Microstructural evolution of SKD11 tool steel during multi-stage thixoforming and subsequent heat treatments

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1 Microstructural evolution of SKD11 tool steel during multi-stage thixoforming and subsequent heat treatments Yi Meng 1, a *, Hui-Min Zhou 1, Jia-Lin Gan 1 and Sumio Sugiyama 2 1 College of Materials Science and Engineering, Chongqing University, Shazheng Street No.174, Shapingba Qu, Chongqing, China 2 nstitute of Industrial Science, The University of Tokyo, Komaba 4-6-1, Meguro Ku, Tokyo, Japan a mengyi@cqu.edu.cn Keywords: semisolid; tool steel; heat treatment; phase transformation. Abstract. The application of semisolid forming (SSF) is inhibited significantly by the inhomogeneous distributions of microstructure and mechanical properties in the products manufactured by SSF. Beside of forming parameters including forming temperature, isothermal holding time, and forming speed, post heat treatments of SSF is one of the most important facts affecting the microstructure and mechanical properties of the SSF-manufactured products. In this study, heat treatments including annealing, quenching, and tempering different experimental conditions were conducted on the SKD11 tool steel samples manufactured by multi-stage thixoforming with different forming parameters. The microstructures in different regions of specimen processed by different experimental conditions were investigated by using a field-emission scanning electron microscopy (FE-SEM), an energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometer (XRD). Base on the results of microstructural analyses, the microstructural evolution of SKD11 tool steel during multi-stage thixoforming and subsequent heat treatments with varies experimental parameters were discussed and summarized. Meanwhile, the relationship between microstructure and mechanical properties of SKD11 tool steel processed by multi-stage thixoforming and subsequent heat treatment was also discussed. Introduction Although SSF technology innovated by Flemings et al in 1970s can be employed to realize net-shape forming with lower forming load and less micro and macro-defects, it has not been applied widely in industrial production of metal products to replace the conventional forging and casting [1,2]. The narrow semisolid temperature intervals, high working temperature, and liquid segregation took place during SSF were considered as the main obstacles hindering SSF technology in the industrial application of ferrous alloys [3,4]. Owing to the different formabilities of different phases in semisolid slurry, liquid segregation occurred during SSF and led to inhomogeneous distributions of microstructure and mechanical properties of the products [5,6]. To ensure the quality of these products manufactured by SSF, liquid segregation should be controlled and inhibited effectively [7]. To achieve this aim, various investigations were conducted by different researchers. The effects of forming parameters including forming temperature, forming speed, and forming load on liquid segregation in semisolid slurries during SSF were studied [8]. Owing to the complex microstructural evolution of ferrous alloys at high temperatures, normalization heat treatments were also used to improve the homogeneities of microstructure and mechanical properties of ferrous parts processed by SSF [9]. Based on the flexible motion ability of a mechanical servo press, multi-stage semisolid forming strategies was proposed in our pervious study to control and inhibit the liquid segregation during SSF of ferrous alloys and non-ferrous alloys [10]. In compared with the ones manufactured by conventional SSF, the products manufactured by multi-stage semisolid forming exhibited more homogenous distributions of microstructure and mechanical properties. However, liquid segregation in these products could not be eliminated completely. Thus, subsequent heat treatments were

