Contact Press / Media
Dr. Holger Friedrich
Fraunhofer-Center for High Temperature Materials and Design
95448 Bayreuth, Germany
Phone +49 921 78510-300
Fax +49 921 78510-001
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:
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.