Solution annealing restores the corrosion resistance of austenitic stainless that has been forged or welded: heat it to 1,050-1,100 °C, hold one hour per 25 mm of thickness so chromium carbides dissolve into the steel, then quench rapidly in water or forced air so they cannot re-form on the way down. It needs a furnace that holds that temperature evenly, and it applies to austenitic grades, not to ferritic or martensitic ones.
Part of Stainless Steel Production
The heat treatment that dissolves chromium carbides in austenitic stainless after hot work or welding, and the quench that keeps them dissolved.
Why The Quench Is Part Of The Treatment
After forging or welding, chromium carbides may have formed at grain boundaries as the metal cooled slowly through the carbide-forming range (the sensitization of Carbon Control). The carrier’s cure is to heat to 1,050-1,100 °C so the carbides dissolve back into the steel, then cool quickly so they do not come out again. The soak and the quench are one operation: a soak followed by slow cooling puts the carbides back.
The carrier’s forging text says not to quench austenitic stainless, which contradicts this quench from 1,050-1,100 °C. As I read it, the forging line means a quench will not harden the steel as it does carbon steel (my recollection is that austenitic steel does not harden by quenching); it does not forbid the corrosion-restoring quench. See Austenitic (300 Series) for the same point.
What You Need
- A furnace that holds 1,050-1,100 °C evenly and a way to check it (Pyrometric Cones).
- A water quench tank or a forced-air blast. Quench Tank and Water Access covers sizing a quench tank.
- Long tongs and a clear path from furnace to quench, short enough that the part does not cool in the air first.
- Pickling and passivation equipment for afterwards.
Method: One Anneal
Step 1: Clean. Remove oil, grease and anything else that can carburise or stain at heat.
Step 2: Heat to 1,050-1,100 °C and wait until the part is uniformly hot through its thickness.
Step 3: Hold. One hour per 25 mm of thickness, per the carrier.
Step 4: Quench quickly. Move the part from furnace to water or air blast with no delay; thin sections can distort in water, so use forced air for them.
Step 5: Descale and passivate (Passivation Treatment).
Worked Example
Hold time at the carrier’s rate of 1 hour per 25 mm:
- 5 mm plate: 5 / 25 = 0.2 hours, about 12 minutes.
- 12 mm plate: 12 / 25 = 0.48 hours, about 29 minutes.
- 50 mm bar: 50 / 25 = 2 hours.
For very thin parts this rule gives a hold shorter than the time the furnace needs to bring the core to temperature; time that heat-up first, then start the hold. The carrier does not give a minimum, and I cannot source one.
Common Mistakes
- Slow cooling after the soak. The carbides re-form and the work was wasted.
- Under-heating. Below the carrier’s 1,050 °C the carbides do not fully dissolve; check the furnace.
- Treating a ferritic or martensitic part this way. The carrier’s text applies to austenitic stainless.
- Quenching a hot assembly into cold water. Thin or unevenly heavy parts can warp or crack.
Safety
Furnace and quench
The furnace is at 1,050-1,100 °C and the part is white to yellow hot when pulled. Quenching in water produces a burst of steam; stand back and keep the quench tank clear of oil, as a tank with oil floating on it can ignite. Hot scale flakes off during the quench. Wear eye protection, a face shield, leather gloves and apron.
What’s Next
- Learn next: Passivation Treatment - rebuilding the surface film after heat
- Combine with: Welding Stainless - the usual reason for annealing