Microstructure Development in Powder Metallurgy Steels: Effect of Alloying Elements and Process Variables

Abstract:

Microstructure of the powder metallurgy (PM) steels and especially mechanism of its formation differs significantly from the microstructure of the conventional steels even if composition will be exactly the same. The difference is not only connected to the presence of the pores, which are inalienable feature of the PM parts. Presence of the prior inter-particle boundaries, which can be contaminated by residual oxides, as well as microstructure heterogeneity are another characteristic features of the microstructure of PM steels. Microstructure heterogeneity is connected to the PM manufacturing process: powder mix, consisting of the base powder and additional alloying elements is compacted and then sintered. Fully prealloyed powder is not always possible to use in standard press & sintering route due to the solid solution strengthening of the ferrite resulting in bad powder compressibility. Hence, in order to provide good powder compressibility only pure iron or low-alloyed (typically <3 wt.%) powders are used. Required alloying elements and carbon (added as graphite) are further admixed in the powder form and are distributed during sintering by diffusion into iron particles at high temperatures. To assure satisfactory distribution of alloying elements, oxide layer, covering surface of the powder particles and hindering mass-transfer of the alloying elements, has to be removed first. This can be done by gaseous reducing agents as hydrogen and carbon monoxide. However, their cost and/or purity are of issue for modern alloyed PM steels. Admixed carbon, additionally to its function as alloying element, plays a role of effective reducing agent at higher temperatures. Paper summarizes the main features of microstructure formation during the whole sintering cycle (heating and isothermal sintering) and effect of alloying additives (different carbon sources, alloying elements) and processing parameters (sintering atmosphere composition, temperature profile) on the microstructure formation during conventional sintering process. Results indicate that for successful sintering of alloyed PM steels with homogeneous defect-free microstructure, hydrogen-rich atmospheres and high-temperature sintering are required.

• Periodical: Materials Science Forum (Volume 782)
• Pages: 467-472
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.782.467
• Online since: April 2014
• Authors: Eduard Hryha*, Lars Nyborg
• Keywords: Carbon Dissolution, Microstructure Formation, Oxide Reduction, Sintered Steel, Sintering Atmosphere

References

[1] Höganäs Handbook for Sintered Components, Höganäs AB, Höganäs, Sweden, (2004).

[2] E. Hryha, P. Zubko, E. Dudrová, L. Pešek, S. Bengtsson, An application of universal hardness test to metal powder particles, J. Mater. Process. Technol. 209 (2009) 2377-2385.

DOI: 10.1016/j.jmatprotec.2008.05.038

[3] S. Sainz, V. Martinez, M. Dougan, F. Baumgaertner, F. Castro, Sinterability, Hardenability and Mechanical Properties of Mn-Containing PM Steels through the Use of a Specially Designed Fe-Mn-C Master Alloy, Adv. Powder Metall. Part. Mater. 2006 (2006).

[4] E. Hryha, E. Dudrova, The Sintering Behaviour of Fe-Mn-C Powder System, Correlation between Thermodynamics and Sintering Process, in M. Tadashi (Ed. ), Application of Thermodynamics to Biological and Materials Science, InTech, Rijeka, 2011, pp.573-602.

[5] E. Hryha, E. Dudrova, L. Nyborg, Critical Aspects of Alloying of Sintered Steels with Manganese, Metall. Mater. Trans. A 41A (2010) 2880-2897.

DOI: 10.1007/s11661-010-0357-5

[6] R. Bidulský, M. Actis Grande, E. Hryha, J. Bidulská, The Effect of Manganese on Microstructure Development and Diffusion Characteristics in Admixed Fe-Mn-C Sintered Steels, Proc. World PM2010, 3 (2010) 113-120.

DOI: 10.1515/htmp-2015-0107

[7] L. Čiripová, E. Hryha, E. Dudrová, A. Výrostková, Prediction of Mechanical Properties of Fe-Cr-Mo Sintered Steel in Relationship with Microstructure, Mater. Des. 35 (2012) 619-625.

DOI: 10.1016/j.matdes.2011.10.011

[8] E. Hryha, C. Gierl, L. Nyborg, H. Danninger, E. Dudrova, Surface Composition of the Steel Powders Pre-Alloyed with Manganese, Appl. Surf. Sci. 256 (2010) 3946-3961.

DOI: 10.1016/j.apsusc.2010.01.055

[9] A. Salak, Manganese Sublimation and Carbon Ferromanganese Liquid Phase Formation During Sintering of Premixed Manganese Steels, Int. J. Powder Metall. 16 (1980) 369-379.

[10] E. Navara, Alloying of Sintered Steels with Manganese, Proc. VIth Int. PM Conference, Brno, 1 (1982) 143-154.

[11] E. Hryha, L. Nyborg, A. Malas, S. Wiberg, S. Berg, Carbon control in PM sintering: industrial applications and experience, Powder Metall. 56 (2013) 5-10.

DOI: 10.1179/0032589912z.00000000085

[12] E. Hryha, L. Nyborg, Oxide Transformation During Sintering Of Prealloyed Water Atomized Steel Powder, Proc. World PM2010, 2 (2010) 268-275.

[13] E. Hryha, L. Nyborg, L. Alzati, Effect of Carbon Source on Oxide Reduction in Cr-Prealloyed PM Steels, Proc. World PM 2012, (2013) 16A-T9-11 (CD-ROM).

DOI: 10.1179/0032589914z.000000000191