Creep cavitation in iron—I. Sulfides and carbides as nucleation sites

Abstract

The nucleation of creep cavities in iron containing controlled additions of C, P and S has been studied using slow strain rate tensile tests ( ϵ ̇ = 1.1 × 10 −6 and 4.4 × 10 −5 s −1) at te and 800°C. The test materials all had the same 100 μm grain size and showed approximately the same UTS at 700°C. When failure occurred by grain boundary cavitation a ductility minimum was found at 700°C and the ductility (% RA) was less than about 40%. The cavitation process depends strongly on the S content: if the S content is above the solubility limit, cavities nucleate easily on grain boundary sulfides and the ductility is very low. In this S regime additions of P can improve the ductility, but the mechanism of this improvement has not been identified. If the S content is well below the solubility limit (about 1 wtppm), the material is always very ductile, even though there are many small (~ 0.1 μm dia.) oxide particles on the grain boundaries that are potential nucleation sites. Carbides were never seen to nucleate cavities, even when the C concentration is as high as 4 times the solubility limit at 700°C. Auger analyses of the grain boundary facets of one of the brittle alloys showed that both S and C segregate to the grain boundaries.