Upper bainite:

　　Upper bainite forms in the upper part of the bainite transformation temperature range, with various morphologies, mainly divided into three types:

　　1. Carbon-free (carbide-free) bainite 2. Granular bainite 3. Feathery upper bainite

　　

 

　　Schematic diagram of upper bainite microstructure

　　a. Carbon-free bainite b. Granular bainite c. Carbon-free bainite d. Feathery upper bainite

　　

 

　　1) Ultra-low carbon bainite:

　　

 

　　Bainite microstructure of ultra-low carbon bainite steel, OM

　　The ultra-low carbon bainite microstructure is similar to that of lath martensite, and it is difficult to distinguish them morphologically. TEM identification is needed.

　　2) Carbon-free (carbide-free) bainite:

　　When the upper bainite microstructure contains only bainite ferrite and retained austenite, but no carbides, this bainite is carbide-free bainite, or carbon-free bainite. This type of bainite usually appears in low-carbon low-alloy steels.

　　

 

　　

 

　　The ferrite laths in carbon-free bainite are arranged in parallel, with wider dimensions and spacing. The inter-lath regions are rich in carbon austenite, or its product during the cooling process. It often appears under the following conditions:

　　1. Since Si and Al are insoluble in cementite, the formation of cementite is delayed. Therefore, in the upper bainite of silicon steel and aluminum steel, residual austenite is often retained at room temperature without the precipitation of cementite, forming carbon-free bainite.

　　2. In low-carbon alloy steels, after the formation of bainite ferrite, cementite has not yet precipitated, and the bainite ferrite is still austenite. Carbon diffuses sufficiently into the austenite, making the austenite tend to be stable and retained, forming carbide-free bainite.

　　3) Granular bainite:

　　

 

　　

 

　　

 

　　

 

　　4) Feathery upper bainite

　　Feathery upper bainite is composed of lath ferrite and discontinuous carbides distributed between the laths.

　　The carbides between the bainite ferrite laths are thin plate-like cementite, and the microstructure shows a feathery morphology.

　　

 

　　Recently discovered feathery upper bainite

　　

 

　　SEM GCr15 steel feathery bainite microstructure

　　

 

　　High-carbon steel feathery bainite microstructure, OM

　　Lower bainite microstructure

　　Lower bainite forms in the temperature range below the nose temperature of the bainite C-curve.

　　Lower bainite includes classical lower bainite, columnar bainite, and near-bainite.

　　

 

　　60Si2CrV steel lower bainite microstructure (a) OM and (b) SEM

　　

 

　　23MnNiCrMo steel lower bainite

　　

 

　　

 

　　SEM GCr15 steel lower bainite microstructure

　　

 

　　Granular-columnar bainite

　　Actual bainite microstructure in steel:

　　In actual steel, microstructures with an organic combination of bainite and martensite are often found.

　　Upper bainite and low-carbon lath martensite have similar morphologies, but the dislocation density in upper bainite is lower than that in martensite.

　　High-carbon plate martensite and lower bainite have similar morphologies, but the former has a twinned substructure, while twins are rarely seen in lower bainite.

　　Integrated microstructure of lath martensite and lower bainite in 23MnCrNiMo steel

　　

 

　　

 

　　Diversity of bainite microstructure morphology

　　Upper and lower bainite are traditional bainite classifications accepted by various schools of thought.

　　There are many other names for bainite morphologies, such as carbon-free bainite, granular bainite, near-bainite, reverse bainite, columnar bainite, special lower bainite, granular microstructure, etc., as well as recently discovered bainite with martensite-like morphologies, such as equilateral triangular bainite, “N” bainite, butterfly bainite, etc. They may all be transformations of typical bainite.

　　The diversity of bainite microstructure morphology illustrates the extreme complexity of bainite phase transformation.

　　Substructure in bainite

　　Bainite ferrite is composed of smaller "subunits".

　　Subunits usually nucleate near the ends of the already formed ferrite, growing by longitudinal elongation and thickening.

　　When the growth of subunits is hindered, nucleation is triggered again, forming new subunit cores on the side of the top of the ferrite lath (upper bainite) or the tip of the ferrite needle (lower bainite).

　　The repeated nucleation and growth of subunits constitute the nucleation and growth process of ferrite in bainite.

　　SEM 60Si2CrV steel lower bainite sheet and the morphology of subunits inside

　　

 

　　Schematic diagram of bainite substructure unit

　　

 

　　STM Fe-0.5C-3.3Mn upper bainite substructure

　　

 

　　Fine substructure of lower bainite

　　It is generally believed that lower bainite strips are composed of subunits. Observations of lower bainite in Fe-1.0C-4.0Cr-2.0Si steel show that lower bainite strips are composed of sub-strips, and sub-strips are composed of subunits. The subunits are parallel to each other and are approximately parallelogram-shaped. As shown in the figure. Using scanning tunneling microscopy, it was found that subunits exist inside the lower bainite ferrite, as shown in the figure.

　　Fe-1.0C-4.0Cr-2.0Si steel lower bainite

　　

 

　　Fine sub-units of lower bainite in STM Fe-1.0C-4.0Cr-2.0Si steel, the sub-units are quadrilateral.

　　

 

　　Sub-platelets of bainitic ferrite in medium carbon Mn-Si steel, high-resolution electron microscopy (HREM) morphology. A) Bright field image, b) Dark field image

　　

 

　　Twin sub-units are also quadrilateral.

　　Twinning of bainite

　　

 

　　

 

　　The discovery of twinning was a surprise to the shear school and a shock to the diffusion school. The diffusion school does not acknowledge the existence of twins in bainite.

　　Fang Hongsheng believes that lower bainite is approximately disc-shaped, composed of sub-platelets, which are in turn composed of sub-units, and the sub-units are composed of even smaller super-sub-units.

　　Scanning tunneling microscopy (STM) morphology of Fe-1.0C-2.0Si-4.0Cr lower bainite: raised features.

　　

 

　　Atomic force microscopy (AFM) morphology of Fe-0.2C-2.5Mn-1.5Si-0.6Cr lower bainite: raised bainite sub-units.

　　

 

　　Height 2～3nm, width approximately 60～80nm.

　　It is pointed out that the raised features are tent-shaped.

　　Higher density dislocation substructure

　　The dislocation density in bainite is not as high as in martensite, but there is also a higher density dislocation substructure, as shown in Figure 5-11. Some believe that there is a higher density of dislocations inside the bainite sub-units, ρ=1010cm-2.

　　

 

　　Relationship between dislocation density and temperature in ferrite

　　TEM image of lower bainite in P20 steel, higher density dislocation substructure

　　

 

　　Orientation relationship between bainitic ferrite and retained austenite

　　Retained austenite is often present in bainite. In carbide-free bainite, a large amount of retained austenite remains at room temperature. In granular bainite, there are M/A islands, and in lamellar bainite ferrite, there are austenite films.

　　Studies have shown that both K-S and N-W relationships exist between bainitic ferrite and austenite. As shown in the figure.

　　High-resolution lattice image of bainitic ferrite and austenite with K-S relationship

　　

 

　　The nature and definition of bainite

　　Bainite is essentially an integrated structure that is an organic combination of bainitic ferrite and θ-cementite (or ε-carbide). The structure often contains inclusions of phases such as retained austenite and martensite. The constituent phases are numerous and the morphology is variable.

　　Upper bainite forms in the higher temperature region, and lower bainite forms in the lower temperature region below the "nose temperature". The main differences in the microstructure are the morphology of the bainitic ferrite and the morphology and precipitation location of the carbides.