Hardened Steels For More Efficient Engines

Combustion engines still have a lot of potential to save energy and reduce emissions. This is the trend towards smaller motors with the same or even higher performance. Engines with a smaller displacement consume less fuel thanks to their lower weight, less friction and less waste heat. However, downsizing means that highly stressed components, such as components of diesel injection systems, are exposed to even higher mechanical and thermal loads. Diesel injection systems must have higher injection pressures and better injection accuracies in order to meet the requirements of downsizing. Therefore, the injection nozzles have to be made from particularly resistant materials.

Hardening of low-alloy steels in the surface layer

An attractive and inexpensive option is the use of low-alloy steels, i.e. types of steel that contain a maximum of five percent by mass of other metals in addition to iron. They are soft to work with and are then hardened for use in order to achieve a hard surface with a tough core. Scientists at the Engler Bunte Institute at KIT are working on a new process for case hardening steel, low-pressure carbonitriding: at temperatures between 800 and 1050 degrees Celsius and total pressures below 50 millibars, the surface layer of the components to be hardened is specifically enriched with carbon and nitrogen and then hardened by quenching.

So far, ammonia has been used almost exclusively as a nitrogen donor in low-pressure carbonitriding in conjunction with a carbon donor, usually ethyne or propane. The researchers at KIT have now examined other gases and gas mixtures for their suitability for low-pressure carbonitriding and experimentally tested their effectiveness in enriching a component surface layer with carbon and nitrogen in a thermal balance. The scientists from Karlsruhe, together with researchers from Robert Bosch GmbH in Stuttgart, discovered that methylamine (CH3NH2) and dimethylamine ((CH3) 2NH) as process gases lead to a good enrichment of the surface layer with carbon and nitrogen.

Process gas methylamine reduces the process steps

When using methylamine for low-pressure carbonitriding, only one instead of two gases is required, and the otherwise usual two process steps can be reduced to one. Compared to ammonia as a nitrogen donor in combination with a carbon donor, methylamine achieves a higher nitrogen enrichment of the surface layer. Since carbon is also introduced at the same time, the process time is significantly reduced. Methylamine also allows carbonitriding at significantly higher temperatures, which further shortens the process time. The methylamine is also better used as process gas, which allows a reduction in the amount of gas used.

KIT scientists are now working on further optimizing low-pressure carbonitriding with amines. The main aim is to improve the uniformity and the free adjustability of the entry of carbon and nitrogen. The next goal is to transfer the process from the laboratory scale to the pilot scale. (qui)