Influence of multi-pass laser hardening of normalized AISI 4140 on the grain size

Abstract Laser hardening is a process to improve surface functionalities by controlling the surface properties of targeted steel, such as hardness, residual stress and microstructure. This study aims in investigating the hardness and grain size based on multi-pass laser hardening. This was conducted on normalized AISI 4140 steel with a controlled maximum temperature in each pass below melting point with different parameter combinations, followed by the Vickers hardness test, electron microscopic investigations for determining the grain size of the martensite and electron probe microanalysis for determining quantitative carbon content of the phase-transformed surface zone. The results showed that with increasing number of passes the hardness and the hardening depth increased. Meanwhile, the grain size of martensitic grains decreased with cycle numbers and with the distance to surface after multi-cycle re-austenitization. With increasing number of paths, the grain size distribution and the carbon content distribution, which were initially inhomogeneous due to ferrite and pearlite segregation lines, became more homogenous. These findings show that multi-pass laser hardening may be promising regarding the functional properties due to increasing the hardening depth and grain refinement as well as homogenization of the martensite grains.