Dragutin Lisjak

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.

Determination of Steel Carburizing Parameters by Using Neural Network

The article discusses the application of neural networks for calculating the laws of complex processes, with carbon diffusion processes in steel carburizing classified among them. Empirical and mathematical models for the Carbomaag carburizing process, which have been proven in practice, are presented for the determination of technological carburizing parameters resulting in a required carbon concentration profile in the carburized layer. A comparison between the mathematical model (MM) and the neural network model (NNM) and the empirical model < EM) with respect to the time required for shallow and great carburizing depths is given special attention in this article. The results of the empirical carburizing model were used as a neural network training set and compared with the results of computer simulation of the MM. A comparison of results obtained at shallow and great carburizing depths shows that the NNM approximates much better the EM than the MM. It is assumed that the carrying out of a larger number of experiments and the repeated neural network training with new sets of experimental data would result in increasingly better solutions. Thus, the disadvantages of theoretical models, i.e., MMs, would be avoided.

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.