Physical and tribological properties of Nitrided AISI 316 stainless steel balls

AISI 316 austenitic stainless steel balls (diameters 5.0 and 12.0 mm, typical hardness 250 HV0.3) and flat samples (20×20×2.0 mm) were nitrided by a pulsed glow discharge Ar/N2 plasma. Hardness of the ball surfaces was analyzed using Vickers indentation. Thermal stability of the nitrided balls (diameter 12.0 mm) was studied using a furnace to heat them in air for 8 hours at temperatures up to 700.0 ℃ and then, after cooling to room temperature, the surface hardness of the heated balls was re-measured. Scanning electron microscopy and X-ray diffraction were used to study the microstructures, composition and phase formation of the nitrided sublayers. Unlubricated pin-on-disc wear testing was used to evaluate the wear resistance of nitrided stainless steel balls (5.0 mm diameter) and the results were compared with similar testing on hardened Cr-Steel balls (5 mm diameter with hardness of about 650 HV0.3). All the test results indicated that the nitrided AISI 316 austenitic stainless steel balls have advantages over the hardened Cr-Steel balls in terms of retaining high hardness after heat treatment and high resistance to sliding wear at room temperature under higher counterpart stress. These properties are expected to be beneficial for wide range of bearing applications.