Dislocation-based modeling of long-term creep behaviors of Grade 91 steels

Zhao, Jingling; Gong, Jiang Hong; Saboo, Abhinav; Dunand, David C.; Olson, Gregory B.

doi:10.1016/j.actamat.2018.02.001

Cited by 55 publications

(18 citation statements)

References 32 publications

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“…El Rayes and El-Danaf [10] found that constant strain rate tensile tests at elevated temperatures revealed minor differences between the deformation mechanisms of retired and as-produced 12% Cr steel through the calculation of stress exponents and activation energies after correcting for the influence of threshold stresses. These stresses arise from the attractive force between the MX carbonitride particles and detaching dislocations and they are themselves dependent on material degradation through the coarsening of precipitates [11,12]. However, typically, the creep rate-controlling mechanisms of 9%–12% Cr steels are often investigated by means of conventional creep testing rigs, which involve long testing times (>10,000 h) due to the slow creep rates that are associated with diffusional creep mechanisms experienced at service conditions.…”

Section: Introductionmentioning

confidence: 99%

Creep Damage Assessment of Ex-Service 12% Cr Power Plant Steel Using Digital Image Correlation and Quantitative Microstructural Evaluation

et al. 2019

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The lifetime of steam pipelines in long-term operation in coal-fired power plants are limited due to material damage that resulted from creep exposure. In the present study, the authors comparatively assess the damage of ex-service 12% Cr piping steel with varying degrees of exposure while using accelerated creep tests that employ digital image correlation (DIC) as well as microstructural investigation that is based on electron microscopy. The DIC technique, which allows multiple creep curves to be measured at temperatures ranging from 550–600 °C from a single specimen, revealed higher Zener–Hollomon parameters for a high damage material with a high void density when compared to a material with lower damage and lower void density. Both of the material states showed similar hardness values, subgrain sizes, and boundary character, despite the difference in void densities. Slightly higher inter-particle spacing of MX precipitates results in a lower threshold stress of 79 MPa for the high damage steel when compared to 97 MPa for the low damage material. Besides large Laves phase particles (>0.2 µm) that are found in the higher damaged materials that result in solid solution depletion, the most prominent microstructural damage indicator was a lower density of M23C6 precipitates. Therefore, the observations indicate that the Zener–Hollomon parameter and M23C6 particles are good damage assessment indicators between the most extreme damage states and they predict a lower damage level for a medium void density material.

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Section: Introductionmentioning

confidence: 99%

Creep Damage Assessment of Ex-Service 12% Cr Power Plant Steel Using Digital Image Correlation and Quantitative Microstructural Evaluation

et al. 2019

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“…For conventional metallic materials, time-dependent elevated temperature creep deformation can be represented by the creep strain-time curve, which is usually distinguished by primary, secondary and tertiary stages. Upon loading, the creep rate quickly decreases during the primary stage and then reaches a steady stage, i.e., the secondary stage, before dramatically increasing due to the formation of cracks during the tertiary stage, leading to the final fracture [31][32][33]. Figure 2c,d demonstrate the tensile creep curves of the studied Zr-based BMG under different creep parameters.…”

Section: Resultsmentioning

confidence: 99%

“…The inset of Figure 3c shows the bright field TEM images and corresponding SAED patterns taken from the crystals. These crystalline phases were identified by TEM Creep is the time-dependent plastic strain at constant testing temperature and applied stress [31][32][33]. For conventional metallic materials, time-dependent elevated temperature creep deformation can be represented by the creep strain-time curve, which is usually distinguished by primary, secondary and tertiary stages.…”

Section: Resultsmentioning

confidence: 99%

Tensile Creep Characterization and Prediction of Zr-Based Metallic Glass at High Temperatures

Wang

Pan

Shi

et al. 2018

Metals

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Abstract:The high temperature creep behaviors of a Zr-based bulk metallic glass (BMG) are studied by uniaxial tensile creep experiments under applied stresses of 50-180 MPa at temperatures of 660-700 K. The microstructural observations of the BMG samples after creep tests show that crystalline phases can be detected under high temperature or high applied stress. Constitutive models for predicting the high temperature creep behaviors of the studied Zr-based BMG are established based on the θ projection method. The creep activation energy and stress exponent are also calculated to establish the creep model. The parameters of the established models are found to be closely associated with the applied stress and temperature. The results show an excellent agreement between the measured and predicted results, confirming the validity of the established model to accurately estimate the high temperature creep curves for the Zr-based BMG. Moreover, based on the classical diffusion creep theory, a schematic model is proposed to describe the creep behaviors of BMGs from the framework of free volume theory.

