Improved homogenization of Ni in sintered steels through the use of Cr-containing prealloyed powders
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NTRODUCTION
NICKEL, Cu, and Mo are the most frequently used alloying elements in powder metal (PM) steel products. Among these three elements, Ni has the slowest diffusion rate into iron, and thus the full benefits of Ni alloying are lost. As a result, heterogeneous microstructures and insufficiently satisfactory mechanical properties are often found in sintered Ni-containing steels, such as in diffusionalloyed FD-0405 (Fe-4Ni-1.5Cu-0.5Mo-0.5C) and sinterhardened FLN4-4408 (Fe-4Ni-0.8Mo-0.75C), which are the standard materials contained in the Metal Powder Industries Federation (MPIF) standards.[1] These Ni-containing steel compacts usually consist of pearlite, bainite, martensite, and a substantial amount of Ni-rich areas.[2,3] The Ni-rich areas were found to be lean in carbon and were soft and were located at sintered necks or in pore-rich areas, which are the most vulnerable sites during mechanical testing.[3–7] To improve the alloy and microstructure homogenization and the mechanical properties, elimination of the weak Ni-rich/C-lean areas is necessary. An intuitive approach to solving this problem is to use prealloyed steel powders. However, the poor compressibility of the hard prealloyed powder has prohibited its wide application. Another alternative is to employ smaller and more spherical Ni powders or to use Ni-coated iron powders.[8,9,10] The use of fine carbonyl iron powders, such as in the metal injection molding process, also provides sintered compacts a more homogenized microstructure due to the short diffusion distance.[9,10] The uniformity of Ni may also be influenced by other alloying additives. The effect of adding Mo and Cu has been shown to be insignificant, as was demonstrated by the inhomogeneous Ni distribution in sintered FD-0405 M.W. WU, Graduate Student, and K.S. HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei, 106 Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted June 1, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
and FLN4-4408 compacts.[2,3,11] The carbon, on the other hand, has a negative effect because Ni and C tend to repel each other due to their high interaction energies.[12] Because Cr is a strong carbide former, which could change the chemical potential of carbon in Ni-rich phases, it is possible that the presence of Cr would alleviate the repelling effect between Ni and C and help the Ni homogenization. However, the use of Cr as an alloying element is rare for regular PM industries. The difficulty comes from its high oxygen affinity. Thus, low dewpoint and hightemperature sintering are required to prevent the oxidation of Cr.[13] This problem can be alleviated by reducing the activity of Cr using Cr-containing prealloyed powders. Wu et al. reported that the amount of soft Ni-rich/C-lean areas in the Fe-4Ni-1.5Cu-0.5Mo-0.5C compact can be reduced when 0.5 wt pct Cr is introduced in the 316L stainless steel powder form.[2] The Cr addition helps homogenize Ni and C and promote the f
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