Cédric Chauvy

Prevention of Weld Metal Reheat Cracking During Cr-Mo-V Heavy Reactors Fabrication

CrMoV heavy reactors fabrication has undergone some weld metal reheat cracking issues during the end 2007 / beginning 2008. This paper addresses this particular problem and explains the methodology used to solve it. Usual techniques, such as chemical factors and required NDE, have shown their limits, underlining the fact that this problem was unpredictable. Special mechanical testing as well as very accurate chemical analyses have allowed the authors to find the root cause. Very small amounts of particular impurities were responsible for the cracking and a criterion is then proposed to ensure that the problem will not come out again.Copyright

Thick Plates in Grade 91 for Fourth Generation Nuclear Reactors

To face the future challenge of global energy supply, taking into account the depletion of fossil fuels and global warming issues, the main nuclear energy users are strongly involved in a research program to fourth-generation reactor technology. This new generation will work at high temperatures between about 450 °C and 550 °C. Extensive studies have been launched worldwide to assess steel grades able to meet these new operating conditions. One of the candidates is Vanadium enhanced 9% Chromium steel grade (EN 10028-2 X11CrMoVNb 9-1 – ASTM A387 grade 91 class 2 – ASME SA387 grade 91 class 2). To meet the future needs in nuclear energy, Industeel improved its conventional 9Cr1MoVNb steel devoted to the fabrication of steam lines in thermal power plants. Preliminary studies revealed the feasibility of thick plates in this enhanced grade. Thick plates, 140 and 210 mm thick, have been hot rolled from a 82 metric tons ingot. Welded joints have then been prepared. Tests performed on both base metal and welded zones proved the excellent mechanical properties of the grade, especially regarding toughness property. This work demonstrated the industrial feasibility of very thick plates made of grade 91 for nuclear applications. This contribution is a review of the work done as well as the results obtained on the plates.© 2010 ASME

Potential Detrimental Consequences of Excessive PWHT on Pressure Vessel Steel Properties

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.

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

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.Copyright

Standard Procedure to Test 2-1/4Cr-1Mo-V Saw Filler Material Reheat Cracking Susceptibility

In late 2007 and early 2008, 2 1/4Cr1MoV heavy reactors fabrication has undergone some weld metal reheat cracking issues that became a serious situation as roughly 25 large vessels were affected. Strong efforts from the whole production chain were put in trying to isolate the root cause of the crack appearance and solve this particular matter. Many investigations were conducted by independent laboratories in the first months of 2008.In particular, a hot tensile Gleeble® test indicated a decrease in ductility in the critical temperature range of Intermediate Stress Relieving (ISR) treatment (650–680 °C) of the affected welding consumables compared to those that did not cause cracking. This mechanical test was successfully combined with high sensitivity chemical analyses (Glow Discharge Mass Spectroscopy – GDMS) to find the root cause of such batch-to-batch differences in ductility. A chemical composition factor, called Kfactor, was defined and statistically linked to the Gleeble® test in July 2008. Altogether, it permitted to solve the issue.Even if reliable and well documented, the Gleeble® test was originally developed to understand the root cause of the cracking. But due to lack of other solutions, its role was largely extended and it became a kind of standard test to qualify 2 1/4Cr1MoV SAW filler material. It was decided to optimize this test, keeping its general philosophy but making it feasible by a larger number of laboratories. In order to do that, a one-year Joint Industrial Program (JIP) was proposed to the community, accepted, sponsored and launched at the beginning of 2010. The target was to create a new test protocol able to discriminate batches of filler material.The objective of this paper is to summarize the investigations performed and the optimization of the original Gleeble® test during the project that led to the definition of a new hot tensile mechanical test successfully benchmarked by independent laboratories and now balloted to be incorporated in American Petroleum Institute (API) recommended practice API RP 934-A as a new appendix.Copyright

Hydrogen and High Temperature Resistant V-Modified 9Cr-1Mo Heavy Plates Devoted to New Generation High Performance Petrochemical Reactors

The efficiency of petrochemical reactors is intimately related to process parameters, i.e. service temperatures and pressures. Low alloyed ferritic materials, such as 2 1/4 Cr1Mo(V) and 3Cr1Mo(V) steel grades, are widely used for many years to build heavy wall reactors. This is mainly due to their good mechanical properties at high temperatures under high hydrogen partial pressures and good resistance to High Temperature Hydrogen Attack (HTHA). Depending on the grades, the ASME Code gives limitations in terms of maximum temperature that can limit the use of these low alloy grades. Moreover, above a given temperature, maximum allowable stresses are driven by the creep behaviour, leading to a strong lowering of the assumed resistance and hence to extra-thickness and weight. Many developments were done concurrently to increase the efficiency of petrochemical processes. In particular, this can lead to increase service temperatures and therefore actual pressure vessel wall temperatures. Indeed, more and more temperatures around 500°C are likely to be used, leading to reduced choice in terms of permitted steel grades. The low alloy vanadium-enhanced grades are not allowed (except using specific code case) whereas the usual grades have reduced creep allowable stresses. With a view to allowing strong improvements in admissible process parameters, a vanadium-modified 9Cr1Mo creep strength enhanced material with advanced hydrogen resistance and improved toughness was developed. Very thick plates (up to 200mm thick) were produced and tested. This contribution reports both mechanical and metallurgical assessments performed on these heavy plates. Evaluations of hydrogen resistance (HTHA) as well as creep resistance under high hydrogen pressure are also reported. The V-modified 9Cr1Mo grade exhibits an excellent behaviour in hydrogen rich environment, showing therefore some advantages in terms of service conditions. The manufacturing of heavy plates has made significant progress in the recent years, allowing thick products to be manufactured with good homogeneity and mechanical behaviour. Taking into account the maximum use temperature as well as the allowable stresses as described in the ASME BPV Code section VIII division 2, the V-modified 9Cr1Mo grade will clearly be of great interest to companies wishing to enhance the efficiency of their refining/petrochemical processes. The question of welding must also be addressed more specifically to finish validating the 9Cr1MoV option.Copyright

Estimation of CTOD in the Case of Very Thick Plates by Knowing CVN Value at a Given Temperature

Material properties assessment at given temperature and thickness is of primary importance for steelmakers. Generally, a list of mechanical properties requirements, namely tensile, Charpy V-Notch (CVN), fracture mechanics, as well as chemical or heat treatments limits are furnished by customers. Subsequently, the best compromise has to be found by the steel producer in order to reach these requirements. Concerning tensile and CVN properties, experience is large and metallurgists are used to determine the best product optimizations so as to reach the requirements’ values. However, optimization is generally more complicated regarding fracture mechanics. Tools are therefore needed in order to evaluate these properties with reference to conventional (i.e. tensile and CVN) properties. The objective of the present paper is to present some rules that can be used to extrapolate Crack Tip Opening Displacement (CTOD) values from CVN and tensile properties. Recent example is given to illustrate this methodology. In addition, special attention will be paid to the comparison of estimated and measured CTOD values.© 2010 ASME