On 2018-08-08 14:48:54
How to prevent the black structure of Carbonitriding parts in the process of hea
In our heat treatment process, the control of parameters is very important. Today, a small editor from Wuxi heat treatment professional factory will tell you that the black structure in the heat treatment process refers to the black spot, black belt and black mesh structure in the carburizing layer found in the metallographic inspection after the carbonitriding of steel parts, which are collectively referred to as black structure, also known as internal oxidation.
The black structure appears in the heat treatment process of the alloy steel. The water vapor, oxygen and carbon dioxide in the furnace gas interact with the alloying elements in the steel at high temperature and are oxidized, which makes the alloying elements in the surface and sub surface "depleted". It reduces the stability of austenite and shifts the isothermal transformation diagram to the left. After quenching, it passes through the transformation zone of Troostite and bainite and finally forms the mixed structure of oxide, troostite and bainite. Because troostite and bainite are easy to corrode, they are black under light microscope. Black net and black belt do not corrode during polishing, sometimes they can be seen, but they are clearer after corrosion.
The oxygen atoms in the atmosphere with high oxygen potential will diffuse inward along the austenite grain boundary. Due to the high energy of the product boundary, the alloy elements will move from the crystal to the grain boundary, and quickly react with the active oxygen atoms on the grain boundary to form oxides. The alloy elements on the grain boundary will also diffuse outward along the grain boundary to meet the oxygen atoms that diffuse inward to form oxides. Therefore, there are many oxides in the grain boundary and surface layer. Compared with the binding force of oxygen at 860 ℃ at high temperature, the elements in front of the Fe elements have the priority to form internal oxidation black spots with oxygen junctions, which mostly occur in the pores 40 μ m away from the surface. The main reason is that the nitrogen potential of furnace gas is too high in the initial stage of CO infiltration, which makes the nitrogen content in the heat treatment layer high. In addition, if the time of CO infiltration is too long, the carbon concentration will increase, which will promote the decomposition of nitrides and the denitrification process, and make atomic nitrogen become molecular nitrogen and form holes.
Most of the black belt processed by heat treatment is in the range of 0-30 μ m surface layer. The oxides, nitrides and small carbide particles of the formed alloy elements "impoverish" the alloy elements in austenite and reduce the hardenability. The formation of non martensitic structure black net is generally formed on the inside of the black belt. Carbon nitrogen diffuses along the austenite grain boundary, forming carbon nitrogen compounds of Mn, t and other elements in the grain boundary, reducing the alloy elements in the vicinity of the grain boundary, and easily forming troostite during quenching. In the same way, a black net will also be formed at the transition of the infiltration layer.