Effect of Gradient Microstructure and Strength Mismatch on Cryogenic Toughness of Fe-Ni Fusion Boundary

In this paper, axial magnetic field (AMF) was applied in gas metal arc welding to produced 9Ni steel joints filled with nickel-based alloy. The gradient microstructure and the cryogenic toughness of Fe-Ni fusion boundary were investigated. Under 0 mT AMF, the unmixed zone (UMZ) and partial mixing zone (PMZ) are observed. UMZ has a dual-phase microstructure of lenticular-like martensite and austenite. At low temperature, the retained austenite transforms into martensite, which aggravates the residual strain. PMZ is solid solution of austenite microstructure. UMZ has a nanohardness twice that of PMZ. Charpy impact test and CTOD tests are conducted at low temperature to evaluate the toughness of fusion boundary. The toughness decreases with the increment of mismatch degree. An abnormal drop of fracture toughness and brittle fracture occurs at 0 mT fusion boundary. Under loading, a high strain gradient occurs between UMZ and PMZ due to the high strength mismatch. Crack preferentially initiates and propagates along the interface. When UMZ and PMZ are eliminated under 12 mT AMF, the fusion boundary has a more compatible deformation and no brittle fracture occurs. It demonstrates that AMF decreases the strength mismatch and improves the cryogenic toughness of Fe-Ni fusion boundary.