Microstructure and mechanical properties of the modified layer obtained by low temperature plasma nitriding of nanocrystallized 18Ni maraging steel

In the present study, low temperature plasma nitriding of nanocrystallized 18Ni maraging steel has been carried out at 360 °C from 1 to 24 h in a mixed gas of 25%N2 + 75%H2. The surface phase constitutions and microstructures of the nitrided layer have been investigated by X-ray diffraction analysis, transmission electron microscopy and optical microscopy. Nanoindentation and microhardness tests have been performed to determine the surface hardness and the hardness profile in the nitrided layer. The plasticity of the nitrided surface has been analyzed based on the nanoindentation results. The results show that at the initial stage of nitriding, the surface phase consists of a solid solution of nitrogen in α-Fe, and nanoscale nitrides and aging phase are formed with increasing of treatment time. The surfaces nitrided for 8 and 16 h possess the highest hardness. The plasticity factor calculations suggest that the nitrided surfaces have a good wear resistance and possess excellent plasticity.

► A 20 μm nitrided layer was formed on nanocrystallized maraging steel by nitriding at 360 °C for 1 h.
► A new phase FeN0.076 with higher hardness was found when the nitriding time was up to 8 h.
► The nitrided surface layer possesses excellent plasticity and it decreases with nitriding time.