In Situ Observation of Phase Transformations in the Coarse-Grained Heat-Affected Zone of P91 Heat-Resistant Steel During
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It is well established that the coarse-grained heataffected zone (CGHAZ) of the welded joints could form heterogeneous microstructures after welding, such as retained d-ferrite, retained c-austenite, and martensite, which to a large extent dictate the mechanical properties of the welded joints.[1] In addition, reheat cracks and type III creep cracks may occur in the CGHAZ, which deteriorate the creep rupture strength of the heatresistant pressure vessel components.[2–4] The as-weld microstructure in the CGHAZ has been shown to be responsible for the subsequent structural evolution during post-weld heat treatment as well as in service at high temperatures.[5] Consequently, it is important to elucidate the phase transformation and microstructure evolution in the CGHAZ during the welding thermal cycle.
YANG SHEN and CONG WANG are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] BO CHEN is with the School of Engineering, University of Leicester, Leicester LE1 7RH, UK. Manuscript submitted on March 9, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Previous CGHAZ studies were primarily based on post-mortem examinations performed at ambient temperature and/or mathematical calculations. Sawada et al.[6] systematically investigated the nonequilibrium microstructure of the heat-affected zone (HAZ) in the as-welded P91 steel and revealed that the CGHAZ exhibited the highest hardness because of the martensite structure with fine laths and high dislocation density. Ueshima et al.[7,8] by means of mathematical analysis, focused on the redistribution of solutes at d/c interfaces during solidification. However, experimental evidence remains scarce pertinent to phase transformations and microstructural evolution during continuous heating and cooling typical of welding. Such information will certainly enrich our understanding of the kinetics of microstructural evolution during welding and enable developing a comprehensive computational model for CGHAZ. One of the powerful in situ approaches to investigate phase transformations and microstructural evolution in steels is high-temperature confocal scanning laser microscopy (CSLM), which breaks the predicament of traditional metallographic experiments and is capable of tracking real-time morphology and performing quantitative analysis.[9–11] Zou et al.[12] utilized CSLM for in situ observations of ferrite laths growth behaviors in the HAZ and found that the increase of cooling rate was beneficial to the formation of acicular ferrites instead of ferrite side plates. Dippenaar et al.[13] observed the development of d-ferrite recovery substructure in low carbon steels though CSLM. It was proposed that the sub-boundary structures in d-ferrite could play a role in modifying austenite decomposition products. CSLM can provide real-time observations, which are not readily available by in situ time-resolved X-ray diffraction using high-energy synchrotron radiation (HEXRD), as the latter technique can only present lattice parameter
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