Single-Layer Weld Overlay on the Inner Wall of Hydrogenation Equipment: Technical Analysis and Qualification Framework
1. Definition and Technical Principles
Single-layer weld overlay on the inner wall of hydrogenation equipment refers to the deposition of one continuous layer of a hydrogen-resistant or corrosion-resistant alloy onto the inner surface of a pressure vessel, reactor, or heat exchanger designed for hydrogenation service. Unlike multi-layer overlay schemes that typically involve a transition layer followed by a work layer, the single-layer approach consolidates all protective functionality into one weld pass or a single strategic layer, demanding superior metallurgical compatibility, dilution control, and residual stress management.
The technical principle is rooted in the need to create a metallurgically sound barrier between the base material (typically low-alloy or Cr-Mo steel such as 1.25Cr-0.5Mo, 2.25Cr-1Mo, or 9Cr-1Mo) and the aggressive high-temperature hydrogen environment. The overlay alloy—commonly a high-nickel austenitic stainless steel (e.g., 309L, 310L), a Ni-Cr-Mo alloy (e.g., Alloy 6, Alloy 625), or a duplex stainless steel—must exhibit:
- Resistance to High-Temperature Hydrogen Attack (HTHA): The overlay material must maintain microstructural stability under sustained exposure to high hydrogen partial pressure (typically 1.0–7.0 MPa H₂) at elevated temperatures (350–500°C), preventing methane bubble formation and intergranular cracking in the base material.
- Resistance to Hydrogen Embrittlement: The overlay must not be susceptible to hydrogen-induced cracking (HIC) or sulfide stress cracking (SSC) under cyclic loading conditions typical of startup/shutdown operations.
- Mechanical Compatibility: The overlay must maintain adequate ductility and toughness at operating temperature while resisting thermal fatigue from repeated thermal cycling.
- Low Dilution with Base Metal: Since a single layer provides no buffer, the dilution ratio between the overlay and the base material must be tightly controlled to ensure the overlay's protective composition is not compromised.
The single-layer approach is particularly relevant for equipment where the base material already provides adequate structural strength and moderate HTHA resistance (e.g., 2.25Cr-1Mo per API 941), and the overlay serves as a supplementary barrier against localized corrosion, erosion-corrosion, or specific chemical attack (e.g., from hydrotreating catalyst fines, sulfur compounds, or chlorides).
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s portfolio, single-layer inner-wall weld overlay for hydrogenation equipment occupies a specialized niche that bridges structural pressure equipment fabrication and surface engineering. Its business positioning is defined by the following characteristics:
- Technical Route: Primarily executed via TIG (GTAW) or pulsed-arc TIG welding for precision control, with MIG (GMAW) as a supplementary option for thicker overlay requirements or larger diameters where productivity demands are higher.
- Equipment Class: Targets high-pressure hydrogenation reactors, hydrogenation reactors (加氢反应器), hydrotreater vessels, hydrocracker reactors, and associated heat exchangers operating under ASME Section VIII or GB/T 150 pressure vessel codes.
- Market Segment: Serves the petroleum refining, petrochemical, and coal-to-chemicals industries, particularly projects involving FCC hydrotreating, distillate hydrotreating, gas oil hydrotreating, and hydrocracking units.
- Value Proposition: Provides a cost-effective alternative to full cladding (e.g., explosion welding or hydraulic explosive bonding) for applications where only the inner wall requires protection, reducing material cost while maintaining code compliance and service integrity.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The single-layer weld overlay on hydrogenation equipment inner walls is designed to achieve the following objectives simultaneously:
- HTHA Mitigation: Extend the service life of Cr-Mo steel equipment beyond the limits defined by API 941 by introducing a hydrogen-impermeable or hydrogen-resistant surface layer that reduces hydrogen ingress into the base metal.
- Corrosion Protection: Protect against acidic corrosion from H₂S, NH₃, HCN, and other sulfurous/nitrogenous compounds generated during hydroprocessing, as well as chloride pitting and erosion-corrosion from catalyst fines.
- Residual Stress Reduction: A properly designed single-layer overlay can be used to introduce compressive residual stresses on the inner surface, improving fatigue life and resistance to stress corrosion cracking (SCC).
- Dimensional Restoration: In repair and maintenance scenarios, the overlay restores worn or eroded inner surfaces to original dimensions while simultaneously providing renewed protection.
3.2 Value to Product Delivery and Customer
- Cost Optimization: Single-layer overlay uses 40–60% less overlay material than multi-layer schemes, directly reducing material costs for high-alloy consumables (e.g., Alloy 625 wire at $80–120/kg).
