Advances in Physical Metallurgy and Processing of Steels. Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel.

Abstract

The creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure are critically reviewed on the basis of experimental data. Special attention is directed to the following three subjects: creep mechanism of the ferritic steel, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use.The high Cr ferritic steel is characterized by its fine subgrain structure with a high density of free dislocations within the subgrains. The dislocation substructure is the most densely distributed obstacle to dislocation motion in the steel. Its recovery controls creep rate and rupture life at elevated temperatures. Improvement of creep strength of the steel requires a fine subgrain structure with a high density of free dislocations. A sufficient number of pinning particles (MX particles in subgrain interior and M 23 C 6 particles on sub-boundaries) are necessary to cancel a large driving force for recovery due to the high dislocation density. Coarsening and agglomeration of the pinning particles have to be delayed by an appropriate alloy design of the steel.Creep rupture strength of the high Cr ferritic steel decreases quickly after long-term use. A significant improvement of creep rupture strength can be achieved if we can prevent the loss of rupture strength. In the steel tempered at high temperature, enhanced recovery of the subgrain structure along grain boundaries is the cause of the premature failure and the consequent loss of rupture strength. However, the scenario is not always applicable. Further studies are needed to solve this important problem of high Cr ferritic steel. MX particles are necessary to retain a fine subgrain structure and to achieve the excellent creep strength of the high Cr ferritic steel. Strengthening mechanism of the MX particles is another important problem left unsolved.KEY WORDS: steel for elevated temperature service; creep; strengthening mechanism; alloy design; microstructure; microstructural degradation.

Finite element modelling of creep cavity filling by solute diffusion

“…These cavities grow under the influence of stress which can lead to failure. Generally, strategies are adopted to prevent creep damage to form, or slow down its growth [1,2]. An alternative method in the form of self healing has been proposed by Laha and coworkers [3] and Shinya [4], where selective precipitation of supersaturated solute at creep cavities hinders the creep cavity growth in stainless steels.…”

The effect of shot peening on notched low cycle fatigue

“…These are fully tempered martensitic steels characterised by well formed laths and a high dislocation density of the order 10 14 m -2 [2]. Relatively large Cr 23 C 6 particles are present at grain and subgrain boundaries and finer MX [(V,Nb)(C,N)]…”

Effect of Long-Term Service on Microstructure and Mechanical Properties of Martensitic 9% Cr Steel

“…7, 8; Tables 2, 3). Especially as the literature data (Ref 27,28) show that the main mechanism of strengthening in martensitic steels of the 9-12% Cr type is the strengthening with grain boundaries and the solution strengthening constituting, respectively, 33 and 29% of the yield strength increase. Nevertheless, in the longer term, these precipitates will be subject to the process of coagulation during service, which leads to a reduction of their number, assuming their constant volume fraction as per OstwaldÕs law (Ref 5,14,16).…”