Larson Miller Research Articles

Residual lifetime assessment of cold-reheater pipe in coal-fired power plant through accelerated degradation test

Non-destructive testing (NDT) is the process of evaluating and inspecting materials without damage. The purpose of present study is to evaluate the residual lifetime of materials by proposing a new concept of non-destructive testing. The residual lifetime of service-exposed 1.25Cr-0.5Mo (P11) steel cold-reheater pipes in a coal-fired power plant, which were used for approximately 23 years at 310 °C, was investigated with accelerated degradation tests of a fresh P11 steel. The hardness and the microstructure of the P11 steel aged at 550 °C for 1000 h were similar to those of the service-exposed cold-reheater pipe. The creep-rupture tests of the steel aged at 550 °C for 1000 h were performed under various temperature and stress conditions together with the fresh steel. The creep-rupture properties analyzed using a Larson–Miller parameter (LMP) approach suggested that the residual lifetime of the service-exposed pipes is approximately 53% at the current conditions. The proposed methodology is expected to serve as a useful tool to check the timing of materials replacement and to maintain safety in a coal-fired power plant.

Experimental and numerical analysis of creep and damage behavior of P92 steel by small punch tests

The creep properties of the P92 steel were evaluated by small punch creep (SPC) tests and uniaxial creep (UC) tests. In addition, finite element (FE) methods using different creep damage models were adopted to analyze the variation of creep damage and deformation process for SPC tests, and the validity of FE methods was confirmed by the comparison of experimental deformation and damage accumulation. The rupture time, fracture positions and crack growth behaviors were dependent on the creep damage model. The friction coefficient was also discussed to analyze the effect on the creep properties. On the basis of these results, the average equivalent stress at the center of the specimen was proposed to correlate the load in SPC tests and the stress in UC tests. Then, the relationship of experimental data and FE results was established. The Larson–Miller master curves obtained from FE results indicated that the strain-based model was an appropriate creep damage model to evaluate the long term creep life. The strain-based model was more suitable for SPC tests to evaluate the creep properties for P92 steel.

Accelerated Creep Life Assessment of In-Service Power Plant Components

Structural metals used in plant components are subject to aging from a combination of fatigue, creep, and corrosion. Exposure to elevated temperatures promotes creep. Aged metals lose toughness, or the ability to absorb energy at stress above the yield point and cannot endure an occasional high load without fracturing. Creep is one of the most critical factors for determining the structural integrity of components. The main objective of the current study is to assess the remaining creep life of various 20-year old power plant engineering components such as the high temperature fasteners. Due to time constraints, the approach followed in this study was to utilize the accelerated high temperature creep testing in addition to Scanning Electron Microscopy (SEM) analysis to assess the remaining life of 4 different samples. The accelerated high temperature creep tests were conducted at a stress level of 61 MPa and at a temperature of 1000°C for samples Sample 1 (original), Sample 2, Sample 3 and Sample 4; these samples were collected from different parts of the power plant. SEM analysis was carried out for all the samples. The results of the accelerated high temperature tests were compared to similar materials’ theoretical creep data using Larson Miller curve. The Larson Miller actual creep lives of the tested samples were much higher than the experimental ones, which suggest that the samples are critically aged. SEM analysis on the other hand, showed that all samples have high percentage of creep voids

Critical assessment 31: on the modelling of tertiary creep in single-crystal superalloys

ABSTRACTModelling of tertiary creep in single-crystal superalloys – operative over the majority of the temperature/stress regime of relevance – is assessed. Traditional, empirical approaches are useful for the prediction of component life; a modern trend is for more physically faithful models which account for microstructure and alloy composition. Calculations are made using different methods, to identify advantages/disadvantages, in an effort to approach best practice. The predictions are tested against experimental data. The creep resistance can be estimated to within approximately one-half of a Larson–Miller parameter, from an input of alloy composition, deformation conditions, and estimates of microstructural parameters, rate laws (and parameters therein), and physical constants. Different merit indices are considered to identify the most appropriate for compositional optimisation.

