Detection Of Creep Damage Research Articles

Creep Damage in Steam Pipelines and the Use of Metallography in Verifying the Possibilities of Testing

With high operating parameters of the medium in medium-pressure and high-pressure steam pipelines of fossil power plants, creep damage occurs, especially in welded joints leading to complete rupture of the pipe wall in the last phase. Detection of creep damage at an early stage before major cracks may occur can prevent these accidents. For these purposes, phased array ultrasonic testing was performed using a high frequency probe. This testing was performed on real welded joints cut from the power plant. To verify the possibility of detection, metallographic analysis on the tested locations was performed. When comparing the results, it is possible to refine the interpretation of the ultrasonic data and the metallographic results can also be used in the eventual qualification of non-destructive testing.

Use of a stepped preparation to improve the damage tolerance of grade 91 steel welds

ABSTRACT Damage equivalent to that seen over very long times in Grade 91 steel welded components was successfully formed in laboratory, feature sized samples tested under controlled conditions. This paper compares the creep behaviour of welds made using a typical preparation with results for novel step weld geometries. When well-designed the step geometry offsets the heat affected zone (HAZ) in the through thickness of the weld which would be equivalent to the pressure boundary in a pipe. The step in the weld profile thus provides benefit to in-service performance because it prevents the rapid link up of aligned creep cavitation that forms in the HAZ of welds made using typical preparations. The results demonstrate that a well-designed step weld geometry increases the creep rupture life and improves damage tolerance. In turn, the increased duration of stable crack growth promotes the opportunity for detection of creep damage using nondestructive testing methods.

Analysis of the possibility of creep damage detection in T24 heat resistant steel with the help of magnetic nondestestructive testing methods

The paper presents the result of an analysis of applicability of various electromagnetic methods of nondestructive evaluation for creep damage detection in a novel heat resistant steel - T24 grade. Two sample sets, cut out from membrane wall tubes, were investigated – the as-delivered one and another exploited for 36,000 h in a power plant. There are described results of magnetic hysteresis loops B(H), Barkhausen noise (BN) and magnetoacoustic emission (MAE) signals measurements. Changes of the B(H) loops shape are observed mainly in the “knee” regions. The BN signal is practically unaffected by exploitation. The most strongly changing signal is the MAE signal, yet the change is not very easy to quantify as it concerns signal envelopes shape not overall intensity. The paper describes various possible signal parameters that can be used. The best one seems to be the ratio of the total pulse count for a quarter of magnetisation (demagnetisation) to the one for the magnetisation half-period. The important issue is in that case a proper choice of the threshold level in order to obtain good sensitivity to creep damage level and reasonably low stochastic pulse count scatter.

The use of the potential drop technique for creep damage monitoring and end of life warning for high temperature components

Electrical potential drop (EPD) is a well-established technique for the measurement of crack initiation/growth in metals. Two variants exist, one using AC excitation, the other using DC. EPD provides crack dimensions (principally depth) in contexts such as fracture/fatigue testing, and in-field NDE. Whilst it has been employed for on and off-line assessment of creep damage, use within a non-lab (i.e. industrial) context is limited by connection issues and, significantly, data interpretation – especially with regard to detecting subtle changes in EPD over general background ‘noise’. We describe, here, a methodology where high sensitivity detection of creep damage can be achieved by looking for a characteristic ‘signature’ within data. This is based on the combination of AC-EPD with its DC equivalent so as to generate a synergistic approach to damage detection. The methodology has been successfully applied to a semi-industrial context to provide prior warning of failure in excess of several weeks.

水素昇温脱離特性に及ぼすクリープボイドの影響

For the clarification of the creep damage detection mechanism by hydrogen, the simulation technique for investigating the effect of creep void on hydrogen thermal desorption characteristics of pure iron has been developed. The dissolved hydrogen was calculated by a kinetics calculation in consideration of trapping and dissociation of hydrogen in voids. In the interaction between hydrogen atoms and voids, the effect of dissociation and adsorption, recombination of hydrogen at the void surface was also taken into account. In addition, hydrogen diffusion inside of samples was calculated according to the diffusion equation. The void number density was used as an input parameter of the simulation. The hydrogen evolution curve obtained from the simulation did not accord with the experimental one. The latter might include the hydrogen desorption from the damages which were too small to be observed by a scanning electron microscope.

