Long-term Creep Rupture Strength Research Articles

A new tensile creep model for predicting long-term creep strengths with short-term test data for creep resistant alloys

The difficulties in using the conventional Norton creep model to rationalise short-term creep data and to subsequently predict long-term creep rupture strengths for creep resistant alloys are presented and analysed. The results of this study show that these difficulties can be resolved if a new tensile creep model that integrates the tensile strength at creep temperature is applied to rationalise the short-term creep data. This is illustrated with the creep and tensile strength data measured for a grade of Ni-based superalloy. Based on this new tensile creep model, the activation energy of creep determined is independent of stress and the stress exponent is not influenced by temperature. Consequently, the model constants obtained from the short-term creep data can be applied together with the Monkman-Grant relationship to make the long-term creep rupture strength predictions at different temperatures. The factors affecting the reliability of the predictions made by this method are also analysed.

Design of a new 11Cr martensitic steel and evaluation of its long-term creep rupture strengths

A new grade of 11 wt% Cr steel was designed in which no nitrogen was added to minimise the driving force of the Z-phase formation and hence to maximise the stability of the fine MX precipitate particles. But, 6 wt% Co was added to completely suppress the δ-ferrite formation in the matrix. The creep data were measured in the range of 650–725 °C at different stresses and rationalised on the basis of a new creep model, which revealed that the effect of stress on creep was equivalent to that of temperature. The model parameters so determined was used to predict the 100,000 h creep rupture strengths. The results showed that these long-term creep rupture strengths were higher than those for the P92 Steel, which is a 9 wt% Cr and the strongest steel grade among commercial 9–12 wt% Cr steels at present. The reliability of the long-term creep rupture strength predictions for the new steel is also analysed.

Verification of Long-term Creep Rupture Properties and Microstructure Stability of Ni based Alloy 45Ni-24Fe-23Cr-7W-Ti Parent Metal and Weldment

ABSTRACT To clarify the long-term creep rupture strength and microstructure stability of Ni-based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti, ASME Code Case 2684), creep tests were conducted up to about 70,000h and 117,000h for the weldment and parent metal respectively, and the microstructure of crept specimens was experimentally investigated. The weldment showed comparable strength with the parent metal. The metallurgical microstructure characterization and quantitative evaluation on the precipitation behavior indicated the precipitation strengthening of Laves phase, M23C6 carbide and αCr phase in the HAZ (Heat Affected Zone) and parent metal contributing to the long-term creep rupture strength stability of weldment. Satisfactory microstructural stability was proven without distinct degradation during long-term creep at 700°C up to around 117,000h. Furthermore, the internal pressure creep test of circumferential weldment was performed to verify the damage and failure morphology of actual components under the multiaxial loading

Long-term creep rupture strength prediction for a new grade of 9Cr martensitic creep resistant steel (G115)—An application of a new tensile creep rupture model

A method is presented to firstly rationalise the short-term creep rupture strength data and then to predict the 100,000 h creep rupture strengths of a new 9Cr grade of martensitic creep resistant steels (G115) at different temperatures. The method is made possible by a new tensile creep rupture model, which is formulated by combining a new tensile creep model that integrates tensile strength at creep temperature with the Monkman–Grant relationship. On the basis of this new tensile creep rupture model, the activation energy of creep rupture determined for the steel G115, which is 293 ± 25 kJ/mol, does not depend on stress or stress range, and the creep rupture stress exponent depends only on stress range but not on temperature. The model parameters determined from short-term creep rupture strength data can then be used to predict the long-term creep rupture strengths at different temperatures for this new steel grade. The reliability of the predictions is analysed and the microstructural evolutions occurring during creep that may cause premature creep rupture and hence lead to possible over-predictions are also delineated. Based on the prediction results obtained, the highest applicable temperature of this new steel grade for power plant applications is determined to be slightly higher than 625 °C.

