Influence of Microstructure on the Fatigue Performance of PM Steels
PM²TEC 2003, in Las Vegas, USA. Authors: Ola Bergman, Anders Bergmark. The fatigue properties of PM steels are mainly depending on the microstructure and density. Focus of this article is to quantify the mechanical performance of PM steel materials based on mixed, pre-alloyed and diffusion-alloyed powders in relation to the microstructures. Fatigue performance is the target parameter and this has decided the amounts of graphite added to the base powders. With one exception, test samples were sintered for 30 minutes at 2050°F/1120°C in 90%N2/10%H2. Fatigue tests have been made in displacement controlled four-point plane bending on modified ISO 3928 specimens. No machining of the test bars was made. All materials have been tested at densities close to 7.1 g/cm³. The PM steel microstructures are classified according to their fatigue performance. Sinter-hardened Astaloy CrM + 0.35%C with its martensitic microstructure has the highest fatigue limit of the tested materials (54 ksi/370 MPa). The diffusion-alloyed materials exhibit somewhat lower bending fatigue limits with D.HP + 0.8%C (48 ksi/330 MPa) followed by D.AE + 0.8%C (40 ksi/280 MPa). These materials demonstrate pronounced heterogeneous microstructures characterized by a continuous network of martensite with islands of softer structures. The pre-alloyed Astaloy CrL + 0.8%C with its mainly lower bainitic microstructure reaches a fatigue limit of 38 ksi/260 MPa. Astaloy Mo + 0.8%C has a microstructure dominated by upper bainite and reaches a fatigue limit of 31 ksi/210 MPa. The FC-0208 has a fatigue limit of 32 ksi/220 MPa and the microstructure consists mainly of fine pearlite.