Potential Detrimental Consequences of Excessive PWHT on Pressure Vessel Steel Properties

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
Cédric Chauvy ◽  
Lionel Coudreuse ◽  
Patrick Toussaint
Keyword(s):  
Heat Treatment ◽  
Base Metal ◽  
Base Material ◽  
Heat Treatments ◽  
Pressure Vessels ◽  
Critical Parameters ◽  
Thick Wall ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Stress Relieving

During fabrication of Pressure Vessels, steels undergo several heat treatments that aim to confer the required properties on the entire equipment, including welds and base metal. Indeed, the production heat treatment of the base material, which leads to achieve the target properties, is most of the time followed by post weld heat treatment (PWHT). The aim of such treatments is to insure a good behavior of the welded zones in terms of residual stresses and obviously properties such as toughness. Generally, many simulated PWHT (up to 4 or more) are required for the testing of the base material, which can affect its properties and even lead to unacceptable results. In some cases for fabrication purposes an intermediate Stress relieving treatment can be required. Special attention is paid on C-Mn steels (e.g., SA/A516 from ASME BPV Code) with the effect of thickness and Ceq (International Institute of Welding Carbon equivalent formula: see page 3) requirements on the final compromise between properties and heat treatments. In particular, toughness and ultimate tensile strength (UTS) are the critical parameters that will limit the acceptance of too high PWHT. Although micro-alloying is a mean to increase the resistance to PWHT, this leads to difficulties in softening the heat affected zones. This solution is therefore not the best one considering the whole equipment optimization. Finally, the manufacturing process can play a major role when specifications are stringent. Quenching and tempering (Q&T) can indeed provide better flexibility in terms of PWHT and improved toughness for given Ceq and thickness. The case of Cr-Mo(-V) steels, which are widely used in the energy industry, is also addressed. Indeed, PWHT requirements for increasing the toughness in the weld metal can lead to decrease the base metal properties below the specification limits. For example, the case of SA/A387gr11 is very typical of metallurgical changes that can occur during these high PWHT leading to a degradation of toughness in the base metal. Another focus is made on the Vanadium Cr-Mo grade SA/A542D that must withstand very high PWHT (705 °C and even 710 °C) because of welds toughness issues. Optimization has therefore to be done to increase the resistance to softening and to guarantee acceptable microstructure, especially in the case of thick wall vessels. Some ways for improvement are proposed on the basis of the equivalent Larson–Miller parameter (LMP) tempering parameter concept. The basic philosophy is to fulfil the need for discussion between companies involved in pressure vessels fabrication so that the best compromise can be found to ensure the best and safest behavior of the equipment as a whole. In particular, the tempering operation can sometimes be done at lower temperature than PWHT in order to offer the best properties to the final vessel.

Potential Detrimental Consequences of Excessive PWHT on Pressure Vessel Steel Properties

2010 ◽  
Author(s):  
Ce´dric Chauvy ◽  
Lionel Coudreuse ◽  
Patrick Toussaint
Keyword(s):  
Heat Treatment ◽  
Base Metal ◽  
Base Material ◽  
Heat Treatments ◽  
Pressure Vessels ◽  
Critical Parameters ◽  
Thick Wall ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Stress Relieving

During fabrication of Pressure Vessels, steels undergo several heat treatments that aim to confer the required properties on the entire equipment, including welds and base metal. Indeed, the Quality heat treatment of the base material, which leads to achieve the target properties, is most of the time followed by Post Weld Heat Treatment (PWHT). The aim of such treatments is to insure a good behaviour of the welded zones in terms of residual stresses and obviously properties such as toughness. Generally, many simulated PWHT (up to 4 or more) are required for the testing of the base material, which can affect its properties and even lead to non acceptable results. In some cases for fabrication purposes an intermediate Stress relieving treatment can be required. Special attention is paid on C-Mn steels (e.g. SA/A516 from ASME BPV Code) with the effect of thickness and Ceq (IIW Carbon equivalent formula: see page 3) requirements on the final compromise between properties and heat treatments. In particular, toughness and UTS are the critical parameters that will limit the acceptance of too high PWHT. Although micro-alloying is a mean to increase the resistance to PWHT, this leads to difficulties in softening the heat affected zones. This solution is therefore not the best one considering the whole equipment optimisation. Finally, the manufacturing process can play a major role when specifications are stringent. Quenching and tempering can indeed provide better flexibility in terms of PWHT and improved toughness for given Ceq and thickness. The case of Cr-Mo(-V) steels, which are widely used in the energy industry, is also addressed. Indeed, PWHT requirements for increasing the toughness in the weld metal can lead to decrease the base metal properties below the specification limits. For example, the case of SA/A387gr11 is very typical of metallurgical changes that can occur during these high PWHT leading to a degradation of toughness in the base metal. Another focus is made on the Vanadium Cr-Mo grade SA/A542D that must withstand very high PWHT (705°C and even 710°C) because of welds toughness issues. Optimisation has therefore to be done to increase the resistance to softening and to guarantee acceptable microstructure, especially in the case of thick wall vessels. Some ways for improvement are proposed on the basis of the equivalent LMP tempering parameter concept. The basic philosophy is to fulfil the need for discussion between companies involved in pressure vessels fabrication so that the best compromise can be found to ensure the best and safest behaviour of the equipment as a whole.

