Fasteners can be heat treated using vacuum technology, which is environmentally friendly and offers the advantages of high controllability. Since vacuum furnace quenching utilizes a gas (nitrogen or helium for example), post-washing is not required like it is with other technologies that quench in oil or other liquid. Vacuum processing is growing more rapidly than any other technology, and a gradual shift is occurring from atmosphere to vacuum processing.

The last decade has seeing double-digit growth and an increased vacuum market share (Fig. 1) throughout the Americas as end-users require higher quality and repeatability of part performance due to increasingly sophisticated and demanding fastening applications, led by the aerospace and automotive industries.

Continuous (Fig. 2) and batch (Fig. 3) vacuum furnace designs are common throughout the industry. In the automotive industry, for example, high-volume throughput requires automated systems integrated into the manufacturing flow. In the nuclear or medical industry, meanwhile, smaller discrete loads are preferred. In either manufacturing scenario, vacuum equipment is quite capable of meeting the productivity demand.

Solution annealing and aging of aerospace fasteners is an example of the use of batch vacuum technology. For example, 17Cr4Ni (Carpenter Custom 630) stainless steel fasteners are loaded into baskets (Fig. 4), heated to 1040°C (1900°F) at a vacuum of 10^-4 Torr, held at temperature for 30 minutes, then quenched in 32°C (90°F) nitrogen. This is followed by age hardening at a temperature of 480°C (900°F) and a 10^-5 Torr vacuum for 65 minutes, then cooling to 52°C (125°F). This process provides bright, shiny parts having a hardness of 42-44 HRC.

References

1. Herring, Daniel H., Atmosphere Heat Treatment, Volume 1, BNP Media, 2014

2. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012