Nickel-Based Alloy 625 Weld Overlay Procedure for Tube Sheets
1. Definition and Fundamental Principles
Nickel-based Alloy 625 (UNS N06625, WNR 2.4856) is a precipitation-strengthened austenitic nickel-chromium-molybdenum superalloy renowned for its exceptional resistance to corrosion, oxidation, and stress-corrosion cracking across a wide range of chemical environments. When applied as a weld overlay on tube sheets, Alloy 625 serves as a sacrificial protective layer that shields the base carbon or low-alloy steel from aggressive process media while maintaining mechanical integrity under thermal cycling and pressure loading.
The fundamental principle of tube sheet weld overlay involves depositing one or more layers of Alloy 625 onto the face of a carbon steel or low-alloy steel tube sheet using qualified welding processes—typically Gas Tungsten Arc Welding (GTAW/TIG) for the critical first layer and potentially Gas Metal Arc Welding (GMAW/MIG) for subsequent buildup layers. The metallurgical bonding between the dissimilar materials is achieved through controlled dilution management, ensuring the interface retains sufficient Alloy 625 content (typically ≥80% by chemical composition at the first layer) while avoiding excessive dilution from the base metal that would compromise corrosion resistance.
The precipitation hardening mechanism in Alloy 625 relies on the formation of Ni₃(Nb,Ti) gamma-prime (γ') and Ni₄(Nb,Ti) mu (μ) phases during solution treatment (typically at 980–1040°C for 1 hour followed by air cooling). This microstructural feature provides the overlay with high tensile strength (≥700 MPa) and yield strength (≥550 MPa) even at elevated temperatures, making it uniquely suited for tube sheet applications where mechanical strength and corrosion resistance must coexist.
2. Category and Business Positioning
Within the company's technology portfolio, the Alloy 625 tube sheet weld overlay procedure falls squarely within the TIG/MIG Weld Overlay technology route. This positioning is critical for several reasons:
- Process Category: The procedure qualifies as a critical welding qualification under ASME Section IX (QW-451 for weld overlay qualification) and NB/T 47014, representing the highest complexity tier within the company's weld overlay capabilities.
- Product Line Alignment: This procedure directly supports the manufacturing of high-integrity heat exchangers, reactor internals, distillation column internals, and pressure vessels operating in highly corrosive service (H₂S, chlorides, acids, and high-temperature oxidizing environments).
- Competitive Differentiation: Mastery of Alloy 625 tube sheet overlay positions the company to serve the oil & gas, petrochemical, nuclear, and power generation sectors where Alloy 625 is the material of choice for tube-to-tubesheet joints in severely corrosive service.
3. Technical Purpose and Value
The primary technical purposes of Alloy 625 weld overlay on tube sheets are:
- Corrosion Barrier Creation: Establishing a continuous, crack-free, and fully dense nickel-based alloy layer that isolates the base steel from corrosive process fluids, including concentrated H₂S environments (NACE MR0175/ISO 15156 compliance), hydrochloric acid solutions, and high-temperature chloride-containing media.
- Stress-Corrosion Cracking (SCC) Resistance: Providing immunity to chloride-induced SCC, a failure mode that is catastrophic in carbon steel tube sheets exposed to aqueous chloride environments.
- Thermal Fatigue Mitigation: Alloy 625's superior thermal fatigue resistance compared to austenitic stainless steels (304L, 316L) extends the service life of tube sheets in cyclic thermal duty applications such as fired heaters and heat recovery units.
- Weldability to Dissimilar Base Metals: Alloy 625 serves as an excellent transition filler metal between carbon steel base plates and austenitic/nickel alloy tube materials (Inconel 625, Hastelloy C-276, Monel 400), reducing residual stress and cracking susceptibility at the tube-to-tubesheet weld joint.
The business value is realized through reduced maintenance intervals, extended asset life (typically 3–5× the life of unprotected carbon steel tube sheets), and elimination of unplanned shutdowns due to tube sheet corrosion failure.
