Constant Load Creep Research Articles

Creep behavior of MAR-M 302 Co-base superalloy

MAR-M 302 a Co-base superalloy is currently used for nozzle guide vanes and stator blades in military and commercial aircraft-turbine engines. These components are usually operated at 1073–1273 K. In this study, the uniaxial constant load creep behavior of MAR-M 302 superalloy over temperature range of 1088–1198 K and stress range of 155–321 MPa was investigated. Concerning creep as thermally activated phenomenon, the investigated alloy followed empirical equation $$ \dot{\varepsilon } = A\sigma^{n} \exp \left( { - \frac{Q}{RT}} \right) $$ with n values ranging mainly from 7 to 10. The mean apparent activation energy was calculated as 525 ± 29 kJ/mol. The correlation between rupture life and creep characteristics was studied by different prediction methods that are based on the iso-stress lines and those relating the secondary or steady state creep rate to rupture life.

Effect of microstructure on creep and creep crack growth behaviour of titanium alloy

Creep and creep crack growth behaviour of a near α titanium alloy has been investigated at 600°C which is affected by primary α content. The alloy was heat treated at different temperatures so as to obtain different levels of equiaxed primary α in the range from 5 to 24 %. Constant load creep tests were carried out at 600°C in the stress range 250 to 400 MPa till rupture of the specimens. Creep crack growth tests were carried out at 600°C. Creep data reveals with increase in primary α content leads to creep weakening. On similar lines maximum creep crack growth resistance is associated with the alloy with lowest primary α content. Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces in the interior of the specimen.

SANS Investigation of γ´ Precipitate Morphology Evolution in Creep Exposed Single Crystal Ni Base Superalloy

The creep degraded nickel base single crystal superalloy CMSX-4 of two axial orientations [001] and [111] was investigated with aim to assess the structure degradation. Constant load creep tests were conducted in the stress/temperature ranges of 250–780 MPa/750 – 50°C resulting in rupture time variation from 50 to 4000 hours. A combination of scanning electron microscopy (SEM) and non-destructive small-angle neutron scattering method (SANS) was used to investigate the directional coarsening (rafting) of the gamma prime (γ') precipitates in relation to the stress and temperature applied as well as to the initial crystallographic orientation of the specimens. The SANS results are discussed in terms of the correlation with the raft development, the axial orientation of specimen, the creep parameters and the mechanical properties.

Analysis of Creep Curves of Haynes 230 Superalloy

The creep behaviour of the solid solution strengthened nickel-based superalloy Haynes 230 has been investigated under constant load and temperature conditions on as received, after conventional solution treatment, and on overaged conditions. The experimental results have shown a very strong dependence of the creep curve shape with the applied stress/temperature: in the tests performed at high stresses/low temperatures, the primary/decelerating stage takes an important portion of the creep curve. At these test conditions, the accelerating creep is mainly caused by the increase of the applied stress with the strain as it happens in constant load creep tests. In the tests performed at low stresses/high temperatures, the primary stage is very small and the following accelerating creep is characterized by different accelerating creep stages. The analysis of the creep curves on the as received and overaged alloys, has shown that a large portion of the accelerating creep at low stresses/high temperatures is caused by microstructural instability.

Creep and Low Cycle Fatigue Characterisation of IN718 TIG Butt-Welds at 620°C

The testing setups, results and analysis of constant load creep and low-cycle high temperature fatigue tests of tungsten inert gas (TIG) butt-welded, thin-section INCONEL 718 (IN718) specimens are presented. The main objectives were to determine the effect the welds have on failure time and analyse any differences in their failure behaviour. It was found that although welded IN718 may exhibit comparatively little loss of tensile strength, its ductility and creep and high temperature fatigue properties are severely compromised due to its changed microstructure.

