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In order to accurately predict the minimum creep rate under the actual working conditions of compacted graphite cast iron cylinder head, this paper conducted creep experiments in the temperature range of 723.15–823.15 K and the stress range of 100–300 MPa. The creep damage evolution and damage mechanisms under test conditions were also analyzed. Then the data fitting performances were analyzed and compared from three aspects: the optimization‐seeking algorithm, the form of the objective function and the construction of the creep constitutive model. Finally, the chosen optimization algorithm combined with the proposed objective function and creep constitutive model resulted in 100% of the predicted values within the 3 times error band and 26.67% of the predicted values within the 1.5 times error band. This improves the prediction accuracy while reducing the effect of data overfitting. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mechanical Strength / Jixie Qiangdu is the property of Zhengzhou Research Institute of Mechanical Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

ASME 2021 code provides constitutive equation for high temperature creep of 316H stainless steel. The paper aims to help engineers and researchers correctly evaluate the strain and creep damage of high-temperature equipment by using the equation. Firstly, the physical mechanism of the items was explained. Secondly, the relationship between crucial parameters with temperature and stress was analyzed. Thirdly, the applicable stress and time range of the 316H creep constitutive equation was briefly introduced. And fourthly, the prediction values of the constitutive equation was compared with the data value from the isochronous stress-strain curve in ASME code. Results show that the constitutive equation is described by three items, in which the first creep stage is described by the rapid initial transient and transient items, and the second creep stage is described by the steady-state creep item. The equation’s applicability of evaluating the strain and creep damage of high-temperature equipment is limited by the starting time of the third creep stage. Moreover the key parameters of the equations, such as εs, εr, decay too rapidly to be accurately predicted at high stress. This research finds that due to item n increases with increasing temperature, the minimum creep rate m and transient creep rate constant r decrease in 1 000-1 075 ℉. Meanwhile, based on contrasting the rapid transient creep rate constants s of the original paper and ASME code, item D of parameter s is incorrect in the latter. Based on comparing predicted strain obtained by equation and data from ASME code isochronous stress-strain curves, the paper finds that predicted values at 950, 1 050 and 1 150 ℉ will agree with isochronous stress-strain curves, but larger than the datum from curve at 1 000 ℉, resulting in a relatively conservative strain prediction. Based on above four research and results, the paper concludes that the high temperature creep constitutive equation of 316H stainless steel in the ASME code can be used to evaluate high-temperature equipment structural integrity at 950, 1 050 and 1 150 ℉, but the result of 1 000 ℉ is conservatism.

A modified Monkman-Grant equation, which can provide a more accurate means of predicting creep rupture life than the standard Monkman-Grant formula, has been investigated for Gr.92 using creep data in the NIMS Creep Data Sheets at 550 to 750°C. The tr versus minimum creep rate ε ˙ min plot, which is called the Monkman-Grant relation, deviates downward at low stresses and long times. Better correlation of the tr with the ε ˙ min is obtained by the replacement of tr with (tr /εr), where εr is the total or rupture strain. The (tr /εr) is inversely proportional to the ε ˙ min over a wide range of stress, temperature and test duration, and the magnitude of data scattering is only a little bit even at low stresses and long times. The creep life of Gr.92 can be predicted by evaluating the ε ˙ min, together with the εr evaluated from the stress and or ε ˙ min dependence. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The creep behavior of steels is influenced by factors such as lattice structure, defects, and stress conditions. Given the high cost and time required for creep tests, accurately predicting creep life and minimum creep rate is essential. This study analyzed 9–12% chromium (Cr) steel using a dataset of 1496 entries covering material composition, mechanical properties, minimum creep rate, and creep life. Nine machine learning (ML) models were developed, with the artificial neural network (ANN) achieving the highest prediction accuracy, evidenced by a coefficient of determination (R 2) of 0.9973 and a root mean square error (RMSE) of 0.045. A dual-target neural network model provided R 2 values of 0.9853 for creep life and 0.9838 for minimum creep rate. Additionally, an empirical equation based on gene expression programming (GEP) achieved an R 2 exceeding 0.9741. This study offers novel insights into the design of 9–12% Cr steel with enhanced creep life. [Display omitted] [ABSTRACT FROM AUTHOR]

