Vessel Welds Research Articles

Proposed specimen for reheating cracking susceptibility and mechanical behaviour assessment in 2.25Cr–1Mo steel pressure vessel weld joint

Post-welding heat treatment (PWHT) is carried out on 2.25Cr–1Mo steel used in pressure vessels, to relieve welding residual stress and diffused hydrogen and to soften the weld joint. However, it is known that reheating cracks often occur during PWHT. A standard specimen is available to pre-estimate the existence of cracks in pressure vessels according to rules such as those in Japanese Industrial Standard WES 3005. However, these specimens do not reflect the shape of the pressure vessels and do not reproduce the actual cracks correctly because of non-cylindrical welding. In the present paper, a numerical analysis has been conducted for various models, which can reproduce the occurrence of reheating cracks in the welded joint of a pressure vessel nozzle. The present paper also investigates mechanical behaviour such as the relief history of welding residual stress as well as the accumulated history of creep strains. Through the above processes, the mechanical behaviour of the welded pressure vessel nozzle joints has been elucidated and a model of reheating crack initiation has been selected. In addition, experiments on reheating crack initiation have been carried out for the WES 3005 specimen, and a new susceptibility assessment specimen has been proposed and developed to pre-estimate the existence of actual reheating cracks correctly.

A microstructural examination of irradiated reactor pressure vessel weld samples

An important contribution to the life extension of Magnox nuclear power stations comes from the assessment of microstructural changes in reactor pressure vessel (RPV) weld metal after extended in-service exposure to a neutron flux. A project was undertaken to remove through-thickness weld samples from an RPV at a power station undergoing decommissioning. Sampling was carried out in a region of the reactor where the RPV was exposed to a high fast neutron irradiation dose at a low operating temperature. Each sample was cut into several specimens for mechanical tests to establish fracture toughness, chemical analysis and dosimetry data. This paper describes some of the preliminary results obtained from a microstructural characterisation programme. Samples were cut in various positions along the weld and at different depths through the vessel wall. These samples were examined by a number of techniques to evaluate the macro and microstructure after different amounts of neutron irradiation. In-situ micro-analysis and fractography was carried out on specimens after mechanical testing.

In-service electrochemical potential data in a deaerator feedwater storage vessel; the influence of a noble metal coating

The present paper describes the two separate test runs where the Electrochemical Potential, ECP, of two weldments, one in the normal oxidised condition and one coated with a layer of palladium, were monitored “in situ” in the storage vessel of a deaerator system in a 20 MW boiler. Although both the test runs, called gamma and delta, were dogged with abnormal operational conditions, viz. numerous off-load events occurred in one run and intermittent condenser leaks in the other, it was established that the Pd coated weld exhibited ECP levels which were consistently lower than the normal oxidised weld. Indeed the differences varied from 100 to 180 mV and averaged 125 mV. When the normal oxidised weld ECP data from the earlier test runs where little, or no, upset operational conditions occurred was compared to the present oxidised data sets it was evident that the initial conditions after going on-load were critical in determining the ECP levels attained for each test. If off-load events or oxygen leaks were prevalent during the initial 400 h, where the hysterisis effects were noticed, the subsequent ECP levels were approximately 200 mV more positive than those ECP levels attained from a test run where no upset conditions were present during the initial stages. Through the use of the beneficial effects arising from Pd coated welds, it was shown that the ECP values in real working deaerator storage vessel welds could be “conditioned”, i.e. lowered, to levels which were well below the critical ECP levels required to induce environmental assisted cracking, EAC, viz. E CRIT, during operation. Other locations in a boiler, viz. those that are water filled, which are prone to EAC can also be treated in a similar manner

A direct fracture toughness model for irradiated reactor vessel weld material based on reference temperature

