The key to resolving the problem of surface decarburization lies in the heat treatment process. Since the decarburization and the oxidation of the steel wire are carried out at the same time, as long as the steel wire is kept in contact with air as little as possible during the heat treatment process, the purpose of improving decarburization can be achieved. For example, American Spring Wire Company used double lead groove oil quenching and tempering to the valve spring steel wire, and achieved ideal results. The production process is: the first molten lead tank is used to austenitize the steel wire. The lead liquid is heated to 871℃ in advance. The steel wire is passed in the lead liquid to achieve austenitization, and then enters the oil tank for quenching, and then enters the second Two molten lead tanks are tempered (482°C). Since the steel wire is not in contact with air in the molten lead, decarburization is effectively avoided. The main problem that needs to be solved with this process is how to prevent lead dust pollution.
The atmosphere that causes surface decarburization is mainly oxidizing gas such as oxygen, water vapor, and carbon dioxide. When these oxidizing gas come into contact with the surface of the heated steel wire, oxidation and decarburization occur at the same time; because the solid solution carbon in iron has a greater affinity for these gas, the carbon on the surface is removed first. The above-mentioned gases are generally brought in from outside the furnace; iron oxide scale, rust on the surface of steel products, and residues on the surface of the steel wire after the cold drawing will also decompose after heating in the furnace and react to generate some oxidizing gases. We can control the atmosphere in the furnace purposefully to make 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; below the equilibrium point, oxidation and decarburization will not occur. The specific value of the equilibrium point should be calculated based on the carbon content and temperature of iron.
Besides, blowing neutral protective nitrogen into the furnace is also an effective measure. Nitrogen entering the furnace can disperse the oxidizing atmosphere in certain sections, while maintaining a positive pressure in the furnace, preventing air infiltration, and reducing or avoiding decarburization of steel parts.
Post time: Nov-10-2023