The Fracture and Strengthening Mechanisms of Ni-Containing Powder Metal Steels
Date Issued
2007
Date
2007
Author(s)
Wu, Ming-Wei
DOI
zh-TW
Abstract
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.
Subjects
粉末冶金鋼
燒結
破壞機制
熱力學模擬計算
顯微組織
相鑑定
powder metallurgy
steel
sintering
fracture
thermodynamic calculation
microstructure
phase identification
Type
thesis
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