Post-Weld Heat Treatment Effects on Microstructure and Properties of SMAW Weld Overlay Clad Layers on Duplex Stainless Steel
1. Definition and Technical Background
Post-Weld Heat Treatment (PWHT) is a controlled thermal cycle applied after the completion of a weld overlay or cladding operation to relieve residual stresses, promote microstructural homogenization, and optimize the mechanical and corrosion-resistance properties of the deposited metal. When applied to weld overlay deposits on duplex stainless steel (DSS) substrates — typically grades such as UNS S31803 (2205), UNS S32750 (2507), or UNS S32205 — PWHT becomes a critical process variable that directly governs the phase balance, hardness, pitting resistance, and long-term service life of the clad component.
Shielded Metal Arc Welding (SMAW) overlay cladding on duplex stainless steel introduces unique metallurgical challenges. Unlike austenitic stainless steels, duplex grades derive their strength and corrosion resistance from a balanced two-phase microstructure consisting of approximately 40–60% ferrite and 40–60% austenite. During welding, the rapid heating and cooling cycles inherent to SMAW can cause phase transformation imbalances, including excessive ferrite formation, sigma phase precipitation, and chromium nitride (Cr₂₃C₆) or chromium oxide (Cr₂O₃) segregation at grain boundaries. PWHT serves as a corrective and stabilizing intervention to address these issues.
2. Metallurgical Principles of PWHT on Duplex Steel Overlay Deposits
2.1 Phase Equilibrium and Thermal Stability
Duplex stainless steels are designed to maintain a ferrite-austenite (α+γ) phase balance within a specific temperature window. The equilibrium phase diagram for Cr-Ni-Mo-N-Fe alloys shows that the two-phase field narrows significantly above approximately 1100°C and below approximately 550°C. During PWHT of SMAW overlay deposits on DSS substrates, the objective is to:
- Solutionize unwanted intermetallic phases: Sigma (σ) phase (Cr₂₅Co₇), chi (χ) phase (Fe₂₃Cr₆), and Laves phase (Fe₂Cr) can precipitate during slow cooling or prolonged exposure to the 600–1000°C range. These phases are Cr-rich and Mo-depleted, creating localized regions of severely reduced pitting and crevice corrosion resistance.
- Rebalance the α/γ ratio: If the weld metal has deviated from the target ferrite content due to thermal cycling during multi-pass overlay welding, a solution heat treatment in the range of 1020–1150°C followed by rapid quenching can restore the intended phase balance.
- Relieve residual stresses: SMAW overlay welding generates high residual stresses (typically 200–400 MPa) due to differential thermal expansion between the deposit and the substrate. Stress relief annealing at 600–700°C can reduce these stresses, though this temperature range requires careful control to avoid sigma phase formation.
2.2 Impact of Cooling Rate on Microstructure
The cooling rate following PWHT is as critical as the peak temperature. A slow air cool or furnace cool after a solution treatment can allow time-dependent precipitation of intermetallic phases. Conversely, a rapid water quench or forced-air quench preserves the solution-treated microstructure but may introduce new residual stresses. The optimal practice for duplex steel overlay deposits is a solution treatment at 1050–1150°C followed by immediate water quenching, as specified in relevant material standards.
3. Key Process Parameters and Implementation Points
3.1 PWHT Parameter Selection for Duplex Steel SMAW Overlay
| Parameter | Typical Range | Rationale |
|---|---|---|
| Treatment Type | Solution Heat Treatment (SHT) | Restores α/γ balance; dissolves sigma and chi phases |
| Peak Temperature | 1020 – 1150 °C | Within the two-phase field; avoids complete austenitization or full ferritization |
| Soak Time | 1 – 3 hours (depending on thickness) | Ensures uniform temperature throughout the deposit and heat-affected zone |
| Cooling Method | Water quench (preferred) or forced air | Suppresses intermetallic precipitation; preserves solution-treated microstructure |
| Heating Rate | 100 – 150 °C/h (up to 800°C), then unrestricted | Prevents thermal cracking in the substrate during ramp-up |
| Maximum Substrate Temperature | ≤ 1150 °C | Exceeding this temperature causes excessive grain growth and phase instability |
3.2 SMAW Overlay Welding Parameters (Pre-PWHT)
The quality of the overlay deposit prior to PWHT directly influences the effectiveness of the heat treatment. Key SMAW welding parameters for duplex steel overlay include:
| Parameter | Recommended Value | Notes |
|---|---|---|
| Electrode Type | ERNiCrMo-3 (UNS A5.15) or equivalent Ni-based | Ni-based filler avoids sigma phase susceptibility in the weld metal |
| Deposition Rate | 3 – 8 kg/h | Balanced to control interpass temperature |
| Interpass Temperature | ≤ 150 °C (strict control) | Higher interpass temperatures promote sigma phase nucleation |
| Weld Leg Length | ≤ 50 mm (staggered pattern) | Minimizes heat input accumulation and local thermal distortion |
| Heat Input | 0.5 – 1.5 kJ/mm | Lower heat input preserves the ferrite content in the weld metal |
| Travel Speed | 150 – 300 mm/min | Higher speed reduces heat input and limits grain coarsening |
3.3 Post-PWHT Microstructural Evaluation
After PWHT, the overlay deposit should be evaluated for:
- Ferrite content (Austenite/Ferrite ratio): Target 35–65% ferrite, measured via magnetic ferrite gauge (ASTM E490) or metallographic analysis (ASTM E562).
