Eutectic growth of unidirectionally solidified iron-carbon alloy

Abstract

Unidirectional solidification experiments were conducted on high purity iron-carbon eutectic alloys containing 0.007-0.449 wt% sulphur to study their solidification behaviour and influence of sulphur on it. Rod specimens of 16 mm diameter X 135 mm long were melted and moved at a constant speed through a vertical furnace. The temperature gradient at the solid-liquid interface was 45°C/cm and the solidification rates were varied from 1 to 250 mm/hr. The inter-graphite spacing, λ, of specimens solidified with a macroscopic planat interface changed in accordance with the relationship λ = Av - 1 2 as found for normal lamellar eutectics, where A is a constant and v is the solidification rate. A small amount of sulphur decreased the intergraphite spacing markedly but more sulphur did not decrease the spacing any more. While, in the solidification condition under which eutectic cells were formed, sulphur increased the intergraphite spacing. The critical value of G/v for the morphological stability of microscopic interface was estimated about 10 X 10 6 deg sec/cm 2 theoretically. When G/v is less than this value, the interface of the austenite becomes unstable and the depth of the austenite groove increases with the proceeding of solidification. The branching and the pinching off of the graphite would occur to cancel the instability of the groove. Colony breakdown of the macroscopic planar interface was accelerated by increasing the sulphur content and/or by decreasing the value of G/v. The stability of the observed macroscopic planar interface is higher than the theoretical expectation.