Surface Behaviors of NM400 Steel Quenched by Laminar Plasma Jet

As a commonly used wear resistance material, the NM400 steel is widely used in various mechanical parts. For prolonging the service life of these mechanical parts, various surface quenching methods are always adapted to improve their wear resistance. As a novel surface quenching method, a homemade laminar plasma surface quenching system was used to quench the NM400 steel for studying the quenching mechanism and the coupling mechanism of the main quenching parameters in this article. First, based on the orthogonal experimental design method, the corresponding experiments were conducted to decide the optimal processing conditions by using the orthogonal experimental range analysis. Sequencing, the microstructures, and wear resistance behaviors of the NM400 steel hardened under optimal processing conditions were tested. Besides, for revealing the hardening mechanism of NM400 steel, a corresponding homemade thermal-solid coupling model was built to explore the temperature distribution of the sample. Finally, the corresponding microstructure and wear resistance were discussed in detail based on the temperature field simulation and experimental results. The study results show that: 1) the NM400 steel could get the best-hardened effect under the following processing conditions: the quenching velocity, quenching distance, and arc current are 160 mm/min, 50 mm, and 110 A, respectively; 2) the microstructure in the hardened zone (HZ) is transformed from ferrite and cementite into lath martensite, while a little part of the microstructure in the heat-affected zone (HAZ) is transformed into martensite and residual austenite; and 3) the microhardness and the absorption work of the HZ are improved from 245 HV0.2 to 415 HV0.2 and 7.52 to 8.36 J, respectively, while the volume wear loss rate of the HZ is decreased from 1.13E-5 to 6.16E-6 mm3/(N $\cdot $ m).