Characterization of martensite-austenite constituents and micro-hardness in intercritical reheated and coarse-grained heat affected zones of API X70 HSLA steel

摘要

The thermal cycles that high strength low alloy (HSLA) low carbon steels experience during welding inevitably affect the microstructure and mechanical properties of the weld joints. It is generally agreed that the coarse grained heat affected zone (CGHAZ) and the intercritical reheated coarse-grained heat affected zone (ICRCGHAZ) in one-pass and multi-pass welding, respectively, have the poorest microstructure and mechanical properties relative to the rest of the steel joint. The martensite-austenite (M-A) constituents formed in the HAZ regions predominantly govern property deterioration in the CGHAZ and the ICRCGHAZ by promoting the initiation and propagation of micro-cracks. However, the characteristics of the M-A constituents, i.e., fraction, size, shape and aspect ratio, are strongly affected by the welding heat input. Accordingly, in this study, the aim was to evaluate the influence of a cold wire addition in conventional tandem submerged arc welding (TSAW + cold wire) on the microstructure and micro-hardness in the CGHAZ and ICRCGHAZ. Microstructural characterization showed that a lower fraction of finely distributed M-A constituents associated with smaller prior austenite grains (PAGs) formed in the CGHAZ of the steel welded by the TSAW + cold-wire process (CW-OD weld) relative to that welded by the conventional TSAW process (TW-OD weld). Furthermore, the fraction of M-A constituents in the ICRCGHAZ of the CW-OD weld was decreased. A higher fraction of slender shaped M-A constituents in the ICRCGHAZ of the TW-OD weld resulted in an increase in the micro-hardness. The microstructure and micro hardness changes associated with cold wire addition were attributed to the lower welding heat input and, consequently, the faster cooling rate with a corresponding reduction in the retention time in the austenitic region and ferrite/austenite region in the CGHAZ and ICRCGHAZ regions, respectively.