In vacuum heat treating, backfilling typically uses inert or non-reactive gases to protect the material, control cooling rates, and maintain the integrity of the treated surface. The most commonly used gases are:

1. Nitrogen (N₂):

• Most Common Backfill Gas: Nitrogen is readily available and relatively inexpensive, making it the most widely used backfill gas for vacuum heat treating.
• Suitable for Many Metals: It works well for steels, especially tool steels, high-speed steels, and stainless steels, where nitrogen reactivity is not an issue.
• Moderate Cooling Rates: Nitrogen provides moderate cooling rates, which are adequate for many vacuum heat-treating processes.

2. Argon (Ar):

• True Inert Gas: Argon is non-reactive with virtually all metals and alloys, making it ideal for materials sensitive to nitrogen, such as certain stainless steels, titanium alloys, and nickel-based superalloys.
• Good Cooling Properties: Argon has a similar cooling rate to nitrogen, though it is more costly. It is often used when absolute material cleanliness and non-reactivity are required.

3. Helium (He):

• Highest Thermal Conductivity: Helium has the best heat transfer characteristics among inert gases, providing the fastest cooling rate, which is valuable in processes requiring rapid quenching.
• Expensive: Because of its high cost, helium is typically reserved for high-value or specialized parts, such as high-speed steels, aerospace alloys, or critical components where fast cooling is essential to achieve desired mechanical properties.

4. Hydrogen (H₂) (used sparingly and with caution):

• Reducing Atmosphere: Hydrogen is occasionally used in controlled quantities as a backfill gas for materials that benefit from a reduction in oxide layers, as it can prevent oxidation and remove residual oxides.
• High Cooling Rate but High Risk: Hydrogen cools faster than nitrogen and argon but is flammable and requires strict control measures due to its reactivity, making it more common in specialized applications like sintering and brazing than in general heat treating.

5. Gas Mixtures (Helium-Nitrogen or Argon-Helium Blends):

• Custom Cooling Rates: By using a mixture of helium with nitrogen or argon, operators can tailor the cooling characteristics and manage costs while still achieving desired processing conditions.
• Cost Efficiency: Blends allow for faster cooling than nitrogen alone without the high expense of pure helium.

The selection of backfill gas in vacuum heat treating depends on factors like the material’s reactivity, the desired cooling rate, cost considerations, and the specific metallurgical properties required by the application.