Creep Test Specimens Research Articles

The Pin-Loaded Small One-Bar Specimen in Use to Determine Uniaxial and Multiaxial Creep Data

AbstractTwo novel small specimen creep testing techniques are presented in this paper. The pin-loaded small one-bar specimen (OBS) and the small notched specimen with four loading pins (SNS4) are designed to determine the remaining lifetime for the high-temperature components. The small OBS is suitable for use in obtaining both uniaxial creep strain and creep rupture life data and the SNS4 is designed to obtain the multiaxial behaviour using small material samples. The specimens can be made from small material samples removed from the component surface or from the heat-affected zone. The specimens can be loaded through pin connections for testing. A conversion relationship and conversion factor have been obtained and used to convert the OBS creep data to the corresponding uniaxial data. For validation two materials have been used, P92 and P91 steels at 650°C. The advantages of these testing techniques are highlighted; the recommendations for future research are also given.

Comparative Study on the Determination of Creep Properties by Using Different Types of Small Specimens

The application of small specimen creep test techniques in determination of creep properties of materials in-service has been increasing. There exists a number of small specimen creep testing techniques, such as sub-size uniaxial specimen, small punch specimen, shear punch specimen, impression specimen, three-point bending specimen, cantilever beam specimen, beam specimen with fixed constraint, circular ring specimen and tow-bar specimen. The diversity in specimen types and dimensions will affect the evaluation of creep properties. It is hence necessary to make a comparison among different small specimen creep tests. A comprehensive review has been made on the effect of the specimen dimension, stress state, conversion equation, creep properties, strain sensitivity and other influence factors. The results show that different small specimens result in different creep curves. Influenced by many factors, the small punch specimen has a very complicated data interpretation. With a large equivalent gauge length, the cantilever beam and circular ring specimens can obtain rather accurate creep data. Large deformation effect exists for the three-point bending, cantilever beam and circular ring specimens, which can be controlled by the critical load approach. Because of the low frictional sensitivity, the beam specimen with fixed constraint can obtain creep rupture data of high accuracy. A code of practice is then recommended for selecting a proper small specimen to evaluate the creep properties for in-service components.

A small cantilever beam test for determination of creep properties of materials

ABSTRACTThe application of small specimen creep test techniques in the evaluation of creep properties of materials in‐service has been increasing. To obtain the creep data accurately and conveniently, a new creep test method with small cantilever beam specimens is proposed. Analytical equations are derived that can convert the load to equivalent uniaxial stress and the displacement rate to equivalent uniaxial strain rate. Three types of the cantilever beam specimens are designed. The optimal configuration of the cantilever beam specimens is recommended with the aid of finite element method, which is further validated by the cantilever beam and uniaxial specimen tests. The results show that parameters obtained from the cantilever beam tests correspond reasonably well with those from uniaxial tests at low stress levels. With a relatively large equivalent gauge length, the cantilever beam specimen allows the small creep strain rate data obtained with a high accuracy.