Creep Behavior and Microstructural Evolution of a Fe-20Cr-25Ni (Mass Percent) Austenitic Stainless Steel (Alloy 709) at Elevated Temperatures

Understanding creep properties and microstructural evolution for candidate materials of the next-generation nuclear reactors is essential for design and safety considerations. In this work, creep tests were carried out at temperatures ranging from 973 to 1073 K and stresses 40 to 275 MPa followed by microstructural examinations of a Fe-20Cr-25Ni (mass pct) austenitic stainless steel (Alloy 709), a candidate structural material for the Sodium-cooled Fast Reactors. The apparent stress exponent and activation energy were found to be 6.8 ± 0.4 and 421 ± 38 kJ/mole, respectively. The higher activation energy relative to that of lattice self-diffusion together with the observation of dislocation-precipitate interactions in the crept specimens was rationalized based on the concept of threshold stress. The threshold stresses were estimated using a linear extrapolation method and found to decrease with increased temperature. By invoking the concept of threshold stresses, the true stress exponent and activation energy were found to be 4.9 ± 0.2 and 299 ± 15 kJ/mole, respectively. Together with the observation of subgrain boundary formation, the rate-controlling mechanism in the Alloy 709 was conclusively determined to be the high-temperature dislocation climb. Three types of precipitates were identified in the crept samples: Nb(C, N), Z-phases of sizes between 20 and 200 nm within the matrix and M23C6 with sizes between 200 and 700 nm within the matrix and on grain boundaries. Further, the analysis of creep rupture data at high stresses indicated that the Alloy 709 obeyed Monkman–Grant and modified Monkman–Grant relationships with creep damage tolerance factor of ~ 5. Using the Larson–Miller parameter, it was concluded that the Alloy 709 exhibited superior creep strengths relative to the other advanced austenitic steels.

Creep life assessment craze damage evolution of polyethylene methacrylate

AbstractTests have been conducted to investigate the influence of temperature, stress, and crazing for polyethylene methacrylate (PMMA) creep life. Creep life decreases with stress in a logarithmic relation, and rupture strains increase with decreasing stress. To assess the creep life of PMMA, the Larson–Miller parameter is first introduced to correlate stress and temperature with rupture time. The important role of crazing in polymer material damage, especially, the craze density and craze growth rate in the stable creep stage, is demonstrated. The craze initiation time is related to the tensile strength and to the stress and temperature at the elastic limit, and the craze initiation time can be predicted by an empirical model subsequently presented in this paper. Based on the connection between craze growth and creep rupture time, thereafter, a modified Monkman–Grant model is presented to predict creep life. The curve of craze density over creep time has the same two distinct stages as the curve of strain over time. In the final sections, a critical assessment of creep damage is also presented to provide a convenient method to predict residual life using only craze density.

Short-Term Creep Data Based Long-Term Creep Life Predictability for Grade 92 Steels and Its Microstructural Basis

Long-term creep life (5000–100,000 h) predictabilities, based on the short-term creep life data (~ 5000 h), for Grade 92 steel were investigated among major creep life prediction models, Larson–Miller parameter (LMP), normalized power law (NPL) and Wilshire models. The NPL and Wilshire models showed superior short-term creep data based long-term creep life predictabilities to the LMP model. In particular, the Wilshire model showed relatively accurate predictions (within an error range of 7%–10%), which seemed to be due to reasonable coupling of the normalized stress with the temperature-dependent rupture life term in a form of the cumulative distribution function. Both NPL and Wilshire models, calibrated by the short-term creep life data, showed a transition in creep mechanism at a similar normalized stress. Thermodynamics-based kinetic simulation (MatCalcTM) results for major precipitates (M23C6, MX and Z phases) of Grade 92 steel suggested that the creep transition is associated with coarsening of M23C6 precipitates at high temperatures (above 600 °C), which led to the degradation of the creep property.

