Laser hardening of TiN-coated steel

Abstract

The surface hardness of hardened carburizing steel is about the same as that required for the substrate of hard physically vapour-deposited coatings in practical applications. Accordingly, by changing the normal hardening- coating sequence to the reverse sequence coating-hardening, it should be possible to obtain usable hard coatings also on carburizing steels.This idea was studied in the present work. The hardening treatment was performed with radiation from a 2.5 kW CO2 laser using different beam scanning velocities and laser power densities. The tracks of the beam on the TiN-coated carburized steel samples were studied with regard to substrate hardness, hardening depth and microstructure. The coating was studied using scratch tests to determine the critical load and possible adhesion defects.It was found that the hardness of the substrate was excellent and the maximum depth of hardening was 0.4 – 0.5 mm. The coating showed good adhesion provided that overheating was avoided, excessive laser power densities or insufficient beam velocities resulting in circular detachments of the coating and finally in incipient melting. The first signs of these detrimental phenomena appeared in the neighbourhood of sample edges where dissipation of absorbed heat was, for geometrical reasons, more restricted than in the middle parts of the sample. On the basis of the thermal expansion coefficients for steel and the coating some estimates were made concerning the stress relations between the coating and the substrate during a hardening cycle. It seems that compressive stresses are the probable reason for the detachments.