Surface hardening of low-alloy 15CrNi6 steel by CO2 laser beam
The study investigates laser transformation hardening in the low-alloy 15CrNi6 case-hardening steel. The effect of process parameters such as beam power, beam diameter and travel speed on microstructure, case depth and hardness was examined. In addition, the effect of surface preparation was also investigated. For this purpose, all specimens were initially sandblasted, and some were additionally coated with graphite in order to enhance surface absorptivity. In most cases, a heat affected zone (HAZ) formed below the surface. 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 carburization was found for certain process conditions in the graphite-coated specimens. The transition area consisted of a dispersion of fine carbides in a ferrite 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 approximately 0.6 mm were obtained, without surface melting. The additional use of graphite coating enhanced, in most cases, the coupling of laser-beam to the surface resulting in greater HAZ depths. (C) 1999 Elsevier Science S.A. All rights reserved.