Analysis of the deformation and damage development of a dissimilar weld operating in the creep regime

The analysis of the deformation and damage behaviour of stress rupture tests with specimens out of the dissimilar metal weld seam 12% Cr-steel welded with a nickel base electrode for alloy 800 exhibits two competing processes:

• - Crack initiation occurs along the melting line due to high thermal stresses;

• - Creep deformation and damage concentrates in a heat affected zone of the ferritic 12% Cr steel due to long term stresses. The velocity of stress relaxation determines the resulting damage mechanism. At high temperatures with predominant creep deformation the cracks initiated in the melting line arrest and the creep deformation is concentrated in the heat affected zone (HAZ). At lower temperature the fracture area along the melting line increases. Long term tests at 535°C lead to lower stress rupture values compared to the scatterband of X 20 CrMoV 12 1 due to the reduced cross section after crack initiation in the melting line.

The analysis of stress rupture tests leads to the conclusion that grinding of melting line cracks is a reasonable measure because of sufficient stress relaxation.     

Relation between size and density of nonmetallic inclusions and the J-integral at crack initiation

Two quantitative relations for the calculation of the fracture toughness of ductile materials available in the literature in mathematically closed form — the relation of Stroppe relating the critical value J_c of the J-integral to microstructural data as well as data of tensile tests and the relation of Hahn and Rosenfield relating fracture toughness and data of tensile tests — are applied to three conditions of different toughness of the 12% Cr-steel X 20 CrMoV 12 1.The microstructural parameters necessary for the calculation of the J-integral at crack initiation such as type, size, density and arrangement of nonmetallic inclusions as well as precipitates were determined for the three material conditions, X 20 CrMoV 12 1 optimized (low sulphur content), X 20 CrMoV 12 1 conventional (higher sulphur content) and the aged similar weld metal. The tensile tests and J—R-tests were performed at 150°C, where the energy absorbed corresponds to the upper shelf.Comparing the J_i-values calculated according to the equation of Stroppe with the J_i-values experimentally determined it is shown that the calculated values fall into the scatterband of the experimentally determined ones, showing a good agreement of calculated with the measured values for the three material conditions. In the case of the optimized X 20 CrMoV 12 1 the value of J_i, calculated according to the relation of Hahn and Rosenfield, corresponds to the measured one. However in the case of the conventional X 20 CrMoV 12 1 and the similar weld metal no more correspondence is found.     

Stress intensity factors for complete circumferential surface cracks at the outer wall of a pipe loaded by stress gradients

Stress intensity factors were calculated for complete circumferential surface cracks at the outer wall of a pipe by means of the weight functions method. The pipe with an inner radius to wall thickness ratio of 10 is loaded by axial tensile stress varying through the wall. The results are compared to stress intensity factors of circumferential cracks at the inner wall of a pipe.     

Modeling of weld residual stresses in core shroud structures

This paper presents a computational model to predict residual stresses in a girth weld (H4) of a BWR core shroud. The H4 weld is a multi-pass submerged-arc weld that joins two type 304 austenitic stainless steel cylinders. An axisymmetric solid element model was used to characterize the detailed evolution of residual stresses in the H4 weld. In the analysis, a series of advanced weld modeling techniques were used to address some specific welding-related issues, such as material melting/re-melting and history annihilation. In addition, a 3-D shell element analysis was performed to quantify specimen removal effects on residual stress measurements based on a sub-structural specimen from a core shroud. The predicted residual stresses in the H4 weld were used as the crack driving force for the subsequent analysis of stress corrosion cracking in the H4 weld. The crack growth behavior was investigated using an advanced finite element alternating method (FEAM). Stress intensity factors were calculated for both axisymmetric circumferential (360°) and circumferential surface cracks. The analysis results obtained from these studies shed light on the residual stress characteristics in core shroud weldments and the effects of residual stresses on stress corrosion cracking behavior.     

