In this work the reduction during sintering of Mn and/or Cr prealloyed steels has been studied. In inert atmospheres, the reduction of surface and especially of internal oxides in these steels is shifted to markedly higher temperatures than in Fe-C or conventionally alloyed PM steel grades, reduction being not yet completed at 1300°C. At least at higher alloy element contents, the main reduction starts about 1000°C compared to 700°C for Fe-C, in both cases removal of surface oxides being the first major process. Sintering in high purity H2 results in partial reduction in the 400°C range, H2O being formed as the product, but removal of the remaining oxides requires more or less the same high temperatures as in inert atmospheres, and the reaction product is mainly CO, which confirms that at temperatures >1000°C carbon is the reducing agent in any sintering atmosphere.

No abstract

The Masteralloy (MA) alloying route has a great potential for reducing the alloying costs in sintered steels, while allowing the introduction of innovative alloying systems. However, in order to achieve an efficient use of the alloying elements, the particle sizes needed are often below 25 µm, which means that for standard gas atomization a significant fraction of the batch has to be discarded or at least recycled. This work evaluates the performance of steels containing MA powders obtained with a novel atomization technique (Ultra-High-Pressure Water atomization) that allows the production of low-cost powders with low oxygen contents, rounded morphologies and mean particle sizes as low as 6 microns. Mechanical properties, dimensional variations and interstitial contents were measured in steels containing different MA compositions sintered at either 1120 °C or1250 ºC in N 2 -5H 2 atmospheres. Already with less than 3 wt.% of alloying elements these steels present excellent combinations of properties, reaching strength levels of 560-915 MPa and hardness 220-260 HV10, combined with elongations of 1.3-3.2% and impact energies around 20-30 J/cm2.

The combination of alloying elements in the form of a masteralloy (MA) powder gives the possibility to protect oxygen-sensitive elements against oxidation and to promote the formation of a liquid phase that enhances the sintering mechanisms. As compared to the prealloying approach, the MA route has lower impact on compressibility and provides more flexibility in the selection ofthe final composition. Knowledge of the chemical aspects of sintering combined with the possibility to tailor the properties of sintered steels through the use of specific MA compositions and with the development of novel atomizing methods to produce MA powders may, in the near future, position the MA approach as a very interesting alternative to conventional alloying methods. In this work, sintered steels containing cost-effective Fe–Mn–Si masteralloysare processed at increasing temperatures in the range between 1120 and 1300 °C. The combination with different base powders provides a good overview of the properties that can be obtained with this alloying approach. Besides, the evaluation of microstructure and mechanical properties as a function of temperature allow understanding the real benefits of increasing the sintering temperature, in order to find an appropriate balance between the economic requirements and the material performance.