Study of residual stress by neutron diffraction in friction stir welded armor steel

Friction stir welding (FSW) is a solid-state process that minimizes heat-affected zone (HAZ) softening due to its lower heat input compared to arc welding, making it suitable for joining armor steels with excellent ballistic resistance. However, research on the residual stresses induced by FSW in such materials remains limited. This study examines the residual stresses in three types of FSW welds: similar rolled homogeneous armor (RHA) steel, similar high-hardness armor (HHA) steel, and a dissimilar weld between the two. Residual stresses were measured using neutron diffraction in the three principal directions to determine their magnitude and distribution in the welded armor steels. The results were correlated with weld microstructure and hardness. Neutron diffraction revealed an M-shaped residual stress pattern in the longitudinal direction for all welds, with the highest stress magnitudes located in the softened HAZ. Maximum tensile residual stresses reached approximately 70% of the yield strength of the materials. Additionally, full width at half maximum (FWHM) consistently indicated low levels of microstrain in the softened region due to over-tempering and grain growth in the HAZ.