Conventional heat treatment of a component is performed in a furnace whereby the component is continuously heated to a high temperature and quenched to get a hardened material. The furnace heating and tempering process take several hours which is expensive over a long run and less flexible. Laser transformation hardening is an attractive heat treatment technology which can be used to enhance the surface properties of highly stressed components such as cams, gears, and bearings without altering its bulk properties. The highly intense laser beam rapidly heats up the irradiated surface above its austenitization temperature which cools down instantaneously (self-quenching) as the laser moves away from the spot producing a hardened surface. The fast heating and cooling generate a non-equilibrium phase transformation of which very little is understood. An attempt was made to improve the surface properties of steel through solid solution hardening and microstructure refinement using a 250 W fiber laser. To identify the effect of various parameters on laser hardening, scanning conditions such as beam spot size, scan rate, power input, surface condition and overlap ratio were controlled. The change in hardness and morphology of laser treated surface were carefully investigated. The results show the surface hardness increased above 800 HV after laser treatment compared to 260 HV of the as-received specimen. It is found that austenitization has the highest effect on hardness achieved and can be controlled by proper choice of laser parameters and scanning rates.
Unwanted removal of carbon from surface may occur during laser surface processing of steels despite the short interaction time and thermal cycle. However, no attention is paid in literature to investigate this phenomenon systematically. This paper presents two different scenarios during laser surface processing of steels: complete absence of decarburization for an alloy steel but decarburization with depth up to 70 μm for a plain carbon steel, showing that alloying elements tend to retard decarburization process by reducing the mobility of carbon in austenite. Further analysis reveals that the laser induced decarburization is dependent primarily on peak temperature and austenitization kinetics.
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