Single-Layer Stripping Electroslag Weld Overlay Technology for Hydrogenation Equipment Shell Inner Wall
1. Definition and Fundamental Principles
Single-layer stripping electroslag weld overlay (also referred to as single-pass stripping electroslag cladding) is an advanced welding overlay process in which a single pass of electroslag welding is performed along the interior surface of a pressure vessel shell, producing a metallurgically sound, corrosion- and hydrogen-resistant cladding layer. The "stripping" designation distinguishes this variant from conventional electroslag welding in that the slag pool is continuously removed and refreshed throughout the welding cycle, ensuring consistent thermal input, uniform slag composition, and superior dilution control between the base metal and the overlay consumable.
The process exploits the intense, stable thermal energy of the electroslag arc—where an electric current passes through a molten slag pool—to achieve deep, uniform penetration into the base material while simultaneously depositing a high-alloy overlay metal. The stripping mechanism ensures that the slag pool composition remains consistent by eliminating the accumulated slag that would otherwise alter the heat balance and chemical environment of the weld pool. This is critical for hydrogenation equipment, where even minor compositional variations in the cladding layer can compromise resistance to high-temperature hydrogen attack (HTHA) and sulfidation corrosion.
The thermodynamic cycle of the process involves three sequential phases within each welding pass: (1) establishment of the slag pool and initial melting of the base metal surface; (2) continuous stripping of spent slag and replenishment with fresh flux to maintain stable arc geometry and heat distribution; and (3) controlled solidification of the weld metal under the protective slag blanket. The single-layer approach means that the entire cladding thickness is achieved in one pass, requiring precise control of deposition rate, travel speed, and heat input to achieve the specified cladding thickness (typically 6–12 mm) in a single operation.
2. Category and Business Positioning
This technology falls within the weld overlay category of Cladding Technology Shanxi Co., Ltd's manufacturing portfolio, specifically under the electroslag welding sub-category. It represents a specialized extension of the company's TIG/MIG weld overlay capabilities, leveraging electroslag welding's advantages—high deposition rate, deep penetration, and excellent metallurgical homogeneity—for applications where large-diameter, thick-walled pressure vessels require internal cladding of the entire shell circumference.
In the company's three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—this process occupies the weld overlay domain but addresses a niche that conventional TIG/MIG overlay cannot economically serve: large shell diameters (typically above DN 2000) with thick base plates (40–120 mm) where multi-pass TIG overlay would be prohibitively time-consuming. The electroslag stripping variant provides a bridge between standard weld overlay and heavy-fabrication cladding, enabling the company to qualify for hydrogenation reactor projects that demand full-circumference internal cladding at production-viable rates.
Business positioning: This capability targets the oil refining, petrochemical, and hydrogen production sectors, where high-pressure hydrogenation reactors and converters require internal cladding layers resistant to HTHA (governed by NACE MR0175 and API 941). The technology positions the company as a qualified vendor for critical hydrogen service equipment, a segment with high margins and long-term maintenance contracts.
3. Technical Purpose and Value
The primary technical purpose of single-layer stripping electroslag weld overlay on hydrogenation equipment shell inner walls is to provide a continuous, defect-free corrosion- and hydrogen-resistant barrier on the interior surface of the pressure vessel. This barrier must withstand operating conditions of 350–450°C, hydrogen partial pressures up to 8.0 MPa, and prolonged exposure to sour service environments containing H₂S, CO, and trace sulfides.
3.1 Engineering Value
- HTHA Resistance: The overlay alloy (typically 9Cr-1Mo, 12Cr, or 15Cr-5Ni based on API 941 Class classification) raises the Cr and Mo content of the surface layer well above the threshold required to resist carbon depletion and internal cracking under high-temperature hydrogen attack.
- Corrosion Resistance: The high-alloy cladding layer resists sulfidation and oxidation at operating temperatures, extending vessel service life from approximately 8–10 years (bare carbon steel) to 25–30 years.
