Wear behaviour of plasma nitrided tool steels

Plasma nitriding of three grades of tool steels, namely H13, D2 and a speci al purpose proprietary tool steel, referred to as L7', has been explored in an effort to enhance the working life of roll entry (RE) guides in wire ro d rolling mill that are subjected to a complex wear mode including impact, sliding and rolling. In the case of H13 and D2 steels, plasma nitriding is found to improve the life of guide rolls by two to three times depending up on the type of tool steel. The working life of the guide rolls made from L7 ' steel, however, was observed to be lower after plasma nitriding due to so ftening of the substrate at plasma nitriding temperature. The cross-section normal to wear scar and the surface of worn-out rolls were characterised b y scanning electron microscopy (SEM) to understand the wear mechanisms. The SEM examination of worn-out surfaces revealed signatures for the adhesion, abrasion, delamination and tribochemical (oxidative) modes of wear. In the case of rolls without plasma nitriding, adhesion was one of the important causes of wear in all the tool steels. Delamination wear occurred in H13 st eel and both delamination and microcutting modes of wear contributed to the overall damage in D2 steel rolls. L7' steel showed breaking of surface oxi de film, indicating tribochemical wear. Plasma nitriding decreased the adhe sive wear substantially. Delamination was found to be the primary mode of w ear in nitrided H13 steel rolls. Abrasive wear contributed to damage in nit rided D2 steel rolls. Severe roll damage occurred in L7' steel, primarily b y microcutting, due to softening during plasma nitriding. The working Life of the rolls has been deliberated upon in the light of wear mechanisms obse rved in the different tool steels. (C) 1999 Elsevier Science S.A. All right s reserved.