Effects of Gas Flow Ratio on the Properties of Tool Steel Treated by a Direct Current Plasma Nitriding Process

Abstract

Nitriding treatments were conducted on tool steel (SKD 61) at a temperature of 500oC for 5 hr using high vacuum direct current (DC) plasma, with ammonia and argon as source gases. The structural and compositional changes produced in the nitrided layers by applying different ratios of Ar to NH3 (nAr/nNH3) were investigated using glancing x-ray diffraction (GXRD), optical microscopy, atomic force microscopy (AFM), micro-Vickers hardness testing, and pin-on-disc type tribometer. Nitriding case depths of around of 50 mm were produced, varying slightly with different ratios of nAr/nNH3. It was found that the specimen surface hardness was 1150 Hv with nAr/nNH3=1, increasing to a maximum value of 1500 Hv with nAr/nNH3=5. With a further increase in ratio to nAr/nNH3=10, the surface hardness of the specimen reduced slightly to a value of 1370 Hv. These phenomena were caused by changes of the crystallographic structure of the nitride layers, i.e the g'-Fe4N phase only was observed in the sample treated with nAr/nNH3=1, and the intensity of the g'-Fe4N phase were reduced but new phase of e'-Fe3N, which was known as a high hardness, with increasing nAr/nNH3. Also, the relative weight loss of counterface of the pin-on-disc with unnitrided steel was 0.2. And that of nitrided steel at a gas mixture (nAr/nNH3) of 1, 5, 7, and 10 was 0.4, 0.7, 0.6, and 0.5 mg, respectively. This means that the wear resistance of the nitrided samples could be increased by a factor of 2 at least than that of unnitrided steel.