Effects of TIG Surface Melting and Chromium Surface Alloying on Microstructure, Hardness and Wear Resistance of ADI

Microhardness and wear resistance of different microstructures formed by TIG (tungsten inert gas) surface melting and chromium surface alloying (using ferrochromium) of ADI (austempered ductile iron) were studied. Surface melting resulted in the formation of a ledeburitic structure in the melted zone, and this structure has a hardness up to 896 HV as compared to 360 HV in that of ADI. Moreover, chromium surface alloying resulted in the formation of different structures including: (1) a hypereutectic structure consisting of primary (Fe, Cr)7C3 carbides and a eutectic matrix of transformed austenite (into martensite and retained austenite), as also (Fe, Cr)7C3 carbides, with a hardness of 1 078 HV; (2) a hypoeutectic structure consisting of the same eutectic along with transformed primary austenite, with a hardness of 755 HV; and (3) a ledeburitic structure with an acicular morphology and a hardness of 896 HV. The results also indicated that surface melting reduced the wear rate of the ADI by approximately 37%. Also, chromium surface alloying yielded a superior wear behavior and reduced the wear rate of the treated specimens by about 38% and 70%, depending on the structures formed.