Induction Heating Stress Improvement Research Articles

The Effect of Induction Heating Stress Remedies on piping reliability

To address the effect of Intergranular stress corrosion cracking (IGSCC) on the probability of a leak in BWR piping, we have introduced some changes to the model of IGSCC for the Probabilistic fracture mechanics PRAISE code. This semi-empirical model is based on experimental and field data compiled from several sources. Using probabilistic techniques, the model addresses various stress corrosion phenomena, including crack growth, crack initiation, and linking of multiple cracks. The model also considers the effect of residual stresses in addition to cyclic stresses resulting from normal plant operation. Residual stresses which are an inherent result of conventional welding processes have long been known to play an important role in the failure of structural components. Such stresses can have a beneficial or detrimental influence, depending primarily on whether they are tensile or compressive at the inside surface of the pipe. Residual stresses undoubtedly are influential in both the initiation and propagation of cracks. The purpose of this paper is to review the influence of residual stresses on the reliability of piping under IGSCC. A procedure such as induction heating stress improvement (IHSI) is a means of altering the residual stress pattern in a pipe. Examples of the use of this technique to the initiation and growth of stress corrosion cracks in pipes are presented for various pipe sizes and residual stress distributions. The effectiveness of IHSI as an eternal countermeasure for IGSCC depends upon the level of service stress as well as the crack size and its large benefit is immediately apparent.

Effects of remedial actions on small piping reliability

This article describes probabilistic calculations that address intergranular stress corrosion cracking of stainless steel piping; a degradation mechanism of major concern to nuclear pressure boundary integrity. The objective is to simulate the cracking of stainless steel piping under intergranular stress corrosion cracking conditions, and to evaluate the structural reliability using remedial actions for intergranular stress corrosion cracking that are limited to benefits of in-service inspections and the induction heating stress improvement process. The results show that an effective in-service inspection requires a suitable combination of flaw detection capability and inspection schedule, and it has been suggested that the residual stresses could be altered to become favorable, thereby improving the reliability piping.

原子炉一次系配管の高周波加熱による残留応力の改善 (IHSI) に関する理論解析と実験

原子炉一次系配管の高周波加熱による残留応力の改善 (IHSI) に関する理論解析と実験

ICONE15-10244 DEVELOPMENT OF COOLING TECHNIQUES FOR INDUCTION HEATING STRESS IMPROVEMENT OF REACTOR RECIRCULATION INLET NOZZLE

Induction heating stress improvement (IHSI) has been used in nuclear power plants to reduce residual stress in welded sections of pipes by generating temperature differences between the inner and outer surfaces of the pipes. The outer metal surface is heated by induction heating, and the inner surface is cooled by flowing water. However, it is difficult to obtain a sufficient temperature gradient in the places where the flow stagnates and the heat transfer cannot be enhanced. In the present study, we developed cooling techniques for a reactor recirculation inlet nozzle with a closed end and very narrow annular channel. Computational fluid dynamics (CFD) analyses, half-scale tests, and full-scale tests were conducted to investigate the flow and cooling characteristics. One million grids of a reactor recirculation inlet nozzle model were used for the CFD analysis. Detached eddy simulation (DES) was used as the turbulence model to evaluate the unsteady phenomena of the jet flow and temperature distribution. The experimental apparatuses used for the half-scale tests were made of acryl to visualize the flow, and heat transfer coefficients were measured at the welded portions. In the full-scale tests, the temperature differences between the inner and outer surface of the recirculation inlet nozzle were measured, and reduction of the residual stress was verified. It was confirmed that the jet flow moved up and down when two jet nozzles were arranged symmetrically. The turbulence due to self-sustained jet fluctuation was effective for uniform cooling in the reactor recirculation inlet nozzle. The flow did not stagnate around the welded portion. The heat transfer coefficients at the welded portion were evaluated using an equation with Reynolds and Nusselt numbers in half-scale tests. It was also verified in full-scale tests that the temperature difference between the inner and outer surfaces of the recirculation inlet nozzle was approximately 490℃, which satisfied the criteria of 446℃.