4. Key Process and Implementation Points
4.1 Base Metal Preparation
Surface preparation is the foundation of a successful Alloy 625 overlay. The tube sheet face must be machined to a minimum Ra 6.3 μm surface finish, free of scale, oxide, oil, and other contaminants. The recommended preparation sequence is:
- Mechanical grinding with P80–P120 grit followed by P240–P320 grit finishing
- Acetone or solvent cleaning immediately prior to welding (within 2 hours)
- Preheating to 150–250°C for carbon steel base plates (ASTM A105, A216 WCB, A350 LF2) to reduce hydrogen-induced cracking susceptibility
- For low-alloy steels (ASTM A387 Gr. 11, A387 Gr. 22), preheat to 200–300°C with interpass temperature control at ≤300°C
4.2 Welding Process Parameters
The following table summarizes the recommended parameters for single-layer and multi-layer Alloy 625 weld overlay on carbon steel tube sheets:
| Parameter | First Layer (GTAW/TIG) | Subsequent Layers (GTAW/TIG) | Buildup Layers (GMAW/MIG) |
|---|---|---|---|
| Process | GTAW (QW-101) | GTAW (QW-101) | GMAW (QW-111) |
| Filler Metal | ERNiCrMo-3 (AWS A5.11) | ERNiCrMo-3 (AWS A5.11) | ERNiCrMo-3 (AWS A5.16) |
| Filler Diameter | 1.6 mm / 2.4 mm | 2.4 mm / 3.2 mm | 1.2 mm / 1.6 mm |
| Shielding Gas | Argon 99.99% (5–10 L/min) | Argon 99.99% (5–10 L/min) | Argon 99.99% (10–20 L/min) |
| Current (DCEN) | 80–120 A | 120–180 A | 150–250 A |
| Voltage | 10–14 V | 12–16 V | 18–24 V |
| Travel Speed | 50–80 mm/min | 70–120 mm/min | 150–250 mm/min |
| Deposition Rate | 0.3–0.5 kg/h | 0.5–1.0 kg/h | 1.5–3.0 kg/h |
| Interpass Temperature | ≤150°C | ≤200°C | ≤200°C |
| Weld Leg Angle | 10–15° | 10–15° | 15–20° |
| Weld Stringer Angle | 10–15° | 10–15° | 15–20° |
4.3 Multi-Layer Buildup Strategy
The overlay thickness requirement is typically 1.5–3.0 mm for standard service and 3.0–5.0 mm for severe service conditions. The recommended layering strategy is:
- Layer 1 (Critical Bond Layer): GTAW with ERNiCrMo-3, minimum 0.8 mm deposited thickness, bead width 6–8 mm. This layer must achieve ≥80% Alloy 625 composition by spectrographic analysis to ensure adequate corrosion resistance at the base metal interface.
- Layer 2 (Transition Layer): GTAW with ERNiCrMo-3, 0.8–1.2 mm deposited thickness. Composition typically achieves 85–95% Alloy 625 content with reduced dilution effects.
- Layer 3+ (Buildup Layers): GMAW with ERNiCrMo-3 wire, 1.0–1.5 mm per layer. These layers achieve ≥95% Alloy 625 composition and provide the required final overlay thickness.
- Final Layer: GTAW with ERNiCrMo-3, 0.5–0.8 mm, providing a smooth, uniform surface suitable for machining or direct service.
4.4 Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) of Alloy 625 overlay is generally not recommended at temperatures exceeding 425°C, as this can cause sigma phase precipitation and intergranular carbide formation that severely degrades corrosion resistance. For tube sheets requiring stress relief of the base plate, the following approach is applied:
- If base plate requires PWHT above 425°C (e.g., 595°C for A387 Gr. 11), the overlay must be applied after PWHT of the base plate.
- If overlay must be applied before PWHT, the PWHT temperature must be limited to ≤425°C with a maximum soak time of 2 hours, followed by furnace cooling to ≤100°C.
- Post-overlay solution treatment (980–1040°C, 1 hour, air cool) may be applied to the entire component if distortion tolerances permit, to restore full precipitation hardening response and optimize corrosion resistance.
