Advances in surface engineering using TIG processing to incorporate ceramic particulates into low alloy and microalloyed steels - a review

The application of surface engineering techniques to improve the surface properties of carbon steels using high powered lasers for transformation hardening and surface melting is well established. Based on this previous research, a tungsten inert gas torch (TIG) technique has more recently been explored for the surface modification of steels, as a much cheaper option to lasers. In the present research, initial studies compared the preheat temperature recorded on a low alloy steel with Ar, He and N protective shielding gases over a single track length. The effect of overlapping 17 tracks on the temperature variation for three different gases was also explored. These studies lead to Ar being the chosen gas for the next stages of the work. During TIG processing, incorporation of fine TiC or SiC ceramic particulates into the liquid steel was investigated, with the aim of obtaining a uniformly high hardness in a crack and porous- free melt zone of sufficient length and depth to provide improved wear resistance over the parent steel. TiC particulates of 45-100µm size were preplaced on a low alloy steel, and following TIG processing, the hardness increased from the as-received steel value of ~200 Hv to~800 Hv, due to some dissolution and re-precipitation of TiC particulates. The incorporation of the more economic SiC particulates of ∼5μm or ∼75 μm size preplaced on a microalloyed steel was investigated. Single track surface zones were melted by a tungsten inert gas torch, and the effect of two energy inputs, 420 and 840 Jmm−1, compared. The results showed that the samples melted using 420 Jmm−1 were crack-free. Analytical microstructural and XRD studies established that both sizes of SiC particulates dissolved, and that some of the hardness increase recorded was due to formation of a high carbon martensite. A potential method of decreasing SiC particulate dissolution by generating a high Fe–Si liquid, thereby retaining the ceramic in the microalloyed steel after processing, was found to show promise.