NbC Precipitation and Deformation of SS 347H Crept at 850°C

Niobium stabilized 347H stainless steel is a candidate alloy for fuel cladding of Generation IV Supercritical Water-Cooled Reactors (SCWR). The creep properties and microstructural evolution of SS 347H are vitally important for the proposed SCWR challenging design conditions. The high temperature mechanical properties and phase stability of stainless steels is linked to formation, dissolution and coarsening of precipitates [1]. In solution annealed and aged condition, type 347 stainless steel forms NbC precipitates which stabilise the steel against grain boundary precipitation of M23C6 to reduce sensitizati knowledge, microstructural investigation on crept specimens at 850°C is not available. In this study precipitation and deformation behaviours of a commercial SS 347H plate (17%Cr, 9%Ni, 0.36%Nb, 0.04%C) in the as received, aged at 850°C for 1244h (i.e., TEM sample taken from the head of creep specimen) and the creep conditions were examined using the FEI TEM -ray detection technology operating at 200kV. In Fig 1(A) typical NbC precipitates within the matrix in the as received condition are shown. Figure 1(B) is an HRTEM image of one of these precipitates that appears to be coherent with the matrix. An overlay of Nb EDX map on a high angle annular dark field (HAADF) image of this sample is shown in Fig 1(C), highlighting uniformly distributed precipitates. In the aged condition, Fig 2 presents a series of micrographs showing distribution of the NbC precipitates within the matrix and also at grain boundaries. In the crept sample, Fig. 3(A) is a STEM-HAADF image with CBED inset representing [011] zone axis and Fig. 3(B) shows the corresponding EDX elemental map highlighting the NbC precipitates of the crept specimen. These precipitates were found to be aligned towards slip direction (Fig. 3(B)). The slip lines visible in Fig 3(A) were analysed in HRTEM image of Fig 3(C), where the inset is the corresponding fast Fourier transform (FFT). From the FFT pattern the ( 11 1) spot was used to form the masked inversed FFT shown in Fig 3(D), illustrating the effect of slip in ( 11 1) planes. Figure 3(E) is a dark field TEM image formed using (1 11 ) reflection of a cubic NbC precipitate circled in Fig 3(B). Figures 3(F) and (G) compare the deformation in the matrix of the aged and the crept samples. In the high resolution STEM images, Moire fringes magnify the effect of defects within the matrix, and are indicative of the deformation. Arrows point to the image of the dislocations in Moire fringes and illustrate severe deformation in the crept sample as opposed to the aged sample. Figure 3(G) also implies that the NbC precipitate (circled in 3(B)) is oriented towards slip lines of creep deformation. In all samples larger NbC precipitates were also found in grain boundaries. It is interesting to note that no sigma, Z-phase (CrNbN) or M23C6 precipitates were observed in the aged specimens. Based on published data presence of the sigma phase in the aging regime we used is expected [3]. This study shows that NbC precipitates were oriented towards slip direction during creep at 850°C and can hinder the slip process. Study of longer creep test specimens is underway.

On the Determination of Material Creep Constants Using Miniature Creep Test Specimens

Full size creep test specimens, i.e., conventional uniaxial creep test specimen and Bridgman notch specimens are usually used to determine the full set of material constants for any creep model. However, in many situations, sufficient material is not available for theses specimens to be manufactured from it. Therefore, small creep test specimens have been introduced and used to determine (i) creep constants and (ii) the remaining life time for engineering components. Two commonly used small creep specimen types, i.e., the impression and the small ring creep tests, are used in this paper to determine the steady state creep constants. However, these specimen types are limited for use in determining the secondary creep properties, i.e., they are unable to determine the full set of material creep constants for creep damage models. In this paper the recently developed small two-bar creep test specimen and the newly developed small notched specimen test are described and used to determine a full set of material constants for Kachanov and Liu-Murakami creep damage models. The small notched specimen manufacturing, loading and testing procedures are described in this paper. P91 steel at 600 °C and (Bar-257) P91 steel at 650 °C have been used to compare the material constants obtained from the small two-bar and the small notched creep test specimens with those obtained from the conventional uniaxial creep test specimens and Bridgman notch specimens. The results show remarkably good agreement between the two sets of results.

A Basis for Selecting the Most Appropriate Small Specimen Creep Test Type

Many components in conventional and nuclear power plant, aero-engines, chemical plant etc., operate at temperatures which are high enough for creep to occur. These include plain pipes, pipe bends, branched pipes etc., the manufacture of such components may also require welds to be inserted in them. In most cases, only nominal operating conditions (i.e., pressure, temperatures, system load, etc.) are known and hence precise life predictions are not possible. Also, the proportion of life consumed will vary from position to position within a component and the plant. Hence, nondestructive techniques are adopted to assist in making decisions on whether to repair, continue operating or scrap certain components. One such approach is to use scoop samples removed from the components to make small creep test specimens, i.e., sub-size uniaxial creep test specimens, impression creep test specimens, small punch creep test specimens, and small ring (circular or elliptical) creep test specimens. Each specimen type has its own unique advantages and disadvantages and it may not be obvious which one is the most appropriate test method to use. This paper gives a brief description of each specimen and associated test type and describes their practical limitations. The suitability of each of the methods for determining “bulk” material properties is described and it is shown that an appropriate test type can be chosen.