Numerical Simulation of CrMo Steels Thick Welds Intermediate Postweld Heat Treatments

2005 ◽  
Author(s):  
Maryline Clerge´ ◽  
Christian Boucher ◽  
Sylvain Pillot ◽  
Philippe Bourges
Keyword(s):  
Heat Treatment ◽  
Complex Shape ◽  
Heat Treatments ◽  
Pressure Vessels ◽  
Post Weld Heat Treatment ◽  
Treatment Conditions ◽  
Stress Relieving ◽  
Creep Resistant Steels ◽  
Postweld Heat ◽  
Hydrogenation Treatment

During manufacturing, complex shape welded pressure vessels are submitted to numerous intermediate heat treatments after each weld (de-hydrogenation treatment - DHT and/or intermediate stress relieving treatment - ISR) before final Post Weld Heat Treatment (PWHT). The present study aims at analysing and optimising the intermediate heat treatment conditions regarding the resulting mechanical properties (tensile strength and impact. strength) of CrMo and CrMoV creep resistant steels. Hydrogen behaviour in weld metal and HAZ, and residual stresses evolution have been assessed by numerical modelling and experimental measurements on welded specimens representative of big pressure vessels: butt welds and set in nozzle welds of 150 mm wall thickness. The optimised conditions are compared to usual construction codes and buyer’s requirements.

Effect of Pre-Strain on Mechanical Properties of Pressure Vessel Steel Grades: Assessment of Stress Relieving / Post Weld Heat Treatments Efficiency at Regenerating Properties

2014 ◽  
Author(s):  
Sylvain Pillot ◽  
Carole Baudin ◽  
Stéphanie Corre ◽  
Deborah Heritier ◽  
Cédric Chauvy ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
High Temperature ◽  
Pressure Vessels ◽  
Low Alloy Steels ◽  
Pressure Vessel Steel ◽  
Cold Forming ◽  
Stress Relieving ◽  
Steel Grades ◽  
Weld Heat

Ensuring mechanical properties of carbon and low alloy steels after deformation is of major concern since the building process of heavy (i.e. thick-walled) pressure vessels may be directly impacted. Indeed, thick plates encounter forming and welding operations that may modify as-delivered properties. From both technical and economical points of view, cold forming is usually preferred. This technique is nowadays widespread and new rolling equipments display sufficient power to handle plates up to at least 250mm thick. Current limitations are now mainly related to maximum admissible strain in materials and regulation rules resulting from construction codes. The ASME Boilers and Pressure Vessels Construction Code on the American side and the EN 13445 Unfired Pressure Vessels Construction Code on the European side, both allow the use of as-strained material up to maximum 5% plastic (i.e. permanent) strain without any subsequent heat treatment operation. Above 5% plastic deformation, on one hand the European code requires a full quality treatment (meaning high temperature austenitization treatment, then cooling in air (normalizing – N) or in accelerated conditions (quenching – Q or accelerated cooling – NAC), followed by a Tempering treatment T) and on the other hand the ASME code only requires Tempering that can even be carried out using the mandatory Post Weld Heat Treatment (PWHT) needed by welded zones. However, it is of high importance to note that thick vessels are always submitted to a final PWHT to insure sufficient toughness in welded zones. This final PWHT is performed whatever the deformation obtained during plate rolling. In practice, there are no thick vessels made out of plates in as-strained conditions. Avoiding a full quality treatment as demanded per EN 13445 rules is of major interest for fabricators as it allows to decrease the delivery time, the risk of appearance of problematic issues (uncontrolled deformations of the vessel during high temperature treatments…) and significantly reduces the overall fabrication costs. This paper focuses on the effect of strain on conventional mechanical properties for steel grades widely used for the fabrication of heavy pressure equipments (i.e. tensile properties, hardness, Charpy V toughness) for different strain levels. In particular, it points out and discusses PWHT effects on properties of various pre-strained materials, showing that there is no need for full quality heat treatment.

