Recast Zone and Unmelted Zone of the White Layer Induced by Microsecond Pulsed Laser Surface Processing of 42crmo4 Alloy Steel
T. Zhao, S. Eckert and F. Vollertsen
A common characteristic known as white layer can be observed at the surface of various steels machined by different processes. This paper studies the substructures of white layers induced by microsecond pulsed laser processing of DIN 42CrMo4 alloy steel. Besides recasting morphologies, grinding marks were still retained on both sides of the outer surface of white layers, while the highest temperature in the experiment was above the boiling temperature. Retained microstructural bandings were partially eliminated in white layers. These results indicated that the unmelted material in this process can also be changed to white layers through the solid-state phase transformation. The boundary between the recast zone and the unmelted zone can be regarded as the initial melting point isotherm, which is not that boundary between the entire white layer and the surrounding transition layer. Explanation based on deep welding and granular impact was made to describe the shaping of the white layer.
Keywords: Yb-fibre laser, 42CrMo4 alloy steel, microstructural banding, grinding marks, cavities, cracks, recast layer, solid-state phase transformation