Analysis and Design of a Small, Two-Bar Creep Test Specimen

In this paper, a new small-sized (two-bar) specimen type, which is suitable for use in obtaining both uniaxial creep strain and creep rupture life data, is described. The specimen has a simple geometry and can be conveniently machined and loaded (through pin-connections) for testing. Conversion relationships between the applied load and the corresponding uniaxial stress, and between the measured load-line deformations and the corresponding uniaxial minimum creep strain rate, have been obtained, based on the reference stress method (RSM), in conjunction with finite element analyses. Using finite element analyses the effects of the specimen dimensions on reference stress parameters have been investigated. On this basis, specimen dimension ratio ranges are recommended. The effects of friction, between the loading pins and the specimen surfaces, on the specimen failure time, are also investigated. Test results obtained from two-bar specimen tests and from corresponding uniaxial specimen tests, for a P91 steel at 600 °C, are used to validate the test method. These results demonstrated that the specimen type is capable of producing full uniaxial creep strain curves. The advantages of this new, small, creep test specimen, for determining uniaxial creep data, are discussed and recommendations for future research are given.

Small ring testing of a creep resistant material

Many components in conventional and nuclear power plant, aero-engines, chemical plant etc., operate at temperatures which are high enough for creep to occur. These include steam pipes, pipe branches, gas and steam turbine blades, etc. The manufacture of such components may also require welds to be part of them. In most cases, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components are not possible. Also, the proportion of life consumed will vary from position to position within a component. Hence, non-destructive techniques are adopted to assist in making decisions on whether to repair, continue operating or replace certain components. One such approach is to test a small sample removed from the component to make small creep test specimens which can be tested to give information on the remaining creep life of the component. When such a small sample cannot be removed from the operating component, e.g. in the case of small components, the component can be taken out of operation in order to make small creep test specimens, the results from which can then be used to assist with making decisions regarding similar or future components. This paper presents a small creep test specimen which can be used for the testing of particularly strong and creep resistant materials, such as nickel-based superalloys.

몽크만·그랜트관계에 기초한 소형펀치 크리프시험 데이터와 일축 크리프시험 데이터의 관계

The relationship between the small-punch creep test and the conventional creep test was investigated experimentally using a method similar to that of the Monkman?Grant relationship. Uniaxial and small-punch creep rupture tests were carried out on 9Cr-2W ferritic steel(Commercial Grade 92 steel: X10CrWMoVNb 9-2) at elevated temperatures. From the relation derived in the same manner as the Monkman?Grant relation, a correlation between the displacement rate in response to the small-punch creep test and the strain rate in the uniaxial creep test was found, and the creep life was calculated using this relation. Furthermore, the failure modes of the small punch creep test specimens were investigated to show that the fracture was caused by creep.

Characterization of the creep properties of heat resistant 9–12% chromium steels by miniature specimen testing

A miniature specimen geometry is presented that allows to extract small samples for tensile creep tests directly from critical components of steam generators in power plants like e.g. superheater tubes. In this way, material can be tested which has been subjected to the full processing chain of the component. The specimens then exhibit all microstructural modifications that are present in the component after the shaping process and heat treatment, and representative mechanical properties are determined. Similarly, specimens can be extracted from pre-corroded test pieces or used components after service to determine the impact of complex aging/loading/oxidation conditions during service on the mechanical properties of the material. The applicability of the miniature specimen test method is demonstrated in a comparative creep study involving standard and miniature specimens of P91 tempered martensite ferritic steel. Test results indicate satisfactory accuracy/repeatability of the method. Comparison with large scale specimen data reveals an influence of specimen size on the obtained creep behavior. These size effects need to be considered for a correct interpretation of results from miniature specimen creep tests.

