Effect of carbon content on carbide morphology and mechanical properties of AR white cast iron with 10-12% tungsten

Carbide morphologies of white cast iron containing 22% Cr and 10-12% tungsten with different carbon contents (2.34-3.20 wt.%) were investigated. Results indicated that for the as-cast alloys with no heat treatments, the addition of carbon changes the morphology of carbides during air-cooling in the presence of tungsten. Light microscopy analysis revealed that for an alloy with 2.3 wt% carbon, chromium carbides possess coarse gray appearance (GA). Increasing the carbon content reduced the coarse GA zones volume fraction while a finer GA zones emerged. The coexistence of coarse and fine GA phases came to an end at 2.8 wt% carbon, at which only fine GA zones spread throughout the chromium carbide phase. Scaling up the carbon content to 3.2 wt% led to the formation of tungsten carbide and austenite in a eutectic reaction. Both fine and coarse GA zones vanished while the tungsten carbides acquired fishbone-like morphology. Upon heat treatment, the coarse GA zones vanished completely and turned into island appearance (IA) of chromium carbide. On the contrary, the finer GA zones remained unchanged after heat treatment and they coexisted with the IA. After heat treatment, the fishbone morphology shattered apart, however, the hyper chromium carbides remained unchanged. X-ray diffraction, EDS, and electron microscopy identified the coarse GA zones microstructure to be mainly dendritic chromium carbide together with tungsten carbide and austenite phase residing in-between the chromium dendrites. After carrying out the proper heat treatment protocols, the chromium carbide in the coarse GA zones in the as-cast structure dissolved into tungsten carbide and martensite that were dispersed within the chromium carbide with island appearance (IA). Mechanical wear and hardness tests showed that the samples with higher IA volume fraction attained better wear resistance and higher hardness after heat treatment. The enhancement in the mechanical properties could be attributed to (i) the precipitation of chromium carbide in the form of IA morphology inhibited the crack propagation in the matrix. (ii) The precipitation of tungsten carbide both improved the matrix wear tolerance and promoted the transformation of austenite to a more wear-resistant and harder martensite phase. (C) 2012 Elsevier B.V. All rights reserved.