Influence of Oxidation on Estimation of Long-Term Creep Rupture Strength of 2.25Cr–1Mo Steel by Larson–Miller Method

AbstractThe influence of oxidation on the estimation of long-term creep rupture strength is investigated for 2.25% chromium (Cr)–1% molybdenum (Mo) steel specified as JIS STBA 24, JIS SCMV 4 NT, and ASTM A542/A542M by the Larson–Miller method using creep rupture data in the National Institute for Materials Science (NIMS) Creep Data Sheets at 450–650 °C for up to 313,000 h. The creep rupture data exhibit a change in slope of the stress versus time to rupture curves due to oxidation in air during 600 °C creep tests at 15,000–40,000 h and 650 °C tests at 2000–3500 h for different size specimens, which indicates degradation in creep life by the oxidation. The estimated 100,000 h creep rupture strength using regression analysis is increased by the elimination of long-term data degraded by the oxidation. Several metallurgical factors, such as the initial strength represented by the 0.2% proof stress at the creep test temperature and the concentration of aluminum (Al) impurity, also affect the creep life of the tested steel.

Assessment of Long-Term Creep Rupture Strength of T91 Steel by Multiregion Rupture Data Analysis

Creep rupture strength of creep strength enhanced ferritic steels is often overestimated, and its evaluated value has been reduced repeatedly. In this paper, the cause of the overestimation is discussed, and the creep rupture strength of T91 steel is assessed with its updated creep rupture data. Effects of residual Ni concentration on the creep rupture strength and necessity of F factor in T91 steel are also discussed. Decrease in activation energy Q for rupture life in long-term creep is the cause of the overestimation, since conventional time–temperature parameter (TTP) methods cannot deal with the change in Q. Due to the decrease in Q, long-term creep rupture strength evaluated decreases as longer-term data points are added or shorter-term data points are discarded in the conventional TTP analysis. The long-term region with small values of activation energy and stress exponent is named region L2 in this paper. Region L2 appears in all the heats of T91 steel and plate products of Gr.91 steel. Since service conditions of the T91 steel are usually in region L2, the creep rupture strength under the service conditions should be evaluated from the rupture data in region L2 only. The 5 × 105 hrs rupture strength at 550 °C decreases from 129 MPa (evaluated from the whole data of T91 steel) to 79 MPa (evaluated from the data in region L2 only) with increasing cut-off time for data selection. The 105 hrs rupture strength at 600 °C also decreases from 87 MPa (whole data) to 70 MPa (region L2 only) despite sufficient number of long-term data points at 600 °C. Careful consideration on the data selection is necessary in evaluation of creep rupture strength of the T91 steel. A multiregion rupture data analysis (MRA) is helpful to select data points belonging to region L2.

Predicting long-term creep-rupture property of Inconel 740 and 740H

The prediction of long-term creep-rupture strength values for Inconel 740 and 740H is made at temperatures ranging from 650°C to 850°C by using Larson–Miller (LM) and Manson–Haferd (MH) methodologies. Then, the predicted creep-rupture strength values are compared with the ones reported by Wilshire–Scharning (WS) model, a new approach for life assessment. The results show that the LM method is easy to cause either underestimation or overestimation of long-term properties depending on the selected data. Compared with the LM and WS methods, predictions produced by MH method have a relatively good consistency with the experimental data over 30 000 h, and the 105 hours creep-rupture strength extrapolated from rupture life of 5000 h or more than 30 000 h are almost same. This suggests that the MH method is a better method for predicting the long-term creep-rupture strength of Inconel 740 and 740H alloys.

