Internal Pressure Creep Tests Research Articles

Creep Life Assessment Method for Super304H Boiler Tubes Based on HTS-SQUID Gradiometer

This paper presents a novel method for the creep life assessment of Super304H (KA-SUS304J1HTB, ASME code: 2328) boiler tubes based on a discrete-type eddy-current-testing (ECT) HTS-SQUID nondestructive evaluation (NDE) system. Six specimens were subjected to internal pressure creep tests at a stress level of 110 MPa and a temperature of 700°C. The specimens were inspected at various creep test times using our NDE system, which included an ECT probe and a HTS-SQUID gradiometer. By studying the physical characteristics and microstructures of the specimens at different test times, we found that the electrical conductivity and permeability of the specimens varied with an increasing test time. The detected signals may have been generated from creep voids and/or surface microcracks. Based on the NDE inspection results, creep life assessment curves that can be used for assessing the remaining creep life of the Super304H boiler tubes were plotted. The results confirmed the possibility of assessing the remaining creep life using our HTS-SQUID-based system in a nondestructive manner.

Verification of Long-term Creep Rupture Properties and Microstructure Stability of Ni based Alloy 45Ni-24Fe-23Cr-7W-Ti Parent Metal and Weldment

ABSTRACT To clarify the long-term creep rupture strength and microstructure stability of Ni-based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti, ASME Code Case 2684), creep tests were conducted up to about 70,000h and 117,000h for the weldment and parent metal respectively, and the microstructure of crept specimens was experimentally investigated. The weldment showed comparable strength with the parent metal. The metallurgical microstructure characterization and quantitative evaluation on the precipitation behavior indicated the precipitation strengthening of Laves phase, M23C6 carbide and αCr phase in the HAZ (Heat Affected Zone) and parent metal contributing to the long-term creep rupture strength stability of weldment. Satisfactory microstructural stability was proven without distinct degradation during long-term creep at 700°C up to around 117,000h. Furthermore, the internal pressure creep test of circumferential weldment was performed to verify the damage and failure morphology of actual components under the multiaxial loading

Self-healing behavior of Cr-coated Zr alloy cladding in high temperature steam oxidation process

Cr-coated Zr alloy claddings, prepared via physical vapor deposition (PVD), were used to study the damage evolution under a series of high-temperature steam oxidation conditions. Through-thickness cracks were initially produced within the Cr coat by internal pressure creep test. The Cr-coated samples with cracks were then oxidized at temperatures from 800 °C to 1200 °C in water steam environment. Microstructures of the samples before and after oxidation were characterized in comparison by scanning electronic microscopy, transmission electronic microscopy and energy dispersion spectrometry. The results showed that a large proportion of cracks would be self-healed when the temperature increased to 1000 °C. At the places where the cracks have survived, the corresponding Zr substrate regions were oxidized to form semi-spherical or fan-shaped ZrO2. However, the maximum depth of ZrO2 under the unhealed cracks was smaller than that of the uncoated claddings at the same oxidation conditions.

Thermal creep behavior of CZ cladding under biaxial stress state

Thermal creep is a key property of zircaloy cladding. CZ developed by CGN is a new zircaloy used as PWR fuel cladding. This research is devoted to investigating the thermal creep behavior of CZ and build the thermal creep model of CZ. Twenty internal pressure creep tests were conducted, and the ranges of temperature and Tresca stress were 320–430 °C and 70–300 MPa, respectively. Real-time creep data were analyzed by separating primary creep and steady-state creep. Based on Soderberg model and creep test data, CZ thermal creep model is derived. As a whole, the mean value and the standard deviation of P/M of CZ saturated primary creep strain are very close to these from steady-state creep rate, however, the predictive effect of primary creep is less satisfactory. Four conditions, where there exists large deviation between predicted values and test data, are 320 °C and 300 MPa, 350 °C and 190 MPa, 380 °C and 160 MPa, 380 °C and 190 MPa, respectively. As primary creep was much smaller than steady-state creep in long-time operation, the thermal creep model built can be applied to predict the thermal creep behavior of CZ cladding.

Creep and damage anisotropies of 9%Cr and 14%Cr ODS steel cladding

Two Oxide Dispersion Strengthened (ODS) steel tubes, obtained by means of hot extrusion and cold rolling, are studied as potential materials for cladding applications in sodium-cooled fast reactors. A 9%Cr martensitic tube (Fe9Cr1W-0.3Ti-0.25Y2O3) with isotropic microstructure is compared to a 14%Cr ferritic tube (Fe14Cr1W-0.3Ti-0.25Y2O3) with elongated grains and a strong texture. Tile and notched ring specimens are used to perform uniaxial creep tests at 650 °C in the longitudinal and transverse directions of the tubes. The 14%Cr ODS steel tube exhibits strong mechanical anisotropies, while the 9%Cr presents only a slight anisotropy in creep resistance, presumably due to the influence of specimen geometry on mechanical fields. Both intergranular and ductile zones have been observed on the fracture surfaces of the uniaxial creep specimens, suggesting the competition of two damage mechanisms. Damage for both tubes appeared anisotropic, with orientated cracks in the rolling directions on the ring specimens and the 14%Cr tile specimens. Internal pressure creep tests also performed at 650 °C show complementary results for both tubes in agreement with ring specimens’ ones, validating their use for hoop properties assessment. Overall, while the 14%Cr ODS steel tube is strong in the longitudinal direction, the 9%Cr tube proves almost no remarkable anisotropy in creep strength.

