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  • Author: C. Gierl-Mayer x
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Abstract

Sintering of Cr-prealloyed PM steels requires atmospheres with good quality – low oxygen potential – to achieve satisfactory sintering results. But during heating even the best atmospheres may be oxidizing, the system turns to reducing conditions only at high temperatures, which can be monitored by thermal analysis. During the dewaxing process, oxidizing conditions are favourable for effective dewaxing without sooting and blistering. However, this may result in some oxygen pickup during heating, and then the final properties of the produced parts may be strongly influenced by this intermediate oxidation. This study demonstrates the behaviour of artificially oxidized steels (Fe-C and Fe3Cr-0.5Mo-C) during the sintering process by stepwise sintering. Iron and steel powder were slightly oxidized and then pressed and sintered at different temperatures. In parallel, as a second approach, pressed samples were oxidized and then sintered. Density, hardness and impact energy were measured and dilatometry/MS was used for online monitoring of the sintering process. The starting oxygen content of 0.20 to 0.30 wt% is high enough to change the sintering behaviour of the materials, but still leads to rather good properties. Thermal analysis showed that most of the oxygen picked up was present as iron oxides on the surface which were reduced by hydrogen at rather low temperatures, confirming that these were iron oxides, which also holds for the Cr-prealloyed variant. The biggest influence on the final performance was exerted by the final carbon content and the microstructural development of the material.

Abstract

For powder metallurgy products, high density is an essential requirements to obtain maximum mechanical properties. Here, supersolidus liquid phase sintering (SSPLS) is an effective means to attain high sintered density, as known from PM high speed steels. In the present work it is shown that this technique can also be applied to Cr prealloyed low alloy steel grades. Supersolidus sintering through indirect heating requires precise control of temperature and also the atmosphere, to avoid uncontrolled changes of the carbon level. Higher C contents are beneficial here since they enable lower temperatures and result in wider temperature windows for sintering. The temperatures necessary for SSLPS at moderate C levels are fairly high for standard sintering furnaces, therefore induction sintering was studied in this work. It showed that, as was to be expected, also here precise temperature control is required, but for any carbon level tested a sintering temperature could be identified that yielded high sintered density and good shape retention. The high density attained, in combination with the very high temperatures, results in pronounced grain growth, this process no more being inhibited by the presence of pores, which is undesirable but can however be remedied by suitable heat treatment.

Abstract

Phosphorus as an alloy element is quite common in powder metallurgy, the contents industrially used being markedly higher than those present in wrought steels. In this study, the influence of phosphorus addition through different P carriers was investigated. PM steels of the type Fe-0.7%C-x%P (x = 0.0 … 0.8%) were manufactured by pressing and sintering in H2. It showed that Fe3P is the best phosphorus carrier, resulting in fine and regular microstructure and in high impact energy data at 0.3 … 0.45%P while red P and also Fe2P showed a tendency to agglomeration, with resulting secondary porosity. At high P levels the mechanical properties tend to drop, for the tensile strength at P > 0.60%P while for the impact energy the threshold is 0.45%P. The dimensional behaviour of Fe-C-P can be related to PM aluminium alloys, expansion by transient liquid phase being followed by shrinkage by persistent liquid phase, at least at higher temperatures. In contrast to the dimensional behaviour, degassing and reduction is hardly affected by the phosphorus content.