Mechanical properties of layers obtained by DC-pulsed plasma nitriding on a low-alloy steel (AISI 4140)

An industrial DC-pulsed plasma nitriding (DCPPN) equipment was used to obtain samples of an AISI 4140 low-alloy steel with five different configurations: substrate, substrate + diffusion zone, diffusion zone, substrate + diffusion zone + compound layer, and diffusion zone + compound layer. The microstructures of the samples were investigated by scanning electron microscopy and X-ray diffraction. Microhardness and tensile tests were performed, and the fracture mechanisms for the different materials were finally analyzed and discussed. The DCPPN produced an increase in the elastic modulus as well as in the yield strengths, together with a pronounce ductility loss, associated to the brittle intergranular fracture surface due to the nitride precipitation in the grain boundaries.