2 considered as one of the most potential methods to eliminate liquid segregation in ferrous part manufactured by SSF. In this study, with the aim of establishing an effective subsequent heat treatment strategy following the multi-stage thixoforming of ferrous alloys to improve the mechanical properties of products, the effects of multi-stage thixoforming and the subsequent heat treatments on a ferrous alloy were studied experimentally. On the basis of the results of microstructural analysis and Vickers hardness measurements, the microstructural evolution and change of mechanical properties of a commercial hot-rolled SKD11 tool steel during multi-stage thixoforming and the subsequent heat treatments with various processing parameters was investigated in this study. 1. Experimental procedures 1.1 Starting material. A commercial SKD11 tool steel obtained from hot rolling was selected as the starting materials. The chemical composition of this commercial hot-rolled SKD11 tool steel is listed in Table 1. According to the result of differential scanning calorimetry (DSC) analysis, the semisolid temperature range of SKD11 is from 1200 to 1342 C. The micrographs of SKD11 at room temperature and the ones cooled from different temperatures are shown in Fig. 1. As shown in Fig.1a, eutectic compounds were distributed along the rolling direction in the hot-rolled SKD11 tool steel. When temperature excessed 1200 C, partial melting took place in the regions with lower melting points (grain boundaries and regions rich in alloying elements) in the SKD11 tool steel. Liquid phase transformed to eutectic compounds during cooling of semisolid slurry as shown in Fig. 1b. The morphologies of eutectic compounds transformed from liquid phase were different with those of original ones. According to our previous research, the original and new-formed eutectic compounds were identified as M6C-type and M7C3-type carbides, respectively [14]. Liquid fraction of semisolid slurry increased with the increasing temperature. Semisolid slurry with globular microstructure was obtained at 1300 C, as shown in Fig. 1c. Owing to the completely dissolution of the original eutectic compounds, only M7C3-type carbides transformed from liquid phase were observed in SKD11 tool steel cooled from 1300 C. Table 1 Chemical composition of SKD11 steel used in this study, mass/%. C Si P S V Cr Mn Ni Cu Mo Fe Bal. Fig. 1 Micrographs of SKD11 at room temperature and the ones cooled from different temperatures. 1.2 Experiments As shown in Fig. 2, multi-stage thixoforging includes a partial melting, a fast first compression, a partial solidification, and a secondary compression. Subsequent heat treatments include an annealing, a quenching, and two passes tempering. In this study, multi-stage thixoforging was simplified and simulated physically by double-passes semisolid compression tests on a multistage hot compression

3 test machine with controllable free-motion capability. In partial melting, cylindrical specimen cut from commercial SKD11 tool steel were heated to higher semisolid temperature with a heating rate of 20 C/s, and held isothermally for 20 s. In fast first compression, the cylindrical specimen was compressed with a higher strain rate with a stroke of 4 mm. In partial solidification, the compressed specimen was cooled to lower semisolid temperature and held isothermally for 20s. In secondary compression, the specimen was compressed once again with a lower strain rate with a stroke of 2 mm. In the end of multi-stage thixoforming, the specimen was cooled to room temperature by cooled water immediately. Subsequent heat treatments were conducted by using a resistance furnace. In the annealing treatment, all of the RAP-processed specimens were held at 850 C for 3 h and then cooled in a furnace. The subsequent quenching was carried out by heating the specimens to 1000, 1050, or 1100 C and cooling them in air after isothermal holding for 480 s. Following the quenching treatment, specimens were tempered twice at temperatures ranging from 200 to 600 C for 1.5 h and subsequently cooled in air. Fig. 2 Schematic illustration of multi-stage thixoforming and subsequent heat treatmetns. After polishing and etching in 10% nitric acid/alcohol solution, specimens processed under various experimental conditions were observed by using a JEOL JSM-5600 scanning electron microscopy and a Keyence VH-5500 optical microscope. Hardness in different regions of these specimens were measured by using a Shimadzu Vickers hardness tester under a load of 0.2 kg and a dwell time of 10 s. In this study, the positions used for microstructural observation and hardness measurement were conducted at least three times in same regions of specimens. The average values of raw data were calculated and used. 2. Results and discussion SKD11 samples with more homogenous microstructure and smoother surface could be manufactured by multi-stage thixoforming including partial melting with a heating rate of 20 C/s and an isothermal holding at 1280 C for 20 s, and fast first compression with a stroke of 4 mm and a strain rate of 2.0/s at 1280 C, partial solidifications to 1240 C with a cooling rate of 2 C/s, and secondary compression with a stroke of 2 mm and a strain rate of 0.2/s at 1240 C.

4 Fig. 3 Micrographs of different regions of SKD11 specimen compressed at 1280 C with a stroke of 6 mm and a strain rate of 2.0/s and then cooled to 1240 C with a cooling rate of 2 C/s to 1240 C with a cooling rate of 2 C/s, and secondary compressions with a strain rate of 1.0/s. In partial melting stage, semisolid slurry with globular microstructure was obtained. In fast first compression stage, rotation of solid particles among the liquid matrix was dominated. Owing to the different forming behaviors of liquid and solid phase, liquid from in center region of the sample flow outward and resulted in liquid segregation. The outflow of liquid phase was inhibited by higher strain rate in this stage, because higher strain rate reduced the time for liquid phase to form paths for outflow. In particle solidification stage, liquid phase transformed to solid grains partially and resulted in larger size and higher volume fraction of solid particles. Owing to the decreased volume fraction of liquid phase, sliding and plastic deformation of solid particles were dominated during the secondary compression. Moreover, the lower strain rate decreased to forming load effectively in secondary compression stage. Detail description of multi-stage thixoforming was provided in our previous report [10].

