Hot Deformation Characteristics of 9Cr-1.5Mo-1.25Co-VNb Ferritic-Martensitic Steel
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DUCTION
OWING to their excellent thermophysical properties characterized by high thermal conductivity and high creep resistance, Ferritic-Martensitic (F-M) steels with high levels of Cr (9 to 12 wt pct) have been regarded as potential candidates for use in ultra-supercritical (USC) power plants.[1,2] F-M steels with 9 to 12 wt pct of Cr were initially developed for fossil fuel-fired power plants in the 1960s, and their creep rupture strength has gradually increased over the decades with the addition of Nb, V, and other alloying elements.[3–5] 9 to 12 wt pct Cr steels have utilized the strengthening mechanisms of solution hardening, dislocation hardening, and precipitation hardening to improve the creep properties at high temperatures.[5] Among the strengthening mechanisms, precipitation hardening plays a decisive role. Generally, 9 to 12 wt pct Cr steels are normalized and tempered, and their microstructures therefore typically consist of tempered martensite.[6] JOONOH MOON, TAE-HO LEE, CHANG-HOON LEE, and SEONG-JUN PARK are with the Steel Department, Metallic Materials Division, Korea Institute of Materials Science, 797 Changwondaero, Seongsangu, Changwon, Gyeongnam, 51508, Republic of Korea. Contact e-mail: [email protected] JONH-HO SHIN and JONG-WOOK LEE are with Doosan Heavy Industries & Construction, 555 Gwigok-Dong, Seongsangu, Changwon, Gyeongnam, 51711, Republic of Korea. Manuscript submitted December 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
During the tempering process, Cr-enriched M23C6 carbides precipitate along prior austenite grain boundaries (PAGB) and martensite laths. In addition, the lath interior areas are decorated by fine MC (M: Nb and/or V) carbides that can contribute to strengthening.[6] These precipitates impart beneficial effects on the creep properties. Previous work has reported that M23C6 carbides disturb the recovery of the dislocation substructure[7] and that the MC carbides themselves obstruct the movement of dislocations, which increases the creep strength.[8] Meanwhile, many attempts have been made to control the precipitation and coarsening behavior of these precipitates in steels because fine particles which are uniformly dispersed are the most effective means of improving the strength.[5,9,10] Boron (B) is a typical alloying element which controls the coarsening rate of the M23C6 carbide; B dissolves into M23C6 carbides and reduces their coarsening. As reviewed above, many works have reported beneficial effects of precipitates on the creep properties. However, a negative effect has also been reported.[11] Heo et al. found that intergranular cracking under tensile stress at an elevated temperature occurred due to decohesion of the grain boundary/carbide interfaces at carbide-free grain boundaries. Thus, it is conceivable that intergranular carbide precipitation during a thermo-mechanical process, such as hot forging or hot rolling, can deteriorate the hot workability.
Recently, the modified 9Cr-1Mo steel, COST FB2, was developed in Europe, and it has received much attention ow
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