The effect of a long post weld heat treatment on the integrity of a welded joint in a pressure vessel steel

Abstract As part of nuclear power plant ageing management, the increased probability of a need of repair welding must be taken into account along with the increase of plant lifetime. An essential prerequisite for successful and safe repair welding is that the applied welding procedures have been properly validated and qualified prior to their use. For instance, if no post-weld heat treatment can be performed and the desired tempering effect has to be based on temper-bead technique, a user needs to scan among several available repair welding procedures. A decision has to be made which of the procedures provides the maximum desired tempering effect with the case in question. This research is a part of a larger experimental effort developing repair welding techniques, and is a part of the Finnish Nuclear Power Plant Safety Research Programme SAFIR2022. The currently studied experimental repair welding case is a low-alloy steel mock-up with an austenitic cladding. Repair welding is assumed to represent a ‘worst-case’ scenario where a postulated linear crack-like defect exists beneath the cladding and might extend across the interface into the reactor pressure vessel steel side. This postulated defect will be removed by machining, and the thereby machined groove will be filled by repair welding using a nickel-base super alloy 52M filler metal by cold metal transfer-gas metal arc welding with a robotic arm. In this paper, different repair welding techniques and alternatives are shortly surveyed based on existing literature. Overall, published documentation was sparse. While only few studies were considered relevant in terms of established links to actual repair cases of under-cladding defects in reactor pressure vessels, others were mainly for modelling and simulation purposes without e.g. cladding groove preparation or the use of irradiation-embrittled material. Most of these procedures were based on the use of nickel-base alloy filler metal in the combination with temper-bead welding technique, with the aim at omitting both preheating and post-weld heat treatment. The main challenge in the repair weld design is to optimise all relevant welding parameters, including the thermal efficiency of temper-bead welding, in order to obtain a sound, defect-free weld with controlled reactor pressure vessel steel heat affected zone maximum hardness. In the simulations presented in the paper, the goal was to compute the resulting deformations, strains and stresses induced by the repair process and make a-priori estimates of the effectiveness of different repair techniques based on the numerical predictions. The numerical analyses allow the comparison of the procedures and enable selecting the one with most efficient combination of weld thermal cycles in terms of tempering and normalisation effects. The prediction of prevailing residual stresses is also important when further application of the component is considered. The paper is followed by Part II, in which the topics of experimental evaluation and material characterization of the repair weld are presented.

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Establishing Recommended Guidance for Local Post Weld Heat Treatment Configurations Based on Thermal-Mechanical Finite Element Analysis

Volume 3: Design and Analysis ◽

10.1115/pvp2016-63581 ◽

2016 ◽

Author(s):

Phillip E. Prueter ◽

Brian Macejko

Keyword(s):

Heat Treatment ◽

Finite Element Analysis ◽

Finite Element ◽

Crack Initiation ◽

Residual Stresses ◽

Pressure Vessel ◽

Post Weld Heat Treatment ◽

Element Analysis ◽

Non Linear ◽

Weld Heat

Post weld heat treatment (PWHT) is an effective way to minimize weld residual stresses in pressure vessels and piping equipment. PWHT is required for carbon steels above a Code-defined thickness threshold and other low-alloy steels to mitigate the propensity for crack initiation and ultimately, brittle fracture. Additionally, PWHT is often employed to mitigate stress corrosion cracking due to environmental conditions. Performing local PWHT following component repairs or alterations is often more practical and cost effective than heat treating an entire vessel or a large portion of the pressure boundary. In particular, spot or bulls eye configurations are often employed in industry to perform PWHT following local weld repairs to regions of the pressure boundary. Both the ASME Boiler and Pressure Vessel (B&PV) Code and the National Board Inspection Code (NBIC) permit the use of local PWHT around nozzles or other pressure boundary repairs or alterations. Additionally, Welding Research Council (WRC) Bulletin 452 [1] offers detailed guidance relating to local PWHT and compares some of the Code-based methodologies for implementing local PWHT on pressure retaining equipment. Specifically, local PWHT methodologies provided in design Codes: ASME Section VIII Division 1 [2] and Division 2 [3], ASME Section III Subsection NB [4], British Standard 5500 [5], Australian Standard 1210 [6], and repair Codes: American Petroleum Institute (API) 510 [7] and NBIC [8] are discussed and compared in this study. While spot PWHT may be appropriate in certain cases, if the soak, heating, and gradient control bands are not properly sized and positioned, it can lead to permanent vessel distortion or detrimental residual stresses that can increase the likelihood of in-service crack initiation and possible catastrophic failure due to unstable flaw propagation. It is essential to properly engineer local or spot PWHT configurations to ensure that distortion, cracking of adjacent welds, and severe residual stresses are avoided. In some cases, this may require advanced thermal-mechanical finite element analysis (FEA) to simulate the local PWHT process and to predict the ensuing residual stress state of the repaired area. This paper investigates several case studies of local PWHT configurations where advanced, three-dimensional FEA is used to simulate the thermal-mechanical response of the repaired region on a pressure vessel and to optimize the most ideal PWHT arrangement. Local plasticity and distortion are quantified using advanced non-linear elastic-plastic analysis. Commentary on the ASME and NBIC Code-specified local PWHT requirements is rendered based on the detailed non-linear FEA results, and recommended good practice for typical local PWHT configurations is provided. Advanced computational simulation techniques such as the ones employed in this investigation offer a means for analysts to ensure that local PWHT configurations implemented following equipment repairs will not lead to costly additional damage, such as distortion or cracking that can ultimately prolong equipment downtime.

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