Arguments for the material choice of X 20 CrMoV 12 1 to be used for the high temperature pipework of THTR 300 as seen from an operators viewpoint

The steel X 20 CrMoV 12 1 has been employed for live steam pipework in power stations in the Federal Republic of Germany for approximately 20 years for temperature from 550 to 530°C. This steel has good creep strength properties which leads to thinner walls being necessary in comparison to inferior high temperature steels. Apart from its good creep resistance it has excellent toughness properties - expressed through rupture elongation and reduction of area from creep rupture tests or through the higher creep strength values for notched samples as against smooth samples. This steel has proved its value as no creep damage on the weld areas has yet occurred.     

Effect of biaxial loading on the fracture behaviour of a ferritic steel component

The effect of biaxial loading on the ductile behaviour of a through-wall crack in a ferritic steel structure under contained yield is of particular interest to the structural integrity argument for reactor pressure vessels. This results from the fact that there are many instances in practice (for example a crack in a circumferential weld), where a significant applied stress is present in the direction parallel to the crack as well as in the perpendicular direction. Two large plate ductile tearing tests have been performed on centre through-crack specimens (75 mm by 2 m by 2 m) manufactured from a ferritic steel. The first test specimen was loaded in uniaxial tension and the second test specimen was loaded biaxially. This paper presents experimental details and results of the two plate tests and describes the analysis work undertaken to interpret the experiments satisfactorily.     

The restraining effect of austenitic cladding on the extension of a three-dimensional crack into the wall of a water-cooled nuclear reactor pressure vessel during a hypothetical overcooling accident

A very simple analysis gives an upper bound to the restraining effect of the austenitic cladding on the stress intensity for the extension, at the deepest point, of a three-dimensional under-clad crack into the wall of a water-cooled nuclear reactor pressure vessel during a hypothetical overcooling accident. By comparing the upper bound results with results for a two-dimensional crack, it is concluded that cladding restraint is unlikely to provide a significant reduction in the stress intensity at the deepest point. This viewpoint is confirmed by additional results for a special case, namely that of a semi-circular under-clad crack.     

An interaction analysis of twin surface cracks by the line-spring model

The fracture mechanics analysis of surface cracks is important for the integrity evaluation of flawed structural components. The objective of this paper is to numerically investigate the interaction effect of twin surface cracks in plate and cylindrical geometries. First the usefulness of the line-spring model is verified by analyzing a single surface crack in a plate, and then the model is extended to twin surface crack in plate and cylindrical geometries. For the case of a finite plate under uniaxial loading, the effect of crack spacing on the stress intensity factor is negligible. However, for the case of a cylinder under moderate internal pressure, a significant increase in stress intensity factor is observed at the deepest point of the surface crack.     

Environmentally-assisted cracking behaviour in the transition region of an Alloy182/SA 508 Cl.2 dissimilar metal weld joint in simulated boiling water reactor normal water chemistry environment

The stress corrosion cracking (SCC) and corrosion fatigue behaviour perpendicular and parallel to the fusion line in the transition region between the Alloy 182 Nickel-base weld metal and the adjacent SA 508 Cl.2 low-alloy reactor pressure vessel (RPV) steel of a simulated dissimilar metal weld joint was investigated under boiling water reactor normal water chemistry conditions. A special emphasis was placed to the question whether a fast growing interdendritic SCC crack in the highly susceptible Alloy 182 weld metal can easily cross the fusion line and significantly propagate into the adjacent low-alloy RPV steel. Cessation of interdendritic SCC crack growth was observed in high-purity or sulphate-containing oxygenated water under constant or periodical partial unloading conditions for those parts of the crack front, which reached the fusion line. In chloride containing water, on the other hand, the interdendritic SCC crack in the Alloy 182 weld metal very easily crossed the fusion line and further propagated with a very high rate as a transgranular crack into the heat-affected zone and base metal of the adjacent low-alloy steel. The observed SCC cracking behaviour at the interface correlates excellently with the field experience of such dissimilar metal weld joints, where SCC cracking was usually confined to the Alloy 182 weld metal.     