Creep damage characterization using non-linear ultrasonic techniques

Abstract This paper describes the use of non-linear ultrasonic techniques for the characterization of material degradation in 99.98% pure copper due to high-temperature creep. Flat dog-bone-shaped specimens were subjected to constant load creep testing at different stress and temperature levels. Creep damage progression was monitored by conducting continuous and interrupted mode creep tests. In the case of continuous loading non-linear ultrasonic (NLU) measurements were conducted after fracture at different locations along the gage length of the sample. For interrupted tests the NLU measurements were conducted on different creep life fractions, through periodic interruption of the creep test. In all cases a through transmission NLU measurement technique was employed. Three different non-linear measurements, namely static displacement, second harmonic and third harmonic, were taken and their responses compared. The NLU measurements were found to be significantly sensitive to the extent of creep damage (∼200–2500% of base level), while the linear ultrasonic measurements, representing the change in longitudinal velocities, were only in the range 10–30% for a comparable creep damage level. NLU measurements carried out on fractured samples suggest that the NLU response was locally high at locations where the creep damage was concentrated, compared with other locations, even within the gage length of the specimen. This was confirmed using micrograph observations. Of the three non-linear measurements, the third harmonic data was most sensitive to creep damage.

Creep Behaviour of Closed Cell Aluminium Foams from Stress Relaxation Tests

Creep behaviour of closed cell aluminium foams and transitions from power law to power law breakdown (PLB) creep regimes are investigated from results of stress relaxation tests (SRT) carried out on Alporas foams with densities between 0.20 to 0.32 g/cm3. Tests were carried out at temperatures between 200°C and 300°C and stress relaxation was measured from the collapse stress under compression of the foams. Under similar foam density, temperature and stress conditions, the values of strain rate calculated from SRT tests were of the same order of magnitude than those previously reported in the literature from conventional constant load creep tests. Under stress values close to the collapse stress, the creep mechanism seems to follow a PLB regime, with values of the power law exponent n=10-17, much higher than those corresponding to the power law creep in the solid material (n=4.4-6.5) and with activation energy values close to Q = 150 KJ/mol. Having in mind the limitations of available creep models for closed cell foams and the need for additional experimental results, the use of SRT testing would offer advantages for the assessment of the high temperature behaviour of aluminium foams, due to its lower testing times and reduced experimental effort.

Primary Creep in Sn3.8Ag0.7Cu Solder. Part I: Theory, Experiments, and Data Reduction

e < 10 � 4 s � 1 ; and temperatures 25� C, 75� C, and 100� C. Creep and stress relaxation measurements showed that transient creep caused faster strain rates during stress relaxation for a given stress than the corresponding minimum creep rate from constant-load creep tests. The extent of strain hardening during primary creep was a function of temperature and strain rate. Data reduction incorporated a fast Fourier transform method to remove spurious data from stress relaxation corresponding to the period of partial strain relaxation during loading.

The stress relaxation and creep behaviour of a manganese-stabilized austenitic stainless steel

The stress relaxation behaviour of 21–4N, a manganese-stabilized austenitic stainless steel, is investigated in terms of the metallurgical state, the application of multiple strain levels during ‘stepped’ stress relaxation testing at 700 °C, the strain level during isostrain stress relaxation tests at 538 °C and 700 °C, and the correspondence with results from constant-load creep tests. The results indicate that for isostrain stress relaxation tests the stress relaxation rate is similar for strains that span both elastic and plastic strain levels. A transition in the stress relaxation behaviour occurs at a stress level approximately equivalent to the tensile stress–strain proportional limit; below this transition the stress–strain rate relationship, or the time predicted for 1 per cent creep strain, obeys a creep power law type of equation. Stress relaxation testing successfully delineates the difference between the creep resistances of two different metallurgical conditions with similar tensile properties using fewer specimens and requiring less time. The time to 1 per cent creep strain determined from the analysis of stress relaxation results is always less than the actual time to 1 per cent creep strain during constant-load creep tests.

Grain-boundary sliding in a TiAl alloy with fine-grained duplex microstructure during 750 °C creep

Constant-load creep experiments at a temperature of 750 °C and a nominal stress of 300 MPa were conducted on a fine-grained Ti–45Al–5Nb–0.2B–0.2C (in at.%) alloy with a duplex microstructure. Microstructures before and after creep (accumulated strain: 9.6%) were analyzed using scanning and transmission electron microscopy (SEM and TEM). TEM analysis after creep indicates that the individual microstructural constituents of the fine duplex microstructure, namely, the equiaxed γ and the lamellar α 2/γ colonies, undergo varying degrees of deformation and develop various substructures. Lamellar grains deform by dislocation creep. They show clear evidence for dislocation and twin activity. In contrast, only few dislocations are found in the equiaxed grains. We show that the regions with small equiaxed γ grains, representing 65–75 vol.% of the microstructure, deform by grain-boundary sliding.

