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Simulationsbasierte Regelung der Laserhärtung von Stahl
Bei der Oberflaechenhärtung mit Hilfe von Laserstrahlen ist eine konstante Einhärtetiefe erwünscht, wobei gleichzeitig Anschmelzungen vermieden werden sollen. Um Anschmelzungen zu verhindern, kann die Temperatur im Auftreffpunkt des Lasers gemessen werden und die Laserleistung entsprechend geregelt werden. Eine konstante Temperatur fährt bei geometrisch komplizierten Bauteilen jedoch nicht zu einer konstanten Einhärtetiefe. In dieser Arbeit wird ein Verfahren aufgezeigt, wobei durch numerische Simulationen eine nichtkonstante Oberflächentemperatur berechnet wird, die eine konstante Einhärtetiefe liefert. Die berechnete Oberflächentemperatur kann als Solltemperatur im realen Prozess benutzt werden.
Phase transformation modeling and parameter identification from dilatometric investigations
The goal of this paper is to propose a new approach towards the evaluation of dilatometric results, which are often employed to analyse the phase transformation kinetics in steel, especially in terms of continuous cooling transformation (CCT) diagram. A simple task of dilatometry is deriving the start and end temperatures of the phase transformation. It can yield phase transformation kinetics provided that plenty metallographic investigations are performed, whose analysis is complicated especially in case of several coexisting product phases. The new method is based on the numerical solution of a thermomechanical identification problem. It is expected that the phase transformation kinetics can be derived by this approach with less metallographic tasks. The first results are remarkably promising although further investigations are required for the numerical simulations.
A molecular dynamics view of hysteresis and functional fatigue in martensitic transformations
Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and ?reverse? shape memory in the high symmetry phase. Processing can change these features: repeated cycling can ?train? the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to creation of persistent localised defects, which are impossible to study using non-atomistic methods. Here we present a molecular dynamics simulation study of this behaviour. To ensure the largest possible system size, we use a two dimensional binary Lennard-Jones model, which represents a reliable qualitative model system for martensite/austenite transformations. The evolution of the defect structure and its excess energy is investigated in simulations of cyclic transformation/ reverse transformation processes with 160,000 atoms. The simulations show that the transformation proceeds by non-diffusive nucleation and growth processes and produces distinct microstructure. Upon transformation, lattice defects are generated which affect subsequent transformations and vary the potential energy landscape of the sample. Some of the defects persist through the transformation, providing nucleation centres for subsequent cycles. Such defects may provide a memory of previous structures, and thereby may be the basis of a reversible shape memory effect.
Numerical cooling strategy design for hot rolled dual phase steel
In this article, the Mo-Mn dual phase steel and its process parameters in hot rolling are discussed. The process window was derived by combining the experimental work in a hot deformation dilatometer and numerical calculation of process parameters using rate law models for ferrite and martensite transformation. The ferrite formation model is based on the Leblond and Devaux approach while martensite formation is based on the Koistinen-Marburger (K-M) formula. The carbon enrichment during ferrite formation is taken into account for the following martensite formation. After the completion of the parameter identification for the rate law model, the evolution of phases in multiphase steel can be addressed. Particularly, the simulations allow for predicting the preferable degree of retained strain and holding temperature on the run out table (ROT) for the required ferrite fraction.