The problem of decarburization on the surface of smls pipe

The problem of decarburization on the surface of smls pipe

The carbon element in the seamless steel pipe (smls) has a great influence on its own performance, so the service life of the seamless steel pipe will be greatly reduced after decarburization treatment, and if the surface of the seamless steel pipe is not decarburized, after cleaning, The hardness and wear resistance of the surface layer of the seamless pipe will be reduced. When quenching, due to the different volume changes of the inner and outer layers, cracks will be formed on the surface of the workpiece. Therefore, how to prevent the decarburization of the seamless pipe surface is a problem that should be paid attention to during the production process.

The key to solving the problem of surface decarburization lies in the heat treatment process. Since the decarburization and oxidation of the steel wire are carried out at the same time, as long as the Wuxi seamless pipe is in contact with the air as little as possible during the heat treatment process, the purpose of improving the decarburization can be achieved. 

The steel wire is austenitized. The lead liquid is heated to 871°C in advance, and the steel wire passes through the lead liquid to achieve austenitization, then enters the oil tank for quenching, and then enters the second lead melting tank for tempering treatment (482°C). Since the steel wire is not in contact with the air in the molten lead, decarburization is effectively avoided. The main problem to be solved in this process is how to prevent lead dust pollution.

The atmosphere that causes decarburization on the surface of seamless pipes is mainly oxidizing gases such as oxygen, water vapor, and carbon dioxide. When these oxidizing gases come into contact with the heated wire surface, oxidation and decarburization occur simultaneously; because the solid solution carbon in iron has a greater affinity for these gases, the carbon on the surface is removed first. The above-mentioned gases are generally brought in from outside the furnace; the iron oxide scale, rust on the surface of the seamless pipe and the residue on the surface of the steel wire after cold drawing will also decompose after heating in the furnace, and the reaction will generate some oxidizing gases. 

We can purposefully control the atmosphere in the furnace to keep it in a reducing state, which can effectively avoid surface decarburization. For example, controlling the ratio of carbon dioxide to carbon monoxide in the furnace, at the equilibrium point, neither oxidation nor decarburization; when the ratio of carbon dioxide exceeds the equilibrium point, oxidation and decarburization will occur; when the ratio is lower than the equilibrium point, no oxidation and decarburization will occur oxidation and decarburization. The specific value of the balance point should be calculated according to the carbon content and temperature of the iron.

In addition, it is also an effective measure to pass neutral protective nitrogen into the furnace from the seamless tube. At the same time, it can maintain positive pressure in the furnace, prevent air infiltration, reduce or avoid decarburization of steel parts.