Bainitic Ferrite Plate Thickness Evolution in Two Nanostructured Steels

Abstract

Bainitic ferrite plate thickness evolution during isothermal transformation was followed at the same holding temperatures in two nanostructured steels containing (in wt.%) 1C-2Si and 0.4C-3Si. A dynamic picture of how the bainitic transformation evolves was obtained from the characterization of the microstructure present at room temperature after full and partial transformation at 300 and 350 °C. The continuous change during transformation of relevant parameters influencing the final scale of the microstructure, YS of austenite, driving force of the transformation and evolution of the transformation rate has been tracked, and these variations have been correlated to the evolution of the bainitic ferrite plate. Instead of the expected refinement of the plate predicted by existing theory and models, this study revealed a thickening of the bainitic ferrite plate thickness as the transformation progresses, which is partially explained by changes in the transformation rate through the whole decomposition of austenite into bainitic ferrite.

Bainite plate thickness reduction and microstructure tailoring by double austempering of Al-rich 3Mn steel

“…12b) with a progress of transformation from 212 nm after 25% up to 235 nm after the transformation completion. This phenomenon is consistent with the literature [48] and is caused by the changes in the yield strength of the austenite from which the bainite grows and the decrease in transformation rate (driving force for transformation) as transformation proceeds. Hardness decreases with a progress of transformation, which is obviously caused by a decrease in high temperature martensite fraction (Figs.…”

“…Al in bainitic TRIP-aided steels is basically added to reduce the harmful effect of carbides on RA stabilization [5,47]. However, Al is also a strong α phase stabilizer, which increases the driving force for bainitic transformation and therefore accelerates the transformation time itself [48,49]. The tested steel, despite the increased content of manganese, balances its slowing effect on the bainitic transformation with the addition of aluminum, which allows for a significant reduction in heat treatment time.…”

Effect of prior martensite formation on the bainite transformation kinetics in high-strength 3% Mn multiphase steel

“…Moreover, the higher tempering effect of martensite occurring at 350 °C results in some decrease in its tetragonality, what further decreases the hardness. Moreover, in both cases, longer times result in the small increase of bainite lath thickness, which could further contribute to some hardness decrease at both temperatures [5].…”

“…The thickness of bainite laths depends on the chemical composition of steel and heat treatment parameters. These factors affect the driving force for bainitic transformation and eventual stabilization of retained austenite [4][5][6][7]. The ultra-fine bainitic microstructure is usually obtained via isothermal treatment at a temperature about 300 °C, above martensite start temperature (M s ).…”

In Situ Tem Observations of the Growth of Bainitic Ferrite in an Fe-0.3c-3Mn-1.5Si-0.15Mo Steel

“…Typically, it might be expected that as the transformation progresses the increase in austenite yield strength as a result of the shrinking austenite grain would tend to reduce lath thickness and aspect ratio [45,46]. However, Ruiz-Jiminez et al [53] note in high and medium carbon steels containing greater than 1.5wt% Si, the thickness unexpectedly increased as the transformation proceeded, an effect which was correlated with the rate of transformation rather than the state of transformation.…”