Dilatometric Analysis of Anisotropic Dimensional Changes in a 16 Pct Cr Stainless Steel with a Planar Banded Structure

Abstract

Anisotropic dimensional changes in a 16 pct Cr ferritic stainless steel possessing a banded structure of alpha + alpha' obtained by hot rolling were studied. Considerable anisotropic transformation plasticity was observed during both the austenitization and the martensite formation reactions. Anisotropy was also observed in the case of the coefficient of thermal expansion (CTE), over a wide annealing temperature range. The observations are shown to be due to the geometrical arrangement of the phases, with ferrite acting as a constraint against the in-rolling-plane straining of the pancaked gamma, thus encouraging exaggerated dimensional changes along the normal direction (ND). Assuming isotropic dimensional change within the rolling plane and combining the dilatometric results in the rolling and normal directions (NDs), the measured dilatation and CTE can be used to determine the volume fraction of alpha'. This alternative phase analysis method is shown to have advantages compared to a conventional image analysis method especially at low annealing temperatures where there are still residues of the tempered martensite.