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.

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

2012 ◽  
Author(s):  
Deborah Heritier ◽  
Sylvain Pillot ◽  
Stéphanie Corre ◽  
Cédric Chauvy ◽  
Patrick Toussaint
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Material Properties ◽  
Pressure Vessels ◽  
Forming Process ◽  
Cold Forming ◽  
Thick Plates ◽  
Maximum Deformation ◽  
Stress Relieving ◽  
Weld Heat

During fabrication of large pressure vessels, thick plates are submitted to numerous process phases that may affect the initial (i.e. as delivered) properties of the material. Regarding the advantages (both technical and economical) of cold forming process, this technique is largely preferred and widely spread. Modern forming presses and rollers are now sufficiently powerful to roll very thick plates (typically up to 250mm thick) devoted to ultra-heavy pressure equipments. As force does not really constitute a limitation anymore, current limitations are now focusing on maximum admissible strain in materials. This particular limit is linked to: - Intrinsic maximum deformation admissible by the material (given by tensile tests), - Regulation rules coming from construction codes. From a practical point of view, the actual limitation comes from the construction codes that are very severe. Main codes (ASME Boilers and Pressure Vessels Construction Code from American side and EN 13445 Unfired Pressure Vessels Construction Code from European side) both give a limit equal to 5% strain for using material in “as-strained” condition without any heat treatment. Above this limit, the philosophy differs from one code to another. While European Code requires a full quality treatment of the strained material (Normalisation or Austenitization / Tempering), American code only requires Tempering, allowing fabricators the possibility of using the mandatory Post Weld Heat Treatment (PWHT) (needed by welded zones) as a tempering treatment to improve welded zone toughness and to regenerate material properties. The purpose of this contribution is to review the effect of pre-strain on mechanical properties (Hardness, Tensile and Toughness transition curves) for different strain levels and to evaluate the ability of typical PWHT to regenerate material properties. Results presented in this paper are based on both recent studies on the most common up-to-date materials as well as on historical data collected in the last decades. This study clearly demonstrates that the required PWHT is efficient enough to regenerate all material properties and that there is no need to apply a full quality heat treatment, even for the highest level of strain. This benefits both the fabricator and the end user as it implies reducing costs and risks of components deformation while maintaining the necessary level of service properties.

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.

scholarly journals Influence of post weld heat treatment for weld joint of P355GH

2018 ◽  
Vol 247 ◽  
pp. 00038
Author(s):  
Sławomir Parzych ◽  
Rafał Dziurka
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Quality Control ◽  
High Temperature ◽  
Weld Joint ◽  
Post Weld Heat Treatment ◽  
Microstructure And Properties ◽  
Welding Method ◽  
Welding Procedure ◽  
Weld Heat

From steel designed to work under pressure and exposed to high temperature apart from the good weldability, good mechanical properties are required. The guidelines set by the regulations require post welding heat treatment above 35mm thick. An important factor affecting the microstructure and properties of the joint made of thick-walled elements is heat treatment after welding. All welding operations must be properly planned before performing welding work. Welding procedure specification (WPS) is a document describing these operations, it is essential for proper determining of basics in planning welding operations and quality control in welding. The purpose of this paper is to compare the properties of joints made by 121 welding method in combination with and without post welding heat treatment.

Issues Related to Creep-Strength-Enhanced Ferritic (CSEF) Steels

2014 ◽  
Author(s):  
Rama S. Koripelli ◽  
David N. French
Keyword(s):  
Heat Treatment ◽  
High Temperature ◽  
Case Studies ◽  
Creep Strength ◽  
Pressure Vessels ◽  
Low Alloy Steels ◽  
Allowable Stress ◽  
Pinning Effect ◽  
Superheater Tube

T-91 and P-91 are the oldest of a new class of creep-strength-enhanced ferritic steels (CSEF) approved for use in boilers and pressure vessels. These newer alloys develop high strength through heat treatment, a rapid cooling or quenching to form martensite, followed by a temper to improve ductility. As a result, these alloys offer a much higher allowable stress which means thinner sections provide adequate strength for high-temperature service. Most of the applications thus far have been a substitute for P-22/T-22. The primary advantages of T91 materials over conventional low-alloy steels are: higher allowable stresses for a given temperature, improved oxidation, corrosion, creep and fatigue resistance. T23 is also considered as a member of the family of CSEF steels. The alloying elements such as tungsten, vanadium, boron, titanium and niobium and heat treatment separate this alloy from the well defined T22 steel. Although, T23 is designated for tubing application, its piping counterpart P23 has a strong potential in header applications due to superior strength compared to P22 headers. Now that T-91 and P-91 have been in service for nearly 30 years, some shortcomings have become apparent. A perusal of the allowable stress values for T-91 shows a drop off in tensile strength above about 1150°F. Thus, start-up conditions where superheaters, and especially reheaters, may experience metal temperatures above 1200°F, lead to over-tempering and loss of creep strength. During welding, the temperature varies from above the melting point of the steel to room temperature. The heat-affected zone (HAZ) is defined as the zone next to the fusion line at the edge of the weld metal that has been heated high enough to form austenite, i.e., above the lower critical transformation temperature. On cooling, the austenite transforms to martensite. Next to this region of microstructural transformation, there is an area heated to just below the austenite formation temperature, but above the tempering temperature of the tube/pipe when manufactured. This region has been, in effect, over-tempered by the welding and subsequent post-weld heat treatment (PWHT). Over-tempering softens the tempered martensite with the associated loss of both tensile and creep strength. This region of low strength is subject to failure during service. Creep strength of T91 steel is obtained via a quenching process followed by controlled tempering treatment. Elements such as niobium and vanadium in the steel precipitate at defect sites as carbides; this is known as the ‘pinning effect’. Any subsequent welding/cold working requires a precise PWHT. Inappropriate and/or lack of PWHT can destroy the ‘pinning effect’ resulting in loss of creep strength and premature failures. Several case studies will be presented with the problems associated with T91/T23 materials. Case studies will be presented, with the results of optical microscopy, scanning electron microscopy, hardness measurements and energy dispersive spectroscopy analysis. One case study will discuss how the over-tempering caused a reduced creep strength, resulting in premature creep failure in a finishing superheater tube. A second case presents the carburization of a heat recovery steam generator (HRSG) superheater tube, resulting in reduced corrosion/oxidation resistance. A case study demonstrates how a short-term overheating excursion led to reheat cracking in T23 tubing. Another case will present creep degradation in T91 reheater steel tube due to high temperature exposures (over-tempering).

