Case hardening steels cover a dominant fraction of engineering steel production. Particularly with recent developments in automotive power train elements, good hardenability, high tensile strength, high fatigue strength and high impact resistance are required. With a share of up to 40 %, the material and the heat treatment process take a main part of the overall manufacturing costs. Production costs can be reduced by shortening the overall process time from the melting to the finishing. New developments in carburising technology with an increased carburising temperature offer the possibility of time and cost reduction. Simultaneously a precise grain size control during austenitising is required. A homogenous former austenite grain size distribution results in a more uniform performance of the mechanical properties. This paper summarises the results of recent research regarding the demands on steels for a high temperature carburising process. The possibility to prevent undesired grain growth by increasing the grain coarsening temperature using microalloying elements as well as the impact on the production process is discussed. The results show that a considerable cost reduction of case hardening components can be achieved by the use of increased carburising temperatures. These temperatures however need adapted materials to prevent austenite grain growth with its deteriorating effects on the mechanical properties. In comparison to conventional case hardening steels, the grain coarsening temperature of a microalloyed steels can be raised up to 1050 deg C. Precipitates of the microalloying elements Nb, Ti act as grain boundary pinning particles and thus prevent grain growth. The benefit is a reduced overall process time of about 40 %. Large gears - with a case depth >2mm - can also be heat treated by direct quenching, whereas conventional steels require a single hardening. A more homogeneous grain size distribution furthermore causes a reduction of distortion. This offers the possibility of savings of hard machining operations with an according decrease of production costs.
Microalloying in case hardening steels - a challenge for the lean production of gears
Mikrolegieren von Einsatzstählen - eine Herausforderung für die schlanke Produktion von Getrieben
2005
8 Seiten, 7 Bilder, 2 Tabellen, 12 Quellen
Conference paper
English
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