A Study On Advanced Plasma Nitriding For42CrMo Steel

Plasma nitriding is a process to form nitriding layer using glowdischarge technology in a vacuum chamber, in which thin gas containingnitrogen is introduced. In the vacuum, high-voltage electrical energy isused to form plasma, through which nitrogen ions are accelerated toimpinge on the workpiece (as cathode, and vessel wall or extra metalplate as anode). This ion bombards the workpiece, loses energy, producesactive nitrogen, and diffuses into the interior.In this paper, AISI4140steel was chosen as the experimentalmaterial. the effect and mechanism of DC plasma nitriding catalyzed bypre-oxidation was investigated for42CrMo. Furthermore, AISI4140steelwas are nitrided by active screen plasma nitriding at500℃、550℃withdifferent nitriding time from2~6h. Additionally, AISI4140steel arenitrided by active screen plasma nitriding under gas pressure from180~340Pa. In this study, the nitride layer, phase structure, surfacemorphology and SFE of pre-oxidation surface were investigated bymeans of optical microscopy(OM), microhardness tester, X-raydiffraction(XRD), scanning electron microscopy (SEM) and contactangles, respectively.The results show that pre-oxidation had significant catalysis effecton plasma nitriding, and the maximum compound layer thickness of15μm was obtained after nitriding at500℃for4h under the optimumpre-oxidation (PO) condition of300℃and30min, which was more thantwo times thicker than that without pre-oxidation. The maximumeffective diffusion layer of570μm was obtained, which was muchthicker than that without pre-oxidation. XRD results showed that theoxide formed during oxidizing could be transformed to nitride afternitriding. The possible catalysis mechanism is due to the formation of auniformly nano-iron oxide particle along with some nanocracks andnanopores on the surface, which has the highest surface free energy. Theresults about Active screen plasma nitriding show that Active screenplasma nitriding (ASPN)is a novel nitriding technology, whichovercomes many inherent problems associated the conventional DC plasma nitriding (DCPN), such as difficulties in maintaining a uniformchamber temperature, obtaining uniform nitrided cases and gradualhardness distribution. Friction performance were improved under theoptimum gas pressure of260Pa.