- Schedule Efficiency: Reduced welding passes translate to shorter welding times, fewer interpass inspections, and faster turnaround for equipment fabrication or repair.
- Qualification Leverage: A qualified single-layer WPS (Welding Procedure Specification) for hydrogenation service can be extended to multiple base material and overlay material combinations, building a reusable qualification database.
- Customer Assurance: Provides documented, code-compliant protection for critical hydrogenation assets where failure consequences are severe (high-pressure hydrogen release, environmental damage, production shutdown).
4. Key Process and Implementation Points
4.1 Base Material Preparation
Surface preparation is critical for single-layer overlay because there is no subsequent layer to mask surface defects. The inner wall must undergo the following preparation sequence:
- Machining: The inner surface must be machined to a minimum Ra of 3.2 μm (125 μin) to ensure uniform heat input distribution. Any existing weld seams, repair patches, or surface discontinuities must be ground flush or identified for overlay inclusion.
- Cleaning: Solvent degreasing (acetone or equivalent) followed by mechanical grinding to a bright metal finish within a 25 mm band around the overlay zone. No oil, rust, scale, or coolant residue may remain.
- Preheating: Base material preheat temperatures must be controlled per the WPS and applicable code (typically 150–300°C for Cr-Mo steels per ASME IX or GB/T 3375). Preheat must be verified with calibrated thermocouples at the weld line and a minimum of 50 mm away.
- Heat-Affected Zone (HAZ) Management: For Cr-Mo base materials, the HAZ must be identified and, where applicable, post-weld heat treated (PWHT) after overlay to relieve residual stresses and temper the HAZ microstructure.
4.2 Welding Process Parameters
The following table summarizes typical TIG weld overlay parameters for single-layer application on hydrogenation equipment inner walls:
| Parameter | Typical Range (TIG) | Notes |
|---|---|---|
| Welding Current | 80–200 A | Depends on overlay thickness and wire diameter; pulsed mode preferred for dilution control |
| Pulse Frequency | 50–150 Hz | Higher frequency reduces spatter and improves bead profile |
| Pulse Current | 120–250 A | Controls penetration and dilution |
| Background Current | 20–60 A | Maintains arc stability between pulses |
| Wire Feed Speed (if MIG) | 2.0–4.5 m/min | Calibrated to achieve target dilution ratio |
| Shielding Gas | 100% Ar or 98% Ar + 2% O₂ | Argon for Ni-based and austenitic SS; trace O₂ for improved wetting on Cr-Mo |
| Gas Flow Rate | 15–25 L/min | Back-of-bead purging required for full-penetration overlay on thick walls |
| Interpass Temperature | ≤ 300°C (Cr-Mo base) | Must not exceed material-specific limits to avoid HAZ embrittlement |
| Travel Speed | 30–80 mm/min | Slower speeds increase dilution; must be balanced with heat input |
| Heat Input | 0.8–2.5 kJ/mm | Higher heat input increases dilution; single-layer requires careful control |
| Overlay Thickness | 3–6 mm (typical) | Minimum 3 mm for HTHA service; may be reduced to 2 mm for corrosion-only protection |
4.3 Dilution Control and Metallurgical Considerations
Dilution is the single most critical variable in single-layer overlay. Because there is no subsequent layer to compensate, the overlay composition must be designed to tolerate the expected dilution range while maintaining its protective properties.
- Target Dilution Ratio: Typically 20–40% base metal dilution for austenitic stainless steel overlay on Cr-Mo steel. For Ni-based alloys on low-alloy steel, dilution may be tolerated up to 50% depending on the alloy system.
- Dilution Measurement: Each WPS qualification must include spectrographic analysis (OES or XRF) of the overlay at multiple positions (center, toe, and 1/4 thickness) to verify composition. Acceptance criteria are defined by the WPS and applicable standard (e.g., ASME IX QW-451, GB/T 12469).
- Microstructure Requirements: The overlay must be predominantly austenitic (for SS overlay) or Ni-rich austenitic/ferritic (for Ni-based overlay) with no detrimental delta-ferrite, carbide precipitation, or intermetallic phases. A ferrite content of 5–15% is acceptable for duplex overlay but must be controlled to avoid 475°C embrittlement.
