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標題: 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
Microstructural and abrasive characteristics of Fe-Cr-C hardfacing alloy with various carbon contents
作者: 張家銘
Chang, Chia-Ming
關鍵字: Hardfacing alloy;硬面合金;Microstructure;Solidification behavior;Wear behavior;顯微組織;凝固行為;磨耗行為
出版社: 材料科學與工程學系所
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本研究目的為探討不同碳含量對高鉻Fe-Cr-xC硬面合金顯微組織與磨耗性質之影響,利用鎢極惰性氣體遮護電弧銲接法(Gas Tungsten Arc Welding, GTAW)將六組不同配比之石墨、鐵粉與鉻粉混合作為合金填料,然後銲覆在A36低碳鋼基材上。銲覆層之顯微結構藉由光學顯微鏡、電子顯微鏡及X-ray繞射分析進行探討,再以熱分析搭配顯微組織觀察進行凝固行為分析。最後利用乾砂磨耗試驗分析其耐磨耗性質,再觀察磨耗面探討磨耗行為。
本研究在合金填料中添加六種不同配比石墨經銲覆後分別會產生以Fe–Cr相、(Cr,Fe)23C6與 (Cr,Fe)7C3組成的亞共晶、近共晶與過共晶組織。在顯微組織的型貌可發現初晶Fe-Cr 相與初晶碳化物之形貌有著明顯不同的差異。初晶Fe-Cr 相屬於樹枝狀結構,而(Cr,Fe)7C3碳化物則是以六方形的形態存在。本研究發現初晶相之形貌是由於固液界面類型的不同所造成的。初晶Fe-Cr 固溶體為non-faceted固液界面,故易於成長為樹枝狀結構。而(Cr,Fe)7C3碳化物屬於faceted固液界面,因此傾向生長成多方狀結構。在共晶結構方面,Fe-Cr+(Cr,Fe)23C6共晶組織以層狀形態存在,而Fe-Cr+(Cr,Fe)7C3共晶組織以柱狀形態存在。此外,在本研究亦發現由於Si 原子在(Cr,Fe)7C3碳化物中的溶解度相當低,因此在凝固過程中(Cr,Fe)7C3碳化物會排斥Si原子至基地相。初晶(Cr,Fe)7C3碳化物周圍會產生無析出區,且其共晶組織會在其邊緣以異質成核方式成核。由熱分析可知,當銲覆層碳含量增加時,其液相線溫度會隨之降低,而共晶溫度則不隨碳含量改變。此外,(Cr,Fe)7C3碳化物在熱分析方面具有寬大的峰表示其具有較慢的溶解與形成速率。在橫截面分析方面,在沿母材與銲覆層的界面會有一平面晶成長凝固。此外,當銲覆層成份落在亞共晶組成時,平面晶前端會有樹枝狀晶產生;而當落在過共晶區域時,在平面晶前端會有共晶成長的產生。

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.
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