For bright annealing of 304 stainless steel (StSt), partial pressure control of hydrogen is often the more practical approach. However, it depends on the specific setup of your furnace and process requirements.

Here’s why:

1. Hydrogen as a reducing agent: Hydrogen is essential in the bright annealing process because it acts as a reducing atmosphere, preventing oxidation and helping to achieve the desired bright, scale-free surface. However, a full backfill of hydrogen is not always necessary.

    

2. Partial pressure control: Typically, annealing can be performed with hydrogen at partial pressures. This allows for lower hydrogen consumption while still providing sufficient coverage to prevent oxidation. By maintaining a controlled hydrogen pressure (often with nitrogen as a diluent), you can ensure enough reducing atmosphere without running at full hydrogen pressure.

    

3. Full backfill considerations: If you opt for a full backfill of hydrogen (i.e., using 100% hydrogen at atmospheric pressure), you will need to take extra precautions regarding safety, as hydrogen is flammable and poses explosion risks in enclosed spaces. This approach may also be more costly in terms of hydrogen usage.

    

4. Process control: Using partial pressure control gives more flexibility in controlling the atmosphere inside the furnace. Many bright annealing furnaces are designed with the capability to control the hydrogen concentration at a lower level (e.g., 50%-75%), balancing cost, safety, and process performance.

    

5. Desired surface finish: If achieving the highest possible surface brightness is the goal, a higher hydrogen concentration may be beneficial. In this case, you might increase the hydrogen level but still keep it under full backfill pressure.

In conclusion, for most applications, partial pressure control with hydrogen is recommended, as it provides a balance between cost, safety, and the required surface finish. However, if your process has specific requirements for ultra-high brightness, you could consider increasing the hydrogen concentration, though not necessarily a full backfill.