Decomposition behaviour of M‐A constituent and recovery of toughness deterioration of HAZ toughness in 780 and 980 MPa class HSLA steels welded with high heat inputs and its improvement

Summary This paper describes an investigation of the decomposition of M‐A constituent by PWHT (post weld heat treatment) together with its effect on the toughness of the weld CGHAZ (coarse‐grained heat affected zone) of 780 and 980 MPa class HSLA steels. For all steels tested, the area fraction of the M‐A constituent decreases with an increasing PWHT temperature and time, but the toughness dependence of the PWHT parameters of the 980 MPa class steels is quite different from that of the 780 MPa class steels. The toughness of the weld CGHAZ of the 780 MPa class steels is increased as the M‐A constituent is decomposed into ferrite and cementite by the PWHT. The weld CGHAZ of the 980 MPa class steels, however, shows embrittlement at PWHT temperatures above 823 K, though the toughness increases with the PWHT time at temperature below 623 K. The foregoing results suggest that the toughness of HSLA steels impaired by high heat input welding can be significantly improved by PWHT at a temperature around 623 K.