Type IV creep damage is a problem in high-temperature steam piping made of high chromium steel at thermal power plants, and a method for evaluating the remaining life is required. In this study, we considered that void’s initiation and growth can be expressed by initiation rate f, growth rate h, and initiation start time t 1 , and that stress and TF affect f, h and t 1 . We also proposed the method to estimate f, h and t 1 by measuring the change of the distribution of radius of voids during creep test. The creep test conditions are (1) test temperature of 650 C, maximum principal stress σ 1 of 79.5MPa, and TF of 2.5 ~ 3.0, and (2) test temperature of 650C, maximum principal stress of 71.5MPa, and TF of 2.5 ~ 3.0. The influence of σ 1 to f, h and t 1 was quantified by comparing the result of test (1) and that of test (2).

In this study the effect of Widmanstätten-type morphology α 2 plates on creep has been investigated by preparing nearly equiaxed γ (N γ ) and nearly equiaxed γ having Widmanstätten-type α 2 plates within grain (Wα 2 ). Creep tests were conducted at 1073 K under constant stresses, high stress and low stress, in air. At the high stress, Wα 2 shows creep rate smaller than N γ in transient stage, both specimens show similar minimum creep rate and the creep strain at minimum creep rate is 3 % for Wα 2 and 10 % for N γ, since N γ shows prolonged primary region. In acceleration stage, both show similar behavior with rupture time of about 50 h and rupture elongation of 60 %. At the low stress, on the other hand, reverse behavior occurs, that is, W α 2 shows creep rate higher than Nγ in transient stage. The regions near grain boundaries progressively deformed for both specimens at high stress level, whereas deformed region is extended within grain interiors. From these results it is suggested that α 2 plate act as the obstacle for dislocation motion in the γ matrix at high stress and that interfacial dislocation promote the creep deformation at low stress.

A 10%Cr martensitic steel with 3%Co and 0.008%B exhibits extremely long creep rupture time of approximately 40000 h under an applied stress of 120 MPa at a temperature of 650°C. The steel’s microstructure after creep tests interrupted at different creep stages was examined by transmission and scanning electron microscopy. It was shown that superior creep resistance of this steel was attributed to slow increase in creep rate at the first stage of tertiary creep whereas the rapid acceleration of creep rate took place only at the short second stage of tertiary creep. Transition from minimum creep rate stage to tertiary creep was found to be accompanied by coarsening of Laves phase particles, whereas M 23 C 6 – type carbides demonstrated high coarsening resistance under creep condition. Strain-induced formation of Z-phase does not affect the creep strength under applied stress of 120 MPa due to nanoscale size of Z-phase particles.

The creep behavior of a γ-TiAl based alloy at 1073 K was investigated, examining three different microstructures: equiaxed γ (Eγ), γ/γ fully lamellar (FLγ), and equiaxed γ with α 2 phase on grain boundaries (Eγα 2 ). The aim was to understand the influence of lamellar interfaces and grain boundary α 2 phase on creep behavior. Initially, creep rates were consistent across all specimens upon loading. However, Eγ exhibited a gradual decrease in creep rate compared to Eγα 2 and FLγ. Notably, the minimum creep rate of Eγ was one order of magnitude lower than that of Eγα 2 and FLγ. Conversely, Eγα 2 and FLγ displayed a slight acceleration and the longest rupture strain, albeit with the shortest rupture time compared to Eγ. Upon microstructural analysis of of the creep-test specimens, it was observed that numerous dynamic recrystallized grains (DXGs) and sub-grains formed along grain boundaries and interiors in Eγ, whereas they were limited to the region along grain boundaries in FLγ. In contrast, very few DXGs were formed in Eγα 2 . These findings indicate that γ/γ interfaces inhibit the extension of DXGs into grain interiors, suggesting that the grain boundary α 2 phase effectively suppresses the formation of DXGs.

The aim of this work was to reveal the effects of trace elements on the creep properties of nickel-iron base superalloys, which are the candidate material for the large components of the advanced-ultrasupercritical (A-USC) power generation plants. High temperature tensile and creep properties of forged samples with seven different compositions were examined. No significant differences were observed in the creep rate versus time curves of the samples, of which contents of magnesium, zirconium, manganese and sulfur were varied. In contrast, the curves of phosphorus-added samples showed very small minimum creep rates compared to the other samples. The creep rupture lives of phosphorus-added samples were obviously longer than those of the other samples. Microstructure observation in the vicinity of grain boundaries of phosphorus-added samples after aging heat treatment revealed that there were fine precipitates consisting of phosphorus and niobium at the grain boundaries. The significant suppression of the creep deformation of phosphorus-added sample may be attributed to the grain boundary strengthening caused by the fine grain boundary precipitates.

