These enable precise control of metallurgical processes and open up new possibilities for demanding applications.

To meet the requirements of quenching technology, both oil and high-pressure gas quenching can be offered. Regarding CO₂-neutral operating media, high-pressure gas quenching with hydrogen is being rediscovered alongside the classic gases nitrogen and helium. As described in [1], the influencing factors of high-pressure gas quenching can be divided into three categories: physical influencing variables, system-specific factors including peripherals, and the characteristics of the batch and its composition.

Figure 1: Modern vacuum hardening furnace with hydrogen quenching and LPC system (Source: Aichelin Group)

Figure 1: Modern vacuum hardening furnace with hydrogen quenching and LPC system (Source: Aichelin Group)

Figure 2 shows the dependence of the heat transfer coefficient on pressure and gas type. Due to its physical properties, hydrogen proves to be an ideal quenching medium. Thanks to its lower gas density, helium and hydrogen require significantly less power for gas circulation, making pressures of up to 20 bar economically viable.

Figure 2: Effect of quenching pressure and gas type on the average heat transfer coefficient (Source: Hoffmann, F.; Gondesen, B.; Lohrmann, M.; Lübben, Th.; Mayr, P.: Possibilities and Limitations of Gas Quenching. HTM 53 (1998) 2, pp. 81–86)

Figure 2: Effect of quenching pressure and gas type on the average heat transfer coefficient (Source: Hoffmann, F.; Gondesen, B.; Lohrmann, M.; Lübben, Th.; Mayr, P.: Possibilities and Limitations of Gas Quenching. HTM 53 (1998) 2, pp. 81–86)

When examining hydrogen-nitrogen mixtures with regard to quenching performance, it becomes apparent that mixtures exhibit a higher heat transfer coefficient than pure hydrogen, which largely depends on the flow conditions [2]. The superior quenching effect of hydrogen combined with higher pressures compared to nitrogen enables martensitic hardening even of components with larger cross-sections. Table 1 lists three cold-work steels as examples, showing the through-hardenable cross-section for different media and quenching pressures [3]. This presents an alternative to oil quenching while simultaneously improving the dimensional stability of the heat-treated components. Alternatively, the increased cooling rate allows the use of more cost-effective materials. Low-alloy case-hardening steels such as 1.7131 or 1.7147, which have proven effective in combination with oil quenching, achieve comparable hardness values with adapted high-pressure gas quenching using hydrogen.

Potential of Low-Pressure Carburizing

In the following, the potential of vacuum technology in the field of low-pressure carburizing (LPC) will be examined in more detail from an application engineering perspective, focusing on the reduction of process time through an increase in process temperature.

Case hardening as such can be regarded as a combination of carburizing and subsequent martensitic hardening. On an industrial scale, gas carburizing dominates today. The equilibrium atmosphere allows in-situ measurement and thus targeted control of the carbon potential. This process is complemented by low-pressure carburizing, in which acetylene (ethyne) is used as the carbon-donating gas. The process takes place at pressures below 10 mbar, with pulse and diffusion phases alternating. The specific advantages and disadvantages of gas and low-pressure carburizing are described in the works of Edenhofer [4] and Liedtke [5]. Essential for both processes is the temperature dependence of the mass transfer and diffusion coefficients, which can be described using an Arrhenius approach. An increase in process temperature therefore allows a significant reduction in process time. Figure 3 illustrates this for two different process temperatures with different alloy systems for the low-pressure carburizing process.

Figure 3: Process times for low-pressure carburizing of pulsator wheels (Source: Steinbacher, M.; Stenico, A.: Reliable High-Temperature Carburizing of Fine-Grained Stabilized Steels in Atmospheric and Vacuum Furnaces, Final Report on AiF Research Project 86ZN, Bremen, July 2006)

Figure 3: Process times for low-pressure carburizing of pulsator wheels (Source: Steinbacher, M.; Stenico, A.: Reliable High-Temperature Carburizing of Fine-Grained Stabilized Steels in Atmospheric and Vacuum Furnaces, Final Report on AiF Research Project 86ZN, Bremen, July 2006)

Despite the increased energy required for heating the batch, the energy savings from the shortened process time are dominant, contributing to improved energy efficiency and a reduced CO₂ footprint. At this point, attention should be drawn to the aspect of fine-grain stability — potential issues of grain growth due to the elevated process temperature must be taken into account. However, the work of Steinbacher and Stenico [6] shows that this can be compensated by appropriate measures, so that the process reliability of high-temperature carburizing is assured. The investigations into tooth root and pitting load-carrying capacity show no negative effects. To suppress nitrogen effusion, a minimum pressure of 200 mbar was maintained during the diffusion phase.

The two selected materials (20MnCr5 and 18CrNiMo7-6) differ in their hardenability. This allows them, in combination with various quenching media — in particular high-pressure gas quenching with hydrogen, gas quenching with nitrogen or helium, and classic oil quenching — to cover a broad spectrum of component geometries.

Conclusion

In order to meet both the economic and ecological demands of our time, a holistic evaluation of the various technological approaches in the field of heat treatment is more important than ever. Depending on the application, either atmospheric heat treatment or vacuum system technology may prove advantageous. Particularly in the area of quenching technology, established technologies must be reassessed in light of changed boundary conditions and to exploit new technological potential. High-pressure gas quenching with hydrogen in modern vacuum systems can represent a forward-looking approach in this regard.

Literature

[1] Hoffmann, F.; Gondesen, B.; Lohrmann, M.; Lübben, Th.; Mayr, P.: Möglichkeiten und Grenzen des Gasabschreckens. HTM 53 (1998) 2, pp. 81-86
[2] Laumen, Ch.; Holm, T.; Lübben, Th.; Hoffmann, F.; Mayr, P: Hochdruck-Gasabschrecken mit Wasserstoff. HTM 53 (1998) 2, pp. 72-80
[3] Altena, H.: Hochdruck-Wasserstoffabschreckung. HTM 50(1995) 1, pp. 27-30
[4] Edenhofer, B.: Einsatzhärten – Ein Prozess mit neuen Entwicklungen und Perspektiven. HTM Journal of Heat Treatment and Materials, vol. 56, no. 1, 2001, pp. 14-22
[5] Liedtke, D.: Stand des Einsatzhärtens aus industrieller Sicht. HTM Journal of Heat Treatment and Materials, vol. 64, no. 6, 2009, pp. 323-337
[6] Steinbacher, M.; Stenico, A.: Prozesssicheres Hochtemperatur-Aufkohlen feinkornstabilisierter Stähle in Atmosphären- und Vakuumöfen, Abschlussbericht zum AiF-Forschungsvorhaben 86ZN, Bremen, Juli 2006