Probabilistic fatigue-creep life reliability assessment of aircraft turbine disk

Life consumption is one of the serious concerns for turbine disk performance and maintenance cost. The low cycle fatigue of turbine disk coming from the creep and their interaction has a direct influence on life and reliability of an aero engine. So, in this paper, the finite element analysis fatigue creep on Nickel base super alloy GH4133 turbine disk at 650 °C is computed based on the material properties, load and performance parameter of the low cycle fatigue as a random factor. The LCF Manson coffin model for cycles to failure (Nf ) and Larson Miller (Nc) are calculated. Damage fraction summation is used to calculate the low cycle fatigue creep life and fitted by two parameter Weibull distribution. Therefore, Monte Carlo simulation is used in random sampling from two parameter Weibull distribution to estimate the reliability assessment of turbine disk.

Damage mechanism of nickel-based creep-resistant alloys strengthened by the Laves phase at the grain boundary

ABSTRACTThis study proposes a design guideline for polycrystal Ni-based model alloys with high ductility and 100-MPa creep rupture strength beyond 800°C and 105 h. These alloys are strengthened by both the precipitation of fine γ′ particles inside the grain and the Laves phase at the grain boundary. For investigating the damage mechanism, transformation from the non-equilibrium Laves phase to the σ phase at the grain boundary and formation of the equilibrium needle-like Laves phase inside the grain are promoted by increasing the Fe concentration. The rupture time of Fe-free alloys significantly increases because of the equilibrium Laves phase at the grain boundary owing to a suitable Mo equivalent. In particular, W addition can help achieve high-temperature creep strength. The precipitate-free zone (PFZ) is predominantly formed by prior migration at the grain boundary without precipitation. Creep rupture occurs at the precipitation/matrix interface in the PFZ. Therefore, transformation control from the Laves to the σ phase at the grain boundary suppresses creep degradation. Consequently, a Ni-based alloy with strength >100 MPa and rupture elongation >20% at 800°C and 105 h is fabricated using Larson–Miller parameter conversion, and the alloy design guideline’s validity is confirmed.

Improvement of long-term creep life extrapolation using a new master curve for Grade 91 steel

This study aims to improve the long-term creep life extrapolation of Grade 91 (Gr. 91) steel using a new master curve of a hyperbolic sine (“sinh”) form. In the master curve for extrapolating creep rupture life, a controversial point of a polynomial form, which has usually been used in time–temperature parametric methods, was preliminarily indicated. The validity of the new master curve was verified. A large body of creep rupture data for Gr. 91 steel was prepared from different sources of available worldwide data at 500 °C to 700 °C for Gr. 91 steel. The material constants of the Larson–Miller, Orr–Sherby–Dorn, and Manson–Harferd parameters were obtained using the polynomial and sinh forms on the basis of the rupture data. Long-term creep rupture life was extrapolated for up to 60 years using the two forms that defined the master curves. Results of long-term creep life extrapolation showed that the master curve of the polynomial form exhibited overestimation due to the divergent curves in the low stress ranges, whereas the master curve of the sinh form revealed goodness in the low stress ranges beyond the experimental data. The proposed sinh form was superior to the polynomial form. The sinh form could be applied to extrapolate the long-term creep rupture life of other heat-resistant steels. The creep rupture stresses predicted by the sinh function were comparable to those of RCC-MRx code.

Thermo-Mechanical Fatigue Life Assessment of a Gas Turbine Rotor Through Reliability Approach

Turbine rotor is a critical and life-limiting component in gas turbine engines. The thermo-mechanical fatigue (TMF) life of a turbine rotor was studied using reliability method. The fatigue life was estimated using (a) Marrow’s model and (b) Smith–Watson–Topper model. The creep life was estimated based on Larson Miller equations and finite element analysis. The cumulative fatigue–creep damage was estimated, and the turbine rotor TMF life was estimated against the data variation. The reliability approach takes care of material property variations, load variations and geometrical variations. These variations bring out the scatter in component stress–strain and further into life. The scattered life spells out the component reliability. The TMF life was modeled as Weibull distribution, and the reliability was estimated. The component was tested for structural integrity through hot cyclic spin test, and the results were compared with the predictions. The blade growth and strain estimations using Marrow and SWT–creep methods were found in good agreement with the test values.

