Austenitization Process Research Articles

The nucleation of austenite in ferritic ductile cast iron

Austempered ductile cast iron has recently been receiving increasing attention because of its excellent combination of strength and ductility. Since the austenitization process has a significant influence on the mechanical properties of austempered ductile cast iron, several investigations on the nucleation sites of austenite and diffusion paths of carbon from spheroidal graphite have been reported in ferritic ductile cast iron. However, agreement on this subject has not ben reached. The purpose of this paper is to study the preferential nucleation sites of austenite during austenitization at two austenitizing temperatures in ferritic ductile cast iron. An attempt was made to understand the reasons which give rise to preferential austenite nucleation sites. The carbon diffusion paths from spheroidal graphite were also investigated.

Experimental study of the austenitization process of hypereutectoid steel alloyed with small amounts of silicon, manganese and chromium, and with an initial structure of globular cementite in a ferrite matrix

The austenitization process was examined in steel containing 0.80 wt.% C, 2.02 wt.% Si, 2.06 wt.% Mn and 1.05 wt.% Cr by structure observations in the scanning electron microscope, and microanalytical measurements of structural phases. The initial structure of the material, consisting of globular cementite in a ferrite matrix, was obtained in two different ways, resulting in two types of initial structure. The kinetics of the α → γ transformation in this steel is very slow. The kinetics and the morphology of the structure are significantly influenced by the type of initial structure, and in particular by the morphology of the ferrite grains. The origin of these effects is regarded as being the result of the specific influence of silicon on carbon activity in austenite, and of the redistribution of silicon and manganese at the α- γ phase boundary during the α → γ transformation.

An optical bench

Scuffing failure was induced in samples of SAE 4340 steel which had been quenched and annealed at several temperatures. Examination of worn areas using grazing-incidence X-ray diffraction revealed an austenitic phase at volume fractions of up to 60%. No dependence of austenite fraction on depth was observed for depths of up to 5.0 μm. No clear trend in austenite fraction after scuffing as a function of tempering temperature was found. One wear test was interrupted precisely during scuffing, and the worn areas of that sample showed 15% austenite. A fully scuffed sample of the same tempering temperature contained 60% austenite, while analysis of original material and contact areas prior to scuffing initiation revealed no austenite.Observations from the present work suggest that austenite formation was caused by scuffing and not the other way around. Austenite may have formed due to a rapid austenitization process, mechanical dissolution of carbides, or a diffusionless transformation. Once formed, the austenite may have been stabilized by stress or by small crystallite size, but not likely by compositional change.Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy under Contract W-31-109-ENG-38.