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ELIXIR Résumé de rapport

Project ID: G1RD-CT-2000-00326
Financé au titre de: FP5-GROWTH
Pays: Hungary

Method for optimising preheat conditions for creep resistant steels

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.

Contact

Lenkey GYÖNGYVÉR, (Deputy Director)
Tél.: +36-46-560110
Fax: +36 46-422786
E-mail