Structure-property correlation in laser surface treated AISI H13 tool steel for improved mechanical properties

The present study concerns laser surface hardening (LSH) and melting (LSM) of AISI H13 tool steel using a high power continuous wave diode laser. Depth of surface hardened or melted layer increases with increase in incident laser energy density. Surface melting occurs at a higher laser energy density (>75 J/mm2) and leads to the formation of inhomogeneous microstructure comprising non-uniform distribution of retained austenite, carbides (along inter-dendritic boundary) and martensite with their respective volume fractions varying with depth. Application of intermediate laser energy density (50-75 J/mm2) yields a hardened layer with dispersion of ultrafine mixed carbides (M23C6, M7C3, MC or M2C). Laser treatment with a very low laser energy density (<50 J/mm2) leads to formation of an over-tempered microstructure consisting of low carbon martensite and coarse carbide precipitates. Micro-tensile studies with specially machined samples from the surface melted zone following LSM with a laser energy density of 100 J/mm2 records a high yield strength of 1310 MPa along with poor ductility, marked by brittle failure. On the other hand, a similar sample from laser surface hardened zone treated with a laser energy density of 62.5 J/mm2 yielded even higher yield strength of 1460 MPa with a maximum elongation of 3.6%. Though both LSH and LSM produced higher yield strength compared to hardened and tempered AISI H13 tool steel, LSH yielded a combination of higher elongation (3.6%), than that after LSM (0.97%), with high yield strength and hence was considered a better option.