Thomas Lübben

Hardenability Testing and Simulation of Gas-Quenched Steel

The aim of a joint project between the Stiftung Institut für Werkstofftechnik, Bremen (IWT), the company Ipsen International, and the Faculty of Mechanical Engineering and Naval Architecture (FMENA), the University of Zagreb is to develop a computer program for prediction of hardness on the axial section of axially-symmetrical workpieces of any complex shape, thereupon high pressure gas quenching. The hardenability for the specimens made of tool steel grade EN-90MnCrV8 and the cooling dynamics under two gas pressures are measured using the unique facility at IWT. With developed computer simulation model, the cooling curves at different positions (J) along the end-quenched specimen are determined. Based on them, the cooling time from 800 to 500°C (t 8/5) is determined, and the curves J = f(t 8/5) are derived for different quenching conditions. This curves together with curves of hardness distributions along the end-quenched specimens can serve for the prediction of hardness distribution on the cross-sections of a batch of workpieces cooled in vacuum furnace.

Influence of Rewetting Behavior on the Distortion of Bearing Races

Immersion quenching in evaporable fluids like oil, polymer solutions, or water is a widely used technique in heat treatment shops because this technique provides higher heat transfer coefficients than the most alternative gas quenching techniques. The disadvantages of immersion quenching in evaporating fluids are the complex heat transfer mechanisms which consist of the three phases, film boiling, nucleate boiling, and convection. Especially the transition from film to nucleate boiling—the rewetting of the sample surface—is a complex process which leads to a strong position dependence of the heat transfer coefficient of the cooled work pieces. In particular, immersion quenching of thin-walled rings can result in large changes of ovality with comparable high scattering. To understand these results, two series of experiments were performed. Rings were quenched in oil in a special quenching tank and the rewetting behavior was documented. Furthermore, bearing races were quenched in a standard tank. Before and after heat treatment, these rings were measured by a coordinate measuring system and the distortion was determined. The investigations have clearly shown that the vapor film can have an important influence on distortion generation during oil quenching. The resulting distortion can principally be explained by the observed rewetting behavior.

Anisotropic Transformation Strain and Its Consequences on Distortion during Austenitization

The distribution of segregations, which is introduced in the continuous casting process and modified during succeeding manufacturing steps, is considered as an important “distortion potential carrier” for chemically banded steels. This article presents a recently developed mathematical model for integration of the effect of prior forming and cutting operations into heat-treatment simulations by considering “anisotropic transformation strain (ATS).” The model was justified experimentally by simulating the heating and austenitization of dilatometer specimens machined from the forged discs with distinct orientations with respect to the banded microstructure. After the verification, it is used in conjunction with former experimental work to demonstrate that the distribution of fiber flow is one of the important reasons of the dishing of carburized discs. The model provides promising results for process chain simulation to predict the heat-treatment distortion that cannot be predicted with currently available models.

Modeling of Dimensional Changes and Residual Stresses After Transformation-Free Cooling

Predicting thermal distortions and residual stresses after steel heat treatment is a complex task in which the solution involves the use of a number of process parameters and nonlinear variation of steel properties. Former investigations in transformation-free cooling processes of long cylindrical work pieces in a gas nozzle field showed a typical behavior of the dimensional changes which indicated the possibility of introducing dimensionless numbers to predict thermal distortions. It was found that the changes in the dimensions of cylinders correlate well with only a few dimensionless numbers which are defined as function of shape and dimensions of components, its initial temperature, temperature of the quenching media, heat transfer coefficient, heat conductivity, heat capacity, density, thermal expansion coefficient, Youngs modulus, Poissons ratio, yield strength, and strain hardening behavior. For a systematic investigation of impacts on dimensional changes and residual stresses after transformation-free cooling, a representative group of 28 austenitic stainless steels was selected from literature. Their properties were statistically analyzed and three representative combinations of steel properties have been selected. The numerical simulations were carried out by use of the commercial finite element (FE) program SYSWELD 2005 with the aim to predict residual stresses and change of dimensions of a long cylinder made of austenitic stainless steel after gas cooling from the high temperature down to room temperature. The FE results were analyzed with the nonlinear regression methods and with genetic programming methods. From these analyses, two dimensionless mathematical models were proposed, one for prediction of thermal distortions and the other for prediction of equivalent residual stresses. The proposed dimensionless regression models allow the portability of the calculation results to similar cooling conditions (temperature independent heat transfer) for a transformation-free cooling of long cylinders made from any austenitic steel selected from the considered representative group of steels.

The influence of geometrical features on rewetting behaviour of cylindrical components

During immersion quenching the transition from film to nucleate boiling - the rewetting of the sample surface - is a complex process which leads to extreme local changes of the heat transfer coefficient. The start of the rewetting process depends on specific geometrical features like edges, boreholes, couplings, etc. From these starting points the rewetting front moves forward. The present paper investigates the influence of geometrical properties on the starting of rewetting and the movement of the rewetting front across the work piece. Cylinders made out of stainless steel with different edge shapes, lengths, and diameters will be considered. The edge configuration varies in a few steps from chamfered edges to spherical formed edges. The quenching was done with high speed oil (Isorapid 277®). The aim of the paper is the gathering of information of the local and time dependent HTC necessary for heat treatment simulations.