Damage Evolution and Life Prediction of a P91 Longitudinal Welded Tube Under Internal Pressure Creep

The clarification of creep damage mechanism and the establishment of remaining life prediction methods of longitudinal welded piping of P91 steel are important subjects to maintain a reliable operation of boilers in thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage with time and to evaluate a life prediction method. Interrupted creep tests were performed for damage observation in addition to rupture tests. Three dimensional finite element creep analyses of the longitudinal welded tube specimens were conducted to identify the stress and creep strain distributions within the specimen during creep. Failure occurred at a heat affected zone (HAZ) without a significant macroscopic deformation. It was found that the initiation of creep voids had concentrated at the midthickness region in the HAZ rather than in the surface. The creep analysis results indicated that the triaxial tensile stress yielded at the midthickness region in the HAZ due to difference of creep deformation property among the base metal, the HAZ, and the weld metal. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the HAZ, resulting in internal failure of the tube specimens. A rupture time prediction method of the welded tube is proposed based on the maximum principal stress and the triaxial stress factor in the HAZ. The void growth behavior in the HAZ was well predicted by the previously proposed void growth simulation method by introducing a void initiation function to the method.

Damage assessment method of P91 steel welded tube under internal pressure creep based on void growth simulation

Development of creep damage assessment methods for longitudinal welded piping of P91 steel is important and an urgent subject to maintain reliable operation of boilers in ultra super critical thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage in a heat-affected zone (HAZ) of the longitudinal welded pipe. Failure occurred at a heat-affected zone without significant macroscopic deformation. It was found that initiation of creep voids had concentrated at mid-thickness region rather than surface. Three-dimensional finite element (FE) creep analysis of the creep tested specimens was conducted to identify stress and creep strain distribution within the specimen during creep. Finite element creep analysis results indicated that triaxial tensile stress yielded at the mid-thickness region of the HAZ. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the heat-affected zone resulting in internal failure of the tube specimens. Void growth behavior in the heat-affected zone was well predicted with the previously proposed void growth simulation method by introducing void initiation function to the method. A “limited strain” was defined as rupture criterion and dependency of the maximum stress and multiaxiality on the “limited strain” was derived by the void growth simulation. Creep damage distribution in the HAZ under the internal creep test was calculated by proposed damage assessment method.

Creep strength of high chromium steels welded parts under multiaxial stress conditions

Internal pressure creep tests as well as uniaxial creep tests were conducted on 9Cr and 12Cr steels welded joints in order to evaluate creep strength of the high chromium steels welded parts under multiaxial stress conditions, where a welding direction of the internal pressure specimen was longitudinal one. Except under high stress conditions, cracks occurred at fine grained HAZ region for both materials, i.e., the type IV mode fracture. Regarding creep voids distribution along thickness direction of the internal pressure specimens, the creep voids were predominantly observed in the middle of the thickness at the HAZ region while they might be negligible in outer and inner surface of the specimen. Finite element analysis which considers HAZ and weld metal properties revealed that the principal stress (hoop stress) and the stress multiaxiality are the largest in the middle of the thickness at the HAZ region under the internal pressure creep conditions. It was suggested that the creep voids remarkably occurred inside the specimens at the HAZ region due to the severe stress state conditions.

C106 スモールパンチクリープ試験によるボイラ用鋼管のクリープ損傷評価(材料強度・規格-(1),OS5 保全・設備診断技術),動力エネルギーシステム部門20周年,次の20年への新展開)

The small punch creep (SPC) test is possible to predict residual creep life at a high accuracy. But, the results of SPC tests cannot be compared with uniaxial creep or internal pressure creep results directly. In this report, the relationship between SPC test results and uniaxial creep test results in STPA23(1.25Cr-0.5Mo Steel) and STPA24(2.25Cr-1Mo Steel) were studied. The obtained relationship between SPC load and equivalent uniaxial creep stress of all materials formed a simple linear equation under the wide range of test temperature and test period. The relationship between SPC test results and internal pressure creep tests results was also studied. The creep life of internal pressure creep influenced material also showed good correspondence with SPC results. Therefore SPC can estimate the residual life of internal pressure creep influenced materials.

