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Researchers develop faster, less process-intensive case-hardening

22 November 2015

Scientists of KIT's Engler-Bunte Institute have developed a promising new process for the case-hardening of steel - low-pressure carbonitration.

Injection pumps and injectors, such as the Bosch Common Rail System CRS3-25, generate pressures of up to 2,500bar, thanks to hardened steel (image source: Bosch)

There is a trend to use smaller internal combustion engines with reduced cylinder capacity that consume less fuel due to their lower weight, reduced friction, and less exhaust heat. This 'downsizing', however, is associated with higher mechanical and thermal loads acting on the already highly loaded components. Diesel injection systems, for example, have to reach higher injection pressures and improved injection accuracies in order to meet the requirements of downsizing, so injection nozzles have to be made of highly stable materials.

An attractive and inexpensive option is the use of low-alloy steels - types of steel containing not more than five mass percent of metals other than iron. Such steels can be machined easily in the soft state and then case-hardened. In this way, a hard surface with a tough core is obtained.

Scientists of KIT's Engler-Bunte Institute are now working on a new process for the case-hardening of steel - low-pressure carbonitration. At temperatures between 800 and 1050°C and total pressures below 50mbar, 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," says Koch.

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, an important factor for more complex component geometries.

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

Their efficiency in enriching the surface layer with carbon and nitrogen was tested using a thermobalance. Together with researchers at Robert Bosch GmbH in Stuttgart, they found that methylamine and dimethylamine process gases resulted in 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 usual two-step process can be reduced to one. Compared with 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 utilisation of methylamine as a process gas is better, so 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.

The results obtained for low-pressure carbonitration with methylamine are described in an article published in The Journal of Heat Treatment and Materials.

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