Behaviour of the M‐A constituent in a simulated HAZ after single and multiple welding thermal cycles: HAZ toughness in 780 and 980 M Pa class HSLA steels welded with high heat input (1st report)

Summary This paper describes an investigation of the effect of the areal fraction of the M‐A constituent on the Charpy absorbed energy (VE) of two types of simulated HAZ ‐ the CGHAZ (coarse‐grained HAZ of a high heat input welding process) and ICCGHAZ (intercritically reheated CGHAZ of a multiple welding process) ‐ in 780 and 980 MPa class HSLA steels. The M‐A constituent is classified into two morphological types: massive M‐A and elongated M‐A. With an increasing cooling time (?t8/5), the M‐A constituent morphology changes from elongated M‐A to elongated M‐A + massive M‐A and then to elongated M‐A + massive M‐A + carbides. The M‐A size also increases. The VE of ICCGHAZ is shown to be somewhat lower than that of CGHAZ in the double thermal cycle test at a temperature of 1073 K where the peak temperature of the 780 MPa class thermal cycle Tp2 exceeds Ac1 due to the formation of massive M‐A. The VE at both 873 K and 673 K, where the peak temperature of the 780 MPa class thermal cycle Tp3 is lower than Ac1 is improved to exceed that at 1073 K which is equal to Tp3 in the triple thermal cycle test. Since the improvement of VE corresponds well with the decrease in the M‐A area fraction, the M‐A constituent is decomposed by the tempering effect of the third thermal cycle, so that the VE of ICCGHAZ appears to be improved. The increase in the total M‐A area fraction in both single and multiple welding thermal cycle tests causes an increase in the areal fraction of massive M‐A whose appearance appears to be the key factor affecting VE deterioration in the CGHAZ and ICCGHAZ of 780 and 980 MPa class HSLA steels.