Minimum Creep Rate Research Articles

Effect of welding on creep damage evolution in P91B steel

Study of creep behavior of base metal (without weld) and welded specimens of P91B steel over a range of temperatures (600–650 °C) and stresses (50–180 MPa) showed similar values of minimum creep-rates for both specimens at higher stress regime (>100 MPa) whilst, significantly higher creep rates in the case of welded specimens at lower stress regime. Considering that welded specimen is comprised of two distinct structural regimes, i.e. weld affected zone and base metal, a method has been proposed for estimating the material parameters describing creep behavior of those regimes. Stress–strain distribution across welded specimen predicted from finite element analysis based on material parameters revealed preferential accumulation of stress and creep strain at the interface between weld zone and base metal. This is in-line with the experimental finding that creep rupture preferentially occurs at inter-critical heat affected zone in welded specimens owing to ferrite-martensite structure with coarse Cr23C6 particles.

Creep Life Degradation and Microstructure Degeneration in a Low-Pressure Turbine Blade of a Military Aircraft Engine

Military aircraft engines operate under arduous environmental conditions with rapid throttle excursions as a part of its mission requirements. Life of an aircraft engine particularly of a military class is limited by the longevity of hot end components. Total technical life of a gas turbine component is dictated by the bulk properties while time between overhaul is decided by the aero-thermal degradation which in turn is influenced by the surface protective coating. In the present research, stress rupture tests were carried out to evaluate the creep life of blades while swelling of the coating and secondary reaction zone (SRZ) were evaluated to assess the coating damage. Further, an investigation was made to assess the degradation in creep life and microstructure degeneration during the service period when turbine blades made of wrought nickel-based alloy is provided with aluminide coating. Minimum creep rate and time to rupture are used as the characteristic parameters to evaluate the degradation in creep life. Coarsening of gamma prime (γ′) and degeneration of carbides was evaluated for microstructure degeneration in the bulk material. Degradation in coating life was evaluated by volume fraction of β-phase and development of SRZ in the coating.

Creep properties of simulated heat-affected zone of HR3C austenitic steel

Tensile creep tests on austenitic stainless HR3C steel pipe in an as-received state and after heat affected zone (HAZ) thermal cycle simulation by means of the GLEEBLE 3800 physical simulator were carried out at 923 and 1023K and an applied stress range between 100 and 350MPa to evaluate the effect of welding on its creep properties and behaviour. It was found that creep testing was conducted in a power-law creep regime. The results show a clear detrimental influence of the HAZ simulation on creep properties of HR3C steel at 923K and for very short creep exposure. However, with increasing time to fracture, the effect of weld simulation becomes less significant. By contrast, no such deterioration of creep properties was detected at 1023K. The evaluated similar values of the stress exponents of the minimum creep rate and the time to fracture are explained using the assumption that both creep deformation and the fracture process are controlled by the same mechanism. The main creep fracture mode was intergranular brittle fracture due to grain boundary creep damage.

Creep behaviour of P91/GTR-2CM/12Cr1MoV dissimilar joint predicted by the modified Theta method

Creep behaviour of P91, 12Cr1MoV steels and the P91/12Cr1MoV dissimilar joint were investigated at 823 K. Results show that the creep strength of P91 is much higher than 12Cr1MoV and than the dissimilar joint. The stress dependence of minimum creep rate and rupture life for both steels and the dissimilar joint obeyed Norton’s power law. The values of stress exponent are similar for 12Cr1MoV steel and the dissimilar joint in high stress region, indicating similar creep mechanism. However, the creep behaviour at 140 MPa for the dissimilar joint showed deviation from the other higher stresses, indicating different creep mechanism as the stress is decreasing. Microstructure showed that creep ruptured on the 12Cr1MoV side of the dissimilar joint as conducted in the high stress region, whereas ruptured on the carbon decarburized zone as conducted in the low stress region. Fracture location changed from 12Cr1MoV base metal to inter-critical heat affected zone as the creep time going. A modified theta equation was proposed to predict the creep behaviour, and the random errors from fitting to experimental creep data were smaller than obtained from the traditional theta method. The predicted creep behaviour showed good agreement with experimental ones.

Transient minimum creep of a γ′ strengthened Co-base single-crystal superalloy at 900 °C

A characteristic creep behavior, with a transient minimum creep rate region, was exhibited in a Co-Al-W-Ta-Ti single-crystal superalloy at 900°C and 420MPa. Interrupted creep tests were performed at various creep regions, the investigation of substructural evolution indicated the configurations of stacking faults were different during various creep regions. The extension of stacking faults and the formation of Lomer-Cottrell locks are hypothesized to be responsible for the accelerating region and steady-state region, respectively.

