Thereby, low-pressure carburizing systems complement the standard vacuum hardening furnaces (Fig. 1), which of course comply with the specifications of AMS2750, CQI-9 and NADCAP.

In order to meet the requirements of quenching technology, both oil and high-pressure gas quenching can be offered. In the context of the discussion about carbon-neutral resources, the topic of high-pressure gas quenching with hydrogen is being rediscovered in addition to nitrogen and helium. It should be noted that helium production is based on fractional distillation of natural gas, while hydrogen can alternatively be produced by electrolysis of water using renewable power.

As shown in [1], the influencing parameters of high-pressure gas quenching can be grouped by physical parameters, influencing factors of the heat treatment equipment, and parameters of the batch. Fig. 2 shows the heat transfer coefficient as a function of pressure and type of gas. Hydrogen is an ideal quenching medium because of its physical properties. Due to the lower gas density, the power requirement for gas recirculation is significantly lower for helium and hydrogen, allowing pressures up to 20 bar to be achieved economically.

The peculiarities of hydrogen-nitrogen mixtures with respect to the quenching effect should also be noted. It has been shown that mixtures have a higher heat transfer coefficient than pure hydrogen, although this depends largely on the flow conditions [2]. The improved quenching effect of hydrogen in combination with an increased pressure compared to nitrogen allows martensitic hardening of components with larger cross sections. Table 1 shows examples of three cold work tool steels with respect to the through-hardenable diameter for different media and quenching pressures [3]. This shows an alternative to oil quenching with improved distortion behavior of the heat treated parts.

Alternatively, the increased cooling rate allows the use of less expensive steel grades. Low-alloy case-hardening steels such as 1.7131 or 1.7147, which have proven themselves in combination with oil quenching, achieve comparable hardness values with adapted high-pressure gas quenching with hydrogen.

 

Conclusion

To meet today’s economic and environmental demands, a holistic view of the various technological approaches to heat treatment is required. Depending on the application, either atmospheric or vacuum heat treatment may prove advantageous. Especially in the field of quenching technology, well established technologies have to be re-evaluated due to changing boundary conditions – under the heading of carbon footprint – and high-pressure gas quenching with hydrogen can be an interesting approach.

References

Hoffmann, F.; Gondesen, B.; Lohrmann, M.; Lübben, Th.; Mayr, P.: Möglichkeiten und Grenzen des Gasabschreckens. HTM 53 (1998) 2, pp. 81-86

Laumen, Ch.; Holm, T.; Lübben, Th.; Hoffmann, F.; Mayr, P: Hochdruck-Gasabschrecken mit Wasserstoff. HTM 53 (1998) 2, pp. 72-80

Altena, H.: Hochdruck-Wasserstoffabschreckung. HTM 50(1995) 1, pp. 27-30

 

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