4.5 Key Implementation Controls
| Control Parameter | Specification | Verification Method | Frequency |
|---|---|---|---|
| Base metal cleanliness | No visible contamination, Ra ≤6.3 μm | Visual + Surface roughness gauge | Each tube sheet |
| Preheat temperature | 150–250°C (CS), 200–300°C (LAS) | Thermocouple / IR pyrometer | Each tube sheet |
| Interpass temperature | ≤150°C (Layer 1), ≤200°C (subsequent) | IR pyrometer | Each pass |
| First layer dilution | ≤20% base metal dilution | OES spectroscopy | Each weld coupon |
| Overlay thickness | ≥1.5 mm (std), ≥3.0 mm (severe) | Ultrasonic thickness gauge | Each tube sheet (grid pattern) |
| Filler metal storage | 20–60°C, RH ≤40%, in original packaging | Environmental monitoring log | Continuous |
| Shielding gas purity | Argon ≥99.99% (4N) | Oxygen analyzer | Each shift |
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- ASME BPV Code Section II, Part D: Material specification for Alloy 625 (UNS N06625 / WNR 2.4856)
- ASME BPV Code Section IX: Welding procedure qualification per QW-451 (Weld Overlay Qualification), QW-101 (GTAW), QW-111 (GMAW)
- ASME BPV Code Section V: NDT acceptance per Article 2 (RT), Article 4 (MT), Article 5 (PT), Article 7 (UT), Article 23 (ET)
- ASME BPV Code Section VIII, Div. 1 and 2: Design and construction requirements for pressure vessels
- NB/T 47014-2011: Chinese national standard for welding procedure qualification
- GB/T 1954-2019: Nickel and nickel alloy castings and wrought products
- ASTM A240/A240M: Chromium and chromium-nickel stainless steel plate (for base material reference)
- AWS A5.11/A5.11M: Specification for nickel and nickel alloy welding electrodes and rods (ERNiCrMo-3)
- AWS A5.16/A5.16M: Specification for nickel and nickel alloy welding wires (ERNiCrMo-3)
- ISO 15156-3: Materials resistant to H₂S in petroleum and natural gas industries
- NACE MR0175/ISO 15156: Materials for use in H₂S-containing environments
- EN 13445: Unfired pressure vessels (European code reference)
- ASME PT-34: Examination of weld overlay deposits
5.2 Acceptance Criteria
| Inspection Method | Acceptance Criteria | Reference Standard | Inspection Coverage |
|---|---|---|---|
| Visual Testing (VT) | No cracks, undercut, porosity >0.5 mm, incomplete fusion visible. Surface smooth, uniform color. | ASME Sec. V Art. 9 / ISO 17637 | 100% of overlay surface |
| Penetrant Testing (PT) | No linear indications (cracks, laps, inclusions). Round indications ≤3 mm acceptable if isolated. | ASME Sec. V Art. 7 / ISO 3452 | 100% of overlay surface |
| Magnetic Particle Testing (MT) | Not applicable to Alloy 625 overlay (non-ferromagnetic). Applied to base metal interface if accessible. | ASME Sec. V Art. 4 | N/A to overlay |
| Ultrasonic Testing (UT) | No indications exceeding acceptance limits. Overlay thickness verified per grid pattern. | ASME Sec. V Art. 7 / ISO 17640 | 10% minimum, 100% for critical service |
| Hardness Testing | Overlay: 180–320 HV (as-welded), 250–380 HV (solution treated). Base metal per material spec. | ASME Sec. V Art. 20 / ASTM E10 | Grid pattern per surface area |
| Chemical Analysis (OES) | Layer 1: Ni ≥60%, Cr ≥20%, Mo ≥8%, Nb+Ta ≥3.5%, C ≤0.10%, Fe ≤10% | AWS A5.11 / ASTM E1473 | Each weld coupon + witness coupons |
| Dye Penetrant (Corrosion) | Overlay passes standard immersion test (e.g., 10% HCl, 60°C, 24h) without intergranular cracking. | ASTM A262 Practice E / NACE TM0172 | Weld coupons |
5.3 Mechanical Test Requirements for WPS Qualification
- Macrographic Examination: No cracks, incomplete fusion, or excessive porosity in cross-section. Layer boundaries clearly visible with no segregation.
- Chemical Composition: OES analysis of each layer to verify dilution control and Alloy 625 content.
- Hardness Survey: Hardness mapping across the overlay-to-base metal interface to identify the dilution zone and verify hardness values are within specification.
- Corrosion Testing: Salt spray test (ASTM B117, 1000 hours minimum) or acid immersion test demonstrating overlay integrity. Intergranular corrosion test per ASTM A262 Practice E.
- Stress Corrosion Cracking Test: For H₂S service qualification, testing per NACE TM0172 (ASTM G150) demonstrating resistance to sulfide stress cracking.
