Vacuum low pressure carburizing heat treatment process of 8620 steel

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8620 alloy steel is an American steel grade. Its main composition is similar to that of domestic 20CrNiMo steel. It is often used in the forging of key components such as gear shafts, valves in pressure vessels, and inlet and outlet pipe seats. It has high hardenability and superior welding and processing performance. Generally used after carburizing. Vacuum low pressure carburizing heat treatment technology, as a representative of advanced heat treatment technologies such as clean, precise, non-oxidizing and low emission, has become a hot spot of current heat treatment research.

The picture shows an 8620 alloy steel valve. This valve body is used in the automotive industry, requiring high precision, good wear resistance, good carburizing uniformity, no oxidation on the inner and outer surfaces, and mass production, which requires high product performance consistency.

Compared with traditional gas carburizing, the advantages of low pressure vacuum carburizing are as follows:

(1) The product has no internal oxidation (black structure), which can significantly improve the surface fatigue performance of the parts and greatly improve the reliability and service life of the product.

(2)The heat treatment distortion of the product is small, and it can even replace the press quenching, reduce the processing amount in the later stage, and save the processing cost.

(3) The control precision of the seepage layer is high, and the computer simulation control precision can reach ±0.05mm. The carbon content of the vacuum carburizing surface does not need to be controlled by carbon potential, and the precise control of the surface carbon content can be achieved by controlling the carburizing pressure and the carburizing gas flow. The heat-treated workpiece has good repeatability and stable quality.

(4) The surface quality of the product after treatment is good: no oxidation, no decarburization, and maintains the silver-gray and bright shape of the metal’s true color, which can save cleaning and shot peening processes.

(5) Under the condition of low pressure and high temperature, the carburizing process can be greatly shortened and the productivity is high. The carburizing temperature range spans a wide range, from low temperature carburizing to the highest carburizing temperature up to 1050 °C. For deep carburizing, the process time can be greatly saved, and it is more conducive to complete the carburizing process of special steel grades.

(6) High production efficiency: high temperature carburizing can greatly shorten the production cycle, effectively save time and energy costs (the consumption of auxiliary gas is greatly reduced), and can reduce costs and improve production efficiency.

(7) There is no fire curtain, no exhaust port, no oil fume, and the working environment is clean. It is a safe and environmentally friendly heat treatment equipment.

(8) It has wide adaptability and great flexibility, and can realize on-line production (vacuum carburizing + high pressure gas quenching). Applicable parts include: hole type parts, such as needle valve body injector. Gears, especially large gears, minimize the difference between the infiltration layer of the tooth tip and the tooth root, and have no non-martensitic structure and internal oxidation. For thin-walled parts or parts with complex shapes, through vacuum carburizing and high-pressure gas quenching, the distortion energy is minimized, and the press quenching mode can be eliminated.

The heat treatment of 8620 steel adopts low-pressure pulse carburizing under vacuum conditions. The carburizing medium is acetylene and high-purity nitrogen fully mixed in a certain proportion. After 90min, the sample was taken out after deep cooling and slowly warmed to room temperature, and then tempered at 170°C. The specific process curve is shown in the figure:

The workpiece is placed at 8 vertex positions and 1 center position of the material frame. At the same time, in order to prevent the material frame from moving during quenching, a large workpiece is placed as a counterweight. The shape of the workpiece is shown in the figure:

8620 steel adopts a combination of vacuum low pressure carburizing and cryogenic treatment, which can reduce the amount of retained austenite on the surface and improve the surface hardness. After treatment, the surface hardness is between 57~60HRC, the depth of the effective hardened layer is 0.7~0.8mm, and the depth deviation of the effective hardened layer of the samples in the same heat is less than 0.1mm, and the carburizing uniformity is good, which meets the product process requirements.

Equipment selection: The RVC series vacuum carburizing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum carburizing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.