Reduced stress welding process for nuclear plant piping

This paper discusses the development and application of FineLine™ Welding (FLW). FLW is a modified Gas-Tungsten-Arc Welding mechanized process developed by GE Nuclear Energy. The FLW process offers significant improvements over standard and narrow-gap welding. For piping, FLW reduces the weld groove width and volume of weld metal compared to standard ‘V’ or J-Bevel welds by approximately 30% for smaller pipe and 70% or more for larger sizes. Also, extremely low heat inputs are achieved, and typically result in: (1) significant reductions in welding elapsed time, particularly for thicker walls, (2) compressive/very low tensile residual stresses on the pipe inside surface, and (3) improved heat-affected-zone microstructure, with reduced shrinkage/distortion. The improved residual stress and metallurgical states provide increased resistance to stress corrosion cracking in high temperature, oxidizing water. The FLW process is successful in eliminating the need for secondary stress improvement process such as induction-heating stress improvement. Verifications of the stress improvement were performed on standard V-groove and narrow-gap welds, which were compared to thin and thick-wall stainless steel pipe FLW welds. Testing included strain-gage, X-ray diffraction, magnesium chloride and metallographic evaluations. The results show significant improvement in the residual stress level and uniformity of the FLW welds.

Elasto-plastic analysis of thin shells under thermal transients

The objective of this paper is to use a thermal-elasto-plastic algorithm developed by the authors in order to find the temperatures and thermal stresses that are produced by time-dependent thermal loads, such as the ambient temperatures and prescribed surface heat fluxes. These loads must be given as functions of time. With the thermal conductivities and surface film coefficients given for heat conduction, and the elastic and plastic material properties for stress analysis, the outcome is the temperature, displacement, and stress distributions that are produced by the thermal transients. We apply the analysis to thin shells; that is, objects that have one dimension, wall thickness, smaller than any other length. Even though the ideas given here appy to shells of general shape, we limit the analysis in this paper to cylindrical shells and consider axisymmetric thermal loads only. For verification, the results of a simple problem are compared with those of an established finite element code, and the agreement is noted. The algorithm presented here is well suited for the calculation of residual stresses that are produced by cyclic thermal loads. It is applied to the induction heating stress improvement (IHSI) in pipes. The general features of the IHSI are reproduced.

Recent topics of flaw prevention

Flaws resulting from corrosion are still a significant issue in nuclear power plants. Prevention of these flaws is one of the biggest themes of plant maintenance and new countermeasures have been developed year by year. In this paper three newly proposed techniques are presented: (1) ceramics coatings on flange seal surface by surface hardening due to laser-alloying to prevent pitting corrosion; (2) Induction Heating Stress Improvement (IHSI) on nozzle safe-end with thermal sleeve by using forced cooling; and (3) TIG Torch Heating Stress Improvement (THSI) to reduce welding residual stress on small diameter pipes.

サーマルスリーブ付ノズルの高周波加熱応力改善法の開発

The Induction Heating Stress Improvement (IHSI) technique is regarded as one of the most effective remedies for intergranular stress corrosion cracking (IGSCC) cocuring in the heat affected zone (HAZ) of susceptible stainless steel in some boiling water reactor piping systems. In this process, cooling water must flow at a velocity high enough to keep the inside surface of the pipe relatively cool, creating a high temperature gradient through the pipe wall. However, nuclear plant pipings, where forced flowing during this process may be difficult, have been welded. This paper presents computer simulation analyses and experimental verifications of post-IHSI residual stress distribution of the welded portion of a nozzle with a thermal sleeve, where forced water cooling is difficult. From these analyses and experiments a new IHSI technique is developed. This technique consists of new heating method which has temperature grandient through both the pipe wall and longitudinal direction.