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Hot cracking in overlay | Excessive sulfur/phosphorus, inadequate dilution control, high travel speed | Overlay failure, loss of corrosion protection | Use low-S, low-P filler metal; maintain proper current/travel speed ratio; ensure adequate layer overlap (≥50%) |
| Excessive dilution | High heat input, wide bead width, insufficient first-layer thickness | Reduced corrosion resistance, potential SCC susceptibility | Control heat input ≤15 kJ/mm for Layer 1; narrow bead width (6–8 mm); verify by OES after Layer 1 |
| Sigma phase formation | PWHT above 425°C, prolonged exposure at 600–900°C | Severe loss of corrosion resistance, embrittlement | Limit PWHT temperature to ≤425°C; apply overlay after base plate PWHT when possible |
| Intergranular carbide precipitation | Carbon pickup from base metal, sensitization temperature range (600–900°C) exposure | Intergranular corrosion, reduced toughness | Use low-carbon filler (C ≤0.08%); avoid sensitization temperature range; solution treat if required |
| Hydrogen-induced cracking (HIC) | Absorption of hydrogen from moisture, high restraint, inadequate preheat | Delayed cracking in base metal near overlay | Preheat to 150–250°C; use dry shielding gas; bake filler metal per manufacturer spec; post-weld bake if required |
6.2 Process Risks
- Tungsten contamination: In GTAW, tungsten electrode contamination with carbon steel base metal introduces iron into the weld pool, reducing Alloy 625 content. Control: Use dedicated tungsten electrodes for Alloy 625 welding; grind or replace tungsten if contamination is suspected; use tungsten with 2% thorium oxide (WTh-2) or lanthanum oxide (WLa-1.5) for improved arc stability.
- Porosity from moisture: Hydrogen porosity from atmospheric moisture or contaminated filler metal. Control: Store filler metal in desiccated container; use argon with dew point ≤-60°C; ensure proper gas coverage with trailing gas cup for GTAW.
- Undercut and incomplete fusion: Insufficient heat input or poor technique at layer boundaries. Control: Maintain consistent travel speed; use proper torch angle; ensure ≥50% bead overlap; perform VT of each layer before proceeding.
- Distortion of tube sheet: Thermal distortion from sequential weld passes on large-diameter tube sheets. Control: Use symmetric welding sequence; employ back-of-bead cooling (water cooling) where permitted; limit heat input per pass; use backing plate with thermal mass.
6.3 Inspection Risks
- Incomplete NDT coverage: Missing defects in overlay due to insufficient grid pattern or missed areas. Control: Define grid pattern per surface area (e.g., 100 mm × 100 mm grid for 10% UT coverage); use calibrated UT equipment with Alloy 625 reference blocks.
- False acceptance of dilution: Inadequate spectrographic sampling missing the critical interface zone. Control: Sample Layer 1 at multiple points including the toe region where dilution is highest; use calibrated OES equipment with Alloy 625 reference standards.
- Non-detectable defects: Planar defects (laps, cold shuts) that may not be detected by UT alone. Control: Supplement UT with PT for 100% surface coverage; use TOFD or phased array UT for critical applications.
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
The Alloy 625 tube sheet overlay procedure is the flagship application within the company's TIG/MIG weld overlay capability. This route provides the highest precision, finest grain structure, and most reliable dilution control for critical overlay applications. Specific applications include:
- Heat Exchanger Tube Sheets: Overlay of Alloy 625 on carbon steel (A105, A216 WCB) or low-alloy steel (A387 Gr. 11, Gr. 22) tube sheets for service with Inconel 625, Hastelloy C-276, or Monel 400 tubes in chlorinated, acidic, or high-temperature oxidizing environments.
- Reactor Internals: Overlay of distribution plates, support plates, and heat exchanger tube sheets in hydrocrackers, reformers, and methanol synthesis reactors.
- Distillation Column Internals: Overlay of tray support plates and demister pads in acid gas treating units and sour water strippers.
- Offshore Platform Equipment: Overlay of heat exchanger tube sheets and pressure vessel heads for subsea production systems exposed to marine environments and H₂S-containing hydrocarbons.
7.2 Hydraulic Explosive Bonding Route
While Alloy 625 tube sheet overlay is primarily a weld overlay application, the company's hydraulic explosive bonding capability supports complementary scenarios:
- Large-Surface Cladding: For large-diameter tube sheets (≥1500 mm) where weld overlay would require excessive deposition time and carry higher distortion risk, hydraulic explosive bonding of Alloy 625 cladding plate (2–5 mm) onto the tube sheet face provides a full-surface corrosion barrier. The bonded interface achieves metallic bond strength exceeding 200 MPa (shear) with minimal dilution.
- Multi-Layer Cladding: Hydraulic explosive bonding of Alloy 625 as the outer corrosion-resistant layer onto a stainless steel (304L or 316L) intermediate layer bonded to carbon steel substrate, providing a cost-effective multi-layer cladding solution for severe service.
- Repair and Retrofit: For in-service tube sheets where weld overlay repair is not feasible due to geometry or accessibility constraints, hydraulic explosive bonding provides a non-thermal alternative for corrosion protection restoration.
7.3 Explosion Welding Route
Explosion welding (explosive cladding) extends the Alloy 625 cladding capability to specialized applications:
- High-Integrity Cladding: For tube sheets requiring the highest metallurgical bond quality with minimal interfacial diffusion, explosion welding of Alloy 625 cladding onto carbon steel provides a single-pass, high-energy bonding process with bond strength typically exceeding 300 MPa (shear) and minimal intermetallic compound formation.
