Oxidation and decarburization of forgings during heating process
Oxidation is a phenomenon in which a flange forging blank (metal raw material) chemically reacts with an oxidizing gas in the heating process, forming an oxide skin on the metal surface of the forging. For example, when steel is heated to a high temperature, ferrum on the surface layer reacts with oxygen, carbon dioxide, water, and form FeO, Fe3O4, Fe2O3. High-temperature oxidation is actually a diffusion process. As the ferric iron diffuses from the inside to the surface, the oxygen in the furnace gas is adsorbed on the surface of the billet and diffused internally, forming three layers - the outermost one is Fe2O3, which accounts for 10% of the thickness of the whole layer, the middle Fe3O4 layer accounted for 50% and the innermost FeO layer accounted for 40%.
Oxidation is mainly affected by the type of metal materials, the performance of furnace gas, heating temperature and time.
The degree of oxidation depends on what kind of metal material it is. Like billet steel, under the same conditions, the oxygenated burning loss of different grades of steel is also different. The burning loss of low carbon steel is larger than that of high carbon steel. This is due to the reaction with high carbon steel produces more carbon monoxide and reduces the amount of iron oxide produced. When the material contains alloying elements such as Cr, Ni, Al, Si, Mo, etc., an oxide film which is dense and not easy to fall off is formed on the surface of the steel material, and the continued oxidation of the metal during the heating process can be prevented.
The properties of the gas in furnace can be divided into oxidizing furnace gas, reducing furnace gas, and neutral furnace gas, which are determined by the air supply during the fuel combustion. The furnace gas may be fully composed of oxidizing gases such as O2, CO2, H2O, SO2, etc., and produce a relatively thick scale. In the reducing furnace gas, if a sufficient amount of reducing gases like CO is contained, it will not, or barely oxidize the metal materials. Normal electric resistance furnace gas heated in air medium belongs to oxidizing furnace gas.
The heating temperature is the most important factor affecting the oxidation rate of the metal. The higher the temperature, the greater the diffusion rate of metal and gas atoms, and the more intense the oxidation, the thicker the scale formed. At 200-500 ℃, the surface of the steel material can only form a very thin oxide film. When the temperature rises to 600-700 ℃, it will start to oxidize significantly and form scale. If the temperature rises to 850-900 ℃, the oxidation rate of the steel increases sharply and the oxide layer of the billet is thicker.
The longer the billet is heated in an oxidizing space, the larger the diffusion of oxygen and the thicker the scale formed. Especially in a high temperature stage, the effect of heating time is more pronounced.
The oxygenated burning loss of metal is very harmful. Under normal circumstances, 0.5%-4.0% of the metal is oxidized and burned off every time the steel is heated. At the same time, the oxide scale also aggravated the wear of the mold and reduces the surface quality of the forging. Besides, the scale also accelerates the wear of the cutting edge of the tool during machining. Therefore, reducing or eliminating oxygenated burning of the metal during heating is very important for forging production.
Reducing oxidation methods in the forging heating process includes: under the premise of ensuring the quality of forgings, try to shorten the heating time, especially at high temperature. Under the condition of complete fuel combustion, minimize the excess air in order to avoid excessive oxygen in the furnace, and try to reduce the moisture in the fuel and use oxidation-free heating methods. The furnace should maintain a small positive pressure to prevent the inflow of cold air.
When the steel material is heated, the carbon on the surface and the oxidizing gas in the furnace react chemically, the phenomenon that the surface of the steel material and the carbon content decrease is called decarburization, a result of diffusion. One the one hand, the oxygen in the furnace gas diffuses into the steel, and on the other hand, the carbon in the furnace gas diffuses to the outer surface, so that the steel forms a decarburized layer with a low carbon content on the surface. From the whole process, the decarburization layer can be formed only when the decarburization rate exceeds the oxidation rate.
The factors affecting decarburization are the same as oxidation, which is mainly affected by aspects such as chemical composition of the material, furnace gas composition, heating temperature and time.
