Volume 78, Issue 2

Within the research work of the Transregional Collaborative Research Center TRR 136, process signatures were determined for processes that generate essentially thermal loading of the component surface layer. These process signatures represent correlations between material modifications (e. g. changes of residual stress, microstructure, hardness profile, etc.) and characteristic values of the material loading. The underlying idea is that the material does not know any processes but only loads caused by processes. In particular, the same loads lead to the same material modifications regardless of the kind of process. A decisive advantage of this concept over conventional approaches is the possibility of calculating the necessary internal material loads backwards on the basis of concrete specifications for the material modifications. If there are additional correlations between internal material loads and process quantities as well as between process quantities and machining parameters, the necessary machining parameters can be determined, too. In this paper, this procedure will be introduced using the example of one-sided induction hardening of cuboidal components made of 42CrMo4. The determination of the process signature by experimental and numerical investigations is described and work for experimental verifications are presented.

To elucidate the mechanisms of residual stress generation in quenched steel parts has been a longstanding problem. Qualitative explanations for the changes in the stress distribution in quenched cylinders appeared in the 1930s when it became possible to measure residual stresses in specimens. The explanation at that time used the concept of thermal and transformation stresses, which is still included in current textbooks. This concept is referred to here as the estimated stresses-based approach. To simplify the explanation of stress generation due to the combined effects of temperature change and phase transformations, quenching experiments were devised using Fe-Ni alloys in which only martensitic transformation occurs. On the other hand, since heat treatment simulations provide strains resulting from thermal, phase transformation, plastic, transformation plastic, and creep phenomena, a method to elucidate the mechanism using these strains was devised in the early 2000s and named the simulated strains-based approach. This paper contrasts the estimated stresses-based and simulated strains-based approaches to the mechanism of residual stress generation in quenched Fe-Ni alloy cylinders and highlights the superiority of the latter.

Gas impingement jets are widely applied in industrial cooling processes. In continuous heat treatment lines of steel, aluminium and copper strips, impingement jet nozzle systems are utilised to achieve rapid cooling or heating. The heat transfer depends on the flow but also on the geometric parameters such as nozzle to strip distance and the nozzle shape. The key challenge while designing cooling sections is to determine the performance of those nozzle systems or their Nusselt number respectively. Jet cooling sections are challenging to model with computational fluid dynamics or in an experimental set up. Yet, RANS-turbulence models are a cost-effective way to predict Nusselt numbers. In this work the capability of the ANSYS generalized k-omega (GEKO) two-equation turbulence model to determine the local and integral Nusselt number of an impinging air jet is evaluated. The results are contrasted to experimental investigations.

Carbonitriding enhances properties of many steels and is therefore an attractive alternative for surface hardening of steel components in the mechanical industry. However, pore formation in the carbon and nitrogen enriched surface layer may occur under certain process conditions. For a given steel and case depth specification, pore formation can be managed by reducing the nitrogen activity of the carbonitriding atmosphere below a defined limit, depending on process temperature and process time. Recent progress in process control allows automatic and independent adjustments of the carbon and nitrogen activities and corresponding potentials of the carbonitriding atmosphere. This study contributes to the practical evaluation of pore formation limits under selected carbonitriding conditions for a range of commonly used engineering steel grades.