- Hardness: Typical target ≤ 350 HV for solution-treated duplex overlay; values exceeding 400 HV may indicate intermetallic precipitation.
- Intermetallic phase content: Metallographic examination per ASTM E3 with etchants such as Kalling's reagent or Vilella's reagent to identify sigma, chi, and Laves phases. Acceptance criterion: no continuous intergranular phase network.
- Pitting corrosion resistance: ASTM G48 (Salt Spray) or ASTM G150 (ECP) testing to verify PREN (Pitting Resistance Equivalent Number) of ≥ 34 for 2205-equivalent deposits.
4. Applicable Standards and Acceptance Criteria
4.1 Material and Welding Standards
| Standard | Relevance |
|---|---|
| ASTM A240 / ASTM A276 | Material specifications for duplex stainless steel plates and forgings |
| ASTM A5.15 | Specification for Ni-Cr-Mo cast welding electrodes (ERNiCrMo-3) |
| ASME Section IX, Part 3 (QW-400) | Welding procedure qualification for SMAW; covers PWHT requirements |
| NACE MR0175 / ISO 15156 | Sulfide-resistant materials for oil and gas; includes PWHT requirements for high-sulfide environments |
| GB/T 24511 | Chinese standard for welding procedure specification and qualification |
| GB/T 20878 | Chinese standard for stainless and heat-resistant steel and alloy designations |
| ASTM E490 | Standard for determination of ferrite content in austenite-ferrite stainless steel weld metals |
| ASTM G48 / ASTM G150 | Corrosion testing methods for pitting and crevice resistance |
4.2 Acceptance Criteria for PWHT-Processed Duplex Overlay Deposits
- Visual and Dimensional Inspection: No cracks, porosity, or undercut exceeding 0.5 mm depth per ASME Section V, Article 1.
- Hardness: Maximum 350 HV for the overlay deposit; gradient from substrate to deposit should be within 100 HV per ASME B31.3.
- Ferrite Content: 35–65% ferrite in the weld metal (ASTM E490); 35–55% ferrite in the HAZ (ASME Section IX).
- Intermetallic Phases: No continuous sigma phase network; isolated sigma particles ≤ 5% area fraction (ASTM E3 metallographic evaluation).
- Corrosion Resistance: PREN ≥ 34 for 2205-equivalent deposits; no pitting in ASTM G48 Practice B (6% FeCl₃, 60°C, 24h).
- Non-Destructive Testing: Magnetic particle inspection (MT) per ASTM E709 or liquid penetrant inspection (PT) per ASTM E165 to detect surface cracks and defects.
5. Common Risks and Controls
5.1 Sigma Phase Precipitation
Risk: Exposure of the overlay deposit or HAZ to temperatures in the 600–1000°C range for extended periods (even during PWHT if parameters are poorly controlled) can nucleate sigma phase (Cr₂₅Co₇), which depletes the matrix of chromium and molybdenum, causing severe pitting and intergranular corrosion.
Control: Limit interpass temperature during welding to ≤ 150°C; use solution heat treatment at 1050–1150°C with rapid quenching; avoid stress relief annealing in the 600–1000°C range for duplex steels.
5.2 Excessive Ferrite Formation
Risk: Low heat input or high cooling rate during SMAW overlay can shift the microstructure toward full ferrite, reducing toughness and increasing susceptibility to intergranular corrosion and stress corrosion cracking (SCC).
Control: Optimize heat input to 0.8–1.5 kJ/mm; use Ni-based filler metals to promote austenite formation; verify ferrite content via ASTM E490 after each pass.
5.3 Thermal Cracking in the Substrate
Risk: During PWHT, if the heating rate is too rapid or the peak temperature exceeds 1150°C, the duplex substrate can experience grain boundary cracking due to thermal gradients and phase transformation stresses.
Control: Use controlled heating rates of 100–150°C/h up to 800°C; employ thermocouples at multiple locations on the component to monitor temperature uniformity; limit peak temperature to ≤ 1150°C.
5.4 Quench Cracking
Risk: Water quenching after solution treatment can cause thermal shock and cracking in thick sections or components with complex geometry, particularly if the substrate has residual hydrogen from the welding process.
Control: Pre-heat the component to 200–300°C before quenching; use controlled quench media (e.g., polymer quenchants) for thick sections; perform hydrogen bake-out (200–300°C for 2–4 hours) before PWHT.
