Characterization of the multi-pass weld metal and the impact of retained austenite obtained through intercritical heat treatment on low temperature toughness

In this study, we explore the relationship between the microstructure and low temperature toughness of weld metal obtained from a real multi-pass weld joint (up to 55 mm) by submerged arc welding, which was developed for high strength (yield strength over 550 MPa) and heavy wall pipe fitting applications with composition of 0.1 wt% C, 2.0 wt% Mn and other micro-alloys. The study indicated that the necklace-type coarse martensite-austenite (M-A) constituent formed in the interlayer heat affected zone (IHAZ) of weld metal was responsible for low impact energy of 39 J at −40 °C. To enhance the toughness, conventional tempering and new intercritical heat treatment were designed. The results suggested that there was insignificant effect on toughness through conventional tempering, but obvious improvement through combination of quenching plus intercritical annealing and tempering. The impact energy was increased to ~98 J. The microstructure that benefit toughness primarily comprised of intercritical lath-like ferrite or acicular-type ferrite, bainite/martensite, and fine acicular retained austenite, with average size of ~0.3 μm. Retained austenite with volume fraction of ~6% was formed by the enrichment of Mn and Ni in reversed austenite during intercritical tempering process.