Microstructural characterization of low alloy steel A508 – 309/308L stainless steel dissimilar weld metals

Highlights: • Welding process caused microstructural complexities at the HAZ. • A correlation between microstructural features and mechanical properties of LAS 508 was drawn. • Sigma phase precipitation at the austenite/ferrite interface boundary of the 309/308L SS. • HAZ region had the highest hardness, followed by LAS 508 and 309/308L SS respectively. In this study, the microstructure and microhardness of low alloy steel A508 welded with 309/308L stainless steel are investigated. The microstructure evaluations show that the heat affected zone of the A508 base metal changes, from tempered bainite to an uneven mixture of bainite and martensite, depending on the intensity of the heat flow from the welding process. Different recrystallization processes at the heat affected zone of the base metal are responsible for the changes of the microstructure, which lead to differences in the grain sizes, strain level, and yield strength in this region. The greatest deformation level is observed in the grain refined region with the highest heat influx, which leads to a high level of grain refinement and stress in this region, followed by the partial grain refined region and base metal matrix region, respectively. The interface region between the base metal and weld metal is observed to be very complex. At some regions of the 309/308L, adjacent to the fusion boundary type II boundaries, resulting from the allotropic δ – γ transformation and migration of steel phases/boundary at high temperature of welding were observed, alongside the type I boundaries. The 309/308L stainless steel consists of vermicular ferrite distributed along the austenite matrix, while ternary sigma secondary phase was observed to precipitate at the interface of the ferrite/austenite boundary. The yield strength of the grain refined region, partial grain refined region, and base metal matrix in A508 were estimated to be 598 MPa, 600 MPa, and 595 MPa, respectively. Finally, the heat affected zone of the base metal showed the highest hardness followed by the base metal matrix and weld metal, respectively.