Creep Rupture Tests Research Articles

Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe2Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe2Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).

Microstructural Changes of 22Cr-9Mo-3Fe-4Nb Ni-Base Superalloy during Creep

A new Ni based superalloy is considered as bucket material because of high fatigue resistance with fine grain. However a bucket is usually exposed to high temperature for a long time, so creep rupture tests were conducted to understand the microstructure evolution during creep deformation under various stresses at temperatures of 566, 593, 621, 649 and 670 ℃. The hardness applied to the crept specimen of these alloys increased until the value of LMP reached 25, and decreased while the value of LMP exceeded 25. The microstructure analysis with TEM, SEM and OM were conducted on ruptured specimen to observe the changes of γ″ morphology, carbides, and fracture mode. As a result of investigating microstructural changes and creep properties, it can be concluded that the effect of grain size on creep properties is dominant regardless of the size of γ″. (Received December 5, 2016; Accepted January 14, 2017)

Determination of Mo-Rich Laves-Phase in Weld Metals of a Creep-Resistant 9% Cr-Steel Using Light Optical Microscopy

Creep-resistant 9% Cr-steels feature different phases in their weld metal microstructure; especially after creep-rupture testing. Laves-Phase in such alloying concepts is widely studied, but there is still a need for a simple identification technique utilizing light optical microscopy. The Lichtenegger–Bloech color tint etchant was utilized to contrast the weld metals microstructure in as-welded condition, after post-weld heat treatment and creep-rupture tested. Proof of the presence of Laves-Phase after creep-rupture tests in the weld metal of a 9% Cr-steel GX13CrMoCoVNbNB9-2-1 (CB2) was performed using Lichtenegger–Bloech and light optical microscopy. To validate Laves-Phase within the analyzed microstructure, scanning electron microscopy and energy dispersive x-ray spectroscopy were performed. For a thorough analysis, phase analysis was done in selected regions of interest. The results show that the Laves-Phase in this type of steel can be easily contrasted and quantified at comparatively low lateral resolutions by Lichtenegger–Bloech, with no further need of analysis required.

On cyclic yield strength in definition of limits for characterisation of fatigue and creep behaviour

AbstractThis study proposes cyclic yield strength as a potential characteristic of safe design for structures operating under fatigue and creep conditions. Cyclic yield strength is defined on a cyclic stress-strain curve, while monotonic yield strength is defined on a monotonic curve. Both values of strengths are identified using a two-step procedure of the experimental stress-strain curves fitting with application of Ramberg-Osgood and Chaboche material models. A typical S-N curve in stress-life approach for fatigue analysis has a distinctive minimum stress lower bound, the fatigue endurance limit. Comparison of cyclic strength and fatigue limit reveals that they are approximately equal. Thus, safe fatigue design is guaranteed in the purely elastic domain defined by the cyclic yielding. A typical long-term strength curve in time-to-failure approach for creep analysis has two inflections corresponding to the cyclic and monotonic strengths. These inflections separate three domains on the long-term strength curve, which are characterised by different creep fracture modes and creep deformation mechanisms. Therefore, safe creep design is guaranteed in the linear creep domain with brittle failure mode defined by the cyclic yielding. These assumptions are confirmed using three structural steels for normal and high-temperature applications. The advantage of using cyclic yield strength for characterisation of fatigue and creep strength is a relatively quick experimental identification. The total duration of cyclic tests for a cyclic stress-strain curve identification is much less than the typical durations of fatigue and creep rupture tests at the stress levels around the cyclic yield strength.

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.

Effect of orthorhombic case on the creep rupture of Ti-22Al-25Nb (at%) orthorhombic alloy

Creep rupture tests and tensile tests of Ti-22Al-25Nb alloy with bimodal size lamellar O phase microstructure were carried out at 650°C and 700°C. The fracture surface analyses indicate that the tensile fracture originated from the center of the cylinder sample while the creep rupture started at the surface. The different fracture mechanism of creep rupture samples is ascribed to the orthorhombic cases (O cases). It is found that the sub-surfaces of the creep rupture samples were enriched with the element oxygen. The interstitial oxygen, an alpha stabilizer for titanium alloys, promoted the transformation from B2 phase to O phase. Thus the sub-surfaces contained more acicular O phases than the substrates. The sub-surfaces with more O phase component are named as O cases. The dissolved interstitial oxygen and the added O phase component increased the strength but reduced the ductility and the toughness in the O cases. It was the incompatibility of the creep strains between the O cases and the substrates that made the creep rupture to originate at the surfaces.