The higher the carbon content of the steel, the greater the tendency to decarburize. Certain alloying elements, like Si, increase the tendency to decarburize, while some alloying elements such as Cr, Mn, etc., can prevent decarburization. The medium with the strongest decarburization ability in the composition of the furnace gas is water vapor, followed by CO2, and O2. Increasing the amount of CO, or heating in a neutral medium or a weakly oxidizing medium can reduce the tendency of decarburization. The higher the heating temperature and the longer the heating time, the more serious the decarburization. When the temperature is over 1000 ℃, the decarburization is weak due to strong oxidation. At higher temperatures, there will be violent decarburization due to the loss of protection of the oxide scale.
Oxidation is mainly affected by the type of metal materials, the performance of furnace gas, heating temperature and time.
The degree of oxidation depends on what kind of metal material it is. Like billet steel, under the same conditions, the oxygenated burning loss of different grades of steel is also different. The burning loss of low carbon steel is larger than that of high carbon steel. This is due to the reaction with high carbon steel produces more carbon monoxide and reduces the amount of iron oxide produced. When the material contains alloying elements such as Cr, Ni, Al, Si, Mo, etc., an oxide film which is dense and not easy to fall off is formed on the surface of the steel material, and the continued oxidation of the metal during the heating process can be prevented.
The properties of the gas in furnace can be divided into oxidizing furnace gas, reducing furnace gas, and neutral furnace gas, which are determined by the air supply during the fuel combustion. The furnace gas may be fully composed of oxidizing gases such as O2, CO2, H2O, SO2, etc., and produce a relatively thick scale. In the reducing furnace gas, if a sufficient amount of reducing gases like CO is contained, it will not, or barely oxidize the metal materials. Normal electric resistance furnace gas heated in air medium belongs to oxidizing furnace gas.
The heating temperature is the most important factor affecting the oxidation rate of the metal. The higher the temperature, the greater the diffusion rate of metal and gas atoms, and the more intense the oxidation, the thicker the scale formed. At 200-500 ℃, the surface of the steel material can only form a very thin oxide film. When the temperature rises to 600-700 ℃, it will start to oxidize significantly and form scale. If the temperature rises to 850-900 ℃, the oxidation rate of the steel increases sharply and the oxide layer of the billet is thicker.
The longer the billet is heated in an oxidizing space, the larger the diffusion of oxygen and the thicker the scale formed. Especially in a high temperature stage, the effect of heating time is more pronounced.
The oxygenated burning loss of metal is very harmful. Under normal circumstances, 0.5%-4.0% of the metal is oxidized and burned off every time the steel is heated. At the same time, the oxide scale also aggravated the wear of the mold and reduces the surface quality of the forging. Besides, the scale also accelerates the wear of the cutting edge of the tool during machining. Therefore, reducing or eliminating oxygenated burning of the metal during heating is very important for forging production.
Reducing oxidation methods in the forging heating process includes: under the premise of ensuring the quality of forgings, try to shorten the heating time, especially at high temperature. Under the condition of complete fuel combustion, minimize the excess air in order to avoid excessive oxygen in the furnace, and try to reduce the moisture in the fuel and use oxidation-free heating methods. The furnace should maintain a small positive pressure to prevent the inflow of cold air.
When the steel material is heated, the carbon on the surface and the oxidizing gas in the furnace react chemically, the phenomenon that the surface of the steel material and the carbon content decrease is called decarburization, a result of diffusion. One the one hand, the oxygen in the furnace gas diffuses into the steel, and on the other hand, the carbon in the furnace gas diffuses to the outer surface, so that the steel forms a decarburized layer with a low carbon content on the surface. From the whole process, the decarburization layer can be formed only when the decarburization rate exceeds the oxidation rate.
The factors affecting decarburization are the same as oxidation, which is mainly affected by aspects such as chemical composition of the material, furnace gas composition, heating temperature and time.
The higher the carbon content of the steel, the greater the tendency to decarburize. Certain alloying elements, like Si, increase the tendency to decarburize, while some alloying elements such as Cr, Mn, etc., can prevent decarburization. The medium with the strongest decarburization ability in the composition of the furnace gas is water vapor, followed by CO2, and O2. Increasing the amount of CO, or heating in a neutral medium or a weakly oxidizing medium can reduce the tendency of decarburization. The higher the heating temperature and the longer the heating time, the more serious the decarburization. When the temperature is over 1000 ℃, the decarburization is weak due to strong oxidation. At higher temperatures, there will be violent decarburization due to the loss of protection of the oxide scale.
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