Improvement in Surface Properties of M50NiL Steel by Plasma Nitriding with Rare Earth Addition

The quenched M50NiL steel was plasma nitrided at 560 °C with and without rare earth (RE) addition. The effect of pure RE lanthanum (La) on the surface properties of plasma nitrided M50NiL steel was investigated using pin-on-disk tribometer. The treated surfaces were characterized using SEM equipped with EDS, XRD and microhardness tester. The results show that the presence of La obviously changes the surface morphologies and phase structures of the nitrided layers. The Fe4N phase appeared on the plasma nitrided surface is replaced by the LaFeO3 phase after introducing La into nitriding atmosphere. A suitable amount of La addition could accelerate the nitriding process by regulating the nitrogen concentration on the nitrided surface. The wear rates of plasma nitrided specimens are apparently lower than that of untreated one. The involvement of La could further reduce the wear rates of plasma nitrided specimens. The specimen plasma nitrided with La addition of 2.5 g demonstrates the best wear resistance with the wear rate of 3.01 ± 0.4 × 10−5 mm3/Nm. The transition of wear mode between adhesion and abrasion is mainly due to the differences in the phase structures and their proportion in the nitrided layers.