Weld overlay repairs from conception to long-term qualification

The weld overlay was initially developed in 1982 as an interim repair for intergranular stress corrosion cracking (IGSCC) flawed welds in boiling water reactor stainless steel piping. Since that time extensive work has been performed to qualify the weld overlay repair technique for longer-term service. Analytical studies of the weld overlay process, experimental programs on laboratory and field specimens, tests to demonstrate the beneficial effects of the weld overlay over its service life, and programs to develop nondestructive examination (NDE) techniques for use in inspecting weld overlays have all yielded positive results. These programs have been sponsored by the utility industry, either individually, through EPRI sponsorship or as part of the BWR Owners' Group (BWROG) effort, and by the US Nuclear Regulatory Commission (USNRC). Weld overlay repairs serve several design functions. These include: structurally reinforcing the flawed location to restore code margins to failure, providing an IGSCC-resistant barrier to crack growth, and imposing a favorable residual stress distribution in the inner portion of the flawed component to inhibit further flaw initiation and growth. The qualification programs confirm that these design functions are maintained with long-term service. A regulatory barrier to long-term operation with weld overlays was the need periodically to inspect overlays and thereby demonstrate continued integrity. Although IGSCC flaws have generally been detected by ultrasonic methods, the repair of these flaws by weld overlay made continued monitoring of the repaired flaw by conventional ultrasonic techniques difficult. Recent developments in ultrasonic examination equipment and techniques, including surface preparation criteria, have enhanced the inspectability of weld overlay repairs and the underlying base material. The original acceptance of weld overlay repairs by the USNRC was for limited service (one or two fuel cycles of operation). However, more recent regulatory guidance, including the recently issued NUREG-0313, Revision 2, recognizes the above technical advances, and provides criteria for acceptance of weld overlays as long-term repairs. Several US utilities have been successful to date in obtaining USNRC approval for extended operation with weld overlays. Some overlays at these plants are currently entering their fourth successful fuel cycle of operation.     

内燃机轴瓦新型镀层

为了满足现代内燃机高性能高指标和绿色环保对内燃机轴瓦越来越苛刻的要求,Daido公司最新开发了一种双层电镀层。经过试验台和样机的装机试验证实,轴瓦的各项性能指标大大提高。在流体动力工况下工作的轴瓦,采用合成(树脂质)镀层是Miba公司近年来的研究成果。已正式推介到发动机中应用。     

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.     

Development of probabilistic fracture mechanics analysis codes for reactor pressure vessels and piping considering welding residual stress

Probabilistic fracture mechanics (PFM) analysis codes for reactor pressure vessels (RPVs) and piping, called as PASCAL (PFM Analysis of Structural Components in Aging LWRs) series, have been developed. The PASCAL2 (PASCAL version 2) evaluates the conditional probability of fracture of an RPV under transient conditions including pressurized thermal shock (PTS) considering neutron irradiation embrittlement of the vessels. Recent improvements to PASCAL2 are related to the treatment of weld–overlay cladding. The results using the improved code indicate that the residual stress by weld–overlay cladding affects the fracture probability to some extent. The PASCAL-SP (PASCAL – Stress corrosion cracking at welding joints for Piping) evaluates the probabilities of failures including leakage and breaks of safety-related piping complying with Japanese regulation and rules. Effects of welding residual stress distribution as well as inspection accuracy are focused in this study. Residual stress distributions have been determined by parametric FEM analyses and incorporated into the code.     

相山铀矿田深部找矿论述

相山铀矿床田是我国最大、最富的火山型铀矿田。至今已发现邹家山、沙洲、云际、石洞等铀矿床,为华南铀成矿省北部赣杭构造火山岩铀成矿带与南部大王山—于山花岗岩型铀成矿带的复合叠加部位。该区成矿条件优越,深部找矿潜力巨大。通过深入研究该矿田的铀物质的来源以及成矿作用对深刻揭示其成因、形成机制,以及对其深部找矿和发展中国铀成矿理论等都有重要意义。     

Nation-wide transmission overlay design and benefits assessment for the U.S.

A U.S. nation-wide transmission overlay is a high capacity, multi-regional transmission grid, potentially spanning all three interconnections, designed as a single integrated system to provide economic and environmental benefits to the nation. The objective of this paper is to identify benefits to building a national transmission overlay and to lay out essential elements to facilitate continued dialog on this topic. A preliminary study performed on a national scale using a long term investment planning software illustrated that a national transmission overlay, under a high renewable penetration scenario, could result in cost-reduction of between one quarter trillion and one-half trillion dollars over a 40-year period, while promising to increase infrastructure resilience and flexibility.     

1000MW汽轮机高压内缸硬质合金堆焊技术的应用

阐述了硬质合金司太立21堆焊技术的试验及应用过程,通过采用焊接试验件模拟产品堆焊的方法,确定了在高压内缸密封面上堆焊司太立21的可行性,并得出优良的焊接工艺参数来指导生产。实践证明,堆焊后的密封面完全满足设计要求。     

Measurement and simulation of residual stress in type 304 weld overlay stainless steel pipe

Repair by welding overlay is a commonly used method mainly employed to rebuild piping systems suffering from intergranular stress corrosion cracking (IGSCC). It is desirable that the overlay welding technique, by attaching an overlay weld to the pipe and sustaining a heat sink of flowing water inside the pipe, induces a compressive residual stress at the inner surface of the welded pipe for prevention of IGSCC. A better understanding of the effect of a welding overlay repair on the residual stresses at the inner and outer surfaces of weld overlay is thus required. To obtain this understanding, it is necessary to investigate the distribution of residual stresses on the welded pipe.

In this study, the hole-drilling strain-gauge method was adopted to determine the residual stresses at the inner and outer surfaces of the weld overlay pipe. The incremental drilling technique was used on pipes with outside diameters of 267 mm. In addition, the Weld 3 code was applied to simulate the residual stress distribution for comparison and verification with the measured results.

The results obtained from the experimental and from the computational methods are in good agreement. The residual stress at the inner surface of the pipe is compressive with a magnitude approaching the yield stress of the material; that at the outer surface is tensile, also with a magnitude close to yield stress but smaller than the compressive stress. The experimental residual stress magnitude is generally greater than that from computation. This observation can be attributed to several factors including applied mechanics, temperature distribution, original residual stress, strain gauge location, mechanical grinding and the oxidation layer.     

钴基合金耐磨堆焊工艺及应用

本文通过对煤化工设备中灰锁防磨衬板的堆焊,详细介绍了钴基合金焊条电弧焊耐磨堆焊的焊接工艺参数、焊接过程的控制、采取的防变形措施及热处理工艺.     

Welding overlay analysis of dissimilar metal weld cracking of feedwater nozzle

Inspection of the weld between the feedwater nozzle and the safe end at one Taiwan BWR showed axial indications in the Alloy 182 weld. The indication was sufficiently deep that continued operation could not be justified considering the crack growth for one cycle. A weld overlay was decided to implement for restoring the structural margin. This study reviews the cracking cases of feedwater nozzle welds in other nuclear plants, and reports the lesson learned in the engineering project of this weld overlay repair. The overlay design, the FCG calculation and the stress analysis by FEM are presented to confirm that the Code Case structural margins are met. The evaluations of the effect of weld shrinkage on the attached feedwater piping are also included. A number of challenges encountered in the engineering and analysis period are proposed for future study.