Stable processes with high product quality

For the optimization of process parameters of industrial heat treatment processes, HTL offers a special methodology based on individually developed ThermoOptical measurement systems (TOM). This method consists of several steps and is very flexible in its application. The optimization goals are always defined in dialogue with the customers. Depending on the requirements, HTL helps to minimize reject rates, to optimize product attributes, to increase transfer rate or/and to minimize energy consumption. The wear of furnace components can also be minimized by optimization of the process parameters.

Heat treatments in the industry have to be stable processes resulting in high product quality and low costs. The costs consist of the depreciation for the - generally capital-intensive - furnace plants, the labor costs for maintenance and operation as well as the costs for wear parts, consumables and energy. In order to keep the depreciation costs low, a high throughput for the material is striven for. This generally also improves the energy efficiency of the heat treatment. However, the shortening of the temperature cycle results in higher thermal stresses in the material and the temperature-loaded furnace components (linings, carriers, capsules, etc.). This leads to higher reject rates for the heating product, respectively shorter maintenance cycles, which in turn increase the manufacturing costs.

For the optimal operation of a furnace, there are process parameters that lie between too low and too high throughput. Finding the optimal process parameters in industrial heat treatment requires special tools. With a simple  stretching or compression of the time-temperature cycle, the optimum cannot be found, because the critical process zones relevant to the result and the thermal energy required for the material are not sufficiently taken into consideration. The otherwise very powerful design of experiments fails very often when optimizing temperature-time cycles, because the heating and cooling rates, exposure times and temperatures are strongly correlated with each other. Thus, the principal effects to be examined are obscured by interaction effects in statistical test plans. In addition to the temperature cycle, the result of the heat treatment also depends on further parameters: The furnace atmosphere and the local interaction between the material and the furnace atmosphere, the arrangement of the material in the furnace and its thermal and chemical interaction with the environment.

HTL offers a special methodology for the target-oriented optimization of the parameters for industrial heat treatment processes and the improvement of process understanding. Four steps are required:

1. In the first step, the industrial heat treatment process is simulated in a special laboratory furnace. The so-called ThermoOptical measurement systems (TOM) of HTL, developed specifically for this purpose, accurately reproduce the atmosphere of the industrial furnace. The TOM furnaces have measuring probes to determine in situ, i.e. during the heat treatment, the material properties relevant for the description of the material change in the heating process. The measurements are usually consucted on small samples of material, because they can be heated under precisely defined conditions, e.g., without large temperature gradients.
2. In the second step, the measured data about process kinetics is parameterized using numerically robust methods are used. The results are transferred to a finite element (FE) model. In the FE model, the thermal, mechanical and chemical effects that occur during the heat treatment are coupled to one another. By means of the FE methodology, the sample size is scaled up to the component size and geometry. In this way, stresses and deformations occurring in the component can be already determined in the FE simulation. The process parameters are varied in order to determine optimal process conditions on the computer.
3. The optimized process conditions are then checked by heat treatment in the laboratory furnaces. Larger samples or components are used for this purpose. In these experiments, the process parameters are fine-tuned. If necessary, interactions with the furnace environment are investigated here as well – if not already during the previous steps. The process parameters are determined from the point of view of the material (so-called worm’s-eye view), which is optimal with regard to the product quality.
4. Finally, the optimized process parameters are transferred back to the industrial furnace. If necessary, investigations on the industrial furnace with regard to furnace atmosphere, gas flow and temperature distribution will be carried out. In this phase, FE simulations can be helpful as well for scaling up the optimized process to the production scale. Since the conditions in large industrial furnaces are generally not ideal, for example, because of temperature differences in the useable volume, adjustments to the process parameters may be necessary. For example, temperature-time cycles can be optimized, so that, despite inhomogeneous temperatures in the industrial furnace, the scattering of the product properties becomes minimal. The optimized process parameters are finally transferred to the user.

The methodology developed at HTL for the optimization of heat treatment processes can be used very flexibly. Thus, the optimization of a heat treatment process can also be focused on its particularly critical subprocesses. The optimization targets are defined by the customer. Depending on the customer's requirements, HTL minimizes reject rates, optimizes product characteristics, increases throughput rates and minimizes energy consumption. The wear of furnace components can also be minimized when optimizing the process parameters. Applications are in the production of silicate ceramics, oxide ceramics, non-oxide ceramics, refractory products, powder metallurgical products as well as in metal production. Both the product quality and the costs, energy and material efficiency of the heat treatment processes can be optimized with the methods of HTL.

In addition to the optimization of the process parameters, a potential analysis of production furnaces can also be carried out by HTL. HTL has developed a mobile furnace testrig for this purpose. This is used on site in order to determine furnace properties, while the oven is running. From the data, conclusions can be drawn about possible improvements in the production furnace with regard to product quality or energy efficiency.