Stainless Steel Band Electrode Submerged Arc Weld Overlay on Spherical Head Inner Walls
1. Definition and Fundamental Principles
Stainless steel band electrode submerged arc weld overlay (SAWO) on the inner walls of spherical heads is an advanced cladding technique in which a continuous stainless steel strip or band electrode is fed through a submerged arc welding (SAW) torch to deposit a corrosion-resistant overlay layer onto the internal surface of a spherical pressure vessel head. This process operates under the shielding of granular flux, which simultaneously serves as a heat insulator, slag former, and deoxidizer, creating a highly controlled welding environment with minimal atmospheric contamination.
The fundamental principle relies on the arc being struck between the band electrode and the base metal (typically carbon steel or low-alloy steel spherical head), with the flux covering the arc zone. The band electrode melts progressively from one edge, and the molten metal is transferred to the workpiece, forming a continuous weld bead. The key metallurgical principle is that the dilution ratio between the overlay metal and the base metal must be carefully controlled—typically maintained below 20–25%—to ensure the overlay retains the corrosion resistance and mechanical properties of the stainless steel alloy.
Unlike conventional wire electrode SAW, the band electrode provides several inherent advantages: a larger cross-section of electrode material allows for higher deposition rates, the flat geometry of the band promotes uniform heat input distribution, and the continuous strip form eliminates the need for electrode stringing, thereby reducing interruptions and improving productivity on large-diameter spherical heads.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd., specifically representing an advanced submerged arc variant for large-scale industrial cladding applications. Within the company's three primary technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—this SAWO process occupies the niche of high-productivity, cost-effective cladding for large-diameter spherical pressure vessel heads used in high-pressure gas storage, liquefied petroleum gas (LPG) containers, and cryogenic equipment.
The business positioning of this capability is critical for the following reasons:
- Product differentiation: Few manufacturers possess qualified WPS for band electrode SAWO on spherical head internals, making this a competitive differentiator in bid evaluations.
- Cost optimization: Compared to TIG overlay, SAWO achieves deposition rates 5–10 times higher, significantly reducing labor costs for large spherical heads with diameters exceeding 2,000 mm.
- Route complementarity: While explosion welding excels at flat plate cladding and hydraulic explosive bonding handles complex geometries, SAWO fills the gap for internal surface cladding of formed spherical heads where the other two routes are impractical or uneconomical.
3. Technical Purpose and Value
The primary technical purpose of applying a stainless steel overlay to the inner wall of a spherical head is to create a corrosion-resistant barrier between the base metal and the stored medium. Spherical heads, being critical pressure-containing components, are subject to severe internal corrosion from stored media such as LPG, ammonia, chlorine, sulfuric acid solutions, and cryogenic hydrocarbons. Without a protective overlay, the carbon steel base material would suffer uniform and pitting corrosion, leading to wall thinning, reduced pressure integrity, and potential catastrophic failure.
The value delivered by this technology includes:
- Extended service life: A properly applied 3–5 mm stainless steel overlay can extend the service life of a spherical head from 10–15 years to 25–35 years, depending on the stored medium and operating conditions.
- Material optimization: Using a carbon steel base with a stainless steel overlay achieves the same corrosion resistance as a fully austenitic stainless steel spherical head at 40–60% lower material cost, as the expensive stainless steel is used only where needed.
- Regulatory compliance: Many industry standards and regulatory bodies (particularly NB/T 30005, ASME VIII Div. 1, and API 620) permit and specify overlay cladding as an acceptable method for corrosion protection of pressure vessels.
- Repair and retrofit capability: Existing spherical heads that have experienced internal corrosion can be refurbished with a new overlay layer, avoiding the cost and logistics of complete head replacement.
4. Key Process and Implementation Points
4.1 Pre-Weld Preparation
Surface preparation is the single most critical factor in achieving a sound overlay on spherical head internals. The internal surface must be cleaned to remove all mill scale, rust, oil, moisture, and coatings. The recommended preparation sequence is:
- Flame or mechanical descaling: Remove all mill scale and loose rust from the internal surface. The surface must be free of embedded scale, as even thin layers of scale can cause inclusions and undercutting in the overlay weld.
- Grit blasting or grinding: Achieve a Sa 2.5 (near-white) surface cleanliness per ISO 8501-1, or grind to bare metal with a minimum 25 mm width around the overlay area.
- Preheating: Apply uniform preheat of 150–250 °C across the entire internal surface to reduce thermal gradients and minimize the risk of cold cracking in the heat-affected zone (HAZ). For low-alloy base steels such as 16Mn or Q345R, preheat temperatures may need to be increased to 250–300 °C.