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“…where M is the Taylor factor (2.9 for bcc metals), l the shear modulus of the matrix (59.3 and 57.0 GPa at 923 K and 973 K respectively [49] ), b the Burgers vector (0.248 nm), k the edge-to-edge inter-precipitate spacing Models for threshold stress in creep have been discussed in terms of dislocation climb over precipitates, either in the form of local climb, in which the dislocation follows closely the matrix-precipitate interface, inducing a sharp bend in the dislocation line, or general climb, in which the climb portion of the dislocation extends smoothly from the matrix-precipitate interfacial region to the glide plane. [12,[52][53][54][55] The threshold stress normalized by the Orowan stress (r th /r Or ) predicted by local climb is about 0.4-0.7, [56] the correct order of magnitude for reported alloys, [9,40] as well as our designed steel. However, the dislocation configuration in local climb with the sharp bend is highly unstable and is thus unlikely to occur.…”

Section: B Mechanical Propertiesmentioning

confidence: 91%

“…They rely on dispersion strengthening by MX precipitates, where M = (V,Nb,Ti) and X = (C,N), M 23 C 6 precipitates, and Laves phase Fe 2 (W,Mo) and solid-solution strengthening by W or Mo. [5][6][7][8][9][10][11][12] When exposed to these elevated temperatures for extended periods, there are concerns about microstructural degradation due to the metastability of MX, coarsening of M 23 C 6 and the Laves phase, as well as Z phase formation (Cr(Nb,V)N). [13][14][15][16][17][18][19][20][21][22][23][24] In particular, the Z phase has been held responsible for the reduction of creep strength and failure of these steels after extended operation, as its precipitation results in the dissolution of finely dispersed MX.…”

Section: Introductionmentioning

confidence: 99%

Design and Characterization of a Heat-Resistant Ferritic Steel Strengthened by MX Precipitates

Zhang

et al. 2019

Metall Mater Trans A

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The long-term performance of current heat-resistant ferritic steels used in steam generators is primarily limited by microstructure degradation as a result of coarsening or transformation of precipitates. To improve the microstructural stability, a heat-resistant ferritic steel was designed via computational thermodynamics to exclusively contain metal carbonitride (MX) precipitates as the primary means for strengthening at elevated temperatures. The volume fraction of precipitates is 0.35 vol pct, about one-seventh of P91. These MX precipitates are either V-rich or Nb-rich with plate-like or spheroidal morphology, respectively. The precipitate size remains almost constant at 973 K for up to 3000 hours aging. Electron diffraction analysis revealed a Baker-Nutting orientation relationship between the precipitate and the matrix. Consistent with the thermodynamics-based design, M 23 C 6-, Laves-, and Z phase were not detected. The creep threshold stress, derived from high-temperature compressive creep tests, are evaluated to be 63 ± 1 and 43 ± 2 MPa at 923 K and 973 K, respectively, on par with or slightly better than P91. This study reveals that MX precipitates in ferritic steels coarsen slowly at temperatures up to 973 K and that a relatively small volume fraction of MX precipitates can provide effective long-term creep performance at elevated temperatures.

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Dislocation-based modeling of long-term creep behaviors of Grade 91 steels

Cited by 55 publications

References 32 publications

Creep Damage Assessment of Ex-Service 12% Cr Power Plant Steel Using Digital Image Correlation and Quantitative Microstructural Evaluation

Creep Damage Assessment of Ex-Service 12% Cr Power Plant Steel Using Digital Image Correlation and Quantitative Microstructural Evaluation

Tensile Creep Characterization and Prediction of Zr-Based Metallic Glass at High Temperatures

Design and Characterization of a Heat-Resistant Ferritic Steel Strengthened by MX Precipitates

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