Study on Bainitic Transformation by Dilatometer and In Situ LSCM

Abstract

This study investigates the bainitic transformation kinetics of carbide-free bainitic steel with Si + Al and carbide-bearing bainitic steel without Si + Al, as well as the phase transformation and microstructure through in situ high-temperature laser scanning confocal microscopy. Results show that bainitic ferrite plates preferentially nucleate at the grain boundary. New plates nucleate on previously formed ones, including two dimensions which appear on a plane where a three-dimensional space of bainitic ferrite forms. Nucleation on the formed bainitic ferrite is faster than that at the grain boundary in some grains. The bainitic ferrite growth at the austenite grain boundary is longer and has a faster transformation rate. The bainitic ferrite growth on the formed bainitic ferrite plate is shorter and has a slower transformation rate. The location and number of nucleation sites influence the thickness of the bainitic ferrite. The higher the number of plates preferentially nucleating at the original austenite grain boundary, the greater the thickness of the bainitic ferrite.

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

“…to directly observe the bainite transformation has found increasing use in the metals research community [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], complementing more traditional methods such as (post mortem) metallography and dilatometry (not yielding morphological or local information). This relatively inexpensive technique has a number of advantages as it enables real time observation of bainite sheaves, allowing individual kinetic measurements, identification of probable nucleation sites and so on, but lacks a high spatial resolution and in the end remains a surface analysis technique.…”

“…Nevertheless, the transformations in binary Fe-C alloys [5], the Fe-C-Mn [6] and Fe-C-Ni systems [7,8], as well as alloys of more complex compositions [9][10][11][12][13][14][15], including superbainitic steels [16][17][18][19][20], have been investigated with this technique under a range of conditions including continuous cooling and isothermal holding. Under optimal conditions measurements of individual lengthening rates could be made and nucleation sites could be identified.…”

Microstructure and properties of an advanced high-strength steel after austempering treatment

“…The bainite transformation kinetics of CFB steels is very complicated and many factors such as alloying compositions and the austenisation conditions can affect it. Most commonly used alloying elements except for Al are reported to retard bainitic transformation in conventional steels [10], although some recent research argues differently in CFB steels [11,12]. Si is reported to postpone the starting time but advance the end time of the bainite transformation [11].…”

“…Most commonly used alloying elements except for Al are reported to retard bainitic transformation in conventional steels [10], although some recent research argues differently in CFB steels [11,12]. Si is reported to postpone the starting time but advance the end time of the bainite transformation [11]. The effect of austenisation conditions and cooling history, which decides the austenite grain size and uniformity of alloying elements, on the evolution of the bainite transformation in CFB steels is not yet certain.…”