Z-Ultra ready to use: New chromium steels for high-temperature applications
(December 21, 2016) As the most important industrial construction material, with more than 2,500 grades, steel is highly specialized for diverse applications. Even the smallest changes of the composition can modify the material structure on an atomic scale and improve material properties on the macroscale. The consortium of the EU-project Z-Ultra, led by the Fraunhofer Institute for Mechanics of Materials IWM, has developed new 12% chromium alloys for high-temperature applications that are up to 30% stronger than traditional 9% chromium steels and withstand higher temperatures and pressures for a longer period of time. Atomistic simulations supported the development of the new steel alloys in a targeted manner.
Higher operating temperatures in gas and coal power plants mean higher efficiencies and, therefore, less CO2 emissions per kilowatt-hour of electricity. However, the temperature capacity of real materials is naturally limited. The materials used in power plants (usually steels) lose their strength with increasing temperature and no longer withstand the stresses prevailing in turbines and pipelines. In addition, corrosion increases significantly with increasing temperature. For this reason, generations of engineers have worked on an ongoing improvement of steels so that operating temperatures of 615 °C are possible with today's 9% chromium steels, compared to a maximum of 300 °C 100 years ago.
More chromium in the steel has advantages and disadvantages
In order to further increase the operating temperature, a higher chromium content in the steel is necessary. The element chromium has the pleasant property of forming a protective chromium oxide layer on the steel surface, and it does so all the more effectively the higher the chromium content is. The thereby improved corrosion protection allows not only for higher temperatures, but also the use of biological waste and other renewable fuels, the combustion products of which can be very aggressive. "Now, unfortunately, there's a catch, which has so far prevented the use of higher chromium contents: the remarkable strength of what are currently the best heat-resistant steels is due to finely dispersed nitride particles," explains Prof. Hermann Riedel, Project Manager at the Fraunhofer IWM. At these operating temperatures chromium atoms can migrate into these particles, converting them into the so-called Z-phase. At the expense of the fine nitrides, these coarse Z-phase particles grow, which are of no use in terms of strength. "In the current 9% chromium steels, this undesirable transformation lasts for decades, whereas at 12% chromium content, it leads to an intolerable loss of strength in one year," says Riedel. For this reason, the 12% chromium steels have not yet been useable in power plants, since they are designed for a service life of more than ten years.
The trick: Use the Z-phase as a stabilizer
"In the project Z-Ultra, we have set the goal of influencing the coarse, brittle Z-phase in its growth in such a way that it is no longer harmful, but instead makes the steel more stable," explains Riedel. "We have looked for and found alloy compositions and manufacturing processes which 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 developed in the project are about 30% stronger than the best conventional 9% chromium steels, have a lifetime which is 10 times higher under the same load conditions, and are considerably more corrosion resistant.
Tubes made of the new materials were tested under conditions close to those in the superheater of a power plant heat exchanger: hot water vapor 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 developed uniformly – thicker on the outside than on the inside. Some pipes have also been tested in real power plant operation. In the meantime, they have been dismounted, examined and used again for long-term tests in a coal-fired power plant.
"In order to show their practicality, the steelmaker involved has produced a large twelve-ton forging, since it is not only the chemical composition of the steel that is responsible for the material properties, but also the manufacturing process, particularly the heat treatment," explains Riedel. Finally, it is important to maintain the outstanding material properties when welding the pipelines and other power plant parts. One focus of the project was therefore the development of suitable welding processes, including rings from the large forging as a model for welded turbine rotors.
Simulation tools for targeted alloy development
The steel developers were continuously guided by atomistic simulations in the process of adjusting the exact compositions of the new steels and the parameters of the forging process. In order to speed up the material development through the use of numerical simulation methods, the scientists at the Fraunhofer IWM used atomic and thermodynamic simulations to explore questions such as "What is the exact formation process of the Z-Phase?" and "What happens during the production and later during operation on the atomic scale?" They specifically investigated the behavior and the influence of the different alloying constituents and improved the atomic composition of the alloy with their results. For example, it was possible to determine at which content of carbon, nitrogen, niobium or tantalum the fastest or slowest process of Z-phase transformation takes place. Atomic simulations have contributed significantly to identifying the individual steps in this complex transformation process as well as to understanding their interdependencies.
Under the leadership of the Fraunhofer Institute for Mechanics of Materials IWM, six other research institutes as well as a steel producer, a power plant operator and an engineering consultancy company from the EU and from the eastern partner countries Ukraine, Georgia and Armenia participated in the EU-funded project Z-Ultra.
Steel is the ideal material for components in high-temperature applications up to 600 °C, as can occur in power plants or in the chemical industry. In the 1980s, the development of 9% chromium steels was a major step forward, as a result of which the application temperature could be increased from 540 up to 615 °C. At those temperatures, components of 9% chromium steel last about 20 to 30 years. In the meantime, 12% chromium steels have been developed that can withstand even higher temperatures but have a lower component lifetime. In order to improve these new steels so that they qualify for industrial applications, the EU project Z-Ultra was launched.
The 12% chromium steels are interesting, since thermal electricity generation from fossil fuels is still expected to be an important part of electricity generation for many years to come: it will compensate for fluctuations in the electricity generation of renewable energies in the power grid. For economically aspiring countries inside and outside of the EU, the number of coal and gas power plants is also expected to increase. It is therefore all the more important to increase their efficiency so that the consumption of fossil fuels and CO2 emissions remain as low as possible.