High precision stress relaxation tests (SRT) at temperatures between 550C and 700C were performed on serviced and reheat treated T91, 9%Cr steel. The service exposure was 116,000 hours at steam temperatures to 550C. Constant displacement rate (CDR) tests were also run at 600C on notched specimens for the two conditions. Specimens, heat treated after service, were stronger at the lower test temperatures in terms of both tensile strength and creep strength. This difference was reflected in the CDR results, which also suggested a lower fracture resistance in the heat treated condition. Thus, service exposure appears to have softened the alloy and enhanced its resistance to fracture, with no evidence of embrittling reactions. Based on the analysis of the SRT tests, projections were made of the times to 1% creep and the times to rupture as well as direct comparisons with minimum creep rate data'. When plotted on the basis of a Larson- Miller parameter (C=30), the calculated values compared well with actual long time rupture testing for exposed and re-heat treated specimens, and generally showed higher precision. The longest test time was about eighteen months for the stress rupture data compared with the use of one machine for a few weeks for the SRT data. The latter actually covered a far greater range of creep rates and projected lives. The SRT test is especially consistent at higher parameter values, i.e., higher temperatures and/or lower stresses. This method of accelerated testing is now being applied to a wide range of alloys for fossil power plants for composition and process optimization, design analysis, and life assessment.

The Cr and W effect on the creep strength of ferritic steels were studied using the new strengthening hypothesis, precipitation strengthening mechanism, by examining the residual aligned precipitates consisting of W and Cr. In 2 mass% W-containing steel, the increase in Cr content up to 10 mass% resulted in the creep life extension. However, the Cr content higher than 11 mass% decreased the creep life. In 9 mass% Cr-containing steel, the increase in W content decreased the creep deformation rate with creep time. However, it also shortened the time to reach the minimum creep rate. Therefore, optimum Cr and W contents possibly resulted in the optimum alloy design. To understand the effect of W and Cr contents on creep strength, the precipitation strengthening hypothesis by the precipitates at the block boundary must be introduced. The residual aligned precipitation line is supposedly an effective obstacle for the dislocation motion at the interparticle space of the aligned precipitates. The new hypothesis will be activated after block boundary migration. It occurs during the acceleration creep period. On the basis of the hypothesis, creep strength was expressed as the summation of threshold creep stress and effective internal creep stress. According to the experimental data of microstructure recovery, the effective internal stress decreased with creep deformation and consequently vanished. In such cases, creep strength is decided only by the threshold stress of creep. Integrating all, we concluded that the creep deformation mechanism of ferritic creep-resistant steel possibly transits from the viscous dislocation gliding mode to the microstructure recovery driven type mode during the acceleration creep.

In this study, we have examined the creep of a novel austenitic heat resistant steel of Fe-20Cr- 30Ni-2Nb (at.%) steel at 1073K in steam and air atmospheres. Our studied steels were Fe-20Cr- 30Ni-2Nb (base steel) and that with 0.03 at. %B (B-doped steel) . The addition of boron is to intentionally increase the area fraction of Laves phase on grain boundaries (ρ). The specimen with ρ = 43% (base steel pre-aged at 1073 K/240 h) exhibits the rupture life of 262 h, whereas the rupture life of the specimen with higher ρ of 80% (B-doped steel pre-aged at 1073 K/240 h) is 833h, which is about three times longer than that of the specimen with ρ = 43%. The specimen with ρ = 80% exhibits smaller creep rate than those with lower ρ than 43% in the entire creep stage. In addition, all specimens show the creep rupture strain of about 60%. The creep rupture life is almost same to that tested under air, whereas the creep rupture strain is slightly smaller (a few percent) than that under air. In the surface of the creep ruptured specimen in steam, the intergranular oxides associated with voids or cavities are often present and grow along grain boundaries to over 100 μm in depth. The intergranular oxidation occurs more extensively in steam rather than air. These results demonstrate that stable Fe 2 Nb Laves phase on grain boundary could increase the creep resistance of the present steel at 1073K without ductility loss in steam as well as air, resulting in the pronounced extension of rupture life. The intergranular oxidation accelerated by steam would not give a serious effect on the creep properties of the present steel below 103 hours in rupture life.

