Martensite morphology and fracture behavior in intercritically treated medium-carbon 2Si-6Ni steel
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Martensite Morphology and Fracture Behavior in Intercritically Treated Medium-Carbon 2Si-6Ni Steel KON B A E LEE and HOON K W O N During intercritical treatment in the ferrite + austenite region, the morphology o f austenite (i.e., martensite after quenching) formed from the initial structure o f martensite is influenced by alloying elements, m This variation in martensite morphology, in turn, plays a significant role in controlling the tensile properties in low-carbon dual-phase steels.I2] In recent years, Kwon and his co-workers [3 71 reported that the morphology o f gain-boundary martensite (GBM) acted as an important factor in controlling the impact toughness and fracture behavior in experimental mediumcarbon alloy steels intercritically treated. Continuous, coarse GBM led to intergranular fracture in the 6Ni steel, compared to transgranular fracture observed in its absence in a 2Si steel.[3,4.5~ In addition, in a 2Si-3Ni steel, a transgranular-to-intergranular fracture transition occurred as discontinuous, fine GBM was changed to a continuous, coarse one with increasing intercritical treatment temperature, t6,TJ KON BAE LEE, Graduate Student, and HOON KWON, Associate Professor, are with the Department of Metallurgy and Materials Engineering, College of Engineering, Kookmin University, Seoul 1367 0 2 , Korea. Manuscript submitted July t 7 , 1992. 508--VOLUME
24A, FEBRUARY 1993
In this study, the morphology o f GBM and fracture behavior o f a 2Si-6Ni steel intercritically treated have been compared with a 2Si-3Ni steel, tyj In addition, the effect o f tempering on fracture behavior has been investigated. The chemical composition o f the alloy used in this study is 0.37C, 2.04Si, 5.94Ni, 0.006P, and 0.006S (wt pct). In order to obtain an initial martensite structure, Charpy impact specimens were austenitized in a flowing argon atmosphere at 1200 °C for 1 hour and oil-quenched. These were then intercritically treated in the range o f 680 °C to 740 °C for 1 hour and water-quenched. To investigate the effect o f tempering, the intercritically treated specimens were tempered at 600 °C for 1 hour and waterquenched. Microstructures o f specimens intercritically treated in the range o f 700 °C to 740 °C are shown in Figure 1. At 720 °C, the coarse GBM was partly formed, as the amount and connectivity of martensite increased with an increase in intercritical treatment temperature. At 740 °C, continuous, coarse (mostly blocky) GBM was formed. This variation in GBM morphology is similar to that seen in the range o f 730 °C to 770 °C for the 2Si3Ni steel, t71 although the 2Si-3Ni steel showed continuous, coarse (plate-like) GBM at 750 °C. The hardness o f the mixed region o f fibrous martensite and ferrite in the 2Si-6Ni steel is compared with that o f the 2Si-3Ni steel in Figure 2. It increased with increasing intercritical treatment temperature. Continuous, coarse GBM is formed at temperatures o f about 730 °C and 750 °C in 2Si-6Ni and 2SiJ3Ni steels, and the hardness at those temperatures is about Hv 510 and 420, respe
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