There is a low carbon layer on the surface of copper casting, including a complete decarburization layer showing ferrite structure and a semi decarburization layer with low carbon content. The depth of decarburized layer is closely related to casting quality, size, thickness and processing conditions. In general, it is not easy to produce a small amount of non decarburized investment castings under correct and wrong vacuum casting conditions. Generally, the total thickness of decarburized layer is 0.3-0.6 mm, and 0.7-0.9 mm in severe cases.
Cause: during casting cooling, oxygen in air reacts with carbon in molten steel and carbon in cementite and austenite, and decarburization occurs on casting surface.
化学反应如下：2C + O2 = 2CO C + O2 = CO2
The chemical reaction is as follows: 2C + O2 = 2CO C + O2 = CO2
In conclusion, the chemical reactions at high temperature, medium temperature or low temperature have obvious spontaneous tendency.
matters needing attention:
1. Adjust the temperature of shell and casting properly to accelerate the cooling rate of casting.
2. Add carbides, such as carbon sand, activated carbon, anthracite, etc. to the sand or sand filling of the shell reinforcement layer.
3. Artificial repair gas will be produced at the periphery of the mold, such as covering the mold shell with a cover after casting, dropping kerosene into the cover or sprinkling appropriate waste wax, which can effectively reduce the decarburized casting layer on the surface.
4. In the heat treatment process of casting parts, it should also be carried out in recovery atmosphere. The key method to prevent decarburization of casting surface is to lay a layer of coke particles on the casting and prepare the sealing of furnace mouth in advance.