Importance of columnar grains in dictating hot ductility of steels

The hot ductility of an as cast austenitic stainless steel slab having a co arse columnar grain structure emanating from the surface to the interior ha s been established in the temperature range 700-1000 degrees C. Tensile spe cimens were taken in three directions, namely, parallel and transverse to t he casting direction in a plane close to the top surface of the slab, and i n the through thickness direction, The average dimensions of the columnar g rains in these directions were 1, 2, and 8 mm, respectively. The ductility was highest in the through thickness direction and lowest parallel to the c asting direction, this being the direction of stressing during the straight ening operation in continuous casting. For the lowest ductility failures, t he cracks were always intergranular. These observations can be explained in terms of the crack size being related to the grain size perpendicular to t he tensile axis, this being greatest for samples tested parallel to the cas ting direction. In this direction, cracks can readily propagate a long dist ance before meeting a new grain and having to change direction. It is recom mended that to restrict transverse cracking, columnar grains should be avoi ded. MST/4398.