Continuous Cooling Transformation of CLAM Steel in Heat Affected Zone
JIANG Zhi-zhong1,2, HUANG Ji-hua1, HU Jie1, CHEN Shu-hai1
1. School of Materials Science and Engineering,University of Science and Technology Beijing,Beijing 100083,China;
2. Institute of Nuclear Energy Safety Technology,Chinese Academy of Sciences,Hefei 230031,China
Abstract：By means of the physical simulation, the coarse grained zone in heat affected zone (CGHAZ) of CLAM steel during cooling process in the welding thermal cycles was conducted by using of the Gleeble1500 simulator. The effects of cooling rates(ωc)on microstructure transformation and Vickers hardness were investigated through microscopic observations, hardness tests as well as thermodynamic calculation with Thermo-calc software, and the SH-CCT diagram of the CGHAZ for CLAM steel was finally established. The results showed that the SH-CCT diagram exhibited only two transformation regions, i.e. low-temperature lath martensite (LM) region and high-temperature proeutectic ferrite (α-ferrite) region. 0.25℃/s was the critical cooling rate for phase transformation from supercooled austenite to full martensite. When the ωc＞0.25℃/s,the microstructure appearances of CGHAZ had no significant changes and consists of lath martensite and a small amount of δ ferrite within this range, the occurrence of δ ferrite was attributed to incomplete δ→γ transformation in the high temperature stage. When the ωc＜0.25℃/s, the CGHAZ showed dual phase of α-ferrite and lath martensite because of the γ→α transformation. The content of α-ferrite increased with the decreased cooling rate. When the ωc=0.04℃/s, the microstructure of CGHAZ was completely transformed to α-ferrite and carbides.
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