The primary applications for our furnaces are advanced materials (ceramics), glass, laboratory applications including dental laboratories and especially thermal-processing technology.

The demand for improved temperature uniformity in combination with increased productivity as well as stable and reliable processing equipment is increasing significantly. Hence, compliance with specifications like AMS 2750E and CQI-9 is more frequently requested by our customers. Living up to these requirements is more challenging for continuous furnaces than for batch furnaces.

At first, a suitable measuring setup must be determined for the specific process. Looking at the heat treatment or tempering of springs as a bulk good, this needs to be done within the charged furnace and appropriate test pieces holding the thermocouples.

In terms of the process itself, a fast and homogeneous heat-up without any overshoots due to overheating, for example, is required as a first step. Afterward, the springs need to dwell for a certain time with a stable and homogeneous temperature uniformity across the charged area in accordance with the requirements of CQI-9. In order to comply with these requirements, an improved furnace technology is required.


Continuous Furnace with Jet Heating

The technical solution to meet the defined challenges is a continuous furnace with jet heating in the heat-up zone and conventional forced convection in the following dwell zones. Figure 1 shows one of these furnaces for the heat treatment of springs at a working temperature of up to 500°C (932°F) with a throughput of 300 kg/hour. Its heated length is approximately 3,600 mm (12 feet), and the useful band width is 1,100 mm (43 inches). Other realized furnace models are able to heat treat 400 kg/hour, and the largest ones even up to 600 kg/hour with a heated length of less than 6,000 mm and the same process cycle time and productivity.

Within the first zone, the springs are rapidly heated to the required setpoint without overheating to ensure a homogeneous quality of spring properties. A specially designed air-guidance frame is the technological feature that realizes this huge benefit. The heated air hits the bulked springs from the top with a very high velocity and volume flow rate down to the conveying belt level (Fig. 2).

After hitting the belt, the air is recycled and heated up to repeat the process. With this technology, Nabertherm realizes short and very homogeneous heat-up times at a high productivity without overshoots due to over-temperature, which are not allowed by CQI 9. For springs, typical bulking heights of 50 mm for small springs with a high bulk density can be heated up quickly.

In the second and third heating zones of the furnace, the springs dwell at set temperature for the required amount of time to achieve the customer-requested properties. Within these heating zones, a conventional forced-convection air-guidance frame (including baffles) is used to optimize the airflow onto the charge and achieve the required temperature uniformity over the belt width (Fig. 2).

In other words, an optimal temperature uniformity over the belt width can be obtained to meet the strict regulations of automotive market suppliers. The furnaces can be optionally designed to fulfill Nadcap or CQI-9 norms. For example, the furnace shown in figure 1 achieves a temperature uniformity of better than +/-10°C in loaded condition according to CQI-9.

The measurement setup is shown in figure 3 and a typical measurement report in figure 4. For the measurement, five specimens of springs welded in the shape of typical bulking formations with tubes to fix the thermocouples are set into the bulked goods. The specimens are evenly distributed over the workspace (useful band width), and the measurement is conducted during continuous operation of the furnace with the designated charge throughput.


Conveying System and Automation

The conveying system is a stainless steel mesh belt driven by chain wheels (Fig. 5). The huge benefit of using stainless steel is that the system can be run lubrication-free. This is beneficial for many reasons. For example:

  • Reduction of maintenance cost
  • No contamination of the final product with lubricants or burnt residues
  • No evaporation/fumes are released into the working environment due to decomposing or burning lubricants


The furnaces can be adapted to different modes and requirements for loading and unloading. From completely manual loading and unloading for larger springs to fully automized loading and unloading, nearly everything is possible. The belt width can also be divided by separators along the movement direction in order to treat different springs or different tolerance levels of one spring at the same time. The customer can choose between the following solutions:

  • Manual loading and unloading of large springs
  • Manual loading of large springs and passive unloading by a slide into a carrier box or onto a cooling track
  • Automatic loading by a slide from a pre-process and unloading onto a slide into a box or onto a cooling track
  • Belt width divided into three lanes – one wide central lane and two smaller ones to the left and right side of the belt. During the pre-process, the coiling machine automatically measures the springs and separates them into three tolerance levels (too short, OK, too long), which are directly transported into the furnace in separate lanes. All springs are treated identically in the process, and the tolerance level of the shorter and longer springs can be corrected by consecutive milling. Figure 1 shows this setup with the required handling devices in front and behind the furnace.

    The aforementioned examples affirm that fully automized lines are possible and can easiliy be realized.


Summary and Outlook

Nabertherm continuous furnaces allow a highly productive in-line tempering process for springs as bulk goods or as single pieces. With this technology, a faster heat-up is achieved without using over-temperatures that might influence the final properties or result in overshooting, which is not allowed by CQI-9.

Due to the faster heat-up phase, the dwell cycle under homogeneous temperature distribution can either be extended without changing floor space or the floor space can be reduced with a shorter furnace length. By the semi-modular furnace design with one or more heating zones and the following dwell zones, the furnaces can be easily adapted to different throughput requirements by extending or decreasing the overall length.

Choosing the right furnace provides productivity gains at a high quality level as a competitive edge not only for springs, as reported in this article, but also for other bulk goods.

For more information: Contact Roland von Bargen, project manager, Nabertherm GmbH, Bahnhofstr. 20, 28865 Lilienthal / Bremen (Germany); tel.: +49 (4298) 922-165, fax: +49 (4298) 922-129; e-mail:; web: Nabertherm is a Germany-based company offering furnaces since 1947, with industrial furnaces covering nearly all heat-treatment needs.