- Crack Sensitivity: Single-layer overlay is susceptible to hot cracking (solidification cracking) due to the high dilution and the presence of sulfur/phosphorus segregants from the base metal. Mitigation strategies include:
- Using low-sulfur, low-phosphorus filler metals (S ≤ 0.015%, P ≤ 0.025%)
- Incorporating grain refiners (Nb, Ti, Zr) in the filler metal
- Employing pulsed-arc TIG to reduce peak temperature and promote equiaxed grain growth
- Using a backing layer of compatible material to reduce dilution during the first pass
4.4 Weld Sequence and Thermal Management
For large-diameter hydrogenation reactors (typically 2.0–4.0 m ID), the single-layer overlay must be applied in a controlled sequence to manage cumulative thermal distortion and residual stress:
- Segmentation: Divide the inner circumference into equal segments (typically 8–16 segments) and weld in a balanced, opposing sequence to minimize barrel distortion.
- Heat Input Distribution: Alternate between diametrically opposite segments to ensure uniform thermal expansion and contraction.
- Temperature Monitoring: Continuously monitor the interpass temperature at multiple points around the circumference; do not exceed the WPS-specified limit.
- Stress Relief: After overlay completion, apply PWHT per ASME VIII Div. 1 UW-40 or Div. 2 Part 5. The overlay must be included in the PWHT cycle or protected (e.g., with insulating blankets) if the overlay alloy is not heat-treatable.
4.5 Post-Overlay Processing
- Visual Inspection: 100% visual inspection per ASME IX QW-191 or GB/T 3375 for undercut, porosity, lack of fusion, and profile irregularities.
- Dimensional Verification: Measure overlay thickness at intervals not exceeding 300 mm along the weld length and at each segment boundary using ultrasonic thickness measurement (UT) or magnetic thickness gauge (for ferromagnetic base).
- Surface Finish: Final machining or grinding to achieve the specified surface roughness (typically Ra ≤ 1.6 μm for catalytic service, Ra ≤ 3.2 μm for general service).
- Post-Weld Heat Treatment: Where required by the base material specification or design code, apply PWHT at the specified temperature and soak time. For 2.25Cr-1Mo, a typical PWHT is 700–750°C for 1 h per 25 mm of thickness.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Applicability | Key Requirements |
|---|---|---|
| ASME BPV Section VIII Div. 1 | Design and construction of hydrogenation pressure vessels | Welding per Section IX; overlay acceptance per UW-40 |
| ASME BPV Section VIII Div. 2 | Alternative design rules for hydrogenation equipment | Fracture mechanics-based acceptance; overlay qualification per Part 5 |
| ASME Section IX | Welding procedure qualification | WPS/PQR qualification per QW-451 (overlay); essential variables for dilution control |
| API 941 | HTHA resistance of Cr-Mo steels | Defines maximum temperature-hydrogen partial pressure limits; overlay may extend service envelope |
| NACE MR0175/ISO 15156 | Sour service materials | HIC/SSC resistance requirements for overlay in sour hydrogenation service |
| GB/T 150 | Chinese pressure vessel code | Design, fabrication, and inspection of pressure vessels including overlay |
| GB/T 3375 | Welding procedure qualification | Chinese equivalent to ASME IX for WPS/PQR qualification |
| GB/T 12469 | Welding consumables for overlay | Chemical composition and mechanical property requirements for overlay filler metals |
| NB/T 47013 | NDT methods for pressure equipment | UT, RT, MT, PT acceptance criteria for overlay welds |
| ASTM A240 / A568 | Overlay material specifications | Stainless steel and Ni-alloy plate/wire composition requirements |
| ASTM B366 / B564 | Ni-based alloy specifications | Alloy 6, 625, 825 composition and property requirements |
5.2 Acceptance Criteria
- Visual (VT): No undercut deeper than 0.5 mm or 10% of overlay thickness (whichever is less); no surface porosity, cracks, or lack of fusion. Acceptance per ASME IX QW-191 or NB/T 47013.2.
- Ultrasonic Testing (UT): 100% UT per NB/T 47013.3 or ASME V Article 4 for indication of lack of fusion, cracks, and volumetric defects. Acceptance: no indication exceeding the specified size for the applicable code (typically no indication > 2 mm length for critical service).
- Positive Material Identification (PMI): 100% OES or XRF verification of overlay composition at not less than 3 points per 1000 mm of weld length. Acceptance: composition within the WPS-specified range and dilution within ±10% of the qualified value.
- Hardness Testing: Hardness survey of the overlay and HAZ per ASME IX QW-451. Acceptance: overlay hardness within the specified range (typically 200–350 HV for austenitic SS, 250–400 HV for Ni-based alloys); HAZ hardness not exceeding the base material's PWHT hardness by more than 40 HV.