- Thick Cladding Applications: Where overlay thickness requirements exceed 5 mm, explosion welding of 6–10 mm Alloy 625 cladding plate provides economic advantages over multi-layer weld overlay, with superior mechanical properties throughout the cladding thickness.
- Non-Ferrous to Ferrous Bonding: Explosion welding uniquely enables direct bonding of Alloy 625 to dissimilar base metals (including copper alloys, titanium alloys) where welding is impractical, supporting specialized heat exchanger designs.
8. Qualification Building and Customer Value
8.1 Qualification Portfolio Development
Mastery of the Alloy 625 tube sheet weld overlay procedure contributes directly to the company's qualification portfolio in the following ways:
- ASME Section IX Qualification: Each qualified WPS (Welding Procedure Specification) for Alloy 625 overlay on specific base metals (A105, A216 WCB, A387 Gr. 11, A387 Gr. 22, A350 LF2) expands the range of products the company can legally manufacture under ASME "U" stamp or "R" stamp authorization.
- NB/T 47014 Qualification: Chinese national standard qualification for Alloy 625 overlay enables participation in domestic nuclear, power, and petrochemical procurement programs requiring NB/T 47014 compliance.
- Material Qualification Matrix: Building a comprehensive matrix of qualified combinations (base metal × filler metal × process × thickness range × preheat range) provides manufacturing flexibility and reduces the need for requalification when customer specifications vary.
- Welder Qualification: Each Alloy 625 overlay WPS requires qualified welders (ASME Section IX QW-301/QW-302) with demonstrated competency in GTAW and GMAW of nickel-based alloys, building institutional knowledge and reducing personnel risk.
8.2 Customer Value Proposition
The Alloy 625 tube sheet overlay capability delivers measurable customer value through:
- Cost Reduction: Compared to using a fully Alloy 625 tube sheet (material cost 15–20× carbon steel), overlaying 1.5–3.0 mm of Alloy 625 onto a carbon steel tube sheet reduces material cost by 60–80% while providing equivalent corrosion protection at the process-exposed surface.
- Weight Reduction: For offshore and mobile applications, carbon steel tube sheets with Alloy 625 overlay weigh 3–5× less than solid Alloy 625 tube sheets, reducing structural loading and installation costs.
- Service Life Extension: Independent field data demonstrates that Alloy 625 overlay on carbon steel tube sheets in H₂S/chloride service extends service life from 2–3 years (unprotected carbon steel) to 15–20+ years, providing 5–10× improvement in time between shutdowns.
- Design Flexibility: The ability to provide qualified Alloy 625 overlay on various base steel grades allows customers to optimize design for mechanical requirements (base metal) and corrosion requirements (overlay) independently, rather than being constrained by a single material selection.
- Compliance Assurance: Full traceability from qualified WPS, qualified welders, NDE reports, and material certifications provides customers with complete documentation packages for regulatory compliance (ASME, NB/T, NACE, ISO 15156).
8.3 Product Delivery Excellence
The systematic approach to Alloy 625 tube sheet overlay—encompassing WPS qualification, welder certification, process parameter control, multi-stage NDT, and chemical verification—ensures consistent, repeatable product quality. This systematic approach enables the company to:
- Deliver tube sheets with guaranteed overlay thickness uniformity (±0.2 mm across the entire surface)
- Provide complete traceability documentation for each tube sheet (WPS, WPQ, welder ID, material certs, NDE reports, hardness maps, OES results)
- Meet tight delivery schedules through optimized multi-layer welding sequences and parallel processing of multiple tube sheets
- Minimize rework rates through in-process monitoring (interpass temperature, bead appearance, dilution verification after Layer 1) and early detection of non-conformances
9. Conclusion
The Nickel-Based Alloy 625 Weld Overlay Procedure for Tube Sheets represents a cornerstone capability within Cladding Technology Shanxi Co., Ltd's TIG/MIG weld overlay technology route. Through rigorous process control, comprehensive qualification, and systematic quality management, this procedure enables the production of high-integrity, corrosion-resistant tube sheets that serve critical applications across the oil & gas, petrochemical, power generation, and nuclear industries. The procedure's alignment with international standards (ASME, AWS, NACE, ISO) and Chinese national standards (NB/T, GB/T) ensures global market acceptance, while the integration with hydraulic explosive bonding and explosion welding capabilities provides customers with a comprehensive cladding solution portfolio addressing the full spectrum of corrosion protection requirements.