不同碳含量對Fe-Cr-C硬面合金顯微結構及磨耗特性之研究

Abstract

This study discussed the effect of carbon content on microstructural and abrasive characteristics of high chromium Fe-Cr-C hardfacing alloys. Six fillers which consisted of chromium and iron powder with different graphite additions were deposited on A36 low carbon steel by gas tungsten arc welding (GTAW). Optical microscope, electron microscope and X-ray diffraction were used to investigate the microstructural constituents. Thermal analysis was used to study solidification behavior. Wear resistance was estimated with sand wheel wear test, and the worn surfaces were observed by optical microscope. This research produced hypoeutectic, near eutectic, and hypereutectic microstructures of Fe–Cr phase, (Cr,Fe)23C6, and (Cr,Fe)7C3 carbides on hardfacing alloys, respectively. Morphology of primary Fe-Cr phase with dendrite-like was different from that of primary carbides with hexagonal shape due to solid/liquid interface. The non-faceted interface trended to dendrite growth; faceted interface preferred to polygonal growth. For morphology of eutectic structure, Fe-Cr+(Cr,Fe)23C6 eutectic colony was lamellar, and Fe-Cr+(Cr,Fe)7C3 eutectic colony was rod-like. Moreover, this study found primary (Cr,Fe)7C3 carbides rejected Si atom to remained liquid during solidification process. Pricipitate-free zone occurred in the vicinity of primary (Cr,Fe)7C3 carbides. Eutectic colony hetergenuous nucleated in the ledge of primary carbide. Thermal analsis showed that increase of carbon content casused liquid temperature decreasing, but eutectic temperature was invariable. For cross-section observation, the cross-section analysis indicated epitaxial solidification with planar front growth at the interface between hardfacing and the substrate. Furthermore, eutectic growth existed in the near interface as hardfacing composition fell in the hypereutectic region. Surface hardness of hardfacing alloy increased with carbon content. The increasing eutectic colony enhanced hardness of hypoeutectic hardfacing layer. Hardness of hypereutectic hardfacing layer increased when the fraction of primary carbide increased. With regard of wear test, the relationship between wear loss and sliding distance was linear. The increase in carbon content of hardfacing layer resulted in wear loss. Wear loss was in inverse proportion to hardness of hardfacing layer. Worn surface observation showed that the wear mechanisms in hypoeutectic microstructure were ploughing and micro-cutting. Level of plastic groove transited from severe to mild when the eutectic colony of Fe-Cr+(Cr,Fe)23C6 increased. However discontinuous ploughing and carbide pull-out existed in hypereutectic microstructure. Among all kinds of microstructure the hypereutectic of Fe-Cr and (Cr,Fe)7C3 has the highest wear resistance.