Influence of laser hardening on mechanical properties of a low alloy steel

Abstract

An investigation was carried out to examine the influence of laser hardening on the mechanical properties of 300M steel. The mechanical properties(including high cycle fatigue strength) of 300M steel surfaces hardened by several conventional techniques such as shot peening, carbonitriding, and induction hardening have been compared with the mechanical properties of CO₂ laser hardened specimens. The effects of such sUlface coatings as manganese phosphate and colloidal graphite on the laser hardening were also examined. The influence of all these hardening methods on the nature and magnitude of the resulting residual stresses was studied. The results of the present investigation indicate that the induction hardening produces the largest improvement in high cycle fatigue strength. Laser hardened specimens with manganese phosphate coating also showed significant improvement in high cycle fatigue strength over untreated specimens. In contrast, the high cycle fatigue strength of the graphite coated laser treated specimens was lower than that of untreated specimens. Test results also show that laser hardening produced surface hardness comparable to that achieved by induction hardening. However, the ductility of the laser hardened specimens (both manganese phosphate and graphite coats) was significantly lower than that of the induction hardened specimens. The major problem appears to be the formation of a diffusion bonded phosphate layer on the interface of manganese phosphate coated specimens.The lower fatigue strength and ductility of the laser treated graphite coated specimens appear to be a result of surface melting. Induction hardening introduced significant amounts of compressive residual stresses into the surface of these specimens.