The Modification Of Carbides In High-chromium Cast Irons

High-chromium cast irons (HCCIs) have been widely used in many abrasive environments, such as. mining, metallurgy, coal grinding and abrasive slurries milling etc. because of the high hardness, exceptional wear resistant ability and excellent corrosion resistance. HCCIs contain a great amount of Cr (12%-30%), and during the solidification, Cr and Fe will be reacted with C and a high volume fraction of M7C3 will be formed. The hardness of M7C3 is very high (about HV1200-1800), and these carbides will be distributed isolated in the matrix, so these carbides'deterioration of toughness is much slighter than the carbides that grow to be net-like. That is the reason why HCCIs possese higher toughness and wear resistance than other Cr-contained materials. Although M7C3 will be grown to be isolated blades during the solidification, they will grow to be large sized rods and blades under the normal cast conditions, such as sand casting. These large sized M7C3 will be easily broken when suffered to high strikes, and the duration of HCCIs will be decrease. So, it is important to modify the morphology of the carbides existed in HCCIs. It has been grown to be a hotspot to enhance the mechanical performance of HCCIs by improving the size, morphology and distribution of carbides.In this study, K/Na contained modifier, Sr, B and Zr were used to modify the carbides in HCCIS under laboratory condition, and the modification mechanisms were discussed. The Optical Microscopy, FESEM were used to observe the microstructure of HCCIs, carbides contained in HCCIs were extracted and their three dimensional morphology was observed by FESEM. XRD was used to identify the phases contained in HCCIs. It is found that, after the addition of K/Na contained modifier and Sr, the large sized primary were modified to be smaller rods and blocks, and the hollow eutectic carbides tubes were modified to be solid carbides rods, and the morphology of these carbides was turned to be more regular. Besides that, carbides were distributed much more homogeneously in the matrix. The eutectic carbides were refined after the addition of B, with the B addition amount increased, fine primary M7C3 carbides were found in eutectic HCCIs, and the eutectic carbides and matrix were both refined. The modification is much better when B and Zr are both existed in HCCIs.The modification mechanisms of K/Na modifier and Sr were discussed. It is deduced that K/Na and Sr can be played as surface-active elements in HCCIs and will be absorbed on the fast growth planes of carbides which will finally prohibit the carbides growth and modify the morphology of carbides. The modification of B4C and Zr should not only be due the absorption of Zr on the fast growth planes of carbides but also because of the formation of B and Zr contained borides in the HCCIs which can be acted as the nucleus of the heterogeneous nucleation during the formation of carbides.The hardness and toughness of HCCIs before and after the modification were examined. It is found that, after the modification, the hardness and toughness were all enhanced.