- Structural Integrity: The single-layer approach eliminates inter-pass defects (lack of fusion, slag inclusions between passes) that are inherent in multi-pass overlay, providing a more homogeneous and reliable barrier layer.
- Cost Efficiency: Despite higher consumable cost per kilogram, the single-pass deposition rate (typically 8–15 kg/h) significantly reduces total welding hours compared to multi-pass TIG overlay for equivalent cladding thickness.
3.2 Customer Value
For end-users (refineries, hydrogen producers, petrochemical complexes), this technology delivers:
- Reduced unplanned shutdown frequency due to cladding failure
- Extended inspection intervals under NB/T 47013 and ASME Section V
- Lower lifetime cost of ownership for critical hydrogenation units
- Compliance with international codes (ASME VIII Div. 1/2, NB/T 47003, TSG 21) for pressure vessel certification
4. Key Process and Implementation Points
4.1 Base Metal Preparation
The shell inner surface must be prepared to a surface roughness of Ra ≤ 6.3 μm within the cladding zone. The cladding area is typically marked with a 50–80 mm overlap band on each side of the weld joint to ensure full coverage across circumferential and longitudinal welds. Preheating to 150–250°C (depending on base material carbon equivalent) is required to prevent cold cracking in the heat-affected zone.
4.2 Consumable Selection
| Parameter | Specification | Notes |
|---|---|---|
| Overlay Alloy (Weld Wire) | 9Cr-1Mo / 12Cr / 15Cr-5Ni (per API 941) | Selected based on HTHA severity and operating conditions |
| Flux Composition | Basic flux (CaO-SiO₂-Al₂O₃ system), low S and P | Low sulfur content (≤0.02%) critical for HTHA resistance |
| Wire Diameter | Ø 3.2 mm – Ø 5.0 mm | Selected based on shell thickness and required cladding thickness |
| Flux Coverage | Continuous fresh flux supply via stripping mechanism | Slag pool depth maintained at 30–50 mm |
4.3 Welding Parameters
| Parameter | Typical Range | Control Objective |
|---|---|---|
| Welding Current | 400–800 A (DC) | Ensure adequate base metal melting and dilution control |
| Welding Voltage | 32–42 V | Maintain stable slag pool and arc geometry |
| Travel Speed | 80–200 mm/min | Control cladding thickness and bead width |
| Wire Feed Speed | 2.5–5.5 m/min | Maintain consistent deposition rate |
| Heat Input | 3.5–6.0 kJ/mm | Balance dilution rate against solidification cracking risk |
| Inter-pass Temperature | Not applicable (single layer) | Post-weld cooling rate controlled by post-heat treatment |
4.4 Stripping Mechanism Operation
The stripping mechanism is the defining feature of this process. It consists of a mechanical or hydraulic device that continuously removes solidified slag from the trailing edge of the weld while simultaneously feeding fresh flux into the leading edge. The stripping rate must be synchronized with the travel speed to maintain a constant slag pool volume. Key operational parameters include:
- Stripping gap: 5–15 mm between the stripping shoe and the weld surface
- Stripping force: 0.5–2.0 kN (adjustable based on slag viscosity at operating temperature)
- Flux replenishment rate: Matched to stripping rate to maintain slag pool depth of 30–50 mm
- Slag temperature monitoring: Infrared pyrometry at 1400–1600°C slag pool temperature
4.5 Dilution Control
Dilution is the critical quality parameter in this process. The overlay layer must achieve a minimum alloy content (e.g., Cr ≥ 9% for 9Cr-1Mo overlay) while maintaining a metallurgical bond with the base metal. Dilution is controlled through:
- Pre-melting of base metal surface: A controlled pre-pass with lower current (200–300 A) creates a 1.0–2.0 mm molten base metal layer before the main overlay pass
- Flux composition: Basic flux with high CaO content reduces base metal dissolution into the slag pool
- Travel speed optimization: Higher travel speeds reduce dilution but may compromise fusion; optimal speed determined through qualification welds
- Post-weld metallographic verification: Dilution measured at 25%, 50%, and 75% depth of cladding layer; maximum allowable dilution typically 30% for HTHA service
