On 2018-08-08 14:48:54
Vacuum heat treatment of iron and steel materials technical problems
Technical problems of vacuum heat treatment of iron and steel materials! Vacuum heat treatment is a new type of heat treatment technology which is carried out in vacuum (less than one atmospheric pressure) in whole or in part. Vacuum heat treatment can achieve almost all of the conventional heat treatment processes, but the quality of heat treatment is greatly improved. Compared with the conventional heat treatment, the vacuum heat treatment can achieve no oxidation, no decarburization, no carburization, can remove the phosphorus chips on the surface of the workpiece, and has the function of degreasing and degassing, so as to achieve the effect of surface bright purification. The following problems should be paid attention to during the vacuum heat treatment.
1、 Carbon increase.
Vacuum oil quenching is the main process of vacuum heat treatment. The technical difficulty in the development of vacuum oil quenching technology is the carburization of vacuum oil quenching. The surface of steel is activated when it is heated in high temperature in vacuum, and the activated carbon atoms formed by the decomposition of quenching oil in the process of quenching in vacuum penetrate into the surface of the workpiece, which makes the carburizing layer form. This carburizing phenomenon is more serious with the increase of quenching temperature. The result of carburization is to reduce the fatigue performance. In order to prevent this phenomenon, it is necessary to select high quality vacuum quenching oil and improve the heat treatment process. Vacuum quenching oil requires low saturated vapor pressure, good chemical stability, less residual carbon and impurities, low acid value; the key is to ensure the stability of long-term use. In the process of vacuum quenching, the appropriate heating vacuum degree should be selected, and the method of filling gas first and then oil or gas oil two-stage cooling should be adopted. In addition, there is carburizing problem in solution treatment with vacuum oil quenching furnace. In the operation of the vacuum oil quenching furnace, the oil attached to the material fork and the material basket of the vacuum oil quenching furnace is brought into the heating chamber in the operation process to become the carburizing agent. Carburization will not only increase the cold work hardening rate, but also worsen the corrosion resistance and affect the strengthening mechanism of the alloy. Solutions include: scrubbing the oil adhering to the fork basket of vacuum oil quenching furnace; solution heating after wrapping to prevent the possibility of carbon contacting with alloy; but for austenitic stainless steel. For precipitation hardening stainless steel and other steel with low cooling speed, the best way is to use vacuum gas quenching furnace. Generally, high purity liquid argon is used as gas source for gas cooling, and the oxygen content should be less than 0.0002%.
2、 Quenching speed problem.
Under low vacuum pressure, the cooling capacity of vacuum quenching oil decreases, which may not achieve the purpose of quenching for some steels. Therefore, before quenching, a high purity neutral or inert gas is filled into the quenching chamber to create a certain pressure on the oil surface, so that the steel can be fully quenched and a bright surface can be obtained. The lowest liquid surface pressure which can obtain the same quenching hardness as that under atmospheric pressure is called critical quenching pressure. For the steel with poor hardenability, the method of filling gas before oil should be used for quenching. The oil surface pressure should be higher than the critical quenching pressure. Generally, the regulation is about 5 ′ 104pa, but it should not be lower than 1 ′ 104pa. In recent years, the development of vacuum compression quenching has been rapid. It can replace the traditional air cooling, partial oil cooling or staged quenching. It can achieve the purpose of controlled cooling and reasonable cooling. In order to improve the cooling rate of vacuum gas quenching, the following methods can be adopted: (1) using large heat exchanger to reduce the temperature of quenching gas; (2) increasing the gas flow rate and flow; (3) changing the unidirectional air flow to 360 ° circumferential high-pressure jet flow or alternating up, down, left and right; (4) selecting the gas with high heat conduction capacity. Considering the safety and cost, the best cooling rate, the lowest cost and safety is 80% HE20% N2 gas mixture. Theoretically, the cooling condition of he at 2 ′ 106pa has reached the cooling speed of oil cooling, while that of H2 at 4 ′ 106pa has reached the cooling capacity of water cooling.