Laser surface hardening of bearing steels
Date of Issue2019-04-24
School of Mechanical and Aerospace Engineering
Advanced Remanufacturing and Technology Centre
Laser surface hardening has emerged as a promising technique to improve the wear resistance of machine components such as bearings, gears and camshafts. However, a lack of thorough understanding of the underlying mechanism coupled with the stringent requirements on surface properties still hinders its widespread application in industries. This thesis investigates various aspects of laser surface hardening of bearing steels (such as effect of operating parameters and laser types, microstructural changes, corrosion properties and improving effectivity) with an aim to elaborate the underlying mechanism and improve surface properties. The laser hardening has been demonstrated using a continuous wave fiber laser which produces hard outer case owing to the martensitic transformation due to rapid heating and self-quenching. Hardness as high as 844 HV is achieved near the surface with depths up to 500 μm. A parametric study on the effect of various laser operating parameters such as defocus distance, power, speed and overlapping has been carried out which can be a significant input for designing of laser hardening system. Moreover, millisecond laser is shown to have similar hardening effect as continuous wave laser using lower power than continuous wave laser. Further investigations with ultrashort pulsed lasers exhibit ablation capability than surface hardening. This can have potential application in surface cleaning and repair. Additionally, the corrosion properties of gradient microstructure produced by laser hardening are also explored by performing layer-by-layer electrochemical tests. The study shows that an improved corrosion resistance is found at the surface layer due to formation of chromium rich oxide layer and phase transformation, while the sub-surface does not show significant improvement in corrosion properties. Moreover, a novel gas assisted laser hardening is proposed to improve the effectiveness of laser hardening and achieve superior surface properties. The tests are performed in a specially designed chamber for better controllability of the process. Among different gases used, propane is found to have profound beneficial effect on the steel surface with maximum hardness reaching as high as 900 HV even for low carbon steel. This is attributed to the influx of carbon from the gas into steel which forms a carburized region near the surface. Depending on the parameters used, different microstructures are obtained in the near surface region ranging from eutectoid to eutectic composition. The potential applications of this technique to modify surface properties are also discussed.