Manufacture of Advanced 2.25Cr-1Mo-V Pressure Vessel Steel for High Pressure and High Temperature Hydrogen Service

2004 ◽  
Author(s):  
T. Hasegawa ◽  
A. Narita ◽  
H. Iga ◽  
S. Inoue
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Pressure Vessel ◽  
Base Material ◽  
Postweld Heat Treatment ◽  
Pressure Vessels ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Manufacturing Technologies ◽  
Welding Consumables

At this moment, the materials which are applied for high pressure and high temperature services are selected from conventional 2.25Cr-1Mo, 3Cr-1Mo-V and 2.25Cr-1Mo-V steels. Especially, the application of 2.25Cr-1Mo-V steel is increased extremely rather than the other steels in order to take the advantage of higher strength and better resistance to hydrogen damages such as hydrogen attack, hydrogen assisted crack growth and also hydrogen induced disbonding. In order to achieve these properties, the vanadium element is added intentionally into base material and welding consumables. Due to the addition of vanadium, the engineer have recognized that there is some possibility of cracking in the weldment due to the higher hardness and insufficient manufacturing practices caused by lack of knowledge for preheating, intermediate/final postweld heat treatment and other factors. The manufacturers have to establish the proper technologies to avoid this problem during fabrication stage while the pressure vessels are to be fabricated in their shop and/or fields. In this paper, the manufacturing technologies of 2.25Cr-1Mo-V steel pressure vessel through actual fabrication is presented herewith.

Measurements of Stress During Thermal Shock in Clad Reactor Pressure Vessel Material Using Time-Resolved In-Situ Synchrotron X-Ray Diffraction

2018 ◽  
Author(s):  
Sam Oliver ◽  
Chris Simpson ◽  
Andrew James ◽  
Christina Reinhard ◽  
David Collins ◽  
...  
Keyword(s):  
Thermal Shock ◽  
Pressure Vessel ◽  
Pressure Vessels ◽  
Reactor Pressure Vessel ◽  
Pressure Vessel Steel ◽  
X Ray Diffraction ◽  
Time Resolved ◽  
Vessel Steel ◽  
X Ray ◽  
Reactor Pressure

Nuclear reactor pressure vessels must be able to withstand thermal shock due to emergency cooling during a loss of coolant accident. Demonstrating structural integrity during thermal shock is difficult due to the complex interaction between thermal stress, residual stress, and stress caused by internal pressure. Finite element and analytic approaches exist to calculate the combined stress, but validation is limited. This study describes an experiment which aims to measure stress in a slice of clad reactor pressure vessel during thermal shock using time-resolved synchrotron X-ray diffraction. A test rig was designed to subject specimens to thermal shock, whilst simultaneously enabling synchrotron X-ray diffraction measurements of strain. The specimens were extracted from a block of SA508 Grade 4N reactor pressure vessel steel clad with Alloy 82 nickel-base alloy. Surface cracks were machined in the cladding. Electric heaters heat the specimens to 350°C and then the surface of the cladding is quenched in a bath of cold water, representing thermal shock. Six specimens were subjected to thermal shock on beamline I12 at Diamond Light Source, the UK’s national synchrotron X-ray facility. Time-resolved strain was measured during thermal shock at a single point close to the crack tip at a sample rate of 30 Hz. Hence, stress intensity factor vs time was calculated assuming K-controlled near-tip stress fields. This work describes the experimental method and presents some key results from a preliminary analysis of the data.

A52M/SA502 Dissimilar Metal RPV Repair Weld: Evaluation of Different Techniques

2020 ◽  
Author(s):  
Heikki Keinänen ◽  
Pekka Nevasmaa ◽  
Juha Kuutti ◽  
Caitlin Huotilainen ◽  
Iikka Virkkunen ◽  
...  
Keyword(s):  
Heat Treatment ◽  
Nuclear Power ◽  
Filler Metal ◽  
Post Weld Heat Treatment ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Repair Welding ◽  
Reactor Pressure Vessel Steel ◽  
Nickel Base ◽  
Reactor Pressure

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.

The Consequences of Macroscopic Segregation on the Transformation Behavior of a Pressure-Vessel Steel

2014 ◽  
Vol 136 (3) ◽  
Author(s):  
E. J. Pickering ◽  
H. K. D. H. Bhadeshia
Keyword(s):  
Mechanical Properties ◽  
Pressure Vessel ◽  
Large Scale ◽  
Pressure Vessels ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Chemical Segregation ◽  
Compositional Variations ◽  
Grade 3 ◽  
Kinetics Of

It is important that the material used to produce high-integrity pressure vessels has homogeneous properties which are reproducible and within specification. Most heavy pressure vessels comprise large forgings derived from ingots, and are consequently affected by the chemical segregation that occurs during ingot casting. Of particular concern are the compositional variations that arise from macrosegregation, such as the channels of enriched material commonly referred to as A-segregates. By causing corresponding variations in microstructure, the segregation may be detrimental to mechanical properties. It also cannot be removed by any practically feasible heat treatments because of the large scale on which it forms. Here we describe an investigation on the consequences of macrosegregation on the development of microstructure in a pressure-vessel steel, SA508 Grade 3. It is demonstrated that the kinetics of transformation are sensitive to the segregation, resulting in a dramatic spatial variations in microstructure. It is likely therefore that some of the scatter in mechanical properties as observed for such pressure vessels can be attributed to macroscopic casting-induced chemical segregation.

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