Application of industrial gases in heat treatment

Application of industrial gases in heat treatment

During the mechanical processing process, mechanical parts must be heat treated by placing them in various heating furnaces for heating. After reaching a predetermined temperature, they are kept warm for a period of time, then released from the furnace, and then cooled to complete a heat treatment process. In the machinery manufacturing industry, most of the parts processed are steel materials. When steel parts are heated in a furnace, the surface will be oxidized at 500°C, that is, decarburization occurs. If the blank is processed, there will be a machining allowance later to ensure that the oxidation and decarburization layer is removed. If it is the final heat treatment process, only a small amount of grinding work is left on the part. If the oxidative decarburization layer is deep and cannot be removed by final processing, the performance of the parts after heat treatment will be greatly reduced.

The decarburization phenomenon of steel parts during heating is due to the presence of oxygen in the heating medium. As long as the oxygen is isolated from the heating, the oxidative decarburization phenomenon can be avoided. This requires not to heat in an air furnace, usually in a salt bath furnace. To use a salt bath to isolate oxygen, the salt bath must be deoxidized. The processed salt residue and steam also pollute the environment. Vacuum furnaces are also used for processing, but the sealing technology requires high requirements and the furnace cannot be made too large, which limits its application.

Gas-protected furnaces are widely used in industry. During the heat treatment process, a variety of gases are used, including argon protection, nitrogen-based protection, and a large number of nitrogen-based protective atmospheres.

Nitrogen-based protection can prevent oxidative decarburization of steel parts and greatly improve the surface quality of heat-treated parts, especially when dealing with some tools and molds with complex shapes. After they are quenched, the cavity will no longer be processed. If there is oxidative decarburization, It will greatly reduce the hardness of the surface layer, that is, reduce its wear resistance and service life. By using neutral heating in a nitrogen-based protective atmosphere, any oxidative decarburization phenomenon will no longer occur on the working surface, which improves the quality of heat treatment on the surface of the workpiece and extends the service life of the workpiece.

In heat treatment equipment, in order to use different gases for protection, there is a multi-purpose furnace or fluidized furnace, which can use nitrogen and various carriers in different proportions to perform nitriding, nitrocarburizing (soft nitriding) , carburizing and other chemical heat treatments.

It provides protection for the heating treatment process based on industrial gases, and can prepare various carrier gases above for various chemical heat treatments, which not only facilitates the heat treatment process of materials, but also greatly improves the heat treatment efficiency.

Nitrogen-based protective atmosphere uses pure nitrogen (99.99%) or industrial nitrogen as the raw material gas, adding appropriate hydrocarbons (such as natural gas, propane, etc.), and if necessary, adding certain gases that participate in the reaction, such as hydrogen, ammonia, carbon dioxide, Air, etc., to produce a mixed gas with ammonia as the main component. This type of gas does not contain or contains certain reducing gases and can be widely used in various heating processes, such as bright heat treatment, chemical heat treatment, brazing, powder metallurgy sintering and other processes.

Nitrogen used for heat treatment can be roughly divided into the following types:

1. Pure oxygen generally refers to protective gas containing more than 99.99% nitrogen.

2. Amino neutral protective gas refers to a protective gas that does not oxidize, decarburize or carburize steel. This kind of protective gas also has certain reducing properties. Because it has protective properties for steels with various carbon contents, as long as the heating cycle is the same, steels with different carbon contents can be processed in the same furnace, and can be used for quenching, annealing, tempering, etc. at high, medium and low temperatures. Heat treatment process to achieve bright effect. Commonly used neutral gases include the following:

1. Nitrogen + hydrogen: This protective gas has certain reducing properties and weak decarburization properties. The hydrogen content in the gas is generally controlled between 0.5% and 3%.

2. Nitrogen + carbon monoxide + hydrogen: This protective gas can be used for non-oxidation, non-decarburization, and non-carburization heat treatment of steel structures, tool steels, and bearing steels, such as carbon monoxide content 0.5%~1% and hydrogen 1%~2% Annealing and quenching of tool and die steel, high-speed steel, and bearing steel are carried out in protective gas. In a nitrogen-based atmosphere with a carbon monoxide + hydrogen content of 2%, high-speed steel with a carbon content of 1% is heated to 1200°C, and there is basically no decarburization after 40 minutes. The preparation of this protector can be obtained by purifying industrial nitrogen with methanol.

3. Nitrogen-based carbon potential atmosphere: This is a nitrogen-based atmosphere with a high content of active ingredients. Usually, an appropriate amount of additives (hydrocarbons or oxygen-containing derivatives of hydrocarbons) can be added to nitrogen to obtain a carbon potential atmosphere for carburizing treatment.

4. Nitrogen-methanol protective gas: This is a nitrogen-based atmosphere currently widely used abroad. Control the ratio of nitrogen to methanol so that carbon monoxide: hydrogen: nitrogen = 1:2:2 in the atmosphere.

The advantages of using nitrogen-based atmosphere heat treatment: First, it saves energy. Compared with endothermic atmospheres, using nitrogen-based atmosphere can save fuel consumption by 25% to 85%. Second, the gas source is abundant. The preparation of nitrogen source in a nitrogen-based atmosphere mainly comes from air, and the gas source is very abundant. Third, it can improve product quality. The nitrogen-based atmosphere contains less carbon monoxide and hydrogen, which greatly reduces hydrogen embrittlement and internal oxidation. Usually the endothermic atmosphere is a reducing gas for steel due to its high carbon monoxide and hydrogen content. But carbon monoxide is an oxidizing agent for elements such as chromium, manganese, strontium, molybdenum, and titanium. Therefore, the endothermic atmosphere is a bright heating atmosphere for carbon steel, while a black oxide is formed on the heating surface of alloy steel. For example, stainless steel and bearing steel have high chromium content. Since chromium has a strong affinity with oxygen, chromium is oxidized in the atmosphere of carbon monoxide and carbon dioxide. The carbon monoxide content in the endothermic atmosphere reaches about 25%, so the heat treatment results for most stainless steel, bearing steel, and high-chromium steel in the endothermic atmosphere are not ideal. An oxide layer will form on the surface of the steel. Similarly, chromium will also oxidize in water atmosphere. Therefore, for high chromium alloy steel, the use of endothermic atmosphere is not suitable from theoretical analysis. The use of nitrogen-based atmosphere can reduce the oxidation degree of alloy elements and improve the quality of heat treatment. Fourth, it has wide adaptability. Nitrogen-based atmosphere is suitable for heat treatment of various types of carbon steel, alloy steel and stainless steel, as well as non-ferrous metals such as copper and aluminum. Fifth, it has good safety. Nitrogen is a neutral gas, non-toxic, does not pollute the environment, has no explosion hazard, and is easy to transport, manage and use.

Regarding the application of industrial gases in heat treatment, comprehensive nitrogen-based atmosphere heat treatment has obvious advantages. Therefore, key enterprises and projects in China have adopted foreign advanced gas source devices and nitrogen-based atmospheres for various heat treatments.