This could explain why Fig. 9 Hardness profiles of rings in each operation process showing effect of various pre heat treatments

6 J. Cent. South Univ. (2014) 21: the hardness increases with the decrease of cooling rate. Due to the increasing internal dislocation density and work hardening after cold rolling, the hardness of ring samples after annealing process C, B and A increases to 283.6, and 239.4, respectively. 4 Conclusions 1) After spheroidizing annealing, the lamellar pearlite transforms into granular pearlite in 100Gr6 steel rings. 2) The particle size of cementite grains and the tensile strength increase and the elongation decreases with the decrease of annealing cooling rate. 3) Annealing cooling rate has minor effect on the yield strength of 100Cr6 steel. Carbide particle almost has no plasticity, and its tiny uniform dispersion in the matrix is beneficial to the improvement of the plasticity. In a certain range, the faster the annealing cooling and the more the fine carbide particles, the better the plasticity of GCr15 steel. 4) The hardness of the rolled ring is higher than that before rolling. References [1] HUA Lin, HUANG Xing gao, ZHU Chun dong. Ring rolling theory and technology [M]. Beijing: China Machine Press, 2001: 1 2, 27, 80. (in Chinese) [2] HU Zheng huan, HUA Lin. Technology of rotary metal forming [M]. Beijing: Chemical Industry Press, 2010: , (in Chinese) [3] JOHNSON W, MACLEOD I, NEEDHAM G. An experimental investigation into the process of ring or metal type rolling [J]. International Journal of Mechanical Sciences, 1968, 10(6): [4] HAWKYARD J B, JOHNSON W, KIRHLAND J, APPLETON E. Analyses for roll force and torque in ring rolling with some supporting experiments [J]. International Journal of Mechanical Sciences, 1973, 15(11): [5] YANG D Y, KIM K H. Rigid plastic finite element analysis of plane strain ring rolling [J]. International Journal of Mechanical Sciences, 1988, 30(8): [6] HAHN Y H, YANG D Y. UBET analysis of roll torque and profile formation during the profile ring rolling of rings having rectangular protrusions [J]. Journal of Materials Processing Technology, 1991, 26(3): [7] HUA L, ZHAO Z Z. The extremum parameters in ring rolling [J]. Journal of Materials Processing Technology, 1997, 69(1/2/3): [8] YAN F L, HUA L, WU Y Q. Planning feed speed in cold ring rolling [J]. International Journal of Machine Tools and Manufacture, 2007, 47: [9] HAN Xing hui, HUA Lin. 3D FE modeling of cold rotary forging of a ring workpiece [J]. Journal of Materials Processing Technology, 2009, 209(12/13): [10] HUA L, ZUO Z J, PAN L B. Research on following motion rule of guide roller in cold rolling groove ball ring [J]. Journal of Materials Processing Technology, 2007, 177: [11] HUA L, QIAN D S, PAN L B. Deformation behaviors and conditions in L section profile cold ring rolling [J]. Journal of Materials Processing Technology, 2009, 209: [12] GUO L G, YANG H. Research on plastic deformation behaviour in cold ring rolling by FEM numerical simulation [J]. Modeling and Simulation in Materials Science and Engineering, 2005, 13(7): [13] WANG W, YAN W, ZHU L, HU P, SHAN Y Y, YANG K. Relation among rolling parameters microstructures and mechanical properties in an acicular ferrite pipeline steel [J]. Materials and Design, 2009, 30: [14] RYTTBERG K, WEDEL M K, RECINA V, DAHLMAN P, NYBORG L. The effect of cold ring rolling on the evolution of microstructure and texture in 100Cr6 steel [J]. Materials Science and Engineering: A, 2010, 527: [15] SHAO Y C, HUA L, WEI W T, WU M. Numerical and experimental investigations into strain distribution and metal flow of low carbon steel in cold ring rolling [J]. Materials Research Innovations, 2013, 7(1): 49 57(9). [16] WU M, HUA L, SHAO Y C. Influence of the annealing cooling rate on the microstructure evolution and deformation behaviours in the cold ring rolling of medium steel [J]. Materials and Design, 2011, 32: [17] WU Min, Research on microstructure evolution in pre heat treatment and cold ring rolling [D]. Wuhan: Wuhan University of Technology, (in Chinese) [18] CUI Zhong xin, LIU Bei xing. Metallographic and principles of heat treatment. Harbin: Harbin Institute of Technology Press, 1998: (in Chinese) [19] SETO Hao. Bearing steel [M]. Beijing: Metallurgical Industry Press, [20] LIU Zong chang. Pearlite transformation and annealing [M]. Beijing: Chemical Industry Press, 2007: 19, 21. (in Chinese) (Edited by YANG Bing)