Surface hardening of low-alloy 15CrNi6 steel by CO2 laser beam

The study investigates laser transformation hardening in the low-alloy 15Cr Ni6 case-hardening steel. The effect of process parameters such as beam pow er, beam diameter and travel speed on microstructure, case depth and hardne ss was examined. In addition, the effect of surface preparation was also in vestigated. For this purpose, all specimens were initially sandblasted, and some were additionally coated with graphite in order to enhance surface ab sorptivity. In most cases, a heat affected zone (HAZ) formed below the surf ace. This consisted of two discrete areas, a surface layer and a transition area between this surface layer and the base metal. The microstructure of the surface layer was found to consist of lath martensite, while carburizat ion was found for certain process conditions in the graphite-coated specime ns. The transition area consisted of a dispersion of fine carbides in a fer rite matrix. A substantial increase in surface hardness was achieved, by a factor of 2.5 times the base metal hardness. Depths of the HAZ up to approx imately 0.6 mm were obtained, without surface melting. The additional use o f graphite coating enhanced, in most cases, the coupling of laser-beam to t he surface resulting in greater HAZ depths. (C) 1999 Elsevier Science S.A. All rights reserved.