Effect of Deep Cryogenic Treatment on Microstructure and Mechanical Properties of Friction Stir Welded TRIP590 Steel Joints

In this study, friction stir welding was first applied to the 1.4 mm thick TRIP590 steel sheets at a constant transverse speed of 100 mm/min and different rotation speeds from 200 to 500 rpm. Then, the obtained joints received deep cryogenic treatment in liquid nitrogen for 24 and 48 h, respectively. It was revealed that the content of retained austenite in the stir zone of the welded joints decreased from 3.3% to 0.2% when the rotation speed increased from 200 rpm to 500 rpm. The stability of retained austenite increased due to grain refinement and work hardening at low rotation speeds. After deep cryogenic treatment of the welded joints, the retained austenite in the stir zone partially transformed into martensite, which led to the precipitation of nano-sized carbide in the ferrite matrix and the release of local stress. As a result, both the strength and plasticity of the stir zone after 48 h of deep cryogenic treatment increased from 798 MPa, 15% to 927 MPa, 17% for the 200 rpm joint, and from 914 MPa, 14% to 1086 MPa, 16% for the 300 rpm joint during the tensile tests.