Coarsening of Ferritic Grains of Sintered Iron during Ar₃ Transformation

Abstract

It has been reported by H. Kuroki, T. Wakabayashi and Y. Tokunaga (Journal of the JSPM, 17 (1971), 307, in Japanese) that most of the commercial iron powders show an.abnormal coarsening of ferritic grains in the density range of 6.8-7.3 g/cm³. In the present work, some attempts were made to clarify the cause of this coarsening.
Compacts, 8×8×20mm, were prepared from carbonyl iron powder (gaf'L') following the procedure of pressing under 4 t/cm², presintering at 800°C for 30min, re-pressing under 3-11 t/cm² and sintering at 850-1200°C for 5 min-48 hr in dry hydrogen.
Results were summarized as follows:
The coarse grains were formed during the Ar₃ transformation and had a tendency of growing in a columnar structure with the axis along the direction of the heat flow.
This structure was observed not only in sintered iron but also in cast iron of high purity, and the structure was promoted by a very small content of oxygen, but suppressed by that of carbon.
In low density compacts, this type of coarsening was controlled by the sticking of grain boundaries to pores.
In high density compacts, fine grains were formed in some cases, as the result of deoxidation and/or carburization caused by the lubricating oil enclosed in pores under high re-pressing pressure.