Fatigue Characteristics of Laser Surface-Hardened Cast Irons

Contrary to expectations, laser surface heat treatment has a deleterious effect on the fatigue performance of pearlitic gray and ductile cast irons. A 1.2 kW, continuous wave, CO₂ gas laser, operating in square beam mode, was employed to heat-treat the surfaces of standard fatigue specimens. Rotational-bending fatigue tests were then conducted on untreated (as-cast) and laser treated specimens. Results indicated that the effect on fatigue behavior of case depth, microstructure and hardness of laser-hardened surface layers were opposite to that observed in carbon and low alloy steels. A fracture model based on the presence of graphite, residual stresses, and strain-induced transformation is postulated to explain the adverse effect of laser hardening of cast irons.