Leistungen
The principal aim of the project was the elimination of PWHT and ISR from the welding cycle during manufacture or repair welding. The production of Guideline document on Fast Repair Methodology without PWHT was seen as a mechanism to disseminate the findings of the project and advise the wider community on the best practice to achieve these goals.
The Guideline document reviews the problems associated with the removal of PWHT and then proposes the best practices to be adopted to undertake welding without PWHT for the range of materials covered in the Elixir Project. A summary of recommendations is given within the Guideline Document.
The contents of the Guidelines will give welding engineers confidence to undertake repair without PWHT or in more complex cases identify the work needed to ensure a satisfactory repair without PWHT. Any repair done without PWHT will bring commercial benefit to both manufacturer and plant user. Elimination of PWHT from the welding cycle will improve the working conditions of those involved in repair welds.
Those partners involved with the implementation of repair welding will utilise this result in their consultancy and services to industry thus promoting the findings of the Guidelines. The availability of the Guidelines to the industrial community will further aid dissemination and acceptance of the repair welding without PWHT.
A model to compute and simulate the Heat Affected Zones produced by welds has been developed, being able to compute the mechanical properties of these metallurgically hetereogeneous regions. It is possible to estimate hardness, yield strength, ultimate tensile strength as well as the toughness at the different regions from a very detailed microstructural description.
It is closely connected to other results of the project able to compute thermal profiles, microstructes, etc.
On the fracture model, a weakest link hypothesis is used. It is assumed that the active zone of the piece or specimen under load (its plastic zone) can be divided in a number of volume elements, so that the failure of one of them causes macroscopic failure of the piece.
Each volume element is characterized by its stress state, depending on the macroscopic loading state and by its (probably path dependent) plastic state. Then the fracture probability is obtained from the integral extended to the whole active volume, within the plastic region. Now, it is in the deduction of F where the necessary succession of microcrack nucleation at a particle, propagation through the particle/matrix boundary and further propagation through the first encountered grain boundary must be included.
The innovative aspect of the ELIXIR project was to achieve elimination of intermediate stress relief (ISR) in fabrication welds and a validated repair procedure without PWHT. An integrated approach involving sophisticated computer welding simulation and innovative RS measurement techniques had to be developed to achieve this goal.
The production of Guidelines document on Advanced Simulation Procedures for Fabrication & Repair Welds was seen as a tool to advise the welding engineer to achieve this goal in a straightforward and economic manner. The Guidelines will inform the user of FE software for weld simulation about the best suited code for his specific problem, the material data necessary for the calculations, the proper element types, the importance of boundary conditions, techniques to simulate a moving heat source and last but not least the economical implications of numerical weld simulation. The exchange of experimental and numerical data serves for mutual cross fertilization and favours further developments in both fields.
The availability of the Guidelines to the industrial community will aid dissemination and acceptance of the numerical welding simulation techniques to solve the essential problems of welding mentioned above.
The aim of the work was to develop and validate optimal technological parameters for creep resistant steel welded joints, especially regarding the elimination of martensite in austenitic welded joints. The work has been performed jointly with University of Miskolc, Department for Mechanical Engineering.
Creep resistant steels are mainly applied in fossil power plants as material of pipes and/or pressure vessels operating at high pressure and at elevated temperature. Besides these steels are frequently used in oil refineries, gas processing plants and chemical industry. They are predominantly designed as materials of boiler tubes, steam and smoke pipes, therefore they usually work in slightly oxidising atmosphere. Both for having significant creep limit at high temperature and strong resistance against oxidisation, Cr and Mo content of creep resistant steels rises with temperature.
Operating temperature of pipes located at different sections of thermal power stations can be very different from each other, e.g. main steam pipes can work at 500-700°C, while operating temperature of pipes of heat exchangers can be as low as 200-250°C.
Inside a given section steels with the same composition are used. Since the composition of filler material applied for welding of these steels is usually very similar to one used for the base materials, the welded joint is called homogeneous one.
For economical reason the composition of the pipes located at different temperature sections is not the same, and where these pipes have to be connected, the chemical composition of steels in a given joint is different. In these welded joints the Cr content of filler material is equal to the Cr content of one of the base materials or falls between their Cr contents. The selected filler material is very often austenitic stainless steel or 70%Ni * 20%Cr type Ni base alloy. Joints in which the composition of the base metals is different or the composition of filler material is different from both base metals called as heterogeneous ones.
In the weld and in a part of the heat affected zone of the martensitic creep resistant steels the previously formed austenite transforms into martensite during cooling down and because of this they are often cracked. In order to prevent the crack formation, steels have to be preheated. The temperature range of preheating temperature advised by manufacturers is 200¿350°C, which is very wide (max. 150°C).
With respect to the fact, that the chemical composition of the different steel heats are different, easy to realise, that the optimum value of the preheating temperature can not be the same for different steel heats.