Effect of oxidation on creep data: Part 2 – A procedure for assessing the effect of oxidation on constant-stress or constant-load creep curves using the Theta-projection concept

A procedure based on Theta-methodology coupled with the knowledge of oxidation kinetics is envisaged for quantitatively assessing the effect of oxidation on creep curves. The procedure is based on the generation of a series of real constant-stress creep curves, at different stress and temperature levels, in inert atmosphere, where the effects due to oxidation are kept to a minimum level. Stress enhancement factors due to the effect of area reduction on specimen cross-section with plastic deformation and oxidation are defined for constant-stress or constant-load creep testing. These factors can be used in the integration of the strain-rate equation related to the 4θ - parameter analysis, to derive constant-stress or constant-load curves in air using either the strain hardening or time hardening theories. Although systematic constant-stress creep data in vacuum are not yet available to test the methodology effectively, a preliminary simulation is done to demonstrate how the model works, to check its performance and the possibilities of analysis.

Effect of oxidation on creep data: Part 1 – Comparison between some constant load creep results in air and vacuum on 2¼Cr–1Mo steel from 600 to 700°C

A description is given of two chambers specially developed for creep testing in controlled environment which accommodate inside them the complete load train, with the extensometry system and thermocouples. The creep machines used in this project present the advantage of allowing performance of both constant load or constant stress testing, using interchangeable profile cams. A set of preliminary creep results on 2¼Cr – 1Mo was obtained at 600, 650 and 700°C at four stress levels in each case, under constant load, in air and vacuum. The data were examined according to the conventional methodology of creep data analysis by the identification of parameters from the Norton, Arrhenius and Monkman-Grant relations and the application of some parameterization techniques as a first attempt to establish a systematic comparison between creep properties in air and vacuum in this steel.

Microstructural Evaluation of the Zirconia Plasma Sprayed Coating on Ti-6Al-4V Alloy after Creep Test

The titanium affinity by oxygen is one of main factors that limit the application of their alloys as structural materials at high temperatures. The objective of this work was evaluating the microstructure of the of the zirconia plasma sprayed coating Ti-6Al-4V alloy in creep test. Yttria (8 wt.%) stabilized zirconia (YSZ) (Metco 204B-NS) with a CoNiCrAlY bond coat (AMDRY 995C) was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco. Constant load creep tests were conducted with Ti-6Al-4V alloy in air and in nitrogen atmospheres in uncoated samples and in air in coated samples at stress level of 56 MPa at 700°C. The microstructural analyze show that the predominant mechanism in the test conditions was characterized by formation and coalescence of micro cavities with shape and size varieties. The cavities are corrugated in discontinue regions, like inclusions and points involving dislocation. The size and shape of dimples are governed by number and distribution of nucleated micro cavities and by stress internal level present in the material.

A Comparison between CoNiCrAlY Bond Coat and Zirconia Plasma Sprayed Coatings on Creep Tests

Thermomechanical and electrical properties of zirconia-based ceramics have led to a wide range of advanced and engineering ceramic applications like solid electrolyte in oxygen sensors, fuel cells and furnace elements and its low thermal conductivity has allowed its use for thermal barrier coatings for aerospace engine components. A comparison between CoNiCrAlY bond coat and zirconia plasma sprayed coatings on creep tests of the Ti-6Al-4V alloy was studied. The material used was commercial Ti-6Al-4V alloy. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco Type 9 MB. Constant load creep tests were conducted on a standard creep machine in air on coated samples, at stress levels of 520 MPa at 500°C to evaluate the oxidation protection on creep of the Ti-6Al-4V alloy. Results indicate that the creep resistance of the ceramic coating was greater than metallic coating.