4.6 Post-Weld Heat Treatment (PWHT)
Following completion of the cladding operation, the vessel shell undergoes PWHT per the applicable code (ASME VIII Div. 1 UCS-56 or NB/T 47003). Typical PWHT parameters:
- Temperature: 620–680°C for 9Cr-1Mo base/overlay combinations
- Soak time: 1 hour per 25 mm of section thickness (minimum 4 hours)
- Heating rate: Limited to 178°C/h (or 28 mm per hour, whichever is less)
- Cooling rate: Controlled below 178°C/h down to 300°C, then air cool
5. Applicable Standards and Acceptance Criteria
5.1 Design and Fabrication Standards
- NB/T 47003 — Technical Specification for Pressure Vessel Fabrication (Chinese National Boiler Standard)
- ASME BPV Code Section VIII, Division 1 and Division 2 — Rules for Construction of Pressure Vessels
- TSG 21 — Safety Technical Supervision Regulation for Fixed Pressure Vessels
- GB/T 150 — Pressure Vessels (Chinese National Standard)
- API 941 — Materials and Welding Requirements for High-Temperature Hydrogen Attack Resistance
5.2 Welding Procedure Standards
- ASME Section IX — Qualification of Welding, Brazing, and Filler Metal Procedures (QW-441 for electroslag welding)
- NB/T 47014 — Qualification Test Methods for Welding Procedure of Pressure Vessels
- GB/T 985 — Groove Dimensions for Arc Welding
- ISO 15614-11 — Qualification Testing of Welding Procedures for Metallic Materials (Electroslag Welding)
5.3 Inspection and Acceptance Standards
| Inspection Method | Standard | Acceptance Criteria |
|---|---|---|
| Visual Inspection (VT) | GB/T 3323 / NB/T 47013.1 | No cracks, undercut, or surface discontinuities; cladding coverage 100% within marked zone |
| Ultrasonic Testing (UT) | NB/T 47013.2 / ASME V Article 4 | No indications above acceptance threshold; bond integrity confirmed at full cladding thickness |
| Flaw Detection (FDT/EMAT) | NB/T 47013.6 / ASME V Article 8 | No delamination or lack of fusion at base metal/cladding interface |
| Hardness Testing | GB/T 230 / ASME II Article 1 | Overlay layer hardness within specified range (e.g., 200–260 HB for 9Cr-1Mo) |
| Metallographic Examination | NB/T 47013.9 | No cracks, segregation, or excessive dilution; grain size within acceptable limits |
| Chemical Analysis | GB/T 223 / ASTM E415 | Overlay composition within API 941 specified ranges at all measured depths |
5.4 Hydrogen Attack Specific Requirements
- NACE MR0175/ISO 15156 — Materials for Use in H₂S-Containing Environments (hardness limits, microstructure requirements)
- API 941 — Materials selection based on Nelson Curve (temperature vs. hydrogen partial pressure)
- NB/T 47006 — Materials for Pressure Vessels (HTHA-resistant material specifications)
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Excessive dilution | High heat input, slow travel speed, insufficient pre-melting control | Qualification welds to establish parameter window; in-process dilution monitoring via spectral analysis |
| Solidification cracking | High S/P content in consumable, rapid cooling, unfavorable grain orientation | Low-sulfur flux and wire (S ≤ 0.015%); controlled cooling rate; PWHT |
| Hydrogen-induced cracking (HIC) | Diffusible hydrogen trapped in overlay layer | Hydrogen baking at 200–250°C for 2 hours post-weld; low-hydrogen consumables |
| Intergranular corrosion susceptibility | Carbon precipitation at grain boundaries in Cr-Mo overlay | Stabilized overlay composition (Nb or Ti addition); PWHT at appropriate temperature |
6.2 Process Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Slag inclusion | Inadequate stripping, flux contamination, slag pool instability | Automated stripping synchronization; flux drying at 300°C for 2 hours; slag pool level monitoring |
| Weld geometry deviation | Travel speed variation, wire stickout change, strip shoe misalignment | Automated welding equipment with real-time feedback; stickout monitoring; strip shoe alignment verification |
| Base metal distortion | Excessive heat input, asymmetric heating | Back-ironing support; symmetric welding sequence; controlled heat input |
