Causes and Prevention of Surface Fissures on Hot-worked Steel Products (2nd Report)

Abstract

Microscopic examination of the enriched surface layers caused by oxidization of steels and the mechanism of surface fissures. The enriched layers formed on the surface of steel due to oxidization while heating and the cracks on the samples by hot-bending were examined microscopically, and the mechanism of the formation of surface fissures was investigated. The results obtained were as follows : (1) When commercial steels are heated in oxidizing atmosphere, the tramp elements in steel, such as Cu,Sn and As are steadily enriched on the surface of steel with the increase of scaling. These elements are usually dissolved in the steel matrix so long as the degree of enrichment is comparatively low, but as the enrichment is further developed, a non-ferrous liquid phase appears. It is noticed that the non-ferrous liquid phase is never found penetrating deeply into the matrix during the heating. (2) The scaling loss of samples necessary for non-ferrous liquid phase to be produced decreases as the amounts of Cu, Sn and As in steel are increased and the heating temperature is lowered, and the amount of the liquid to be eventually produced increases with the increase of scales. (3) The scalinig loss of samples necessary for non-ferrous liquid phase to be produced agrees exactly with the critical scaling loss for forming surface fissures mentioned in the 1st report, and the crack severity is proportional to the amount of the liquid. (4) The surface fissure is considered to be a kind of stress-corrosion crack brought about when external stress is applied to a steel sample with non-ferrous liquid phase existing on the surface. (5) The enriched phases, when not accompanying non-ferrous liquid, do not cause the usual fissures.