By Matthias Rink – Ipsen International GmbH
In many heat treatment processes (e.g. hardening or case hardening), the transformation into a martensitic structure plays an essential role. This transformation causes the hardness increase. To achieve this martensitic transformation, the components/batches have to be rapidly quenched from hardening temperature (usually 850-900 °C) to a temperature of (usually) 60-80 °C. The available duration for this depends on the chemical composition of the material (proportions of alloying elements) and the size of the component.
In simple terms, with increasing alloying element content, quenching can take longer as well as the martensitic transformation (martensitische umwandlung). A corresponding cooling curve is shown in Figure 1 (rapid cooling [schnell kühlung]). However, if the quenching lasts too long, no martensitic transformation takes place. Instead, a ferritic/bainitic (ferritisch/bainitische) structure will develop, which cannot achieve the required hardness values. Reasons for this could be a too low alloying element content (possibly also a wrong material) or changed component or batch size/density. Larger/heavier components have a higher heat capacity and therefore need a longer time to transfer the heat to the quenchant (usually oil, salt or gas). Figure 1 (slow cooling [langsame kühlung]).
In the case of batches with many layers and thus a large component surface, the quenching oil heats up as it flows through the batch. The warmer the quenching oil, however, the less heat can be transferred and thus slows down the quenching process of the components in the upper layers. In such a case, an uneven distribution of hardness values from the bottom (higher hardness values due to colder oil) to the top (lower hardness values due to warmer oil) can occur.
To solve this, the quenching effect has to be increased in order to achieve the desired structure and/or the required uniform hardness values over the entire batch. However, since in most cases the design of the oil bath itself (capacity and depth of oil coverage) and the oil quality cannot be varied, the only possibility is to change the speed of the oil bath circulator (in this case to increase it). In conventional, already installed furnace systems there are only three options for the speed of the oil circulation:
- No circulation
- Slow circulation (approx. 750 U/min)
- Fast circulation (approx. 1200 U/min)
In addition, the power of the motors for the oil circulation is usually relatively low (approx. 2.5 kW).
In order to be able to achieve significant improvement, Ipsen has developed the Super Quench® oil bath design. The SuperQuench is a slightly deeper oil bath in which the oil flows through guiding channels and from bottom to top through the charge. Since the guiding channels below the batch are ending in a register (see Figure 2 and 3) that exactly corresponds to the batch base area, it can be guaranteed that the quenching oil flows uniform through the batch and does not flow past it uselessly to the side.
The motor rating of the oil bath circulator has also been increased drastically to 7.5 kW per motor. In addition to the increased motor power, the circulation speeds can also be continuously adjusted up to 1500 rpm (for a short time even up to 1800 rpm), so that the optimal quenching speed can be programmed segment by segment in the Ipsen Carb-o-Prof® 4 for each component/batch.
Even with the SuperQuench, the quenching should be realized as fast as necessary (in order to achieve the hardness requirements for the component), but still be carried out as slowly as possible (in order to avoid the inevitable thermal induced stresses and therefore to minimize distortions).
As already mentioned above, the use of the SuperQuench leads to an increase in the flow rate of the oil through the batch with a simultaneous flow equalization across the batch cross section (see Figure 5).
This means that, on the one hand, components made of relatively low-alloyed steels can still be martensitically hardened and, on the other hand, good results can also be achieved in the top layers of a batch (where ‘only’ 3-4 layers could be charged with conventional quenching baths it now 5-6 layers). It is precisely this possibility that increases the throughput of the furnace system and increases productivity significantly.
In summary, it can be said that by using the Ipsen SuperQuench, both the quenching intensity and the quenching uniformity can be increased, which leads to better quenching results with improved uniformity and possibly also to an increase in productivity. Due to the variable speed of the oil circulation motors, the hardness values of the components to be treated can be optimized and distortions can be minimized. The frequency converter and suitable programming mean that the total energy requirement of the quenching bath is even lower, despite the higher motor power of the oil bath circulator.
We would be happy to advise you in more detail on the various options that the SuperQuench system offers for chamber furnaces, so that you can make the right choice. For more information, please feel free to contact us at [email protected].
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