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Correct use and maintenance of vacuum brazing furnace

In the vacuum furnace, it is estimated that it is used for brazing to account for about 10%. A large number of early products mainly meet the strict requirements of aerospace products, and the trend in recent years has shifted to general industrial applications.

Vacuum brazing produces cleaner, finely machined, metallurgical quality products, some of which cannot be formed into final shape without vacuum brazing. Lower cost due to elimination of post-braze cleaning. This method can also be used to manufacture products composed of many small, low-cost parts, instead of machining or welding together large blocks. certainly. Vacuum brazing rarely poses environmental hazards.

The popularity of vacuum brazing furnaces is beneficial to both users and manufacturers. At present, there are many furnaces to choose from, and a large number of vacuum furnace products have flooded the market. Vacuum brazing equipment is usually self-feeding, fully automated, and does not require much experience or training to master the operation. In fact, vacuum protection operators only need to strictly abide by the operating procedures and press the buttons and switches at the right time to operate. Except for the loading, running, unloading of equipment and notification of maintenance personnel when the equipment is not functioning normally, the operator does not need to do any work. Unlike machine tools, there is no need to change tools or lubricate parts. However, for new users, these conditions and conditions are easy to be misunderstood, and it is easy to cause the illusion of safety, and they have to wait until they have to, before submitting an application for maintenance. Specialization in vacuum handling has created new maintenance problems for less experienced users. Vacuum furnaces are more abrasive for brazing than for heat treatment alone. If regular maintenance is not carried out, various problems will appear in the new furnace before long, causing the vacuum furnace to be out of service.

Practical Operation Technology of Vacuum Furnace

Vacuum furnace managers should use good judgment when approving materials to run in vacuum brazing furnaces. The relationship between meeting the current production needs and the long-term operation of the equipment must be correctly weighed, and the hazards caused by the processing of contaminated workpieces in the vacuum brazing furnace must be weighed.

There are three general requirements for the maintenance of vacuum brazing furnaces:

1) The type of product material entering the furnace;

2) The cleanliness of the material;

3) Regular maintenance of system functions.

A vacuum furnace is not a universal tool. Care must be taken not to allow materials that could introduce contaminants into the furnace. Most ferrous metals and various non-ferrous metal parts are easier to braze. Austenitic stainless steels such as 304, 316, 321 and 347 are the most commonly used materials for aerospace components that do not require hardenability. Usually 304 and 3l6 stainless steel, no special requirements, 1800. It is easy to braze above F (982°C), but it should be cooled rapidly in the sensitive temperature range of F (900°C-425°C). Since the composition of stainless steel 321 and 347 contains carbon: 0.08 silicon: 1.00 manganese: 2.00 phosphorus: 0.045 sulfur: 0.030 nickel: 9.00-12.00 chromium: 17.00-19.00 titanium: 5C-0.70, it requires extra care in pre-cleaning to Avoid forming a hard oxide film that hinders wetting and forms stains. Ferritic stainless steel is rarely used for brazing, but hardenable martensitic stainless steel is more commonly used, such as 410, 416, 418 and 440C. Martensitic stainless steels were chosen because of their excellent dimensional stability. Brazing filler metals rarely experience poor wetting problems with these base materials. Precipitation-hardening stainless steel and other high-strength steels, heat-resistant steels, etc. are widely used materials for vacuum brazing components in the aerospace industry, and they require some additional measures to ensure brazing wettability. These alloys contain more than 2% refractory metal elements in total, requiring chemical cleaning and surface nickel plating (according to AMS 2403) in the brazing joint area before assembly to ensure proper wetting properties of the brazing filler metal. Under ideal laboratory conditions, chemical cleaning and nickel plating are unnecessary, but under daily production conditions, this guarantees the success of brazing.

Avoid Vacuum Furnace Contamination

Before entering the vacuum brazing furnace, all parts and components to be welded should be properly cleaned. The cleaning methods can be wiped with degreased acetone, ultrasonic cleaning or dry grinding (such as glass abrasive grains). The pre-cleaning of the workpiece is only the first step in keeping the vacuum brazing furnace system in the designed functional state. Assuming that the brazing furnace is clean and all operating steps are in compliance with the regulations, pre-cleaning the workpiece is a guarantee for successful vacuum brazing. However, if the furnace is polluted due to the unclean workpiece being put in the previous time, and it is used carelessly or misused, it can be expected that the brazing will not be successful this time. Many operators use brazing furnaces for vacuum heat treatment, often resulting in neglected sources of contamination of the vacuum furnace due to the introduction of improperly cleaned workpieces containing residual hydrocarbons, workshop dirt and elements with high vapor pressure. It condenses in the cold zone of the furnace and enters its surrounding environment at high temperature, thus affecting the quality of brazing. The pollution of the castings to the vacuum brazing furnace is serious. During the repeated production process, the castings are constantly outgassing, which will soon affect the quality of the components brazed in the same furnace. Doing so often results in costly overhauls and cleaning or redoing of internal heating zone components. Degassing of any material should be done in vacuum and not in a vacuum brazing furnace. Brazing filler metals containing zinc, cadmium and lead should be avoided, or used only when the partial pressure of argon or nitrogen is higher than 500um, and the temperature is lower than its vapor pressure, and the use of flux should be avoided as much as possible.

