Ever higher demands are being placed on the materials used in the construction of modern diesel engines for freight and passenger ships: Tighter emissions standards and rising oil prices call for greater energy efficiency, while at the same time increasing amounts of horsepower are being extracted from ever smaller engines. This progress has resulted in a significant increase in operating temperatures in the combustion chamber.
These high temperatures are pushing the materials currently used in this area to their limits. To meet the increased demands ThyssenKrupp VDM has optimized its proven high-strength nickel superalloy Alloy 80A for use in the valves of large two-stroke marine diesel engines. High-temperature corrosion resistance has been significantly improved by an increased chromium content, while the strength of the material has been maintained by the use of further alloying elements.
Modern large marine diesel engines in the 4,000 to 80,000 kW power range can weigh up to 2,300 tons. The valves used in them are up to two meters in size and weigh 300 kilograms. They are exposed to high mechanical stresses and at risk from hot gas corrosion. The combustion chamber is subject to very high temperatures, which in combination with the heavy oil used in large diesel engines attack the surface of the valve stems causing heavy corrosion. The material used for the valves therefore has to be particularly corrosion-resistant and long-lasting. The aim in developing materials for valve stems is to further improve these properties. ThyssenKrupp VDM is responding to this challenge with its new superalloy, developed in close cooperation with the Finnish engine maker Wärtsilä and the valve manufacturer Märkisches Werk Halver.
As an experienced manufacturer of components for large marine diesels, Märkisches Werk Halver offered the infrastructure necessary to validate and implement the new material in the valve stem application. "The performance of the valve stem is closely linked to the quality of the material," says project worker Rainer Weigelt from Märkisches Werk Halver. "Meeting all the requirements in one material is a major challenge but is the only way to achieve effective, optimized combustion processes with reduced pollutant emissions."
"In ThyssenKrupp VDM we had an outstanding materials producer with great expertise in high-temperature corrosion on our side," says Dr. Dietmar Schlager, who headed the project for Wärtsilä. The engine manufacturer has wide-ranging knowledge of the requirements placed on materials and engine components, as well as extensive valve test capabilities. In one of the first tests a valve was made from the new material and fitted in the engine of a container ship. "The results of long-term field testing were very promising," explains Schlager. "Now we're testing six further valves and we expect these field tests to deliver more information on production readiness."
The new material also has great potential in other areas. It can be used wherever high demands are placed on high-temperature corrosion resistance in connection with aggressive oil . "We see outstanding prospects for the material in the auto area as well," says Dr. Jutta Klöwer, project manager at ThyssenKrupp VDM, "particularly given the trend towards more powerful engines with higher combustion chamber temperatures."