Maile: Thinner tube walls make power plants more flexible

KraftwerkForschung.info: What did you discover about the highly heat-resistant steels on the high-temperature material test facility at the GKM power plant in Mannheim?

Maile: We investigated how the materials behave under real conditions in practice, whereby they didn’t behave any worse than was forecasted by us in calculations and laboratory tests. The materials have also shown good creep rupture strength in practice. We have gained knowledge by qualifying the materials under the aforementioned practice conditions. We can use this for confirming the characteristic values and behaviour from the laboratory tests.

We have also garnered practical experience in terms of processing the materials. Through the applied heat, the welding influences the material and component behaviour in accordance with the chemical composition. This is because when welding a heat affected zone is created with a different microstructure and special stress conditions. Our project partners have optimised the process and modified the subsequent heat treatment so that the positive properties of the materials also remain when welding.

With the test facility we were able to show that the developed welding processes enable us to largely eliminate the risk of cracks forming. This can therefore ensure the optimum strength of the weld connection.

KraftwerkForschung.info: What was the result of the test facility inspection in May 2013?

Maile: We closed down the test facility, in other words we removed the pressure and heat, in order to check whether damage to the components could be found or identified. For these materials and components, there is still potential for improving the destruction-free testing methods for nickel-based alloys. By means of ultrasonic and endoscopic testing, we have conducted relatively exhaustive tests as to whether cracks can already be detected. Until now no cracks have been found on the tubes and fittings. That corresponds to our expectations that we have formed through the accompanying numerical calculations.

KraftwerkForschung.info: Don’t you have to run the test facility for longer in order to obtain more robust results about the service life of the components?

Maile: We have conducted around 1,700 load changes to date. We have therefore not yet reached the critical figure we have calculated of 2,000 to 2,500 load changes.

KraftwerkForschung.info: When will you exceed this figure?

Maile: We will already achieve more than 2,000 load changes this year. We have calculated that damage could occur above this critical figure. That means that cracks could form. However, our calculations entail a certain amount of uncertainty. At all events we want to ensure that the test facility does not have to be unexpectedly shut down as a result of damage, since this would impair the power plant process. We will achieve this by monitoring it and further optimising the computational forecasts on the crack initiation. By validating the computational methods it will be possible to check the soundness of our calculation.