Effect of intermediate heat treatments on failure rates of boron-modified 9Cr-2 W steel tubes during drawing

Abstract

The manufacturing process of a cladding tube comprises cold pilgering, intermediate heat treatment, and drawing. Intermediate heat treatment after pilgering can result in failure during drawing. In this study, boron-modified 9Cr-2Wsteel tubes obtained via various heat treatments are investigated for their grain structure, secondary particle segregation, and texture. Elongated grains are observed in as-pilgered and tempered tubes, whereas normalized as well as normalized and tempered tubes exhibit equiaxed grain structures owing to a phase transformation from gamma to alpha'. Even though boron segregations are observed in all the tubes subjected to heat treatment, carbon and chromium segregations decreased in the normalized heat-treated tubes. Inverse pole figure and orientation distribution function texture studies indicate that the normalized tubes show the preferred texture direction, and that both alpha- and gamma-fibers developed equally for numerous slip bands. Finally, the failure rate of the as-pilgered tubes (approximately 90%) reduced significantly to 32% after normalized heat treatment, whereas the failure rate of the tempered tubes is comparable to that of the as-pilgered tubes.