Research and Development of Heat-Resistant Materials for Advanced USC Power Plants with Steam Temperatures of 700 °C and Above

Materials-development projects for advanced ultra-supercritical (A-USC) power plants with steam temperatures of 700 °C and above have been performed in order to achieve high efficiency and low CO2 emissions in Europe, the US, Japan, and recently in China and India as well. These projects involve the replacement of martensitic 9%−12% Cr steels with nickel (Ni)-base alloys for the highest temperature boiler and turbine components in order to provide sufficient creep strength at 700°C and above. To minimize the requirement for expensive Ni-base alloys, martensitic 9%−12% Cr steels can be applied to the next highest temperature components of an A-USC power plant, up to a maximum of 650°C. This paper comprehensively describes the research and development of Ni-base alloys and martensitic 9%−12% Cr steels for thick section boiler and turbine components of A-USC power plants, mainly focusing on the long-term creep-rupture strength of base metal and welded joints, strength loss in welded joints, creep-fatigue properties, and microstructure evolution during exposure at elevated temperatures.

A method for long-term creep–rupture strength prediction based on a small sample of experimental results, smoothed bootstrapping and time–temperature parameters

A method is presented that enables predictions of long-term creep–rupture strength based on a small sample of short-term experimental results. Smoothed bootstrapping in combination with the most commonly used time–temperature parameters is used for the determination of the optimal values of coefficients. For ten metals, creep–rupture strengths are evaluated for both full-size and sub-size data sets. Predictions in the case of small data sets always lie on the conservative side, whereas the confidence interval of predicted strengths decreases with an increasing number of experimental results. However it is shown here that for the evaluated materials, usable interim predictions of creep–rupture strengths can be achieved.

Long-term creep-rupture strength prediction for modified 9Cr–1Mo ferritic steel and type 316L(N) austenitic stainless steel

The prediction of long-term creep-rupture strength values for mod. 9Cr –1Mo ferritic steel and 316L(N) austenitic stainless steel is made using several life prediction methodologies at 773, 823 and 873 K. Creep-rupture strength values have been predicted following the Larson –Miller parameter, the Orr – Sherby –Dorn parameter, artificial neural network, and Wilshire approaches for rupture lives up to 60 years (5.2566105 h) using creep-rupture data available in the literature. It has been demonstrated that the prediction of creep-rupture strength values using these approaches are comparable. Creep-rupture strength values have been also evaluated using linear extrapolation of average creeprupture strength values given in French Nuclear Design Code RCC-MR for durations more than 36105 h. Predicted creep-rupture strength values using the literature data are found to be higher than those obtained from RCC-MR for both 9Cr –1Mo steel and 316L(N) SS. This suggested that the RCC-MR data are conservative and can be safely used for a design life of 60 years.

Alloy Design of Creep- and Oxidation-Resistant 9%Cr Steel for High Efficiency USC Power Plant

Ferritic heat resistant steels are strongly desired to expand their maximum use temperature up to 650°C for application to the next highest temperature components of highly efficient, low emission ultra-supercritical (USC) power plant with maximum steam temperature of 700 °C. This minimizes the requirement of expensive nickel base superalloys. Critical issues for the development of ferritic steels for 650 °C USC boilers are the improvement of oxidation resistance as well as long-term creep rupture strength, including welded joints. The optimized combination of boron addition and dispersion of nanometer-sized vanadium nitrides significantly improves long-term creep strength of 9% Cr steel and also exhibits no degradation in welded joints at 650 °C. The protective Cr2O3-rich scale forms on the surface of 9% Cr steel by pre-oxidation treatment in Ar gas, which significantly improves the oxidation resistance in steam at 650°C.

Long-term creep-rupture strength prediction for modified 9Cr–1Mo ferritic steel and type 316L(N) austenitic stainless steel

The prediction of long-term creep-rupture strength values for mod. 9Cr –1Mo ferritic steel and 316L(N) austenitic stainless steel is made using several life prediction methodologies at 773, 823 and 873 K. Creep-rupture strength values have been predicted following the Larson –Miller parameter, the Orr – Sherby –Dorn parameter, artificial neural network, and Wilshire approaches for rupture lives up to 60 years (5.2566105 h) using creep-rupture data available in the literature. It has been demonstrated that the prediction of creep-rupture strength values using these approaches are comparable. Creep-rupture strength values have been also evaluated using linear extrapolation of average creeprupture strength values given in French Nuclear Design Code RCC-MR for durations more than 36105 h. Predicted creep-rupture strength values using the literature data are found to be higher than those obtained from RCC-MR for both 9Cr –1Mo steel and 316L(N) SS. This suggested that the RCC-MR data are conservative and can be safely used for a design life of 60 years.

