Hideaki Kushima

Effect of stress on the creep deformation of ASME Grade P92/T92 steels

Abstract The stress versus strain curve of ASME Grade P92/T92 steels was analysed and the stress dependence of the creep deformation behavior was investigated. Good correspondence between 50% of 0.2% offset yield stress and a proportional limit was observed over a range of temperatures from 550 to 700°C. Stress dependence of the minimum creep rate was divided into two groups of high and low stress regimes with a boundary condition of 0% offset yield stress. Creep deformation in the low stress regime was considered to be controlled by dislocation climb. Large stress dependence of the minimum creep rate in the high stress regime was equivalent to the strain rate dependence of flow stress observed in the tensile test. Creep strength in the high stress regime is assumed to be influenced by plastic deformation, and should be excluded for a long-term creep strength evaluation.

Inherent creep strength and long term creep strength properties of ferritic steels

Abstract Inherent creep strength is defined as a constant creep strength independent of time and microstructure. Sigmoidal inflection of a relation between stress and time to rupture is explainable in terms of a loss of creep strength followed by the advent of inherent creep strength. Although a large variation of short term creep rupture strength is observed, inherent creep strength is evaluated to be almost the same for several low alloy Cr-Mo steels. Creep rupture strength of the 12Cr-1Mo-1W-0.3V steel is higher than the other steels investigated in the short term region, however, it decreases with decrease in stress abruptly and it is almost the same as common inherent creep strength for low alloy Cr-Mo steels. On the other hand, creep rupture strength of the modified 9Cr-1Mo steel is higher than the other steels even at low stresses.

Influence of normalizing heat treatment on precipitation behaviour in modified 9Cr–1Mo steel

Abstract Precipitation behaviour during normalizing heat treatment has been investigated on modified 9Cr – 1Mo steel. Heterogeneously distributed spherical MX particles and platelet M3C were observed in the as-normalized condition. The number of the precipitates decreased with increasing normalizing temperature and no precipitates was observed after normalizing at 125°C. The size of MX increased with an increase in normalizing temperature up to 1200°C. For MX, not only size, but also composition of metallic elements was influenced by the normalizing temperature. Since equilibrium composition of MX depends on temperature, an MX particle with non-equilibrium composition dissolves and precipitation of it takes place with its equilibrium composition at the normalizing temperature. A phase field diagram of NbX–VX quasi binary system in modified 9Cr –1Mo steel was experimentally determined. It has been supposed that precipitation of M3C takes place during cooling from normalizing temperature in the surrounding area of MX particles where the concentration of niobium and vanadium in matrix is poor.

Effect of creep deformation on Z phase formation in Gr.91 steel

Abstract In order to make clear the effect of creep deformation on Z phase formation in Gr.91 steel, creep interrupted tests were performed for 10 000, 20 000, 30 000, 50 000 and 70 000 h at 600°C under 70 MPa. The time to rupture was 80 736·8 h at 600°C under 70 MPa. Z phase was observed in the vicinity of prior austenite grain boundaries in the grip and gauge portions after creep deformation for 10 000 h and more. Number density of Z phase particles increased with increasing creep time in the grip and gauge portions. The number densities of MX carbonitrides particles in the grip and gauge portions clearly started to decrease after 30 000 h corresponding to increase in number density of Z phase. In the gauge portion, the number density of MX carbonitrides particles was almost the same as that of Z phase particles after creep rupture, meaning that a large number of MX carbonitrides particles were disappeared. The number density of Z phase particles in the gauge portion was 2·5 times of that in the grip portion after creep rupture. This indicates that creep deformation promotes the Z phase formation.

Long-term Creep Strength of Creep Strength Enhanced Ferritic Steels

Overestimation of long-term creep strength of creep strength enhanced ferritic steels is caused by inflection of a relation between stress and time to rupture. Creep rupture strength of those steels has been re-evaluated by a region splitting analysis and allowable tensile stress of some steels regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code in Japan has been reduced. A region splitting analysis method evaluates creep rupture strength in the short- and the long-term individually, which is separated by 50% of 0.2% offset yield stress. Inflection of stress vs. time to rupture curve is attributable to longer creep rupture life with a stabilized microstructure of creep strength enhanced ferritic steels, since tensile strength property, which determines short-term creep rupture strength, remains the same level. Accuracy of creep rupture strength evaluation is improved by region splitting analysis. Delta (δ) ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite at the normalizing temperature. It increases driving force of diffusion and promotes recovery of tempered martensite adjacent to δ-ferrite. Concentration gap may be generated also in heat affected zone (HAZ), especially in fine grain HAZ, and it has possibilities to promote recovery and, therefore, to decrease in creep strength.

