Liquid Phase Sintering of Boron-Containing Powder Metallurgy Steel with Chromium and Carbon
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POWDER metallurgy (PM) steels are widely used to produce automobile and mechanical parts. Their mechanical properties can be improved by adequate microstructures, alloy designs, and processing parameters.[1–9] However, the mechanical performances of PM steels are inferior to those of wrought steels due to their porosity, which ranges from 5 to 15 vol pct. Liquid phase sintering (LPS) is a practical process for decreasing the porosity of PM steels.[10–29] Copper,[26,27] phosphorus,[28,29] and particularly boron[10–25] are effective alloying elements for LPS in PM steels. Boron induces a eutectic reaction (Fe + Fe2B fi L) and improves the densification. The influence of boron in the LPS of various PM steels, including alloy steels,[10–18] stainless steels,[19–21] maraging steels,[22] and high-Cr steels,[23–25] has been extensively investigated. Among the material systems of boron-containing PM alloy steels, Fe-B[10–13,18] and Fe-B-Mo[10–14,18] are two MING-WEI WU, Associate Professor, and WEN-ZHANG CAI, Master Student, are with the Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, No. 1, Sec. 3, Zhong-Xiao E. Rd., Taipei 10608, Taiwan, ROC. Contact emails: [email protected]; [email protected] YU-CHI FAN, Graduated Master Student, and HER-YUEH HUANG, Associate Professor, are with the Department of Materials Science and Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei, Yunlin 63201, Taiwan, ROC. Manuscript submitted July 17, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
major systems investigated. German et al.[10] investigated the Fe-(0~1.1 wt pct)B-(0~5 wt pct)Mo steels and found that at a given boron content, increasing the molybdenum concentration leads to the formation of FeMo2B2 compound and thus reduces the amount of liquid phase. However, Dudrova´ et al.[11] indicated that the LPS densification of Fe-B-1.5Mo steel is better than that of Fe-B, even though the added 1.5 wt pct Mo reduces the amount of liquid phase available at 1473 K (1200 C). Selecka´ et al.[12] also found that the sintered density after LPS of Fe-0.4B is improved by increasing the molybdenum content from 0 to 1.5 wt pct. Sarasola et al.[13] found that the liquid phase forms at 1448 K (1175 C) in Fe-0.3B steel with molybdenum content lower than 1.5 wt pct. In Fe-0.3B-3.5Mo steel, however, the temperature of eutectic reaction is higher than 1473 K (1200 C). The influences of carbon on the LPS of boron-containing PM steels have also been studied.[15–18,22] Xiu et al.[17] found that increasing the carbon content from 0.2 wt pct to 0.44 wt pct improves the sintered density of Fe-B-Mo-C steel. Sercombe[22] showed that the onset of the LPS temperature of maraging steel with carbon is ~60 K (C) lower than that of maraging steel without carbon. The carbon lowers the melting point, so carbon-containing liquids can be formed. In contrast, Liu et al.[16] indicated that the effect of carbon on LPS densification is not obvious in Fe-0.3B-1Ni-1Mo-C steel with up to 0.4 wt pct C. R
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