Combustion engines still have potential to save energy and reduce emissions; a current trend is to use downsized engines of the same or even increased power. Engines with a reduced cylinder capacity consume less fuel due to their smaller weight, smaller friction, and smaller amount of exhaust heat. This downsizing, however, is associated with higher mechanical and thermal loads acting on the already highly loaded components such as diesel injection systems.

Diesel injection systems have to reach higher injection pressures and improved injection accuracies in order to meet the requirements of downsizing. Hence, injection nozzles have to be made of highly stable materials.

An attractive and inexpensive option is the use of low-alloy steels—i.e. types of steel containing not more than five mass percent of metals other than iron. Such steels can be machined well in the soft state and are then hardened via a process such as carbonitriding for use.

Carbonitriding enables carbon and nitrogen atoms to diffuse interstitially into the low-alloy steel, creating barriers to slip and increasing the hardness and modulus near the surface. The harder surface resists wear better.

So far, low-pressure carbonitration has been carried out nearly exclusively using ammonia as a nitrogen donor together with a carbon donor—e.g., ethyne or propane. The KIT scientists have now studied other gases and gas mixtures for suitability for low-pressure carbonitration.

The new carbonitration process is being developed by the scientists at KIT’s Engler-Bunte Institute. At temperatures between 800 and 1050 °C and total pressures below 50 millibars (0.73 psi), the surface of the components to be hardened is specifically enriched with carbon and nitrogen and subsequently hardened by quenching.

The project, headed by David Koch, is aimed at studying the fundamentals of low-pressure carbonitration and developing this process to maturity in cooperation with research and industry partners.

Low-pressure carbonitration combines the advantages of low-pressure processes with those of atmospheric carbonitration, Koch explains. Atmospheric carbonitration damages the surface of the components treated by oxidation. This can be prevented by low-pressure processes. In addition, a more homogeneous hardness profile is generated in the component, in particular in case of complex component geometries.

The team tested the efficiency of the other gases and mixtures in enriching the surface layer with carbon and nitrogen using a thermobalance. Together with their Bosch colleagues, they found that methylamine (CH₃NH₂) and dimethylamine ((CH₃)₂NH) process gases cause a good enrichment of the surface layer with carbon and nitrogen.

When using methylamine for low-pressure carbonitration, only one gas instead of two is required and the usually applied two process steps can be reduced to a single one.

Compared to the use of ammonia as nitrogen donor together with a carbon donor, methylamine alone reaches a higher nitrogen enrichment in the surface layer. As carbon enters the surface layer in parallel, the process duration is shortened considerably. Methylamine also allows for carbonitration at much higher temperatures, which additionally shortens the process duration. Moreover, the degree of utilization of methylamine as a process gas is better, as a result of which the amount of gas used can be reduced.

The KIT scientists are now working on further optimizing low-pressure carbonitration with amines. Work focuses in particular on improving the homogeneity and free adjustment of carbon and nitrogen input. The next goal is to transfer the process from the laboratory to the pilot scale.

Resources

• D. Koch, L. Hagymási, T. Waldenmaier, S. Bajohr, and R. Reimert (2015) “Low Pressure Carbonitriding with Amines” HTM Journal of Heat Treatment and Materials 70, 4, 171-182 doi: 10.3139/105.110263