The master curve method has opened a new means to acquire a directly measured material-specific fracture toughness curve based on testing a small number of replicate specimens. This process enables, for the first time, the construction of a material-specific fracture toughness curve for an irradiated material directly from fracture tests. Currently, only an inferred fracture model is available through a combination of the ASME Boiler and Pressure Vessel Code and a regulatory guide from the U.S. Nuclear Regulatory Commission. This approach uses the fracture toughness curve of a generic, unirradiated reactor vessel steel that is shifted by a reference temperature ( RT NDT) based on Charpy impact test data. The master curve method yields a key material parameter called reference temperature, T 0, which indicates the location of the transition range fracture toughness curve on the temperature axis. When a small number of pre-cracked Charpy specimens were tested at several different fluence levels, the material specific reference temperatures can be shown as a function of fluence. One such model for the WF-70 weld material is presented in this paper. The irradiated specimen data and analyses from Oak Ridge National Laboratory (ORNL) and the B&W Owners Group (B&WOG) are utilized for this model. This model is based on fracture toughness data, independent of Charpy impact energy levels, percent shear, and most importantly, material properties of unirradiated condition.

Department of energy’s annealing prototype demonstration projects

One of the challenges utilities face in addressing technical issues associated with the aging of nuclear power plants is the long-term effect of plant operation on reactor pressure vessels. These vessels are exposed to neutrons during the operation of a reactor. For certain plants, this exposure can cause embrittlement of some of the vessel welds, which can shorten the useful life of the vessel. This reactor pressure vessel embrittlement issue has the potential to affect the continued operation of a number of US pressurized water reactor plants. However, the properties that are degraded by long-term irradiation can be recovered through a thermal annealing treatment of the vessel steel. Although a dozen Russian-designed and several US military vessels have been successfully annealed, US utilities concluded that an annealing demonstration using a US reactor pressure vessel was a prerequisite before annealing a licensed US nuclear power plant. In May 1995, the Department of Energy and Sandia National Laboratories initiated a program to evaluate the feasibility of annealing US licensed plants using two different heating technologies. One team completed its annealing prototype demonstration in July 1996, using an indirect gas-fired furnace at the uncompleted Public Service of Indiana’s Marble Hill nuclear power plant in southern Indiana. The second team’s annealing prototype demonstration using a direct heat electrical furnace at the uncompleted Consumers Power Company’s nuclear power plant at Midland, Michigan, was scheduled to be completed in early 1997, but has now been delayed indefinitely. This paper describes the Department of Energy’s annealing prototype demonstration program and the results to date for each project.

Boat sampling and inservice inspections of the reactor pressure vessel weld No. 4 at Kozloduy NPP, Unit 1

The paper deals with reactor pressure vessel (RPV) boat sampling performed at Kozloduy Nuclear Power Plant, Unit 1, from August to November 1996. Kozloduy NPP, Unit 1 has no reactor vessel material surveillance program. Changes in the material fracture toughness resulting from the fast neutron irradiation which cannot be monitored without removal of the vessel material. Therefore, the main objective of the project was to cut samples from the RPV wall in order to obtain samples of the RPV material for further structural analyses. The most critical area, i.e. weld No. 4 was determined as a location for boat sampling. Replication technique was applied in order to obtain precise determination of the weld geometry necessary for positioning of the cutting tool prior to boat sampling, and determination of divot depth left after boat sampling and grinding of sample sites. Boat sampling was performed by electrical discharge machining (EDM). Grinding of sample sites was implemented to minimize stress concentration effects on sample sites, to eliminate surface irregularities resulting from EDM process, and to eliminate recast layer on the surface of the EDM cut. Ultrasonic, liquid penetrant, magnetic particles, and visual examinations were performed after grinding to establish baseline data in the boat sampling area. The project preparation activities, apart from EDM process, and the site organization lead was entrusted to INETEC. The activities were funded by the PHARE program of the European Commission.