With the increase in engine power density, the temperature and stress carried by the cylinder head during operation also increase. The thermal engine fatigue life prediction of the cylinder head needs to consider accurate and reasonable creep-constitutive models and parameters. In view of the wide range of temperature and stress working conditions of the compacted graphite cast iron (CGI) cylinder head, the creep test of CGI under the conditions of temperature 450~550 °C and stress 100~300 MPa was carried out, and CGI under the conditions of wide temperature and stress was proposed to characterize a creep-constitutive model for minimum creep rate. Research indicated that under wide temperature and stress conditions, CGI was more prone to creep damage than under low load, and creep deformation was dominated by grain boundary sliding (GBS), intragranular dislocation glide (IDG), and dislocation climb (IDC). With the deformation mechanism-based true stress (DMTS) creep model, combined with the multiobjective optimization method, a creep-constitutive model of CGI was constructed, and 73% of the predicted values of the model were within twice the error range. Compared with the linear regression method, the multiobjective optimization method could still fit the accurate model parameters in the case of small samples. [ABSTRACT FROM AUTHOR]

With the increase in engine power density, the temperature and stress carried by the cylinder head during operation also increase. The thermal engine fatigue life prediction of the cylinder head needs to consider accurate and reasonable creep-constitutive models and parameters. In view of the wide range of temperature and stress working conditions of the compacted graphite cast iron (CGI) cylinder head, the creep test of CGI under the conditions of temperature 450~550 °C and stress 100~300 MPa was carried out, and CGI under the conditions of wide temperature and stress was proposed to characterize a creep-constitutive model for minimum creep rate. Research indicated that under wide temperature and stress conditions, CGI was more prone to creep damage than under low load, and creep deformation was dominated by grain boundary sliding (GBS), intragranular dislocation glide (IDG), and dislocation climb (IDC). With the deformation mechanism-based true stress (DMTS) creep model, combined with the multiobjective optimization method, a creep-constitutive model of CGI was constructed, and 73% of the predicted values of the model were within twice the error range. Compared with the linear regression method, the multiobjective optimization method could still fit the accurate model parameters in the case of small samples.

The influence of boron nitride (BN) particles, W and Mo concentrations and creep test conditions of stress, temperature and test duration on the creep ductility parameter λ in UK R5 has been investigated for five heats of W-Mo-balanced 9Cr steel with 1.5% Mo equivalent at 600, 650 and 700 oC. With increasing time to rupture tr, the λ at first decreases only a little bit, then increases for up to a time showing a maximum peak and finally significantly decreases. The BN particles are responsible for the decreases in λ at long times by accelerating the formation of creep voids. The evaluation of creep rupture ductility based on the λ should be restricted to the long-time region at low stresses after reaching a maximum peak in λ, where the λ decreases with increasing tr mainly by a decrease in total elongation. [ABSTRACT FROM AUTHOR]

The creep deformation behaviour and its effect on the creep life and rupture ductility are investigated for W-Mo-balanced 9Cr steels at 600 °C to 700 °C. The creep life is described as tr = 1.5/[ ε ˙ min (dln ε ˙ /d ε)], where ε ˙ min and dln ε ˙ /d ε reflect the creep deformation behaviour in the transient and acceleration creep regions, respectively. The creep life is also correlated with the time to minimum creep rate tm as tr = gtm, where g slightly increases with decreasing stress. The logarithm of creep rate linearly increases with strain in the initial stage of acceleration creep region. This is followed by a decrease in dln ε ˙ d ε with strain at high stresses, resulting in ductile failure at large strains, while it is followed by an increase in dln ε ˙ /d ε with strain at low stresses, resulting in rupture at small strains. [ABSTRACT FROM AUTHOR]

In this study, the effect of cold work and annealing treatment on the creep behaviour of Zr-2.5Nb material is investigated. The creep properties of Zr-2.5 wt.% Nb pressure tube material was evaluated under two material conditions: i) cold-worked and stress-relieved (CWSR) and ii) annealed. The annealing treatment was carried out in a vacuum furnace at 800 °C, corresponding to the two-phase (α+β) region of the Zr-Nb phase diagram, with a 30-minute soaking time and then furnace-cooled. Constant load creep tests were performed as per ASTM 139-2011 using flat creep specimens with their axis along the axial direction of the pressure tube at 450 °C under three normalized stress levels corresponding to 0.7, 0.8 and 0.9 times the yield strength of the respective material condition. The minimum creep rate in the annealed material was found to be two orders lower with a wide range of steady-state creep regimes as compared to the CWSR material condition. The change in texture after annealing and creep exposure was found to be marginal. The two-order difference in creep rate could be attributed to the elongated grain structure and to the presence of high density of dislocations in CWSR material. [ABSTRACT FROM AUTHOR]

