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Vacuum Heat Treatment of Aviation Parts Made of High Strength Steel

In the field of aviation, especially in aircraft manufacturing, a large number of ultra-high strength steel structures are needed. Due to the requirements of aviation for safety and service life, such parts often have very high process requirements, and they need to cooperate with advanced heat treatment technology to meet the required process requirements. Among them, vacuum heat treatment is the most widely used, most effective and cost-effective heat treatment method.

Vacuum quenching can completely avoid the harm of “hydrogen embrittlement” during heating in methanol cracking atmosphere furnace and the phenomenon of oxidative decarburization during heating in ordinary electric furnace. Compared with the current process, it can save a long time of low-temperature tempering dehydrogenation process, and also save some grinding, correction, sandblasting man hours, with significant economic benefits. Good process reproducibility and stable product quality.

In terms of mechanical properties, the mechanical properties after ordinary air electric furnace quenching and vacuum furnace quenching are compared. When the plasticity index is equal, the tensile strength and yield strength of vacuum furnace quenching are improved, and the impact toughness is improved more obviously, which fully shows the advantages of vacuum quenching.

Take the large shaft part of the flat tail as an example, which is the horizontal stabilizer shaft of the aircraft. As a key force bearing component, it mainly bears the bending and torsional stresses transmitted from the flat tail, as well as the alternating cyclic loads such as tension and compression. Because it is close to the engine and has a certain temperature and gas effect, it requires that the material used to make the large shaft with flat tail should not only have high strength and rigidity, but also have good comprehensive properties such as stress corrosion resistance. These harsh use conditions determine that it must have excellent mechanical properties and manufacturing accuracy. Through vacuum quenching, cold treatment, tempering and other processes, the mechanical properties of the product and the hardness of the designated position on the shaft fully meet the technical requirements. The surface is bright without decarburization, carbon increase, nitrogen increase and grain boundary oxidation. Basically no deformation, no correction, greatly reducing machining allowance.

Technological process:

(1) Clean and degrease

The water-based detergent was used as the degreasing detergent before and after vacuum quenching, and a special cleaning tank was prepared. After the workpiece is clamped, it is lifted into the tank with a crane for soaking and washing. If necessary, compressed air is introduced for mixing. The bath temperature is 40-60 ℃, and the time is 5-10min. It can also be manually scrubbed at room temperature, and then dried and charged.

(2) Hanging

Vacuum furnace heating mainly depends on radiation, and a certain gap must be kept between workpieces to avoid shielding; Because the whole operation process is completed by mechanical automatic operation in a closed environment, it is required to keep the balance and reliability when mounting and hanging the workpiece. It is better to use a special quenching fixture and hang it on the automatic lifting appliance of the vacuum furnace in a free state.

(3) Vacuum quenching

When the heating chamber of the vacuum furnace reaches the vacuum working pressure, the workpiece shall be put into it. On the premise of maintaining the working vacuum, preheating, heating and insulation shall be carried out according to the requirements of the process curve. The holding time is the heating lag time+calculated according to the “shortest quenching holding time” specified by the section size of the workpiece. Before the end of insulation, the oil tank shall be vacuumized to less than 6.7Pa about 20min in advance, heated to 40~60 ℃, stirred and fully degassed. After the workpiece is quenched into the oil, fill it with nitrogen and control the oil level pressure at 4 × 10*～8 × Within 10 * Pa. After the workpiece is thoroughly cooled (~20min), put it into the atmosphere and take it out for cleaning and degreasing.

(4) Cold treatment

After quenching to room temperature, cold treatment shall be carried out within 1h. First, lay 200~300mm thick solid CO2 in the cold treatment tank, pour in an appropriate amount of alcohol, then put in the workpiece, and then add solid CO2 and alcohol until immersion. Cover it and keep it under – 65 ℃ for 1h, and then take it out and naturally warm it in the air. The residual refrigerant in the cold treatment tank can be covered for heat preservation, and only a small amount of solid CO2 and alcohol can be added into the next heat to meet the requirements below – 65 ℃.

(5) Tempering

After the workpiece temperature rises to room temperature, it shall be charged and tempered within 4h. The vacuum tempering furnace is used for tempering in the protective atmosphere. Hang it after discharging, and use blower for cooling.

(6) Correction: the deformation is very small, so no correction is required.

(7) Inspection

Selection of vacuum heat treatment equipment: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for vacuum heat treatment of aviation high-strength parts. Good temperature control accuracy and uniformity, and excellent vacuum system ensure the effective vacuum heat treatment process. SIMUWU focuses on the manufacturing 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 gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc, which are widely sold in developed and developing countries.