Quantification of the toughness distribution in a heavy section submerged arc multilayer reactor pressure vessel weldment

In a working procedure qualification test weld representing a heavy section circumferential reactor pressure vessel (RPV) weld tested in 1968, lower toughness values were observed in the top layer region compared to those found in the filler region. Gleeble simulation, extensive microscopic evaluation, diligent Charpy V-notch testing and modelling of the bead sequence and distribution of alloying elements was applied to explain this effect. It could be revealed that the microstructure of the weld metal is the most important factor influencing the toughness. When an ‘as welded’ microstructure is partly or fully reaustenitised by the adjacent multilayer beads, the microstructure transforms and the toughness increases. In the filler region, 85% of the cross-section consists from transformed microstructure, whereas in the top layer only 20% are transformed. It is quite evident that, accidentally, the notch tip of Charpy samples in 1968 were placed in untransformed microstructures. When the top layer on the inner surface of the RPV is weld cladded by austenitic stainless steel, full transformation occurs and the toughness representing the filler region can be taken into account for safety evaluations.

Radiographic inspection of piping and vessel welds in existing plants : Sprague, J.L. Energy Week '96: American Society of Mechanical Engineers and American Petroleum Institute Energy Week Conference and Exhibition, Houston, TX (United States), 21 Jan. –2 Feb. 1996, pp. 217–220 (1996) ISBN 0964873184

Radiographic inspection of piping and vessel welds in existing plants : Sprague, J.L. Energy Week '96: American Society of Mechanical Engineers and American Petroleum Institute Energy Week Conference and Exhibition, Houston, TX (United States), 21 Jan. –2 Feb. 1996, pp. 217–220 (1996) ISBN 0964873184

Status of ITER research and development in the USA

The US Home Team, in close collaboration with the International Thermonuclear Experimental Reactor (ITER) Joint Central Team and other Home Teams, performs certain technology research and development tasks for the ITER Project. This paper provides an overview of the current status of such tasks. The US Home Team devotes most of its effort to the central solenoid model coil and the divertor cassette projects. Additional efforts are devoted to safety issues, first wall materials, vacuum vessel welding, ion cyclotron heating components, fueling and tritium components and diagnostics. In general, good progress has been made towards meeting the objectives of the engineering design activities phase.

Analysis of the Type IV creep failures of three welded ferritic pressure vessels

Creep rupture and creep crack growth assessments are performed on three 0.5Cr–0.5Mo–0.25V welded pressure vessels using reference stress methods. Each test vessel failed by steam leakage through the Type IV location of a weld. Estimated rupture times and failure locations are compared with the experimental results. The location of failure is shown to be correctly predicted and the calculated time to failure to be conservative with respect to the test results in each case. Estimates of creep crack growth from a pre-existing defect in the Type IV region of one vessel weld are also described. Conservative predictions of growth close to vessel failure are obtained.

In-service corrosion potential measurements in a series of feedwater deaerator systems in small peat-fired steam raising units

This paper gives an assessment of a series of studies aimed at establishing the corrosion potentials prevalent in working deaerator systems of small 20–40 MW peat fired steam raising units. Two Ag AgCl reference electrodes were inserted into each deaerator and their tips positioned at different feedwater storage vessel weld run locations. As well as corrosion potential, water chemistry parameters, unit load and feedwater temperature details were recorded. It was shown that the reference electrodes: (i) sensibly recorded the corrosion potential in deaerator feedwater storage vessel weldments, (ii) the level of corrosion potential was similar at different weld locations, (iii) the agreement between both readings in any vessel was excellent, and (iv) exhibited a range in durability with the 0·1 m reference electrode being better than the 3·0 m. An initial potential hysteresis, where corrosion potential (CP) values remained at higher levels than those predicted from potential-dissolved oxygen trends, was consistently observed in all three deaerators. Also, various sudden changes in corrosion potential were recorded and it was shown that these coincided with either normal working unit load fluctuations or off-load events. A significant influence of surface condition was evidenced, in that a bright clean weld surface recorded a CP value which was consistently over 150 mV more positive than that shown by an oxidised weld surface. Finally, it was concluded that the present deaerator system corrosion potential data compared well with other reported potential-dissolved oxygen trends and exhibited reasonable agreement with modern day empirical and deterministic models which predicted crack growth behaviour in stainless and low alloy steels in hot aqueous environments.

