Hardness analysis of welded joints of high manganese austenitic cryogenic steel for LNG tanks

Objective Due to the outstanding low cost and superior cryogenic performance, high manganese austenitic cryogenic steel is proposed to replace expensive 9Ni steel and become a competitive steel material for the construction of clean energy LNG tanks. In application, determining hardness and its range of welded joints takes an important role in welding and testing. The aim is to investigate hardness and its variation of welded joints of high manganese austenitic cryogenic steel to provide guidance for the actual welding and testing process. Methods In this paper, three welding methods were used to prepare welded joints of high manganese austenitic low-temperature steel by submerged arc welding (SAW), shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW), and microstructure and morphology were observed by optical microscope and Vickers hardness test was carried out. Results The result shows that hardness changes from low to high from weld center to base metal. The maximum hardness was detected in the heat-affected zone (HAZ) near base metal. Hardness of base metal after several welding thermal cycles was higher than that after one or two welding thermal cycles. Hardness of welding HAZ with different welding methods can be divided into five characteristic zones. Conclusion Hardness of welded joints of high manganese austenitic cryogenic steel is all lower than 296 HV, the highest hardness appeared in SMAW joints located in the L₁ line of Ⅳ zone. The test data is expected to provide the basis for hardness test methods of welded joints of high manganese austenitic cryogenic steel.