The evolution of dislocation density during heat treatment and creep of tempered martensite ferritic steels

https://www.tandfonline.com/doi/pdf/10.1088/1468-6996/9/1/013002

Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants

On the effect of long-term creep on the microstructure of a 12% chromium tempered martensite ferritic steel

Premature breakdown of creep strength is a serious problem to be solved in long-term creep of advanced high Cr ferritic steels. The material studied was ASTM grade 92 steel crept at 550–650 °C for up to 63 151 h. Stress exponent for rupture life decreases from 17 in short-term creep to 8 in long-term creep, confirming the breakdown in the steel. The steel shows ductile to brittle transition with increasing rupture life, and the breakdown accords with the onset of brittle intergranular fracture. Creep cavities are nucleated at coarse precipitates of Laves phase along grain boundaries. These findings suggest the following story of the breakdown of creep strength. Laves phase precipitates and grows during creep exposure. Coarsening of Laves phase particles over a critical size triggers the cavity formation and the consequent brittle intergranular fracture. The brittle fracture causes the breakdown. The coarsening of Laves phase can be detected non-destructively by means of hardness testing of the steel exposed to elevated temperature without stress.

Causes of breakdown of creep strength in 9Cr-1.8W-0.5Mo-VNb steel

There are rather few quantitative data on the microstructure of the 9-12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 ◦ C. The same measurements were conducted in the same P91 steel in the as received conditions. Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 ◦ C up to 113,431 h. Electron Backscatter Diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure.

/pdf/evolution-of-dislocation-density-size-of-subgrains-and-mx-1ynp9fk96i.pdf

Evolution of dislocation density, size of subgrains and MX-type precipitates in a P91 steel during creep and during thermal ageing at 600 °C for more than 100,000 h

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.

https://www.jstage.jst.go.jp/article/isijinternational1989/41/6/41_6_641/_pdf

Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel

Effect of W on creep strength of martensitic steels was investigated paying special attention to microstructural degradation during creep. Though tempered martensitic lath structure is stable at the elevated temperatures without stress, its recovery takes place substantially during creep. After the recovery, lath width and dislocation density in lath interior reached the stationary values determined by creep stress. There was no difference in the stationary values between the two steels with (TAF650 steel) and without (Mod.9Cr–1Mo steel) W. However, recovery processes of the lath structure are significantly different between the two steels. The growth of lath width and the annihilation of dislocations in lath interior are slower in W containing TAF650 steel than those in Mod.9Cr–1Mo steel. Accumulation of creep strain is suppressed in TAF650 steel because of the slow recovery of its lath structure. The retardation of the recovery of lath structure results in the lower creep rate and the higher creep rupture strength of the W containing steel. The slow recovery of lath structure in the W containing steel is ascribed to pinning effect of M23C6 and Laves phase (Fe2W) precipitated on lath boundaries.

Effect of W on recovery of lath structure during creep of high chromium martensitic steels

The deformation mechanism map of 2.25Cr—1Mo steel was examined by creep data obtained over a wide range of creep rates down to 10 −11 s −1 . The stress dependence of minimum creep rates of the steel is similar to that of particle strengthened materials: low, high, and low stress exponent, respectively, in high (H), intermediate (I), and low (L) stress regions. The stress exponent and activation energy for creep rate suggest dislocation creep controlled by lattice diffusion as the deformation mechanism in regions I and L, including service conditions of the steel. Transition to diffusion creep occurs at a lower creep rate than what is expected in the deformation mechanism maps. Region H appears above athermal yield stress. During loading in this region, athermal plastic deformation takes place by dislocation glide mechanism, and then dislocation creep starts. The dislocation creep in region H is different from the one in regions I and L due to the plastic deformation during loading. A modified creep mechanism map of 2.25Cr—1Mo steel is proposed on the basis of the experimental results.

Examination of deformation mechanism maps in 2.25Cr-1Mo steel by creep tests at strain rates of 10-11 to 10-6 s-1

Synopsis : Several microstructural changes take place in a material during the course of creep. These changes can be a measure of creep life. In this paper, microstructural changes in Mod.9Cr-1Mo steel were studied and it was examined which is a good measure of creep life. Microscopic structural changes, such as void growth, lath structure uniformly oriented to the tensile axis and elongation of grains, are evident only in the necked portion of ruptured specimens. These macroscopic structural changes are not useful for creep life assessment. Lath width increases and dislocation density within lath decreases with increasing creep duration. These changes in dislocation substructure start in the early stage of creep life, and cause the increase of strain rate in the tertiary creep stage. The lath width and the dislocation density reach a saturated value before rupture. The saturated values are independent of temperature, and uniquely related to creep stress normalized by shear modulus. The extent of these microstructural changes are greater at lower stresses under which the material is practically used. These facts suggest that the lath width and the dislocation density within lath can be a useful measure of creep life. Hardness of crept specimens is closely related to the lath width and the dislocation density within lath. The changes of these microstructural features can be evaluated by the measurement of hardness.

https://www.jstage.jst.go.jp/article/tetsutohagane1955/83/7/83_7_466/_pdf

Microstructural Changes during Creep and Life Assessment of Mod.9Cr-1Mo Steel

Omega method has been proposed for predicting rupture life from creep strain e-time t data. The method is based on a linear relation between logarithm of creep rate e and strain, namely a creep curve without the primary and secondary creep stages. Such a linear relation, however, seldom holds in real creep data. In this paper, the original equation is modified to the following form: e=e 0 +1/Ω{In(l+ζt)-In(1-ηt)} where e 0 , Ω, ζ and η are parameters characterizing a creep curve. The first and second terms in the parentheses describe primary creep and tertiary creep, respectively. The equation is applied to ferritic steels. The four parameters of the equation can uniquely be determined for a creep curve with a curved In e-e relation. The equation can well reproduce creep curves with a prominent primary creep stage. Because of the high reproducibility, the modified equation can predict rupture life with higher accuracy at an earlier stage of creep than the original equation.

Kota Sawada

Papers

Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel

Effect of W on recovery of lath structure during creep of high chromium martensitic steels

Examination of deformation mechanism maps in 2.25Cr-1Mo steel by creep tests at strain rates of 10-11 to 10-6 s-1

Microstructural Changes during Creep and Life Assessment of Mod.9Cr-1Mo Steel

Improvement of Omega Method for Creep Life Prediction