Internal Pressure Creep Test and Life Evaluation for Full Size Reheat Elbow of Mod.9Cr-1Mo Steel

In order to establish the life assessment method for the welded hot reheat elbow of modified 9Cr-1Mo steel, an internal pressure creep test is conducted with a full size elbow component. The results are following, (1) : The elbow component finally leaked at weld metal. The rupture time was 1606 hours. The creep damage seemed to be initiated near the inside surface of the elbow and to grow towards the outside surface along to the fine-grained HAZ of the seam weld. In outside neighborhood, the crack that had initiated along the fine-grained HAZ changed the direction of initiation into the weld metal. (2) : The results obtained by the Time of Flight Diffraction (TOFD) inspection for detecting cracks under the surface at the seam weld on the intrados of the elbow show the presence of a crack near the inner surface of the elbow at the end of life. (3) : The fracture life of the full size elbow internal creep test based on the stress level at the mean diameter formula was slightly shorter than the uniaxial creep test results.

Damage characterization of a P91 steel weldment under uniaxial and multiaxial creep

Clarification of creep damage mechanism and establishment of remaining life assessment methods of boiler pipings of P91 steel with weldment are important subjects to maintain reliable operation of thermal power plants. In order to characterize the creep damage of a P91 steel weldment and to discuss creep rupture life prediction methods, uniaxial creep tests on a P91 steel cross weld and internal pressure creep tests on a longitudinal welded tube specimen were conducted. Three-dimensional finite-element creep analysis of the longitudinal welded tube specimen was conducted to identify stress and creep strain distribution within the specimen. Creep rupture time of the weld joints was reduced significantly from that of the base metal due to Type IV failure. It was found from an examination of creep damage in interrupted cross weld specimens that creep voids have already initiated around 20% creep damage, and that the number of voids inside the specimens was larger than those at the surface of the specimens. Creep analysis results indicated that triaxial tensile stress yielded at mid-thickness in the heat affected zone (HAZ) of the tube specimen due to differences of creep deformation properties of the base metal, the HAZ and the weld metal. It was suggested that triaxial stress states caused acceleration of creep damage evolution in the HAZ resulting in internal failure of the tube specimens. Rupture life of the longitudinal welded tube specimen under internal pressure creep is predicted from the circumferential steady-state creep strain rate at the HAZ obtained from creep analysis using a three-materials finite-element model of the tube specimen.

OS0707 厚肉試験体を用いた内圧クリープ試験によるSuper 9Cr鋼のクリープ損傷評価(OS07-02 多軸疲労・クリープ,OS07 多軸疲労および多軸クリープ)

OS0707 厚肉試験体を用いた内圧クリープ試験によるSuper 9Cr鋼のクリープ損傷評価(OS07-02 多軸疲労・クリープ,OS07 多軸疲労および多軸クリープ)

Full size internal pressure creep test for welded P91 hot reheat elbow

In order to establish a life assessment method for welded modified 9Cr–1Mo steel hot reheat elbow, an internal pressure creep test was conducted with a full size test component. As a result, the fracture mode of the component has been clarified and a life prediction method has been established. Furthermore, the appropriate creep damage detection procedures have been proposed.

Observation of type IV cracking in welded joints of high chromium ferritic heat resistant steels

Type IV cracking in welded joints of high Cr ferritic heat resistant steels at 923 K is investigated in the present study. First, internal pressure creep tests were performed to investigate the features of cracking at 923 K and at stresses of 108–167 MPa. Results showed that at low stresses cracks occurred in the fine grained heat affected zone (FGHAZ), showing low deformation, a vast amount of creep voids and short creep life: these were identified to be type IV cracks. However, as the stress was increased the crack locations moved further away from the fusion line, the creep void area density decreased, and the slope of the main crack ratio curve increased. At high stress, cracks occurred in the base metal and were identified to be normal cracks. Second, creep tests were carried out on one pass welded specimens to clarify the relationship between the creep property deterioration in the FGHAZ and the weld thermal cycle. Type IV cracking was repeated successfully and results showed good correspondence with the position of cracks among the softened region, the highest creep void area density and the fracture location. Both internally pressurised and one pass welded specimens showed that at low stress the FGHAZ was the weak zone and creep voids played an important role in type IV cracking.

Estimation of Creep Damage for the Components of Mod. 9Cr-1Mo Steel. 3rd Report, Creep Damage Assessment for Seam Weld in Internally Pressurized Pipe.

