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Vacuum Detection Technology of Vacuum Annealing Furnace

1 Overview

Vacuum annealing is an important process in the production of intermediate and finished products. Vacuum annealing has the functions of preventing oxidation, vacuum degassing, preventing surface embrittlement of titanium and refractory metals, uniform heating by vacuum heating, small temperature difference between inside and outside, and small thermal stress; vacuum annealing technology is a high-precision, high-quality, energy-saving and Clean and pollution-free processing and manufacturing technology, widely used in various metal materials, alloy materials, crystal materials and other stress relief treatment, bright annealing, vacuum degassing, etc. How to measure the vacuum degree and detect the leakage of the vacuum system is an important issue in the use and maintenance of the vacuum annealing furnace.

2 Vacuum measurement

2.1 Definition of vacuum measurement

Vacuum measurement is the technique of measuring the thinness (ie vacuum) of the gas in a container or a certain part of the space. The degree of vacuum is usually expressed by the total pressure P of the gas in the container. High pressure means low vacuum; conversely, low pressure means high vacuum. Therefore, in the modern vacuum measurement technology, the vacuum gauge is usually used to directly or indirectly measure the pressure, thereby reflecting the degree of vacuum. The measurement unit of gas pressure is Newton/m2, referred to as Pa (Pa). Traditionally, the gas pressure unit is millimeter mercury column. The conversion of various pressure units: 1 standard atmospheric pressure=101325.0 Pa=14.7Psi=760mmHg=29.92inHg=760Torr =760000micron=1Bar

2.2 Classification of vacuum gauges

The device used to measure the pressure of the gas in the container, that is, the degree of vacuum, is called a vacuum gauge. According to the measurement principle of vacuum gauge: direct measurement vacuum gauge and indirect measurement vacuum gauge.

3 Vacuum leak detection

3.1 Overview

Vacuum leak detection is to solve the existence of leaks or gaps in the vacuum system or container, and determine the leakage rate for repair. The leaks referred to in vacuum technology are very small, and their cross-sectional shapes are different, so it is impossible to express their size with specific dimensions, and there are various leak paths. Therefore, the leak rate (abbreviated as leak rate) is generally used to indicate the size of the leak, that is, the amount of gas entering the vacuum system or container through the leak. Its physical meaning is pressure × volume/time, and the unit of leak rate is Pascal × Cubic meters per second, recorded as Pam3/s.

3.2 Leak detection methods

The vacuum system or container cannot be completely leak-proof. Therefore, according to the actual working requirements of the vacuum system, a reasonable leak rate value should be formulated. As long as the leak rate of the leak hole is lower than the specified value, these leak holes are allowed. If it is found that the leak rate exceeds the specified value, it means that there is a leak in the vacuum system or container, and we need to conduct a vacuum leak inspection. There are many vacuum leak detection methods, which can be divided into pressure leak detection method, vacuum leak detection method and other leak detection methods according to the state of the tested parts.

3.2.1 Charge leak detection method

The pressure leak detection method is to pump the system or container to be tested to a vacuum, and then fill it with a leak-indicating substance with a certain pressure. A certain method or leak detection instrument detects the leak-indicating substance leaking from the leak hole outside the container to be inspected, determines the location of the leak hole, and estimates the size of the leak rate of the leak hole.

Commonly used pressure leak detection methods include water pressure method, air pressure method, ultrasonic method, helium mass spectrometer leak detector nozzle method, nitrogen leak detection method, etc. The principle of the ammonia leak detection method is to evacuate the vacuum system or container, paste a developing tape with an indicator sensitive to ammonia at the suspected leaking hole outside the wall of the container, and then fill the container with a pressure higher than one atmosphere. Ammonia gas, when there is a leak, the ammonia gas escapes through the leak to change the color of the developing belt. Based on this, the location of the leak can be found, and the size of the leak can be roughly estimated from the development time and the size of the discolored area.

3.2.2 Vacuum leak detection method

The vacuum leak detection method is to pump the detected vacuum container or vacuum system and the detection element of the leak detection instrument into a vacuum state, and apply the leak-indicating substances to the suspicious parts outside the tested container or system in turn. If there is a leak in the container or system to be inspected, the leak-indicating substance will not only enter the container or system through the leak hole, but also enter the space where the detection element of the leak detection instrument is located, so that the leak is detected by the sensitive element. substance, to the location and size of the leak.

Commonly used vacuum leak detection methods include high-frequency spark leak detector method, vacuum gauge method, mass spectrometer method, ion pump leak detection method, etc. The principle of the ion pump leak detector is to pump the vacuum system or container and the ion pump to a vacuum, and then apply the leaking gas outside the tested container. When the leaking gas enters the ion pump through the leak hole, the pump current will change. , according to this change, the leak of the vacuum system can be detected, and the size of the leak can be estimated.

3.2.3 Other leak detection methods

In addition to pressure leak detection and vacuum leak detection, other vacuum leak detection methods. Commonly used methods include fluorescence method, back pressure method, radioisotope leak detection method, etc.

The back pressure leak detection method is a combination of pressure leak detection and vacuum leak detection, and is mostly used in the non-destructive leak detection technology of seals. It is to use the back pressure chamber to first fill the leak detection gas into the test piece through the leak hole, and then make the leak detection gas leak out from the test piece in a vacuum state. A method for determining the total leak rate of a test piece. The process can be divided into three steps: pressurization, purification and leak detection.

The fluorescent leak detection method is to dissolve a fluorescent substance in an organic solvent, pour the solution into the container to be inspected, or make the inspected part come into contact with the solution. If there is a leak, the solution can seep into the leak due to capillary action. On the other side, after the solvent evaporates, a dry fluorescent substance is deposited on the other side of the leak hole. At this time, if it is irradiated with an ultraviolet lamp, it can emit fluorescence, which means that there is a leak hole there.

4 Conclusion

Through the introduction of the vacuum measurement methods and vacuum leak detection methods commonly used in the use and maintenance of vacuum annealing furnaces, the article provides strong empirical and theoretical support for the use and maintenance, testing and research of vacuum annealing furnaces in the future. With the increasing of vacuum science and technology, vacuum gauges and vacuum leak detectors should be further developed in terms of diversification, automation, small size and beautiful appearance.