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture tr exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in tr with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe2(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening. [ABSTRACT FROM AUTHOR]

Abstract The relation of creep rupture life to minimum creep rate, namely Monkman-Grant (MG) relation is examined by using a creep database of modified 9Cr-1Mo steel. The database covers wide ranges of creep rupture life up to 1.2 × 105 h and of minimum creep rate down to 7.6 × 10−8 h−1. The MG plot exhibits a low overall value of MG exponent (p = 0.85) together with wide scatter of data points at long rupture life. Causes of the low p value and the wide scatter are discussed in the present study paying attention to change in creep curve shape with creep rupture life. There are four regions H, M, L 1 and L 2 showing different creep behavior in the steel. Regional MG relations are examined also in each region. Values of p are less than unity and change among the four regions. The p value is especially low (0.62) in long-term region L 2. The regional p values less than unity result in the low overall p value of 0.85. The especially low regional value of p in the long-term region causes the wide scatter of data points in the overall MG plot. A modified Monkman-Grant equation (Eq. (29)) is proposed taking account of creep deceleration rate A (=−1/(d ln ε ˙ / d ε), where ε ˙ and ε are the creep strain rate and the creep strain) in primary creep and creep acceleration rate B (=1/(d ln ε ˙ / d ε)) in tertiary creep. The rates characterize shape of a creep curve. The modified equation can explain the difference of p values among the four regions. [ABSTRACT FROM AUTHOR]

A modified theta projection model is proposed for the description of creep behaviour. The model shows very good flexibility in fitting the entire test data of the creep-resistant steel 12Cr1MoV over the three creep stages. The capability of describing the primary to secondary stage makes it easier to precisely evaluate the steady state or minimum creep rate using the model. The model is derived and modified from the original theta projection method. It also keeps the four indeterminate coefficients and can meet the convergence condition easily during regression by the least squares method. In this paper, the minimum creep rates of 12Cr1MoV are also obtained and discussed. [ABSTRACT FROM AUTHOR]

A modified version of the Monkman-Grant equation, which can provide a more accurate means of predicting creep rupture life than the standard Monkman-Grant formula, has been investigated for Gr.91 using creep data in the NIMS Creep Data Sheets at 450–725 °C. The maximum time to rupture t r was 1.2 × 105 h. The t r versus minimum creep rate ε ˙ min plot, which is called the Monkman-Grant relation, deviates downward at low stresses and long times. The difference between the maximum and minimum t r at a same ε ˙ min becomes more significant with decreasing stress and increasing test duration. Better correlation of the t r with the ε ˙ min is not obtained by the replacement of t r with (t r / ε r), where ε r is the total or rupture strain. The magnitude of data scattering is larger in the (t r / ε r) versus ε ˙ min plot than in the t r versus ε ˙ min plot. The (t r / ε m), where ε m is the strain to minimum creep rate, is inversely proportional to the ε ˙ min over a wide range of stress, temperature and test duration and the magnitude of data scattering is only a little bit even at low stresses and long times. The (t r / ε m) versus ε ˙ min plot gives us more reliable relation for Gr.91 than the t r versus ε ˙ min and (t r / ε r) versus ε ˙ min plots. • Monkman-Grant eq. with "strain to minimum creep rate" accurately predicts creep life. [ABSTRACT FROM AUTHOR]