Effect of oxidation on creep data: Part 1 – Comparison between some constant load creep results in air and vacuum on 2¼Cr–1Mo steel from 600 to 700°C

A description is given of two chambers specially developed for creep testing in controlled environment which accommodate inside them the complete load train, with the extensometry system and thermocouples. The creep machines used in this project present the advantage of allowing performance of both constant load or constant stress testing, using interchangeable profile cams. A set of preliminary creep results on 2¼Cr – 1Mo was obtained at 600, 650 and 700°C at four stress levels in each case, under constant load, in air and vacuum. The data were examined according to the conventional methodology of creep data analysis by the identification of parameters from the Norton, Arrhenius and Monkman-Grant relations and the application of some parameterization techniques as a first attempt to establish a systematic comparison between creep properties in air and vacuum in this steel.

Effect of oxidation on creep data: Part 2 – A procedure for assessing the effect of oxidation on constant-stress or constant-load creep curves using the Theta-projection concept

Effect of oxidation on creep data: Part 2 – A procedure for assessing the effect of oxidation on constant-stress or constant-load creep curves using the Theta-projection concept

Effect of coating on the creep behavior of the Ti–6Al–4V alloy

The influence of the plasma-sprayed coatings and of the atmosphere on creep of the Ti–6Al–4V alloy was investigated. Yttria partially stabilized zirconia (YSZ) with CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on a standard creep machine in air and nitrogen atmospheres on uncoated samples and in air on coated samples, at stress levels of 520 MPa at 500 °C, 319 MPa at 600 °C and 56 MPa at 700 °C. Results indicated that the creep rates in nitrogen and of the coated alloy were lower than those of the uncoated in air.

Stress Relaxation Testing For Analysis of 12Cr1MoVG Steel for Creep Strength Evaluation

The design of pressurized components at elevated temperature is limited by available mechanical properties that often do not span appropriate range of stress, time, strain and temperature. To improve the design process, a new approach based on short-time stress relaxation test (SRT) is introduced in steel tensile creep analysis. The experiments are implemented using SRT and tested for creep properties of 12Cr1MoVG steel at 560 °C. The results of creep curves agree well with the traditional constant load creep test data. The possibility to apply SRT approach is shown promising in the design of the pressurized component at elevated temperature.

Creep behaviour of Fe–Mn–Al steel from 500 to 800 °C : Part 1: Analysis of constant-load creep data

The creep properties of an Fe–34.5Mn–9.2Al–1.8Si–0.95 (wt.%) alloy were determined by constant-load tests at four temperature levels ranging from 500 to 800 °C and stresses from 30 to 560 MPa. Correlations among stress, temperature, secondary creep rate, rupture time and creep ductility were obtained and results are compared to literature data. Parameters of the Norton, Arrhenius and Monkman–Grant relations are discussed as an attempt to gain some understanding on the mechanisms controlling creep behaviour of the material in the stress/temperature range investigated. The investigated alloy exhibited potential creep law with n values ranging mainly from 3 to 5 and an m value of the Monkman–Grant relation close to 1. The apparent activation energy, in the temperature range of 600 to 800 °C, takes values of 376 ± 38 kJ/mol. Microstructural instability was observed in crept samples, mainly from 500 to 600 °C, which was associated with the lower creep ductility observed in this temperature range.

Analysis of Indentation Creep

AbstractIncreasingly, indentation creep experiments are being used to characterize rate-sensitive deformation in specimens that, due to small size or high hardness, are difficult to characterize by more conventional methods like uniaxial loading. In the present work we use finite element analysis to simulate indentation creep in a collection of materials whose properties vary across a wide range of hardness, strain rate sensitivities, and work hardening exponents. Our studies reveal that the commonly held assumption that the strain rate sensitivity of the hardness equals that of the flow stress is violated except for materials with low hardness/modulus ratios like soft metals. Another commonly held assumption is that the area of the indent increases with the square of depth during constant load creep. This latter assumption is used in an analysis where the experimenter estimates the increase in indent area (decrease in hardness) during creep based on the change in depth. This assumption is also strongly violated. Fortunately, both violations are easily explained by noting that the “constants” of proportionality relating 1) hardness to flow stress and 2) area to (depth)2 are actually functions of the hardness/modulus ratio. Based upon knowledge of these functions it is possible to accurately calculate 1) the strain rate sensitivity of the flow stress from a measurement of the strain rate sensitivity of the hardness and 2) the power law exponent relating area to depth during constant load creep.