Effects of Compressor Fouling and Compressor Turbine Degradation on Engine Creep Life Consumption

The effect of engine degradation in the form of compressor fouling and compressor turbine degradation on the creep life consumption of the high-pressure (HP) turbine blades of an LM2500+ industrial gas turbine engine is investigated in this work. The degradations are flow capacity degradation and isentropic efficiency degradation. An engine model was created in Cranfield gas turbine performance and diagnostics software, pythia. Blade thermal and stress models were developed together with the Larson–Miller parameter (LMP) method for creep life analysis. The percentage decreases in creep life due to each effect were examined. For the engine considered, compressor degradation has more impact on engine creep life toward peak power operation, while HP turbine degradation has more impact on creep life at lower power levels. The results of this work will give engine operators an idea of how engine components creep life is consumed and make reasonable decisions concerning operating at part loads.

A Reliability Assessment Method for High Speed Train Electromagnetic Relays

The condition of electromagnetic relays is crucial to the safe and normal operation of the high-speed train (HST). At present, there are no proper testing and assessment methods for the condition of electromagnetic relays. Electromagnetic relays experience aging and failure over their service time, which causes many potential security problems. To solve these problems, the failure mechanisms of electromagnetic relays are analyzed. The Fick diffusion theory and the Larson–Miller method are used to deduce the degradation models for contact resistance and closing time. Based on these models, an approach to calculate relay life is proposed. To verify the effectiveness of the proposed method, a test platform was designed and built. The accelerated life test was carried out by using Siemens 3TH electromagnetic relays (3TH4244-OLF4, Siemens, Suzhou, China). Contact resistance and closing time were measured using a test platform. The least squares fitting was used to calculate the residual sum of squares and the unknown parameters in the degradation models with the test data. The optimal degradation model was chosen according to the fitting results. Relay life at four temperature stress levels was predicted by the optimal degradation model and failure threshold of the relays. The result indicates that the life of the electromagnetic relays at ambient temperature is 11,874 days and prediction error is about 5%, which provides important guidance for the maintenance of electromagnetic relays.

Microstructure evolution and residual life assessment of service exposed Cr35Ni45 radiant tube alloy

Abstract The degradation of material microstructure was frequently encountered in radiant tubes during high temperature service, resulting in their premature failure. Effect of service time on the phase transformation and oxidation behavior of the Cr35Ni45 radiant tubes was analyzed using scanning electron microscopy (SEM), electronic probe coupled with wavelength dispersive spectrometer, X-Ray diffraction (XRD) and transmission electron microscopy (TEM). The orthorhombic M7C3 and the face-centered cubic NbC primary carbides in the as-cast tube transformed into the face-centered cubic M23C6 and the face-centered cubic G phase, respectively, during service at 1000 °C. Metallographic observation showed that a continuous Cr2O3 layer and a discontinuous SiO2 layer formed on the inner surface of the radiant tubes due to the oxidizing environments, and a precipitate free region was found beneath the oxide layers. Mechanical properties of the tubes decreased with the increasing degree of microstructure degradation as service time prolongs. Residual life assessment of the serviced tubes was conducted by nonlinear fitting Larson–Miller parameter versus rupture testing stress. In spite of lower mechanical properties for the serviced tubes compared to as-cast tube, residual life assessment indicates that they have considerable residual life at the service temperature.

On the physical basis of a Larson-Miller constant of 20

Larson-Miller (LM) and Orr-Sherby-Dorn (OSD) parameters are widely used when formulating and predicting creep rupture life tr. The LM constant C and the activation energy Q in the OSD parameter characterize temperature dependence of tr. Q = QLSD (QLSD: activation energy for lattice self-diffusion) and C = 20 are often assumed in the formulation. Creep rupture datasets of 11 heats of nickel alloy A617, 304H and 304J1 stainless steels, 6 heats of A3004N aluminum alloy and four Mg-Al alloys are formulated with the parameters for determining C and Q of each dataset. Based on the correlation between C and Q among similar materials, it is discussed what is the C value equivalent to Q = QLSD. It is also examined how the C value changes with melting temperature of the material. The value of C equivalent to Q = QLSD varies from 14 to 9.5 depending on average values of log(rupture life) and of reciprocal temperature of the dataset. These values of C point out that C = 20 is not equivalent to Q = QLSD. The equivalent C values are similar in all the materials independently of their melting temperatures.

