Effect of impurity content on creep crack growth resistance in 1Cr1Mo0.25V ferritic steels

Abstract

The effect of impurity content on creep crack growth (CCG) rate and, more generally, on hot ductility of a typical lCrlMo0.25V ferritic steel was evaluated. Four heats intentionally doped with various amounts of impurities were characterized after heat treatments simulating the industrial thermal cycle taking place in a 1000-mm-diameter high-pressure rotor at two positions: near the outside surface and at the center in the cases of air cooling and oil quenching, respectively. Results indicate that the highest crack growth rates occur in the grade with a low P content (40 ppm) and Sn and Sb values (100 to 200 ppm) comparable with those characteristic of commercial steels. A marked reduction in brittleness is achieved only through a substantial reduction in the amount of Sn and Sb, even when medium-to-high P levels (100 ppm) are present. Creep resistance in terms of both time to rupture and minimum growth rate is not influenced by the impurity content, at least within the range of stresses investigated. Auger analyses on crept specimens demonstrate the presence of a selective segregation of impurity elements similar to that found in other ferritic steels: P is the only segregating element at non-cavitated grain boundaries, while cavitated areas contain Sn, Sb, and Cu in addition to P. The embrittlement at high Sn and Sb levels depends on two factors: at low P levels, cracks rapidly propagate under surface diffusivity control; at high P levels, excess P segregates at the grain boundaries, and crack propagation proceeds by an intergranular decohesion mechanism.