Region Splitting Analysis on Creep Strength Enhanced Ferritic Steels

Overestimation of long-term creep strength of creep strength enhanced ferritic steels is caused by inflection of a relation between stress and time to rupture. Creep rupture strength of those steels has been re-evaluated by a region splitting analysis and allowable tensile stress of some steels regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code in Japan has been reduced. A region splitting analysis method evaluates creep rupture strength in the short- and the long-term individually, which is separated by 50% of 0.2% offset yield stress. Inflection of stress vs. time to rupture curve is attributable to longer creep rupture life with a stabilized microstructure of creep strength enhanced ferritic steels, since tensile strength property, which determines short-term creep rupture strength, remains the same level. Accuracy of creep rupture strength evaluation is improved by region splitting analysis. Delta ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite at the normalizing temperature. It increases driving force for diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to that in dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength.Copyright

Creep Strength Assessment of High Chromium Ferritic Creep Resistant Steels

Degradation mechanism and life prediction method of high chromium ferritic creep resistant steels have been investigated. In the high stress condition, easy and rapid extension of recovered soft region results in significant decrease in creep strength, however, ductility is high. In the low stress condition, extension of recovered soft region is mainly controlled by diffusion and it is slow, therefore, deformation is concentrated in the recovered soft region along grain boundaries and ductility is extremely low. Delta-ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite phases at the normalizing temperature. It increases driving force of diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength. It has been confirmed the advantage of region splitting analysis of creep rupture strength for high chromium ferritic creep resistant steels, through a residual error analysis. It is important to avoid a generation of concentration gap in order to improve stability of microstructure and to maintain high creep strength.

Contribution of Z-Phase Precipitation to Recovery of Martensitic Structure in High Chromium Creep Resistant Steel

The precipitation site, main metallic composition and number density of Z phase have been investigated in T91 in order to clarify the influence of Z phase formation on recovery of martensitic structure and creep strength degradation. The Z phase particles were mainly present around prior austenite grain boundaries and/or packet boundaries in the steels crept at 550oC and 600oC. The Z phase particles were found in specimens crept at 550oC to 650oC. There was no indication of Z phase formation up to about 62475.0 h at 500oC and 14106.5 h at 700oC. The Nb content of Z phase observed at 550oC was lower than that at 600oC. The number density of Z phase measured at 550oC was lower that that at 600oC, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550oC in contrast with 600oC.

Development of Ferritic Heat‐Resistant Steels Based on New Materials Design Concept for Advanced High Efficiency Power Plants

To save fossil fuel resources and to reduce CO2 emission, considerable efforts have been directed toward the research and development of heat-resistant materials that can help in improving the energy efficiency of power plants by increasing their steam temperature and pressure. Instead of the conventional 9–12Cr ferritic heat-resistant steels with a tempered martensitic microstructure, the authors have proposed “Precipitation Strengthened 15Cr Ferritic Steel” based on the new material design concept that is a solid-solution treated ferrite matrix strengthened by intermetallic compounds. In this study, the creep strength of the developed 15Cr ferritic steels with various Ni contents was investigated. As a result of the creep tests for the 15Cr steels at 923, 973, 1023 K up to about 65 000 h, it was found that the creep strength of the 15Cr steels was two times higher than that of the conventional 9Cr ferric heat-resistant steels at every temperatures. The creep rupture lives of the steel were 10 to 100 times longer than that of the conventional versions. The creep rupture strength of the 15Cr steels after 100 000 h was estimated to be 100 to 130 MPa at 923 K and 45 to 70 MPa at 973 K from Larson-Miller parameter method. The authors believe the precipitation strengthened 15Cr ferritic steel is a candidate material of the high-temperature structural components in high-efficient thermal power plants.

Creep Deformation Analysis of Grade 91 Steels and Prediction of Creep Strength Properties

Creep deformation behavior was analyzed on Grade T91 steels and an empirical equation was derived to express creep strain as a function of time. Creep rupture life prediction was investigated by means of the derived empirical equation. Influence of creep rupture strain on creep rupture life predicted by the derived equation was small over a range of creep rupture strain from 0.05 to 0.30, especially creep rupture strain of 0.10 and above. Prediction accuracy of creep rupture life of the equation was better than that of creep rupture data analysis with a Larson-Miller parameter. Heat-to-heat variation of long-term creep rupture strength was appropriately observed. Minimum creep rate, onset of tertiary creep, and time to specific strain are easily predicted by the equation.Copyright