Multi-cycle phase transformation during laser hardening of AISI 4140

Abstract

Laser hardening is a process that modifies not only hardness, but also residual stresses near the surface. However, limited information is available on the effect of multi-cycle phase transformation on the hardness and residual stresses, which could be a possibility to achieve different material modifications. In this paper, the effects of multi-pass laser hardening on the material modifications of quenched and tempered as well as ferrite/pearlite AISI 4140 steel have been investigated. The multi-cycle phase transformation process has been carried out using a continuous wave laser with a rectangular beam shape using different schemes, without heat accumulation between each cycle. Results showed that material properties were modified with accumulation effects within the given pass up to 16 cycles. With increasing passes, it was found that the induced compressive residual stress at the surface decreased despite of the initial material state, while the hardness in the subsurface area and hardening depth increased differently, which depended on the initial heat treatment condition. These results indicated the possibility of tailoring the material hardness and surface residual stress by the given heat treatment condition, laser process and cycle number in the multi-pass laser hardening.