Microstructure-property relationship for AISI 304/308L stainless steel laser weldment

Abstract

Laser welding is attractive for numerous applications requiring materials joining, due to its low energy input. However, it is unknown how the laser welding process influences the microstructure-property relationship across the weldment. The objective of this study is to determine the strengthening mechanisms in the weld metal, base metal, and heat affected zone (HAZ) of an AISI 304/308L stainless steel (SS) laser weldment. Scanning electron microscopy (SEM) with electron backscattered diffraction (EBSD) scanning and transmission electron microscopy (TEM) was used to evaluate the microstructure, and static nanoindentation was used to evaluate strength across the weldment. Although the HAZ has a finer dendritic grain structure, its higher hardness compared to the base and weld metal cannot be explained by the Hall-Petch relationship. Therefore, a new strengthening model for weldments that considers the evolution in grain boundary size and orientation angle, as well as dislocation density, precipitation, and solid solution is proposed. Notably, core-shell Ti-C-N precipitation, which provides Orowan dislocation bypass strengthening, is found to be a major contributor to strengthening in the HAZ. The proposed model predictions fall within 10% of experimentally measured properties for all three regions of the analyzed weldment.