Plasma nitriding of 42CrMo low alloy steels at anodic or cathodic potentials

The nitriding of low alloy steel has been carried out at anodic potential in a space enclosed by an active screen that consists of two cylinders with different diameter. These two cylinders made up a hollow cathode in a discharge system. The difference in diameter of the two cylinders is about 8–10 mm to maintain strong discharge between them. They can also be heated rapidly to the required temperature for treatment. The sample to be nitrided was held at the same potential as that of the anode used in the discharge and heated through heat radiation from the hot cylinders and by electron bombardment. Electrons bombarded the surface of the sample even though the intensity of bombardment was low because of the anodic sheath. To illustrate the effect of the anodic potential on the nitriding a comparison was made between nitriding at anodic and cathodic potential (general plasma nitriding). The phase composition, the compound layer thickness and the surface topography of the nitrided layer, as well as its properties, were investigated using X-ray diffraction, scanning electron microscopy and microhardness tester. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarization tests in 3.5% NaCl solution. The results showed that anodic plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the low alloy steel.