Elevated-temperature shear creep evolution and life prediction of sintered nano-silver lap-shear joint

Nano-silver paste, as a state-of-the-art die-attach material, has great application potential on elevated temperature, high power wide-band semiconductor packaging. During operation of semiconductor devices, sintered nano-silver bond-lines have to withstand creep deformation caused by elevated operating temperature and high switch frequency. In this study, the shear creep behavior of nano-silver sintered lap-shear joint was studied over the shear stress range from 3.5 to 8 MPa and the elevated temperature range from 498 to 598 K. Four kinds of constitute equations, namely three-parameter theta projection concept, Kachanov damage evolution, Larson–Miller parameter relationship and Monkman–Grant relationship, were employed to describe the shear creep behavior and predict the shear creep rupture life of the joint. For nano-silver sintered lap-shear joint, the three-parameter theta projection could quantitatively describe the shape of individual creep curves at any specified stress and temperature. The creep damage evolution of the joint, which was investigated by Kachanov damage evolution, was temperature dependent. The prediction accuracy of creep rupture life using these four kinds of constitutive equations was also evaluated.

Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

ABSTRACTIn this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.

Creep-Life Usage Analysis and Tracking for Industrial Gas Turbines

Creep-life usage analysis and tracking of first-stage turbine rotor blades of an aeroderivative industrial gas-turbine engine are investigated in this study. An engine performance model is created, and blade thermal and stress models are developed for the calculation of the blade material temperatures and stresses at different sections of the blade. A creep-life model is developed based on the Larson–Miller parameter method by taking inputs from the thermal and stress models. An integrated creep-life estimation system is developed by bringing together the engine performance model, the blade thermal and stress models, the creep-life model, and a data acquisition and preprocessing model. Relative creep-life consumption analysis using new concepts developed in this research is introduced for the analysis of creep-life consumption of the gas-turbine engine operating for a period of time; these concepts include equivalent creep life and equivalent creep factor. The developed algorithms have been applied to the creep-life tracking of an aeroderivative gas-turbine engine using its field test data. The results show that it is able to provide a quick evaluation and tracking of engine creep-life consumption and provide very useful information for gas-turbine operators to support their operation optimization and creep-life consumption monitoring.

High-Temperature Mechanical Properties of P91 Weld Metal

Abstract Increasing temperature is one of the key factors for improving the efficiency of steam power plants. Important metallurgical phenomena are activated at such high temperatures and creep resistance becomes a driving criterion for the material selection. Ferritic steels, including 2Cr, 9Cr, and 12Cr steels, are among the best candidates; their continuous development and optimization with the addition of Mo, V, Nb, and W have resulted in a significant improvement in creep strength together with a good weldability. This study investigates the high-temperature mechanical properties of two Grade 91 welded plates and focuses on the creep behavior, proposing a modified expression of the Larson–Miller parameter for the estimation of critical combinations of temperature, stress, and time, which could lead to rupture. The suggested parameter, which is highly sensitive to temperature, is able to outline the criticality of the welding and it is useful for predicting the duration of a creep test with the specimen rupture.

Creep Rupture Analysis and Life Estimation of 1.25Cr-0.5Mo, 2.25Cr-1Mo and Modified 9Cr-1Mo Steel: A Comparative Study

This paper highlights a comparative assessment of creep life of 1.25Cr-0.5Mo, 2.25Cr-1Mo and modified 9Cr-1Mo steels based on accelerated creep rupture tests. Creep rupture test data have been analysed and creep life of the above mentioned materials have been assessed using Larson Miller parameter at the stress levels of 60 and 42 MPa for different temperatures. Limiting steam temperatures for minimum design life of 105 h at 42 and 60 MPa for the above mentioned steels have also been calculated. Microstructural studies for the three above mentioned steels are also done.