Potential drop detection of creep damage in the vicinity of welds

Recent research indicated that directional Alternating Current Potential Drop (ACPD) measurements could be exploited for nondestructive evaluation of creep damage. Unfortunately, creep damage is often concentrated in welded sections where the inspection is less straightforward. In this work, a directional square-electrode ACPD probe with spring-loaded contact pins was used to evaluate different levels of creep damage in the vicinity of welds. The electric transfer resistance was measured in two orthogonal directions to detect creep-induced material damage. This is particularly difficult close to weld/base metal interfaces because the electric conductivity usually changes significantly from one side of the interface to the other and therefore exhibits an apparent anisotropy caused by the influence of the boundary. The feasibility of directional ACPD evaluation of creep damage in the vicinity of such transition regions was studied by experimental, analytical and numerical means. Finite element simulations validated the analytical and experimental results and offered an opportunity to further investigate secondary effects, such as the inclination angle of the interface relative to the surface and the influence of an extended transition zone.

A review of non-destructive techniques for the detection of creep damage in power plant steels

The assessment of creep damage in steels employed in the power generation industry is usually carried out by means of replica metallography, but the several shortcomings of this method have prompted a search for alternative or complementary non-destructive techniques, ranging from ultrasonic to electromagnetic methods, hardness measurements and nuclear techniques. A critical review of the main results obtained to date in the secondary and tertiary stages of creep is presented in this paper, and the advantages and disadvantages of each method are discussed. Ultrasonic and potential drop techniques appear to be the most promising, but further research is needed before they are fully mature for deployment in the field.

Detection of creep damage in a nickel-based superalloy turbine bucket using eddy current imaging

Due to elevated temperatures, excessive stresses and severed corrosion conditions, turbine engine components are subject to creep processes that limit the components life such as a turbine bucket. The failure mechanism of a turbine bucket is related primarily to creep and corrosion and secondarily to thermal fatigue. As a result, it is desirable to assess the current condition of such a turbine component. This study uses the eddy current (EC) nondestructive evaluation technique in an effort to monitor the creep damage in a nickel base superalloy, 7FA stage 2 turbine bucket after service. The experimental results show a significative electrical conductivity variations in EC images on the creep damage zone of nickel-based superalloy samples cut from a turbine bucket. Thermoelectric power measurements were also conducted in order to obtain a direct correlation between the presence of material changes due to creep damage and the electrical conductivity measurements. This research work shows an alternative nondestructive method in order to detect creep damage in a nickel-based superalloy turbine bucket.

The characterisation of creep damage using non-destructive Barkhausen noise detection techniques

Industry today sees an ever-increasing demand for power and energy. Due to the environmental, time and financial constraints of building new power generation and petrochemical processing facilities it has become feasible to run existing plant and equipment beyond their original design lives. Considering this and the demand to achieve greater production targets there is an increased requirement for plant and equipment inspection. The causes, identification and consequences of creep damage on equipment within the power and energy industries are examined. The Electricity Supply Boards (ESBs) current approach to Non-Destructive Evaluation (NDE) of creep damage, its advantages and disadvantages are presented. This project concentrates on using Barkhausen noise techniques for the NDE of creep damage in Cr-Mo-V steels, as used in the manufacture of power generation and chemical processing plant and equipment. The two main Barkhausen noise detection methodologies, Magneto-Acoustic Emission (MAE) and Magnetic Barkhausen Noise (MBN), and how they can be applied to creep damage detection will be discussed. Barkhausen noise techniques will be shown to offer an alternative NDE solution.

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.

Full-size internal-pressure creep test for welded P91 hot reheat piping

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

Ultrasonic Creep Damage Detection by Frequency Analysis for Boiler Piping

Boiler piping of fossil-fuel combustion power generation plants are exposed to high-temperature and high-pressure environments, and failure of high-energy piping due to creep damage has been a concern. Therefore, a precise creep damage assessment method is needed. This paper proposes a nondestructive method for creep damage detection of piping in fossil-fuel combustion power generation plants by ultrasonic testing. Ultrasonic signals are transformed to signals in a frequency domain by Fourier transform, and a specific frequency band is chosen. To determine the creep damage, the spectrum intensities are calculated. Calculated intensities have a good correlation to life consumption of the weld joints, and this method is able to predict the remaining life of high-temperature piping, which has been already installed.

A study on detection of creep damage before crack initiation in austenitic stainless steel

The magnetic property changes in Type 304 stainless steel as creep damage progresses. The microstructure related to this magnetic property changes has been investigated by using SEM-EDX, SEM-EBSP, magneto-optical Kerr effect microscope and magnetic force microscope. One reason for magnetic property changes is the transformation from face-centered cubic to body-centered cubic phases with lower chromium content. These magnetic body-centered cubic phases are observed in grains along the stress axis or near the triple points, at which are sites of severe deformation. On the other hand, for aged materials magnetic phases are observed only at grain boundaries. The formation of magnetic phases is related to creep damage in Type 304 stainless steel.

