Larson-Miller Parameter Research Articles

Behaviour of dissimilar weld joint of steels FB2 and F during long-term creep test

abstractCreep test to the rupture of both the dissimilar weld joint made of FB2 and F martensitic steels and the base materials was carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa. Creep rupture strengths of the weld joint and the base materials were evaluated using Larson-Miller parameter. Assessment of microstructure development and changes of hardness was correlated with the creep strength. Critical zones of creep damage were determined. At lower temperatures and higher stresses the weld joint ruptured in the base material of F steel unaffected by welding, while at higher temperatures and lower stresses rupture occurred in the intercritical heated and fine-grained parts of heat affected zone of steel F. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centres for cavities. As the fine grained heat affected zone of F steel was the softest part of the weld joint, many cavities originated and initiated causing failure of samples.

Creep Life Prediction of Alloy 718 for Automotive Engine Materials

This study was carried out to investigate the creep life at the high temperature of the Alloy 718 for automotive engine components using the initial strain parameter method (ISPM). Creep tests have performed at elevated temperatures in the range of 550 oC to 700 oC in this work. We also carried out constant stress creep tests. The initial strains were measured during 1 minute after loading. Both the creep stress and rupture time depend on the initial strain. We calculated the creep life of Alloy 718 by using the creep life prediction equations obtained from the ISPM. Then, we compared the creep life predicted by the ISPM to the Larson-Miller parameter (LMP). The experimental rupture time and the calculated rupture time by using the ISPM agreed with a confidence level of 95 %. The creep life predicted by using the ISPM was in very good agreement with the creep life predicted using the LMP method.

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.

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.

Creep performance and damage mechanism for Allvac 718Plus superalloy

The creep behavior, creep damage and creep life of 718Plus alloy were investigated at temperatures ranging from 650 to 750 °C in air. The fracture surface, microstructure evolution and deformation mechanisms were studied by optical, scanning and transmission electron microscope (SEM and TEM). The results show that creep curves of 718Plus alloy consist of primary stage and tertiary stage without steady-state region. The Monkman-Grant relationship of 718Plus alloy can be described by tr = 9.32E-2/(ε̇min)0.75 and apparent creep activation energy of 718Plus alloy is about 572.7 kJ/mol. Further investigation shows creep damage of alloy at 650 °C can be mainly ascribed to the effect of grain boundary sliding (GBS) and microtwinning. The alloy shows transgranular fracture at high stress, while it presents ductile fracture at low stress due to the nucleation, growth and linkage of creep voids. At 750 °C, the alloy failed in the form of ductile fracture mainly due to the microstructure instability and dislocation glide. At higher stress of 700 °C, the effect of GBS is mainly responsible for the creep damage of 718Plus alloy, while GBS, dislocation glide and microstructure instability work together and lead to the creep damage of alloys at lower stress of 700 °C. The fracture mechanism map and fracture models for 718Plus alloy were constructed. Remaining life assessment of 718Plus alloys was also evaluated by Larson-Miller parameter method.

Determination of creep constitutive model for 28-48WCo alloy based on experimental creep tests at 817–982 °C

The high-chromium high-nickel alloy is widely used as a heat-resistant material for components in plants under elevated temperatures, and thus, prediction of its creep deformation and rupture life is required for safe design. In this study, a creep constitutive model for the 28-48WCo alloy was determined by employing the Sherby-Dorn equation. Creep deformation tests were conducted at temperatures of 817, 871, 927 and 982 °C, under an applied stress ranging from 27.58 to 82.74 MPa. High temperature tension tests were also conducted to measure the change in elastic modulus with temperature variation. The model provided good predictability of the minimum creep strain rate in the range of experimental test conditions, with a coefficient of determination of 0.92. The creep rupture life was characterized using the Larson-Miller parameter. Creep design curves were proposed to estimate the creep rupture life of the 28-48WCo alloy at a temperature range of 817–982 °C.

