The gold annealing microstructure of 20 steel (after quenching in water at 860℃) is shown in Figure 1. A single ferrite grain with an average ferrite grain size of about 5μm can be clearly observed in the specimen.
The average size of the ferrite grains is slightly larger than that of the parent material, so it can be inferred that the hot deformation affected the grain growth of the material and increased the grain size. At the border of the ferrite grains, there are some Widmanstatten structures, as can be seen in the micrograph. This suggests that the material was likely subjected to cooling during the plastic deformation process.
The remainder of the microstructure consists of a few small, roundish carbides randomly distributed in the ferrite matrix, confirming the presence of alloying elements in the steel. This evidence suggests that the steel underwent partial precipitation of the alloying elements during the quenching process.
Additionally, there is evidence in the micrograph of retained austenite in the ferrite matrix. The retained austenite grains are relatively small, with an average grain size of 1-2µm. This suggests that the quenching and subsequent tempering treatments resulted in partial transformation of the austenite in the steel and inhibited further grain growth.
From the micrograph, it is evident that 20 steel exhibits a good combination of mechanical properties and formability. This is due to its combination of ferrite grains of medium size (5µm), a homogeneous dispersion of carbide particles, and the presence of retained austenite.
Overall, it can be concluded that 20 steel is a well-balanced material suitable for applications that require good formability and strength. Its combination of ferrite matrix with Widmanstatten structures, dispersed carbides, and retained austenite ensure that it has good fatigue resistance and ductility.
