Materials Science and Engineering A 532 (2012) 373–380 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering A jo ur n al hom epage: www.elsevier.com/locate/msea Strain-induced coarsening of nanoscale precipitates in strength enhanced high Cr ferritic steels Hassan Ghassemi Armaki a,∗ , Ruiping Chen a , S. Kano a,c , Kouichi Maruyama a , Yasushi Hasegawa b , Masaaki Igarashi c a Graduate School of Environmental Studies, Tohoku University, 6-6-02 Aobayama, Sendai 980-8579, Japan b Nippon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan c Corporate Research and Development Laboratories, Sumitomo Metal Industries, Ltd., 1-8 Fuso-Cho, Amagazaki 660-0891, Japan a r t i c l e i n f o Article history: Received 4 February 2011 Received in revised form 28 October 2011 Accepted 30 October 2011 Available online 9 November 2011 Keywords: High Cr ferritic steels Strain-induced coarsening M23C6 precipitates Breakdown of creep strength Static recovery a b s t r a c t Strain-induced coarsening of precipitates, mainly M 23 C 6 , has been studied in several types of Gr. 122, Gr. 92 and Gr. 91 steels in a wide range of creep conditions. Strain-induced coarsening of precipitates should be separately evaluated in high stress and short-term creep region (or called “H”), where precipitates and subgrains are thermally stable, and low stress in long-term creep region (or called “L”), where ther- mal coarsening of precipitates and subgrains appear. Creep plastic deformation does not accelerate the coarsening of precipitates during the primary and secondary creep regions in short-term creep region “H”. However, the coarsening of M 23 C 6 precipitates occurs during the acceleration creep region and increases with decreasing of applied stress. The onset of long-term region “L” is accompanied by the thermal coars- ening of M 23 C 6 precipitates as well as strain-induced coarsening of M 23 C 6 precipitates during secondary and acceleration creep regions and maybe in primary creep region under very low creep stress. In long- term region “L”, recovery of the dislocation substructure is controlled simultaneously by creep plastic deformation and pinning force from precipitates. On the other hand, the coarsening of precipitates is due to both thermal coarsening and strain-induced coarsening of precipitates. In fact, the stability of precipi- tates is the most important factor for the suppression of subgrain recovery in long-term region “L” rather than short-term creep region “H”. In long-term region “L”, greater thermal recovery of subgrains and M 23 C 6 precipitates due to higher creep temperature or/and longer creep rupture life results in greater strain-induced coarsening of M 23 C 6 precipitates. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Strength enhanced high Cr ferritic steels are widely used for high temperature components for fossil power plants because of their high strength at high temperature and the advantage of small ther- mal stress in cyclic operations. After conventional heat treatment, the transformed microstructure exhibits elongated subgrains and during heat treatment, the precipitation of M 23 C 6 particles occurs on the prior austenite and subgrain boundaries and fine MX parti- cles appear in the subgrain interior. For achieving high resistance against creep at elevated temperatures, a stable microstructure containing a fine subgrain structure with fine precipitates at the subgrain boundaries and in the subgrain interior is needed [1–10]. The microstructural stability of ferritic heat resistant steels is mainly controlled by the stability of precipitates that are able to ∗ Corresponding author. Present address: Graduate School of Engineering, Brown University, Providence, RI, USA. Tel.: +1 401 2257152/+81 22 795 7326; fax: +81 22 795 7325. E-mail address: hasan gm2003@yahoo.com (H. Ghassemi Armaki). retard the growth rate of subgrains. The role of pinning obstacles (precipitates) against dislocation motion and sub-boundary migra- tion (or growth of subgrains) is the key parameter in strength enhanced high Cr ferritic steels. Pinning force of sub-boundaries from precipitates is determined not by diameter but primarily by spacing of particles [10]. Therefore we measured the spacing instead of the diameter. The following equation is widely known on Ostwald ripening by following equation: d 3 - d 3 0 = kt (1) and  = d   6V  1/3 (2) where d is the diameter and  is the interspacing and V the volume fraction of precipitates. Therefore,  3 -  3 0 = k   6V  t (3) If the volume fraction V is kept constant, the Ostwald ripening can be analyzed by measuring the particle spacing, too. In the case 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.105