Annealing furnace

The structure, heating technology, temperature control, and furnace atmosphere of the annealing furnace directly affect the quality of the processed product. In the casting heating furnace, increasing the heating temperature of the metal can reduce deformation resistance, but excessive temperature can lead to grain growth, oxidation, or overburning, severely impacting the quality of the workpiece. During the heat treatment process, if the steel is heated to a point above the critical temperature and then rapidly cooled, it can increase the hardness and strength of the steel; if heated to a point below the critical temperature and cooled slowly, the hardness of the steel can be reduced while its toughness is improved.

To achieve accurate dimensions and smooth surfaces of workpieces, or to reduce metal oxidation for purposes such as protecting molds and minimizing machining allowances, various low-oxygen or non-oxygen heating furnaces can be selected. In a low-oxygen or non-oxygen flame furnace, the incomplete combustion of fuel produces a reductive gas, which can heat the workpieces to reduce oxidation burn loss to below 0.3%.

可控气氛 furnaces utilize artificially prepared atmospheres, enabling processes like gas carburizing, carbonitriding, bright quenching, normalizing, and annealing within the furnace for the purpose of altering the microstructure and improving the mechanical properties of workpieces. In an active particle furnace, the combustion gases of the fuel or other fluidizing agents applied externally are forcibly passed through a layer of graphite particles or other inert particles on the furnace bed. Workpieces buried in this particle layer can undergo intensive heating and various non-oxidizing heat treatments like carburizing and nitriding. In a salt bath furnace, the molten salt acts as the heating medium, preventing oxidation and decarburization of the workpieces.

Melting cast iron in the furnace is usually affected by factors such as coke quality, blowing method, charge condition, and air temperature, making the melting process difficult to stabilize and not conducive to obtaining high-quality iron. The hot blast furnace can effectively increase the temperature of the iron, reduce alloy loss, and decrease the oxidation rate of the iron, thereby enabling the production of cast iron.

With the emergence of the coreless induction furnace, the cupola furnace is gradually being replaced. This type of induction furnace is not restricted by any cast iron grade and can quickly switch from melting one grade of cast iron to another, which is beneficial for improving the quality of iron. Some special alloy steels, such as ultra-low carbon stainless steel and steel used for roller and steam turbine rotors, require the molten steel produced by open hearth furnaces or general electric arc furnaces to be further refined in a refining furnace through vacuum degassing and argon stirring to remove impurities, resulting in high-purity, high-volume superior molten steel.

If there are impurities on the workpiece surface, especially organic matter or ash, heating will undoubtedly affect the oxide scale. The oxide scale formed on the stainless steel surface where the flame of gas or oil directly contacts and where it does not contact will differ. Therefore, the workpiece should not be directly exposed to the flame. If there is residual oxide scale on the local area of the workpiece before heat treatment, there will be differences in the thickness and composition of the oxide scale between the areas with and without oxide scale after heating, leading to an uneven surface after acid washing. Thus, attention should not only be given to the subsequent heat treatment but also to the intermediate heat treatment and acid washing.