Study Of Hypereutectic High Chromium Cast Irons Containing Niobium

High chromium cast irons（HCCI） are excellent wearable materials. Their exceptional abrasive and erosive wear resistance result primarily from the emergence of M₇C₃ carbides. In most industrial cases, HCCI are used as hypoeutectic alloys. It is generally believed that when the carbon content exceeds the eutectic point, bulky primary carbides M₇C₃ are formed in the hypereutectic HCCI, whose toughness therefore decreases. In this study, in order to improve the toughness of hypereutectic HCCI, we investigate different approaches in refining the primary M₇C₃ carbides:（1） NbC particles appeared in the hypereutectic HCCI when Nb was added into the alloy. The primary carbides of the hypereutectic high-Cr cast iron were refined gradually with the increase of the content of Nb.（2） with the increase of the content of Nb, the impact toughness of the hypereutectic HCCI increased gradually. the impact toughness of the specimen with 3.0wt%Nb has the biggest impact toughness,7.1 J/cm2,which is about 2.8 times of that of the specimen without Nb. However, the content of Nb do not have much effect on the micro-hardness of the specimen.（3） water quenching experiment and theoretical calculation both indicated that NbC precipitated before M₇C₃ primary carbides.（4） the refining mechanism of Nb on M₇C₃ primary carbides is as following: the mismatch of NbC on M₇C₃ at the low index surfaces, such as （110）_NbC//（0101）_M7C3,（111）_TiC//（0101）_M7C3,（111）_NbC//（0001）_M7C3 And （100）_NbC//（0001）_M7C3 is less than 12%,so NbC can act as the substrates for heterogeneous nucleation of primary M₇C₃ carbides, which results in significant refinement of the final grain size.（5） Wear test and actual application indicated that the hypereutectic HCCI with 3.0wt%Nb Good abrasion resistance.