Effect of Carbon, Nitrogen and Nickel on Long-term Creep Rupture Strength of 10 Cr Steels Containing Boron

Microstructure and precipitates of creep ruptured specimens were investigated in order to understand the effects of carbon (C), nitrogen (N) and nickel (Ni) on long-term creep rupture strength of 10Cr heat-resistant steels containing boron (B). The low-N steels showed higher creep rupture strength than the high-N steels. In long-term creep rupture region such as over 10,000 h at 650°C, the deterioration of creep rupture strength was not observed in the low-N, high-C steel. On the other hand, the creep rupture strength of the low-N, low-C steel dropped to the strength level of the high-N steels. The addition of N to the B containing steels promoted the recovery of microstructure. The formation of coarse BN in the high-N steels led to the decrease of the amount of effective B that dissolved in M23(C,B)6 and suppressed its coarsening. From EDS analysis of precipitates, the fraction of M23C6 in the low-C steels was less than that in the high-C steels, while the fraction of coarse Laves phase in the low-C steels was more than that in the high-C steels. In the low-N, low-C steel, the coarsening of precipitates caused the deterioration of the creep rupture strength after 10,000 h exposure. Ni lowered Ac1 and Ac3 transition temperature, but it did not affect the fraction of precipitates according to the calculation of thermodynamic equilibrium using Thermo-Calc. It is concluded that C and N are more effective to the stability of microstructure than Ni.

Novel Concept of Creep Strengthening Mechanism using Grain Boundary Fe2Nb Laves Phase in Austenitic Heat Resistant Steel

ABSTRACTThe creep behavior of a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%), strengthened by intermetallic Fe2Nb Laves phase, has been examined. Particular attention has been given to the role of grain boundary Laves phase in the strengthening mechanism during long-term creep. The creep resistance increases with increasing area fraction (ρ) of grain boundary Laves phase according to equation ε/ε = (1−ρ), where ε0 is the creep rate at ρ = 0. In addition, the creep rupture life is also extended with increasing ρ without ductility loss, which can yield up to 77% of elongation even at ρ = 89%. Microstructure analysis revealed local deformation and well-developed subgrains formation near the grain boundary free from precipitates, while dislocation pile-ups were observed near the grain boundary Laves phase. Thus, the grain boundary Laves phase is effective in suppressing the local deformation by preventing dislocation motion, and thereby increases the long-term creep rupture strength. This novel creep strengthening mechanism was proposed as “grain boundary precipitation strengthening mechanism” (GBPS).

Effect of specimen size and shape on creep rupture behavior of creep strength enhanced ferritic steel welds

The creep strength enhanced ferritic (CSEF) steels such as Grades 91, 92, 122, 911, 23 and 24 have become very important key materials for high efficiency fossil-fired power plants for last decades, however the long-term creep rupture strength and strength reduction in welds due to Type IV failure of these steels are serious problem to be urgently resolved. In order to use CSEF steel welds safely setting new weld strength reduction factors have internationally discussed. For instance ASME Boiler and Pressure Vessel Code Committee recently developed a new creep strength reduction factors for CSEF steels intercritically post-weld heat treated to be 0.5 based upon the creep rupture data obtained for standard creep specimens. However it is needed to make further consideration on the specimen size/shape effect on the creep strength of the welds to determine more appropriate weld strength reduction factors and joint influence factors. Present report provides comprehensive creep rupture test results of the specimens with various size and shape, and full size components dealing with creep rupture location/behavior and specimen size/shape effect on the creep rupture strength of CSEF steel welds.

