Enhanced Impact Toughness of Heat Affected Zone in Gas Shield Arc Weld Joint of Low‐C Medium‐Mn High Strength Steel by Post‐Weld Heat Treatment

Abstract

In the present study, two different post weld heat treatments (PWHT) are carried out on gas shield arc welded low‐C medium‐Mn high strength steel to explore the effect of PWHT on microstructure, microhardness, and low temperature toughness of heat affected zone (HAZ). The results show that when the weld joint is PWHT at 630 °C for 30 min, the microhardness of HAZ is homogeneous and almost identical to base metal (BM). The microstructure of HAZ comprises of tempered martensite and strip‐shaped reversed austenite. The impact energy of fusion line (FL), coarse‐grained HAZ (CGHAZ), and fine grained HAZ (FGHAZ) at −40 °C is enhanced by 29 J (from 42 to 71 J), 42 J (from 68 to 110 J), and 33 J (from 140 to 173 J), respectively, compared to the weld joint, which is PWHT at 200 °C for 120 min. The formation of reversed austenite with certain stability in the HAZ is the main toughening mechanism when the weld joint is PWHT at 630 °C. Reversed austenite enhances the impact energy significantly through the transformation‐induced plasticity (TRIP) effect, which releases the stress concentration, improves the crack initiation energy, and effectively prevents the propagation of cleavage crack.