• The present study compares the thermal creep behaviour of different generations of the Zr-2.5Nb pressure tube used in Indian Pressurized Heavy Water Reactors. • Pressure tubes manufactured from new route were found to be more creep resistant than the tubes manufactured from old route. • Both grain size and aspect ratio of the new route tubes were higher than that of old route tubes. • Higher extrusion ratios incorporated in fabrication of new route tubes produced stronger texture, which resulted in better creep properties. • Stress exponent and activation energy values revealed that the creep deformation mechanism was dislocation creep mechanism (dislocation climb – rate controlling mechanism, dislocation glide – strain producing mechanism). Indian Pressurized Heavy Water Reactors (IPHWRs) uses Zr-2.5Nb alloy pressure tube (PT) in cold work and stress relieved condition. These PTs are subjected to extreme conditions of stress, temperature and irradiation. Since past three decades of PHWR operation in India, fabrication routes of PTs for IPHWR has undergone several modifications to enhance its in-reactor performance. The deformation of PT in PHWR occurs due to irradiation creep, irradiation growth and thermal creep. In the present study, the different generations of PTs used in IPHWR are tested for its thermal creep behaviour. The accelerated creep rupture tests were carried out using flat tensile specimens with its axis parallel to the longitudinal direction of the PT in the temperature range of 350 to 450°C under stresses in the range of 0.7 to 0.9 times of yield strength at the respective temperatures. The minimum creep rates were evaluated along with the stress exponents and activation energies. The values of stress exponent and activation energy revealed the underlying creep mechanisms responsible for the creep deformation. Microstructural examination was carried out to understand the need and efficacy of the new fabrication route. The observed thermal creep behaviour of all the generations of pressure tubes were rationalised in terms of their respective texture and microstructures. Indian Pressurized Heavy Water Reactors (IPHWRs) uses Zr-2.5Nb alloy pressure tube (PT) in cold work and stress relieved condition. These PTs are subjected to extreme conditions of stress, temperature and irradiation. Since past three decades of PHWR operation in India, fabrication routes of PTs for IPHWR has undergone several modifications to enhance its in-reactor performance. In the present study, the thermal creep behaviour of different generations of PTs used in IPHWR is examined by carrying out accelerated creep rupture tests in the temperature range of 350 to 450°C under stresses in the range of 0.7 to 0.9 times of yield strength at the respective temperatures. The study revealed that the rupture life of the PTs following the new routes were higher than that of old route at 350 and 400°C. At 450°C all PTs had similar rupture life. A similar correspondence was made with minimum creep rate. The tubes manufactured by the new routes were more creep resistant than the old route tubes. [Display omitted] [ABSTRACT FROM AUTHOR]

Creep rupture properties of austenitic stainless steel foil, 347SS, used in compact recuperators have been evaluated at 700 °C in the stress range of 54-221 MPa to establish the baseline behavior for its extended use. Creep curves of the foil show that the primary creep stage is brief and creep life is dominated by tertiary creep deformation with rupture lives in the range of 10-2000 h. Results are compared with properties of bulk specimens tested at 98 and 162 MPa. Thin foil 347SS specimens were found to have higher creep rates and higher rupture ductility than their bulk specimen counterparts. Power law relationship was obtained between the minimum creep rate and the applied stress with stress exponent value, n = 5.7. The value of the stress exponent is indicative of the rate-controlling deformation mechanism associated with dislocation creep. Nucleation of voids mainly occurred at second-phase particles (chromium-rich MC carbides) that are present in the metal matrix by decohesion of the particle-matrix interface. The improvement in strength is attributed to the precipitation of fine niobium carbides in the matrix that act as obstacles to the movement of dislocations. [ABSTRACT FROM AUTHOR]

Abstract: Extensive creep testing was carried out on 1Cr1Mo¼V low alloy forging and casing steels in as received (normalized and tempered) and aged condition. Both the steels exhibited considerable secondary and tertiary stages in the creep tests conducted in the temperature range between 813 and 873K (540°C and 600°C). The primary stage, though present, was rather negligible in higher temperature ranges. Casting steel showed wedge type cavities which grew as cracks along the grain boundaries. In the case of forged steel, the voids were elliptical and flat which developed during the tertiary stage of creep deformation. Tertiary creep deformation and creep ductility of the two steels investigated have been analyzed based on the type of voids developed during the tertiary stage. Based on detailed microstructural investigations using scanning and transmission electron microscope, prevailing damage mechanisms such as structural transformation, particle coarsening, grain boundary thickening have been identi ied and applied for remaining life assessment of the two steels. Hardness and other mechanical properties were observed to decrease slightly on aging; both for rotor forging and casing casting steel. The softening occurred due to dissolution of M3C and Mo2C carbides and coagulation of others resulting in reduced creep strength. Gradual fall of creep strength at intermediate aging times was due to recovery in ferrite, gradual depletion of solid solution carbides from the ferrite matrix, metastability and transitional character of precipitated carbides. Creep crack growth studies have also been carried out on both the steels. Life assessment calculations have also been carried out using creep crack growth methodology. [Copyright &y& Elsevier]