Fractography of Sinteraustempered and Sinterhardened Fe-3Mn-0.8C PM Steels

Sintered steels with the addition of manganese are widely used in industry because of their attractive mechanical properties. The main problem of using manganese in powder metallurgy steel production is its high affinity for oxygen. The choice of proper sintering parameters can significantly improve the properties of the final product. For the present investigations Höganäs iron powder grade NC 100.24, low-carbon (1.3%C) ferromanganese Elkem (Eramet Norway Sauda – formerly Elkem Manganese Sauda) and graphite powder grade C-UF were used as the starting powders. Mixture of powders, containing 3% Mn and 0.8% C, was prepared in Turbula mixer for 30 minutes. Following mixing, “dog bone” compacts were pressed at 660 MPa according to PN-EN ISO 2740 standard. Sintering of compacts was carried out in the laboratory tube furnace at 1120 and 1250°C for 60 minutes in air or pure nitrogen (99.999%N2). The present investigation deals with the comparison of two heat treatment routes: sinterhardening (SH) and sinteraustempering (SAT), carried out both in air and pure nitrogen. Mechanically tested steels were investigated using JEOL JSM 700F completed with EDS. Metallography tests were carried out on 3% Nital etched samples. In this paper the results of mechanical tests and metallography and fractography investigations are shown. According to the results obtained, it can be assumed that for sinteraustempering higher temperature does not influence mechanical properties of sintered steels. For both methods using pure nitrogen as sintering atmosphere gave better results.