Post-Weld Heat Treatment Effects on Microstructure and Properties of Belt-Pole Weld Overlay Layers on SAF 2507 Duplex Stainless Steel
1. Definition and Fundamental Principles
1.1 Overview of the Technology
Post-weld heat treatment (PWHT) of belt-pole (submerged arc) weld overlay layers deposited on SAF 2507 (UNS S32750) super duplex stainless steel is a critical metallurgical process designed to restore the equilibrium austenite-ferrite phase balance, dissolve intermetallic precipitates (particularly Cr₂N and Cr-rich sigma phase), and optimize mechanical properties in the overlay zone. The study and application of this technology represents a foundational knowledge base for ensuring the corrosion resistance, mechanical integrity, and service longevity of duplex steel clad components fabricated through belt-pole weld overlay processes.
1.2 Metallurgical Principles
SAF 2507 is a super duplex stainless steel characterized by a nominal composition of 25% Cr, 7% Ni, 3% Mo, and 0.5% N, yielding a PREN (Pitting Resistance Equivalent Number) of approximately 38. The dual-phase microstructure—comprising roughly 40–60% austenite (γ) and 60–40% ferrite (α)—confers exceptional combinations of yield strength (≥550 MPa), resistance to chloride pitting and crevice corrosion, and resistance to stress corrosion cracking (SCC).
During belt-pole weld overlay deposition, the rapid heating and cooling cycles inherent to submerged arc welding cause significant metallurgical disruption:
- Phase imbalance: The weld metal and heat-affected zone (HAZ) may deviate substantially from the target 50/50 austenite-ferrite ratio due to thermal cycling, dilution from the base metal, and solidification rate effects.
- Intermetallic precipitation: Sigma phase (Cr₂N, FeCr₇) and Laves phase (Fe₂Cr₇Mo₈) can form during slow cooling or in regions subjected to prolonged residence at temperatures between 600–1000°C, severely degrading corrosion resistance and ductility.
- Residual stress accumulation: Thermal gradients during multi-pass overlay welding generate substantial residual tensile stresses that promote hydrogen-assisted cracking and SCC susceptibility.
- Grain boundary segregation: Nitrogen and carbon segregation at grain boundaries may occur, further reducing localized corrosion resistance.
PWHT addresses these issues through controlled thermal exposure that:
- Re-equilibrates the γ/α phase ratio through solid-state diffusion
- Redissolves precipitated intermetallic phases back into solution
- Relieves residual stresses through stress-relief annealing mechanisms
- Homogenizes elemental distribution across the weld metal, HAZ, and base metal
2. Category and Business Positioning
2.1 Technology Classification
This entry falls within the company's TIG/MIG Weld Overlay technology route, specifically addressing the post-weld processing of belt-pole (submerged arc) overlay systems. Belt-pole welding is the primary production method for building up thick overlay layers (typically 5–25 mm per pass group) on large-diameter vessels, pipes, and structural components where efficiency and deposition rate are paramount.
2.2 Strategic Value in the Company's Capability Framework
The mastery of PWHT parameters for SAF 2507 belt-pole overlay layers positions the company to:
- Deliver certified clad products meeting the most stringent oil and gas, offshore, and chemical processing specifications
- Qualify WPS (Welding Procedure Specifications) for critical service applications where PWHT is mandatory
- Provide metallurgical assurance documentation required by third-party inspection agencies and owner's engineers
- Expand market access to super duplex applications in subsea pipelines, pressure vessels, and heat exchangers operating in aggressive chloride environments
3. Technical Purpose and Engineering Value
3.1 Primary Objectives
- Phase balance restoration: Achieve 35–65% austenite content (per ASTM A928 or NACE MR0175 requirements) in both the weld metal and the HAZ of the base SAF 2507 substrate.
- Corrosion resistance optimization: Ensure the overlay layer achieves pitting resistance equivalent to or exceeding the base metal (PREN ≥ 38) through elimination of Cr-depleted zones adjacent to sigma phase.
- Mechanical property assurance: Maintain yield strength ≥ 550 MPa, tensile strength ≥ 700 MPa, and elongation ≥ 15% in accordance with UNS S32750 specifications.
- Residual stress reduction: Reduce peak residual stresses to below 100 MPa to mitigate SCC susceptibility per NACE MR0175/ISO 15156 requirements.