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.

Parametric Methods for Determining the Characteristics of Long-Term Metal Strength

A large number of parametric methods were proposed to calculate the characteristics of the long-term strength of metals. All of them are based on the fact that temperature and time are mutually compensating factors in the processes of metal degradation at high temperature under the action of a constant stress. The analysis of the well-known Larson-Miller, Dorn-Shcherby, Menson-Haferd, Graham-Wallace, and Trunin parametric equations is performed. The widely used Larson-Miller parameter was subjected to a detailed analysis. The application of this parameter to the calculation of ultimate long-term strength for steels and alloys is substantiated provided that the laws of exponential dependence on temperature and power dependence on strength for the heat resistance are observed. It is established that the coefficient C in the Larson- Miller equation is a characteristic of the heat resistance and is different for each material. Therefore, the use of a universal constant C = 20 in parametric calculations, as well as an a priori presetting of numerical C values for each individual group of materials, is unacceptable. It is shown in what manner it is possible to determine an exact value of coefficient C for any material of interest as well as to obtain coefficient C depending on stress in case such a dependence is manifested. At present, the calculation of long-term strength characteristics can be performed to a sufficient accuracy using Larson-Miller’s parameter and its refinements described therein as well as on the condition that a linear law in logσ–Р dependence is observed and calculations in the interpolation range is performed. The use of the presented recommendations makes it possible to obtain a linear parametric logσ–Р dependence, which makes it possible to determine to a sufficient accuracy the values of ultimate long-term strength for different materials.

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.

Time-dependent creep analysis and life assessment of 304 L austenitic stainless steel thick pressurized truncated conical shells

This paper presents a semi-analytical solution for the creep analysis and life assessment of 304L austenitic stainless steel thick truncated conical shells using multilayered method based on the first order shear deformation theory (FSDT). The cone is subjected to the non-uniform internal pressure and temperature gradient. Damages are obtained in thick truncated conical shell using Robinson's linear life fraction damage rule, and time to rupture and remaining life assessment is determined by Larson-Miller Parameter (LMP). The creep response of the material is described by Norton's law. In the multilayer method, the truncated cone is divided into n homogeneous disks, and n sets of differential equations with constant coefficients. This set of equations is solved analytically by applying boundary and continuity conditions between the layers. The results obtained analytically have been compared with the numerical results of the finite element method. The results show that the multilayered method based on FSDT has an acceptable amount of accuracy when one wants to obtain radial displacement, radial, circumferential and shear stresses. It is shown that non-uniform pressure has significant influences on the creep damages and remaining life of the truncated cone.

Creep-Fatigue Interaction Life Consumption of Industrial Gas Turbine Blades

This paper presents the creep-fatigue interaction life consumption of industrial gas turbine blades using the LM2500+ engine operated at Pulrose Power station, Isle of Mann as a case study. The linear damage summation approach where creep damage and fatigue damage are combined was used for the creep-fatigue interaction life consumption of the target blades. The creep damage was modelled with the Larson-Miller parameter method while fatigue damage was assessed with the modified universal slopes method and the damage due to creep-fatigue interaction was obtained from the respective life fractions. Because of the difficulty in predicting the life of engine components accurately, relative life consumption analysis was carried out in the work using the concept of creep-fatigue interaction factor which is the ratio of the creep-fatigue interaction life obtained from any condition of engine operation to a reference creep-fatigue interaction life. The developed creep-fatigue interaction life consumption analysis procedure was applied to 8 most of real engine operation. It was observed that the contribution of creep to creep-fatigue interaction life consumption is greater than that of fatigue at all ambient temperatures. The fatigue contribution is greater at lower ambient temperatures as against higher ambient temperatures. For the case study, the overall equivalent creep-fatigue factor obtained was 1.5 which indicates safe engine operation compared to the reference condition. The developed life analysis algorithm could be applied to other engines and could serve as useful tool in engine life monitoring by engine operators.

