Enhanced Densification of PM Steels by Liquid Phase Sintering with Boron-Containing Master Alloy

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TRODUCTION

POWDER metallurgy (PM) steels for high-performance applications are of significant interest because of their low costs, flexibility in alloy design, and high volume production. However, reaching high density is not viable through conventional pressing and sintering which limits the usage of PM steel components in high-performance and demanding applications. Hence, achieving high density is essential to obtain the demanding mechanical properties that will enable PM parts to serve as an alternative to conventionally manufactured parts. High densities can be achieved either through pressure-based or sintering-based techniques or by combining both.[1–4] Sintering with liquid phase forming additives activates the sintering and enhances the final MAHESWARAN VATTUR SUNDARAM, EDUARD HRYHA, and LARS NYBORG are with the Department of Industrial and Materials Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden. Contact e-mail: [email protected] KUMAR BABU SURREDDI is with the Materials Science, Dalarna University, 791 88 Falun, Sweden. ANGELA VEIGA and FRANSISCO CASTRO are with the CEIT, Paseo de Manuel Lardiza´bal 15, 20018, San Sebastia´n, Spain. SIGURD BERG is with the Ho¨gana¨s AB, 263 83 Ho¨gana¨s, Sweden. Manuscript submitted June 10, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

densification through the liquid phase sintering mechanisms (LPS).[5,6] Elements with lower atomic number such as P, B, and C are well suited for activating sintering of iron-based powders due to the difference in electronic structure, phase composition, and grain boundary cohesion.[7] Lower amounts of such liquid-forming additives results in better densification by the liquid phase sintering (which is a dominant mechanism) than the surface energy activation.[5] Boron addition significantly improves the density of the PM steels by forming a liquid with iron at ~1447 K (1174 C) due to the eutectic reaction cFe+Fe2BfiL.[8] Once the liquid forms, more iron will be dissolved in the liquid, which results in persistent liquid formation. In PM steels, boron is introduced either as elemental boron[7,9–14] or ferroboron,[15–20] or master alloy,[21–24] or compounds like hBN or B4C.[25,26] Enhanced density levels were reached with elemental boron addition for both the carbonyl and water-atomized powders.[7] Ferroboron tends to agglomerate, resulting in inhomogeneous density distribution during sintering.[26] Selecka´ et al. [15] observed an increase in density by introducing ferroboron in Fe-Mo-prealloyed systems only when the boron content reaches beyond 0.2 wt pct. The design of master alloy is based on proper selection of alloy systems which can melt at lower melting temperatures so as to enable liquid phase sintering.[27] The elements such

as Mn, Si, Cr, Ni can also be introduced without affecting the base powder compressibility.[28] When boron is introduced in the master alloy, it provides the flexibility of generating a liquid phase from the melting of the master alloy and the eutectic reaction between iron and boron. In t

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