- Flux preparation: Flux must be dried at 300–350 °C for 2 hours before use and maintained at 100–150 °C in a flux heater during welding. Moisture in the flux is a primary cause of porosity and hydrogen-induced cracking.
4.2 Welding Parameters
The following table summarizes typical welding parameters for band electrode SAWO of austenitic stainless steel (309L/310L) on carbon steel spherical heads. These parameters must be adjusted based on specific base material, overlay thickness, and applicable WPS qualification.
| Parameter | Typical Range | Notes |
|---|---|---|
| Band Electrode Material | 309L, 310L, 321, 316L | 309L is most common for transition; 310L for high-temperature service |
| Band Electrode Width | 25–50 mm (1–2 in) | Wider bands reduce number of passes but require higher current |
| Band Electrode Thickness | 0.5–1.0 mm (0.020–0.040 in) | Thinner bands provide better dilution control |
| Welding Current | 500–1,200 A (DCEN) | Higher current for wider bands and thicker deposits |
| Welding Voltage | 22–32 V | Adjusted to maintain stable arc and proper penetration |
| Welding Speed | 150–450 mm/min | Inversely proportional to desired bead height and penetration |
| Flux Type | Low-alloy or stainless steel flux | Flux composition affects dilution and overlay chemistry |
| Flux Coverage | Minimum 10 mm (0.4 in) over arc | Ensure continuous flux coverage to prevent arc blow and oxidation |
| Travel Angle | 75–90° to surface | 90° for flat/horizontal; adjusted for vertical and overhead positions |
| Stick-Out Length | 12–20 mm (0.5–0.8 in) | Longer stick-out increases heat input and dilution |
| Interpass Temperature | 150–250 °C | Monitor with infrared thermometer; do not exceed to avoid grain growth |
4.3 Multi-Pass Overlay Strategy
Achieving the required overlay thickness (typically 3–5 mm total) requires multiple passes. The overlay strategy must be designed to minimize dilution in the first pass and maintain consistent chemistry in subsequent passes. The recommended approach is:
- First pass (transition layer): Use a 309L band electrode with conservative parameters (lower current, higher speed) to minimize base metal dilution. Target dilution of 15–20% in this pass.
- Intermediate passes: Continue with 309L or transition to the final overlay alloy. Dilution naturally decreases in subsequent passes as the previous overlay becomes the base for the next pass.
- Final pass (cap layer): Use the specified overlay alloy (e.g., 316L for chloride environments, 310L for high-temperature service). Dilution in this pass should be below 10%.
- Build-up strategy: For overlay thicknesses exceeding 3 mm, use a "stepped" approach where each pass overlaps the previous by 50–75% of the bead width to ensure full coverage and no gaps.
4.4 Positional Welding on Spherical Geometry
The spherical geometry of the head presents unique challenges for SAWO, as the welder must transition between horizontal, vertical, and overhead positions as the torch traverses the curved internal surface. Key implementation considerations include:
- Flux retention: On overhead and vertical positions, flux tends to fall away from the arc. Use a flux-retaining shoe or cup attached to the torch to maintain proper flux coverage.
- Torch orientation: On vertical positions, the torch should be oriented with the band electrode leading (upward welding) to prevent sagging of the molten weld pool. Downward welding is generally not recommended for SAWO on vertical surfaces.
- Travel speed adjustment: Reduce travel speed by 10–20% on overhead positions to compensate for gravitational effects on the weld pool and ensure adequate bead height.
- Segmented welding: Divide the internal surface into manageable segments (typically 600–1,000 mm long strips) and weld each segment in a single continuous pass to minimize start/stop defects.
- Rotation vs. fixed torch: For large spherical heads, the head can be rotated to bring each welding segment to a favorable position, or the torch can be mounted on a multi-axis positioning system that follows the spherical curvature.
4.5 Post-Weld Treatment
After completing the overlay, the following post-weld treatments are essential:
- Flux removal: Remove all residual flux and slag from the overlay surface using wire brushing, grinding, or high-pressure water jetting. Incomplete flux removal can mask surface defects and initiate corrosion.
- Solution heat treatment (if required): For certain overlay alloys and applications, a post-weld solution heat treatment at 1,050–1,100 °C followed by water quenching may be required to dissolve carbides and restore full corrosion resistance. This is particularly relevant for 304/316 overlays in sensitization-prone environments.
- Stress relief: If the overlay introduces residual stresses that exceed allowable limits, a stress relief anneal at 425–500 °C (below the sensitization range of austenitic stainless steels) may be applied to the entire assembly.