Pressure vessel nozzle repair

The dissimilar metal weldment joining Boiling Water Reactor (BWR) nozzles to safe-ends is one of the more complex configurations in the entire recirculation system. Field installation techniques typically specify that a special shop weld deposit (butter) be placed on the end of the nozzle prior to final shop post-weld heat treatment (PWHT) in order to facilitate field welding without PWHT. The shop weld deposit is normally an Inconel or stainless steel and does not require additional field heat treatment after welding. Soon BWR vessels used Inconel 182 manual shielded metal arc electrodes to weld deposit the butter. This manual process produces a deposit which is now known to be susceptible to intergranular stress corrosion cracking (IGSCC), especially under severe conditions such as crevices and/or cold work.Recently two BWRs have experienced, IGSCC of I-182 weld deposits in which axially oriented cracking progressed into the low alloy steel nozzle material. Although IGSCC extension from weld butter into nozzle material has been observed in other BWRs, these cases appeared to be the deepest. At Taiwan Power Company's Chinshan Unit 2 a repair has been completed in which the defect was verified visually, the old butter and defect removed by machining, the butter restored with PWHT and a new safe-end installed. At Carolina Power and Light's Brunswick Unit 1 a large number of deep axial indications have been identified on nine nozzles by ultrasonic examination. Weld overlay repairs using a temperbead procedure have been completed as a temporary measure while preparing for a permanent repair at the next refueling outage.Details of the cracking observations and repair activities are discussed. Concurrent repairs/replacements of safe-ends will also be described.     

State of the development and application techniques of the steel X 20 CrMoV 12 1

In most European and many other countries, the martensitic steel X 20 CrMoV 12 1 is used for the most highly stressed components of conventional power stations, such as superheater tubing and main steam pipe systems. This steel permits the use of much thinner wall thicknesses and a more flexible design as compared with lower alloyed steels. In the operation of such plants, there is the additional advantage of higher start-up velocities as opposed to the heavy wall piping systems. Accordingly, within the variety of high temperature steels, the X 20 CrMoV 12 1 steel is found to be the optimum link between the low-alloyed ferritic-bainitic and the high-alloyed austenitic steels.From the viewpoint of alloying, the high creep rupture strength of this steel is only attained by 12% chromium and the addition of molybdenum and vanadium. The high hardness level of the martensite formed in this steel following hot forming or welding has to be taken into account in further processing.     

Creep damage behaviour of 12% Cr steel

The creep damage behaviour of different batches of 12% Cr steel X 20 CrMoV 12 1 was investigated. An overall damage map as function of consumed life fraction was developed. This classification is being compared to that of TRD 508 guideline. Generally in X 20 CrMoV 12 1 cavitation and microcrack formation is shifted to higher life fractions. Recommendations for procedures with power plant components are given.     

Fatigue crack growth under residual stress field in low-carbon steel

The influence of residual stress on fatigue crack growth was experimentally and analytically investigated for surface crack. Fatigue tests were performed on straight pipe components of low-carbon steel having a circumferential inner surface crack in laboratory air environment. Some of the test pipes had been subjected to special heat treatments so as to have compressive or tensile residual stresses along the inner surface.The results show that the compressive residual stress remarkably suppresses the surface crack growth while the tensile residual stress doesn't accelerate the crack growth very much.The crack growth analyses were conducted by the application of power relationship between ΔK and . The stress intensity factors due to the non-linear stress field were calculated by the weight function method. The analyses resulted in a confirmation of the behavior of the crack growth observed in the experiments.     