Evaluation of intergranular cracks on the ring header cross at Grand Gulf Unit No. 1

A metallurgical investigation was performed on a sample of cracked ring header cross material from the Grand Gulf Unit No. 1 Nuclear Power Station. The cracks were located in a 6–7 in (15–17·5 cm) width band running circumferentially below the cross to cap weld with a similar band above the cross to discharger pipe weld. The indications were up to 19 mm in length and 6·0 mm in depth. This particular sample was cut from a cross which had not seen actual service but which had been used to qualify the induction heating stress improvement (IHSI) technique for the Grand Gulf units. The base material was SA 182 material manufactured to SA 403-type WP 304 stainless steel. The investigation consisted of visual/dye penetrant examination, chemical analysis, hardness testing, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. The evaluated cracks were intergranular and initiated on the forging's exterior surface. The grain size of the material was larger than ASTM 00 and no definitive corrosive species were found by Energy Dispersive Spectroscopy (EDS). The cracking is considered to be the result of the forging having been overheated/burned during manufacture.

IHSI application to the welded joint with small cracks

Induction Heating Stress Improvement (IHSI) has been proved as a strong countermeasure for intergranular stress corrosion cracking (IGSCC). IHSI is also expected to suppress further crack extension of existing cracks. The applicability of IHSI to a flawed pipe was investigated using a thermo elasto-plastic FEM program. IGSCCs with depths up to 50% of the pipe thickness are considered. It is found that there is a large margin for the extension of the crack during IHSI. Effectiveness of IHSI as an eternal countermeasure for IGSCC depends upon the level of service stress as well as the crack size. A long circumferential crack will be prevented from further extension by IHSI if the depth is under 30% of the thickness and the service stress is under 0·75 S m . For practical use, a simplified procedure to predict IHSI effectiveness for preflawed pipe is proposed making use of calculation of K by the superposition principle.

Weld overlay of BWR flawed piping

The occurrence of intergranular stress corrosion cracking (IGSCC) in the reactor water clean-up (RWCU) system and the recirculation systems in Boiling Water Reactors (BWR) has plagued utilities in the United States over the past few years. There are those who believe that if significant IGSCC exists, the only long-term solution is to replace the affected piping. Nevertheless, there are also those who contend that if only moderate flaws exist, the application of induction heating stress improvement (IHSI) and/or the installation of a hydrogen injection system may provide for a long-term solution to the problem. In the meantime, flawed pipes have been repaired with a weld overlay procedure to allow utilities the necessary time to plan and prepare for a permanent resolution. This paper describes the evolution of weld overlay repair designs and suggests that if the obstacle of inspectability can be overcome, it may be possible to consider the weld overlays themselves as the long-term solution.

Evaluation of stainless steel pipe cracking: Causes and fixes

This paper discusses (1) studies of impurity effects on susceptibility to intergranular stress corrosion cracking (IGSCC), (2) intergranular crack growth rate measurements, (3) finite-element studies of the residual stresses produced by induction heating stress improvement (IHSI) and the addition of weld overlays to flawed piping, (4) leak-before-break analyses of piping with 360° part-through cracks, and (5) parametric studies on the effect of through-wall residual stresses on intergranular crack growth behavior in large diameter piping weldments. The studies on the effect of impurities on IGSCC of Type 304 stainless steel show a strong synergistic interaction between dissolved oxygen and impurity concentration of the water. Low carbon stainless steel (Type 316NG) appear resistant to IGSCC even in impurity environments. However, they can become susceptible to transgranular SCC with low levels of sulfate or chloride present in the environment. The finite-element calculations show that IHSI and the weld overlay produce compressive residual stresses on the inner surface, and that the stresses at the crack tip remain compressive under design loads at least for shallow cracks.

外面高周波誘導加熱によるステンレス鋼配管溶接部内面の残留応力改善

A new technique called induction heating stress improvement (IHSI) has been developed in Japan, as a countermeasure for pipe cracking in boiling water reactors. The objective of this method is to improve the residual stress distribution by means of heating the outside surface and cooling the inside surface of pipe weldments.Prior to the application for this process on boiling water reactor piping, numerous experiments including full sized mock-up tests and stress analysis using finite element method were conducted in order to verify the effectiveness of IHSI. Thus, IHSI has successfully been applied to the plants under construction, as well as the operating plants.In this report, the essential information regarding IHSI application described above are summarized.