Influence of TiB2 nanoparticles on elevated-temperature properties of Al-Mn-Mg 3004 alloy

Two contents (1.5% and 3%) of TiB2 nanoparticles were introduced in Al-Mn-Mg 3004 alloy to study their effects on the elevated-temperature properties. Results show that TiB2 nanoparticles were mainly distributed at the interdendritic grain boundaries with a size range of 20–80 nm, which is confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Therefore, the volume fraction of the dispersoid free zones is greatly reduced and the motion of grain boundaries and dislocations is inhibited more effectively at elevated temperature. After peak precipitation heat treatment, the yield strengths in the alloy with 3% TiB2 addition at room temperature and 300 °C were increased by 20% and 13% respectively, while the minimum creep rate at 300 °C was reduced to only 1/5 of the base alloy free of TiB2, exhibiting a considerable improvement of elevated-temperature properties in Al-Mn-Mg alloys.

Study on High Temperature Creep Test of P92 Material Based on the Minimally Invasive Technique

The tensile and creep properties of P92 steel have been studied using a precision tensile and creep test machines for miniature and conventional specimens under various stress level at 625°C. The results showed that the data stability of miniature plate specimens is high whether at room or high temperature tensile tests. Compared with the conventional plate specimens, tensile strength, yield strength and total elongation is slightly lower, the uniform elongation is higher for miniature plate specimens at room temperature. By contrast, the tensile strength and uniform elongation is slightly higher, and the yield strength and total elongation is lower at high temperature. Besides, there had similar creep curves between miniature and conventional specimens, and the creep rupture time and minimum creep rate are closer under the same stress. By comparing the power law creep index and damage tolerance factor at the second creep stage, it can be derived that the creep mechanism is identical for the micro and conventional specimens, which is controlled by the dislocation movement.

Effect of Q-Al5Cu2Mg8Si6 phase on mechanical properties of Al-Si-Cu-Mg alloy at elevated temperature

The effect of Q-Al5Cu2Mg8Si6 phase on creep and tensile properties of Al-Si-Cu-Mg alloy at elevated temperatures was investigated. Microstructure observation clarifies that the Q-Al5Cu2Mg8Si6 phase is distributed in the Al matrix. The creep stress exponent is in the range of 4.20–4.67 and activation energy for creep varies from 218 to 226kJ/mol with increasing the volume fraction of Q-Al5Cu2Mg8Si6 phase. The stress exponent and activation energy suggest that dislocation climb is the dominant creep mechanism. In addition, slight variation of the stress exponent and activation energy indicates that the creep mechanism do not change with the increase of Q-Al5Cu2Mg8Si6. The relation between the minimum creep rate and creep fracture life are fitted by Monkman-Grant equation, which indicates that the creep fracture life is increased with the increase of Q-Al5Cu2Mg8Si6 phase. The ultimate tensile strength at 250°C increases from 157MPa to 199MPa with the volume fraction of Q-Al5Cu2Mg8Si6 phase varying from 3.2% to 6.4%. However, the ultimate tensile strength at 350°C only increases 9MPa with increasing the Q-Al5Cu2Mg8Si6 phase. The slight increase in ultimate tensile strength at 350°C indicates that the Q-Al5Cu2Mg8Si6 phase is not effective in improving the ultimate tensile strength at 350°C.

Effects of minor Cu and Mg additions on microstructure and material properties of 8xxx aluminum conductor alloys

Abstract

Deformation-mechanism-based modeling of creep behavior of modified 9Cr-1Mo steel

A deformation-mechanism-based true-stress creep model is proposed for studying the creep behavior of modified 9Cr-1Mo steel in this research. Constant-load creep test is conducted on modified 9Cr-1Mo steel in forged form (F91). The creep data obtained in the present study and those reported from the National Institute for Materials Science (NIMS, Japan) on modified 9Cr-1Mo steels processed by different means are analyzed. It is revealed that the relationship of minimum creep rate versus applied engineering stress exhibits distinct power exponent n in three stress regions, which are associated with different deformation mechanisms. The proposed model considers three well recognized deformation mechanisms: dislocation glide, dislocation climb, and grain boundary sliding. The analyses of the experimental data show that this deformation-mechanism-based model can describe fairly well the entire creep deformation process consisting of primary, steady-state, and tertiary creep.

