Evaluation of structural stability and creep resistance of 9-12% Cr steels

Abstract

During creep exposure of modified chromium steels lowering of solid solution strengthening due to precipitation of Laves phase as well as coarsening of all precipitates causes degradation of creep resistance. Two distinct domains of the stress dependence of creep rate and time to rupture have been observed in precipitation strengthened modified chromium steels. The stress characterizing the transition between these domains was found to be closely related to the Orowan stress. This stress consists in these steels of the contribution from large particles on subgrain boundaries (mainly M23C6 and during the limited time also Laves phase) and from small precipitates (Nb(C,N) and VN) inside subgrains. This has to be considered when measuring the interparticle spacing and calculating Orowan stress. Larson‐Miller parametric equation is used to elucidate the necessity of long‐term creep testing. By means of two heats of CrMoVNbN steel it is shown that reliable extrapolation of creep properties is possible only in a stress and temperature domain in which only one creep creep rupture mechanism operate. In the high stress domain Larson‐Miller constant CLM is well above 30 while in the low stress domain this constant does not exceed 25. When the extrapolation is based mainly on short‐term creep tests, the CLM constant is close to that valid in the high stress domain and therefore it overestimates long‐term creep strength.

Characterization of the carbides in the steel X20CrMoV12.1 used in thermal power plants

“…In order to simulate the evolution sequence of carbides, some specimens of the same material were prepared, but they were first quenched in oil at a temperature of 1040 • C. This temperature ensured the solution of all carbide particles in austenite. [3] Specimens of steel were then tempered from 1 to 1344 h at 800 • C.…”

Investigations on the growth kinetics of Laves phase precipitates in 12% Cr creep-resistant steels: Experimental and DICTRA calculations

“…Other authors [11] have shown that the size of the Laves phase is of great significance to the creep properties. On the basis of short-term creep tests, other investigations concluded that Laves phase particles grow and coarsen very rapidly [12,13].…”

Creep-to-rupture of the steel P92 at 650°C in oxygen-controlled stagnant lead in comparison to air

“…The VN-precipitates are highly resistant to coarsening at 600 and 650°C, contributing to long-term creep strength of the material [4,10]. The Laves phase particles may strengthen the steel, but the solid solution strengthening of the matrix is decreased, especially after coarsening of Fe 2 (W,Mo) [6,[11][12][13]. Dislocation density decreases and the sub-grain width increases by factor of 3-5 during aging at stress applied [4].…”