Interpass Temperature Control for Duplex Stainless Steels (≤150°C)
1. Definition and Fundamental Principles
Interpass temperature control for duplex stainless steels is a critical thermal management practice in weld overlay and cladding fabrication, mandating that the base metal and deposited weld metal between successive passes remain at or below 150°C. This requirement applies specifically to high-alloy austenitic-ferritic duplex stainless steels such as SAF 2205 (UNS S31803) and SAF 2507 (UNS S32750), which are inherently susceptible to microstructural degradation when subjected to elevated thermal exposure during multi-pass welding operations.
The metallurgical basis for this requirement stems from the unique dual-phase microstructure of duplex stainless steels. In the as-welded condition, these alloys maintain approximately a 40:60 to 50:50 austenite-to-ferrite ratio, which provides their signature combination of high yield strength (typically ≥450 MPa for 2205 and ≥550 MPa for 2507), excellent pitting and crevice corrosion resistance (PREN ≥32 for 2205 and ≥38 for 2507), and superior resistance to chloride stress corrosion cracking. However, when the weld metal or heat-affected zone (HAZ) is held in the critical temperature range of approximately 600°C to 950°C for extended periods, several detrimental phase transformations occur:
- σ-phase precipitation: A brittle intermetallic compound (Cr₂N-based, with formula σ = Cr₁₄Mo₆N₈) that forms preferentially in the ferrite phase. The σ-phase is extremely hard (HV 1200–1600), non-magnetic, and catastrophically reduces ductility and fracture toughness. Its formation is thermodynamically favored at temperatures between 600°C and 900°C, with kinetics accelerating above 700°C.
- Chromium nitride (Cr₂N) precipitation: Nitrogen in solution combines with chromium to form fine Cr₂N particles within the ferrite matrix, depleting the surrounding matrix of both Cr and N. This reduces the PREN of the affected zone and creates localized anodic sites that initiate pitting corrosion.
- Phase ratio imbalance: Prolonged thermal exposure in the 800–950°C range promotes the dissolution of ferrite into austenite (γ-phase), shifting the microstructure toward a predominantly austenitic condition. This eliminates the beneficial properties of the ferrite phase and increases susceptibility to hot cracking during subsequent welding passes.
The interpass temperature limit of 150°C is established to ensure that residual heat from previous passes dissipates sufficiently before the next pass is deposited. This prevents the cumulative thermal input from pushing the prior pass material into the critical σ-phase precipitation temperature window. The 150°C threshold provides a substantial safety margin below the onset of σ-phase formation, accounting for thermal gradients within the weld bead and the local temperature peaks that occur during subsequent pass deposition.
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s capability framework, interpass temperature control for duplex stainless steels is classified under the category of Process Temperature Control and Cooling (过程温控与降温), specifically addressing the Material Layer Temperature (材料层温) technical direction. This positioning reflects the company's systematic approach to thermal management as a cross-cutting quality control function that underpins all three primary technology routes.
The business positioning of this capability is strategic rather than incidental. Duplex stainless steel cladding products—particularly 2205 and 2507 overlays on carbon steel or low-alloy steel substrates—represent high-value, specification-critical deliverables in the oil & gas, offshore engineering, pulp & paper, and chemical processing industries. The mandatory nature of this requirement (as indicated by the "2205/2507 mandatory" designation) means that failure to implement rigorous interpass temperature control directly results in non-conforming products, rejected inspections, and potential service failures. This capability therefore serves as a differentiator in competitive bidding, as not all fabricators demonstrate documented, systematic compliance with duplex steel thermal management protocols.
Within the company's quality management system, this capability interfaces with:
- WPS/PQR qualification: Interpass temperature limits must be specified in the Welding Procedure Specification and verified during Procedure Qualification Record testing.
- NDT planning: Temperature-controlled welds produce cleaner microstructures that yield more predictable ultrasonic testing results, reducing false indications and rework cycles.
- Customer audits: Documented temperature monitoring records provide objective evidence of process control during customer site inspections and third-party certification audits.