Endeavouring to get higher (but in reality only virtual) safety, welding engineers often prescribe unnecessarily high preheating temperature. Welding, which carried out at the preheating temperature, higher than the necessary can cause not only extra costs, but can increase the crack sensitivity in the cooling period. Similarly, it is just wrong to use lower preheating temperature, than the optimum one, since in these cases the joint can be cracked even during welding operation.
Preheating temperature can be considered optimal one at which:
- Austenite to martensit ratio is independent on the heat composition,
- There is enough amount of austenite in the microstructure of the joint to avoid the formation of too high tensile residual stresses during post-welding cooling.
It has been proved that the optimal value of the preheating temperature depends on the Ms temperature characterising the chemical composition of a given creep resistant steel heat, therefore the effect of the wall thickness on the cooling rate is negligible.
In order to determine the optimal preheating temperatures a lot of experiments have been carried out. The suggested preheating temperature is within the practically used wide interval, but the calculation method based developed has a great advantage, since the quantity of martensite, formed during welding can be followed by attention.
Importance of the developed method is much greater for welding of X20 CrMoV 12 1 steel grade, because its martensite is far harder due to higher carbon content. Unquestionable, that in recent times quite less amount of new products are manufactured from this steel grade, but when the repairing and/or renewing of old products have to be done this method gives higher safety and reliability.
During fabrication and repair in service, pressure vessels are submitted to various Intermediate Stress Relieving (ISR) and Post Weld Heat Treatments (PWHT) following requirements proposed by construction codes.
In some cases these requirements have appeared too stringent (ISR in place of Dehydrogenation Treatment DHT) or on the contrary too imprecise.
The determination of optimised Heat Treatment conditions depends on the purpose of this treatment and therefore on the dehydrogenation, the tempering and the stress relieving behaviour of the materials to be concerned.
It has been shown that DHT is able to remove hydrogen almost completely for the CrMo steels but not for the CrMoV steels. ISR soacking temperature can be slightly decreased allowing keeping good impact strength even after several ISR.
Besides mechanical characterisation, numerical simulation has been used for the determination of hydrogen behaviour and the evolution of residual stresses.
A guideline for optimising ISR for the heat resisting steels has been issued as a project deliverable to help applying the optimised procedures.
Dissemination of the results:
- Hungarian Seminar - Miskolc (Hungary) -23rd of June 2004.
- ESOPE 2004 (International conference on pressure vessels)- Paris (France) - 28th to 30th of September 2004.
- General assembly of Systus users - Paris (France) - 11th of October 2004.
- Soudage et Techniques Connexes - the French Technical review on welded construction - To be published.
- PVP 2005 - Denver (USA) - 17th to 21st of July 2005.
The aim of the work done is to determine the hydrogen behaviour of CrMo(V) steel during welding. The introduction of hydrogen during welding is something dangerous that must be taken into consideration for prescription of post-weld heat treatments. Different cases of heat treatments have been tested (measurement of H2 content before and after HT, measurement of H2 diffusion kinetics in every zones of the joints...), to generate safer Industrial Heat Treatments procedures.
Hydrogen diffusion measurements have been made on 10CrMo9.11 and 13CrMoV9.11 parent materials, simulated martensitic and bainitic HAZ and weld metals and diffusion laws defined for each material. The high trapping of hydrogen behaviour of the 13CrMoV9.11 was noted compared to the 10CrMo9.11 material.
The numerical hydrogen diffusion model within Sysweld was capable of simulating the hydrogen diffusion from the degassing test specimens.
The Sysweld numerical model successfully simulated the HAZ and microstructure of the simulated welds.
Hydrogen content measurements made on 150mm thick weldments in 10CrMo9.11 and 13CrMoV9.11 after DHT and ISR showed that DHT alone was sufficient to remove all the diffusible hydrogen in standard 10CrMo9.11. In the case of the vanadium enhanced 13CrMoV9.11, DHT alone is not sufficient to remove all the remaining hydrogen and ISR is required to remove the remaining hydrogen. The poor performance of the 13CrMoV9.11 is due to its greater ability to trap hydrogen. These measurements give end-users and manufacturers of pressure vessels a very practical feel for the efficiency of the DHT and IST treatments in these grades of steel.
The Sysweld numerical simulation of the 150mm thick section weldments and the Industrial Case Studies confirmed the relative findings of the hydrogen measurements on the 150mm thick weldments, i.e. DHT is sufficient for 10CrMo9.11 but that DHT and ISR is required for 13CrMoV9.11.
It has been shown the Sysweld numerical models can be used to predict hydrogen content in weld zones and show the relative differences between geometries, weld procedures, materials and heat treated condition but does tend to give a conservative overestimate of the hydrogen content.
Two improvements to the laws currently governing hydrogen diffusion within Sysweld are recommended:
- Hydrogen diffusion should be modelled using a second degree experimental law. This corresponds to a second degree polynominal law in log-log plot. This would provide improved predictions in the medium temperature range (200-300°C) where the existing linear laws do not fit well.
- Additional accounts needs to be taken of the effects of trapped hydrogen.
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