High-temperature deformation and fracture mechanisms of an advanced heat resistant Fe-Cr-Ni alloy

In this work creep-strain test ranging from 180 to 240MPa at 973K, 998K, and 1023K were conducted to investigate the high temperature deformation and fracture mechanisms for Sanicro 25 alloy. The relationship between minimum creep rate and applied stress followed Norton's law. Apparent stress exponent values of 8, 6.5, and 5 were obtained at 973K, 998K, and 1073K, respectively, with an apparent creep activation energy value of 437.3–606.1kJ/mole illustrating that the rate-controlled creep occur in Sanicro 25 during creep. The threshold stresses were 129.5, 111.5, and 82.0MPa at 973K, 998K, and 1023K, respectively. The true creep activation energy and the true stress exponent were found to be 271.6kJ/mole and 3, respectively. The presence of nanoscale MX and Cu precipitates were observed and theoretical calculations of threshold stresses confirmed that shearing of the nano-Cu precipitates occurred at 973K, while the dislocations climbed up MX precipitates at 973–1023K. The apparent stress exponent obtained for creep rupture shows that similar mechanisms operate in creep deformation and rupture behavior for Sanicro 25. A damage tolerance factor of ~5 in the alloy indicates that the microstructural degradation such as coarsening of precipitate and subgrain structure is the dominant creep damaging mechanism in the alloy.

Comparison of creep strength and intrinsic ductility for serviced and reheat treated T91 steel based on stress relaxation testing

High precision stress relaxation tests (SRT) at four temperatures were conducted on T91 (9%Cr) steel after extended boiler service, and also after re-heat treatment. Relative differences in creep strength, measured over five decades in strain rate were dependent on test temperature. Using an established correlation between strain rate sensitivity and elongation at failure, intrinsic ductility values as a function of stress and test temperature were determined. The general trend of a minimum in ductility in terms of stress or strain rate was consistent with long term creep rupture data on T91, and with literature data on alloy steels. However, the precision and repeatability of the SRT analysis contrasted with the appreciable scatter and heat to heat variation in traditional testing. It is argued that the current creep strength evaluation based on the nearly constant state measurement from the SRT test is superior to the measurement of stress dependence of minimum creep rate in traditional creep rupture testing. The complexity of primary creep in laboratory testing, which may not be significant at operating stresses where loading strains may be fully recoverable (anelastic), does not apply to the SRT. Since very low strain rates are achieved in a one day test, the procedures for setting of design allowables and design analysis based on the SRT data should not be significantly different from current practice. This technique offers accelerated alloy development and optimisation for creep strength and also ductility, and hence resistance to notch sensitivity.

Experimental Study on Creep Characterization and Lifetime Estimation of RPV Material at 723-1023 K

During the plant operation, nuclear reactor pressure vessel (RPV) is the most critical pressure boundary component for integrity and safety in a light-water reactor. In this paper, the creep behavior and properties for RPV metallic material are studied by conducting constant-temperature and constant-load creep tests at 723, 823, 923 and 1023 K. The θ projection constitutive model was established based on a creep method to describe the high-temperature creep behavior of RPV material. The material parameter θ would be obtained based on experimental data by depending on numerical optimization techniques. The relationship between and among θ, T and σ was evaluated, and the coefficients a i , b i , c i and d i were obtained. Based on the short-term tests at a high temperature, the values for long-term creep data could be predicted in accordance with parameter θ. Moreover, rupture life, the minimum creep rate and the time reaching to an arbitrary strain can be calculated and may be used to evaluate the damage behavior and properties, so as to be used as a reference for design and safety assessment.

Uniaxial creep and stress relaxation behavior of modified 9Cr-1Mo steel

An attempt has been made to establish a correlation between the creep strains rates predicted from a stress relaxation test with that directly obtained from a creep test results on a commercial grade of steel (P91) at three different temperatures. Tests were performed on a set of specimens made from the same plate to exclude the effect of material variability on the test data. These were used to explore if short term stress relaxation test (SRT) performed on the same grade of steel could predict its creep rupture strength. The results suggest that the magnitude of activation energy and stress exponent obtained from SRT is significantly lower than those obtained from the creep test data. This is primarily due to the microstructures that evolved during the two variants of tests which had difference in thermal exposures, and leads to the conclusion that material parameter set obtained from SRT cannot be used to predict creep rate of the steel at any given stress and temperature. It needs an additional conversion factor for the prediction of minimum creep rate.

Investigation of microstructure and mechanical properties of Sn-xCu solder alloys

The microstructure and mechanical properties of five binary pre-aged Sn-Cu alloys with varying Cu contents were investigated. The Cu compositions examined varied from 1 wt% to 5 wt%. Serial aging heat treatment processes were performed at 373, 393, 413 and 433 K with a holding time of 2 h at each temperature. The microstructural characteristics of the tested alloys have been investigated using scanning electron microscope (SEM) and X-rays diffraction (XRD). Typical indentation creep curves were derived from hardness values. The minimum creep rate is increased as the Cu content increased up to 4 wt%, but above this level the trend is reversed. Moreover, increasing aging temperature resulted in an increase in the minimum creep rate for the all tested alloys. The mean values of energy activating the creep process are in agreement with that quoted for dislocation core diffusion mechanism.

