Metal furnace heated in the flame, the main component of the atmosphere surrounding the CO2, CO, H2O, H2, O2 and N2, etc. Including CO2, H2O and O2 to metal oxidation or decarburization; H2 and CO is a reducing gas; N. In addition to the particular metal, in general, is an inert gas, but in general contain a small amount of nitrogen oxide impurities, directly to the heat treatment can also cause oxidation.
Each direction of a reversible reaction, according to their chemical equilibrium constant and the temperature. That is, the furnace gas is an oxidizing or reducing, depending on the temperature of the furnace gas in the ratio of the content of the gas may be. Iron redox shown FIG. For example, at 825 ℃, when the CO and CO2 content and the ratio of H2 and H2O in the ratio greater than 2, the iron is not oxidized, but it is difficult to stabilize combustion control in this state, even if done, when the temperature drop to below 750 ℃, this atmosphere is oxidizing it. Therefore, in general, the flame furnace atmosphere is oxidizing or reducing is not controllable. Therefore, during the heat treatment, a chemical change when allowed, or only allowed to take place there is a change (such as carburization or decarburization etc.), only a radiation tube or muffle furnace electrically heated indirectly heated or modified and pass into the preform, the composition of the controlled atmosphere desirable.
Controlled atmosphere Category:
Controlled atmosphere metallurgical plants commonly used fuel preparation can be divided into the atmosphere, nitrogen-based controlled atmosphere, ammonia preparation of pure hydrogen atmosphere and so on.
Fuel preparation Atmosphere:
General use gas fuel (natural gas, propane, butane, and coke oven gas, etc.) or through incomplete combustion catalytic reaction, removal of CO2 and H2O from the system controlled atmosphere. When the heat released by the combustion process to maintain the reaction proceeds, the resulting gas is called exothermic atmosphere can be divided into Ⅰ, Ⅱ and type Ⅲ type, in order to adapt to different uses; When the excess air coefficient is very low, you must heat to maintain the reaction, the resulting clear endothermic gas atmosphere. Two kinds of combustible gases contained in the atmosphere based on the number, type and concentration can be divided into short type.
Nitrogen-based controlled atmosphere:
Mainly refers to the preparation of industrial nitrogen mixed atmosphere of nitrogen and hydrogen. Preparation exothermic fuel type Ⅱ and Ⅲ type also belong to this atmosphere. General separating oxygen from air or nitrogen pressure swing adsorption proceeds, oxygen is about 0.2% to 2%, only as a general controlled atmosphere diffuse gas; refined by oxygen is less than 10-5 nitrogen, can be used for light less demanding steel protective atmosphere. Should generally be mixed with a small amount of H2 or CH4, by catalytic deoxygenation dry before as a controlled atmosphere.
Preparation ammonia atmosphere:
Is evaporated with anhydrous ammonia through the catalyst interior, external heating to near 1000 ℃ tank reactor, the mixture containing 25% N2 and 75% H2 gas mixture, cooling, drying shall be controlled atmosphere of ammonia decomposition. Add ammonia of incomplete combustion air to remove CO2 and H2O, and then by further purification of the main component is obtained ammonia burner controlled atmosphere of N2.
Purity:
Solution with water or from a coke oven gas, chemical byproduct gas extracted by pressure swing adsorption method.
Controlled atmosphere choice:
Mainly based on the type of material and heat treatment process requirements to choose.
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