Solution To The Surface Decarburization Layer in Investment Casting

Carbon is the first of the five elements of castings and has a great influence on the performance of castings. 

Steel and iron are distinguished by their carbon content. The surface decarburization layer of precision castings is generally not checked, but it is an important factor affecting the surface hardness and wear resistance of castings. If the customer has requirements, it must be controlled. 

The standard for the depth of decarburization layer on the surface of castings is currently only the standard of the Ministry of Machinery Industry in 1991 (8B/5100-91) for reference:

The national standard for the determination of the depth of decarburization layer of steel was formulated in 1987, and then revised in 2008 and 2019. However, if it is for the heat treatment industry, we can refer to the above standards.

Generally, when we test in front of the furnace, we take the molten steel into the sample spoon. Because its bottom is not in contact with the air, the composition tested is the composition of the molten steel, but when testing after the furnace, it will be found that the carbon content is much less. Don’t think it is caused by high-temperature burning. In fact, it is caused by surface decarburization. If you adjust the subsequent ingredients based on this, it is wrong.

The correct approach is to cut the casting or die head, take the middle part to test the composition and hardness, because precision casting is red shell pouring, the casting cools slowly, and the decarburization layer will be relatively larger. According to our actual experience, it will generally reach about 1mm or more.

Under non-vacuum pouring conditions, it is difficult to make precision castings without decarburization, but certain measures can be taken to reduce the decarburization layer. 

For example, graphite powder can be added to the slurry of the transition layer and subsequent layers of the shell, and the amount added accounts for about 10% of the mullite powder. Other conditions remain unchanged. In this way, during the shell mold roasting process, the graphite powder is oxidized to produce CO gas, which increases the gap in the shell mold and increases the air permeability. 

The key is that after pouring molten steel, the equilibrium temperature of molten steel and shell mold is about 1300 degrees. 

At this temperature, graphite powder reacts with oxygen in the air inside and outside the shell to produce CO2 or CO, forming a reducing atmosphere inside the shell, thereby preventing the carbon on the surface of the casting from being oxidized to form a decarburization layer. 

Combined with the box buckle treatment after pouring, the decarburization layer can be controlled within 0.25mm. Generally, graphite powder of about 100 mesh can be used, which will not significantly affect the high temperature strength of the shell, but will affect the disintegration properties.