Vacuum brazing fixtures can also cause contamination

Vacuum brazing fixtures are another common source of contamination in vacuum brazing furnaces. Fixtures should be cleaned together with the workpiece to prevent contamination from being introduced into the brazed joint or vacuum furnace. The fixture material shall be able to withstand the brazing temperatures achieved. in 1900. F (1038°C) damage caused by accidentally using bronze and aluminum as fixture parts during nickel brazing. Catastrophic to the workpiece and severe damage to the furnace interior. For this, downtime ranges from a few days to a few weeks, depending on the availability of key components and the efficiency of repair work. Due to the routine daily brazing work, the aforementioned foreign objects lead to a gradual reduction in the radiation capacity of the heat shield (the surface of the insulation jacket), due to the accumulation of continuous oxidation and wear. Any reduction in radiation capability caused by the product or solder cannot be compensated for by extending the operating life of the internal insulation and also reducing power requirements and consumption. Maintaining the functionality of your vacuum furnace Maintaining optimal function of your vacuum furnace requires strict adherence to a regular maintenance program.

About Vacuum Brazing Solder Spatter

The constant daily operation – brazing various parts with different brazing filler metals – inevitably causes some brazing material splash damage to heating elements and heat shields. If not repaired in time, the deposition of brazing metal will soon corrode and damage the heating element, or cause localized overheating, or cause the heating system to break one or more phases. This introduces temperature inhomogeneity, which is often unnoticed in batch brazing production. Preventing the spatter of vacuum brazing solder can reduce the vacuum, and the repair of vacuum heating elements is relatively easy, which is different from the damage of heat shields. If the heat shield is made of molybdenum, as long as a thin layer of graphite is added to the screen for protection, the hazard of brazing metal splash can be minimized. The brazing metal does not wet and bond to the graphite surface. Simply roll into the cold zone of the stove in small beads. These residues can be easily removed after brazing is complete without causing harm. Spattering of the brazing filler metal can be caused by improper or excessive application of the solder paste, by entrapped gases being ejected from the brazed joint, or by too rapid a heating rate. When molybdenum strip heating elements with a width of 4 to 5 inches (101 to 127mm) and a thickness of 0.025 inches (6.35mm) are usually used, the damaged part should be cut off, and a new molybdenum strip should be bolted in place. Make repairs. This is adequate for short-term use, but after a few such repairs it may be better to replace an entire section, say a third of the loop. The spatter of the brazing filler metal is also easy to stick to the connecting bolts of the patch, and after being reheated, the components will be quickly damaged. This is in contrast to graphite heating elements, which are not susceptible to this type of damage, but are prone to cracking and brittle failure. In this case, the entire element must be replaced. The patch cannot be applied.

Vacuum Brazing Furnace Hot Zone Isolation

There are two basic design ideas for the heat zone isolation of vacuum brazing furnaces according to different materials, any of which can be used. These two programs are usually labeled as . All metal and soft cover design. Each has its own characteristics, according to the user’s intention to choose. The user must consider the advantages and disadvantages of each according to the type of product and the process implemented in the vacuum furnace. Selection should also include other factors such as operating cost, efficiency, ease of repair, and initial investment. The all-metal thermal zone isolation consists of an insulating jacket. It can be cylindrical or rectangular, consisting of more than two layers of aluminum plates, more than three layers of stainless steel plates, and an air gap in the middle. The heating element is internally supported with metal molybdenum or ceramic hardware to isolate the heat shield. The main advantage of this design is that it is not easy to absorb dirt, especially water vapor, and it can also achieve a high degree of vacuum, and it can be kept clean for a long time. The disadvantage is that it needs to consume about five times more power than the soft package solution. 1/1 and cause more heat loss. Maintenance is more difficult and expensive, the initial investment cost is also high, and the molybdenum heat shield in this well is also reported to be easily bonded by brazing spatter. The soft sheath scheme basically consists of a graphite thin layer as the inner layer. The back Murakami consists of a few inches thick ceramic fiber cotton and a steel inflatable sleeve. Due to better insulation, it requires less power and is easier to maintain. Its construction features make it easy to repair in the user’s factory, and the cost is relatively low because molybdenum is not used as a heat shield. As mentioned above, graphite layer heat shields are also immune to brazing filler metal spatter. The replacement of the internal heat shield of the all-metal hot zone, including the replacement of the heating element, usually needs to be purchased from the manufacturer, while the “soft sheath design” can be self-made in the user’s factory. A vacuum braze with an average diameter of 50 inches (1.27m) The cost of the internal heat shield of the welding furnace is about 100,000 yuan, and the replacement of the soft cover, including a new set of heating parts, if the user makes it in the factory, the cost is about a quarter of the above cost. Processing 24 hours a day In the case of production, the replacement of the internal heating elements and heat shields is carried out approximately every two or three years.It depends on the type of vacuum brazing job and the frequency of use.

