In Quenching & Partitioning (Q&P) steels, silicon is added as an alloying element in order to supress cementite precipitation, thus ensuring maximum carbon availability to partition from martensite into austenite. However, despite the addition of silicon, the precipitation of transitional carbides has been reported in recent literature. This raises a question as to the role played by silicon in carbide formation processes.
In order to gain further insight into the effect of silicon, two model alloys have been studied: Fe-1C-1Mn and Fe-1C-1Mn-2Si (wt.%). Quenching and Partitioning cycles have been applied, where the partitioning stage ranged from 30 seconds to 1 hour. By means of dilatometry, X-ray diffraction (XRD) and atom probe tomography (APT), the evolution of the microstructure has been characterised. Owing to the high carbon content within the alloys and the resulting tetragonality effect of the martensite, XRD allows the distinction of the martensite formed during the first quenching that becomes carbon-depleted after the partitioning step (denoted as M1), from the carbon-rich martensite formed in the last quench of the heat treatment (denoted as M2). Furthermore, based on the tetragonality of M2, it is possible to deduce the changes in carbon content occurring within the austenite during the partitioning stage. Dilatometry and XRD results suggest the onset of decomposition of austenite during the final quench, which appears to be delayed for the silicon-containing alloy.
At the atomic scale, it has been observed that the distribution of carbon within M1 is non-uniform, where numerous carbon-rich features were seen surrounded by a carbon-depleted matrix, hinting the presence of carbides in both alloys. However, the effect of silicon can be appreciated in the carbon partitioning process, since a significant difference in the carbon profile across the austenite/M1 interface is visible between the two alloys. It appears that the stability of the austenite via carbon partitioning is favoured by the presence of silicon.