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.

KEYWORDS:

• Friction stir welding
• CO₂ cooling
• Ni-C steel
• retained austenite
• mechanical properties
• transformation-induced plasticity

Acknowledgments

This study received support from the Industry-University Joint Basic Research ‘Heterostructural Control’ of the Japan Science and Technology Agency (JST) and the Global COE Program, and a subvention from the Japan Society for the Promotion of Science for which we should like to express our gratitude.