Luoyang , Henan, China. Keywords: Cr-Mo Steel, Heat Treatment, Microstructure, Impact Toughness.

Transcription

1 2017 International Conference on Mechanical Engineering and Control Automation (ICMECA 2017) ISBN: Effects of Quenching Temperature on Microstructure and Mechanical Properties of 75Cr-Mo Wear-resisting Cast Steel Ya-Bo HUANG 1,2, Wen-Yan WANG 1,2, Ji-Yao LIU 1,2, Jing-Pei XIE 1,2, Kai-Hui XU 1, Shi-Qin SHI 1,2 and Wen-Jun HUANG 1,2 1 College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang , Henan, China 2 Collaborative Innovation Center of Non-Ferrous Materials,Luoyang , Henan, China. Keywords: Cr-Mo Steel, Heat Treatment, Microstructure, Impact Toughness. Abstract. In this paper, metallographic structures, hardness, wear resistance, impact toughness and impact fractures were adopted to study effects of different quenching methods (air cooling + fog cooling) and tempering temperatures on microstructure and mechanical properties of 75Cr-Mo wear-resisting cast steel. The results reveal that quenching stress could be decreased effectively by using "air cooling + fog cooling" comparing with water quenching, thus avoiding initiation of micro crack. the microstructure of the quenched steel mainly comprises pearlite in addition to a little martensite and ferrite. M 23 C 6 carbides, which have no crystal orientation with the matrix, disperses in the matrix. A hardness of 401 HBW and impact toughness of 35 J/cm 2 were obtained for the steel when quenched after insulation for 2 hour at 880. Introduction Ball mill is an grinding equipment which is widely used in mineral processing, chemistry, building materials and other industrial sectors, while liner is an important abrasion-proof parts, abrasion of liner causes great economic losses every year[1]. Currently, high manganese steel is the most commonly used materials in liner. However, only under high stress conditions can high manganese steel play a high wear resistance. If the stress is below the required, the performance of high manganese steel will be limited, the abrasion of liner will become faster and the service life will be reduced, which will lead to the increasing cost of grinding [2]. In this paper, 75Cr-Mo wear-resistant cast steel is used as liner material, to study and optimize the heat-treatment process, while the influence of different quenching temperature on the microstructure and mechanical properties has been analyzed[3~4]. Through this study, we want to find a suitable route for 75Cr-Mo wear-resistant cast steel heat treatment process, which can make75cr-mo wear-resistant cast steel be able to give full play to good wear resistance under different working conditions. Experimental Procedures The chemical composition of 75Cr-Mo steel experiment material is as shown in table 1. The content of high carbon can not only improve the hardness and the hardenability, but also combine with chromium that plays the role of invigorating effect to form stable (Fe, M) 3 C type alloyed cementite, and Moderate and Cr/C ratio can make plenty of carbide in steel to improve the wear resistance of materials. Cr can increase the hardenability of steel, Mo can prevent the temper brittleness[5~6]. Sample preparation: Using Mo wire cutting machine cut the sample into 10 mm x 10 mm x 55 mm shock specimen, then put the specimen into XWL-13-5y type of box-type resistance furnace for heating, the heating temperature of thermal refining is 840, 860, 880 and 900 respectively and insulate for 2 hours. Adopting the method, "air cooling + cold mist cooling to cool the specimen and choose quenching temperature in which organization performs well. metallographic specimen: mechanical grinding and polishing the sample after heat treatment, and then use 4% nitric acid alcohol solution for corrosion. Sample organization and impact fracture morphology after heat treatment can be observed by SM-5610LV scanning electron microscope, the brinell hardness tester can be used to test the hardness of specimens after heat treatment. 70

2 Table 1. Chemical Composition of 75Cr-Mo Steel %. C 0.70~0.80 Mo 0.40~0.60 Cr 0.80~1.10 Mn 0.40~0.60 Si 0.40~0.60 Ni 0.40~0.45 Results and Analysis Microstructure Analysis The organization was a typical pearlite and ferrite mixed organization by using "air-cooled + fog" method as shown in Fig.1, which is caused by slow cooling rate. As the carbon content is higher, the pearlite structure after quenching is granular, and the higher carbon content also makes the material still have martensite after cooling. When the process is carried out, some tiny high-carbon Martensite will appear. Figure1 shows the organization after 840 quenching. It can be seen that organization is uneven distribution; there are many blocky ferrites whose size is bigger. Because many granular types of carbide distribute in the ferrite matrix, the number of granular pearlite increases. Figure1(c) shows that the ferrite and pearlite are island-like interval distribution, pearlite structure in the SEM was similar to the martensitic transformation of the embossed phenomenon [7]. Within the grain orientation is obvious, layered tissue is clearly. As shown in Figure1(d) when the proportion of ferrite is further reduced and evenly distributed, the martensite structure is obvious. (b) (c ) (d ) Figure 1. Microstructure in Different Quenching Temperature (SEM 500 ) 840 ; (b)860 ; (c)880 ; (d)900. Using the Image Pro Plus software to quantitatively analyze the microstructure of the quenched microstructure [8], The ferritic proportion after different temperature quenching are 40.2%, 29.3%, 14.2% and 9.5% respectively, thus it can be seen that with the increase of quenching temperature, solid carbide is dissolved in austenite matrix and carbon supersaturation of transform into martensite after quenching is increased, the degree of carbon content is the main factor which influence the hardness. With austenitizing degree increasing, ferrite content gradually reduces, the martensite content gradually increases, and making the hardness and strength improve significantly. Cr, Mo alloy elements are strong carbide forming elements, these elements are easy to form alloy cementite and restrict the grain boundary migration in the process of austenitization, therefore the 71

