Assessment of microstructural evolution in cold-worked Ti-modified 14Cr-15Ni austenitic stainless steel on creep exposure

Microstructural evolution of cold-worked Ti-modified 14Cr-15Ni austenitic stainless steel on creep exposure has been studied. Creep tests were carried out at 973 K and 225 MPa on the solution annealed steel imparted with 0, 10, 20, 30 and 40 percent cold work. EBSD analysis indicated that cold working increased the fraction of low angle grain boundary resulting in increase in hardness. Creep deformation and rupture strength of the steel increased on cold working to a maximum value at around 20 percent cold work. The fraction of low angle grain boundary decreased accompanied with increase high angle grain boundary fraction on creep exposure, indicating formation of subgrain on recovery of the cold work structure. TEM investigation revealed subgrain formation as well as re-crystallization on creep exposure appreciably at higher cold work percentage, resulting in reduction in creep strength. Precipitation of fine titanium carbides pinning the dislocations was observed on creep exposure. Dislocations were found more effectively pinned by fine precipitates in 20 percent cord worked steel than that in higher cold worked steel. Deformation twins formation on creep exposure was noticed having relatively more abundant in the 20 percent cold work steel than in higher cold work, indicating more sluggish movement of dislocation in the 20 percent cold work steel. XRD investigation indicated preferential grain orientation on creep exposure with predominance presence of 220 peak over the 111 peak on cold working. Complex microstructural evolution in the cold-worked Ti-modified 14Cr-15Ni steel influenced the creep rupture strength quite differently depending on the cold work percentage.