Numerical investigation of frictional effect on measuring accuracy of different small specimen creep tests

Small specimen test methods have distinct advantages in life prediction of components operating at high temperatures. Yet some factors which can be ignored in traditional bulk material creep tests should be considered in the small specimens. In the present paper four different miniature specimens used to evaluate high-temperature creep properties of materials, such as small punch, impression, three-point bending and cantilever beam specimens, are discussed. Combined with Kachanov–Rabotnov creep damage constitutive equations, finite element models are established and applied to analyze the creep properties of small specimens. The effect of friction on the steady and tertiary creep properties is further investigated for four different small specimen tests. The results show that the friction existing between the specimen and clamp has a significant influence on the evaluation of creep properties and is more pronounced than that between the specimen and punch. In comparison with the small punch, impression, and three-point bending specimens with friction constraints the three-point bending specimens with fixed ends and the cantilever beam specimens give better results. Smaller ratio of characteristic punch size to gauge length, fixed constraint and smaller or no sliding between the punch and specimen are accordingly recommended.

Cross-Weld Creep Comparison of Power Plant Steels CrMoV, P91 and P92

Results obtained from high temperature creep tests of ferritic steel CrMoV, P91 and P92 cross-weld specimens are compared. Data are presented normalized with the results of the corresponding parent metals in order to compare the materials tested at different temperatures and stresses, and to identify characteristic cross-weld material trends. All cross-weld failures occurred in the heat-affected zone (HAZ) adjacent to the parent metal, known as type IV, at earlier test durations than for the parent material specimens. The relative performance of the cross-welds was found to decrease with decreasing stress. The CrMoV cross-weld specimens showed by far the lowest weld strength reduction factors and greatest notch strengthening, determined by testing uniaxially loaded bars with semicircular circumferential notches located in the heat-affected zone. The P91 and P92 cross-weld specimens were found to have similar (high) weld strength reduction factors and showed little or no notch strengthening. The failure time of a P92 cross-weld specimen relative to the P92 parent material could be estimated using data from P91 cross-weld and relative parent material tests.

Research on Creep Characteristics of the Double-Base Solid Propellant

The uniaxial tensile creep tests of the double-base solid propellant specimens were carried out at 450C under five different stress levels. The viscoelastic models such as three parameters solid model, Burgers model, generalized Kelvin model, five parameters solid model and exponential model were adopted. The computation equation that can describe the strain rate of the second creep stages of the double-base solid propellant under different applied stress levels was founded on the basis of these creep tests, and the adaptability of the five models on creep characteristics of the double-base solid propellant were studied. The study results shows that the five models can show good agreement with second creep course of propellant under different applied stress levels, but the generalized Kelvin model may have optimal characterize creep curve of the double-base solid propellant.

Theoretical basis and practical aspects of small specimen creep testing

Interest in and the application of small specimen creep test techniques are increasing. This is because it is only possible to obtain small samples of material in some situations, for example, the scoop samples that are removed from in-service components, the heat-affected zones that are created when welds are used to join components and the desire to produce only small amounts of material in alloy development programmes. It is therefore important to review and compare the theoretical basis and practical aspects of each of the small specimen creep testing methods, in order to clearly understand which of the methods is the best for any specific application. This article provides the theoretical basis for each commonly used test method.

Type IV Creep Damage Behavior in Gr.91 Steel Welded Joints

Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and 923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution of Type IV creep damage.

The Selection and Application of Filling Material Based on Long-Term Strength of Backfill Body

High stress makes backfill body present the rheological properties in deep mine. This paper first uses RRTS-ⅢA to carry on creep test of specimen in different proportion, according to ε-t curves under different load levels to determine the specimen’s long-term strength, based on the exponential relationship of rock block and rock body, the long-term compressive strength is derived, combining with concrete parameters of working face, mixture ratios of filling material which meet long-term stability are selected, while analyzing the support resistance of backfill body. The results show that this design can effectively avoid the rheological instability of backfill body.

GS18 微視的クリープ損傷シミュレーションに基づく改良9Cr鋼溶接継手の余寿命評価(GS-4 一般セッション4,一般セッション)

Assessment of Type IV creep damage in pipe welds is important for residual life prediction of fossil power plants. In this paper, the micro-macro combined creep damage simulation on the basis of the grain-boundary-fracture-resistance model is applied to the creep damage simulation of Mod.9Cr steel welds. Both time history of number density of small defects and final failure life of the large size creep test specimen subject to tension are predicted together with damage mechanics concept.

