MECHANICAL-PROPERTIES AND STRUCTURE OF CVD-COATED AND POST HEAT-TREATED STEELS

Post heat treatment of high temperature (HT)-CVD TiN-coated steel subs trates 42CrMo4 (4140) and X155CrVMo12-1 (D2) was investigated. The des ired substrate structures and hardness could be achieved by post quenc h hardening and tempering. The coating structure seemed unchanged unde r light microscopy. The support of the coating by higher substrate har dness was confirmed by higher critical loads in scratch tests. Technol ogical experiments, such as bending tests, alternating bending tests a nd notched-bar tests, were performed. CVD TiN-coated/post hardened and tempered, and uncoated hardened and tempered steel 4140, reached the same bending 0.2%-yield strength, lifetime at alternating bending, and notch toughness. Bending strength and lifetime at alternating bending of CVD TiN-coated D2 were low compared to the uncoated quench-hardene d state because of decarburisation during deposition. These difference s disappeared after post quench hardening and tempering. TEM-investiga tions of the coating/substrate interface were done to explain the stru cture and mechanical properties of CVD-coated and post heat-treated st eels. (C) 1997 Elsevier Science S.A.