Effect of Initial Microstructure on Creep Strength of ASME Grade                        T91 Steel

Sawada, Kota; Sekido, Kaoru; Kimura, Kazuhiro; Arisue, Ko; Honda, Masaki; Komai, Nobuyoshi; Fukuzawa, Norihide; Ueno, Takashi; Shimohata, Nobuaki; Nakatomi, Hitoshi; Takagi, Kenji; Kimura, Tsuyoshi; Nomura, Kayo; Kubushiro, Keiji

doi:10.2355/tetsutohagane.tetsu-2018-066

Cited by 6 publications

(7 citation statements)

References 7 publications

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“…Such differences are also confirmed in the other heats of Grades 91 and 92 than those shown in Table 16 (NIMS, 2007;1014;2018). Furthermore, the reasons for these differences have been studied (Kimura et al, 2013;Sawada et al, 2014a;2014b;2019a), however they have not been sufficiently clarified. Therefore, though the creep strength of Grade 92 is considered to be credible, there is room for further improvement.…”

Section: Degradation Of 100 000 H Rupture Strength Of 9cr Martensitic Steel With High Strengthmentioning

confidence: 80%

“…The negative effect of Ni contained in the specification range of Grade 91 (Ni ≤ 0.40 mass%, hereinafter %) on the long-term rupture strength during the formation of the Z-phase particles was reported by Kimura et al (2013) and Sawada, Kushima, Hara, Tabuchi, and Kimura, (2014a). Sawada et al (2019a) also recently demonstrated the harmful effect of the presence of micro-segregation prior to creep tests on the long-term rupture strength of ASME Grade T91. However, the following issues have not been properly elucidated: The number densities of the Z-phase particles in the Grade 91 ruptured specimens tested at 600 °C are larger than those of Grade 92 (Sawada et al, 2006), the strat time of the Z-phase formation reported by Sawada et al (2007) does not coincide with the inflection point on the stress-rupture curves in high strength martensitic steels like Grade 91, which was highlighted by Yan, Wang, Shan, and Yang (2013).…”

Section: Introductionmentioning

confidence: 84%

“…The reason for the relationship between these values and heat treatment is unclear, but the relationship between the initial microstructure or the microstructures within an early stage of transient creep and long-term rupture strength should be thoroughly investigated in the future. Nevertheless, there may still be numerous potential techniques to improve the long-term rupture strength of high-strength Fe-Cr-Mo steel to mitigate rapid hardening during transient creep and to delay the recovery during accelerating creep, other than reducing Ni, Al, and the degree of segregation and controlling heat treatment (Sawada, Suzuki, Kushima, Tabuchi, & Kimura, 2008;Sawada et al, 2014a;2019a;Sawada & Kimura, 2019b). If new techniques are explored, these may also be applicable to Fe-Cr-W systems.…”

Section: Comparison Between Grade 91 and Grade 92mentioning

confidence: 99%

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Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

Tamura¹,

Abe²

2021

JMSR

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To investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel, the creep curves of Grades T91, T92, and P92 were analyzed along with the experimental steels of 9Cr-1W and 9Cr-4W by applying an exponential law to the temperature, stress, and time parameters. The activation energy (Q ), activation volume (V ), and Larson-Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs due to dislocations that are swept out of the sub-grains, which is followed by strengthening due to the rearrangement of M23C6 and the precipitation of the Laves phase. After Q  reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of several of the weakest boundaries due to coarsening of the precipitates. This activity triggers an unexpected degradation in strength due to the accelerated formation of the Z-phase. Stabilization of M23C6 and the Laves phase is important for mitigating the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for the Cr-Mo systems because Fe2Mo is easily coarsened at ~600 °C as compared to Fe2W. Lowering the hardness and Si content also prevents excess hardening due to the Laves phase, which also mitigates the degradation. The online monitoring of creep curves and the QVC  analysis render it possible to detect signs of long-term degradation under targeted conditions within a relatively short period.

