What is backfill gas?

Backfill gas in vacuum heat treating refers to the process of introducing a gas into the vacuum chamber during the cooling phase of heat treatment. The purpose of backfill gas is to prevent the formation of unwanted oxides or other surface contaminants on the treated material as it cools down. Vacuum furnaces can use a variety of different gases during the processing cycle in partial pressure operation, for backfilling to atmospheric pressure at the end of the processing cycle and for cooling/quenching. The most common of these gases (in order of frequency of use) are nitrogen, argon, hydrogen, and helium. Other common gases include various hydrocarbons and ammonia (for vacuum carburizing/carbonitriding) and specialty gases such as neon (for certain electronics applications).

During the heat treatment process, the material is heated to high temperatures in a vacuum environment to achieve specific metallurgical properties such as improved hardness, strength, or resistance to corrosion. After the desired temperature is reached and the material is held at that temperature for a certain period, it needs to be cooled down. Overall, the use of backfill gas in vacuum heat-treating helps maintain the integrity and quality of the treated material by preventing oxidation and surface contamination during the cooling phase.

Source: https://vacaero.com/information-resources/vac-aero-training/6488-types-backfill-partial-pressure-cooling-gases-vacuum-heat-treatment.html