Electrochemical potential variations in a small 40 MW feedwater storage vessel—the influence of weldment surface condition

This paper describes a study performed to establish the real in situ electrochemical potential (ECP) prevalent in 40 MW deaerator feedwater storage vessel weldments. It was demonstrated that (a) the Ag AgCl reference electrode sensibly recorded the ECP over a period of 1000 h before failure, (b) transients (in the positive, more active direction) were the result of abrupt working unit initial load changes, (c) an initial ECP hysteresis lasting 400–500 h was recorded where the values remained much more positive than those normally associated with low dissolved oxygen levels and (d) a strong effect of weldment surface condition was evident inasmuch as the ECP values of the fresh bright weldment were consistently approximately 150 mV more positive than for the normal oxidised weldment.

Deaerator feedwater storage vessel weldment cracking: some fractographic and crack extension details

The present paper describes the results of a large field surveillance study aimed at understanding and mitigating crack growth in deaerator feedwater storage vessel weldments. It has been demonstrated that this particular type of cracking was widespread in nature, was primarily initiated at the bottom of significant surface corrosion pits and that subsequent subcritical crack growth was the result of a fatigue fracture process. The fatigue crack growth was made up of two different components, viz. an environmental assisted crack, EAC, portion and a pure mechanical portion. Detailed quantitative fractography established that the fatigue fracture surfaces contained a mixture of fanshaped, cleavage-like facets and rough ductile striated regions and that the proportions of these failure modes dictated the rate of fatigue crack extension. Finally the finding that fatigue was the primary crack extension process in deaerator weldments was further strengthened through a comparison of quantitative fractrography and crack geometry evolution with reported data from the literature.

NDE of heavy wall stainless steel cryogenic ASME pressure vessel welds : Sherlock, C.N.; Kruzic, R.W.; Howerton, H.K.; Phillips, J.R NDE for the Energy Industry 1995. Papers presented at The Energy and Environmental Expo '95. The Energy-Sources Technology Conference and Exhibition, Houston, Texas (United States), 29 Jan.–1 Feb. 1995. pp. 29–34. Edited by D.E. Bray. ASME NDE-Vol. 13 (1995) ISBN

NDE of heavy wall stainless steel cryogenic ASME pressure vessel welds : Sherlock, C.N.; Kruzic, R.W.; Howerton, H.K.; Phillips, J.R NDE for the Energy Industry 1995. Papers presented at The Energy and Environmental Expo '95. The Energy-Sources Technology Conference and Exhibition, Houston, Texas (United States), 29 Jan.–1 Feb. 1995. pp. 29–34. Edited by D.E. Bray. ASME NDE-Vol. 13 (1995) ISBN

Size effects on the upper shelf energy of a neutron irradiated pressure vessel weld metal

The methodologies published earlier for predicting the upper shelf energy (USE) of full size Charpy specimens based on subsize test data appear to work satisfactorily for either highly materials (USE > 200 J) or relatively brittle materials (USE ≪ 100 J). A methodology is proposed here that works well for pressure vessel weld materials in both unirradiated and irradiated conditions having USE in the intermediate region (100 J < USE < 200 J). The methodology uses partitioning of the USE into two components, USE p and δUSE ( = USE - USE p). USE p is the absorbed energy for a specimen fatigue-precracked to half the width. The predicted value of the USE of full-size specimens is a sum of two terms. The first term is equal to product of the normalized δUSE of the subsize specimen and the full-size normalization factor for δUSE. The second term is equal to the product of the normalized USE p of the subsize specimen and the fracture volume of the full-size precracked specimen. The predicted values were within about 10% of the measured values for both unirradiated and irradiated materials.