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Section: Degradation Of 100 000 H Rupture Strength Of 9cr Martensitic Steel With High Strengthmentioning

confidence: 80%

Section: Introductionmentioning

confidence: 84%

Section: Comparison Between Grade 91 and Grade 92mentioning

confidence: 99%

See 1 more Smart Citation

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

Tamura¹,

Abe²

2021

JMSR

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“…Recently, the effects of segregation have been studied by Sawada et al [22] on Gr.91 (T91) steel. They investigated the effect of Cr micro-segregation on the overall creep strength and implied that it led to a shorter time to rupture compared to more compositionally homogenous heats.…”

Section: Introductionmentioning

confidence: 99%

“…For several uniaxial creep tests, the variation in M 23 C 6 particle densities and hardness were also correlated to the segregation present. [22] Wang et al also found a variation of hardness along directionally solidified bars of CB2 steel due to the macro-segregation of B, C, Nb, P, Cr, Mo, Si, V, and Mn in steel. [23] The effect of interdendritic enrichment of C, Cr, Mn, and Ni has also been associated with the likelihood of retained austenite in P91 weld deposits by Santella et al [24] The theory of the cause and effect of macro-and micro-segregation has been studied in depth [25,26] but the implications have yet to be systematically researched in 9 to 12 wt pct Cr martensitic boiler steels.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Micro-Segregation on the Quantification of Microstructural Parameters in Grade 91 Steel

Jabar

Siefert²,

Strangwood

et al. 2020

Metall Mater Trans A

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The quantification of key microstructural parameters as a function of aging or creep exposure time is commonplace in the assessment of 9Cr Creep Strength Enhanced Ferritic (CSEF) power-plant steels. In these studies, the sample is either assumed chemically homogenous at the micro-scale or that a material average will be achieved by collecting enough images at random locations. In this paper, the micro-scale chemical homogeneity of two ex-service boiler components, a pipe and a forging, are quantitatively assessed using high sensitivity chemical mapping from µ-XRF. The compositional variation was as expected most pronounced in the larger elements Mo and Nb, where a > 20 pct difference in composition was present between positively and negatively segregated areas. The effect of this micro-segregation on local variations in Laves phase particle characteristics was investigated using SEM images. This showed a factor of two difference in the number of particles and the area coverage between positively and negatively Mo-segregated regions. This result was consistent with the thermodynamic equilibrium predictions of the phase content based on the observed level of segregation.

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Catalog of NIMS creep data sheets

Sawada

Kimura

Abe

et al. 2019

Science and Technology of Advanced Materials

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The background of the NIMS Creep Data Sheet Project, together with the preliminary study and facilities, material selection, and testing method, is summarized. The outcomes from the project are explained, focusing on the long-term creep strength of ferritic and austenitic heat-resistant steels. In some cases, the slope of the stress versus time-to-rupture curve in the long term differed from that in the short term in a manner that was markedly dependent on the type of material. Heat-to-heat variations in creep strength were recognized for ferritic and austenitic steels, even when the chemical compositions of the steels examined were within the range of specifications. The reasons for the heat-to-heat variations were differences in the chemical composition, in the amounts of minor elements, and in the grain size, among others. The existence of inherent creep strength was discovered in the very long term for ferritic heatresistant steels. The amounts of minor solute elements affect the inherent creep strength, independently of precipitation strengthening or the dislocation structure. An inflection point was observed in the tertiary creep stage for a low-alloy steel and for austenitic stainless steels when precipitation occurred during creep. A region-splitting analysis method was proposed for long-term creep strength evaluation for high-chromium ferritic steels. This method was used to review the allowable stress of high-chromium ferritic steels in Japan. A metallographic atlas, time-temperature-precipitation diagram, and fracture-mode map were proposed for ferritic and austenitic steels on the basis of creep-ruptured specimens.

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Effect of Initial Microstructure on Creep Strength of ASME Grade T91 Steel

Cited by 6 publications

References 7 publications

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

The Effect of Micro-Segregation on the Quantification of Microstructural Parameters in Grade 91 Steel

Catalog of NIMS creep data sheets

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