Aerospace material vacuum heat treatment furnace

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Compared with large-scale production in other manufacturing fields, aircraft parts materials are characterized by a wide variety of grades and small batches, resulting in complex and diverse vacuum heat treatment processes and equipment, and difficulties in batching and testing.

Aviation materials are generally expensive and have a long production cycle. Controlling the pass rate of vacuum heat treatment of parts has become an important factor affecting the manufacturing cost and cycle. Therefore, the quality inspection and control of vacuum heat treatment is extremely critical. Application Status of Vacuum Heat Treatment in Aviation Plant

1 Vacuum heat treatment of steel parts

With the substantial increase in the amount of titanium alloys and composite materials used in advanced aircraft, the proportion of steel in aircraft structures has decreased. Due to the advantages of high strength and high modulus, steel still has important uses in parts with limited structural space and high local rigidity requirements, such as the main struts of aircraft landing gear and connecting bolts in important parts. In the aviation factory, since the purchasing state of steel materials is generally in the state of preparatory vacuum heat treatment, final vacuum heat treatment is required after rough machining or finishing, or vacuum heat treatment such as hydrogen removal and stress relief between processes is carried out. Therefore, steel is still the main engine of aviation. The material with the most types of vacuum heat treatment and the largest vacuum heat treatment workload in the factory.

At present, the steels for vacuum heat treatment in aviation OEMs mainly involve high-quality carbon structural steels (such as 45A), low-alloy structural steels (such as 30CrMnSiA), low-alloy ultra-high-strength structural steels (such as 30CrMnSiNi2A, 40CrNi2Si2MoVA), and high-alloy ultra-high-strength structural steels. (such as 23Co14Ni12Cr3MoE), high-strength or ultra-high-strength stainless steel (such as 0Cr16Ni6, 0Cr13Ni8Mo2Al), etc. The types of vacuum heat treatment processes involved include vacuum quenching (or solid solution), vacuum tempering (or aging), vacuum normalizing, vacuum annealing, cold treatment, stress relief, etc. Vacuum heat treatment of steel parts generally controls the mechanical properties (tensile strength, impact, fracture toughness, etc.) and metallurgical properties (decarburization, intergranular oxidation, etc.) of the material.

Vacuum heat treatment equipment for steel parts generally includes vacuum furnace, air furnace, protective atmosphere furnace and salt bath furnace. With the development of vacuum heat treatment equipment, the quenching of parts is gradually dominated by vacuum furnaces to meet the heat treatment requirements of most parts with finished surfaces, and the highly polluting salt bath furnaces are gradually eliminated. Compared with the heat treatment technology of salt bath furnace and air furnace, the oxidation (decarburization) of parts heat treated in vacuum furnace is less harmful, the degree of heat treatment deformation is low, and the comprehensive performance, especially the fatigue performance, is better.

It is worth noting that the selection of the heat treatment system for steel parts should not only ensure the required strength or hardness of the parts, but also pay attention to avoid temper brittleness, and high-strength steel parts need to add hydrogen removal treatment after surface treatment. Failures caused by temper embrittlement, hydrogen embrittlement, etc. also occur from time to time in use.

2 Vacuum heat treatment of titanium alloy parts

At present, most of the titanium alloy structural parts for aircraft are used in the annealed state and solution aging state. For example, the Ti6-AI4-V structural parts widely used in aircraft are almost used in the annealed state. The aviation factory can directly purchase the final vacuum heat treatment blank for machining, and generally no longer do annealing heat treatment for the purpose of improving performance.

However, considering the deformation caused by stress release during the processing of titanium alloy parts, aviation factories often need to carry out residual stress relief annealing during or after processing titanium alloy parts. The residual stress relief annealing of titanium alloy parts mainly controls three aspects:

(1) Control the internal stress. Select the correct vacuum heat treatment parameters to maximize the control and balance of the internal stress of titanium alloy parts.

(2) Control deformation. Titanium alloy parts are subjected to forging, vacuum heat treatment, machining, welding and other processes to generate and superimpose stress, and it is easy to cause warpage and deformation of the parts during the final residual stress annealing process, especially large long beam parts. In order to control the deformation of such titanium alloy parts, it is necessary to customize special fixtures to constrain the easy deformation direction of the parts, vacuum heat treatment together, and sometimes other auxiliary means, such as controlling the residual stress level of raw materials, arranging reserved machining allowances, Increase the number of stress relief or use methods such as “overcorrection and overcorrection”.

(3) Control the surface quality. After the TC4-DT titanium alloy test piece was heated in an air furnace at different temperatures for 1 h, with the increase of the heating temperature, the surface color continued to deepen, from silvery white to light yellow, dark yellow, blue to tan. In order to ensure the surface quality of the parts, vacuum stress relief heat treatment is generally used. Vacuum stress relieved titanium alloy finishing parts require that the surface is not allowed to appear more than light yellow color. Stress relief annealing of titanium alloy parts is generally heat treated in a vacuum furnace. With the large-scale application of large-scale integrated titanium alloy structural parts, the requirements for equipment size are gradually increasing. The size of the equipment currently used has exceeded 1500mm (diameter) x 5000m (length).

3 Vacuum heat treatment of aluminum alloy parts

The aluminum alloy parts for vacuum heat treatment in aviation factories are mainly sheet metal, such as LY12, 2024 and other parts mainly used for skin or sheet metal frame. The remaining large-sized aluminum alloy machined parts are almost all supplied in the final state and processed directly, such as 7050-T7451 pre-stretched thick plates, 2124-T851 pre-stretched thick plates, etc. Affected by the hardenability of aluminum alloys, there are a small number of parts with extra large cross-sections, which require solution aging treatment after rough machining in aviation factories, but there are not many such parts, because aluminum alloys are difficult to resemble titanium after machining. The residual stress of parts is eliminated by stress relief annealing or tempering like alloys or steels. This is because aluminum alloys are sensitive to low temperature (100~200C) long-term heat preservation, which will change the precipitation state of its strengthening phase and affect mechanical properties and corrosion. , so this situation should be minimized in design and manufacturing.

At present, the vacuum heat treatment process types of aluminum alloys in aviation factories mainly include solid solution, vacuum annealing and aging. The equipment is mainly used for vacuum heat treatment in air furnace and nitrate furnace. Generally, it is necessary to test the hardness and electrical conductivity after vacuum heat treatment to detect the state of vacuum heat treatment. The mechanical properties (tensile, etc.) and metallurgical properties (intergranular corrosion susceptibility, etc.) of aluminum alloys are controlled by vacuum heat treatment. However, some problems cannot be solved by vacuum heat treatment alone. For example, in sheet metal forming, the aluminum alloy parts are partially deformed, and the energy accumulated by the deformation can provide grain growth under the normal solution or complete annealing process. The energy causes the local grain size of the part to grow abnormally, which will appear as grain size or surface in the follow-up inspection

Unqualified quality, such problems need to be solved in combination with forming process or design optimization.

Aluminum alloy vacuum heat treatment is gradually replacing nitrate furnace with air circulation furnace to solve the problems of environmental pollution and quenching medium pollution of nitrate furnace.

Equipment selection: The RVGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum tempering furnace, vacuum annealing furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.