In our heat treatment process, the control of parameters is still very important. Today, the editor of the Kunshan heat treatment professional manufacturer will tell you about the black structure during the heat treatment process. The black spots, black bands and black meshes in the seepage layer were found in the lining, which are collectively referred to as black tissue, also known as internal oxidation.

The black microstructure appears in the eutectic layer of the alloy steel during heat treatment. At high temperature, the water vapor, oxygen and carbon dioxide in the furnace gas interact with the alloying elements in the steel and are oxidized, so that the alloying elements in the surface and sub-surface layers are "depleted". The stability of austenite is reduced and the isothermal transformation diagram is shifted to the left. After co-infiltration, it passes through the tortenite and bainite transformation zone and finally forms a mixed structure of oxide, tortenite and bainite. Due to the easy corrosion of tortenite and bainite, they appear black under the light microscope. Black mesh and black belt are not corroded during polishing, and can sometimes be seen, but they are clearer after corroding.

The oxygen atoms in the high oxygen potential atmosphere will diffuse inward along the austenite grain boundaries. Due to the high energy of the grain boundary, the alloying elements will move from the intragranular to the grain boundary, and rapidly chemically interact with the active oxygen atoms on the grain boundary to form oxides. The alloying elements on the grain boundary will also diffuse out along the grain boundary, and meet the oxygen atoms diffused inward to form oxides. Therefore, there are many grain boundaries and surface oxides. At a high temperature of 860℃), the alloy elements are ranked in front of the Fe elements in terms of their binding force with oxygen. They preferentially junction with oxygen to produce internal oxidation black spots, which mostly occur in the pores 40 μm away from the surface layer. The main reason is that the nitrogen potential of the furnace gas is too high in the initial stage of the co-infiltration, which makes the nitrogen content in the heat treatment process infiltration layer high, so the flow rate of ammonia gas and the decomposition rate of ammonia should be controlled. In addition, if the co-infiltration time is too long, the carbon concentration will increase, which will promote the decomposition of nitrides, and the process of denitrification will occur, which will turn atomic nitrogen into molecular nitrogen and form pores.

The heat treatment black belt is mostly in the range of 0~30μm surface layer. The formed small particles of oxides, nitrides and carbides of alloying elements "deplete" the alloying elements in austenite, reduce the hardenability, and form a non-martensitic black network generally formed inside the black belt, carbon and nitrogen. Diffusion along the austenite grain boundary forms carbonitrides of Mn, T and other elements at the grain boundary, which reduces the alloying elements in the vicinity of the grain boundary, and it is easy to form tortenite during quenching. In the same way, a black network will also be formed at the transition of the co-infiltration layer.