EFFECT OF RARE EARTH ELEMENTS ON PLASMA NITRIDING OF 38CrMoAI STEEL

Plaslna nitriding of steels has been intensively investigated and has found many applications in several industrial sectors owing to the excellent combination of properties obtained, i.e. good tribological performance and high fatigue strength. Although some preliminary research work has reported that rare earth elements can speed up most thermochemical treatments, little attention has been paid to the effect of these elements on the plasma assisted thermochemical processes. In the present investigation, plasma nitriding of 38CrMoAl steel has been carried out in order to investigate the effect of introducing rare earth elements on the process and on the structures and abrasive wear resistance of the resultant sUljace layers. Systematic characterisation of the surface layers has been perfonned elnploying optical and electron microscopy (morphology of worn surface), X-ray diffraction (phase identification), energy dispersive X-ray analysis (composition), and microhardness testing. Abrasive wear tests were also conducted using a wheel on wheel configuration with hard iron borides as an abrasive medium under different loads. The results showed that rare earth elements can effectively speed up the nitriding process, and this thicker nitrided case showed improved abrasive wear resistance. Based on the experimental results, the mechanism by which rare earth elements speed up the plasma nitriding process has been discussed in terms of sputtering induced defects (vacancies and dislocations), conjugation of rare earth species to these defects, and lattice distortions surrounding the rare earth atoms, which facilitate fast shortcircuit diffusion of nitrogen.