3. Technical Purpose and Value
3.1 Primary Technical Purpose: Prevention of Brittle Phase Precipitation
The fundamental objective of maintaining interpass temperatures at or below 150°C is to prevent the nucleation and growth of σ-phase and chromium nitride precipitates within the weld metal and HAZ. These brittle phases, once formed, are essentially irreversible under normal service conditions and cannot be eliminated by post-weld heat treatment at temperatures that would themselves cause additional σ-phase formation. The prevention strategy is therefore the only viable approach.
Specifically, this control measure achieves the following:
- Maintenance of corrosion resistance: By preventing Cr₂N precipitation, the nitrogen remains in solid solution within the ferrite phase, preserving the alloy's PREN value and resistance to chloride-induced pitting and crevice corrosion.
- Preservation of toughness: Avoiding σ-phase ensures that the weld metal retains adequate impact energy (typically ≥47 J at -20°C for 2205 welds per ASTM A240 requirements) and fracture resistance.
- Phase ratio stability: Preventing prolonged high-temperature exposure maintains the intended austenite-ferrite balance, ensuring consistent mechanical properties throughout the weld cross-section.
- Cracking resistance: A balanced microstructure with adequate ferrite content provides resistance to solidification cracking during subsequent passes, particularly critical in multi-pass overlay builds.
3.2 Business and Customer Value
The implementation of rigorous interpass temperature control delivers measurable value across the company's operations:
- First-pass qualification success: Reduces the probability of mechanical testing failures during WPS qualification, particularly for Charpy impact and hardness testing in the HAZ.
- Service life assurance: Products delivered with verified microstructural integrity meet design service life expectations, reducing warranty claims and enhancing the company's reputation.
- Regulatory compliance: Satisfies the mandatory requirements of API 579, NACE MR0175, and ASME B31.3 for duplex stainless steel applications in sour service and pressure systems.
- Reduced rework costs: Eliminates the need for post-weld reclamation, re-welding, or complete component rejection due to microstructural non-conformance detected during final inspection.
4. Key Process Implementation Points
4.1 Thermal Input Management Strategy
The "low heat input + rapid cooling" strategy (小热输入+快冷策略) specified in the technical entry is implemented through coordinated control of welding parameters, joint design, and auxiliary cooling measures. The following table summarizes the recommended parameters for TIG weld overlay of 2205 and 2507 duplex stainless steels:
| Parameter | SAF 2205 (UNS S31803) | SAF 2507 (UNS S32750) | Rationale |
|---|---|---|---|
| Maximum Heat Input | ≤ 1.5 kJ/mm | ≤ 1.2 kJ/mm | Limits peak temperature and time at temperature |
| Interpass Temperature | ≤ 150°C (mandatory) | ≤ 150°C (mandatory) | Prevents σ-phase and Cr₂N precipitation |
| Welding Current (TIG) | 80–140 A | 100–160 A | Low current for fine bead control |
| Travel Speed | 150–250 mm/min | 180–280 mm/min | Higher speed reduces heat input |
| Filler Wire Diameter | 1.0–1.6 mm | 1.0–1.6 mm | Thin wire for low deposition rate |
| Shielding Gas | Ar + 2% N₂ | Ar + 2–3% N₂ | N₂ stabilizes ferrite content |
| Backing Gas (if applicable) | Ar + 2% N₂ | Ar + 2–3% N₂ | Prevents back-side oxidation and phase loss |
| Bead Width | ≤ 8 mm (ideally ≤ 6 mm) | ≤ 8 mm (ideally ≤ 6 mm) | Narrow beads cool faster |
| Bead Overlap | ≥ 50% of bead width | ≥ 50% of bead width | Ensures full fusion and uniform cooling |
4.2 Temperature Monitoring Methods
Effective interpass temperature control requires reliable, real-time temperature measurement at the weld location. The following methods are employed in practice:
- Pyrometer (infrared thermometry): Non-contact measurement using a calibrated infrared pyrometer. The operator checks the temperature of the previously deposited bead immediately before initiating the next pass. This is the most common method for production welding.
- Thermocouple monitoring: Type K or Type N thermocouples attached to the weld surface provide continuous temperature logging. This method is preferred for qualification welding and critical production runs where documented evidence is required.
- Thermal paint indicators: Temperature-sensitive paint that changes color at specific thresholds (e.g., 150°C) provides a simple visual confirmation of interpass temperature compliance. Useful as a secondary verification method.
