Characterization and development of FeCrW high speed steel coating for abrasive and impact wear applications

Abstract

XH-7 is a high speed steel alloy developed for use in surface coating to improve the impact and abrasive wear resistance of machine parts and tools. The desired structure of the material is a martensitic matrix with fine carbide and boride precipitates. In order to form these precipitates carbide forming elements such as: W, Cr, V, and Nb, are added to the alloy. It has been found that XH-7 has a tendency to form an area with a ferritic matrix phase when laser cladded. The thesis has been performed in order to solve the problem of scatter in the microstructure of laser cladded XH-7. In order to achieve this the thesis is split into two parts characterization, and development. During the characterization phase simulations, microscopy and mechanical testing is used to identify the phases present in XH-7, explain the solidification behaviour of the material, and explain the reason for the formation of the ferritic matrix. In this part of the thesis it was found that the softer ferritic matrix formed in the slowest cooling regions of the clad, while the faster cooling regions showed a martensitic matrix. Apart from the matrix phase both areas contained the same phases namely: NbC, W2B, and a eutectic structure made up of either W2B or M23C6, and fine grains of matrix phase. It was concluded that the ferritic matrix formed when the formation of NbC depleted the carbon content of the liquid. The reason this did not happen in the martensitic areas is caused by the high cooling rate which allowed austenite to begin solidifying before the carbon in the liquid had depleted significantly. During the development phase several alternative alloys to XH-7 were developed. With the goal of preventing the formation of ferrite. The two most promising were labelled XH-7B and XH-7C.