The heat treatment objectives of duplex stainless steel differ from those of austenitic stainless steel. It must not only ensure corrosion resistance but also precisely control the ratio of ferrite to austenite (typically targeted at 50/50), while guaranteeing high toughness and high strength of the material. The following are the most commonly used heat treatment methods for duplex stainless steel:
1. Solution Annealing
This is the most critical and primary heat treatment method for duplex stainless steel. Almost all duplex stainless steel pipe fittings, flanges and plates must undergo this process. Process:
- Heat the steel to a high temperature range of 1020°C to 1100°C.
- Hold for sufficient time to fully austenitize the microstructure and homogenize the chemical composition.
- Cool rapidly (usually water quenching) to suppress the precipitation of harmful phases.
Purposes:
- Achieve optimum phase ratio: through rapid cooling, austenite precipitates again in the ferrite matrix, finally obtaining a balanced microstructure of approximately 50% ferrite + 50% austenite.
- Dissolve harmful phases: prevent precipitation of carbides (such as Cr₂₃C₆) or intermetallic phases (such as σ phase, χ phase).
- Restore corrosion resistance and toughness: eliminate work hardening, and restore pitting corrosion resistance and stress corrosion resistance of the material.
Applications:
- Final heat treatment for pipe fittings, flanges and plates of all duplex stainless steels (such as 2205, 2507).
- Overall heat treatment after welding (if conditions permit).
2. Stress Relieving
Welding or cold working of duplex stainless steel produces residual stresses, which may lead to stress corrosion cracking (SCC). Process:
- Heating temperature is usually controlled between 200°C and 300°C.
- Note: Prolonged holding of duplex stainless steel in the temperature range of 400°C to 900°C is strictly prohibited, as this will cause rapid precipitation of brittle σ phase, resulting in material embrittlement and complete loss of corrosion resistance.
Purposes:Relieve residual internal stresses caused by welding or cold working, and reduce the risk of stress corrosion cracking. Applications:
- Large welded structural parts.
- Pipe fittings after cold forming (such as cold bending elbows).
3. Post Weld Heat Treatment (PWHT)
The welding heat treatment of duplex stainless steel is very special, and is generally divided into two situations:
1. No Heat Treatment (Most Common)
For most welded duplex stainless steel components (especially 2205), post-weld heat treatment is usually not recommended.Reason: the welding thermal cycle has already caused grain coarsening and phase ratio imbalance (increased ferrite) in the heat-affected zone (HAZ). Further high-temperature tempering will easily fall into the dangerous range of 400–900°C, leading to σ phase precipitation and embrittlement of the joint.Countermeasure: rely on suitable welding consumables (high-nickel filler metals) to compensate for the austenite content in the weld.
2. Solution Annealing (Only for Specific Cases)
If the weldment is very thick or requires extremely high corrosion resistance, post-weld solution annealing may be performed.Process: reheat the entire weldment to 1020–1100°C and water quench.Disadvantage: high cost, and difficult to implement for large structural parts.
4. Heat Treatment After Hot Forming
During hot forming of duplex stainless steel (such as hot-pushed elbows, hot-pressed tees), temperature and cooling rate must be strictly controlled. Process Requirements:
- Forming temperature shall be above 1000°C.
- Rapid cooling (such as water quenching) must be performed immediately after forming.
Purposes:Ensure that austenite precipitates from high-temperature ferrite during cooling to restore the duplex microstructure. Too slow cooling will form a fully ferritic structure, resulting in a significant decrease in toughness and corrosion resistance.
Summary: Key Principles of Duplex Stainless Steel Heat Treatment
- Rapid cooling is mandatory: both solution annealing and hot forming require rapid cooling to maintain duplex microstructure and corrosion resistance.
- Avoid dangerous temperature zone: never allow duplex stainless steel to stay at 400°C to 900°C for a long time — this is the “critical temperature danger zone”.
- Phase ratio is the core: the ultimate goal of all heat treatment is to obtain a 50/50 ferrite/austenite ratio, which provides high strength and high corrosion resistance of duplex steel.
Key Note: On the Material Test Certificate (MTC) of duplex stainless steel, the heat treatment condition is usually marked as “Solution Annealed”, and the cooling method must be specified as “Water Quenched”.
