含鎳粉末冶金鋼的破壞與強化機制

Abstract

含鎳粉末冶金鋼的應用十分廣泛，不過因為鎳通常分佈相當不均勻而使組織中存在許多富鎳區，文獻中都指出這些富鎳區對機械性質影響相當大，但因為這些富鎳區的組織相當細微，所以其真實結構至今還沒被詳細鑑定，因此本研究首先利用EBSD配合成分與微硬度分析來針對燒結鋼中的各種富鎳區進行分類與結構鑑定，其結果顯示共有三種富鎳區，分別為富鎳肥粒鐵、麻田散鐵與沃斯田鐵。 這些富鎳區的形成原因一般被歸因於鎳在鐵中的低擴散速率，但其真正的形成機制至今還沒有被充分瞭解，所以本研究亦將此列為一主題，利用顯微組織與熱力學計算(Thermo-Calc Program)的結果來探討碳與鎳及其他合金元素(鉻、鉬)間的交互作用與分佈情形。其結果顯示鎳與碳之間會有強烈的排斥現象，所以鎳將會迫使碳離開富鎳區，這導致了低強度低碳富鎳區的形成，並成為破裂起始區。而鉻與鉬因為可以減緩鎳與碳之間的排斥現象，所以可以有效消除富鎳肥粒鐵與波來鐵，因而大幅提升了機械性質，其中鉻的效應較佳，可使抗拉強度提升100%以上。 最後為了瞭解不同強化方式對粉末冶金鋼機械性質的影響，本研究也把結果與先前文獻進行討論，結果發現促進組織均勻化與強化顯微組織才是提升粉末冶金鋼機械性質最有效的方法，其效應遠大於改良孔洞特徵和其他因素的影響。

Ni-containing PM steels are used extensively in the industry. Due to the non-uniform Ni distribution, many Ni-rich areas are present in the microstructure. It had been reported that these Ni-rich areas play an important role on the mechanical properties, although their true structures had not been clearly identified. To understand the behaviors of the Ni-rich areas under the mechanical loading, the first objective of this study was thus to identify the actual structures of these Ni-rich areas using electron backscatter diffraction, microhardness, and quantitative analyses. The results show that there are three types of Ni-rich areas: ferrite, martensite, and austenite. It is generally believed that the Ni-rich areas are formed due to the slow diffusion rate of Ni in Fe. But, other mechanisms are still possible. Thus, the second objective was to understand fully the formation mechanism of the Ni-rich areas. The method for eliminating the weak Ni-rich areas is also proposed. Based on the results of microstructures and thermodynamic calculations, the interactions between C and alloying elements, particularly Ni, Cr, and Mo, are analyzed. The results indicate that there is a strong repulsion between C and Ni, which forces C out of the Ni-rich areas. The low-strength Ni-rich/C-lean areas are thus formed and impair the mechanical properties significantly. It is also found that Cr could alleviate the Ni-C repulsion and eliminate the weak ferrite and pearlite, which are responsible for the fracture at low loadings. With Cr additions, the tensile strengths are thus much improved by more than 100%. These results show that improving the microstructural homogeneity and strengthening the microstructures are the most effective method for improving the mechanical properties compared to that of using high sintering temperature, long sintering time, and using fine powders. The effect of improving the properties of the matrix is far better than that of modifying the pore characteristics.