Creep Behavior and Degradation of Subgrain Structures Pinned by Nanoscale Precipitates in Strength-Enhanced 5 to 12 Pct Cr Ferritic Steels

Creep behavior and degradation of subgrain structures and precipitates of Gr. 122 type x Cr-2W-0.4Mo-1Cu-VNb ( x = 5, 7, 9, 10.5, and 12 pct) steels were evaluated during short-term and long-term static aging and creep with regard to the Cr content of steel. Creep rupture life increased from 5 to 12 pct Cr in the short-term creep region, whereas in the long-term creep region, it increased up to 9 pct Cr and then decreased with the addition of Cr from 9 to 12 pct. Behavior of creep rupture life was attributed to the size of elongated subgrains. In the short-term creep region, subgrain size decreased from 5 to 12 pct Cr, corresponding to the longer creep strength. However, in the long-term creep region after 10 4 hours, subgrain size increased up to 9 pct Cr and then decreased from 9 to 12 pct, corresponding to the behavior of creep rupture life. M 23 C 6 and MX precipitates had the highest number fraction among all of the precipitates present in the studied steels. Cr concentration dependence of spacing of M 23 C 6 and MX precipitates exhibited a V-like shape during short-term as well as long-term aging at 923 K (650 °C), and the minimum spacing of precipitates belonged to 9 pct Cr steel, corresponding to the lowest recovery speed of subgrain structures. In the short-term creep region, subgrain coarsening during creep was controlled by strain and proceeded slower with the addition of Cr, whereas in long-term creep region, subgrain coarsening was controlled by the stability of precipitates rather than due to the creep plastic deformation and took place faster from 9 to 12 pct and 9 to 5 pct Cr. However, M 23 C 6 precipitates played a more important role than MX precipitates in the control of subgrain coarsening, and there was a closer correlation between spacing of M 23 C 6 precipitates and subgrain size during static aging and long-term creep region.