Overheating behaviour of a grain-refined low-sulphur steel

Abstract

The effect of cooling from temperatures in the range 1100-1400°C at 2 K min−1 and air cooling on the overheating behaviour of a low-alloy Ni–Cr–Mo steel (A 508 Class 2) containing ~ 10 ppm S at two levels of manganese, i.e. 0.01 and 0·2 wt-% is described. A third steel which contained 80 ppm S and 0·2 wt-%Mn was also investigated. The results show that by decreasing the levels of manganese and sulphur the tendency to overheating can be reduced. However, unless the levels of aluminium and nitrogen are also controlled in the high-purity steel, high-temperature treatments lead to an even greater decrease in toughness as a result of the precipitation of AIN on the prior austenite grain boundaries. By increasing the manganese content, which has been found to lower the solubility temperature of A IN, and/or by air cooling from the high preheat temperatures, any deleterious effects caused by the precipitation of AIN may be reduced. In the steel of high sulphur content, intergranular fracture was observed after heat treating at temperatures above 1250°C. Manganese sulphides were identified on the intergranular facets. A decrease in toughness for this steel occurred after cooling at 2 K min−1 from the highest preheat temperatures; this decrease appears to be related to the rod-shaped morphology of the sulphides.MST/137