- Thermal imaging cameras: Advanced monitoring systems that provide spatial temperature distribution data across the weld zone. Used for process development and audit verification.
4.3 Cooling Strategies
When natural air cooling is insufficient to achieve the 150°C interpass temperature within an acceptable production cycle time, the following auxiliary cooling methods may be applied:
- Air cooling: Directed compressed air (≤ 0.5 MPa) applied to the weld zone after each pass. Must be clean, dry, and oil-free to prevent contamination of the weld surface.
- Water cooling (controlled): Low-flow water spray applied to the base metal adjacent to (not directly on) the weld bead. Requires careful control to prevent hydrogen-induced cracking and thermal cracking. Generally limited to thick-section applications where air cooling is inadequate.
- Thermal mass utilization: Strategic placement of copper thermal mass blocks adjacent to the weld zone to accelerate heat dissipation. Particularly effective for small-diameter pipe overlay applications.
- Pass sequencing optimization: Welding in a pattern that maximizes the time between passes on adjacent areas, allowing natural cooling before the interpass temperature is reached. This is the preferred approach as it requires no auxiliary equipment.
4.4 Welding Technique Considerations
Beyond parameter selection, the welding technique itself plays a critical role in thermal management:
- Continuous welding without pauses: Once a pass is initiated, it should be completed without interruption to minimize the number of thermal cycles. Stop-and-start sequences increase the number of times the material is reheated.
- Root pass optimization: The root pass should be deposited with minimum heat input (lowest practical current, highest practical travel speed) to establish a low-temperature foundation for subsequent passes.
- Filler pass geometry: Avoid excessive bevel angles that require large cross-sectional fill volumes. Instead, use narrow-groove techniques (e.g., back-plate root closure with narrow groove preparation) to minimize total heat input.
- Cover pass management: The final cover pass should be deposited with the same thermal discipline as all preceding passes. The cover pass is particularly critical as it defines the final surface condition and is directly exposed to the service environment.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Relevant Requirements | Applicability |
|---|---|---|
| ASTM A240 / ASTM A999 | Material specification for 2205/2507 plate and sheet; impact test requirements | Base material qualification |
| ASTM A335 / ASTM A213 | Duplex stainless steel tubing and pipe specifications | Clad pipe/tube applications |
| ASME BPV Section IX | WPS/PQR qualification; QW-462 (heat input limits); QW-451 (interpass temperature) | Welding procedure qualification |
| ASME B31.3 | Process piping code; post-weld heat treatment requirements; NDE requirements for duplex SS | Pressure system applications |
| API 5L / API 5CT | Pipeline and casing specifications; cladding requirements for duplex SS overlays | Oil & gas pipeline applications |
| NACE MR0175 / ISO 15156 | Sour service material requirements; duplex SS inclusion; PWHT restrictions | Oil & gas sour service |
| ISO 14343 | Welding of stainless steels; welding consumables for duplex SS | Filler metal selection |
| EN ISO 10043 | Welding consumables for austenitic and austenitic-ferritic stainless steels | European specification compliance |
| GB/T 20878 | Chinese standard for stainless steel chemical composition and technical conditions | Domestic material compliance |
| GB/T 150 | Chinese pressure vessel code; welding procedure requirements | Pressure vessel applications |
| NB/T 47014 | Chinese standard for welding procedure qualification of pressure vessels | WPS qualification (domestic) |
| ASTM E1094 | Standard practice for determining microstructure of welds in stainless steel | Microstructural verification |
5.2 Acceptance Criteria
The following acceptance criteria apply to duplex stainless steel weld overlay products where interpass temperature control is implemented:
- Microstructural examination: Metallographic examination per ASTM E1094 shall reveal no detectable σ-phase in the weld metal or HAZ. Ferrite content in the weld metal shall be between 30% and 65% (per ASTM A240 / ISO 3497 ferrite number requirements).
- Hardness testing: Localized hardness shall not exceed HV 350 (for 2205) or HV 380 (for 2507) in the weld metal or HAZ. Hardness exceeding these limits indicates possible σ-phase or other hard intermetallic formation.
