Ductility and Mass Effect in Large Steel Castings

Abstract

Ductility and mass effect in large steel castings were studied on production scale, using experimental castings of Al killed steel and 0.36%C steel of which dimensions are 200×200×650mm, 300∅×650mm, 600×650mm∅, 1000∅×1050mm and 80×140×180mm respectively under both conditions of single normalizing and double normalizing.
The results obtained are summarized as follows:
1) Fine austenitic grains were obtained by double normalizing, compared with single normalizing, in both high Al-contained cast steel and low Al-contained cast steel. Coarsening temperature can considered to be 1050°C for high Al-contained cast steel and 1000°C for low Al-contained cast steel.
2) Mixed grain obtained in the low temperature range in single normalizing is thought to be primary mixed grain influenced by the initial grain at the as-cast state, and that in the higher temperature range is secondary mixed grain caused by mixture of coarse grain and incompletely grown grain. Primary mixed grain in double normalizing is seemed to be caused by great difference between coarse grain of prior mixed grain and new grain formed by subsequent treatment.
3) Ductility of steel castings is generally more significantly improved by double normalizing than by single normalizing, but injured by mixed grains that appear in the begining of coarsening.
4) Concerning to ductility of sound and segregating zone in large steel castings, low temperature of 1000°C-1050°C is effective for diffusion annealing of sound zone, and high temperature of 1100°C forsegregating zone. Consequently, in accordance with ductility level demanded for cast steel, it is industrially necessary to select optimum conditions of diffusion annealing; diffusion annealing should be carried out in the low temperature range when ductility of sound zone is not enough high, but in the high temperature range when ductility of segregating zone is important.
5) Mass effect generated on casting in large steel castings is greatly influenced by C content in this experimental casting alloyed with small amount of Ni and Cr.
6) As for mass effect generated on heat treatment, ductility of 300mm dia casting hardly decreases in even the inside, but it decreases abruptly for inside castings of 600, 1000mm dia.
If factor of mass effect is considered, it is also possible for steel castings to estimate tensile strength nondestructively from chemical compositions and heat treated conditions.