The effect of NbC morphology on the slurry erosion performance of ferrous alloys

It is often proposed that the wear performance of high-chromium white cast irons could be improved by adding Nb to form NbC. However, NbC can form with a needle-like “Chinese-script” morphology and/or as nodular particles, and clear evidence is not available as to which morphology is more beneficial against slurry erosion, a common application for these alloys. The lack of evidence is due to the difficulty of differentiating between the effect of NbC and that of a second type of carbide, M₇C₃, on slurry erosion resistance. Therefore, we created a martensitic stainless steel resembling the ferrous matrix of a typical high-chromium white cast iron, but without the M₇C₃ phase, that was reinforced with Chinese-script NbC. A second specimen was prepared by substituting a small amount of Nb for Ti to inoculate the NbC, thus forming nodular particles. An AISI 440C stainless steel was used as a commercial benchmark since it has a similar hardness and microstructure. All three samples were subjected to slurry erosion tests against garnet and sand, with the Chinese-script NbC sample being the worst performer in both cases. Analysis of the worn surfaces revealed that the Chinese-script morphology promoted fracturing, resulting in a higher erosion rate.