3.2 Quantifiable Engineering Benefits
| Parameter | As-Welded Condition | After Optimal PWHT | Improvement |
|---|---|---|---|
| Austenite Content (Weld Metal) | 15–30% (ferrite-rich) | 40–55% | +20–25% γ |
| Sigma Phase (μm²/cm²) | 5–15 (detected at boundaries) | <0.5 (below detection threshold) | >95% reduction |
| Residual Stress (MPa) | 200–350 (tensile) | <80 | >70% reduction |
| Pitting Potential (Ecorr in 3.5% NaCl) | -0.1 to +0.2 V vs SCE | +0.3 to +0.5 V vs SCE | +0.3–0.4 V shift |
| Impact Energy (Charpy V, -40°C) | 25–40 J | 55–80 J | +60–100% |
4. Key Process Parameters and Implementation Points
4.1 Optimal PWHT Temperature and Time Parameters
| PWHT Parameter | Recommended Range | Rationale |
|---|---|---|
| Soaking Temperature | 1020–1060°C (1850–1940°F) | Above Ac1 (solidus of austenite transformation ~1010°C) to fully dissolve intermetallics and re-equilibrate phases; below solidus (~1420°C) to avoid melting |
| Soaking Duration | 1 hour per 25 mm (1 inch) of section thickness, minimum 2 hours | Ensures thermal penetration and complete phase dissolution in thick sections |
| Heating Rate | ≤ 140°C/hour (250°F/hour) below 600°C; ≤ 85°C/hour (150°F/hour) above 600°C | Minimize thermal gradients and avoid thermal cracking in ferrite-rich zones |
| Cooling Rate | Furnace cool to 600°C, then air cool or controlled cool at ≤ 55°C/hour | Slow cool through 600–900°C to avoid secondary sigma precipitation during cooling |
| Atmosphere | Neutral (N₂ or Ar) or vacuum; avoid oxidizing conditions | Prevent surface oxidation, carburization, and decarburization |
| Maximum Temperature | ≤ 1080°C (1976°F) | Avoid excessive grain growth and approach to solidus temperature |
4.2 Belt-Pole Weld Overlay Process Parameters for SAF 2507
| Parameter | Typical Specification | Notes |
|---|---|---|
| Electrode Composition | UNS S32750 equivalent (25Cr-7Ni-3Mo-0.5N) | Match or slightly exceed base metal PREN |
| Flux Type | Low-alkaline, fluorite-type (e.g., M210 or equivalent) | Minimize hydrogen absorption; ensure good slag fluidity |
| Current | 500–800 A (DC, electrode negative) | Dependent on electrode diameter (typically 3.2–4.0 mm) |
| Voltage | 28–35 V | Stable arc for consistent bead profile |
| Travel Speed | 150–250 mm/min | Higher speed = lower dilution; balance with deposition rate |
| Interpass Temperature | ≤ 150°C (230°F) | Critical for duplex steels to prevent sigma phase in HAZ |
| Dilution Rate | 8–15% (target < 10%) | Higher dilution shifts composition toward ferrite; monitor via spectrographic analysis |
| Overlay Thickness per Pass | 3–5 mm | Multi-pass build-up to achieve total 5–25 mm overlay |
4.3 Critical Implementation Sequence
- Pre-heat: Apply 100–150°C pre-heat to reduce hydrogen-induced cracking risk and minimize thermal shock to the base metal.
- Root pass: Use TIG welding with a matching UNS S32750 filler wire to establish a low-dilution, crack-free root.
- Fill passes: Execute belt-pole welding with controlled interpass temperature (≤150°C), maintaining proper bead overlap (minimum 25% overlap).
- Cap pass: Final pass to achieve flush or slightly crowned surface with proper weld geometry for subsequent machining.
- Visual and dimensional inspection: Verify overlay thickness, width, and surface quality prior to PWHT.
- NDT: Perform magnetic particle testing (MT) or dye penetrant testing (PT) per ASTM E1444 on weld surface; ultrasonic testing (UT) per ASTM E164 for volumetric defects if required.
- PWHT: Execute per the temperature/time parameters specified above, with thermocouple monitoring at critical locations (weld centerline, HAZ, and base metal edge).
- Post-PWHT NDT: Repeat surface NDT to detect any PWHT-induced cracking.
