Optimization of Microstructure at Ni-C steel joint by friction stir welding with CO₂ cooling

Abstract

The effect of additional cooling by splaying liquid CO₂ during friction stir welding (FSW) on the microstructure change in the stir zone of Fe - 24 wt.%Ni - 0.1 %C alloy steel was investigated. When the cooling rate after the stirring increased by the adoption of a faster welding speed and the use of CO₂ cooling, a larger amount of retained austenite with lath martensite was present in the stir zone. The room temperature tensile test clarified that the retained austenite contributes to the high strength without loss of elongation due to the transformation induced plasticity (TRIP) effect.