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Case Hardening Research at EBI

Case Hardening Research at EBI

David Koch

In the near future, steel parts that are exposed to high stress loads, e. g. diesel fuel injection systems, need to be able to withstand even higher thermo-mechanical loads due to the downsizing of engines and stricter environmental regulations. An attractive and economical alternative to the application of high performance steels is the use of low alloy, and therefore easily machinable steels in combination with a heat treatment process.

To increase the thermo-mechanical resistance of steel parts e. g. atmosphere carbonitriding can be used. However, with this process oxidation effects known as internal or intergranular oxidation damage the surface layer. For many high-tech parts the oxidized layer needs to be treated, which is not always possible. Internal oxidation can be prevented by using low pressure processes, e. g. low pressure carbonitriding.

Low pressure carbonitriding is a case hardening process under development. At temperatures between 800 °C and 1050 °C and at total pressures below 50 mbar the carbon and the nitrogen concentrations in the outer layer of steel parts are increased and the parts are subsequently hardened by quenching. Carbon and nitrogen are brought to the parts by first pyrolyzing gases that contain C and N (typical is the use of acetylene in combination with ammonia) on the surface of the steel parts, followed by diffusion of C and N into the parts. Simultaneously, pyrolysis reactions also occur in the gas phase, during which, in case of carburizing, unwanted compounds like benzene, tar, and soot can be formed. These compounds lead to an increase in maintenance and cleaning frequency, and to a decrease in plant availability. Through interactions between carbon and nitrogen during carbonitriding, amines and cyanides may additionally be formed. To increase the understanding of the process, the gas phase reactions need to be understood. Additionally, for modeling purposes, a kinetic approach describing the gas phase reactions needs to be developed.

The goal of the research project is to gain, together with partners from industry and research institutions, an understanding of the steps that govern low pressure carbonitriding (steps 1-6 in figure 1) to ease the transition from development state to a technical process used in industry. The basis for this research project is knowledge acquired and methodologies developed during previous research projects on low pressure carburizing.

Figure 1: Sub-Steps of Low Pressure Carbonitriding (schematic)

The work packages at EBI deal with:

  • Identification of gases that can be used for carbonitriding
  • Research on gas phase reactions
  • Development of a lab scale flue gas cleaning plant
  • Modeling of gas phase reactions
  • Model verification at an industrial furnace

Existing lab scale plants:

  • Thermobalance (Tmax = 1600 °C, measurement accuracy: 5 µg)
  • Pyrolysis furnace
  • Vacuum furnace
  • Reactor for flue gas cleaning

Existing gas phase analytics:

  • Quadrupole mass spectrometer
  • 4-channel micro gas chromatograph
  • FTIR

Please contact us if you have questions about our research or if you are interested in joint research projects.

Completed Research Theses: