PIETRZYK Maciej Akademia Gorniczo-Hutnicza

Spoluautoři KUZIAK Roman

Hardenability describes the capability of a steel to transform to a given microstructure in a heat treatment process. This has been most successfully applied to martensitic steels. On the contrary bainitic hardenability remains difficult to define. During recent years anew generation bainitic steels with extensive perspective applications [1] have been developed and there is a need for fast evaluation of hardenability of these steels. The objective of the current work was to propose numerical methods of assessing defining bainitic hardenability that could be easily applied in industrial practice. Development of the phase transformation model is the first step of the procedure. Modified Avrami equation was used as a model and coefficients of this model were identified on the basis of dilatometric tests and inverse analysis [2]. A series of low carbon low alloy steels have been subject to this procedure. The developed phase transformation model was applied to evaluate the hardenability of the investigated steels. The objective was to provide a tool, which will help in designing of the composition of bainitic steel with the enhanced hardenability. In order to enable comparison of hardenability of various steels, search for certain general factors which characterize this hardenability, was made. The following features, all based on the relation of the bainite volume fraction and cooling rate (Xb = Xb(Cr)), were considered when defining the hardenability factors: o The area under the curve – the largest is this area the better is the hardenability of steel. o Maximum volume fraction of bainite and the cooling rate, at which this maximum occurs – the larger this maximum is and at lower cooling rate it appears? occurs, the better is the hardenability. Thus, having the mentioned features in mind the following two factors, which are the measure of the hardenability, were suggested: o ratio between maximum volume fraction of bainite and the cooling rate, at which this volume fraction is obtained, o the area under the plot Xb = Xb(Cr) in the range, in which volume fraction of bainite exceeds 0.3. Calculations of these two hardenability factors were performed for a number of steels with different chemical compositions. The results were compared and search for a correlation between the chemistry and the hadenability factor was made. References 1. Kuziak R., Pidvysots’kyy V., Węglarczyk S., Pietrzyk M., Bainitic steels as alternative for conventional carbon-manganese steels in manufacturing of fasteners - simulation of production chain, Computer Methods in Materials Science, 11, 2011, 443 – 462. 2. Pietrzyk M., Kuziak R., Modelling phase transformations in steel, in: Microstructure evolution in metal forming processes, (eds), Lin J., Balint D., Pietrzyk M., Woodhead Publishing, Oxford, 2012, 145-179.