| Equipment malfunction mid-pass | Stripping mechanism failure, flux supply interruption | Redundant flux supply; emergency stop procedures; restart qualification procedures |
6.3 Quality Assurance Controls
- WPS/PQR Qualification: Full qualification per ASME IX QW-441 and NB/T 47014 before production; parameters established through qualification welds with full NDT and metallographic verification
- In-Process Monitoring: Real-time monitoring of current, voltage, travel speed, wire feed speed, and strip shoe position; data logged for traceability
- First-Piece Approval: First cladding pass on each vessel subject to full inspection before production continues
- Welder Qualification: Welders qualified per ASME IX QW-300 or NB/T 47014 for electroslag welding; periodic requalification
- Consumable Traceability: Lot-by-lot chemical analysis of wire and flux; storage under controlled humidity conditions
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Integration
Single-layer stripping electroslag weld overlay complements the company's TIG/MIG weld overlay capabilities in a tiered approach to hydrogenation equipment cladding:
- Small diameter vessels (DN < 2000): TIG weld overlay remains the preferred method due to better geometric control and lower equipment investment. The company's TIG overlay capability (309L transition layer + 316L or 9Cr-1Mo overlay) is applied to reactors, heat exchangers, and smaller pressure vessels.
- Large diameter shells (DN > 2000): Electroslag stripping overlay provides the deposition rate advantage (8–15 kg/h vs. 1–3 kg/h for TIG) required for economic production of large hydrogenation reactor shells.
- Hybrid approach: For vessels with complex geometries, the company may apply TIG overlay to nozzles and headers while using electroslag stripping overlay for the main shell cylinder, ensuring uniform cladding coverage across all internal surfaces.
- Repair and maintenance: TIG overlay is used for localized repair of cladding damage, while electroslag stripping is used for full re-cladding during major overhauls.
7.2 Hydraulic Explosive Bonding Integration
While electroslag weld overlay and hydraulic explosive bonding are fundamentally different processes, they address complementary aspects of hydrogenation equipment protection:
- Hydraulic explosive bonding is used for manufacturing clad plate (e.g., 9Cr-1Mo/SAE 1020 or 316L/SAE 1020) that forms the base material for pressure vessels. The company's hydraulic explosive bonding capability produces large-format clad plates that are then fabricated into vessel shells.
- Electroslag stripping overlay is applied to the interior surface of fabricated shells where additional cladding thickness or a different alloy composition is required beyond what the base clad plate provides.
- Combined approach: For high-severity HTHA service, the company may supply a vessel fabricated from hydraulically bonded clad plate (providing initial cladding) with an additional electroslag stripping overlay layer on the interior surface for enhanced protection. This "dual-layer" approach provides redundant protection against hydrogen attack.
- Quality synergy: The NDT capabilities developed for hydraulic explosive bonding (ultrasonic bond testing, eddy current inspection) are directly applicable to verifying the bond integrity of electroslag weld overlay layers.
7.3 Explosion Welding Integration
Explosion welding (explosive cladding) and electroslag stripping overlay serve different market segments within the hydrogenation equipment domain:
- Explosion welding produces clad plates for pressure vessels where the cladding layer is integral to the base plate through a solid-state bond. This is preferred for applications where the cladding layer must withstand full design pressure and temperature without risk of weld degradation.
- Electroslag stripping overlay is applied post-fabrication to add a cladding layer to an existing carbon steel vessel, or to provide additional protection on the interior surface of a vessel already clad by explosion welding.