Damage mechanics based predictions of creep crack growth in 316 stainless steel

This paper describes a novel modelling process for creep crack growth prediction of a 316 stainless steel using continuum damage mechanics, in conjunction with finite element (FE) analysis. A damage material behaviour model, proposed by Liu and Murakami [1] , was used which is believed to have advantages in modelling components with cracks. The methods used to obtain the material properties in the multiaxial form of the creep damage and creep strain equations are described, based on uniaxial creep and creep crack growth test data obtained at 600 °C. Most of the material constants were obtained from uniaxial creep test data. However, a novel procedure was developed to determine the tri-axial stress state parameter in the damage model by use of creep crack growth data obtained from testing of compact tension (CT) specimens. The full set of material properties derived were then used to model the creep crack growth for a set of thumbnail crack specimen creep tests which were also tested at 600 °C. Excellent predictions have been achieved when comparing the predicted surface profiles to those obtained from experiments. The results obtained clearly show the validity and capability of the continuum damage modelling approach, which has been established, in modelling the creep crack growth for components with complex initial crack shapes.

Correlation of creep strength and microstructural evolution of a boron alloyed 9Cr3W3CoVNb steel in as-received and welded condition

The creep strength of martensitic 9% chromium steels is strongly coupled with the formation and interaction of different precipitate populations like MX (Nb,V)(C,N), M23C6, laves phase and modified Z-phase. Changes in microstructure can be correlated to a change in creep strength. Within this work, a boron-nitrogen balanced 9Cr3W3CoVNb steel is investigated in normalised and tempered (as-received), creep exposed and welded condition. Microstructural characterisation of as-received and creep exposed samples have been carried out by elemental mapping techniques in transmission electron microscopy. The creep strength of the base material has been evaluated at 650°C up to a current testing duration of 24,000 hours. Comparison of creep rupture strength after 20,000 hours shows a benefit in strength of the new grade of about 25 MPa compared to conventional steel grade P92 and 45 MPa to grade P91. The results of the microstructural investigation show only very little changes of the initial microstructure after 10,000 h of creep exposure. The investigations imply a higher microstructural stability of the new steel, compared to other 9% chromium steel grades.In welded condition, the steel shows unique formation of its heat-affected zone. Although the steel experiences several phase transformations during the weld thermal cycle, the original base material grain structure is almost completely retained in the heat-affected zone. The formation of a homogeneous grain refined region is suppressed and, therefore, less susceptibility to Type IV cracking is expected. Creep tests of crossweld specimens at 650°C and various stress levels show similar creep strength to grade P91 base material.

Henry Granjon Prize Competition 2008 Joint Winner — Category B: “Materials Behaviour and Weldability” Evolution of Microstructure and Mechanical Properties of the Heat-Affected Zone in 9Cr Steels

Martensitic 9–12% chromium steels are favoured grades for thick-walled components in ultra super-critically-operated thermal power plants. Welding in all its variations is the major joining and repair technology for such components. Several years of in-service experience and long-term creep testing data of cross-welds of various martensitic steel grades have revealed the inter-critical or fine-grained region of the heat-affected zone as the weakest part of such welded constructions. Within this work, based on long-term creep tests of cross-weld specimens, the predominant failure mode of Type IV cracking is described. Reduced creep strength of cross-welds is related to an enhanced microstructural degradation in the heat-affected zone. Therefore, the formation of the heat-affected zone and resulting microstructure is investigated using advanced microscopy, physical weld simulation and X-ray diffraction. Phase transformations occurring during characteristic weld thermal cycles in the heat-affected zone are recorded by in-situ X-ray diffraction using high-energy synchrotron radiation. The evolution of microstructure in the heat-affected zone is studied by physical weld simulation and subsequent metallographic examination. Within this work, a novel 9Cr-3-W-3Co steel with balanced additions of boron and nitrogen is produced and investigated. The base material shows superior creep strength at650°C up to the current maximum testing time of 19 000 h. Additionally, the new steel has high potential to overcome Type IV failures by the suppression of grain refinement in the heat-affected zone.