On a damage law for creep rupture of clays with accumulated inelastic deviatoric strain as a damage measure

To avoid the dependency on origin of time, an improved damage law for creep rupture of clays is proposed considering the accumulated inelastic deviatoric strain as a measure of damage, instead of incorporating time directly. This law is incorporated into an existing anisotropic elastoplastic-viscoplastic bounding surface model for clays. The performance of the damage law was demonstrated via the simulations of creep rupture tests on undisturbed clays, and generally a good agreement between model simulations and test data was obtained. Discussions on the creep rupture parameters were followed and further improvement was suggested. At present when high quality test data for creep rupture is very limited, the proposed damage law could serve as a practical way to model creep rupture of clays.

Effect of texture on anisotropy at 600 °C in a near-α titanium alloy Ti60 plate

To study the anisotropy of mechanical properties at 600°C in a near-α titanium alloy Ti60, a highly T-type textured plate was prepared, and tensile tests, creep tests and creep rupture tests were carried out at 600°C. Notable anisotropies were observed in all three kinds of mechanical tests. The deformation in TD was much harder than in RD，no matter high or low was the loading stress. The anisotropy of tensile properties was concluded to be related to the different Schmid Factors distribution caused by T-type texture, leading to prismatic <a> slips easily being activated in RD and both prismatic <a> and basal <a> slips hard to be activated in TD. The restriction of creep deformation in TD samples was believed resulting from the effect of T-type texture on self- and solute diffusion. For the anisotropy of creep rupture properties, it was deduced to be a result of the combined anisotropy of diffusion and dislocation slipping.

Creep behavior of dissimilar metal weld joints between P91 and AISI 304

Dissimilar welds between ferritic creep-resistant steels and austenitic stainless steels in power plant service are known to fail prematurely due to a large difference in their thermal coefficient of expansion. To address this problem, in the current work, modified 9Cr-1Mo steel (P91) and austenitic stainless steel (AISI 304) transition joints were produced using friction welding employing three interlayers: Inconel 625, Inconel 600, and Inconel 800H. These interlayers were friction welded one over another on P91 alloy, which was finally friction welded to AISI 304. The joints produced with single and three interlayers were subjected to creep rupture tests at different temperatures and stress levels. The creep test results demonstrated that the proposed approach can help realize striking improvements in creep performance of P91/AISI 304 dissimilar metal welds. Further, analysis of creep resulted in a stress exponent (n) of 3 by incorporating the concept of threshold stress which suggested the creep deformation mechanism to be viscous glide due to solute drag. Creep rupture data was also analyzed using Monkman-Grant relationship and creep damage tolerance factor (λ). Results indicated that the damage mechanism is due to cavity growth by the combined effect of power law and diffusion creep.

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.

Dissimilar metal welds between 9Cr creep strength enhanced ferritic steel and advanced stainless steels—creep rupture test results and microstructural investigations

Modern and future power plants will use more complex materials to withstand the higher steam temperatures and pressures as well as thermal cycling. As such, dissimilar metal welds (DMWs) will be more widespread in new design than before and will need to demonstrate resistance to combinations of severe creep, corrosion/oxidation, and low cycle fatigue. The DMWs between different steels or alloys are an underestimated topic. Experienced and documented premature failures reveal the criticality of DMWs. This study focused on the DMWs between 9Cr martensitic creep strength enhanced ferritic steels and advanced austenitic stainless steels. Two collaborative projects related to 18%Cr and 25%Cr austenitic creep resisting stainless steel delivered data concerning base metal characterization and weldability of similar and dissimilar welding. The specific features of dissimilar welding are further approached in more detail in this analysis. Two main methodologies, i.e., direct welding and buttering techniques using different filler metals were investigated and characterized via uniaxial creep rupture testing and metallographic examination particularly focusing on the fusion line carbide formation and morphology. For the limited exposure times of 10 kh in this project, the results show that aligned and coarse type I carbide formation is occurring at the 9Cr fusion line between A617 type weld metal. The selection of P87 or A82 type filler metals demonstrated isolated to partly aligned carbides, respectively, at the fusion line. To mitigate the risk to corrosion and achieve a marginal increase in creep performance, buttering techniques are recommended.

