Comportement structural lors de trempes étagées de fontes G.S. au molybdène ayant une teneur élevée en silicium

Structural behaviour of spheroidal gray cast iron with molybdenum and a high silicon content during step quenching

Laboratoire de Structure des Matériaux Métalliques, Centre Scientifique d'Orsay, Université Paris Sud

Abstract

Spheroidal Gray Cast Iron (SGC iron) has many industrial applications (for example, in the automotive industry). However, an increase in its working temperature is desirable. This requires both an increase of the transformation temperature and a reduction in the cementite content. With the aim of obtaining these requirements, three types of SGC Iron, with a high percentage of silicon (4wt%) and 0.5, I and I.5wt% Mo respectively have been studied. In this paper, we report the structural behaviour of these alloys during step quenching. For temperatures above 800°C, tranformations obsen’ed are the formation of small globular secondary graphite followed by the formation of ferrite. In the temperature range between 550 and 800°C, transformations which have been observed are : I ) formation of small globular secondary graphite, 2) formation of ferrite, 3) formation of perlite, 4) globularization of cementite, and 5) the graphitization. For temperatures between Ms and 550°C, the formation of small globular secondary graphite is first observed followed by the formation of bainite (upper bainite for T>450°C and lower bainite for T<450°C). The influence of Mo has been found to be, on one hand the separation of the bainite and ferrite-perlite domains and on the other hand, an increase in the incubation time prior to transformation on isothermal curves. Furthermore, this element prevents the formation of silicocarbide. The absence of the three ¡¡hase field (austenite+bainite+carbide) in the case of these alloys is correlated to the presence of Si.