AbstractA new theory for the transition from columnar to equiaxed crystals has been developed. It is proposed that free crystals are formed in the liquid ahead of the growing front and that these crystals grow in the liquid as a result of natural convection. When the free crystals are sufficiently large or sufficiently numerous they physically block the growth of the columnar crystals by adhering to the solidification front. The growth rate of the free crystals is determined by the cooling rate of the mould and the kinetics of the solidification process. The theory indicates that the higher the cooling rate and the slower the solidification process the easier the transition. The calculations are compared with experimental observations on a large steel ingot.MST/272