Importance of columnar grains in dictating hot ductility of steels

Abstract

The hot ductility of an as cast austenitic stainless steel slab having a coarse columnar grain structure emanating from the surface to the interior has been established in the temperature range 700–1000°C. Tensile specimens were taken in three directions, namely, parallel and transverse to the casting direction in a plane close to the top surface of the slab, and in the through thickness direction. The average dimensions of the columnar grains in these directions were 1, 2, and 8 mm, respectively. The ductility was highest in the through thickness direction and lowest parallel to the casting direction, this being the direction of stressing during the straightening operation in continuous casting. For the lowest ductility failures, the cracks were always intergranular. These observations can be explained in terms of the crack size being related to the grain size perpendicular to the tensile axis, this being greatest for samples tested parallel to the casting direction. In this direction, cracks can readily propagate a long distance before meeting a new grain and having to change direction. It is recommended that to restrict transverse cracking, columnar grains should be avoided.