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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Defects and Improvements of Vacuum Furnaces

Vacuum heat treatment technology is widely used in aerospace, electronics, precision machinery. In the production of molds, there are incomparable outstanding advantages of ordinary heat treatment, because vacuum heat treatment has the advantages of non-oxidation, non-decarburization and bright surface of the workpiece; it has degassing and purification functions, can improve the purity of materials, and at the same time improve the fatigue strength of materials, plasticity and toughness, corrosion resistance; the processed workpiece has no risk of hydrogen embrittlement, effectively preventing the surface embrittlement of titanium and refractory metals; small quenching deformation; reliable heat treatment process stability, good repeatability; safe operation; automation High degree; good working environment; no pollution and public hazards. With the development of science and technology, modern vacuum heat treatment technology has also developed rapidly. The future metal surface engineering will rely more on vacuum heat treatment technology and plasma technology, and the future is very broad.

Defect analysis and measures:

Vacuum heat treatment technology belongs to bright heat treatment. Under normal circumstances, the surface of the workpiece after heat treatment should maintain the metallic luster before heat treatment, and the defects of vacuum heat treatment will affect the surface mechanical properties of the workpiece after heat treatment.

For specific problems arising from vacuum heat treatment defects, such as oxidation, uneven hardness, Adhesion; sensitive relays widely used in radio and automatic control, high magnetic permeability alloy 1J79 (permalloy) 4J36 and other magnetic amplifying components do not meet the design requirements. After many experiments and researches, it is not possible to list all the experiments here, but to summarize many experiments and make the following analysis in the following categories. Two sets of equipment are used: 1) SIMUWU vacuum furnace, which is heated by 24 sets of tungsten wires, with a volume of (p300 mm x 600 mm, a vacuum degree of 5 ×10^-5 Pa, a power of 68 W, and a temperature of 1300°C; 2) The vacuum quenching furnace is heated by graphite cloth, the maximum load is 15 kg, the temperature is 1300°C, the power is 30 kW, and the vacuum degree is 7×10^-3Pa.

The following defects are prone to occur during quenching, tempering, and annealing. Oxide films appear at different temperatures.

Generally speaking, there are many oxidation reasons that affect the surface brightness of vacuum heat treatment, among which the three major factors of workpiece, process and equipment are the main factors. The three factors are summarized and analyzed as follows.

1) Material: Contains elements with strong oxygen affinity such as Al, Ti, Si, Cr, V, etc. and elements with high vapor pressure such as Mn, Zn, Cr, etc. 2) There is dirt on the surface of the workpiece, such as water, oil, and oxides. Integral parts, plates and tubular parts with high shape quality.

The temperature is solution treatment at 800~1200°C and aging at 400~600°C. Auxiliary process: cleaning of parts, cleaning medium, drying effect, ambient atmosphere, furnace loading conditions, etc.

Insufficient vacuum pump capacity, leaky piping, contaminated heating material or chamber. The cooling system, the cooling medium is impure, contains moisture, uneven cooling and insufficient cooling capacity. Vacuum furnace air leakage, generally should maintain a certain pressure rise rate, not less than 1.33 Pa/h. Using high-purity gas, the protective atmosphere is reducing. The furnace temperature of the vacuum heat treatment furnace has a significant impact on the brightness. If the oxidation is severe when the furnace is discharged at 500~300°C, the state and brightness of the sample surface will be significantly reduced. It should be discharged below 200°C, and the brightness of the workpiece can exceed 70%. In addition, the presence of a weak oxidizing atmosphere in the heating chamber of the vacuum furnace will also cause surface oxidation, which should attract everyone’s attention. We made a comparison of the spectral analysis of the residual atmosphere in the vacuum furnace after vacuuming for 5 minutes and vacuuming for 40 minutes. , pure metals and metals in alloys will generate steam at a certain temperature and vacuum heat treatment, which will cause depletion of surface elements and cause adhesion.

Alloy elements Ag, Al, Mn, Cr, and Zn that are easier to evaporate are easier to evaporate when the ambient pressure is lower, that is, the vacuum degree is higher and the heating temperature is higher.

Therefore, appropriately reduce the heating temperature, reasonably select the lower limit of the temperature, and reduce and control the vacuum within the range of 5×10^-5Pa. The protective atmosphere is back filled with inert gas, and high-temperature ceramics (better than magnesium hydroxide powder) are placed at the joints of the workpieces. It is especially important for high-temperature alloys, stainless steel, and high-speed steel with high Cr content.

Annealing treatment of U79 and 4J36 magnetic materials and metal materials in vacuum furnaces for comparative experiments. U79C permalloy heat treatment process: heating up to 1100°C, holding for 2 hours, and cooling at 150 ~ 200 C / h Drop to 400°C and cool with the furnace.

Generally, when heating in a vacuum furnace with graphite structure materials, as long as the workpiece is not in direct contact with graphite, the magnetic material will not change significantly. However, for magnetically permeable alloys with high requirements, the lower the carbon content, the better. When the basic composition is basically accurate, in a vacuum furnace with graphite felt insulation and graphite cloth as heating elements, the action of graphite and leaked oxygen will generate CO, which has Carburization. Therefore, when dealing with magnetic alloys with extremely low carbon content/(c)0.01%, the magnetic permeability can not get satisfactory results. Therefore, different results are obtained in vacuum furnaces heated by graphite elements and heated by tungsten wires, and the magnetic permeability can be doubled.

In addition, titanium and its alloys have a strong combination with hydrogen, oxygen, nitrogen and other gases at high temperatures. If heated in such an atmosphere, hydrogen will be absorbed to produce hydrogen embrittlement, and oxygen and nitrogen will be absorbed to produce hardening.

At the same time, the high-temperature metals molybdenum, tungsten, cobalt, button and their alloys have limited solubility to carbon, nitrogen, and hydrogen. In order to prevent embrittlement, the residual content of these elements is required to be minimized as much as possible. Therefore, when dealing with these materials, it is recommended not to use hydrogen furnaces and nitrogen-argon protective atmosphere furnaces, but to use vacuum furnaces to prevent embrittlement of materials. Vacuum heat treatment defects that appear in production, combined with scientific and reasonable analysis, the effects of oxidation, adhesion and different furnace bodies on magnetic conductivity, analysis of defect factors and changing rules, and solutions accumulated from long-term working experience are very good. According to the practical effect, it is concluded through experiments that vacuum furnaces with different heating materials have a certain influence on the magnetic permeability. The processing of different materials has different requirements for vacuum furnaces.