The Deterioration and Improvement of HAZ Toughness in 780 and 980MPa Class HSLA Steels Welded with High Heat Inputs(Report 1). The Behaviour of M-A Constituent in Simulated HAZ with Single and Multiple Thermal cycles.

Abstract

In this research work, the effect of area fraction of M-A constituent on charpy absorbed energy (vE) of both simulated-CGHAZ (Coarse-Grained HAZ for high heat input welding process) and ICCGHAZ (Intercritically-Reheated CGHAZ for multiple welding process) of either 780 or 980 MPa class HSLA steel has been studied.The M-A constituent is classified into two types such as massive M-A and elongated M-A depending on its form. The form of M-A changes from "elongated M-A" to "elongated M-A+massive M-A" and then to "elongated M-A+massive M-A+carbide" according to the increase of cooling time, Δt8/5. Furthermore, the size of M-A increases.It has been clarified that the vE of ICCGHAZ is rather lower than that of CGHAZ as the result of dual thermal cycle test at the temperature of 1073 K where the peak temperature of second thermal cycle, Tp2 exceeds Ac1 due to the formation of massive M-A of large size. However, the vE at both 873 K and 673 K where the peak temperature of third thermal cycle, Tp3 is lower than Ac1 is improved to be more than the vE at temperature of 1073 K which is equal to Tp3 in the triple thermal cycle test. Since these improvement of vE considerably corresponds with the decrease of area fraction of M-A, the M-A constituent is decomposed by the tempering effect of third thermal cycle so that the vE value of ICCGHAZ seems to have been improved.The increase of area fraction of total M-A in both single and multiple welding thermal cycle tests causes the increase of that of massive M-A seems to be the decisive factor for the deterioration of vE in both CGHAZ and ICCGHAZ of either 780 or 980 MPa class HSLA steel.