Controlled atmosphere heat treatment is mainly to prevent oxidation and decarburization, and to achieve maximum control of carburizing and nitriding. In the development trend of non-oxidative heat treatment technology, controlled atmosphere heat treatment is the first.
The sealed box multi-purpose furnace production line is suitable for mass production, such as the automotive industry. Domestic manufacturers are keeping abreast of user requirements and launching various controllable box-type atmosphere furnaces with moderate prices, diverse styles and superior performance. Most of them can optimize the process parameters, predict and control the distribution of carbon concentration, and obtain the ideal concentration distribution and permeation layer structure. It can realize computer management, simple menu design, and friendly man-machine interface.
Controllable atmosphere classification:
The controllable atmosphere mainly includes endothermic gas, exothermic gas, nitrogen-based atmosphere, ammonia decomposition gas, drip decomposition gas, hydrogen and charcoal gas.
In the generator, natural gas, liquefied petroleum gas and other gases are mixed with a certain proportion of air (when the amount of air is small, the mixed gas is partially burned first), and then the mixed gas is heated by a catalyst heated to a high temperature (above 1000 ° C) The unburned part is obtained by thermal cracking (endothermic reaction). Endothermic gas is the most widely used controlled atmosphere. Using it as a carrier gas and bringing an appropriate amount of enriched gas (methane or propane) into the heating furnace, the surface of the low-carbon steel parts can be carburized so that the carbon content reaches the specified requirements. By controlling the amount of enriched gas added, the carbon potential of the furnace gas can be controlled. The carbon potential of the furnace gas refers to the amount of carbon when the furnace gas is in equilibrium with the carbon content of the austenite on the steel surface at a certain temperature, and can be simply understood as the carburizing capacity of the furnace gas. The endothermic atmosphere is widely used in chemical heat treatment processes such as carburizing, carbonitriding and gas nitrocarburizing. It can also be used for bright quenching of medium carbon steel, high carbon steel, alloy tool steel, bearing steel and high speed steel. .
2. Exothermic gas
In the generator, a gaseous fuel such as natural gas liquefied petroleum gas or a liquid fuel such as alcohol and diesel is mixed with more air, so that it is close to complete combustion (exothermic reaction), and the combustion products are initially purified (dewatering) Or highly purified (except for water, carbon dioxide and carbon monoxide). Exothermic gas can be used for bright annealing of low carbon steel, decarburizing annealing of silicon steel sheet, bright quenching of medium carbon steel and high carbon steel, sintering of powder metallurgy and gas nitrocarburizing. The purified exothermic atmosphere can also be used for stainless steel annealing and brazing protection, or as a carrier gas during carburizing.
3. Nitrogen-based atmosphere
Commonly used is high-purity nitrogen obtained by purifying (removing oxygen or air) the industrial nitrogen produced during the oxygen production process, or it can be a gas obtained by evaporating liquid nitrogen. Nitrogen-based atmosphere can be used for heating and protection, or methanol can be added to make its composition close to the endothermic atmosphere. As a carrier gas during carburizing, it can be carburized by adding rich gas (such as propane). Its advantage is that it can save natural gas, liquefied petroleum gas, etc., and the carbon potential can be controlled.
4. Ammonia decomposition gas
Ammonia can be completely decomposed into 3 volumes of hydrogen and 1 volume of nitrogen under a certain temperature and catalyst to form ammonia decomposition gas. You can also mix ammonia and air for partial combustion, and then remove water to purify to obtain ammonia combustion gas (the main component is nitrogen). Both atmospheres contain hydrogen and are hydrogen-water mixed gases. Mostly used for stainless steel high speed steel heat treatment and powder metallurgy sintering. Because ammonia is more expensive, the cost of preparation is higher.
5. Drip decomposition gas
Decomposition gas produced by drip carburizing, which enables the carbon concentration on the steel surface to be controlled, also belongs to a controlled atmosphere. This is the Kabomag method proposed in the early 1960s. The principle is: two organic liquids are dripped into the furnace, and thermal cracking occurs in the furnace. The thermal cracking gas of one liquid (such as methanol) is used as the carrier gas, and the thermal cracking of the other (such as ethyl acetate). Gas is used as an enriched gas to achieve carburizing of steel, and the carbon potential of the furnace gas is controlled by adjusting the dripping amount of the second liquid. The advantage of this method is that it does not use a gas generator, the equipment structure is simple, and it is suitable for mass production.
Hydrogen is a reducing gas. ** Dry hydrogen is difficult to prepare, so the hydrogen used is actually a mixture of trace amounts of water and hydrogen. Whether a metal is oxidized or reduced when heated in hydrogen depends on the ratio of water to hydrogen. But heating with hydrogen protection cannot prevent the decarburization of steel. Annealing of stainless steel and silicon steel sheets, reduction of sintered iron powder by powder metallurgy, can be performed in hydrogen.
7. Charcoal gas
The air is blown into the generator with a blower, and the gas produced by the reaction between the air and the hot charcoal is charcoal gas. Its main components are carbon monoxide and carbon dioxide. Carbon monoxide is a reducing gas, and carbon dioxide is an oxidizing gas.