The bainite structure in steel, cast iron and ferroalloy is extremely complex, which is directly related to the intermediate transition characteristics of bainite transformation. Bainite in steel is essentially the matrix of bainite ferrite, on which the organic combination of θ cementite (or ε carbide) or residual austenite is distributed. Bainite ferrite (BF), carbide, retained austenite, martensite and other phases constitute a complex whole structure.

Bainite structure and morphology of 1 and ultra-low carbon steel

In recent years, the research on carbon content 2, microstructure and morphology of upper bainite,

The upper bainite is formed in the upper part of the bainite transition temperature zone (BS ~ nose temperature), and there are different forms such as feathered bainite, carbonless bainite and granular bainite.

Carbonless bainite is more common in low carbon and low alloy steels. When there are only bainite ferrite and retained austenite in the upper bainite structure without carbide, it is called carbide-free bainite, which is called carbon-free bainite for short.

Ferrite bands in carbon-free bainite are mostly arranged in parallel, with wide size and spacing, and carbon-rich austenite is between bands or the product of its cooling process. After austenitizing at 950℃, 35CrMo steel was kept at 530℃ for 10min, and a carbon-free bainite consisting of bainite ferrite (BF) bands and residual austenite (γ′) was obtained.

The shape of bainite ferrite (α) is irregular, not all strip-shaped, but some block-shaped, and the interface between BF and γ' is partly serrated. There is a carbon-rich γ phase between ferrite bands. Due to the increase of carbon content, it also contains chromium and molybdenum alloy elements. In addition, after transforming into α phase, the specific volume increases and γ phase is squeezed, so the carbon-rich γ tends to be stable and cannot be transformed but remains.

In silicon steel and aluminum steel, it is difficult to form cementite because Si and Al are insoluble in cementite and there is no diffusion of Si and Al atoms. Therefore, in the upper bainite transformation of this kind of steel, cementite is not precipitated, and residual austenite often remains at room temperature, forming carbon-free bainite.

In low-carbon alloy steel, after the formation of bainite ferrite, cementite has not yet precipitated, bainite ferrite is still austenite, and carbon atoms continue to diffuse and enrich into austenite. Due to the expansion of phase transformation volume, the carbon-rich austenite between bainite ferrite is stressed and tends to be stable, and finally remains, forming a carbide-free bainite.

Granular bainite, when supercooled austenite is isothermal in the upper bainite temperature range, after bainite ferrite (BF) precipitates, carbon atoms leave the ferrite and diffuse into austenite, which makes the austenite carbon-rich uneven and increases its stability, and it is difficult to continue to transform into bainite ferrite. These austenite regions are generally granular or strip-shaped, so-called islands, which are distributed on bainite ferrite matrix. This carbon-rich austenite can be partially transformed into martensite during cooling, forming a so-called (M/A) island. This whole structure composed of BF+(M/A) islands is called granular bainite.

Pinnate upper bainite, which contains cementite, belongs to carbide bainite and is a classic bainite structure, and has been observed in recent years. Feather upper bainite is composed of banded bainite ferrite and cementite distributed between bands. The microstructure of classical upper bainite is feather-like, which is the whole structure of BF+θ-M3C. After austenitizing GCr 15 steel, it was isothermal at 450℃ for 40s, and then quenched by water to obtain the complete structure of bainite and martensite.

With the decrease of phase transformation temperature and the increase of carbon content in steel, banded ferrite (BF) becomes finer, dislocation density increases, cementite becomes finer, or particles become smaller and dispersion increases.

3. Morphology of Lower Bainite

There are carbon-free bainite and carbon-containing bainite in the lower bainite. Carbide bainite structure is easy to be obtained in high carbon steel and high alloy Cr-Mo steel, and lower bainite is carbon-free bainite in steel containing more Si. The lower bainite is formed in the lower part of bainite transformation temperature zone (lower than the front end temperature of bainite C curve). It is strip-shaped or bamboo-leaf-shaped, and the slices intersect at a certain angle.