Conditions for achieving high-quality welds in the plasma-arc keyhole welding of structural steels

The present investigation describes the possibilities and the technological conditions for welding structural steels, especially high-strength steels, reproducibly and with high quality. The investigation comprises butt welding with an I-groove in the flat, horizontal vertical and vertical positions, and root welding of thick plates in the flat position. The investigation presents the welding-parameter combinations which results in the best quality welds. It has been shown that mechanized plasma keyhole welding is a very useful method for structural steels. It can compete successfully with common MIG/MAG and SAW welding as well as with electron and laser-beam welding, especially in large applications such as in the welding of large plates, box girders, tanks and pressure vessels, and large diameter pipes.

An advanced ultrasonic system for BWR vessel weld inspection

General Electric have developed a new ultrasonic imaging system for nuclear applications during the past four years. This system, the GERIS 2000, includes a novel ID manipulator and an acquisition system featuring full radio-frequency (RF) waveform recording of all data. The GERIS 2000 examines nozzles from their outside surfaces, and can examine seam welds from either the ID or the OD. During 1992, two European reactor pressure vessels were inspected from the ID surface. Currently, in the United States, examinations are in process on a vessel from the ID at one nuclear plant, and two nozzles from the OD at another plant.

Determination of efficient parameters for argon laser-assisted anastomoses in rats: macroscopic, thermal, and histological evaluation.

The aim of this study was to determine efficient parameters for an argon laser (spot diameter = 200 microns) to obtain and reproduce vessel anastomoses. It was performed in two groups of Wistar rats. In the first group (89 Laser impacts on 10 carotids), the fluence was continuously adjusted from 30 to 3,000 J/cm2 in order to determine efficient sets of parameters (power from 90-200 mW, pulse duration from 0.1 to 5 seconds, pulsed or continuous mode). In the second group, 30 end-to-end carotid anastomoses were performed. The results were evaluated by macroscopic thermal, and histological studies. The second group proved the efficiency of the selected parameters. Vessel welding was obtained with 100 mW, 3 seconds, continuous mode (fluence = 950 J/cm2, irradiance = 320 W/cm2) for a mean temperature of 77 degrees C corresponding to collagen denaturation. In the second group the patency rate was 93% (28/30) with three pseudo-aneurisms and two thromboses. Histological studies noted slight modifications of the media.

NDT performance demonstration in Spain

The experience obtained from the in-service inspection of reactor pressure vessels (RPV) of Spanish nuclear power plants and the participation in several international programs, such as PISC, has shown the need for a performance demonstration, not only for the ultrasonic inspection techniques of RPV, but also for other ISI non-destructive techniques as in the case of eddy current inspection of steam generator tubing. Section XI of the ASME Code, which is applied in Spain for ISI, has incorporated recently the Appendix VIII for performance demonstration of ultrasonic inspection techniques. As a direct consequence of this, a Spanish project for performance demonstration of ultrasonic inspection techniques has been launched recently, which includes the manufacturing of full-scale mock-ups of nozzle to vessel welds, reactor vessel welds, wrought austenitic piping welds and ferritic piping welds of PWR and BWR nuclear power plants from different suppliers. This considerable technical effort will let the different Spanish organizations which are part of the project to participate and colaborate with similar international projects and in particular with a European initiative for performance demonstration.

Atomic number contrast imaging and microanalysis of copper precipitates in irradiated ferritic pressure vessel steels

Abstract Nanometre-scale copper-rich precipitates in irradiated ferritic pressure vessel weld surveillance samples have been imaged directly in the field emission gun scanning transmission electron microscope. This is achieved by using the atomic number sensitivity of the high-angle elastically scattered electron signal. Microanalysis after location using this technique indicates that the precipitates contain phosphorus and manganese as well as copper. Preliminary results show that the smallest (sub-2 nm) precipitates to be observed have a high content of phosphorus and manganese relative to copper while in samples with larger precipitate sizes, in which thermal effects have been significant, copper predominates.