- Macrographic Examination: Cross-sectional macrograph of the overlay showing full penetration, no slag inclusions, and uniform dilution distribution. Acceptance: no lack of fusion, cracks, or excessive dilution zones.
- Micrographic Examination: Microstructure examination of the overlay and the overlay-base metal interface. Acceptance: no intergranular cracking, no excessive carbide precipitation at the interface, and no delta-ferrite stringers in austenitic overlay.
6. Common Risks and Controls
| Risk | Description | Control Measures |
|---|---|---|
| Excessive Dilution | High base metal dilution compromises overlay composition, reducing HTHA and corrosion resistance | Use pulsed-arc TIG; limit heat input; use low-dilution filler metals (e.g., 309L with higher Ni); verify dilution by PMI at qualification |
| Hot Cracking | Solidification cracking in the overlay due to sulfur/phosphorus segregation and high thermal gradient | Use low-S, low-P filler metals; add grain refiners; use pulsed-arc mode; preheat and control interpass temperature |
| Cold Cracking (HIC) | Hydrogen-induced cracking in the HAZ of Cr-Mo base material due to hydrogen pickup during welding | Use low-hydrogen filler metals (diffusible H ≤ 5 mL/100g); apply post-weld bake-out at 150–250°C for 2–4 h; minimize arc time |
| Undercut | Excessive undercut at the overlay toe reduces effective overlay thickness and creates stress concentration | Optimize welding parameters; use proper torch angle (10–15° from vertical); maintain consistent travel speed; inspect and repair per code |
| Residual Stress Exceedance | High residual tensile stress in the overlay promotes SCC and fatigue cracking | Apply PWHT after overlay; use balanced welding sequence; consider shot peening or low-plasticity burn-off to introduce compressive stress |
| Thermal Distortion | Uneven heat input causes barrel distortion of the vessel shell, affecting dimensional accuracy | Use balanced opposing weld sequence; monitor temperature distribution; apply mechanical constraints (jigs, clamps) where feasible |
| Overlay Thinning | Post-overlay machining removes more material than expected, reducing effective overlay thickness below minimum | Overbuild overlay by 1–2 mm above the final machined dimension; verify thickness after machining; include machining allowance in WPS |
| PWHT Degradation | PWHT cycle causes carbide precipitation, sigma phase formation, or intergranular cracking in the overlay | Verify overlay alloy's PWHT compatibility; limit PWHT temperature and soak time; use overlay alloys designed for high-temperature service (e.g., Alloy 625 instead of 309L for high-temperature PWHT) |
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route)
Single-layer inner-wall weld overlay is the primary application of this technology. TIG welding is preferred for the following reasons:
- Precision Control: TIG provides superior control over heat input and dilution, critical for single-layer overlay where there is no margin for error.
- Surface Quality: TIG produces smooth, uniform beads with minimal spatter, reducing post-weld machining requirements.
- Material Compatibility: TIG can be used with a wide range of filler metals, including Ni-based alloys, duplex stainless steels, and high-silicon austenitics, without the wire-feed variability associated with MIG.
- Code Acceptance: TIG is universally accepted by all major codes (ASME, GB, NB, ISO) for overlay applications on critical equipment.
MIG welding is employed as a supplementary or alternative route for the following scenarios:
- Large Diameter Vessels: Where the inner diameter exceeds 3.0 m and TIG productivity is insufficient, MIG (particularly pulsed MIG or CMT) can be used to increase deposition rate while maintaining dilution control.
- Repair Applications: For field repair of worn or eroded hydrogenation reactor internals, MIG provides faster deposition and easier setup than TIG.
- Thicker Overlay Requirements: When overlay thickness exceeds 6 mm, MIG can deposit material more efficiently than TIG while maintaining acceptable dilution through parameter optimization.
7.2 Hydraulic Explosive Bonding (Complementary Route)
While hydraulic explosive bonding (HEB) is typically used for full cladding of large-diameter vessels, it can complement single-layer weld overlay in the following scenarios:
- Hybrid Cladding: A vessel may receive HEB cladding on the outer wall for structural reinforcement and single-layer TIG overlay on the inner wall for corrosion resistance, combining the benefits of both routes.
- Repair of HEB Clad Vessels: When HEB cladding is damaged or worn, single-layer TIG overlay can be used to repair localized areas, providing a cost-effective alternative to full re-cladding.
- Transition Zones: In vessels where HEB cladding does not extend to certain areas (e.g., nozzle transitions, manway areas), single-layer TIG overlay can provide continuous protection across the entire inner surface.