Effects of Ambient Temperature and Shaft Power Variations on Creep Life Consumption of Industrial Gas Turbine Blades

The effects of ambient temperature and shaft power variations on creep life consumption of compressor turbine blades of LM2500+ industrial gas turbine engine are investigated in this work. An engine model was created in PYTHIA (Cranfield University’s gas turbine software) for the analysis. Blade thermal and stress models were developed and used together with Larson-Miller Parameter method for life analysis. Mean life reduction index, which is the propensity of a given effect to reduce engine life by half, is developed for each effect and applied in this research to compare the impacts of the different effects on the blade creep life. It was observed that blade life will be halved when ambient temperature is increased by 8.11 units while 13.64% increase in shaft power reduces blade life by a factor of 2. The results of this work will guide engine operators in making decisions concerning operating at part loads.

High temperature creep properties of a linear friction welded newly developed wrought Ni-based superalloy

AD730™ Ni-based superalloy specimens in solution-treated conditions were linear friction welded. Then, post-weld heat treatment (PWHT), consisting of γ′ sub-solvus solution treatments followed by aging, was conducted on the linear friction welded samples. High temperature creep tests were performed on the as-welded and PWHTed joints at two different temperatures: 700°C under 600 and 750MPa stress levels, and 850°C under 100 and 200MPa stresses. The creep resistance of the PWHTed joints was higher than that of the as-welded samples. The PWHTed joints exhibited better ductility than that of the base material at 850°C, while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of the AD730™ Ni-based superalloy at 850°C was related to a combination of the formation of precipitate-free zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. The Larson-Miller Parameter (LMP) was used to correlate the creep strength, temperature and time to failure for the as-welded and PWHTed samples. LMP values varied between 21.5 × 103 and 24.5 × 103. It was found that in the investigated temperature range, the PWHTed AD730™ has similar creep characteristics as Udimet™720 Li and Inconel 738LC at low values of LMP and better creep properties than those of the Inconel 617 alloy at higher LMP values.

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.

Creep deformation and rupture behaviour of service exposed P91 weld and base steel measured by miniature tensile creep testing

Early cracking had occurred at a weld joint of a service-exposed P91 steel hot reheat pipe branch. In this study, a series of miniature tensile creep tests were conducted for both the base and weld materials extracted from the retired P91 pipe weld. Tests were conducted at various stress levels at 569 °C to measure the creep properties and investigate the failure mechanisms. Microstructural analyses were also carried out for post-test specimens. The measured creep strengths were represented in terms of the Larson-Miller parameter, and compared to literature data. Most of the data fell within the 95% error band of grade 91 steel. However, the weld metal exhibited higher creep rupture strengths. It showed creep brittle fracture, and base metal showed creep ductile behaviour. The weld metal sometimes had non-uniform microstructures, yielding a seriously tempered zones with minor tempered martensites, thereby causing early creep ductile failure of the weld.

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.

Small punch and uniaxial creep fracture behaviours of modified SUS304H steel at various temperatures

Commercial austenitic stainless steel SUS304H with small amount of vanadium addition was used in this study. Small punch (SP) creep and uniaxial tensile creep tests were conducted at 650, 700, and 750 °C to measure creep lives and the minimum displacement rates or the minimum creep strain rates. The measured parameters were compared between the two test methods, seeking empirical relationships among the parameters using Larson-Miller Parameter and Monkman-Grant relation. Magnitude of the applied stress (MPa) in the uniaxial tensile creep test was approximately equal to the applied load value (N) in the SP creep test at all test temperatures. It was shown that during the creep deformation of the SP creep specimen, crack initiation and accompanying crack growth occur simultaneously. Competing failure mechanisms of creep deformation and crack growth may affect the SP creep life and consequently determine the proportionality function, α, in the relation between the SP load and the tensile creep rupture stress in creep tests.