Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels

Abstract

Centrifugally cast heat-resistant HP stainless steels are particularly suitable for applications where service conditions comprise high temperatures and aggressive environments; thus, they are extensively used in reformer furnaces, in which hydrogen production takes place. The demand for better performance has motivated developments in these steels. The addition of Nb and Ti as microstructural modifiers has proved successful in providing a more stable microstructure. In this work yttrium was added to centrifugally cast NbTi-modified HP steels. It was observed that its presence increased the level of fragmentation of the chromium carbides, a positive aspect for creep resistance. The main cause of the fragmentation is the formation of yttrium carbides, which serve as heterogeneous nucleation sites for the other carbides. One of the tubes, with a lower titanium content, showed the best creep performance among those tubes studied owing to the presence of a smaller volume fraction of the deleterious G phase.