In order to establish the assessment method of creep damage for high temperature piping components made of modified 9Cr-1Mo steel with seam weld, subjected to internal pressure, the internal pressure creep tests and FEM stress analysis were done for cylinders with seam weld. In internal pressure creep tests, the creep rupture occurred at fine grained heat affected zone (F. G. HAZ), caused by cracking originated from the formation of voids. The correlation between void area fraction and creep damage was obtained. The morphology of voids were quite similar to that observed in welded joint specimens in uni-axial creep tests. Stress distribution of seam weld cylinder under internal pressure creep showed the higher circumferential stresses in F. G. HAZ where creep rupture occurred. The distribution of circumferential stress and creep damage calculated by using circumferential stress along specimen thickness were quite similar to that of void area fraction in internal pressure creep test. The creep damage in seam weld pipe under internal pressure creep could be estimated by the observation of void area fraction in F. G. HAZ.

Estimation of Creep Damage for the Components of Mod. 9Cr-1Mo Steel. 4th Report. Creep Damage Assessment for Circular Seam Welded Joint in Internally Pressurized Pipe.

In order to establish the assessment method of creep damage for high temperature piping components made of mod. 9Cr-1Mo steel with circular seam welded joint, the internal pressure creep tests and FEM stress analysis were done. Internal pressure creep tests showed the creep rupture occurred in the coarse grained heat affected zone (C. G. HAZ), caused by cracking originated from the formation of voids on grain boundaries. The void parameters such as void area fraction, density of void population and A parameter correlated well with time fraction in internal pressure creep tests. Vickers hardness decreased with creep times and correlated well also with time fraction of internal pressure creep tests. Stress distribution of cylinder with circular seam welded joint under internal pressure creep showed the maximum circumferential stress in C. G. HAZ where creep rupture occurred. The distribution of circumferential stress and creep damage calculated by using circumferential stress along specimen thickness were quite similar to that of void area fraction. The creep damage of circular seam welded joint under internal pressure creep could be estimated by the observation of void and the FEM stress analysis.

516 内圧クリープ試験による改良 9Cr-1Mo 鋼溶接継手のクリープ損傷評価

516 内圧クリープ試験による改良 9Cr-1Mo 鋼溶接継手のクリープ損傷評価

高分子系複合材料 ＧＦＲＰ積層管のクリープ特性評価法の一考察

The purpose of the present work is to estimate the behavior of creep deformation of GFRP pipe under an internal hydraulic pressure where the both ends are free. The GFRP pipe is developed for renewal of an overage drain pipe or gas pipe. It is mainly made up of two layers where the glass fiber mat is used in the inside layer for corrosion resistance and the woven roving cloth is as the outside layer for pressure resistance.Before the internal hydraulic pressure creep test, the tensile creep tests are carried out with the dumbbell type specimen which is cut from the uncured laminated pipe and then is cured. The creep constitutive equation is obtained using the tensile creep data of short-term tests, by taking account of stress, temperature and tensile angle to reinforced glass fiber. Based on this creep constitutive equation, long-term creep behavior can be predicted. A reasonable comparison between the prediction and the tensile experiment is observed at time up to 106 second. The internal hydraulic pressure creep test on the pipe verified the creep constitutive equation.

溶接部のＨＡＺ軟化域を考慮したクリープ損傷解析

The uniaxial creep rupture strength of Mod. 9Cr-1Mo steel welded joints is lower than that of the base metal, due to the existence of the heat affected zone (HAZ) softening region which is formed by welding heat. In this paper, the multiaxial creep damage of welded joints is evaluated analytically using the Finite Element Method (FEM), so as to study the width of HAZ softening region that affects creep damage.To begin with, creep strain equations were established based on the result of uniaxial creep tests. The uniaxial creep damage was evaluated analytically using these creep strain equations, and the result agreed with the experimental result. It was found from the uniaxial creep damage analysis that the effect of HAZ softening region on creep rupture strength is negligible, in the case of relatively narrow HAZ softening region compared with the diameter of welded joints.The analytical result of internal pressure creep tests using the FEM was correlated well with the experimental result. Therefore, the creep damage of welded joints including HAZ softening region can be evaluated analytically.

Material Properties of a Natural UF6Transport Cylinder Vessel at High Temperature

AbstractThe IAEA Regulations for the Safe Transport of Radioactive Material are to be revised in 1996 and the fire test (800°C for 30 min) could become a requirement for the natural UF6 transport cylinder. ASME SA 516 carbon steel is used as the structural material for this type of cylinder. It is very important to obtain high temperature data for SA 516 steel to be able to evaluate the integrity of the UF6 transport cylinder vessel in the fire test. CRIEPI has therefore conducted material tests on SA 516 at high temperatures. The AC1 and AC3 transformation points of actual SA 516 steels have been measured. Tensile tests up to 900°C were conducted using USA, French and Japanese manufactured materials and the influence of phase transformation assessed. Preliminary creep tests show that assessment by creep strength can give a more conservative estimation than using the tensile strength. Creep deformation equations have been obtained using uniaxial creep tests and internal pressure creep tests. In addition, ...