Applicability of an Exponential Law in Creep of Metals

Tamura, Manabu; Esaka, Hisao; Shinozuka, Kei

doi:10.2320/matertrans.44.118

Cited by 16 publications

(19 citation statements)

References 34 publications

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“…[1] was confirmed for rupture data of many kinds of heat-resistant steels and alloys. [17,18] Further, the activation volume is defined as the product of the area swept by dislocations and the length of the Burgers vector when we discuss the microscopic deformation mechanisms by dislocations. However, since it is impossible to measure directly the area swept by dislocations, another interpretation is necessary from a creep-deformation-measuring point of view.…”

Section: Analysis Methodsmentioning

confidence: 99%

“…I and III¢ as shown in Figure 8. The values of Q for time to rupture of many kinds of heat-resistant steels do not exceed 500 kJ/mol, [16][17][18] although Q increases to some extent at high temperatures in some cases. [16] In a high-temperature region higher than 973 K (700°C), neither Laves phase nor Z phase precipitates in high-Cr martensitic steel containing Mo and/or W, [25,29] and therefore, major strengthening factors are considered to be both W and/or Mo, most of which are soluble, and fine MX particles that precipitate stably in the martensitic matrix.…”

Section: A Creep Mechanisms For Grs I-iii¢mentioning

confidence: 96%

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Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

Tamura

2015

Metall Mater Trans A

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Long-term rupture data of 11Cr-2W-0.4Mo-1Cu-Nb-V steel were analyzed using an exponential equation for stress regarding time to rupture as a thermal activation process. The fitness was compared with the usually employed method assuming power-law creep. In the exponential method, rupture data are classified into several groups according to the thermal activation process; the activation energy, Q; the activation volume, V; then, the Larson-Miller constant, C, values are calculated, and a regression equation is obtained for each data group. The fitness level of the equation was satisfactorily high for each group. The values of Q, V, and C were unusually small for a data group where an unexpected drop in rupture strength was observed. The critical issue is how to comprehend signs of degradation within the short term. We can observe several signs at a creep time of approximately one-tenth of the times of the degradation events. The small values of Q and V indicate that completely softened regions form and creep locally, which is consistent with previous observations. From both metallurgical considerations and the variations of Q and V, it is suggested that the rate of the unexpected drop in strength is mitigated after further long-term creep.

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Section: Analysis Methodsmentioning

confidence: 99%

Section: A Creep Mechanisms For Grs I-iii¢mentioning

confidence: 96%

Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

Tamura

2015

Metall Mater Trans A

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“…In recent studies on creep mechanisms, the exponential type equation has been applied to many heat resistant steels by Tamura et al [13][14][15][16]. It is shown that this application is available for summarizing and discussing creep properties.…”

Section: Exponential Type Creep Constitutive Equationmentioning

confidence: 99%

Creep constitutive equation of dual phase 9Cr-ODS steel

Sakasegawa

Ukai

Tamura

et al. 2008

Journal of Nuclear Materials

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“…However, recently availability of a following exponential equation for fitting t r has been re-confirmed. 26,27) …”

Section: Analysis Of Creep Curves Of F82hdmentioning

confidence: 99%

Creep Behavior of Double Tempered 8%Cr-2%WVTa Martensitic Steel

Tamura

Nowell²,

Shinozuka

et al. 2006

Mater. Trans.

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Creep testing was carried out at around 650 C for a martensitic 8Cr-2WVTa steel (F82H), which is a candidate alloy for the first wall of the fusion reactors of the Tokamak type. Rupture strength of the double tempered steel (F82HD) is lightly higher than that of simply tempered steel (F82HS). On the other hand, creep rate of F82HD is obviously smaller than that of F82HS in acceleration creep, though creep strain of F82HD in transition creep, where creep rate decreases with increasing strain, is larger than that of F82HS. Hardness of the crept F82HD decreases with increasing creep strain, which corresponded with the transmission electron microscopy (TEM) observation. On the contrary, X-ray diffraction and electron back-scattered diffraction pattern measurements show that fine sub-grains are created during transition creep. The creep curves were analyzed using an exponential type creep equation and the apparent activation energy, the activation volume and the pre-exponential factor were calculated as a function of creep strain. Then, these parameters were converted into two parameters, i.e. equivalent obstacle spacing (EOS) and mobile dislocation density parameter (MDDP). While EOS decreases with increasing creep strain, MDDP increases with increasing strain during transition creep. The decrease in EOS and the increase in either EOS or MDDP are rate-controlling factors in transition and acceleration creep, respectively. On the other hand, in case of F82HS, EOS increases and MDDP decreases during transition creep. In this case, the decrease in MDDP controls the creep rate during transition creep of F82HS. It is concluded that both EOS and MDDP are representative parameters of the change in substructure during creep.

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Applicability of an Exponential Law in Creep of Metals

Cited by 16 publications

References 34 publications

Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

Creep constitutive equation of dual phase 9Cr-ODS steel

Creep Behavior of Double Tempered 8%Cr-2%WVTa Martensitic Steel

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