Remaining life prediction of the core shroud due to stress corrosion cracking failure in BWRs using numerical simulations

Stress corrosion cracking (SCC) of the welded joints in a reactor core shroud is the primary result of the residual stresses caused by welding, corrosion and neutron irradiation in a boiling water reactor (BWR). Therefore, the evaluation of SCC propagation is important for the safe maintenance of the core shroud. This paper attempts to predict the remaining life of the core shroud due to SCC failures in BWR conditions via SCC propagation time calculations. First, a two-dimensional finite element method model containing H6a girth weld in the core shroud was constructed, and the weld processing was simulated to determine the weld's residual stress distribution. Second, using a basic weld residual stress field, the SCC propagation was simulated using a node release option and the stress redistribution was calculated. Combined with the J-integral method, the stress intensity factors were calculated at depths of 2, 3, 4, 8, 12, 16, 19, 22, 25 and 30 mm in the crack setting inside the core shroud; then, the SCC propagation rates were determined using the relation between the SCC propagation rate and the stress intensity factor. The calculations show that the core shroud could safely remain in service after 9.29 years even when a 1-mm-deep SCC has been detected.     

Environmentally assisted crack growth in a low alloy boiler steel in high temperature water containing oxygen

The crack growth behaviour of the higher strength steel 17 MnMoV 6 4 in deionized high temperature water containing oxygen was investigated with respect to static loading. The tests were performed in an autoclave testing facility at an oxygen content of 8 ppm, a pressure of 70 bar and a temperature of 240°C under quasi-stagnant flow conditions. The stress intensities at the beginning of the tests were 17, 20, 27, 34, 40 and 58 MPa√m. In contrast to the higher loaded specimens no evidence of environmentally assisted cracking was found at stress intensities of 17 and 20 MPa√m.The maximum crack growth rate in the range where only environmetally assisted cracking occured amounted to about 4 x 10^-5 mm/s. The critical stress intensity K_IJ for the onset of stable ductile crack growth in air is 100 MPa√m. On the basis of fractographic studies the crack development found can be ascribed to the “Strain Induced Corrosion Cracking (SICC)” mechanism. This mechanism, used as a working hypothesis, gives a satisfactory explanation for the occurence of stress corrosion cracking of unalloyed and low alloyed steels in high temperature water. SICC is particularly characterized by aggravated corrosive attack occuring as soon as the magnetite/haematite protective layer has been locally disturbed. The stress concentration then just becomes so great that in the region of the resulting crack tips, yield/creep deformation within the critical range of strain rates occurs.     

Proof of the fracture resistance of the pipes in the THTR 300 plant using high temperature burst tests with X 20 CrMoV 12 1 components

An adequate safety margin was demonstrated by fracture mechanics investigation between postulated flaws in components and the critical crack length. These calculations were experimentally verified by pipe burst tests under service-like conditions of the THTR-300 for the provided X 20 CrMoV 12 1 in the begin-of-life condition.The tests show that flaws with a length of the pipe diameter and a depth of 90% of the wall thickness

• - oriented axially in the base metal, and

• - circumferentially in the weld

surely satisfy the leak-before-fracture criteria. The strength behaviour of the pipes can be described conservatively within the relevant technical region by common fracture mechanics failure criteria except the plastic instability criteria.     

An analysis of stress waves in 12Cr steel, Stellite 6B and TiN by liquid impact loading

This research placed emphasis on the computer simulated stress distribution on the surface and in the bulk of the materials which are subjected to the water impact causing erosion damage. The erosion damage was predicted by evaluating the spatial and temporal stress wave distribution generated by water impact pressure on 12Cr steel and Stellite 6B as steam turbine materials and TiN as a hard coating material. There were two distinctive stress wave behaviors. Firstly, the large tensile stress at the surface was developed by the Rayleigh wave component which appeared between the water drop and the Rayleigh wave front. After the Rayleigh wave detached from the water drop, the materials were in the tensile stress state which could be related to fracture initiation. Secondly, the largest tensile stress in the bulk was near the surface due to the Rayleigh wave generated at the surface and decreased due to the enlargement of wave front as the radial distance increased. Rayleigh wave's shape was broadened due to the difference between the contact point velocity and the wave front velocity, while its value decayed exponentially in the depth direction. Also, there may be a tendency to produce a circumferential crack by σ_rr near the surface and a lateral crack by σ_zz in the sub-surface. The tensile stresses in TiN were much lower than those in 12Cr steel and Stellite 6B due to its higher wave velocity.     

