No abstract

Manganese is a standard alloy element in many wrought steels, offering improved strength and hardenability and, in Hadfield steels, also a unique combination of strength, ductility and work hardening behaviour. In sintered steels Mn has been used rarely so far because of two reasons: one is the high oxygen affinity of Mn which promotes oxygen pickup during heat treatment and inhibits removal of oxygen in part through the 'internal getter' effect. The other reason is the high vapour pressure of Mn which results in Mn loss during sintering. In this presentation, these problems are discussed on Mn-alloyed sintered steels with transformation-induced plasticity behaviour. It is shown that by suitable alloying techniques, combined with adjustment of the manufacturing parameters, the problems can be overcome and the benefits of Mn addition can be reaped, obtaining PM steels for which potentially surface densification and hardening can be attained in one run.

For powder metallurgy products, high density is an essential requirements to obtain maximum mechanical properties. Here, supersolidus liquid phase sintering (SSPLS) is an effective means to attain high sintered density, as known from PM high speed steels. In the present work it is shown that this technique can also be applied to Cr prealloyed low alloy steel grades. Supersolidus sintering through indirect heating requires precise control of temperature and also the atmosphere, to avoid uncontrolled changes of the carbon level. Higher C contents are beneficial here since they enable lower temperatures and result in wider temperature windows for sintering. The temperatures necessary for SSLPS at moderate C levels are fairly high for standard sintering furnaces, therefore induction sintering was studied in this work. It showed that, as was to be expected, also here precise temperature control is required, but for any carbon level tested a sintering temperature could be identified that yielded high sintered density and good shape retention. The high density attained, in combination with the very high temperatures, results in pronounced grain growth, this process no more being inhibited by the presence of pores, which is undesirable but can however be remedied by suitable heat treatment.

KurzfassungBei der Wärmebehandlung von Stählen ist die Austenitkorngröße ein wesentlicher Parameter; zu hohe Austenitisiertemperaturen und/oder eine zu lange Dauer führen zu Kornvergröberung mit negativen Auswirkungen auf die mechanischen Eigenschaften. Bei sinterhärtenden Stählen ist Überhitzung aber unvermeidlich, da die Sinterung bei 1120–1280 °C durchgeführt wird. Mittels einer speziellen thermischen Ätzung wird hier gezeigt, dass Sinterstähle in einem breiten Dichte-/Porositätsbereich praktisch überhitzungsunempfindlich sind. Das Austenitkorn wird durch die Restporosität stabilisiert und sogar bei hochdichten Sinterstählen mit Restporosität < 4 % reichen die Poren aus, um das Kornwachstum beim Sintern zu unterdrücken.