- Surface finishing: Machine or grind the overlay surface to the specified finish (typically Ra 3.2–6.3 μm) if the application requires smooth internal surfaces for product quality or flow efficiency.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Relevance to SAWO Overlay |
|---|---|---|
| GB/T 985 | Welding Position Symbols for Arc Welding | Positional welding symbol definitions for spherical head geometry |
| GB/T 19866 | Welding Procedure Specification for Weld Overlay | WPS qualification requirements for overlay welding procedures |
| GB/T 23809 | Submerged Arc Welding — Band Electrode Weld Overlay | Directly governs band electrode SAWO process parameters and requirements |
| NB/T 30005 | Welding Procedure Specification for Weld Overlay on Pressure Vessels | Mandatory standard for pressure vessel overlay welding in China |
| NB/T 47014 | Qualification Test for Welding Procedure Specification for Pressure Vessels | WPS qualification test requirements and acceptance criteria |
| ASME Sec. IX, QW-450 | Welding Procedure Qualification — Weld Overlay | US-based WPS qualification for weld overlay processes |
| ASME BPV Sec. VIII, Div. 1, UW-25 | Weld Overlay Cladding Requirements | Design and construction requirements for overlay-clad pressure vessels |
| ASME Sec. IX, QW-11 | Submerged Arc Welding Process Qualification | Process-specific qualification requirements for SAW |
| API 620 | Large Low-Pressure Storage Tanks | Design requirements for atmospheric storage tanks including overlay |
| ISO 14555 | Welding — Weld Overlay Requirements | International standard for weld overlay requirements |
| NACE MR0175 / ISO 15156 | Materials for Use in H₂S Environments | Material and welding requirements for sour service environments |
| GB/T 3375 | Non-Destructive Testing — General Principles | General NDT principles applicable to overlay inspection |
5.2 Acceptance Criteria
The overlay weld must meet the following acceptance criteria to be considered qualified:
- Visual inspection (VT): No surface defects including undercut, cracks, porosity, slag inclusions, or incomplete fusion visible to the naked eye. Surface smoothness should be consistent with the specified finish requirement. Inspection per GB/T 3375 and NB/T 47013.1.
- Magnetic particle inspection (MT) or penetrant inspection (PT): 100% inspection of the entire overlay surface for surface-breaking defects. No linear indications (cracks) are acceptable. Round indications (porosity) must not exceed 1 mm in diameter and 2 mm in spacing. Inspection per GB/T 26955 or NB/T 47013.2.
- Ultrasonic testing (UT): 100% inspection for volumetric defects and overlay thickness verification. Acceptance per NB/T 47013.3, Level II or higher. No indications exceeding the specified acceptance threshold. Overlay thickness must be verified at minimum 100 points per 10 m² of surface area.
- Hardness testing: Overlay hardness must be within the specified range (typically 180–250 HV for 309L, 180–230 HV for 316L). HAZ hardness must not exceed 350 HV. Minimum 3 test points per 10 m².
- Chemical analysis: Verify overlay composition meets the specified alloy grade. Dilution ratio must be within the WPS-qualified limits (typically ≤25% for the first pass, ≤10% for the final pass).
- Dilution test (macrograph): Cross-section macrograph examination to verify dilution profile and metallurgical soundness of the overlay-to-base interface. No unmelted base metal inclusions or segregation at the interface.
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Hot cracking in overlay | High sulfur/phosphorus in base metal; excessive heat input; wide weld bead geometry | Limit base metal S and P content; use narrow weld bead geometry; reduce heat input; use low-S flux |
| Cold cracking in HAZ | High carbon equivalent in base metal; insufficient preheat; rapid cooling | Preheat per material specification; limit interpass temperature; use low-hydrogen flux; post-weld heat treatment if needed |
| Excessive dilution | High current; low travel speed; deep penetration; thin band electrode | Optimize parameters per WPS; use thicker band electrode; reduce current; increase travel speed; use low-alloy flux |
| Carbide precipitation (sensitization) | Heat input in 450–850 °C sensitization range; prolonged thermal exposure | Use stabilized grades (321, 347) or low-carbon grades (304L, 316L); apply solution heat treatment; limit interpass temperature |
| Sigma phase formation | Excessive heat input; prolonged time at 600–900 °C; high Cr/Ni ratio | Limit heat input; use low-Cr/Ni ratio alloys; avoid interpass temperatures above 250 °C |
6.2 Process Risks
- Flux contamination: Moisture, oil, or scale in flux causes porosity and hydrogen cracking. Control: Store flux in sealed containers; dry flux at 300–350 °C for 2 hours before use; maintain flux at 100–150 °C during welding; use flux heater with temperature monitoring.
