Microstructure and properties of nanobainite steel joint by TIG welding

Nanobainite steel with a tensile strength of 1500MPa was welded by TIG welding process. The microstructure and fracture morphology of the joint were observed and analyzed by optical microscope (OM),scanning electron microscope (SEM),transmission electron microscope (TEM) and Xray diffraction (XRD). And the hardness and tensile strength of the joint were measured. The results show that the weldability of nanobainite steel was poor,because the microstructure in fusion zone and quenched zone were almost brittle martensite with hardness as high as 1000HV. Solute elements in nanobainite steel tended to partition into liquid and hence to segregate toward the boundary between dendrite arms,which formed austenite at the boundary between dendrite arms. Cold cracks were easily found in nanobainite bainitic steel weld which were brittle intercrystalline fracture. Massive carbides grew in the tempered zone,which mainly consisted of M₇C₃ (M:Fe,Cr,Mn) and Fe₃C. And the amount of precipitated cementite increased with the increasing of peak temperature.