3 austenitic grain size is small, organization is also fine after quenching. The rising of austenitizing temperature speeds up the carbon dissolved in the organization, reduces the obstruction to austenitic grain growth from alloying cementite, promotes the grain growth. Single-phase austenitic grain growth is clearly, the resulting microstructure after quenching will become bulky[9]. The Impact Fracture Morphology Analysis The four groups of samples after quenching start impact experiments respectively, and then shot under the SEM, the fracture surface morphology are shown in Figure 2: (b) (c) (d) Figure 2. Impact Fracture SEM Micrographsat Different Quenching Temperature (SEM 500 ) 840 ;(b)860 ;(c)880 ;(d)900. After 840 and 900 quenching, cleavage facet and cleavage step are apparent, toughening litter quantity is less, The brittle fracture mechanism is obvious. Cleavage facet of impact fracture after 860 and 880 quenching is lesser, dissociative level is lower, and more toughening nest around the cleavage stage. After 880 quenching impact fractures is shown in fig. 2 (c). cleavage steps is the mix, larger toughening nest appear around the steps, toughening nest density is larger than that in the other three kinds of quenching temperature. As can be seen from the fig. 2, the impact fracture surface after quenching is a typical quasi cleavage, namely the toughening nest steps distribute around the cleavage fracture. Quasi cleavage fracture is a form of distortion of cleavage fracture, before rupture material occurs plastic deformation, it shows certain toughness. This is of great significance to study the research of wear-resistant materials on impact condition. Mechanical Property Analysis The line of hardness and impact toughness changing with quenching temperature is as shown in Fig. 3. From the diagram, with the increase of quenching temperature, the hardness of materials shows a trend of increase. This is mainly because with the increasing quenching temperature, the alloy elements forming carbide gradually dissolve [10], the dissolved alloy elements go into austenite, austenitic alloy element content is higher, the martensite after quenching alloy element content is higher therefore, leading to lattice distortion, and dislocation motion must overcome higher energy, embodied in the improvement of the hardness, the highest quenching hardness can reach 424 HBW in the fig

4 Figure 3. Hardness and Impact Thoughness Curve at Different Quenching Temperature. With the increase of quenching temperature, it can be seen that the impact of the materials work reduces gradually. This is because with the increase of quenching temperature, the quantity of martensite increases gradually, the material toughness drops, which conform to the mechanical properties on the degree of "strength and toughness is relative" this basic principle. 880 quenching s microstructure is uniform distribution, the different orientation of pearlite clusters absorb shock efficently and the maximum impact energy can reach 40 J/cm2. However, when the quenching temperature reaches 900, the organization is bulky, adversing to the impact toughness, the impact toughness of the material quickly reduces to 16 J/cm2. Taking into account microstructure morphology, impact fracture appearance and mechanical properties test after quenching, we can conclude that material hardness and toughness is good after 880 x2 h quenching. so choosing 880 as the optimum quenching temperature. Substructure Analysis The microstructures of 880 x2 h quenching can be observed by transmission electron microscopy. The results are shown in Fig. From Fig. 5 and (b), it can be seen the presence of lath martensite in the quenching structure, the residual austenite is between the martensite slab, and the sub-structure of martensite is the high density dislocations and parallel-stripes distribution of twins crystal[11]. (b) b) Dislocation Twins 73

5 (c) Carbide Figure 4. Microstructure and Carbide of Quenching at 880 a) Twin; b) Dislocation; c) Carbide; d) Carbide Calibration. Nanoscale carbide distribute on the martensitic matrix {fig. 4(c)}, the precipitation hardening of M23C6 carbides can be demarcated by diffraction pattern. The base of M23C6 [101] share a same spot with α-fe [011], which indicates that there is no apparent crystallographic relationship between them. The dissolution temperature of M23C6 is 950, in the quenching temperature range, M23C6 in the austenitizing heat preservation stage has not completely dissolved, there is still partially retained in the matrix after quenching, Residual M23C6 carbide will greatly improve the quenching strength of the material, which plays a role in improving the wear resistance of materials. Conclusion (1). The microstructure of the quenched steel is composed of pearlite, martensite and ferrite, by using "air cooling + fog cooling" quenching method. When quenched after insulation for 2 hour at 880, a hardness of 401 HBW and impact toughness of 35 J/cm2 can be obtained. (2). The sub structure of the martensite after quenching is twin crystal and high-density dislocation. The fine and dispersed M23C6 carbides, which have no crystal orientation with the matrix, in the matrix can improve strength of the steel. Acknowledgement This research was financially supported by science and technology support plan of Luoyang ( A). References [1] Zhu Jun, Yang Jun. Study and Application of Large Ball Mill Liner [J]. Foundry Technology, 2005, 26 (12): [2] Ma You-ping, Zhou Shu-yi, Li Xiu-lan, et al. Effect of Cr-Mo-V-Ti-Ni Alloying on Solidification Structure of As-cast V-EPC (Vacuum Expandable Pattern Casting) Hjgh Manganese Steel[J]. Special Casting & Nonferrous Alloys, 2012, 32(6): [3] Gao Ling, Zhang Yong-le, Ma Jin-yuan. Research on Improving the High Manganese Steel Lining Board (ZGMn13) Abrasion Resistant [J]. Science & Technology of Baotou Steel (Group) Corporation, 2008, 34 (1): [4] Zhang Jin-rui, Liang Bing, Zhao Li-bing, et al. Technology Development trend on the grinding medium of the ball mill [J]. Non-ferrous Mining and Metallurgy, 2013, 29(1): 42-45, 54. [5] Li Xiu-lan, XIE Wen-ling, Zhou Xin-jun, et al. Influence of Ti, V, Nb and Mo on Morphology and Properties of High Chromium Carbide [J]. Special Casting and Nonferrous Alloys, 2014, 34(8):

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