- Complementary qualification: The company's explosion welding qualification (per ASME IX and GB/T 39656) demonstrates expertise in cladding metallurgy, which supports the electroslag overlay qualification by establishing understanding of base metal/overlay interface metallurgy.
- Product portfolio completeness: Offering all three cladding routes (explosive cladding for plate, hydraulic bonding for large-format plate, and weld overlay for in-situ application) positions the company as a comprehensive cladding solutions provider for hydrogenation equipment manufacturers.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The development and mastery of single-layer stripping electroslag weld overlay technology directly contributes to the company's qualification portfolio in the following ways:
- ASME Stamp qualification: Demonstrates capability for advanced welding processes required for ASME U Stamp or U2 Stamp vessels, expanding the company's certified manufacturing scope.
- NB/T 47014 WPS qualification: Establishes qualified welding procedures for electroslag welding on HTHA-resistant materials, enabling the company to bid for hydrogenation reactor projects requiring code-compliant cladding.
- API 941 compliance: Qualification of overlay procedures that produce layers meeting API 941 chemical and mechanical requirements enables the company to supply vessels for Nelson Curve applications up to 450°C and 8.0 MPa H₂ partial pressure.
- International certification: The technology supports qualification for EN 15614-11, ISO 15614-11, and AWS D14.1, enabling export of cladding services to international markets.
- Welder certification: Training and qualifying welders in electroslag stripping overlay builds a skilled workforce capable of executing complex cladding operations on critical hydrogen service equipment.
8.2 Product Delivery
This technology enhances the company's product delivery capabilities through:
- Large-scale production: The high deposition rate of electroslag stripping overlay enables the company to deliver large hydrogenation reactor shells (DN 3000–6000, shell thickness 60–120 mm) within competitive lead times.
- Full-service capability: The company can offer complete cladding solutions—explosive cladding for base plate, hydraulic bonding for large-format clad plate, and electroslag overlay for interior surface protection—under a single contract.
- Repair and overhaul services: The technology enables the company to offer re-cladding services for existing hydrogenation equipment, creating a recurring revenue stream beyond new vessel fabrication.
- Custom alloy development: The flexibility of electroslag overlay allows the company to develop and qualify custom overlay alloys for specific service conditions, providing differentiated value to customers with unique operating requirements.
8.3 Customer Value
For the company's customers—refinery owners, hydrogen producers, and petrochemical complexes—this technology delivers measurable value:
- Extended asset life: Properly executed electroslag stripping overlay extends vessel service life by 2–3x compared to unclad carbon steel, reducing capital expenditure on replacement equipment.
- Reduced inspection burden: Code-compliant cladding with verified dilution control reduces the frequency and severity of in-service inspection requirements, lowering operating costs.
- Operational reliability: The metallurgical homogeneity of single-layer overlay reduces the probability of cladding failure, minimizing unplanned shutdowns that can cost $500,000–$2,000,000 per day for a large hydrogenation unit.
- Regulatory compliance: Code-compliant cladding per ASME, NB/T, and TSG requirements ensures regulatory approval for operation, avoiding legal and financial penalties.
- Technical partnership: The company's expertise in hydrogenation equipment cladding positions it as a trusted technical partner, enabling collaborative development of next-generation cladding solutions for emerging hydrogen technologies (e.g., high-pressure hydrogen storage vessels, hydrogen cracking reactors).
9. Conclusion
Single-layer stripping electroslag weld overlay technology represents a critical capability for Cladding Technology Shanxi Co., Ltd in the growing hydrogenation equipment market. By mastering this advanced process, the company establishes itself as a qualified provider of code-compliant, high-performance cladding solutions for the most demanding hydrogen service applications. The technology's integration with the company's existing TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding capabilities creates a comprehensive cladding solutions portfolio that addresses the full spectrum of hydrogenation equipment protection requirements—from plate-level cladding through to in-situ surface protection. As the global transition to hydrogen energy accelerates, this capability will be increasingly valuable in serving both traditional hydrogenation processes and emerging hydrogen production and storage technologies.