The microstructures of an advanced Ta-added 9Cr-3Co-2W-Mo steel with increased boron content that has been homogenized at different temperatures were investigated. The chains of coarse W-rich particles were observed in the steel after homogenization at 1150°C for 24 h. These particles remained in the microstructure after normalization and tempering. Such additional dispersion hardening in the initial state of the studied steel decreased the creep rate in transient region. However, the duration of steady state creep and overall creep time was increased in the samples homogenized at 1200°C. Despite of the presence of coarse W-rich particles, the impact toughness of the low-temperature- homogenized steel in the tempered condition was significantly higher than that of the steel homogenized at 1200°C

The creep strength and ductility of Grade P22 steel (2¼ Cr) was measured at 600°C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitization temperatures ranging from 900°C - 1200°C followed by normalization at 900°C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitization temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600°C increased as function of PAG size up to approximately 70µm, but significantly decreased when the average prior austenite grain size measured approximately 108 µm. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.

The paper deals with microstructural evolution in the AISI 316LN + 0.1 wt.% Nb steel during long-term creep exposure at 600 and 625°C. The following minor phases formed: Z-phase (NbCrN), M 23 C 6 , M6X (Cr3Ni2SiX type), η-Laves (Fe2Mo type) and σ-phase. M6X gradually replaced M 23 C 6 carbides. Primary Z-phase particles were present in the matrix after solution annealing, while secondary Z-phase particles formed during creep. Precipitation of Z-phase was more intensive at 625°C. The dimensional stability of Z-phase particles was excellent and these particles had a positive effect on the minimum creep rate. However, niobium also accelerated the formation and coarsening of σ-phase, η-Laves and M6X. Coarse particles, especially of σ-phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.

New Monte Carlo models have recently been developed to predict microstructural evolution in steels and aluminum alloys during heat treatment and high-temperature service. These models can control precipitate type and size distribution, distinguishing between pure lattice and grain boundaries. Consequently, they can forecast the precipitate size distribution within grains and on grain boundaries as a function of time. This paper describes the model validation for ferritic Fe-9Cr P92 steels. The model provides new information over a range of time intervals adding up to the total plant lifetime in an ultra-supercritical plant. This information can be incorporated into continuum damage mechanics models for predicting creep rate and stress rupture life. The paper discusses how this technique is used as a materials development tool to forecast necessary compositional modifications for improving creep properties in ferritic steels.

This study explores methods to enhance the creep strength of 12%Cr martensitic/ferritic steels. The approach focuses on utilizing various precipitates to hinder microstructure coarsening and dislocation movement. A combination of Laves phase (slow precipitation) and MX carbonitrides (dislocation pinning) is used for sustained strengthening. Different MX-forming elements (V, Ta, Ti) are investigated to identify the optimal combination for high quantities of finely distributed strengthening particles. Additionally, cobalt and copper are employed to promote a fully martensitic microstructure and potentially slow down diffusion or provide nucleation sites for Laves phase precipitation. Long-term creep tests confirm the effectiveness of Laves phase precipitation, particularly with tungsten present. Tantalum's influence on both MX precipitation and the Laves phase is also observed. Combining multiple MX-forming elements (V/Ta, V/Ti, Ta/Ti) further improves creep strength, supported by predictions of high MX carbonitride formation from Thermo-Calc calculations. Partially replacing cobalt with copper (1%) also demonstrates positive effects on creep properties.

Fossil fuels continue to be the primary source of energy in the U.S and worldwide. In order to improve the efficiency of fossil power plants, advanced structural materials need to be developed and deployed to meet the need of high temperature creep resistance and corrosion resistance. Examples include creep strength enhanced ferritic (CSEF) steels, austenitic stainless steels, nickel-based superalloys, and oxide dispersion strengthened alloys. Welding is extensively used in construction of fossil power plants. The performance of the weld region can be critical to the safe and economical operation of fossil power plants. Degradations in performance such as reduced creep strength and premature failure in the weld region (e.g. Type IV failure in ferritic steels) are examples of longstanding welding and weldability problems for boiler and other components. In the past, extensive studies have been carried out to characterize the different microstructures in different regions of a weld, and to a certain extent, to establish the correlations between the microstructure and the creep strength. However, the metallurgical or microstructural induced local stress/strain variations have been seldom quantified. In addition, it has been long recognized that, due to the sharp microstructure and property gradients in the weld and HAZ, the standard creep testing procedure for the base metal can produce erroneous results when used for weld testing.