5 Fig. 4 Micrographs in different regions of thixoformed SKD11 tool steel specimen after annealing and quenching from 1050 C. Fig. 5 Vickers hardness of thixoformed SKD11 tool steel treated by different heat treatments with different parameters. Owing to the various contents of alloying elements in former liquid and solid regions of the thixoformed SKD11 tool steel, martensite, austenite, and carbides with various volume fractions were measured in different regions of the thixoformed SKD11 tool steel. This phenomenon was attributed to the different starting temperatures for martensite transformation. As shown in Fig. 4, several grains with martensite structure were observed in the thixoformed SKD11 tool steel specimen after annealing and quenching from 1050 C. Vickers hardness of thixoformed SKD11 tool steel treated by different heat treatments with different parameters are shown in Fig. 5. Dissolution of carbides occurred during annealing treatment and resulted in lower values of hardness. Martensite transformation and precipitation of carbides took place during quenching treatment and led to higher values of hardness. Meanwhile, retrial stress was another reason why hardness increased after quenching. Release of retrial stress and tempering of martensite occurred during tempering and caused lower values of hardness. Retained austenite transformed to martensite when tempering temperature was about 550 C. The new formed martensite increased the hardness of SKD11 tool steel. This phenomenon was named as secondary hardening. Martensite grains and carbides became even coarser when tempering temperature became even higher.

6 Conclusion The main results of this study are summarized as follows. 1. The microstructural evolution of SKD11 samples during multi-stage thixoforming and subsequent heat treatments was investigated experimentally. The effects of process parameters of multi-stage thixoforming and subsequent heat treatments on the microstructural evolution and mechanical properties of SKD11 tool steel were investigated. 2. Different values of Vickers hardness measured in thixoformed SKD11 tool steel processed by heat treatments with different parameters, it was attributed to martensite transformation affected by content of alloying elements. Acknowledgments This study was financially supported by National Natural Science Foundation of China, China (contract No ), Chongqing Natural Science Foundation, China (contract No. cstc2016jcyja1027), Fundamental Research Funds for the Central Universities, China (contract No ), and Grant-in-Aid for Scientific Research on the Innovative Area "Bulk Nanostructured Metals" through MEXT, Japan (contract No ). References [1] M.C. Flemings, Behavior of metal alloys in the semisolid state. Met. Trans. 22A(1991) [2] M. Kiuchi, R. Kopp, Mushy/semi-solid metal forming technology present and future. CIRP Ann. Manuf. Technol. 51(2002) [3] H.V. Atkinson, A. Rassili, Thixoforging steel. Shaker Verlag, Aachen, Germany, 2010, pp [4] D.I. Uhlenhaut, J. Kradolfer, W. Püttgen, J.F. Löffler, P.J. Uggowitzer, Structure and properties of a hypoeutectic chromium steel processed in the semisolid state. Acta Mater. 54(2006) [5] J. Li, S. Sugiyama, J. Yanagimoto, Microstructural evolution and flow stress of semisolid type 304 stainless steel. J. Mater. Process. Technol. 161(2005) [6] J. Li, S. Sugiyama, J. Yanagimoto, Y. Chen, W. Guan, Effect of inverse peritectic reaction on microstructural spheroidization in semisolid state. J. Mater. Process. Technol. 208(2008) [7] R. Song, Y. Kang, A. Zhao, Semi-solid rolling process of steel strips. J. Mater. Process. Technol. 198(2008) [8] W. Püttgen, B. Hallstedt, W. Bleck, P.J. Uggowitzer, On the microstructure formation in chromium steels rapidly cooled from the semisolid state. Acta Mater. 55(2007) [9] Y. Meng, S. Sugiyama, J. Yanagimoto, Microstructural evolution during RAP process and deformation behavior of semisolid SKD61 tool steel. J. Mater. Process. Technol. 212(2012) [10] Y. Meng, J. Zhang, J. Zhou, S. Sugiyama, J. Yanagimoto, Study on the effects of forming conditions on microstructural evolution and forming behaviors of Cr-V-Mo tool steel during multi-stage thixoforging by physical simulation. J. Mater. Process. Technol. 248(2017)