Performance of vacuum unit for vacuum brazing furnace

To maintain excellent brazing quality in a vacuum brazing furnace depends on the performance of the vacuum pump. The vacuum pump needs constant attention. If the vacuum unit is properly maintained, it is more economical to shut down on weekends and holidays. Otherwise, if left idling over the weekend, the mechanical pump will get stuck and be difficult to restart. The results of our experiments show that the general setting is 28 degrees in winter and 26 degrees in summer. This method can save a lot of power. Due to lack of cooling water or overheating: water temperature 140~150. F (60 ~ 66 ℃) above, it is easy to cause the mechanical pump to overheat, thus making noise. Use a portable device to filter the vacuum pump oil once a week, check the oil level and take oil samples, and check the quality of the oil by feeling and appearance. Mechanical pumps are more economical to use high-quality oil, which costs more than three times as much as common industrial oils and keeps the pumps clean for five times longer. The valve and spring should be replaced every three months. Stainless steel springs last longer Diffusion pumps should be disassembled once a year for a major overhaul, including full cleaning. After the diffusion pump oil is contaminated, it can no longer be vacuumed. Therefore, the oil level and oil quality should be checked once a week. It is best to use high-quality silicone-based oil. Practice has proved that the use of this oil can keep the vacuum degree in the range of 10 ~ l0^-5, and the vacuum brazing production is quite satisfactory. The replaced oil can be recycled.

Maintenance of cooling system of vacuum brazing furnace

The cooling system is an important part of the vacuum unit. The waterway should be kept clear, otherwise the water temperature will rise and cause shutdown. This is a problem that is often overlooked when the system is working. When left unattended it can cause major damage to a vacuum furnace. The cooling water is treated with biodegradation chemical auxiliary method, the purpose is to keep the minerals in suspension, reduce the accumulation of sediment in the rubber tube, serpentine tube and water jacket, so as to make the water flow unimpeded. This is generally accomplished with an automatic device that monitors the conductivity of the water, automatically replenishes chemicals, flushes the waterways, and adds fresh water. Combined with an attached vapor filtration system, this method also ensures water pH control and prevents unwanted mineral deposits in the cooling water system. After the system works stably, a water quality test should be carried out at least once a month to prevent over-treatment or under-treatment. The test samples of the vacuum furnace shell and heat exchanger materials should also be placed in the waterway system, and the weight loss inspection should be done once a year to check the effect of the treatment. Special attention must be paid to the coil cooler of the diffusion pump. It should be checked frequently. If clogged, the diffusion pump will overheat and lose function, further causing the diffusion pump heater to burn out, resulting in costly and time consuming repairs. When the serpentine cooling water pipe is blocked, disconnect the waterway from the water source, drain the water and blow it through with compressed air or rinse it with dilute acetic acid solution.

Efficiency of vacuum brazing furnace gas cleaning system

The gas purification system provides a clean medium for the rapid cooling of the workpiece, thus enabling the vacuum brazing operation to be carried out quickly and efficiently. Nitrogen is the most common and cheapest cooling gas and is suitable for most brazing jobs. However, when dealing with a large number of titanium parts, it is necessary to pass argon in the furnace to avoid contamination of titanium materials. Large LPG tanks present the fewest problems and the least chance of contamination. A single gas cylinder confluence gas supply device will cause many leaks due to many joints and human errors in the purification pipeline. When the liquefied gas tank is used, the liquid argon is evaporated through the finned tube evaporator on the outdoor liquefied tank, and then is transported into the large indoor gas storage tank through the pipeline to provide a large amount of gas when the furnace needs to be cooled by forced airflow. Although the gas supplier guarantees that the gas purity at the outlet of the liquefied argon tanker is 99.998%, according to the technical requirements, the dew point of the gas should be checked at the inlet of each furnace. The average dew point is required to be -65. F(-18°C) The whole system should be checked for leaks every six months. In the final analysis, the good operation of the vacuum brazing furnace is only the result of the user’s continuous maintenance. In today’s world, achieving success is nothing more than a compromise between gain and loss and efficiency. What users care about is the good use status of the vacuum brazing furnace.