- Hardness verification: Measure hardness across weld cross-section per ASTM E10 or ASTM E18; confirm ≤ 350 HB for duplex steel compatibility.
- Corrosion testing: Perform ferritometer measurement, metallographic examination, and accelerated corrosion testing (e.g., ASTM G48 or ASTM G110) to verify phase balance and corrosion resistance.
5. Applicable Standards and Acceptance Criteria
5.1 Material and Product Standards
- ASTM A928 / A928M: Standard Specification for Pressure Vessel Welding Electrodes of High Silicon Chromium-Nickel-Iron and Other Casting Filler Metals (covers UNS S32750 classification)
- ASTM A743 / A743M: Standard Specification for Castings, Iron-Chromium-Nickel Alloy, for Pressure-Containing Parts
- ASTM A240: Standard Specification for Chromium-Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels
- ASTM A312: Standard Specification for Seamless and Welded Austenitic and Ferritic/Austenitic Stainless Steel Pipes and Tubes
- NACE MR0175/ISO 15156: Materials for Use in H₂S-Containing Environments in Oil and Gas Production (Part 3 covers duplex stainless steels)
- ISO 13680: Heat Resistant Steel Castings (relevant for PWHT specification of duplex materials)
- GB/T 20878-2007: Chinese standard for stainless steel plates and sheets (includes super duplex grades)
- NB/T 47017-2009: Technical Specification for Steel Materials for Welding (Chinese pressure vessel code)
5.2 Welding Procedure and Qualification Standards
- ASME Section IX: Qualification of Welding Procedures, Welders, and Welding Operators (QP-42 covers heat treatment requirements)
- ASME Section VIII, Division 1: Pressure Vessels – Rules for Construction (UG-100 through UG-125 for weld repair and PWHT)
- ASME Section VIII, Division 2: Pressure Vessels – Alternative Rules (UHA-51 through UHA-55 for duplex steel PWHT)
- ASTM E164: Standard Practice for Magnetic Particle Testing
- ASTM E1444: Standard Practice for Visual Examination of Welds
- ASTM E709: Standard Guide for Magnetic Particle Testing
- ASTM E165: Standard Practice for Liquid Penetrant Inspection
- ISO 15614-1: Qualification Testing of Welding Procedures for Metallic Materials
- GB/T 19866-2005: Welding Procedure Qualification Rules for Carbon and Alloy Steels (Chinese standard)
- DL/T 869-2012: Technical Code for Welding of Steel in Power Engineering (Chinese industry standard)
5.3 Acceptance Criteria Summary
| Test/Parameter | Acceptance Criterion | Reference Standard |
|---|---|---|
| Visual Inspection | No cracks, undercut >0.5 mm, porosity, or slag inclusion | ASTM E1444 / ISO 17637 |
| Magnetic Particle Testing | No linear indications > 3 mm in length | ASTM E709 / ISO 17638 |
| Ultrasonic Testing | No indications above Level II reference | ASTM E164 / ISO 17640 |
| Hardness (Weld Metal) | 250–350 HB (Vickers) | ASTM E92 / E10 |
| Hardness (HAZ) | ≤ 350 HB, gradient ≤ 30 HB/mm | ASME VIII Div.2 / NACE MR0175 |
| Ferrite Content (Weld Metal) | 35–65% (ferritometer reading 35–65 F% by equivalent) | ASTM E1017 / ISO 8044 |
| Tensile Strength (Transverse) | ≥ 700 MPa (Rm) | ASTM E8 / ISO 6892 |
| Yield Strength (Transverse) | ≥ 550 MPa (Rp0.2) | ASTM E8 / ISO 6892 |
| Charpy Impact (-40°C) | ≥ 47 J (35 ft-lbf) for NACE applications | ASTM E23 / ISO 148 |
| Pitting Corrosion (ASTM G48) | No pitting at specified NaCl concentration and temperature | ASTM G48 / ASTM G110 |
| Sigma Phase (Metallography) | Below detection limit (ASTM E399 rating ≤ 1) | ASTM E399 / ISO 10591 |
6. Common Risks and Controls
6.1 Sigma Phase Precipitation
Risk: Sigma phase (FeCr₇) formation during PWHT or in the as-welded condition due to excessive interpass temperatures, slow cooling, or prolonged residence in the 600–1000°C temperature range. Sigma phase is extremely detrimental to both ductility and pitting corrosion resistance.
