Evolution of microstructure in reheated coarse-grained zone of G115 novel martensitic heat-resistant steel

Abstract

Based on the thermal simulation method, a systematical analysis was conducted on the effect of welding peak temperature and the cooling time that takes place from 800 to 500 °C on microstructure, precipitates, substructure and microhardness of the reheated coarse-grained heat-affected zone (CGHAZ) of G115 novel martensitic heat-resistant steel. As revealed from the results, the microstructure of un-altered CGHAZ (UACGHAZ) and supercritically CGHAZ (SCCGHAZ) was lath martensite, and structural heredity occurred. Intercritically reheated CGHAZ (IRCGHAZ) exhibited martensite and over-tempered martensite, and subcritical CGHAZ (SCGHAZ) displayed martensite and under-tempered martensite. The austenite in UACGHAZ and SCCGHAZ was transformed with the diffusion mechanism during the first thermal cycle, but with the non-diffusion mechanism during the second thermal cycle. For this reason, A_c1 and A_c3 during the second thermal cycle were significantly lower than those during the first thermal cycle, and A_c1 and A_c3 were reduced by nearly 14 and 44 °C, respectively. Since the content and stability of the austenite alloy during the second thermal cycle of IRCGHAZ were lower than those during the first thermal cycle, M_s increased by nearly 30 °C. There were considerable precipitates in the over-tempered region of IRCGHAZ, and the Laves phase was contained, which was not conducive to high-temperature creep property. Moreover, the dislocation density and the number of sub-grains in the region were lower, resulting in a sharp decrease in the microhardness, and it was the weak area in the reheated CGHAZ.