Residual stresses in girth butt welded pipes and treatments to modify these

A number of Type 304 stainless steel pipes are used in the primary cooling systems of nuclear plants. Intergranular stress corrosion cracks (IGSCC) were found at some welded joints in these piping systems due to very high tensile residual stress, sensitization of the material due to welding, and corrosive environment, all occurring simultaneously. Investigations have shown that at least one of the above factors must be eliminated to prevent IGSCC. This report describes experimental results on the temperature variations during pipe welding by conventional techniques and by the heat sink welding (HSW) technique. The mechanism of residual stress generation due to welding is also discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It was found that the temperature distribution through the thickness of the pipes was almost uniform for the conventional welding technique, but had a very sharp gradient for HSW. In the pipe axial direction, the temperatures varied sharply for both welding techniques. This implies that the sensitization of metal due to HSW is lighter than that of conventional welding and that the residual stress on the inside surface of the heat sink welded pipe is compressive. The induction heating stress improvement (IHSI) method has been investigated analytically and experimentally. In the IHSI method, a pipe is heated with an induction coil while cold water is pumped through it. This causes a temperature gradient throughout the pipe wall which generates high thermal stresses. This, in turn, generates compressive stresses on the inner surface of the pipe. This method is designed to eliminate tensile residual stresses near the weld heat affected zone on the inner surface. Temperature analysis and subsequent thermoelastic-plastic analysis show that tensile weld residual stresses at a joint were changed into compressive stresses on the inner surface of a pipe. It was confirmed experimentally that these stresses suppressed fatigue crack propagation in the heat affected zone (HAZ) of a welded pipe. Therefore, the IHSI method is effective not only in preventing crack initiation but also in suppressing crack propagation. As for the relaxation of residual stresses, no significant relaxation was measured when external loads were applied at as much as 80% of the yield strength in the experiments.

Qualification of Induction Heating Stress Improvement for Mitigation of Stress Corrosion Cracking

This paper describes the efforts toward qualification of induction heating stress improvement (IHSI) for mitigation of intergranular stress corrosion cracking (IGSCC) in Boiling Water Reactor (BWR) piping. The IHSI process, which is applied to piping after it is fully erected, produces compressive residual stresses on the pipe inside surface in the vicinity of the weld heat affected zone (HAZ). The creation of these compressive stresses has been confirmed by surface and through-wall strain gage and X-ray diffraction residual stress measurements on 4, 10, and 16-in. dia Schedule 80 welded and IHSI treated pipes. Confirmation of increased resistance to IGSCC due to the IHSI process has been accomplished by full-sized 4-in-dia pipe tests in General Electric’s Pipe Test Laboratory. The pipe test utilized an environment of oxygenated high-purity water at 288°C (550°F). Axial loads were applied which exceeded the material 288°C yield strength.

Evaluation of induction heating stress improvement (IHSI) treatment applied to nuclear primary piping

A new technique, IHSI (induction heating stress improvement), has been developed to reduce the tensile welding residual stress in the surface of steel piping for nuclear power plants, so reducing the susceptibility to stress corrosion. It is shown in this paper that the technique is effective and safe even if it is applied to piping which contains a small initial crack which is impossible to detect by ultrasonic inspection.

高周波加熱を用いる応力緩和法の熱解析

Induction Heating Stress Improvement (IHSI) has been developed for the protection against Stress Corrosion Cracking (SCC) which often occurs at the inner surface of austenitic stainless steel pipings and in fact it has already been applied to the several pipings in the boiling water reactors. However, the heating condition during this treatment has been analyzed neither analytically nor experimentally.In this paper, we are concerned with the finite element analysis of electromagnetic field related to the heating condition of pipe during the IHSI treatment. In the analysis, the equation of vector potential component in the circumferential direction is solved assuming the axisymmetry of the geometry. Then, the calculation is made on the induced current in the pipe wall which gives the volumetric heat generation i.e. ohmic heating.Finally, using the solution of heating condition, we analyze the temperature field with the help of the finite element method. The result are compared not only with the calculation based upon the simplified one-dimensional heating condition but also with the experimental values.