Dominant factors of cryogenic toughness of heat-affected zone of welding in high Mn austenitic steel

ABSTRACT

High Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperatures by adding a large amount of Mn which are γ-stabilizing elements. In order to clarify the factors governing the cryogenic toughness of heat-affected zone (HAZ) of welds, investigation of relationship between microstructure and Charpy impact toughness of multi-pass welded joint and a simulated thermal cycle test was conducted for high Mn steel containing 0.5%C-25%Mn-5%Cr. In the MAG welding, HAZ near fusion line showed high absorbed energy, whereas it was minimized at 5 mm towards the HAZ side from fusion line reheated up to around 1073 K. Furthermore, in a simulated thermal cycle test of continuous cooling, the absorbed energy was also minimized at 1073 K. On the other hand, in the simulated thermal cycle test of quenching, the inverse dependence of the grain size on a conventional steel was shown, and the toughness increased as the grain size became coarse. Grain growth occurs according to the maximum temperature of HAZ, and M₂₃C₆ is formed on the grain boundary in the subsequent continuous cooling process. It was clarified that M₂₃C₆ became only initiation site of fracture under Charpy impact loading, and its existence density was the main factor of initiation energy. On the other hand, propagation process is a ductile fracture phenomenon in which γ stability is maintained regardless of the presence of M₂₃C₆.

KEYWORDS:

• Welding
• manganese
• austenite
• Charpy impact toughness
• grain size
• carbide
• ductile fracture
• stacking fault energy