Rupture of spent fuel Zircaloy cladding in dry storage due to delayed hydride cracking

Delayed hydride cracking in the Zircaloy alloy has been considered as a possible degradation mechanism of spent nuclear fuel cladding in interim dry storage. Some recent in-core fuel failures indicated that a long axial crack developed in the cladding was a secondary failure by delayed hydride cracking. The aim of this study is to define the effects of hydride reorientation on the failure of Zircaloy cladding. Different hydride orientations, the amount of zirconium hydride and various cracking types, all have been considered for their effects on the crack growth and stability of the cladding, and have been thoroughly discussed in this paper. A finite element computer code, ANSYS, has been used in conjunction with the strain energy density theory. In summary, the crack propagation will be aggravated if the hydride orientation is shifted from the circumferential to the radial direction. For a larger crack length, the zirconium hydride plays an important role in affecting the crack growth because the strain energy density factor increases as the hydride approaches the crack tip. Furthermore, when thermal effects are considered, a compressive stress exists at the inner side of the cladding, while a tensile stress is found at the outer side of cladding, thus resulting in crack propagation from the outer side to the inner side of the cladding. These findings are in accordance with other experimental results in related literature.     

Effects of thermal load and cooling condition on weld residual stress in a core shroud with numerical simulation

Stress corrosion cracking (SCC) in the heat affected zone is the primary damage form due to weld residual stress, corrosion and neutron irradiation environment in the core shroud of a boiling water reactor. The distribution of weld residual stress around a weld is necessary to be clarified to evaluate the structural integrity of core shroud for SCC. Moreover, studying the effects of welding parameters on residual stress on reducing the residual stress is very important to suppress the initiation and propagation of SCC.In this paper, we used a finite element method (FEM) to clarify the distribution of weld residual stress around the sixth horizontal weld (H6a) between the lower ring and the cylinder in the core shroud. The simulation results of axial stress were consistent with the experimental results at the inside and outside surfaces of the core shroud, respectively. The effects of thermal loads and cooling conditions were also investigated with the same model. We simulated the welding progress with water cooling on the inside and outside surfaces of the core shroud in order to study the influence of cooling conditions on the residual axial stress around the weld. The simulation results indicated that water cooling decreased the residual axial stress at the same side due to changing the temperature-affected fields. Moreover, with fixing the peak temperatures of weld passes, the simulation results of the distribution of residual axial stress by the thermal loads with different heating time were compared. The simulation results suggested that the heating time was expected to be longer and the heat flux to be smaller for reaching the small tension residual axial stress or even compression stress around the H6a weld.     

Toughness behaviour of the steel X 20 CrMoV 12 1 — Results of optimizing the base- and weld metal

The 12% Cr steels are frequently used in German power plants for tubings, pipes, rotors, and blades. The maximum operating temperature is limited by their creep strength properties to about 550°C. There are applications at even higher temperatures. Sufficient materials toughness is required for the base metal and weld metal to withstand sudden load changes. This is of special interest for use in nuclear power plants. Under operating conditions at elevated temperatures microstructural changes occur which greatly influence the toughness properties of both base metal and weld metal. This paper presents the results of ageing treatments at 550°C, carried out with a 12% Cr steel (DIN X 20 CrMoV 12 1) specifically optimized for toughness. The decrease in toughness is already evident at ageing times as low as 1000 h for conventional and optimized material. This drop in toughness is tentatively explained by differences in grain sizes and carbide content (M₂₃C₆ carbides). Detailed investigations indicate that additional carbide precipitation may significantly contribute to the decrease in toughness.