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, Ac1 and Ac3 during the second thermal cycle were significantly lower than those during the first thermal cycle, and Ac1 and Ac3 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, Ms 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.