The wear behaviour of high-chromium white cast irons as a function of silicon and Mischmetal content

The sliding wear behaviour of high-chromium white cast iron (16.8% Cr) has been examined as a function of silicon and Mischmetal alloy additions (1, 2, 3 and 5% Si and 0.1 and 0.3% Mischmetal). Such additions are known to modify the structure, but there is considerable controversy as to the exact effect. Silicon was found to refine the dendritic structure and increased the eutectic carbide volume fraction. However, for contents above 3%, transformation of the austenitic matrix to pearlite occurred in preference to martensite. Mischmetal additions reduced the austenite dendrite arm spacing, but did not have a significant effect on the carbide structure. The wear behaviour was investigated for each alloy in the as-cast (austenitic matrix) and hardened (martensitic) conditions using a block on ring configuration in pure sliding in the load range 42–238 N for a distance of 70 km against a hardened M2 steel counterface. For low loads (42 and 91 N), all the alloys showed a similar wear rate (3×10−4 to 4×10−4 mm3/m), associated with the formation of a thin (∼3 μm) oxide film of Fe2O3, the formation of very fine debris and a small depth of deformation below the worn surface (∼7 μm). For higher loads, wear was a strong function of microstructure, and was associated with a thicker film of the oxides Fe2O3 and Fe3O4 and greater depths of deformation. The iron with 2% silicon exhibited the best performance with a wear rate of 7×10−4 mm3/m and this was attributed to its finer structure and the formation of a thicker oxide film. In contrast, the iron with 5% silicon exhibited the worst performance, with a wear rate of 14×10−4 mm3/m, attributed to the pearlitic matrix. A linear relationship was observed between the depth of carbide fracture and the wear rate. The relationship between microstructure and wear mechanism is discussed.