Metallurgical and mechanical properties of high nitrogen austenitic stainless steel friction welds

Friction welding of high nitrogen austenitic stainless steels was carried out using a pressure servo-control system brake type device. The welding parameters were 2 400 rpm for rotational rate, 70 MPa for 4, 7, 10 and 15s for friction pressure, and 150 MPa for 6s for upset pressure. As the friction time increased, the fully plastically deformed zone (Region I) in the vicinity of the bond-line increased. In contrast, an increase in friction time decreased the region (Region II) where the grains were partly deformed and grown. The TEM examination suggested that the intergranular phases precipitated in the vicinity of the bond-line are Cr2N (Hexagonal, a=0.48113 nm, c=0.44841 nm) and CrN (Cubic, a=0.4140 nm). Tensile test results indicated that high nitrogen stainless steel joints are considerably higher in the tensile strength than the commercial stainless steel SUS31 6L or SUS304 joints. However, for all the welding conditions, the joint strength of high nitrogen HNS-1 or HNS-2 joints was slightly lower than that of the base material. Furthermore, the detailed fractographic observation confirmed that the rupture occurred near the bonding interface. The inferior tensile strength of the nitrogen-containing austenitic stainless steel joint could be attributed to the Cr-nitrides precipitated near the bonding inter-face.