Austenitic Pipe Welds Research Articles

오스테나이트계 배관 용접부의 반대편 원주방향 결점에 대한 초음파검사 검출능 평가

국내 원자력발전소 안전등급 설비의 오스테나이트계열 스테인리스강 배관 용접부는 가동전 · 중검사를 위해 ‘오스테나이트계 배관 용접부 초음파탐상검사 일반절차서(KPD-UT-02)’를 적용하고 있다. 그러나 배관-밸브 또는 배관-이음쇠(pipe-fitting)와 같은 일부 이음부에서는 용접부 설계특성과 용접비드 형상 등에 기인하여 탐촉자의 양쪽 접근이 제한되며 신뢰성 있는 검사가 어렵다. 본 연구에서는 오스테나이트계 배관 용접부의 초음파 탐촉자 접근제한으로 인해 한쪽면 검사가 수행되는 경우 검사체적 충족여부 및 용접부 반대편에 위치한 원주방향 결점의 검출 유효성을 확인하고자 용접부 외면비드가 평편하게 개선 처리된 결점시험편을 사용하여 실험을 수행하였다. 실험결과, 한쪽방향 접근만 가능한 오스테나이트계 배관 용접부의 경우에도 용접부 외면비드를 평편하게 가공처리하면 현행 기량검증 검사절차서를 적용하여 용접부 반대편 검사체적 내에 위치한 모든 원주방향 결점 검출이 가능함을 실험적으로 검증하였다.

Numerical weld modeling — a method for calculating weld-induced residual stresses

In the past, weld-induced residual stresses caused damage to numerous (power) plant parts, components and systems (Erve, M., Wesseling, U., Kilian, R., Hardt, R., Brümmer, G., Maier, V., Ilg, U., 1994. Cracking in Stabilized Austenitic Stainless Steel Piping of German Boiling Water Reactors — Characteristic Features and Root Causes. 20. MPA-Seminar 1994, vol. 2, paper 29, pp.29.1–29.21). In the case of BWR nuclear power plants, this damage can be caused by the mechanism of intergranular stress corrosion cracking in austenitic piping or the core shroud in the reactor pressure vessel and is triggered chiefly by weld-induced residual stresses. One solution of this problem that has been used in the past involves experimental measurements of residual stresses in conjunction with weld optimization testing. However, the experimental analysis of all relevant parameters is an extremely tedious process. Numerical simulation using the finite element method (FEM) not only supplements this method but, in view of modern computer capacities, is also an equally valid alternative in its own right. This paper will demonstrate that the technique developed for numerical simulation of the welding process has not only been properly verified and validated on austenitic pipe welds, but that it also permits making selective statements on improvements to the welding process. For instance, numerical simulation can provide information on the starting point of welding for every weld bead, the effect of interpass cooling as far as a possible sensitization of the heat affected zone (HAZ) is concerned, the effect of gap width on the resultant weld residual stresses, or the effect of the ‘last pass heat sink welding’ (welding of the final passes while simultaneously cooling the inner surface with water) producing compressive stresses in the root area of a circumferential weld in an austenitic pipe. The computer program feresa (finite element residual stress analysis) was based on a commercially available abaqus code (Hibbitt, Karlsson, Sorensen, Inc, 1997. abaqus user's manual, version 5.6), and can be used as a 2-D or 3-D FEM analysis; depending on task definition it can provide a starting point for a fracture mechanics safety analysis with acceptable computing times.

Alternative methods for postweld treatment of austenitic pipe welds to increase the operational safety of BWR plants

Experience has shown that austenitic piping is susceptible to IGSCC in the weld root area under BWR service conditions. Besides non-optimized materials, the residual stresses which are an inherent result of conventional welding processes are also responsible for this susceptibility. In the past, mechanical, thermal and welding post-treatment processes were developed and used with the objective of reducing tensile stresses in the root area. This paper discusses past experience and more recent developments, in particular the latest results with pipe welds treated by means of welding processes (last pass heat sink welding). These measures are suitable for producing compressive stresses in the medium-swept ID-HAZ of austenitic welds, or to at least significantly reduce the tensile stresses and thus practically eliminate the risk of IGSCC.

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.

Evaluation of [formula omitted] estimation scheme for flawed throughwall pipes

The accuracy of the EPRI J- integral estimation scheme for pipes with throughwall cracks and subjected to pure bending was assessed using available experimental data on circumferentially flawed throughwall pipes. The evaluations were performed using elastic plastic J- integral ( J) and tearing modulus ( T) analysis methods. The results indicated that the EPRI J estimation scheme solutions are unnecessarily conservative compared to results from pipe experiments. As a result of these evaluations an improved J estimation scheme is developed, which is shown to have improved accuracy compared to the original EPRI J estimation scheme. These results imply that the flaw evaluation procedures in the ASME Code on austenitic piping welds are conservative. These results also have applications to the leak before break fracture mechanics analyses.