Solidification Processes and Phase Transformations in Steel Alloys

Steel alloys undergo a complex journey from molten metal to finished component, with solidification and subsequent phase transformations dictating microstructure, mechanical performance and defect susceptibility. Upon cooling from the liquid state, nucleation of primary solid phases typically begins with delta-ferrite or austenite, depending on composition and temperature. These nuclei grow into dendritic or equiaxed morphologies, governed by heat extraction rate, local solute enrichment and interfacial energy. In peritectic and hypo-peritectic alloys, a reaction between primary delta-ferrite and remaining liquid produces austenite, often accompanied by volume changes that can induce internal stresses and surface cracks. Solidification microsegregation of carbon, phosphorus and other alloying elements further influences the liquidus-solidus range and the morphology of secondary phases.

Following complete solidification, steel continues to evolve through solid-state transformations. Slow cooling permits formation of pearlite, a lamellar mixture of ferrite and cementite, while moderate cooling rates favour bainite with its fine acicular structure. Rapid quenching can bypass diffusional processes and produce martensite, a supersaturated body-centred tetragonal phase responsible for high strength but limited ductility. Continuous-cooling-transformation (CCT) diagrams serve as roadmaps for tailoring heat treatments and predicting phase fractions as functions of cooling rate. Alloying additions such as silicon, manganese and aluminium adjust transformation temperatures, refine grain size and stabilise specific phases, thereby enabling a wide spectrum of properties from ultra-high-strength sheet to creep-resistant structural steels.

State-of-the-art characterisation techniques—including high-temperature laser scanning confocal microscopy, differential scanning calorimetry and in-situ diffraction—provide direct observation of solidification fronts, peritectic reactions and phase boundary migration. Complementary numerical models integrate thermodynamics, kinetics and heat-transfer to predict grain growth, solute distribution and defect formation in continuous casting and additive manufacturing. Progress in these areas underpins advances in steel quality, sustainability and the development of novel alloys for energy, automotive and infrastructure applications.

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Solidification Processes and Phase Transformations in Steel Alloys publication trend

The graph below shows the total number of articles in solidification processes and phase transformations in steel alloys across all publications each year (not limited to Nature Index journals).

Technical terms

Nucleation: The initial formation of a stable solid phase embryo within the liquid.

Dendritic growth: Tree-like crystal morphology resulting from directional solidification and solute rejection.

Peritectic reaction: A solid-liquid transformation in which an existing solid phase reacts with the remaining liquid to form a new solid.

Austenite: Face-centred cubic iron phase that can dissolve higher levels of carbon at elevated temperatures.

Ferrite: Body-centred cubic iron phase with low carbon solubility, providing ductility.

Bainite: A fine, acicular microstructure formed at intermediate cooling rates via non-martensitic transformation.

Martensite: A hard, supersaturated phase formed by rapid, diffusionless transformation from austenite.

CCT diagram: Graphical representation of transformation start and finish temperatures as functions of cooling rate.

Solute drag: The impediment of grain-boundary motion by segregated alloying elements.

References

1. In situ study and assessment of the phosphorus-induced solute drag effect on the grain boundary mobility of austenite. Acta Materialia (2024).
2. Decomposition of γ-Fe in 0.4C–1.8Si-2.8Mn-0.5Al steel during a continuous cooling process: A comparative study using in-situ HT-LSCM, DSC and dilatometry. Journal of Materials Research and Technology (2023).
3. Characterization of the γ-loop in the Fe-P system by coupling DSC and HT-LSCM with complementary in-situ experimental techniques. Materials Characterization (2021).
4. The Influence of Peritectic Reaction/Transformation on Crack Susceptibility in the Continuous Casting of Steels. Metallurgical and Materials Transactions B (2017).
5. Online Modelling of Heat Transfer, Solidification and Microstructure in Continuous Casting of Steel. IOP Conference Series Materials Science and Engineering (2019).