With this specific application in mind, the research laboratories of ThyssenKrupp VDM have taken an existing material and significantly enhanced its properties. “Alloy 617 B occ” (optimized chemical composition) – corresponding to Nicrofer 5520 occ at ThyssenKrupp VDM – offers high strength and ductility at the same time as good weldability. In the power plant of the future it could be used in tubes, valves and connectors as well as in the form of sheet and plate. Alloy 617 B occ owes its special properties to three factors: First, it is melted and remelted in a vacuum, preventing the pickup of undesirable substances from the air. Second, it contains a minimal amount of boron, whose content has to be precisely defined and controlled. And thirdly, its molybdenum and carbon contents have been optimized to enhance weldability while retaining the same strength. “Alloy 617 is one of the most thoroughly investigated nickel alloys and has been used in pressure vessels for decades,” says Ralf Husemann of Hitachi Power Europe. “Despite this, we wanted to optimize its properties with a view to achieving added operational reliability with A 617 occ. For Hitachi, operational reliability is paramount,” adds Husemann.

From 2012 the new high-performance material is to be tested in pilot plants in parallel with further laboratory investigations. Extensive testing of the new materials is a key requirement for the construction of a 700-degree power plant. A precursor of the now optimized Alloy 617 B occ has been tested successfully for around five years in various laboratories and pilot plants. The results and experience gained played a major role in the development of the new material. “To validate the lab tests it is necessary to manufacture components from the nickel alloys and test them under actual power plant conditions,” says Helmut Tschaffon, head of 700°C R&D activities at E.ON Energie. “This includes demonstrating that the materials can withstand the normal operating stresses in a power plant such as startups, shutdowns and load changes.”

“We are confident that the positive properties of Alloy 617 B occ will be confirmed in the tests and that we can start marketing the material soon,” says Dr. Jutta Klöwer, head of research and development at ThyssenKrupp VDM. “This takes us a big step closer to our goal of building more efficient, low-CO² power plants that can make a major contribution to environmental protection.” Tschaffon, too, is convinced that thanks to the development of Alloy 617 B occ work on building 700-degree power plants can begin in the course of this decade.