Vacuum heat treatment of steel materials
Vacuum heat treatment of steel materials technical problems
The technical problem of vacuum heat treatment of steel material! Vacuum heat treatment refers to a new heat treatment technology that all or part of the heat treatment process is carried out under vacuum (less than one atmosphere). Vacuum heat treatment can realize almost all the heat treatment processes involved in conventional heat treatment, but the quality of heat treatment is greatly improved. Compared with conventional heat treatment, vacuum heat treatment can achieve no oxidation, no decarburization, no carburization, can remove the phosphorus chip on the workpiece surface, and have degreasing and degassing, so as to achieve the effect of surface brightness purification. The following problems should be noted during vacuum heat treatment.
I. carbon enhancement.
Vacuum oil quenching is the main technology of vacuum heat treatment at present. During the development of vacuum oil quenching technology, the problem of carbon enhancement by vacuum oil quenching is a technical difficulty. The surface of steel is activated when heated at high temperature in vacuum, and the activated carbon atoms formed by the thermal decomposition of the quenching oil during the quenching process infiltrate into the surface of the workpiece during the oil quenching process, so as to form a carburizing layer on the workpiece surface. This carburizing phenomenon becomes more serious with the increase of quenching temperature. The effect of carburizing is to reduce fatigue performance. In order to prevent the occurrence of this phenomenon, it is necessary to select vacuum quenching oil with good quality and improve the heat treatment process. Vacuum quenching oil requires low saturation vapor pressure, good chemical stability, less residual carbon and impurities, and low acid value. The key is to ensure the stability of long-term use. In the vacuum quenching process, to choose the appropriate heating vacuum degree, the first gas into the oil or gas-oil two-stage cooling method. In addition, carburizing is a problem in solid solution treatment by vacuum oil quenching furnace. In the operation of vacuum oil quenching furnace, the oil attached to the material fork and basket of vacuum oil quenching furnace is brought into the heating chamber during the operation to become carburizing agent. Carburizing can not only increase the cold hardening rate, but also make the corrosion resistance deterioration and affect the strengthening mechanism of the alloy. The solution includes: scrubbing the oil attached to the vacuum oil quenching material fork basket; After the package, the solid solution was heated to prevent the possibility of carbon contact with the alloy. But for austenitic stainless steel. Precipitation hardening stainless steel and other solid solution treatment cooling speed requirements of the steel, the best way is to use real air quenching furnace, gas cooling generally used as a source of high purity liquid argon, oxygen content should be less than 0.0002%.
Second, quenching speed.
At low vacuum pressure, the cooling ability of the vacuum quenching oil decreases, which may not reach the purpose of quenching for some steels. To this end, before quenching, the quenching chamber filled with high purity of neutral or inert gas, causing a certain pressure on the oil surface, steel can be fully quenched and obtain a bright surface. The minimum surface pressure at which the quenching hardness can be obtained is called the critical quenching pressure. For the steel with poor hardenability, the way of first filling air and then filling oil should be selected for quenching. The oil surface pressure shall be higher than its critical quenching pressure. It is generally adjusted around 5 '104Pa, but should not be lower than 1' 104Pa. In recent years, vacuum pressurized gas quenching has developed rapidly. Vacuum pressurized air cooling can replace traditional air cooling, partial oil cooling or fractional quenching, which can realize controlled cooling and achieve reasonable cooling. In order to improve the cooling rate of true air quenching, the following methods can be adopted :(1) large heat exchanger is adopted to reduce the quenching gas temperature; (2) increase the gas flow rate and flow rate; (3) change a unidirectional airflow is 360 ° circumferential high-pressure jet flow or up and down or so alternating injection; (4) choose the gas with high heat conduction ability. In terms of safety and cost, the best cooling rate, lowest cost and safety is 80% he20% N2 mixture. Theoretically, the cooling condition of He at 2 '106Pa has reached the cooling rate of oil cooling, while the cooling condition of H2 at 4' 106Pa has reached the cooling capacity of water cooling. (kair)