This study is concerned with the creep damage evaluation for the welded joint of modified 9Cr-1Mo steels. A finite element prediction method based on ductility exhaustion approach has been proposed. Degradation of creep ductility under multi-axial stress state has been formulated from the experimental results of notched bar specimens for the base metal and the fine-grained heat affected zone, and has been taken into the damage model. Creep test of welded joint specimen of modified 9Cr-1Mo steel has been conducted to confirm the accuracy of the damage evaluation method. It has been concluded that the predicted trend of creep damage has good agreement with the experimental results, but the predicted rupture time become longer than the experimental results of rupture time.

A methodology is developed for evaluating its creep rupture life from analysis of an on-going creep curve with the aid of an Ω creep curve equation. The method is applied to on-going creep curves of grade 91 steel for evaluating their rupture lives. Quick decrease in creep rupture strength has been reported recently in long-term creep of grade 91 steel. The quick decrease of the steel is discussed by using the rupture lives evaluated. The quick decrease is confirmed in the present study in the time range longer than 3 x 10 4 h at 600°C.

Significant developments have been made in recent years in the description of creep as a damage process. Advanced martensitic steels that expend over half their lives in the tertiary creep stage have been the focus of attention since their use temperatures are being extended to higher temperatures. Data available for assessing the predictability of the damage models are somewhat scarce since long-time exposure testing has generally been for constant temperature and load conditions. In this work, data are reviewed from relatively long-time tests that involved temperature and stress changes. The prediction of several damage models are compared to material behavior. Most of the comparisons are for 9Cr-1Mo-V steel in the temperature range of 538 to 649 C and for times in the range of 10,000 to 80,000 hours.

Titanium is extensively utilized in the aerospace industry due to its low density and excellent mechanical and chemical properties. Given that components in this sector are exposed to temperatures up to 873 K, representing 45% of the metal's melting point, understanding the mechanical properties in this temperature range is crucial for ensuring flight safety. This study focuses on examining the creep behavior of pure titanium to gain insights into its fundamental mechanical response. Creep was observed to occur at stresses exceeding micro-yielding levels around 297 K, primarily attributed to overcoming the pinning effect caused by interstitial atoms. Interestingly, at intermediate temperatures, an inverted primary creep phenomenon was noted, with an activation energy of approximately 240 kJ/mol within this range. This value, significantly larger than those associated with lattice or dislocation-core diffusions, suggests the potential movement of dislocations with interstitial atoms, similar to the diffusion of oxygen or nitrogen within titanium. Moreover, fracture strain exceeded 80% at temperatures surpassing 673 K, possibly resulting from grain boundary diffusion mechanisms akin to superplasticity. The activation energy for this mechanism, at 97 kJ/mol, is adequate for activating grain boundary deformation at intermediate temperatures.

A constitutive equation, with parameters derived from the interpolation of primary and steady state stages of constant load creep curves, has been utilized to estimate the stress relaxation behavior of the martensitic steel X20Cr13, alloy used in many high temperature applications, including heavy duty gas turbines. Creep and stress relaxation tests have been performed at 350°C, close to the negligible creep temperature of the studied alloy for stresses of interest for engineering applications. The creep tests were carried out at stresses below and above the yield stress, whereas, for the relaxation stress tests, the imposed strain was in the range 0.2% to 1.2% with the purpose to have, at the beginning of the tests, the same initial stresses of the performed creep tests. After a stress relaxation period, lasting between 10 to 1000 hours, each specimen was generally reloaded at the initial stress and a new relaxation test, on the same specimen, was carried out. This “reloading procedure”, simulating the re-tightening of bolts, has been repeated several times. The proposed equation has shown to well predict the experimental creep and stress relaxation behavior of the steel under investigation.

Creep strength degradation behavior of normalized and tempered 2.25Cr-1.6W-V-Nb(Gr.23) steel was investigated by conducting extra long-term creep rupture tests. Creep strength drop was observed in long-term creep range at 600°C and above, while signs of a creep strength drop were not identified at 550°C and 575°C. Creep strength drop of around 110 MPa at 600°C was caused not by the effect of oxidation but rather by a change of the deformation mechanism or the weakening of deformation resistance by the microstructural change during creep. The effect of oxidation was significant for causing a further creep strength drop in the range which exceeded 20,000 h in rupture time at 600°C. As a result, the creep strength at 60 MPa and 600°C was almost the same regardless of long tempered or aged steel.