- Impact testing: Charpy V-notch impact energy shall meet minimum requirements: ≥ 47 J at -20°C for 2205 welds (per ASTM A240), ≥ 47 J at -40°C for 2507 welds (per ASTM A999).
- Corrosion testing: Pitting resistance index (PREN) shall be maintained at ≥ 32 for 2205 and ≥ 38 for 2507 weld metal. Electrochemical pitting potential testing in 3.5% NaCl solution shall demonstrate pitting potential ≥ +250 mV vs. SCE for 2205 and ≥ +350 mV vs. SCE for 2507.
- NDT acceptance: Ultrasonic testing per ASME BPV Section V Article 4 or ASTM E2388 shall show no indications exceeding acceptance limits. No linear indications > 3 mm in length shall be accepted in duplex SS welds.
- Temperature records: Documented interpass temperature measurements shall demonstrate 100% compliance with the ≤ 150°C requirement throughout the entire welding sequence.
6. Common Risks and Controls
6.1 Risk Matrix
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| σ-phase formation | Interpass temperature exceeding 150°C; excessive heat input; slow cooling | Catastrophic loss of toughness; reduced corrosion resistance; potential in-service fracture | Mandatory pyrometer checks before each pass; WPS-specified maximum heat input; air cooling where required |
| Phase ratio shift to austenitic | Repeated thermal cycling above 800°C; insufficient nitrogen in filler/gas | Loss of yield strength; increased hot cracking susceptibility; reduced creep resistance | Controlled nitrogen content in shielding gas (2–3% N₂); monitoring of ferrite number during qualification |
| Cr₂N precipitation | Thermal exposure in 600–900°C range; nitrogen depletion from ferrite | Reduced PREN; localized pitting initiation sites; accelerated corrosion in chloride environments | Strict interpass temperature control; appropriate filler metal selection with controlled N content |
| Hydrogen-induced cracking | Excessive water cooling; contaminated filler wire; high hydrogen in shielding gas | Delayed cracking in HAZ; component rejection | Limit water cooling to non-contact methods; use low-hydrogen filler metals; dry gas supply |
| Insufficient fusion | Excessive cooling rate; overly conservative parameter settings | Lack of fusion defects; reduced joint integrity; NDT failures | Balance between thermal control and adequate fusion; maintain minimum current for complete fusion |
| Operator non-compliance | Inadequate training; production pressure to increase speed; lack of monitoring equipment | Uncontrolled interpass temperatures; non-conforming products | Mandatory training; automated temperature monitoring systems; production hold points with documented verification |
6.2 Detailed Risk Controls
σ-phase formation control: The primary risk in duplex stainless steel welding is σ-phase precipitation. This risk is managed through a multi-layered approach: (1) WPS specification of maximum heat input and interpass temperature; (2) real-time monitoring with calibrated pyrometers; (3) documented temperature logs for every production weld; (4) periodic microstructural verification during production audits; and (5) operator certification that includes specific training on duplex steel thermal management.
Production schedule management: A common operational challenge is the conflict between thermal control requirements and production throughput. The cooling time required to reach ≤ 150°C between passes can range from 2 to 15 minutes depending on section thickness, ambient conditions, and cooling method. To manage this, the company implements: pre-planned welding sequences that maximize cooling time between adjacent passes; dedicated cooling stations with forced air capability; and batch production scheduling that allows adequate interpass cooling without compromising delivery timelines.
Environmental controls: Ambient temperature and wind speed significantly affect cooling rates. In outdoor or poorly controlled workshop environments, wind can accelerate cooling (potentially causing hydrogen cracking if too aggressive) or, conversely, warm ambient conditions can slow cooling below acceptable rates. Controls include: minimum ambient temperature of 5°C (or 10°C for 2507) per ASME BPV Section IX QW-305; wind speed ≤ 1.5 m/s at the weld location; and use of welding screens or enclosures for thermal management in variable environments.
7. Application Across Three Technology Routes
7.1 TIG/MIG Weld Overlay Applications
In the TIG/MIG weld overlay route, interpass temperature control is the most directly applicable and frequently implemented aspect of this capability. The following scenarios illustrate its application:
- 2205 overlay on carbon steel pipe: Multi-pass TIG overlay of 2205 filler metal (e.g., ER2209 or ER2594) onto API 5L X65 or X70 carbon steel pipe for sour service applications. Each pass must be deposited at ≤ 150°C interpass temperature to maintain the duplex microstructure of the overlay. The overlay thickness is typically 2–5 mm, requiring 3–8 passes depending on the specific application.
