The effect of nitrogen alloying to the microstructure and mechanical properties of martensitic stainless steel hardfacing

• Prior austenite grain size and the martensitic lath can be effectively refined by nitrogen additive;
• Mechanical properties of the hardfacing are improved, and the fracture mode is changed to ductile transgranular fracture;
• Martensitic lath width of the hardfacing is effectively equal to grain diameter D in the modified Hall-Petch Equation.

The effect of nitrogen additive on the microstructure and mechanical properties of martensitic stainless steel hardfacing was investigated in this work. The phase structure of the hardfacing was analyzed by X-ray diffraction (XRD). The microstructures with and without nitrogen additive were observed by field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The mechanical properties of the hardfacing were measured, and the fracture surfaces were observed by FESEM. The equilibrium phase diagram and phase precipitation rule were calculated by ThermoCalc software. The results show that, by replacing carbon with nitrogen in hardfacing, the martensitic lath is refined, and the segregation and precipitation on prior austenite grain boundary cannot be found. In addition, the mechanical properties of the hardfacing can be improved obviously after composition optimization, in which, the yield strength and tensile strength are increased from 884 MPa and 996 MPa to 1078 MPa and 1554 MPa respectively. Meanwhile, the ductile of the hardfacing is significantly increased. The fracture surface is transformed from brittle intergranular fracture into ductile transgranular one.