LASER SURFACE HARDENING OF CRANKSHAFT

The present work involves systematic study on identification of process parameters and processing conditions for effective laser surface-hardening of automotive crankshaft and its implementation in the industry, utilizing a diode laser integrated to a 6-axis robot and a turn/tilt table. The crankshaft chosen was made of low-alloyed 0.52% C steel and required hardening at two contact regions of bearing/flange seat areas and a pin area (on a different axis than the actual shaft). The subjected areas had features like oil holes, sharp corners and wide areas. The target was to develop laser hardening process resulting in hardened case-depth of above 200 µm with a hardness of 500 - 650 HV at different locations mentioned. Additionally, It was targeted to minimize the processing time and also eliminate any post process machining operations. Suitable robotic processing sequence has been programmed to regulate the power and scan speed at different locations and to complete hardening of all the required surfaces of the actual crankshaft in a single step.

The effect of laser processing conditions on treated layer has been characterized for case dimensions, hardness, microstructure, surface finish and residual stress. Results of the treated layer of crankshaft material, under optimized process conditions, indicated uniform hardness in the range of 500-650 HV with microstructure comprising refined lath martensite and few carbides and about 2.4% retained austenite. The net residual stress was also found to be in the range of −312 MPa indicating compressive condition and as a result envisaging improvement in fatigue life. Various process optimization steps incorporated to obtain required hardened surfaces on a crankshaft at a good processing speed with negligible surface damage are presented. Crankshaft subjected to compressor over-load testing, processed with optimized conditions, indicated improved life and performance envisaging its adoption in automotive industry.