New approach to predict the long-term creep behaviour and evolution of precipitate back-stress of 9–12% chromium steels

Modern advanced 9–12 % Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. The mechanical properties of these steels are significantly influenced by the presence and stability of various precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service, which leads to a varying obstacle effect of these precipitates on dislocation movement. In this work, the experimentally observed creep rupture strength of an modified 9–12% Cr steel developed in the European COST Group is compared to the calculated maximal obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after long time service. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.

Suppression of creep cavitation in precipitation-hardened austenitic stainless steel to enhance creep rupture strength

Extensive creep cavitation in Ti, Nb and Cu containing precipitation hardened austenitic steels was found to limit the usefulness of deformation resistance to increase long-term creep rupture strength. The steels were microalloyed with boron and cerium that resulted in increase in creep rupture strength and ductility of the steels significantly. Grain boundary sliding and creep cavity nucleation and growth in the steels were suppressed greatly on microalloying. Auger spectroscopic analysis revealed the segregation of boron instead of sulfur on cavity surface and the absence of sulfur contamination of grain boundary upon the microalloying. Suppression of creep cavitation through the control of trace elements segregation along with the precipitation hardening increased the creep rupture strength of austenitic stainless steels.

Effect of Cobalt and Boron on Long-term Creep Rupture Strength of 12Cr Cast Steels

Microstructure of creep ruptured specimens is investigated in order to understand the effect of Co and B on long term creep rupture strength of 12Cr cast steels. The specimen with 2.5% Co contains no delta-ferrite and has better tensile and impact properties than Co free specimens including about 1 vol% delta-ferrite. In short-term creep rupture region, Co containing steel has shown better strength than Co free steel, while in long term region such as over 7000 h at 923K, Co free steel has shown better strength. The addition of Co promoted recovery of microstructure. The difference between 20 ppm B and 50 ppm B steels in short-term creep rupture property was not observed, while in long-term region, 50 ppm B steel has a tendency to show better strength. From ATE method, it is considered that some of B have been included in carbide such as M23(CB)6, and retarded coarsening of carbide. According to calculation of thermodynamic equilibrium, solid solution content of W and Mo was reduced by Co addition. Therefore the thermodynamic stabilization of microstructure associated with the solid solution strengthening of W and Mo has decreased and the formation of Laves phase has been enhanced. It has been concluded that improvement of stabilization and enhancement of formation of Laves phase by 2.5% Co addition is one of the reasons of the deterioration of long-term creep rupture strength.

Long-term creep properties of low C–2.25Cr–1.6W–V–Nb steel (T23/P23) for fossil fired and heat recovery boilers

Long-term creep-rupture strength of low C–2.25Cr–1.6W–V–Nb steel (HCM2S; KA-STBA/PA24J1, T23/P23, ASME CC2199) has been studied and reassessed using the oxidation correction method after Nakashiro et al. Their long-term creep strengths are found to be very stable at temperatures between 500 and 600 °C. A severe oxidation of the creep-rupture specimens has been identified at high temperatures above 600 °C and the longer term rupture time tends to decline below the extrapolated curve. An oxidation correction has been found to be effective to evaluate the long-term creep-strength stability of the steel and worked well in the case of 2.25Cr–1Mo steel (ASME T22/P22). It is thus concluded that in low C–2.25Cr–1.6W–V–Nb steel no creep-strength degradation apart from that due to oxidation at high temperatures is expected, in contrast to results on advanced high Cr steels.

Predicted precipitate back-stress and creep rupture strength of the advanced 9–12% Cr steel COST E2

Abstract Modern advanced 9–12% Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. In this work, the experimentally observed creep rupture strength of the European COST alloy E2 is compared to the calculated maximum obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. The applied model for precipitation strengthening is briefly outlined. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after a long time under service conditions. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.