Dissimilar metal welds between martensitic and advanced austenitic high temperature creep resisting steels: Creep rupture test results and fusion line investigation

Modern and future power plants will use more elaborated and complex materials to withstand the higher steam temperatures and pressures as well as thermal cycling. As such, dissimilar metal welds (DMWs) will be much more widespread in new design than before and will need to demonstrate resistance to combinations of severe creep, corrosion/oxidation and low cycle fatigue. The DMWs between different steels or alloys are an underestimated topic. Experienced and documented premature failures reveal the criticality of DMWs. This study focused on the DMWs between martensitic and advanced austenitic stainless steels. Two collaborative projects related to a 18%Cr and 25%Cr austenitic creep resisting stainless steel delivered data concerning base metal characterization and weldability of similar and dissimilar welding. The specific features of dissimilar welding are further approached in more detail in this analysis. Two main methodologies i.e. direct welding and buttering techniques using different filler metals were investigated and characterized via uni-axial creep rupture testing and metallographic examination particularly focusing on the fusion line carbide formation and morphology. For the limited exposure times of 10kh in this project, the results show that aligned and linked up type I carbide formation is occurring when A617 type filler metals are used. The selection of P87 or A82 type filler metals demonstrated isolated to partly aligned carbides respectively at the fusion line. Due to a risk for corrosion and the somewhat better creep rupture strengths at comparable conditions, buttering techniques seem to be recommended above direct welding.

Effect of Cold Working on Creep Rupture Strength of Alloy617

The effect of cold working on the creep rupture strength of Alloy617 was investigated. The creep rupture tests were conducted at temperatures of 700 to 800°C, under stresses from 100 to 350 MPa. At high stress conditions, the creep rupture time of the non-pre-strained samples are similar to those of the pre-strained samples. On the other hand, at low stress conditions, the creep rupture time of the pre-strained samples are longer than those of the non-pre-strained samples. The amount of precipitates near the grain boundaries in the pre-strained sample is higher than that in the non-pre-strained sample. Weak regions such as PFZ and recrystallization grains in the non-pre-strained sample are formed in the early stage of creep compared to the pre-strained sample. At low stress conditions, the precipitates near the grain boundaries in the pre-strained sample play an effective role to pin the grain boundaries and they delay the formation of the weak regions resulting in extension to the creep rupture time.

Effect of Cold Work on Creep Rupture Strength of Alloy263

Creep rupture strength and the microstructure change during creep deformation of pre-strained Alloy263 were investigated. Creep rupture tests were conducted at 1023, 1073 K at stress range from 120 to 250 MPa. Creep strength of the pre-strained samples was higher than that of the non-strained samples. However, rupture strain of the pre-strained samples was much lower than that of non-strained samples. In the pre-strained samples, Ni₃(Al, Ti)- γ’ and M₂₃C₆ inside of the grains precipitated finer than the non-strained samples compared at the same creep time. At grain boundaries, the grain boundary shielding ratio covered by M₂₃C₆ carbide showed almost same value among each samples. However, diameter of the M₂₃C₆ particle decreased in pre-strained samples. Furthermore, dynamic recrystallization was promoted and precipitation free zone (PFZ) was formed around Ni₃Ti-&eta; phase at grain boundary. These observations show that increase in creep-strength of pre-strained sample was due to increase in precipitation strengthening in the grain by fine precipitation of γ’ and M₂₃C₆. In addition, resistance against crack propagation at grain boundary increased by the fine precipitation of grain boundary M₂₃C₆ even though formation of PFZ and promotion of the dynamic recrystallization. It is estimated that Orowan-stress of pre-strained samples was 1.7 times higher than non-strained samples. It is considered that these strengthening effects overcome the weakening effects in the pre-strained samples.

Damage analysis and integrity assessment of a few steam-reformer components at a syn-gas plant

Study deals with damage-analysis and integrity-assessment for a few components of a steam-reformer at a syn-gas generation plant, including catalyst tubes and hot-collectors. Selected components include the ones which either suffered from a fluid-leakage in the service or noticed with a constant high-temperature exposure during normal plant operation. The field investigation activities included visual inspection, dimensional check, die-penetrant test and pressure-drop measurements. Laboratory investigation, made on selective samples, included precise and detailed dimensional-measurement, visual examination, die penetrant test, microstructural characterization, creep-rupture test, etc. As a result of diagnostic analysis, creep was identified as the principal mechanism of degradation of the studied components. Catalyst tubes were found to be in sound condition and, hence, recommended to continue in use for the remaining period of the service-life, even after high-temperature exposure in the past. Other items; however, were noticed with a limited residual creep strength, hence, rejected for future use. Further, recommendations were made for improvement in practices of process- and operation-management, to avoid pre-mature deterioration of the reformer components. In addition, suggestions were given for enhancing the versatility and reliability of plant inspection and monitoring methods, using latest tools and technology of the field. Moreover, suitability and limitation of different analytical approaches regarding interpretation of test and inspection results are highlighted.

