Effect of Initial Microstructure on Long Term Creep Strength of a Low Alloy Ferritic Steel

Kimura, Kazuhiro; Kushima, Hideaki; Baba, Eiji; Shimizu, Tetsuo; Asai, Yoshikazu; Abe, Fujio; Yagi, Koichi

doi:10.2355/tetsutohagane1955.86.8_542

Cited by 10 publications

(8 citation statements)

References 9 publications

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“…The dislocation substructure and the MX particles typical of the conventional steel are not necessary if the precipitates are thermally stable and their spacing is fine enough. Kimura et al 79) have pointed out recently that ferritic steel without the dislocation substructure is superior to martensitic steel in long-term creep strength. This fact advises us against taking the conventional strengthening concept based on the dislocation hardening.…”

Section: Other Elementsmentioning

confidence: 99%

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

2001

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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.

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Section: Other Elementsmentioning

confidence: 99%

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

2001

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“…3) Kimura et al have investigated the effects of initial microstructure on the long-term creep strength of a 0.5Cr-0.5Mo steel, and pointed out that the long-term creep strength of the fully annealed steel with ferrite and pearlite microstructures is higher than that with martensite, tempered martensite and bainite microstructures. 4) Thus, they supposed that the higher creep strength in the long-term regime is associated with its low dislocation density, indicating that a fully annealed microstructure would have advantages for long-term creep strength under a service condition in power generation. 4) Based on this concept, Kimura et al recently investigated the creep properties of fully annealed 15Cr-1Mo-3W-V-Nb ferritic steels at 923 K, and found that an increase in the content of W and Co could improve the creep strength by precipitation strengthening effects of intermetallic compounds and carbide.…”

Section: Introductionmentioning

confidence: 99%

“…4) Thus, they supposed that the higher creep strength in the long-term regime is associated with its low dislocation density, indicating that a fully annealed microstructure would have advantages for long-term creep strength under a service condition in power generation. 4) Based on this concept, Kimura et al recently investigated the creep properties of fully annealed 15Cr-1Mo-3W-V-Nb ferritic steels at 923 K, and found that an increase in the content of W and Co could improve the creep strength by precipitation strengthening effects of intermetallic compounds and carbide. [5][6][7][8] However, the detailed mechanism of strengthening due to W and Co, and availability of such strengthening effects for the longterm creep have not been clearly understood.…”

Section: Introductionmentioning

confidence: 99%

Effects of W and Co on Long-term Creep Strength of Precipitation Strengthened 15Cr Ferritic Heat Resistant Steels.

et al. 2003

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Effects of W and Co on creep deformation and microstructure of fully annealed and precipitation strengthened 15Cr-3W ferritic steels at 923 and 973 K have been investigated, and the strengthening effects of W and Co in a long-term and high temperature service condition have been discussed. The effect of Co addition on the creep rupture strength of the steels is higher than that of the further addition of W in the shortterm. However, the creep rupture life of the steel with 6 mass% W is longer than that of the steel with 3 mass% Co at the low stress condition of 50 MPa and 973 K. The increase in W content promotes the formation of intermetallic compounds such as c phase. The addition of Co promotes the precipitation of fine M 23 C 6 carbide within grains. The precipitation of carbide by Co addition is effective in improving the shortterm creep strength for higher stress regions, whereas the formation of intermetallic compounds by W addition is more effective for lower stress and long-term creep life.

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“…Suppression of the expansion of the degradation area by improving microstructural stability is a key concept for maintaining high creep strength in long-term service (4) . Kimura et al have investigated the effect of initial microstructure on the long-term creep strength of 0.5Cr-0.5Mo steel, and pointed out that the long-term creep strength of the fully annealed steel with ferrite and pearlite microstructures was higher than that of the steels with martensite, tempered martensite and bainite microstruc-tures (5) . They supposed that the higher long-term creep strength of the fully annealed steel was related to its low dislocation density, and a fully annealed microstructure would have advantages for long-term creep strength under service conditions in a power plant (5) .…”

Section: Introductionmentioning

confidence: 99%

“…Kimura et al have investigated the effect of initial microstructure on the long-term creep strength of 0.5Cr-0.5Mo steel, and pointed out that the long-term creep strength of the fully annealed steel with ferrite and pearlite microstructures was higher than that of the steels with martensite, tempered martensite and bainite microstruc-tures (5) . They supposed that the higher long-term creep strength of the fully annealed steel was related to its low dislocation density, and a fully annealed microstructure would have advantages for long-term creep strength under service conditions in a power plant (5) . Based on this concept, Kimura et al have recently investigated the creep properties of 15Cr-1Mo-3W-V-Nb ferritic steels fully annealed at 923 K, and have found that increases in W and Co contents increase the creep strength through the precipitation strengthening effect of intermetallic compounds and carbide (6) - (10) .…”

Section: Introductionmentioning

confidence: 99%

Improvement in Creep Strength of Precipitation Strengthened 15Cr Ferritic Steel by Controlling Carbon and Nitrogen Contents

Toda

Tohyama

Kushima

et al. 2005

JSME Int. J., Ser. A

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The effects of carbon and nitrogen on the creep strength and microstructure of precipitation-strengthened 15Cr ferritic steel have been investigated. The creep rupture lives of 15Cr ferritic steel with the addition of 0.07 mass%N and with a combination of 0.05 mass%C and 0.03 mass%N were extended longer than that of the conventional ferritic creep resistant steel of ASME T92. However, excess addition of carbon and nitrogen resulted in a decrease in long-term creep strength. For the steel with 0.07 mass%N addition, which showed excellent long-term creep strength, many plate-shaped fine precipitates and a few coarse block-type ones were observed. The volume fraction of precipitate free zone in this steel was smaller, and the coarsening rate of the fine precipitates identified as intermetallic compounds in this steel was lower than that of the other steels. The long-term creep strength of the present steel was improved by the addition of 0.07 mass%N through the precipitation strengthening effect of many uniformly distributed stable fine particles.

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Effect of Initial Microstructure on Long Term Creep Strength of a Low Alloy Ferritic Steel

Cited by 10 publications

References 9 publications

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

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

Effects of W and Co on Long-term Creep Strength of Precipitation Strengthened 15Cr Ferritic Heat Resistant Steels.

Improvement in Creep Strength of Precipitation Strengthened 15Cr Ferritic Steel by Controlling Carbon and Nitrogen Contents

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