Microstructure and Properties of 7Ni Steel Shielded Metal Arc Welded Joint for LNG Storage Tanks

With ENiCrMo-6 electrodes as welding materials, 7Ni steel plates with the thickness of 10 mm were subjected to shielded metal arc welding by using a multi-layer and multi-pass welding process with three passes on the front side and two passes on the back side. The microstructure, tensile properties, impact toughness, bending properties and hardness of the joints were investigated. The results show that the forming quality of the welded joint was good, and no defects such as cracks and pores were found in the weld. The weld of the joint was composed of nickel-rich dendritic austenite and precipitated phases, the fusion zone was mainly composed of massive austenite, the coarse-grain heat-affected zone was composed of coarse lamellar martensite and a small amount of residual austenite, and the fine-grain heat-affected zone was composed of fine lamellar martensite and a small amount of residual austenite. The hardness of the weld in the joint was the lowest, being 170−194 HV. The hardness of the heat-affected zone was the highest, being 280−299 HV. The hardness of the base metal was intermediate between the weld and the heat-affected zone. The tensile strength of the welded joint was 763.5 MPa. The average impact absorption energy at −196 ℃ of the weld and the heat-affected zone was 58.2, 59.3 J, respectively. No cracking occurred in the welded joint after side bending tests. The mechanical properties of the welded joints met the requirements of the LNG project.