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Learn about the benefits of vacuum annealing

The self-locking nut of the pallet is a fastener product with complex manufacturing process, high technical difficulty, and high added value. It is mainly used for the connection of sheet metal parts and stamping parts on aircraft, and is widely used in the aerospace field. Product types include single-lug, double-lug, angular, swimming and airtight pallet nuts and other models.
As the aerospace industry’s requirements for high strength, high self-locking and high corrosion resistance pallet nuts are getting higher and higher, pallet nuts manufactured and processed by traditional structural steel materials and processes can no longer meet the requirements. Research shows that the pallet self-locking nut made of martensitic stainless steel 1Cr17Ni2 material can well solve the shortcomings of the original pallet nut’s low performance and meet the harsh working conditions.

After repeated process tests and product trial production, the process flow of the self-locking nut of the 1Cr17Ni2 material pallet is as follows:
Preparing materials → Cold heading → Flattening → Cleaning and drying → Annealing between processes → Secondary flattening → Cleaning and drying → Annealing between processes → Trimming → Punching → Mechanical deburring → Drilling → Hole expansion → Internal chamfering → Tapping Thread → Closing → Cleaning and drying → Final heat treatment → Mechanical surface polishing treatment → Passivation treatment → Inspection → Finished product packaging and storage.
After the flattening process, the 1Cr17Ni2 pallet nut undergoes strong cold work hardening due to the large deformation, making the hardness after flattening reach 520~560HV, which is about 220HV higher than the hardness of the ball after cold heading, which is 300~336HV. Therefore, it is necessary to perform inter-process annealing on the flattened billet to restore and recrystallize the billet, reduce the hardness and brittleness of the material, and restore the plasticity and toughness of the material to facilitate subsequent processing.
The annealing process varies depending on the nut shape and size. Generally speaking, small-sized single-lug and double-lug support plate nuts only need to be annealed twice. For angular and floating support plate nuts, as well as large-sized single-lug and double-lug support plate nuts, three annealings are required to meet the process requirements.

Vacuum annealing process
The annealing equipment uses a vacuum annealing furnace with a maximum operating temperature of 900°C. Clean, remove oil, and dry the 1Cr17Ni2 nut blank, put it into a heat-resistant steel barrel and put it into the furnace, and use the following annealing process.
(1) Small size pallet nuts (M3~M5)
Preheat at 650°C for 30 minutes, use a vacuum annealing process close to the recrystallization temperature, hold at 740°C for 2 hours, and then cool to 495°C and be released. After vacuum annealing, the hardness is relatively uniform, reaching 26~27HRC. The slightly higher hardness can avoid bending deformation of small-sized nuts with thin pallet thickness during cutting and blanking.
(2) Large size pallet nuts (M6~M10)
Preheat at 650°C for 30 minutes, adopt subcritical inter-process annealing, keep at 680-700°C for 2 hours, then cool to 495°C and come out of the furnace, with a hardness of 23-25HRC. The slightly lower hardness is beneficial to the subsequent drilling and tapping processes of large-sized nuts.