6. Application Across Company Technology Routes
6.1 TIG/MIG Weld Overlay Route
While the referenced study focuses on SMAW overlay, the PWHT principles are directly transferable to TIG (GTAW) and MIG (GMAW) weld overlay processes. In TIG overlay of duplex stainless steel cladding, the lower heat input and precise arc control can produce deposits with tighter microstructural control, but the same phase balance and sigma phase risks apply. PWHT parameters (1020–1150°C solution treatment with water quench) remain the same, though the lower heat input of TIG may reduce the severity of microstructural deviations requiring correction.
For MIG overlay, the higher deposition rates and greater heat input can exacerbate ferrite dissolution and grain coarsening, making PWHT even more critical. The study's findings on interpass temperature control and cooling rate sensitivity are directly applicable to optimizing MIG overlay WPS (Welding Procedure Specifications) for duplex steel cladding.
6.2 Hydraulic Explosive Bonding Route
In hydraulic explosive bonding (hydroforming), the PWHT requirements differ fundamentally. The explosive bonding process itself does not involve welding, so there is no weld metal or HAZ requiring solution treatment. However, if the bonded laminate is subsequently subjected to mechanical forming or if the substrate is a duplex steel grade requiring post-forming heat treatment, the PWHT parameters must be carefully managed to avoid sigma phase formation in the bonded interface. The study's insights into temperature-time sensitivity of duplex steel microstructures provide critical guidance for setting post-bonding heat treatment limits on duplex steel clad laminates.
6.3 Explosion Welding Route
Explosion welding (explosive cladding) of duplex stainless steel onto carbon steel or other substrates produces a metallurgical bond with a complex interface microstructure including a molten layer, a deformation zone, and a recast layer. PWHT after explosion welding is sometimes required to relieve residual stresses from the explosive event and to stabilize the interface microstructure. The study's findings on optimal solution treatment temperatures and quenching practices are directly applicable to post-explosion welding PWHT, with the additional consideration that the explosive welding interface may have a different local chemistry and phase distribution than a weld overlay deposit.
7. Contribution to Qualification Building, Product Delivery, and Customer Value
7.1 Qualification Building
Understanding the effects of PWHT on duplex steel overlay deposits is essential for developing and qualifying Welding Procedure Specifications (WPS) and Welding Procedure Qualifications (WPQ) per ASME Section IX or GB/T 24511. The study provides the metallurgical justification for selecting specific PWHT parameters, which must be documented in the WPS and verified through coupon testing. This directly contributes to the company's ability to obtain third-party certifications (e.g., ASME "S" stamp, ISO 3834, NACE MR0175 compliance) for duplex steel clad products.
7.2 Product Delivery
By mastering PWHT parameters for duplex steel overlay, the company can deliver clad pipes, plates, and pressure vessels that meet the most stringent corrosion resistance and mechanical property requirements. This reduces the risk of field failures due to sigma phase-induced pitting or intergranular corrosion, which are among the most common causes of premature failure in duplex steel components in oil, gas, and chemical processing applications.
7.3 Customer Value
The technical knowledge gained from this study enables the company to:
- Provide customers with documented PWHT procedures backed by metallurgical evidence, increasing confidence in product performance and reducing liability exposure.
- Offer value-engineered solutions by optimizing PWHT parameters to minimize processing time and cost without compromising microstructural integrity.
- Address customer specifications requiring NACE MR0175 / ISO 15156 compliance for high-sulfide environments, where PWHT is mandatory for weld overlay deposits on duplex steel substrates.
- Deliver extended service life guarantees by demonstrating through testing that PWHT-processed deposits maintain ferrite balance, hardness, and corrosion resistance over the component's expected service life.
8. Recommended Implementation Protocol
- WPS Development: Incorporate PWHT parameters (1020–1150°C, water quench, ≤ 1150°C max substrate temperature) into the WPS for all duplex steel SMAW overlay procedures.
- Coupon Testing: Qualify the WPS with PWHT-processed coupons tested for ferrite content (ASTM E490), hardness (ASTM E18), intermetallic phase content (ASTM E3), and pitting resistance (ASTM G48).
- Production Monitoring: Implement in-process monitoring of interpass temperature (≤ 150°C), heat input (0.5–1.5 kJ/mm), and PWHT peak temperature (thermocouple verification) during production.
- Post-PWHT Inspection: Perform hardness testing, magnetic ferrite gauge measurement, and NDT (MT/PT) on all production components after PWHT.
- Documentation: Maintain complete records of PWHT parameters, thermocouple readings, and test results for each production batch, in accordance with ASME Section VIII Div. 1, UG-99 and UG-100.
9. Conclusion
Post-Weld Heat Treatment is not merely a procedural step but a critical metallurgical intervention that determines the long-term performance of duplex stainless steel weld overlay deposits. The study of PWHT effects on SMAW overlay microstructure and properties provides the company with actionable, standards-based knowledge that directly enhances qualification capabilities, product reliability, and customer trust. By integrating these findings into WPS development, production protocols, and quality assurance systems, the company can deliver duplex steel clad products that meet the highest industry standards for corrosion resistance, mechanical integrity, and service life in the most demanding industrial environments.