Why Heat Treatment Is Critical for Duplex Stainless Steels

Duplex stainless steels — with their mixed austenite-ferrite microstructure — offer superior strength, corrosion resistance, and stress corrosion cracking resistance compared to standard austenitic grades. However, achieving and maintaining this optimal dual-phase structure requires precise heat treatment control. Improper or absent heat treatment can lead to detrimental phase imbalances, precipitate formation, and significantly reduced performance. This guide covers complete heat treatment and solution annealing procedures for duplex stainless steels in 2026.

The Duplex Microstructure

The defining characteristic of duplex stainless steel is its approximately 50/50 mix of austenite and ferrite phases. This balance provides:

• From austenite: Toughness, ductility, and formability
• From ferrite: Higher strength, chloride stress corrosion cracking resistance
• Combined: Roughly 2x yield strength of standard austenitic grades, excellent pitting and crevice corrosion resistance

The phase balance is highly sensitive to thermal history. The target is 30-70% ferrite (with 40-60% being optimal for most applications).

Solution Annealing: The Essential Heat Treatment

Purpose

• Dissolve any chromium nitrides, carbides, or intermetallic phases that formed during hot working or welding
• Restore the optimal austenite-ferrite phase balance
• Maximize corrosion resistance and mechanical properties
• Relieve residual stresses from forming or welding

Temperature Ranges by Grade

Heating Rate

• For thin sections (<10mm): Can be charged into a preheated furnace at temperature
• For thick sections (>25mm): Heat at maximum 200°C/hour through 600°C range to minimize thermal gradients
• Above 600°C: Full heating rate acceptable

Cooling Requirements

Rapid cooling is essential to prevent formation of detrimental phases during the 900-300°C range:

• Water quenching: Preferred method for all duplex grades, especially super duplex
• Rapid air cooling: Acceptable for thin sections of standard duplex (S31803/S32205) only
• Cooling rate: Must pass through 900-300°C in less than 5 minutes for thick sections
• Never furnace cool: Slow cooling through the critical range causes sigma phase and chromium nitride precipitation

Detrimental Phase Formation

Sigma Phase (σ)

• Forms between 600-1000°C during slow cooling or prolonged holding
• Hard, brittle intermetallic phase rich in chromium and molybdenum
• Reduces toughness, ductility, and corrosion resistance dramatically
• Once formed, requires re-solution annealing to dissolve

Chi Phase (χ)

• Forms in Mo-containing grades between 700-950°C
• Similar effects to sigma phase
• Primarily a concern in super duplex grades (high Mo content)

Chromium Nitrides (Cr₂N)

• Form at grain boundaries between 700-1000°C
• Deplete adjacent areas of chromium, creating localized corrosion susceptibility
• Associated with inadequate solution annealing or slow cooling

475°C Embrittlement

• Long-term exposure (1000+ hours) in the 400-550°C range
• Ferrite phase undergoes spinodal decomposition
• Increases hardness, reduces toughness
• Irreversible without re-solution annealing

Heat Treatment After Welding

Is PWHT Required?

For duplex stainless steel welds, post-weld heat treatment (PWHT) is generally NOT required when:

• Proper welding procedures with controlled heat input are used
• Appropriate filler metals (over-alloyed) are selected
• The weld has been properly solution annealed (for fabricated items)

However, PWHT is required when:

• The component has been heavily cold worked before welding
• Code or specification specifically requires it
• Thick sections have been multipass welded with high heat input

PWHT Parameters for Welded Duplex

• Same temperature range as solution annealing
• For field welds: Local induction heating with trailing water quench
• Verify phase balance after PWHT (target: 30-70% ferrite in weld and HAZ)

Quality Verification After Heat Treatment

Phase Balance Testing

• Ferritoscope: Portable magnetic instrument, measures ferrite number (FN). Target: 30-70% ferrite
• Optical metallography: Laboratory method using etched cross-sections. Most accurate
• ASTM A923: Standard test methods for detecting detrimental intermetallic phases

Mechanical Testing

• Tensile testing per ASTM A370 or equivalent
• Impact testing (Charpy) — especially important after PWHT
• Hardness testing per ASTM E18 (HV or HRC)

Corrosion Testing

• ASTM A923 Method A: Detects detrimental intermetallic phases via impact testing comparison
• ASTM G48: Pitting and crevice corrosion in ferric chloride solution
• ASTM A262: Intergranular corrosion susceptibility

CoreMetal Steel: Duplex Stainless Steel Products

CoreMetal Steel supplies duplex and super duplex stainless steel in plate, sheet, pipe, tube, bar, and fittings. Grades 2205 (S31803/S32205), 2507 (S32750), and Zeron 100 (S32760). All products supplied in solution annealed condition with full certification and phase balance verification.

Contact: Tracy | tracy@coremetalsteel.com | +86 18291910632

Conclusion

Proper heat treatment is essential for achieving the performance advantages of duplex stainless steels. Solution annealing at correct temperatures with rapid cooling ensures optimal phase balance and corrosion resistance. Visit CoreMetal Steel Blog for more technical guides.