- 2507 overlay on 2205 base plate: Upgrading existing 2205 equipment with a 2507 overlay for enhanced corrosion resistance in highly aggressive environments (e.g., offshore oil platform topside structures). The 2507 overlay requires even more stringent thermal control due to its higher alloy content and greater susceptibility to phase degradation.
- Transition layer + duplex overlay: When overlaying duplex stainless steel directly onto carbon steel, a transition layer (e.g., 309L or 316L) is deposited first, followed by the duplex overlay. The interpass temperature control applies to both the transition layer and the duplex overlay passes, with particular attention to the interface between the transition layer and the duplex layer.
- Clad pipe repair: Repair welding of damaged duplex stainless steel cladding on existing piping systems. The repair weld must match the thermal characteristics of the original overlay, requiring interpass temperature control to prevent degradation of the adjacent existing overlay.
For MIG (GMAW) overlay of duplex stainless steels, the interpass temperature requirement is equally mandatory but presents additional challenges due to the higher heat input inherent in the MIG process. Gas metal arc welding with solid wire (ER2209) or flux-cored wire (e.g., ER2594FCA) typically produces higher heat inputs than TIG, necessitating: lower wire feed speeds; increased travel speeds; smaller wire diameters (≤ 1.0 mm); and more aggressive cooling between passes. The company's WPS library includes qualified MIG procedures for 2205 and 2507 overlay with documented interpass temperature compliance.
7.2 Hydraulic Explosive Bonding Applications
In hydraulic explosive bonding (water-jet explosive welding), the interpass temperature control principle is applied differently but remains relevant. Hydraulic explosive bonding uses a water jet to generate a controlled implosion that drives a cladding sheet onto a base plate at supersonic velocities, creating a solid-state metallurgical bond without melting. However, interpass temperature considerations arise in the following contexts:
- Post-bonding weld repair: When hydraulic explosive bonded cladding requires repair welding (e.g., for localized damage or edge sealing), the repair welds must comply with interpass temperature requirements to prevent σ-phase formation in the repair weld and adjacent bonded cladding.
- Multi-pass edge welds: The perimeter of hydraulically bonded clad plate often requires a fusion weld to seal the edges and prevent corrosion ingress. These edge welds, typically deposited with TIG using duplex-compatible filler metal, must maintain ≤ 150°C interpass temperature.
- Thermal effects on bonded interface: Subsequent welding operations on or near the bonded joint (e.g., tack welds, attachment welds, or structural welds) can heat the bonded interface. If the temperature at the bonded interface exceeds 150°C, there is a risk of σ-phase formation in the cladding layer, degrading the bond quality and corrosion resistance. Thermal management during post-bonding operations is therefore essential.
- Clad plate welding for fabrication: When hydraulically bonded clad plates are cut and welded into final components (e.g., pressure vessels, heat exchangers), all welds through the cladding layer must comply with interpass temperature requirements. This includes butt welds, fillet welds, and attachment welds that penetrate through the duplex cladding.
7.3 Explosion Welding Applications
In traditional explosion welding (air-gap explosive welding), the interpass temperature control capability is applied in post-welding operations and in the qualification of explosion-welded clad products for subsequent fabrication:
- Explosion-welded clad plate fabrication: After explosion welding produces the bonded clad plate (e.g., 2205/SAE 1010 or 2507/SAE 1010), the plate is typically cut and welded into final components. All fabrication welds must maintain interpass temperature ≤ 150°C when welding through or adjacent to the duplex cladding layer.
- Explosion-welded clad pipe repair: When explosion-welded clad pipe requires in-service repair, the repair welds must comply with interpass temperature requirements to prevent degradation of the existing cladding.
- Explosion welding parameter optimization: While explosion welding itself does not involve "interpass" temperatures in the traditional welding sense, the thermal effects of the explosion event on the bonded interface must be managed. The peak temperature at the bonding interface during explosion welding is limited by the physics of the process (typically < 400°C for optimized parameters), but the cooling rate and residual thermal stresses must be considered in the overall thermal management strategy.
