The oxidation of forgings is mainly affected by the chemical composition of the heated metal and the heating ring block (such as furnace gas composition, heating temperature, etc.).
1) Chemical composition of metal materials
The amount of scale formed is closely related to the chemical composition. The higher the carbon content of the steel, the less scale is formed, especially when the carbon content exceeds 0.3%. This is because after the carbon is oxidized, a layer of oxidized (CO) gas is formed on the surface of the billet, which acts to suppress the continued oxidation. In alloy steel, the more elements such as Cr, Ni, Al, Mo, Si, etc., the less scale formed during heating, because these elements are oxidized to form a dense oxide film on the surface of steel, and it and steel It has a close thermal expansion coefficient and can be firmly attached to the surface. It is not easy to crack and fall off, so it can prevent further oxidation and protect it. Heat-resistant stainless steel is an alloy steel containing more of the above elements, and when the content of Ni and Cr in the steel is 13% to 20%, almost no oxidation occurs.
2) Furnace gas composition
The composition of the furnace gas has a great influence on the formation of the scale of forgings. The forgings of the same steel grade have different oxide scales under different heating atmospheres. In the oxidizing furnace gas, the scale is formed the most, and it is light gray. It is easy to remove; in neutral furnace gas (mainly containing N2) and reducing furnace gas (including CO, H2, etc.), the scale formed is less black and difficult to remove. In order to minimize the formation and removal of scale, attention should be paid to the control of the composition of the furnace gas at each stage of heating. In general, forgings below 1000 ° C, oxidizing furnace gas should be used for heating, because the temperature is not high, the oxidation process is not very intense, and the formed scale is easy to remove; when the temperature exceeds 1000 ° C, especially in In the high temperature insulation stage, a reducing furnace gas or a neutral furnace gas should be used to reduce the generation of scale.
The nature of the flame furnace gas depends on the amount of air supplied when the fuel is burned. If the air excess coefficient in the furnace is too large, the supplied air is too much, the furnace gas is oxidized, and the metal forms oxide scale. If the air excess coefficient in the furnace is 0.4?0.5, the furnace gas is reduced to form a protective atmosphere. Avoid the formation of scale and achieve no oxidation heating.
3) Heating temperature
The heating temperature is also a major factor in the formation of scales for forgings. The higher the heating temperature, the more intense the oxidation. Before 570 ° C ~ 600 ° C, the forgings are slower to oxidize, the oxidation rate is faster from 700 ° C, and the oxidation is very significant at 900 ° C to 950 ° C. If the oxidation rate at 900 ° C is assumed to be 1, when the temperature reaches 1000 ° C, the oxidation rate is 2, when the temperature reaches 1100 ° C, the oxidation rate is 3.5, and when the temperature reaches 1300 ° C, the oxidation rate is 7, increasing 6 times.
4) Heating time
The longer the forging is heated in the oxidizing gas in the furnace, the larger the amount of oxidative diffusion, the more scale is formed, especially in the high temperature heating stage, so the heating time should be minimized, especially the heating time at the temperature. And the holding time should be as short as possible.
In addition, the forging blank is not only oxidized in the furnace at high temperature, but also in the forging process, although the scale on the billet is removed, if the billet temperature is still high, it is subject to secondary oxidation. The rate of oxidation gradually decreases as the temperature of the billet decreases.
Measures to reduce and prevent scale formation
For the flame heating furnace, according to the heating environment factors affecting the scale formation of the forging, the corresponding measures are taken in the heating process, which can achieve the purpose of reducing the scale generation. The main measures are as follows:
(1) Rapid heating. Under the premise of ensuring the quality of forgings, try to use rapid heating to shorten the heating time, especially to shorten the residence time at high temperature, and to install as little as possible during operation, which also saves energy.
(2) Control the nature of the furnace gas. Under the condition of complete combustion, strictly control the amount of air entering the furnace, and reduce the excess air as much as possible, because too much air will accelerate the oxidation, and there should be excessive reducing gas in the furnace.
(3) Reduce the moisture in the fuel and reduce the oxidation of the surface of the forging by steam.
(4) The furnace should maintain a small positive pressure during the heat preservation, especially in the temperature-heating stage to prevent cold air from entering the furnace.
(5) Medium protection heating. The surface of the blank is isolated from the oxidizing furnace gas to prevent oxidation of the billet upon heating. The medium used is gaseous protective medium (such as pure inert gas, petroleum liquefied gas, nitrogen, etc.), liquid medium (such as glass melt, molten salt, etc.) and solid protective medium (such as charcoal, glass powder, tantalum powder and metal coating, etc.)