Creep life prediction of small punch creep testing specimens for service-exposed Cr5Mo using the theta-projection method

Abstract In this paper, small punch creep tests under different loads and conventional creep tests with various stresses at 550 °C were conducted on service-exposed Cr5Mo material. The theta-projection and modified theta-projection methods were used to analyze the creep curve and the minimum creep rate. The actual fracture time in the small punch creep test was 377 h, and the theta-projection method predicted a time of 339 h, which represents an error of 10.08%. The predicted time from the modified theta-projection method was 316.25 h, which represents an error of 16.11%. The curves predicted by the two models are similar. The theta-projection method is suitable for applications in small punch creep life analysis.

The Microstructural Criterion for Creep Strength Breakdown in a 10%Cr Martensitic Steel

A 10%Cr martensitic steel with 3%Co and 0.008%B tempered at 770°C exhibits no creep strength breakdown at a temperature of 650°C up to an extremely high rupture time of ∼4×104 h under an applied stress of 120 MPa. The minimum creep rate was ∼3×10-11 s-1. Microstructural characterization showed that superior creep resistance associated with a high stability of tempered martensite lath structure. Boundary M23(B⋅C)6 phase particles are highly stable against coarsening under long-term aging and creep conditions. These particles retain their orientation relationship with ferritic matrix unchanged under creep at a temperature of 650°C. As a result, no migration of lath boundaries and their transformation to subboundaries diminishing the long-range elastic stress fields take place. The role of M(C,N) carbonitrides in achieving extraordinary high creep strength consists in hindering the knitting reaction between mobile lattice dislocations and lath boundaries.

Effect of Tempering on Microstructure and Creep Properties of P911 Steel

The microstructure and creep properties of a P911-type steel normalized at 1060°C and then subjected to one-step tempering at 760°C for 3 h or two-step tempering at 300°C for 3 h + 760°C for 3 h were examined. The transmission electron microscope (TEM) observations showed that the tempered martensite lath structure (TMLS) with a lath thickness of 340 nm evolved after both tempering regimes. High dislocation densities of 3×1014 or 5×1014 m-2 retained after one-and two-step tempering respectively. M23C6 carbides with a mean size of 120 nm and V-rich MX carbonitrides having a “wing” shape with an average length of about 40 nm precipitated on high-and low-angle boundaries and within ferritic matrix, respectively. A number of Nb-rich M(C,N) carbonitrides with a mean size of 20 nm precipitated on dislocations during low temperature tempering. The creep tests were carried out under constant load condition at 650°С at applied stresses of 100 and 118 MPa. Analysis of creep rate versus time curves showed that the use of two-step tempering decreases the minimum creep rate providing an increase in the creep strength in long-term conditions.

Deformation, diffusion and ductility during creep – continuous void nucleation and creep-fatigue damage

It is shown that the assumption of unit (negative) slope in the well known Monkman–Grant plot of time to failure against minimum creep rate is too restrictive. By acknowledging observed slopes in the range 0.8–1, a ductility–strain-rate relation is deduced where ductility decreases with reducing strain rate. This in turn has implications for the ductility exhaustion method as applied during stress relaxation in the dwell period of low cycle fatigue tests of austenitic steels at elevated temperature. The simple method is used to calculate the cyclic creep damage in typical tests on austenitic steels in the region 550–650 °C and is compared to other calculations as employed in the R5 high temperature assessment procedure. The assumption of a uniform nucleation rate of grain boundary voids with creep strain goes some way to predicting the slope of the ductility–strain-rate relation. Both the ‘unconstrained’ and ‘constrained’ (lower shelf) regions of void growth are discussed.

A three-stage constitutive model for the creep behaviours of high-Cr steels at elevated temperatures

In this paper, a three-dimensional constitutive model is proposed to simulate the creep behaviours of high-Cr steels at elevated temperatures. In the model, the minimum creep rate and the average creep rupture time at different temperatures and stress levels are predicted by adopting two Larson–Miller parameters. The decrease of the creep rate during the primary creep stage is captured by introducing an internal variable representing the strain hardening effect. The material parameters of the model can be identified by using the conventional experimental results. Both the strain- and stress-driven algorithms are designed to solve the constitutive evolution equations. The response of high-Cr steels during the whole creep procedure can be predicted at a quantitative level by the current model. Further implementing the model into a finite element software, the global creep behaviours of high-Cr components under realistic loading conditions can be simulated.