Hermann Müller

Influence of transformation plasticity on residual stresses and distortions due to the heat treatment of steels with different carbon content

The field of heat treatment of steels offers a large variety of applications for the use of simulation tools. It always includes the development of residual stresses and distortions. The geometry of the part, the composition of the material, the heat treatment process as well as the initial state of the part interact with each other in complex ways and have an influence on the distortion of the part. Using simulation the temporal development of temperature, phases, stresses and distortions while quenching as well as the residual stress distribution and distortion after quenching can be calculated. Transformation plasticity has been proved to be very important for heat treatment simulation. Three steels with identical contents of alloying elements but different carbon contents of 0.2, 0.5 and 0.8 wt. % were analysed. The influence of transformation plasticity during the martensitic transformation on the distortions and residual stresses after quenching of cylinders made out of the three steels was analyzed in simulations and compared to experimental results.

Stresses, residual stresses and distortions developing during brazing of cemented carbide and steel joints with cylindrical geometry

The residual stresses and distortions resulting from the brazing process have a strong influence on failure of the components under mechanical loading in service. X-ray and neutron stress analysis as well as numerical simulations were undertaken in order to better understand the formation of residual stresses in brazed joints of cemented carbide and steel bodies of different steel types. The influence of the dimension of the joint and the phase transformation behaviour of the steel component on the development of the residual stresses and distortions were investigated. A cylindrical geometry was chosen to avoid excessively long runtimes of the FEM simulations. Detailed results will be presented for joints with AISI 1045 (German Grade Ck45) steel bodies.

Simulation of Stress and Residual Stresses due to Gas Quenching of AISI 52100 Steel Cylinders

Steel hardening is a very old manufacturing process to improve the mechanical properties of components. Distortions and residual stresses after temperature balance are influenced by the hardening process. The subsequent rework and the life cycle depend on distortion and on residual stresses. During the heat treatment process the development of stress and deformation cannot be measured in situ. Only numerical methods allow the investigation of temperature, stress, deformation and microstructure development in the whole component during the quenching process. For this purpose heat conduction, phase transformations, and mechanical behaviour of the steel as well as the couplings and the interactions between strain and phase transformation under consideration of transformation plasticity and different volume changes have to be modelled in the used FE-Program. Residual stress and distortion measurements should be carried out to verify the numerically determined results. The choice of the quenching media has a large influence on the development of stress and deformation. For vaporizing liquids like oil the heat transfer depends on the temperature and the location of the specimen. Gas quenching is a modern, efficient process. Compared with oil, gas quenching leads to a more homogeneous and reproducible cooling process and generally to lower residual stresses. In this work numerically determined and experimental results for cylinders quenched with gas in a nozzle system which was developed at our institute are compared and discussed. Different gas pressures and gas media were investigated. Finally selected results of cylinders quenched with gas and oil will be presented and discussed.

Analysis of residual stresses and distortions in brazed joints of cemented carbide and steel

Brazing of cemented carbides to steel bodies gives rise to the development of complex stress states and distortions which influence the fatigue behaviour of the parts. It is quite important to estimate the residual stresses with numerical methods whose agreement is to be guaranteed with experimental characterization of the brazed parts. In this work FEM simulations and X-ray as well as neutron stress analysis were used to examine the residual stresses of brazed samples. Joints with different geometries and dimensions made of cemented carbide and different steels showing different phase transformation behaviours were investigated.

Maß- und Formänderungen von Stahlquadern mit unterschiedlichen Aussparungen beim Härten in Öl- und Wasser

Kurzfassung Die realitätsnahe Erfassung der Randbedingungen spielt in der Simulation des Härteprozesses eine entscheidende Rolle. In dieser Arbeit wird auf die Bestimmung des Wärmeübergangskoeffizienten an quaderförmigen und mit Aussparungen versehenen Modellgeometrien aus 42CrMo4 eingegangen. Durch die Analyse der Wiederbenetzungskinematik wird der Wärmeübergangskoeffizient ortsabhängig definiert. Die Ergebnisse der numerischen Simulation der Temperaturentwicklung werden mit Temperaturmessungen validiert. Des Weiteren werden die berechneten Eigenspannungen nach dem Härteprozess mit röntgenographisch gemessenen Eigenspannungen verglichen. Die Maß- und Formänderung beim Härten in Wasser und Öl wird experimentell und in der Simulation untersucht.