Back purging in a vacuum furnace with nitrogen is often done to remove any residual gases or contaminants that may have entered the furnace chamber during processing, especially after vacuum cycles or before opening the chamber to the atmosphere. In vacuum furnaces processing tool steel or high-speed steel, nitrogen back purging serves as a way to flush out reactive gases like oxygen that can cause oxidation, which is critical for ensuring a clean environment for subsequent loads.

However, when iron is present, especially at high temperatures, there’s a risk of iron reacting with nitrogen to form iron nitrides. Iron nitrides can alter the properties of steel, potentially affecting hardness, brittleness, or machinability. Here’s how back purging works in this context:

1. Cool-Down Phase: Nitrogen is often introduced after the furnace has cooled to a point where the risk of forming nitrides is minimized (usually below 400–500 °C). At lower temperatures, nitrogen is inert and will not react with iron.

2. Controlled Purge Cycles: Multiple cycles of nitrogen purging may be used to ensure all oxygen is removed from the chamber. Since nitrogen is relatively inexpensive, purging until reaching a safe oxygen level is feasible.

3. Inert Gas Usage Consideration: For higher-temperature applications or highly sensitive materials, an inert gas like argon is sometimes preferred, as it does not react with iron even at high temperatures, though it is more costly than nitrogen.

In your case, where tool steels and high-speed steels are processed, nitrogen is typically safe for back purging as long as the temperature and timing are managed properly to avoid nitride formation.