Vacuum Heat Treatment of Metal Bellows

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Metal corrugated pipe is an axisymmetric pipe shell part with elasticity, thin wall and transverse ripple. It requires both elastic properties and pressure resistance, sealing, corrosion resistance, temperature resistance and other properties. It can produce corresponding displacement under the action of pressure, axial force, transverse force or bending moment. Metal corrugated pipe is a functional part. The manufacturing and processing of special metal corrugated pipe involves many processes and technologies such as corrugated pipe design, plastic processing of materials, welding, heat treatment, surface treatment, cutting and performance testing.

Heat treatment is one of the key processes of bellows manufacturing. High quality heat treatment requires not only high surface quality, but also certain formability and comprehensive mechanical properties.

Material specification of corrugated pipe:

GH416 alloy (American grade Inconel 718) is a nickel based deformable superalloy, which has high strength, high elasticity, good corrosion resistance and good processing properties such as forming, welding and cutting. In addition, it has a wide working temperature range (- 253~600 ℃) and is an ideal material for high-pressure metal bellows. Wall thickness of thin-walled pipe 0.15 ～ 0.23mm, diameter φ 18～ φ 225mm。

Selection of heat treatment equipment:

The heat treatment of GH416 alloy thin-walled tubes adopts RVGQ-669-2 double chamber horizontal internal heated vacuum gas quenching furnace produced by SIMUWU. The heating chamber is separated from the cooling chamber by a vacuum gate valve. When the workpiece is cooled in the cooling chamber, the heating chamber will not be affected. The technical indexes of RVGQ-669-2 vacuum gas quenching furnace are as follows: effective heating zone size (width × long × Height) 600mm × 600mm × 900mm, rated charging capacity 200kg, maximum temperature 1300 ℃, furnace temperature uniformity ± 5 ℃, limit vacuum degree of heating chamber 6.6 × 10-3Pa, empty furnace temperature rise time ≤ 30min (empty furnace temperature rises to 1150 ℃ from room temperature), air cooling time ≤ 30min (workpiece decreases from 1150 ℃ to 150 ℃).

The purpose of solution treatment of GH416 alloy thin-walled tubes is to eliminate work hardening, restore the plasticity of materials, and prepare for the forming of corrugated pipes. The effect of solution treatment affects the forming, surface and internal quality of corrugated pipe, so it is the key technology to develop GH416 alloy corrugated pipe.

The factors that affect the quality of vacuum heat treatment of GH416 alloy include pretreatment, heating temperature and vacuum degree, heating time and holding time, charging method, furnace limit vacuum degree, pressure rise rate and furnace temperature uniformity, cooling medium and cooling method.

① Pretreatment: GH416 alloy material has many alloy components and is relatively easy to oxidize, while the corrugated pipe of GH416 alloy material is not allowed to oxidize. In order to achieve bright vacuum solution treatment, the material needs to be strictly cleaned and dried with degreaser and ultrasonic cleaning method before approaching the treatment.

② Process parameters: heating temperature, holding time and vacuum degree are the most important process parameters of vacuum heat treatment. Heating temperature has a decisive impact on the grain size and properties of materials, while holding time has a slight impact. In the heat preservation stage of the heating process, the purpose is to heat the parts evenly, improve the vacuum degree of the heating chamber and the recovery and recrystallization of the materials. The vacuum degree at each stage is a necessary condition to ensure that the material is not oxidized.

③ Charging method: because thin-walled pipes and corrugated pipes are thin-walled parts, thin-walled parts are easy to distort due to the reduction of material strength and their own weight during heating; In the process of positive pressure and high flow rate cooling gas cooling, thin-walled parts are prone to distortion due to the impact of pressure gas in three directions. The influence of the above two aspects shall be comprehensively considered when selecting the charging mode to minimize the distortion. For the solution heat treatment of multi-layer GH416 alloy thin-walled tubes, we use multi-layer tubes to be sleeved and then heat treated together, which can increase the stiffness of single-layer thin-walled tubes and reduce distortion on the one hand, and avoid serious scratches when single-layer thin-walled tubes are sleeved in the solution state on the other hand. Such scratches will significantly reduce the service life of the bellows.

④ The limiting vacuum degree, pressure rise rate and furnace temperature uniformity of the furnace are the main technical indicators of the vacuum heat treatment furnace. It determines the quality of solution heat treatment. The RVGQ series vacuum furnace produced by SIMUWU is the ideal equipment for this process.

⑤ At present, only gas cooling can be used to make the vacuum heat treatment of GH416 alloy thin-walled tubes meet the requirements of bright surface. In order to prevent nitrogen from nitriding GH416 alloy, we use pure argon (99.99%) as the cooling gas. The gas pressure (relative pressure) is 0.08MPa, and the gas manufacturer should also choose.

The above five factors were analyzed, and the vacuum heat treatment process was optimized according to the requirements of surface brightness, no grain boundary display, grain size grade and solid solution degree.

The diagram shows the process curve of solution treatment:

GH416 alloy material is an aging strengthening alloy. Before aging heat treatment, solution heat treatment is carried out first. Its purpose is to dissolve carbides into the matrix as much as possible to obtain single-phase structure, so as to prepare the structure for precipitation of uniform and fine strengthening phase in the future aging precipitation. Another purpose is to obtain uniform and appropriate grain size. Generally, the grain size of the alloy cannot be changed during aging, so the grain size of the alloy after solution will have a great impact on the properties of the alloy. The aging heat treatment of GH416 alloy bellows aims to precipitate a certain amount and size of strengthening phase in the alloy matrix to achieve the maximum strengthening effect of the alloy. A prominent feature of GH416 alloy material is that the reaction to age hardening is very slow, requiring a long time of aging heat treatment. At the same time, in order to adjust the size of the strengthening phase to obtain the best combination of strength and plasticity, aging treatment is carried out in two stages.

Because GH416 alloy is made of high temperature alloy and has high strength at 720 ℃, and GH416 alloy bellows also has certain strength at 1000 ℃, special clamps are not required during the solution and aging heat treatment of GH416 alloy bellows, but only the charging method of the bellows needs to be noted. The figure shows the aging heat treatment process of GH416 alloy bellows：

Both solution heat treatment and post weld heat treatment of thin-walled tubes meet the requirements of bellows forming, with the forming rate of more than 92%. The “solution” and “stage aging” heat treatment achieve the effect of bellows strengthening. The finished product has good formability, high strength and long service life. Vacuum heat treatment can effectively reduce the labor cost, improve the overall quality and increase the production efficiency. It is an excellent process for treating similar alloy parts.

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