A Study on the detection of creep damage in type 304 stainless steel based on natural magnetization

A study on non-destructive technique for creep damage of type304 stainless steel was performed. In this study, we concentrated on the magnetic field change with creep damage. This technique has a possibility that the damage before crack initiation is detected. We report that the results on the magnetic field change during creep damage progress, in order to examine the applicability of this technique based on magnetic field change to time-dependent damage. Though type304 stainless steel is a paramagnetic material, the leakage flux density of creep damaged specimen changes obviously, as compared with that of aged specimen.

Finite element simulation of magnetic detection of creep damage at seam welds

Using appropriately modified magnetization curves for each element of creep-damaged material, a finite element calculation has been carried out to simulate magnetic detection of non-uniform creep damage around a seam weld in a 2.25 Cr 1 Mo steam pipe. The magnetization curves for the creep-damaged elements were obtained from an earlier model for the magnetic effect of a uniformly creep-damaged material as given by Chen, et al. In the finite clement calculation, a magnetic C-core with primary and secondary coils was placed with its pole pieces flush against the specimen in the vicinity of the weld. The secondary emf was shown to be reduced when creep damage was present inside the pipe wall at the cusp of weld and in the vicinity of the cusp. The C-core detected the creep damage best if it completely spanned the weld seam width. Also, the current in the primary needed to be such that the C-core was not magnetically saturated.

Creep Damage Detection for a Steam Turbine Rotor Material after Long-Term Service Using Ultrasonic Technique.

The present paper describes an experimental study on a physical meaning of nondestructive creep damage detection for a steam turbine rotor material after long-term service using an ultrasonic technique. As a result of microstructure observation and density measurements, it was made clear that change in ultrasonic parameters such as attenuation and noise value was found to be attributed to nucleation, growth and coalescence of creep cavities with increasing creep damage. The ultrasonic technique was found to be valid for predicting remanent creep life of a fossil fuel plant steam turbine rotor.

Effects of high temperature creep on magnetic properties of steels (pipeline inspection)

An investigation was performed into detection of creep damage in the welded area of high-pressure, high-temperature steam pipelines. The measurements involved the use of a portable magnetic inspection device called the Magnescope. Results indicated a decline in the coercivity and remanence in creep-damaged pipes, probably due to segregation of impurities occurring during creep. The orientation of the inspection head with respect to the weld line affects the ability to detect creep damage. Measurements taken parallel to the weld line showed greater differences in the coercivity and remanence between the damaged and undamaged specimens. Measurements taken perpendicular to the weld line did not show large differences. >

APPLICATION OF MAGNETICALLY INDUCED VELOCITY CHANGES OF ULTRASONIC WAVES FOR NDE OF MATERIAL PROPERTIES

The technique of nondestructive evaluation (NDE) using magnetically induced velocity changes of ultrasonic waves is described, including its physical basis and the instrumentation used for its measurement. Examples of NDE applications of the method, including stress measurement and creep damage detection, are given. Other potential application areas, such as process control, detection of neutron irradiation damage, characterization of hydrogen attack, and monitoring of stress-relief heat treatment, arc also discussed

超音波の減衰特性を媒介としたＳＵＳ３１６鋼のクリープ損傷の非破壊的検出の試み

In order to attempt the nondestructive detection of creep damage by ultrasonic technique, the changing behavior of the energy attenuation coefficient of ultrasonic shear wave with creep damage was investigated by using SUS316 steel. Two kinds of energy attenuation coefficients, αst and αsL, were measured, in which the ultrasonic shear stress acted in the parallel and normal directions to the loading axis, respectively. The energy attenuation coefficients changed in a reasonable manner in which some intrinsic types of creep damages, such as the deformation of grains, the change of dislocation structure, the progress of aging, and the nucleation of microcracks, were reflected. Moreover, the relative ratio of αst/αsL monotonically increased with creep damage. From the above results, it was concluded that the absolute and relative properties of ultrasonic attenuation may be useful for the nondestructive creep damage detection of materials with high creep rupture ductility, as a potential measure.

Early detection of creep damage by ultrasonic and electromagnetic techniques

Residual lifetime analysis of components of power plants requires information on the degree of damage in the material. In the case of creep damage in components such as pipe bends, it is necessary to detect damage at the stage of micropore formation in order to ensure safe operation. Based on the influence of porosity on physical material properties (density, elastic moduli, electrical resistivity, coercivity), the potential of several NDT techniques for the detection of creep cavities is discussed. Changes in density and elastic moduli can be traced by ultrasonic velocity measurements. Experimental results obtained so far under laboratory conditions show rather good agreement with theoretical estimations. The practical applicability of the techniques used has still to be demonstrated, which is the objective of further work.