scholarly journals The development of production technology of wire rod from chromium-nickel-molibdenum steel grades with subsequent annealing spheroidization

2019 ◽  
pp. 78-82
Author(s):  
S. V. Аudzeyey
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Production Technology ◽  
Full Range ◽  
Physical And Mechanical Properties ◽  
Cold Forming ◽  
Wire Rod ◽  
Automotive Components ◽  
Molybdenum Steel ◽  
Steel Grades

The peculiarity of the rod of chromium-nickel-molybdenum steel grades, used in the production of fasteners and automotive components by cold forming, are the high requirements for the quality of the surface, microstructure and physical and mechanical properties.In the process of development of production technology were developed and implemented measures to minimize the identified design features of heat treatment furnaces, and developed methods for obtaining the most optimal primary hot-rolled metal microstructure for further spheroidizing annealingIn industrial conditions OJSC «BSW – Management Company of Holding «BMC» was mastered the most optimal regimes of heat treatment of wire rod from chromium-nickel-molybdenum steel grades, which required the consumer a full range of physical and mechanical properties of wire rod from steel grades 38ХНГМ, 40XH2MA and 41Х1 used further in the manufacture of fasteners and automotive components by cold forging.

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.

Relaxation of Exemption Requirement of PWHT for SA-508 Grade 1A, by Consideration Surface Welding Residual Stress As Evaluated by Instrumented Indentation Testing Method

2017 ◽  
Author(s):  
Jong-hyoung Kim ◽  
Jun Sang Lee ◽  
Sungki Choi ◽  
Jong-sung Kim ◽  
Dongil Kwon
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Residual Stress ◽  
Power Plants ◽  
Mechanical Tests ◽  
Pressure Vessels ◽  
Post Weld Heat Treatment ◽  
Welding Residual Stress ◽  
Acceptance Criterion ◽  
Weld Heat

Generally, post-weld heat treatment is applied to decrease welding residual stress and improve the mechanical properties and microstructure of weldment, and its performance has been recommended for many years [1, 2]. However, current steel-making technology has improved significantly and, steel toughness levels have generally improved substantially [1]. Additionally for several quenched and tempered steels, it is reported that in some cases, mechanical properties such as tensile strength and impact toughness are degraded after post-weld heat treatment [3]. In addition, for large steel assemblies, post-weld heat treatment can be expensive, so that there is an economic incentive to avoid post-weld heat treatment [2]. The research presented here suggests a way to exempt post-weld heat treatment for SA-508 Grade 1A material, which is used for pressure vessels in nuclear power plants, by considering both mechanical properties and residual stress to simplify the welding procedure. Weldments made of 120 mm thick SA-508 Grade 1A should be post-weld heat treated, according to current ASME BPV Code. In order to increase the PWHT exemption thickness to 120 mm, we performed mechanical tests using welding coupons without PWHT; the test results satisfied current mechanical property criteria. We present a residual stress acceptance criterion based on brittle fracture criteria in this research.

scholarly journals On the impact of post weld heat treatment on the microstructure and mechanical properties of creep resistant 2.25Cr–1Mo–0.25V weld metal

2021 ◽  
Author(s):  
Hannah Schönmaier ◽  
Christian Fleißner-Rieger ◽  
Ronny Krein ◽  
Martin Schmitz-Niederau ◽  
Ronald Schnitzer
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Weld Metal ◽  
Elevated Temperatures ◽  
Stress Rupture ◽  
Pressure Vessels ◽  
Post Weld Heat Treatment ◽  
Microstructure And Mechanical Properties ◽  
The Impact ◽  
Weld Heat

AbstractCreep resistant low-alloyed 2.25Cr-1Mo-0.25V steel is typically applied in hydrogen bearing heavy wall pressure vessels in the chemical and petrochemical industry. For this purpose, the steel is often joined via submerged-arc welding. In order to increase the reactors efficiency via higher operating temperatures and pressures, the industry demands for improved strength and toughness of the steel plates and weldments at elevated temperatures. This study investigates the influence of the post weld heat treatment (PWHT) on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V multi-layer weld metal aiming to describe the underlying microstructure-property relationships. Apart from tensile, Charpy impact and stress rupture testing, micro-hardness mappings were performed and changes in the dislocation structure as well as alterations of the MX carbonitrides were analysed by means of high resolution methods. A longer PWHT-time was found to decrease the stress rupture time of the weld metal and increase the impact energy at the same time. In addition, a longer duration of PWHT causes a reduction of strength and an increase of the weld metals ductility. Though the overall hardness of the weld metal is decreased with longer duration of PWHT, PWHT-times of more than 12 h lead to an enhanced temper resistance of the heat-affected zones (HAZs) in-between the weld beads of the multi-layer weld metal. This is linked to several influencing factors such as reaustenitization and stress relief in the course of multi-layer welding, a higher fraction of larger carbides and a smaller grain size in the HAZs within the multi-layer weld metal.

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