Technical Problems of Vacuum Heat Treatment of Steel Materials
Vacuum heat treatment refers to a new type of heat treatment technology in which all or part of the heat treatment process is performed in a vacuum state (less than one atmosphere). Vacuum heat treatment can realize almost all heat treatment processes that can be 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, and no carburization. It can remove phosphorous chips on the surface of the workpiece, and has the functions of degreasing and degassing, so as to achieve the effect of surface bright purification. Pay attention to the following issues when performing vacuum heat treatment.
The issue of Increasing Carbon.
Vacuum oil quenching is currently the main process of vacuum heat treatment. The technical difficulty encountered in the development of vacuum oil quenching technology is the problem of vacuum oil quenching carburization. The surface of the steel is activated when heated in a vacuum at high temperatures. The activated carbon atoms formed by the thermal decomposition of quenching oil during vacuum quenching infiltrate during the oil quenching process to form a carburized layer on the surface of the workpiece. This carburizing phenomenon becomes more serious as the quenching temperature rises.
The consequence of increasing carbon is to reduce fatigue performance. In order to prevent the occurrence of this phenomenon, good quality vacuum quenching oil should be selected and the heat treatment process should be improved. Vacuum quenching oil requires low saturated vapor pressure, good chemical stability, low residual carbon and impurities, and low acid value; the key is to ensure the stability of long-term use. In terms of the vacuum quenching process, it is necessary to select a suitable heating vacuum degree, and adopt a gas-filled and then oil-filled or gas-oil two-stage cooling method.
In addition, there is a carburizing problem with the vacuum oil quenching furnace for solution treatment. In the operation of the vacuum oil quenching furnace, the oil attached to the fork and basket of the vacuum oil quenching furnace is brought into the heating chamber during the operation to become a carburizing agent. Carbon increase will not only increase the cold work hardening rate but also deteriorate the corrosion resistance and affect the strengthening mechanism of the alloy.
Solutions include: scrubbing clean the oil attached to the vacuum oil quenching charge fork basket; after wrapping, solid solution heating to prevent the possibility of carbon contact with the alloy; but for austenitic stainless steel. For precipitation hardening stainless steels and other steels that require a low cooling rate for solid solution treatment, the best method is to switch to a vacuum quenching furnace. Generally, high-purity liquid argon is used as the gas source for gas cooling, and the oxygen content should be less than 0.0002%.
Second, the Problem of Quenching Speed.
Under low vacuum pressure, the cooling capacity of vacuum quenching oil decreases, and for some steels, the purpose of quenching may not be achieved. For this reason, before quenching, fill the quenching chamber with high-purity neutral or inert gas to create a certain pressure on the oil surface, and then the steel can be fully quenched and a bright surface can be obtained. The lowest liquid surface pressure that can obtain the same quenching hardness as under atmospheric pressure is called the critical quenching pressure. For steel with poor hardenability, it should be quenched by filling the gas first and then oil.
The oil surface pressure should be higher than its critical quenching pressure. Generally, the adjustment is around 5´104 Pa, but it should not be lower than 1´104 Pa. In recent years, vacuum pressurized gas quenching has achieved rapid development. Vacuum pressurized air cooling improves the cooling rate and can replace traditional air cooling, partial oil cooling, or stage quenching, and can achieve controlled cooling to achieve the purpose of reasonable cooling.
In order to increase the cooling rate during vacuum quenching, the following methods can be adopted:
(1) Use a large heat exchanger to reduce the temperature of the quenching gas;
(2) Increase the gas velocity and flow rate;
(3) Change to one-way airflow It is a 360°circumferential high-pressure jet flow or alternately jetting up and down, left and right;
(4) Use gas with high thermal conductivity. In terms of safety and cost, the 80%He20%N2 mixed gas has the best cooling rate, lowest cost, and safety. Theoretically, the cooling condition of 2´106Pa He has reached the cooling rate of oil cooling, and the cooling condition of 4´106 Pa H2 can reach the capacity of water cooling. (Steel Research Institute)