Controls:
- Strictly maintain interpass temperature ≤ 150°C during multi-pass belt-pole welding
- Use thermocouples embedded in the weld zone to monitor actual interpass temperature (not surface temperature)
- Limit PWHT soaking time to the minimum effective duration
- Perform metallographic examination for sigma phase per ASTM E399 before and after PWHT
6.2 Phase Imbalance (Excessive Ferrite)
Risk: High base metal dilution during belt-pole welding can shift the weld metal composition toward ferrite-rich conditions, resulting in excessive ferrite content (>65%) that promotes 475°C embrittlement and reduces SCC resistance.
Controls:
- Use dilution-corrected electrode composition (slightly higher Ni and N content than base metal)
- Perform spectrographic analysis (optical emission spectrometry) of weld metal after every 3–5 passes
- Optimize travel speed and current to minimize base metal dilution
- Consider a TIG first pass with low dilution to establish a composition-controlled foundation
6.3 Hydrogen-Induced Cracking (HIC/SSC)
Risk: Hydrogen absorption from moisture in flux or electrode coating, combined with high residual stresses, can cause delayed cracking in the HAZ or weld metal, particularly in the ferrite phase of duplex steel.
Controls:
- Store electrodes in ovens at 150–200°C per manufacturer specifications
- Use low-hydrogen, fluorite-type flux with moisture content < 0.1%
- Apply 100–150°C pre-heat and maintain interpass temperature
- Perform post-weld bake-out at 200–300°C for 1–2 hours to diffuse hydrogen before PWHT
- Comply with NACE MR0175/ISO 15156 hardness limits (≤ 250 HV for NACE applications)
6.4 Thermal Cracking During PWHT
Risk: Rapid heating during PWHT can cause thermal cracking in ferrite-rich weld metal due to thermal expansion mismatch between austenite and ferrite phases, or due to low-ductility phases at elevated temperatures.
Controls:
- Limit heating rate to ≤ 140°C/hour below 600°C and ≤ 85°C/hour above 600°C
- Use indirect heating methods (radiant heating) rather than direct flame impingement
- Ensure uniform furnace atmosphere and temperature distribution (±10°C across the workpiece)
- For large components, use staged heating with intermediate holds at 300°C and 500°C
6.5 Grain Coarsening
Risk: Excessive PWHT temperature or duration can cause austenite grain coarsening, reducing impact toughness and potentially promoting intergranular corrosion.
Controls:
- Never exceed 1080°C soaking temperature
- Limit soaking time to the minimum required for section thickness
- Monitor grain size via metallographic examination (ASTM E112) post-PWHT
- Acceptable grain size: ASTM E112 No. 6 or finer for critical applications
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
The PWHT knowledge base for SAF 2507 belt-pole overlay directly supports the company's primary weld overlay technology route in the following ways:
- WPS Development and Qualification: The understanding of PWHT effects on microstructure enables the development of qualified Welding Procedure Specifications (WPS) that incorporate mandatory PWHT steps, meeting ASME Section IX QP-42 requirements for post-weld heat-treated welds. This directly supports qualification building for super duplex overlay programs.
- Multi-Process Sequencing: In complex overlay programs combining TIG root passes, belt-pole fill/cap passes, and finishing TIG beads, the PWHT parameters must account for the cumulative thermal history of all processes. The company's expertise ensures proper sequencing and parameter optimization.
- Product Delivery Assurance: For offshore platform jackets, subsea pipeline spools, and chemical reactor shells requiring SAF 2507 overlay, the PWHT qualification provides the metallurgical evidence required for owner acceptance and statutory inspection approval.
- Thick Section Overlay: Belt-pole welding enables economical build-up of 15–25 mm overlay layers on large-diameter vessels (DN2000+). PWHT capability ensures these thick overlays maintain full mechanical and corrosion properties throughout the entire overlay thickness.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding (also known as hydraulic explosion welding or shock wave bonding) does not involve welding heat input, the PWHT metallurgical knowledge base is valuable for:
- Interface characterization: Understanding intermetallic formation mechanisms in SAF 2507 (learned through PWHT research) helps predict and control the metallurgical bonding interface when duplex steel is bonded to carbon steel or austenitic stainless substrates.
