Abstract
In this work, an electric current pulse (ECP) of 500A was applied on a hypereutectic high chromium cast iron (HHCCI) melt before it began to solidify, and the effect of ECP on primary carbides was investigated. The characteristics of the primary carbides were analyzed by X-ray diffraction (XRD), electron probe micro-analyzer (EPMA), transmission electron microscopy (TEM), micro hardness tester, and other techniques. The results showed that ECP not only refined the primary (Cr, Fe)7C3 carbides, but also decreased the average content of Cr in the primary carbides. At the same time, the average value of micro hardness of the primary carbides increased by about 84 Kgf/mm2, which contradicts existing knowledge that hardness increases with an increase in Cr content. XRD analysis showed that the crystal structure of the primary carbides did not change. The results of EPMA indicated that the Cr/Fe ratio gradually decreased from the center to the edges of the carbide particles. Further investigation revealed that the uneven distribution of elements caused by ECP led to an increase in defects (including twins, antiphase boundaries, and dislocations). This increase in defect density is the main reason for the increase in micro hardness instead of the expected decrease. The mechanism of the change in primary carbides was analyzed in detail in this paper, which has provided a new method for the refinement of primary carbides and for improving the properties of primary carbides.
Highlights
M7C3 carbides are the main strengthening phase in hypereutectic high chromium cast iron (HHCCI) due to their greater strength and hardness [1,2].The morphology, distribution, composition, and internal micro-defects of the primary carbides have important effects on the properties of the alloy [1,3,4]
Refinement of Primary Carbides Induced by electric current pulse (ECP) Melt Modification
Fe ion cores with high energy combine more with crystal embryos in the nucleation stage (Figure 7b), which
Summary
The morphology, distribution, composition, and internal micro-defects of the primary carbides have important effects on the properties of the alloy [1,3,4]. Investigation of carbide characteristics and corresponding influencing factors is beneficial to improving the properties of alloys. M7C3 carbides have been widely studied over the past century, which has provided a good foundation of knowledge for their further development. Regarding the morphology of carbides, it has been reported that primary carbides grow as rods while eutectic carbides grow as blades in longitudinal. It has been pointed out that there are defects such as twins, antiphase boundaries, and dislocations in M7C3 carbides [11,12,13,14,15]
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