7.3 Explosion Welding (Complementary Route)
Explosion welding (EW) is primarily used for producing clad plates, which can then be fabricated into hydrogenation equipment components. The relationship with single-layer weld overlay is as follows:
- Pre-Fabricated Clad Components: Explosion-welded clad plates (e.g., 2.25Cr-1Mo / 309L) can be fabricated into reactor shells, and the single-layer TIG overlay is applied as a supplementary protection layer on areas not covered by the clad plate (e.g., weld seams, machined surfaces, repair areas).
- Overlay on EW Clad Surfaces: When explosion-welded clad surfaces require additional protection (e.g., for high-severity service), a single-layer Ni-based alloy overlay (e.g., Alloy 625) can be applied on top of the EW cladding to provide enhanced HTHA resistance.
- Qualification Synergy: Qualification data from single-layer TIG overlay on Cr-Mo steel can be extended to overlay on explosion-welded clad surfaces, reducing the number of separate WPS qualifications required.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The single-layer inner-wall weld overlay trial research for hydrogenation equipment directly contributes to the company's qualification database in the following ways:
- WPS/PQR Development: Each trial produces a qualified WPS/PQR pair for a specific base material / overlay material / welding process combination. These can be extended to similar combinations per ASME IX QW-451 essential variable rules, building a comprehensive qualification matrix.
- Material Qualification: Trial data on dilution, microstructure, hardness, and mechanical properties of the overlay provides material qualification data that can be referenced in future projects, reducing the need for repeat qualification testing.
- NDT Procedure Qualification: The trial includes NDT procedure development and qualification (UT, MT, PT) for overlay welds, establishing repeatable inspection procedures that can be applied to production work.
- Code Compliance Documentation: The trial produces a complete documentation package (WPS, PQR, NDT reports, PMI reports, macro/micrographs, hardness surveys) that demonstrates code compliance and can be submitted to customers and third-party inspectors.
8.2 Product Delivery Value
- Accelerated Project Execution: With qualified WPS/PQR pairs in place, production welding can begin immediately without the delay of qualification testing, reducing project timelines by 4–8 weeks per project.
- Reduced Non-Conformance: Trial data identifies the process window boundaries (heat input limits, dilution limits, interpass temperature limits) that prevent non-conformances during production, reducing rework costs by 30–50%.
- Customer Confidence: A qualified and documented single-layer overlay process provides customers with the assurance that the overlay will perform as specified, reducing the risk of warranty claims and enhancing the company's reputation.
8.3 Customer Value
- Extended Asset Life: Single-layer overlay on hydrogenation equipment can extend the service life of the vessel by 5–10 years by mitigating HTHA and corrosion, providing significant ROI for the customer.
- Reduced Maintenance Cost: A well-qualified overlay reduces the frequency of inspection and repair interventions, lowering the customer's lifetime maintenance cost by 20–40%.
- Regulatory Compliance: The overlay meets the requirements of API 941, NACE MR0175, and applicable pressure vessel codes, ensuring the customer's equipment complies with regulatory requirements and reducing the risk of regulatory penalties.
- Technical Partnership: The trial research demonstrates the company's technical expertise and commitment to quality, positioning the company as a preferred partner for critical hydrogenation equipment projects.
9. Summary and Recommendations
Single-layer weld overlay on the inner wall of hydrogenation equipment is a technically demanding but highly valuable application that requires precise control of dilution, residual stress, and metallurgical compatibility. The trial research described in this entry provides the foundational qualification data and process knowledge necessary to execute this technology reliably in production.
Key recommendations for future development:
- Expand the qualification matrix to include additional base material / overlay material combinations (e.g., 9Cr-1Mo / Alloy 625, 1.25Cr-0.5Mo / Alloy 825) to cover a wider range of hydrogenation service conditions.
- Investigate pulsed-arc TIG and CMT (Cold Metal Transfer) welding as advanced processes that offer superior dilution control and surface quality for single-layer overlay.
- Develop a predictive model for dilution as a function of welding parameters, base material thickness, and filler metal composition, to enable rapid WPS optimization without extensive trial welding.
- Establish a long-term performance tracking program to monitor the in-service performance of single-layer overlay on hydrogenation equipment, providing feedback for process improvement and qualification renewal.
- Integrate single-layer overlay qualification with the company's HEB and EW qualification databases to create a comprehensive surface engineering qualification system that covers all technology routes.
Note: All welding procedures, NDT methods, and acceptance criteria described in this analysis must be applied in accordance with the current editions of the referenced standards and the specific requirements of the applicable project specification. The parameters provided are typical values and must be verified through qualification testing for each specific application.