AID4GREENEST researchers from the University of Oulu (UOulu), have published new research on the effect of solidification-induced segregation on austenite formation and grain growth during re-heating of a forged steel part.
The study, highlighted in the IOP Conference Series: Materials Science and Engineering, presents new experimental and modelling results that shed light on how chemical segregation formed during casting influences microstructural evolution during the reheating of large, forged steel components.
Medium carbon steels alloyed with elements such as chromium, molybdenum, nickel and vanadium are widely used in demanding applications, including large shafts for power plants, where an optimal combination of strength, ductility and creep resistance is required.
Achieving these properties depends on controlling the prior austenite grain size during heat treatment. As austenite nucleates and grows during reheating, the size and distribution of these grains directly influence the subsequent phase transformations that occur upon cooling.
“Although the chemical composition of the steel favours uniformity of elemental distribution throughout the matrix, the solidification process during the casting of the ingot may actually result in extensive macrosegregation of the alloying elements,” explain the publication’s authors Prof. Mahesh Somani, Dr. Aarne Pohjonen, Oskari Seppälä and Rishabh Bharadwaj from UOulu.
“This is especially true in large castings meant for industrial production. Effectively, these castings are influenced by dendritic solidification which results in trapping of solute-rich liquid between the dendritic arms. These interdendritic regions later solidify into solute-rich regions, a phenomenon known as microsegregation”.
In the recent publication, clear variations in prior austenite grain structure were observed in 30CrMoNiV5-11 type steel, and elemental analysis confirmed that these differences were directly associated with solidification-induced segregation.
To better understand and predict this behaviour, the researchers extended a previously developed cellular automata (CA) model describing austenite formation and grain growth during heating.
Segregation effects were introduced into the model by allowing position-dependent variation of nucleation and growth parameters to represent alloy-rich and alloy-poor regions. Through a numerical test case, the researchers demonstrated how these local variations can be included in the numerical simulations to simulate the observed spatially heterogeneous austenite grain structures.
The model allows for reproducing the type of grain size differences observed experimentally, underlining how the incorporation of segregation into the modelling framework is essential for realistically simulating reheating of large, forged components. In future studies, the model’s parameters will be calibrated to correspond to the experimentally observed grain structure variations.
“In future studies, we will aim to fit the model using thermodynamic software and to experimentally observe austenite grain structure that will be obtained from interrupted heating tests, as well as grain growth studies,” the authors state.
References: Pohjonen, A., Seppälä, O., Bharadwaj, R., Somani, M. 2025 IOP Conf. Ser.: Mater. Sci. Eng. 1335 012028