Life Assessment for Cr-Mo Steel Dissimilar Joints by Various Filler Metals Using Accelerated Creep Testing

Accelerated creep rupture tests were performed on T22/T91 dissimilar metal joints to determine the fracture location and rupture time of different weldments. Four configurations of deposited filler metal were tested using gas tungsten arc welding to estimate the service life for Cr-Mo steel dissimilar joints at elevated temperatures in power plants. Results indicated that failure in all configurations occurred in the tempered original microstructure and tempered austenite transformation products (martensite or bainite structure) as type IV cracking at the intercritical area of the heat-affected zone (ICHAZ) for both T22 and T91 sides rather than as a consequence of the different filler metals. Creep damage occurred with the formation of precipitations and microvoids. The correlation between applied stress and the Larson-Miller parameter (PLM) was determined to predict the service life of each material configuration. Calculated time-to-failure based on the PLM and test results for both temperature and applied stress parameters gave a reasonable fit. The dissimilar joints exhibited lower creep rupture compared to the base material indicating creep degradation of the weldment.

Creep behavior of Alloy 617 in high temperature air and helium environments–effect of oxidation damage

The creep behavior of a nickel-base superalloy, Alloy 617, which is considered as a candidate material for the very high temperature gas cooled reactor, was studied. Creep rupture tests were carried out at 800°C, 900°C and 1000°C in static and flowing helium environments as well as in air. Creep rupture life in static helium was longer than that in air, while it was shorter in flowing helium environments. Microstructure observation of the creep tested specimens showed that the shorter creep rupture life in flowing helium was associated with the thicker oxide layer, greater decarburization depth, and deeper internal oxidation happened during the creep tests. The degree of such oxidation damage was quantified for the creep tested specimens and correlated with the creep rupture life in different environments.

Accelerated creep and creep-rupture testing of transverse unidirectional carbon/epoxy lamina based on the stepped isostress method

Accelerated methods for measuring long-term viscoelastic properties of fiber reinforced polymer (FRP) composites are important for reducing the time needed to characterize material behavior. Most of them are based on the time-temperature-stress superposition principle (TTSSP), or some variation. This paper investigates the applicability of the stepped isostress method (SSM) to study tensile creep of unidirectional carbon FRP (CFRP) lamina used in rehabilitating prestressed concrete structures. The SSM employs a load-stepping approach, typically with three to five steps for a single specimen resulting in creep-rupture. Theoretical analysis of the experimental method (previously lacking) is discussed and modifications to the testing and analysis protocol are shown to improve validity of the master curves compared to conventional creep curves and traditional TTSSP creep master curves at room temperature. Finally, three failure criteria are compared for predicting creep-rupture of CFRP laminates based on the accelerated testing method.

The Unified Creep-Fatigue Equation for Stainless Steel 316

Background—The creep-fatigue properties of stainless steel 316 are of interest because of the wide use of this material in demanding service environments, such as the nuclear industry. Need—A number of models exist to describe creep-fatigue behaviours, but they are limited by the need to obtain specialized coefficients from a large number of experiments, which are time-consuming and expensive. Also, they do not generalise to other situations of temperature and frequency. There is a need for improved formulations for creep-fatigue, with coefficients that determinable directly from the existing and simple creep-fatigue tests and creep rupture tests. Outcomes—A unified creep-fatigue equation is proposed, based on an extension of the Coffin-Manson equation, to introduce dependencies on temperature and frequency. The equation may be formulated for strain as ε p = C 0 c ( T , t , ε p ) N − β 0 , or as a power-law ε p = C 0 c ( T , t ) N − β 0 b ( T , t ) . These were then validated against existing experimental data. The equations provide an excellent fit to data (r2 = 0.97 or better). Originality—This work develops a novel formulation for creep-fatigue that accommodates temperature and frequency. The coefficients can be obtained with minimum experimental effort, being based on standard rather than specialized tests.

Creep deformation and rupture mechanism of an advanced wrought Ni[sbnd]Fe-based superalloy for 700 °C class A-USC steam turbine rotor application

The 700 °C class Advanced Ultra-Super-Critical (A-USC) technology with higher thermal efficiency is being developed for next-generation coal-fired power plant in recent years to reduce environmental impacts of energy consumption. A new wrought NiFe-based superalloy with excellent high creep strength and low cost has been developed recently and evaluated as a candidate material for the 700 °C class A-USC steam turbine rotor application. Creep tests were conducted on the new NiFe-based alloy at 700 °C/200 MPa, 700 °C/250 MPa and 725 °C/150 MPa using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The creep-rupture testing results show the new NiFe-based alloy has better creep-rupture strength than the existing commercially available NiFe-based and even some Ni-based alloys. The focus of this study is on understanding the relative important causes of better creep resistance and reduced rupture life in grain interiors and grain boundaries. Detailed creep deformation behaviors and related creep-rupture mechanisms were analyzed and discussed to explore the potential approaches for performance improvements. The results presented are helpful in providing a new approach to design and develop novel high performance alloys for A-USC steam turbine rotor application.