A Novel Ni-Containing Powder Metallurgy Steel with Ultrahigh Impact, Fatigue, and Tensile Properties
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INTRODUCTION
POWDER metallurgy (PM) is a promising method to economically fabricate near net shape parts in the industry. PM steels are widely used for automobile and mechanical parts with complicated shapes and multi-functions. However, the pores in PM steels lead to serious stress concentration at the sintered necks and pore-rich areas during loading.[1,2] The pores also decrease the effective load-bearing cross-section and are thus detrimental to the mechanical properties, as demonstrated by Danninger et al.[3] Among the mechanical properties, the tensile property and hardness have been extensively studied in the literature.[4–8] Toughness is also a predominant property of PM steels because it dominates the failure and the performance under impact loading. Typically, the toughness of PM steels is extraordinary poor due to the high porosity, normally about 5 to 15 vol pct. This unsatisfactory toughness limits high-level applications of PM steels. Various compaction techniques, such as warm compaction and MING-WEI WU, Associate Professor, is with the Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC. Contact e-mail: [email protected] GUO-JIUN SHU, Assistant Research Scholar, is with the Center for Condensed Matter and Science, National Taiwan University, Taipei 10617, Taiwan, ROC. SHIHYING CHANG, Associate Professor, is with the Department of Mechanical Engineering, National Yunlin University of Science and Technology, Douliou 64002, Yunlin, Taiwan, ROC. BING-HAO LIN, Master Student, is with the Department of Materials Science and Engineering, National Formosa University, Huwei 63201, Yunlin, Taiwan, ROC. Manuscript submitted March 22, 2013. Article published online May 23, 2014 3866—VOLUME 45A, AUGUST 2014
double-press/double-sintering, have been used to produce a PM steel with a green density higher than 7.3 g/cm3.[4,6,9] However, these compaction techniques also increase the production cost. The selection of alloying elements is another issue for attaining high-toughness PM steels.[10–14] Several studies have investigated the effects of various alloying elements, including Mo, Cr, Ni, and Cu, on the toughness of PM steels.[10–14] St-Laurent and Chagnon[10] reported that the toughness of Fe-xMo-4Ni-1.5Cu-0.53C steel decreases slightly when the Mo content is increased from 0.5 to 1.5 wt pct. Shanmugasundaram and Chandramouli[11] revealed that 2 wt pct Ni or 1 wt pct Cr obviously impairs the toughness of sinter-forged Fe-0.2C steel. Moreover, the toughness of sinter-forged Fe-2Ni-0.2C steel is slightly decreased by 1 wt pct Cr or 1.5 wt pct Mo. However, Candela et al.[12] found that both 3 wt pct Ni and 3 wt pct Cu increase the impact energy of Fe-3.5Mo-0.7C steel. Wu et al.[13] recently found that 1.5 wt pct Mo impairs the toughness of Fe-0.5C and Fe-4Ni-0.5C, but 1.5 wt pct Cr does not. Furthermore, most studies have indicated that when the tensile strength is improved by adding alloying elements or modifying the microstructures, the impac
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