A Real-Time Dynamic Study on the Synergistic Effects of Grain Size and Cooling Rate on Intragranular Ferrite Formation in Steels

Precise control of non-metallic inclusions is essential for achieving the desired mechanical properties of steels. Generally, fine inclusions are relatively difficult to remove completely during the refining process. They can serve as the heterogeneous nucleation sites for the formation of intragranular ferrite (IGF). This interlocked microstructure is the preferred type in the coarse-grain heat-affected zone of weldments to improve the toughness at low temperatures. In this study, two representative grades of low-alloy steels with 0.2 and 0.4 pct carbon were used to investigate the synergistic influence of the prior austenite grain size (PAGS) and cooling rate on IGF formation. In-situ observation experiments using high-temperature confocal laser scanning microscope, thermodynamic calculations and electron microscopy characterizations were used to investigate this synergistic effect in the two steel grades. It is found that the coarse grains can promote IGF nucleation and growth at an intermediate cooling rate, while smaller grain size requires a higher cooling rate as a driving force for IGF formation. This synergistic effect was verified in the steels with 0.2 and 0.4 pct carbon. The current work provides a quantitative study on the comprehensive influence of PAGS and cooling rate on IGF formation. The obtained findings are validated for general low-alloy steels with different carbon contents, contributing to the development of the ‘Oxide Metallurgy’ concept.