Microstructure and mechanical properties of 9% nickel steel welded by FSW

Avoiding the use of nickel alloy as wire filler material in fusion welding of 9% Ni steels requires high–power density welding processes such as laser and electron beam. However, autogenous welding with these processes reduces the Charpy energy absorbed due to the chemical microsegregation and the precipitation of low-toughness phases. The friction stir welding (FSW), a solid-state welding process that uses a non-consumable tool to stir the material of plates to be welded, may improve the mechanical properties due to the low peak temperatures in the stirred zone (SZ), during processing. This work evaluates the influence of the second pass on the first pass in the 9% Ni steel joint welded by FSW. The samples were welded on both sides with a partial penetration of about 6 mm. The first weld bead reduces the volumetric fraction of retained austenite (VFRA) in the SZ by 89%, and in the thermomechanically affected zone (TMAZ), the retained austenite is mostly located at the deformation bands. The second pass increases the energy in the Charpy V test (subsize) by 17% in the first SZ. The energy in the Charpy V test (full-size) is just 23% lower than that found in the base metal (BM). The second pass reduces the VFRA but improves the Charpy energy absorbed by SZ of the first pass. The volumetric fraction of martensite-austenite (VFMA), VFRA, and grain size are the significant factors in the prediction model for the energy absorbed in the Charpy V test.