New Alloying Systems for Sintered Steels: Critical Aspects of Sintering Behavior

Oxygen-sensitive alloying elements such as Mn, Si, and Cr have a high potential for improving the properties of low alloyed sintered steels while reducing the alloying cost. However, it is necessary to find a way for avoiding, or at least minimizing, the oxidation of these elements especially during the early stages of the sintering cycle. In this study Mn, Si, and Cr were introduced in the form of a master alloy powder designed to be mixed with the iron base powder and provide the final composition of the steel during the sintering process. The reduction/oxidation phenomena taking place during the heating stage were studied by thermogravimetry, dilatometry, and mass spectroscopy, using either reducing (H2) or inert (Ar) atmospheres. The results show how the difference in chemical activity between base iron powder and master alloy causes the so called "internal-getter" effect, by which the reduction of less stable iron oxides leads to oxidation of the elements with higher affinity for oxygen. This effect can be somehow minimized when sintering in H2, since the iron oxides are reduced at lower temperatures at which the reactivity of the elements in the master alloy is lower. However, H2 concentration in the processing atmosphere needs to be carefully adapted to the specific composition of the materials being processed in order to minimize decarburization by methane formation during sintering.Höganäs AB Sweden, financial support provided through the Höganäs Chair IVPublicad

Application of thermal analysis techniques to study the oxidation/reduction phenomena during sintering of steels containing oxygen-sensitive alloying elements

The final publication is available at Springer via https://doi.org/10.1007/s10973-016-5508-5.For the consolidation of steel parts manufactured by powder metallurgy (PM) techniques, removal of the surface oxides covering metallic powder particles is a necessary prerequisite. In PM steels with conventional compositions, reduction of the oxides is easily achieved in traditional sintering furnaces. However, processing steels containing alloying elements with a high oxygen affinity represents a big challenge that requires a deeper understanding of the chemical processes occurring during sintering. In the present work, thermogravimetry analysis coupled with mass spectrometry is used to describe the oxidation/reduction phenomena that take place when sintering steel powders and how these processes are modified by the addition of admixed particles containing oxygen-sensitive elements. Carbothermal reduction processes are studied using pure oxides (Fe2O3, MnO2, Cr2O3 and SiO2) as well as water-atomized Fe powders mixed with small amounts—4 mass/%—of Cr, Mn and Si powders or Fe–Mn–Si–(Cr) master alloy powders. The results show that there is an oxygen transfer from the base iron particles to the oxidation-sensitive elements—“internal getter effect”—taking place mostly through the gas phase. Different alloying elements (Cr, Mn, Si) show different temperature ranges of susceptibility to oxidation. Combination of these oxygen-sensitive alloying elements in the form of a master alloy powder reduces their sensitivity to oxidation. Also, the use of master alloys promotes the concentration of the oxides on the surface of the alloying particles and not in the grain boundaries of the surrounding iron particles—as occurs when using Mn carriers—which should have a beneficial impact on the final mechanical performance.European Union Seventh Framework, People Programme (Marie Curie Actions) FP7/2007-2013911051

Oxide Transformation in Cr-Mn-Prealloyed Sintered Steels: Thermodynamic and Kinetic Aspects

The main obstacle for utilization of Cr and Mn as alloying elements in powder metallurgy is their high oxygen affinity leading to oxidation risk during powder manufacturing, handling, and especially during further consolidation. Despite the high purity of the commercially available Cr- and Mn-prealloyed iron powder grades, the risk of stable oxide formation during the sintering process remains. Thermodynamic and kinetic simulation of the oxide formation/transformation on the former powder surface during heating and sintering stages using thermodynamic modeling tools (Thermo-Calc and HSC Chemistry) was performed. Simulation is based on the results from the analysis of amount, morphology, and composition of the oxide phases inside the inter-particle necks in the specimens from interrupted sintering trials utilizing advanced analysis tools (HRSEM + EDX and XPS). The effect of the processing parameters, such as sintering atmosphere composition, temperature profile as well as graphite addition on the possible scenarios of oxide reduction/formation/transformation for Fe-Cr-Mn-C powder systems, was evaluated. Results indicate that oxide transformation occurs in accordance with the thermodynamic stability of oxides as follows: Fe2O3 -> FeO -> Fe2MnO4 -> Cr2FeO4 -> Cr2O3 -> MnCr2O4 -> MnO/MnSiO (x) -> SiO2. Spinel MnCr2O4 was identified as the most stable oxide phase at applied sintering conditions up to 1393 K (1120 A degrees C). Controlled conditions during the heating stage minimize the formation of stable oxide products and produce oxide-free sintered parts