Metallographic diagram of 20Cr steel (950℃ solid boronizing for 5h)

The microstructure of a 20Cr steel that has undergone solid-dipping of boron at 950°C for 5 hours is a ferrite-pearlite structure. Ferrite grains account for most of the microstructure, with pearlite running through the ferrite grains in thin strands. The pearlite structure is composed of alternating bands of ferrite and cementite, and the cementite has a visible grain size. There is no evidence of any intergranular precipitation or any other microstructural anomalies. The eutectoid composition of the 20Cr steel is cementite that makes up ~13.0 wt.% of the microstructure.

The ferrite grains appear to have a directionally aligned grain structure. This indicates that the solid solution of boron at 950°C has acted to form a coherent grain boundary structure that likely originates from the orientation of ferrite when the steel was annealed above its Ar3 temperature prior to solid-dipping of boron. The grains have a nearly fine but slightly coarse texture.

The microstructure has a relatively uniform distribution of various features, including micro-boundaries, macro-boundaries, and the pearlite structure. The grain boundaries are fairly planar, with the orientation predominantly parallel to each other, but with some slight variations in direction. The grain boundaries appear to be quite smooth, suggesting that the diffusion of boron has been successful in filling in gaps between the grains and providing a uniform boundary structure.

The pearlite structure is composed of alternating bands of ferrite and cementite, with the latter having a visible grain size. The pearlite strands have an elongated, twisted structure, which suggests that they have developed under the action of boron diffusion and shear strain. The cementite has a granular structure with fine grains, indicating that it has been transformed and refined by the diffusion of boron. The presence of pearlite indicates that the tempering of the 20Cr steel has been successful, and that boron has effectively been diffused into the lattice structure.

The central area of the microstructure is occupied by ferrite grains, which are relatively fine and relatively uniform in shape and size. There is no distinguishable evidence of residual austenite in the microstructure, indicating that the annealing process prior to solid-dipping was successful in transforming all of the grains to the ferrite structure. There is no visible destruction of the grain boundaries, which indicates that the solid-dipping process has been effective in preventing the destruction of the grain boundary structure.

In general, the microstructure of the 20Cr steel after solid-dipping of boron at 950°C for 5 hours is typical of steel treated in this manner and the microstructural features are consistent with those expected in such a steel. The structure is composed primarily of ferrite grains, with a banded pearlite structure dispersed throughout. The planar grain boundaries are smooth and the pearlite strands have an elongated, twisted structure. No intergranular precipitation or other microstructural anomalies were observed. The 20Cr steel microstructure appears to be in good condition and suitable for further processing.

The microstructure of 20Cr steel treated by solid solution with boron at 950°C for 5 hours was observed with metallographic method. The structure could be divided into ferrite, bainite and martensite, which can be seen in the microstructure photographs.

The ferrite phase makes up the major part of the structure and presented as banded graphite! The ferrite grains are ellipsoidal in shape, with average grain sizes of about 5 μm. The bainite formed from the bainitic transformation after boron, and it is made of the fine laths which having a width about 2 μm. The martensite phase accounts for a small proportion which is observed as the white blocks distributed in the ferrite grain boundaries.

Based on interaction between the boron and steel, carbides precipitate as secondary particles in ferrite grains and around bainite lath and in martensite blocks. These carbides distribute as tiny and needle-like typical lathi form.

In summary, the microstructure of 20Cr steel treated by solid solution with boron at it 950°C for 5 hours is composed of ferrite, bainite and martensite, and carbide precipitates as secondary particles in these phases.