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New Chrome Steels For High Temperature Applications

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New Chrome Steels For High Temperature Applications
New Chrome Steels For High Temperature Applications

Video: New Chrome Steels For High Temperature Applications

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Video: Stainless Steel and Alloys for High Temperature Applications 2023, January
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Higher operating temperatures in gas and coal-fired power plants mean higher efficiencies and thus less CO2 emissions per kilowatt hour of electricity. The temperature increase is inherently limited. The materials used in power plants, usually steels, lose their strength as the temperature rises and can no longer withstand the loads in turbines and pipelines. In addition, the corrosion increases significantly with increasing temperature. Generations of engineers therefore worked on the further improvement of the steels, so that with today's 9% chrome steels, operating temperatures of 615 ° C are possible compared to a maximum of 300 ° C 100 years ago.

More chrome in steel has advantages and disadvantages

In order to further increase the operating temperature, a higher chromium content in the steel is required. The element chrome has the pleasant property of forming a protective chrome oxide layer on the steel surface and the more effective the higher the chrome content, the more effective. The resulting improved corrosion protection not only allows higher temperatures, but also the use of biological waste and other renewable fuels, the combustion products of which can be very aggressive. "Unfortunately, there is now a horse's foot that has prevented the use of higher chromium contents: The remarkable strength of the best heat-resistant steels currently is based on finely divided nitride particles," explains Prof. Dr. Hermann Riedel, project manager at Fraunhofer IWM.Chromium atoms can migrate into these particles at operating temperatures and thus convert them into the so-called Z phase. At the expense of the fine nitrides, coarse Z-phase particles are created that are useless for strength. "In the current 9% chromium steels, this undesirable conversion takes decades, whereas with a chromium content of 12%, it leads to an intolerable drop in strength in just one year," says Riedel. That is why the 12% chromium steels have so far not been used in power plants, since they are designed for a lifespan of more than ten years.while with a chromium content of 12%, it leads to an intolerable drop in strength in just one year, «says Riedel. That is why the 12% chromium steels have so far not been used in power plants, since they are designed for a lifespan of more than ten years.while with a chromium content of 12%, it leads to an intolerable drop in strength in just one year, «says Riedel. That is why the 12% chromium steels have so far not been used in power plants, since they are designed for a lifespan of more than ten years.

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The trick: use the Z phase as a stabilizer

"In the Z-Ultra project, we set ourselves the goal of influencing the growth of the coarse-grained, brittle Z phase in such a way that it is no longer harmful, but on the contrary makes the steel more stable," explains Riedel. "We looked for and found alloy compositions and manufacturing processes that distribute the Z phase very finely in the steel - this leads to a long-term stable particle structure," says the physicist. The best of the seven alloys newly developed in the project are around 30% stronger than conventional 9% chromium steels, have a service life 10 times longer under the same stress conditions and their corrosion resistance is considerably better.

Pipes made of the new materials have been tested under conditions that come close to those in the superheater of a power plant heat exchanger: hot steam inside and corrosive combustion gases and ash particles on the outside. The tests showed that the corrosion behavior of the materials up to 647 ° C was still very good. The protective oxide layers had grown evenly - thicker on the outside than on the inside. Some pipes were also tested in real power plant operations. They have since been removed, examined and used again for long-term tests in a coal-fired power plant.

"In order to demonstrate practicality, the steel manufacturer involved made a large, twelve-ton forging, because not only the chemical composition of the steel is responsible for the material properties, but also the manufacturing process, in particular the heat treatment," explains Riedel. After all, it is important that the outstanding material properties are preserved when welding the pipelines and other parts of the power plant. One focus of the project was therefore the development of suitable welding processes, right up to rings from the large forged part as a model for welded turbine rotors.

Simulation tools for targeted alloy development

When developing the exact composition of the new steels and the parameters for the forging process, the steel developers were continuously guided by atomistic simulations. In order to accelerate the development of materials through the use of numerical simulation methods, the scientists at Fraunhofer IWM used atomistic and thermodynamic simulations to investigate questions such as "How does the Z phase form?" Or "What happens during production and later in operation atomic scale? «They specifically investigated the behavior and influence of the different alloy components and optimized the atomic composition of the alloy with its results. For example, it can be said at what level of carbon, nitrogen,Niobium or tantalum is the fastest or slowest process of Z-phase transformation. Atomistic simulations made a significant contribution to identifying the individual steps in this complex transformation process and to understanding their interdependencies and influences.

Under the leadership of the Fraunhofer Institute for Mechanics of Materials IWM, six other research institutes as well as a steel manufacturer, a power plant operator and an engineering consultancy from the EU and from the eastern partner countries Ukraine, Georgia and Armenia each participated in the EU-funded Z-Ultra project. (qui)

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