- Multi-layer explosion welding: In rare cases where multi-layer cladding is achieved through sequential explosion welding operations, the interpass temperature between explosion events must be controlled to prevent thermal degradation of the previously bonded layer. The ≤ 150°C requirement applies to the time between successive explosion events.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The interpass temperature control capability is integral to the company's welding procedure qualification program. Every WPS in the company's qualification library for 2205 and 2507 applications includes:
- A specified maximum interpass temperature of ≤ 150°C, verified during PQR testing.
- Documented temperature monitoring records from the qualification weld.
- Microstructural examination results demonstrating absence of σ-phase in the qualification weld metal.
- Impact test results meeting or exceeding minimum requirements, confirming that the thermal management strategy preserves toughness.
This qualification infrastructure enables the company to bid on and execute projects requiring demonstrated capability in duplex stainless steel fabrication, providing a competitive advantage in markets where many competitors lack documented thermal management protocols for high-alloy duplex applications.
8.2 Product Delivery Quality
Rigorous interpass temperature control directly translates to higher first-pass quality rates and reduced rework. The company's quality management system incorporates interpass temperature compliance as a critical quality gate, with the following enforcement mechanisms:
- Production hold points: Welding cannot proceed to the next pass until the interpass temperature has been verified and documented. This is enforced through a combination of operator self-checks and inspector verification.
- Statistical process control: Temperature monitoring data is collected and analyzed statistically to identify trends, drift, or non-conformance patterns. This enables proactive corrective action before non-conforming products are produced.
- Operator certification: All welders performing duplex stainless steel overlay must be certified specifically for this application, with demonstrated competency in thermal management techniques. Certification includes practical assessment of interpass temperature monitoring and control.
- Equipment calibration: All pyrometers, thermocouples, and temperature monitoring equipment are calibrated on a defined schedule (typically every 6 months) to ensure measurement accuracy. Calibration records are maintained and available for customer review.
8.3 Customer Value Proposition
The company's capability in interpass temperature control for duplex stainless steels delivers specific, quantifiable value to customers:
- Reduced life-cycle cost: Products with verified microstructural integrity require fewer in-service inspections, repairs, and replacements. The company provides customers with temperature monitoring records and microstructural verification reports that support reduced inspection intervals and extended service life predictions.
- Compliance assurance: In regulated industries (oil & gas, nuclear, pharmaceutical), the company's documented thermal management practices satisfy regulatory requirements and audit expectations. Customers benefit from reduced regulatory risk and faster project approvals.
- Design flexibility: By demonstrating reliable control of duplex steel thermal properties during fabrication, the company enables customers to specify duplex stainless steel in applications where other fabricators may be reluctant to commit. This expands the customer's design options and potentially reduces overall system cost.
- Warranty confidence: The company's ability to guarantee microstructural integrity through documented thermal management provides the basis for extended warranty periods on duplex stainless steel cladding products, reducing the customer's financial risk.
- Technical partnership: The company's expertise in duplex steel thermal management positions it as a technical partner rather than a simple supplier. Customers benefit from the company's ability to provide welding procedure development, thermal analysis, and microstructural consultation as part of the overall project scope.
9. Conclusion
Interpass temperature control for duplex stainless steels (≤ 150°C) is not merely a procedural requirement but a fundamental metallurgical necessity that underpins the performance and reliability of all 2205 and 2507 cladding products. The implementation of this capability across Cladding Technology Shanxi Co., Ltd.'s three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—demonstrates a comprehensive commitment to quality that extends from raw material selection through final product delivery.
The "low heat input + rapid cooling" strategy, supported by documented monitoring, qualified procedures, and certified personnel, ensures that the unique properties of duplex stainless steels are preserved throughout the fabrication process. This capability is mandatory for 2205 and 2507 applications and serves as a cornerstone of the company's ability to deliver specification-critical products for the most demanding industrial environments. As the global demand for duplex stainless steel continues to grow—driven by the oil & gas industry's shift toward higher-pressure, higher-temperature, and more corrosive service conditions—the value of this capability in ensuring product quality, regulatory compliance, and customer confidence will only increase.