Probing True Microstructure-Hardening Relationship in Simulated Heat Affected Zone of P91B Steels

The inducement of this paper was the revelation of true microstructure-hardening relationship in heat affected zones (HAZs) of P91B steels. This work reports erroneous metallographic practices in revealing true microstructure and establishes prior austenite grain (PAG) hardening and softening phenomenon. Different HAZs were simulated having distinguished microstructural features. True microstructure of simulated and post-weld heat treatment (PWHT) samples were revealed completely and discussed with respect to microhardness indicating the degree of hardening. It was revealed that true microstructure features of sub-HAZs were governed by peak temperature, hold time at peak temperature, PAG size, and microhardness. The simulated samples showed different microstructural features on using different reagents/etchants. Rate of revealing microstructural features and degree of keying force influencing PAG size were observed to be dependent on peak temperature and hold time at peak temperature. However, true-microstructural features of furnace simulated and PWHT samples were independent of reagents’ composition. Degree of hardening varied due to the difference in solid solution strengthening and formation of second phases at a given peak temperature and hold time at peak temperature. In simulated samples, PAG softening was observed for coarse-grained HAZ (CGHAZ) after PWHT, while in furnace simulated samples, softening occurred in inter-critical HAZ (furnace simulated ICHAZ). Interpass temperature with second pass led to the formation of mixed PAGs and PAG hardening. This hardening was retained in inter-critically reheated CGHAZ after PWHT, whereas it was absent in super-critically reheated CGHAZ.