Plasma nitriding and its influence on contact fatigue of sintered steel

Plasma nitriding treatment in combination with shot peening is a promising way of material processing to improve contact fatigue resistance of PM components made of prealloyed Astaloy CrL, powder. This powder was used to manufacture four material variants differing by carbon contents 0.3 % and 0.7 % C, and by surface status (as received and shot peened). Tests were done on a pin-on-disc type device and evaluated in terms of formation of pittings at 50 x 10(6) cycles. They revealed the positive effects of plasma nitriding. In the material with 0.3 % C, the fatigue resistance increased by 65 % and if the nitriding was preceded by shot peening, it increased by 172 %. In the material with 0.7 % C, the corresponding increase was 38 % and 107 %, respectively. The surface nitrided layer, the mechanisms of its damage, and causes of pitting formation and growth depending on Hertzian stresses are described.