In Situ Observation of Martensite Lath Growth Behaviors in the Coarse Grained Heat-Affected Zone of 1.25Cr-0.5Mo Heat-Re
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rrite heat-resistant steels, such as 1.25Cr-0.5Mo steel, offer a combination of high-temperature stability, outstanding creep strength, and eminent corrosion resistance, and are extensively employed in petrochemical, petroleum refining and fossil fired power generating industries.[1–3] Fusion welding techniques, such as shielded metal arc welding (SMAW), flux-cored arc welding (FCAW), and submerged arc welding (SAW), are invariably necessitated to weld such heat-resistant grade steels.[4] However, the microstructures and mechanical properties of the parent 1.25Cr-0.5Mo steel is susceptible to drastic changes during welding thermal cycle. Especially in the coarse grain heat-affected zone (CGHAZ) of the welded joint, which experiences high peak temperature ( Tp Ac3 ) and rapid cooling rate,[5–8] noticeable phase changes, including diffusive austenitic transformation and diffusionless martensitic transformation, may likely occur.[9] Martensite, which is
YANG SHEN and CONG WANG are with the School of Metallurgy, Northeastern University, Shenyang, 110819, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the main microstructure of the CGHAZ, is of practical importance tuning toughness and strength.[10] Consequently, an in-depth understanding of the martensite phase transformation in the CGHAZ is indispensable to secure the overall performance of the weldment. Previous studies elucidated the phase transformations of the CGHAZ from the viewpoints of morphology and crystallography.[11–13] Wang et al.[9] systematically studied the heterogeneous microstructures of the heataffected zone in as-welded 9Cr-1Mo-V-Nb steel and revealed that the CGHAZ exhibited the highest hardness. Sarizam et al.[14] investigated the effect of holding temperature on the variant selection mechanism during bainite transformation in 2Cr-1Mo steel. Yue et al.[15] and Shome et al.[16] mainly focused on the effects of different cooling rates on the microstructure of CGHAZ during continuous cooling transformation process, without considering the effects of absolute temperature. However, most of the above results were based on ex situ observation. Fortunately, in recent years, confocal scanning laser microscope (CSLM), a powerful in situ observation method, has enabled investigations over the kinetic process of phase transformation and microstructure evolution in steels at high-temperature.[17–19] Yin et al.[20] studied the transformation kinetics from dferrite to c-austenite and found that the incoherent d/c interphase boundaries were always unstable with fingerlike morphology, which showed a good agreement between experimental observations and theoretical calculations. Yu et al.[21] investigated the microstructure evolution during CGHAZ thermal cycling of blastresistant steel and confirmed the concurrent refinement of martensite packet size with smaller austenite grain size. Mao et al.[22] revealed five nucleation modes and six types of growth behaviors of bainite laths in reheated weld met
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