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Chamber Furnaces: TQ Versus RTQ

Blog Post
By Dirk Joritz – Ipsen International GmbH

In today’s world, metallic components have to meet ever greater requirements, for example regarding wear or corrosion resistance. To ensure corresponding quality even with comparatively inexpensive raw materials, the components are often subjected to industrial heat treatment. Depending on the quantity of components to be treated and the required flexibility of the plant, either continuously-operated or batch-load furnaces are used.

Continuously operated furnaces are suitable for mass production in the automotive industry, for example. Batch-load systems, also called (single) chamber furnaces, stand out due to their high flexibility: after each batch, the program and consequently the component to be treated can be swapped-out. This makes these systems very popular, for example, with commercial heat-treatment companies.

Chamber furnaces used for hardening and case hardening (carburisation followed by oil quenching) comprise a heating chamber and a directly adjacent oil bath. Depending on the design, a distinction is made between two groups:

  1. Furnaces based on the through quench principle (TQ furnaces)
  2. Furnaces based on the return through quench principle (RTQ furnaces)
TQ and RTQ chamber furnaces

In TQ furnaces, which operate according to the through quench principle, the components to be treated are loaded directly into the heating chamber, which operates above 750 °C, where they are heated up and, if necessary, carburised. The components are subsequently transported to the quenching chamber, where they are usually quenched in an oil bath. This also hardens the surface layer. The components are then removed from the furnace.

In RTQ furnaces, which operate according to the return through quench principle, the components to be treated are first placed in the relatively cold (approx. 120 °C to 140 °C) quenching chamber for a few minutes. After that the batch is loaded into the heating chamber, which operates above 750 °C, where it is heated up and, if necessary, carburised. Subsequently, the components are hardened in the quenching chamber, as in the TQ. The components are then removed from the furnace.

Even though the batch sizes are identical, the actual heat treatment program is the same in both furnaces and the results achieved vary only minimally, there are some differences that need to be taken into account when selecting the appropriate furnace system.

Required space and cost:

The TQ furnace has both a loading and unloading door, so suitable loading systems must be installed both in front of and behind the furnace. Accordingly, this type of furnace requires more space than the RTQ furnace, which manages with only one combined system for loading and unloading. In terms of cost, the two furnaces themselves differ only insignificantly; however, since two charging systems are added to a TQ furnace, procurement is initially somewhat more expensive. However, if a line with at least three TQ furnaces is used for heat treatment, this investment is quickly amortised and the TQ method becomes cheaper than the RTQ method in the long run.

Treatment duration and throughput:

The heat treatment program in the heating chamber is identical in both furnace systems. The difference can be found in the treatment time: this is somewhat longer in the RTQ furnace, as the charge is loaded via the quenching chamber and remains there for a short flushing duration (approx. 15 to 20 minutes). This gives the TQ furnace a slight advantage in terms of throughput, as the next batch can already be loaded into the heating chamber while the preceding batch is still curing in the oil bath. With the RTQ system it is possible that one batch could also be put into the quenching chamber while the previous batch is still in the oil bath (so-called double loading). But this is only possible if the quenching time in the oil bath is significantly longer than the purging time before loading the heating chamber. This means that
with short quenching times, the furnace must first be completely unloaded before the next batch can be treated.

Maintenance and control:

The TQ furnace scores here as well: during normal production it is possible to look into both the heating chamber and the quenching chamber during loading and unloading, for example to observe the condition of the brickwork or possible sooting. With a RTQ furnace, only the quenching chamber can be inspected during operation; inspecting the heating chamber is only possible when the furnace is taken out of production. The TQ furnace is more accessible for maintenance thanks to its two doors. However, it also creates a little extra work because two doors have to be monitored and inspected.

Heat treatment results and process flow:

As mentioned at the beginning, the heat treatment results do not vary significantly between TQ and RTQ furnaces. Nevertheless, a few details should be mentioned: when loading the TQ furnace, air enters directly into the heating chamber. This means that the furnace atmosphere is disrupted and only builds up again after the door is closed. In the RTQ furnace, on the other hand, the batch initially remains in the quenching chamber for a short purging time. The negative effect of the air that has penetrated is compensated for there. When loading the heating chamber, no further air enters the heating chamber and the furnace atmosphere is restored more quickly.

In order to minimise the heating up time in the heating chamber, batches are often preheated to a temperature of 350 °C to 450 °C. In this case, the dwell time in the quenching chamber of RTQ furnaces would be rather detrimental, as the batches lose temperature during this time. This reduces the desired preheating effect.

In summary, it can be said that both furnace systems perform well in different areas and that one type should not be preferred over the other. If, for example, you only have limited space available, a TQ system is probably less suitable. If preheating is of crucial importance to you, a RTQ furnace does not necessarily fit into the concept. That makes individual customer requirements the deciding factor in the selection process.

We will be happy to advise you in more detail on the different types of atmosphere furnaces and their possible applications to ensure that you make the right choice. For more information, please feel free to contact us at [email protected].

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