The sintering process has been a popular topic for presentations at international PM conferences for as long as one can remember. The EURO PM2015 conference in Reims, France, was no exception. Professor Herbert Danninger and his research team at TU Wien (the Technical University of Vienna) have been very active in this sector of PM research and were well represented at EURO PM2015.
For a number of years, low-alloy steel powders containing chromium have been in use for the manufacture of high-strength components, replacing previously used alloy compositions containing expensive nickel and copper. However, due to the high affinity of chromium for oxygen, sintering conditions have to be suitably modified, most commonly by the use of low dew-point nitrogen/hydrogen furnace atmospheres, typically in the ratio of 90% N2 and 10% H2. Over the years, many studies have been made on the process and the resulting material properties. Some of the latest work concerned with the interaction between atmosphere and compacted powder in the sintering furnace was reported in sessions at the EURO PM2015. One of the interesting features was the use of advanced analytical techniques.
Christian Gierl-Mayer and colleagues at TU Wien made a number of comparison experiments with plain iron powder and pre-alloyed low-alloy Cr–Mo steel powder to study the details of oxidation and reduction during sintering, as well as the resulting properties [1]. The metal powders employed were ASC100.29 plain iron powder and Astaloy CrM prealloyed steel powder, both from Höganäs AB. These experiments took a very interesting and illuminating approach. The sintering experiments followed the performance of intentionally pre-oxidized powders. In one series the base powder plus 0.5% graphite was pressed at 600 MPa with die-wall lubrication and then subjected to oxidation at 300 or 400 °C for 30 or 60 min. For the second series the base powders were oxidized before compacting. The oxidation at 300 °C was performed in a drying oven, while the 400 °C treatment was in a tube furnace. Oxygen pick-up for the two types of powder was surprisingly similar, while on the other hand, as expected, the oxidation of the pressed samples was not homogeneous.
The oxidation treatments at 300 °C were followed by two types of sintering experiments: in the first series, sintering was done in a furnace at seven different temperatures from 700 to 1250 °C with a 90/10 N2/H2 atmosphere. In the second series, sintering was performed in a dilatometer coupled with a mass spectrometer, heating in pure hydrogen at 10 °C/min to 1300 °C, holding at temperature for 60 min followed by cooling at the same rate as the heating. The sintered samples were analyzed for carbon and oxygen, as well as the measurement of density, impact value, and hardness, plus examination of microstructure. The dilatometer/MS experiments provided insights into the reactions taking place during the heating cycle by measurement of specific gas species evolved. These tests provided a mass of data from which the authors concluded a number of interesting points.
This article appeared in the May/June issue of Metal Powder Report.