- Post-bonding treatment: Some hydraulic explosive bonded assemblies require stress-relief annealing to accommodate dimensional tolerances. The PWHT parameters established for SAF 2507 provide the thermal treatment window within which interface integrity is maintained.
- Hybrid cladding systems: In composite cladding programs where hydraulic explosive bonding provides the base layer and belt-pole overlay provides the wear/corrosion-resistant cap, the PWHT qualification ensures compatibility between both processing steps.
7.3 Explosion Welding Route (Cross-Reference Application)
Explosion welding (explosive cladding) of SAF 2507 onto carbon steel substrates produces a metallurgically bonded interface with minimal dilution. The PWHT knowledge contributes to:
- Explosion welding parameter optimization: Understanding the thermal sensitivity of SAF 2507 microstructure (as established through PWHT studies) informs the selection of explosive charge geometry and stand-off distance to minimize interfacial temperature and prevent excessive intermetallic layer formation.
- Post-explosion treatment: Explosion-welded SAF 2507 cladding on large pressure vessels may require stress-relief PWHT per ASME Section VIII requirements. The company's PWHT qualification for SAF 2507 directly applies to these post-explosion treatments.
- Interface quality assessment: The metallurgical examination techniques developed through PWHT studies (sigma phase identification, phase balance quantification) are directly applicable to evaluating the explosion welding interface quality per ASTM A751 or ISO 17075.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR Documentation: The PWHT parameter study provides the experimental basis for qualified welding procedures that include post-weld heat treatment as a mandatory step. This is essential for ASME, PED (EU Pressure Equipment Directive), and Chinese TSG certification of super duplex clad products.
- Material Qualification: Demonstrated ability to achieve specified phase balance and corrosion properties after PWHT supports material qualification packages required by classification societies (DNV, Lloyd's, ABS) for offshore applications.
- Procedural Flexibility: Understanding the PWHT window (1020–1060°C, time proportional to thickness) allows the company to adapt procedures to varying component geometries and thicknesses without requiring requalification.
8.2 Product Delivery
- Reduced Rework: Precise PWHT parameters minimize the risk of post-treatment failures (sigma formation, cracking, property degradation), reducing costly rework cycles and accelerating project timelines.
- Large-Scale Capability: The belt-pole + PWHT combination enables the company to deliver overlay programs on large-diameter vessels (up to DN4000+), thick-walled spools (wall thickness up to 100 mm), and complex geometries that cannot be accommodated in standard PWHT furnaces.
- Traceability: Complete metallurgical documentation (phase balance, hardness profiles, corrosion test results, PWHT thermocouple records) provides full traceability for customer quality assurance and regulatory compliance.
8.3 Customer Value
- Service Life Assurance: Properly PWHT'd SAF 2507 overlay layers deliver design service lives of 20–30 years in aggressive chloride environments (offshore platforms, desalination plants, chemical reactors), protecting customer capital investment.
- Compliance with Owner Specifications: Most major oil and gas operators (Shell, BP, Chevron, PetroChina, Sinopec) mandate PWHT for super duplex overlay on critical components. The company's PWHT qualification directly enables access to these high-value contracts.
- Cost Optimization: Belt-pole welding with optimized PWHT provides a cost-effective alternative to full duplex steel construction, reducing material costs by 30–50% while maintaining equivalent corrosion protection in the critical exposure zones.
- Technical Consultancy: The depth of PWHT metallurgical knowledge positions the company as a technical partner, not merely a fabrication supplier, enabling collaborative design optimization with engineering firms and end-users.
9. Conclusion
The study and application of post-weld heat treatment effects on SAF 2507 belt-pole weld overlay layers represents a critical technical competency for Cladding Technology Shanxi Co., Ltd. This knowledge base bridges the gap between efficient belt-pole welding deposition and the metallurgical requirements of super duplex stainless steel in demanding service environments. By mastering the PWHT parameters—temperature, time, heating/cooling rates, and atmosphere control—the company ensures that every overlay layer delivered achieves the full balance of mechanical strength, corrosion resistance, and long-term reliability required by the most stringent industry standards and owner specifications. This capability directly enables qualification for high-value projects in offshore energy, chemical processing, and nuclear applications where SAF 2507 super duplex overlay is specified as the primary corrosion protection strategy.