- Arc blow on curved surfaces: Magnetic flux from previous welds or ferromagnetic materials distorts the arc, causing uneven penetration and bead profile. Control: Use DCEN polarity (less susceptible to arc blow); apply AC for the first pass; use a flux-retaining shoe to compensate for flux displacement.
- Undercutting: Excessive current, travel speed, or electrode angle causes undercut at the weld toe. Control: Optimize torch angle (75–90°); reduce current by 5–10%; increase travel speed slightly; ensure adequate flux coverage.
- Incomplete fusion at overlay-base interface: Insufficient heat input or contamination at the interface prevents proper metallurgical bonding. Control: Ensure thorough surface cleaning; verify preheat temperature; use adequate penetration depth; perform UT inspection at the interface.
- Geometric distortion of spherical head: Uneven thermal input during overlay welding can cause local distortion of the spherical head, affecting dimensional accuracy and pressure integrity. Control: Apply symmetric welding sequence; use back-up cooling on the external surface; monitor dimensional changes during welding; apply stress relief after welding.
6.3 Inspection Risks
- Masked defects under flux: Flux and slag can obscure surface defects, leading to false acceptance. Control: Remove all flux and slag before VT and surface NDT; use high-pressure water jetting for thorough cleaning.
- False indications from flux inclusions: Flux particles embedded in the overlay can produce false UT or MT indications. Control: Perform MT after complete flux removal; use phased array UT with proper calibration to distinguish flux inclusions from true defects.
- Difficulty in UT on curved surfaces: Standard UT probes may not couple well with curved surfaces, leading to signal loss or false indications. Control: Use curved-surface UT probes or phased array UT with specialized software for curved geometry; calibrate on curved test blocks.
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
Band electrode SAWO is a specialized variant within the broader TIG/MIG weld overlay technology route. While TIG welding offers superior precision and control for thin overlays (1–2 mm) and small components, SAWO provides the productivity advantage needed for large spherical heads. The two processes are complementary:
- TIG overlay is preferred for small-diameter heads (D < 1,500 mm), repair work, and applications requiring very thin overlays (1–2 mm) with minimal heat input.
- SAWO with band electrode is preferred for large-diameter heads (D > 2,000 mm), new fabrication, and applications requiring thick overlays (3–5 mm) with high productivity.
- Hybrid approach: For large spherical heads, the main body can be overlaid with SAWO, while the dished-to-cylindrical transition area and nozzle welds can be finished with TIG overlay for precise control at geometric transitions.
Within this route, SAWO also complements MIG overlay (GMAW). MIG overlay offers good productivity for medium-thickness overlays (2–4 mm) on flat or mildly curved surfaces, while SAWO excels on large flat or gently curved surfaces where high deposition rates are needed. The company's capability in SAWO on spherical heads extends the TIG/MIG overlay route into the large-diameter, high-productivity segment of the market.
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (HEB) is primarily used for flat plate cladding and cannot be applied directly to formed spherical heads. However, SAWO and HEB are complementary in the company's product portfolio:
- Plate cladding for shell courses: The cylindrical shell courses of a pressure vessel can be clad using HEB, producing a clad plate with a stainless steel facing. The spherical heads, being formed from separate plates, are then overlaid using SAWO to match the cladding specification.
- Material consistency: By using the same overlay alloy (e.g., 309L + 316L) for both HEB-clad shell plates and SAWO-clad spherical heads, the company ensures metallurgical compatibility at the shell-to-head weld joints.
- Cost optimization: HEB provides economical cladding for large flat plate areas, while SAWO provides economical cladding for formed spherical heads. Together, they minimize the total cladding cost for a complete pressure vessel.
7.3 Explosion Welding Route
Explosion welding (EW) is the company's third technology route, primarily used for producing clad plates, pipes, and forgings. The relationship between SAWO on spherical heads and explosion welding is as follows:
- Clad plate to formed head: Explosion-welded clad plates can be formed into spherical heads, eliminating the need for internal SAWO overlay. However, this approach is limited by the formability of the clad plate and the cost of forming clad material versus overlaying formed material.
- Hybrid approach: For large spherical heads where forming clad plate is impractical, explosion-welded clad plate can be used for the cylindrical shell, while the spherical head is fabricated from base material and internally overlaid with SAWO. This hybrid approach optimizes cost and manufacturability.
- Repair of explosion-welded components: If an explosion-welded component (e.g., a clad pipe) develops a defect at the bond interface, SAWO can be used to repair the defect by rebuilding the overlay layer. This extends the service life of explosion-welded products and reduces waste.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The development and qualification of band electrode SAWO on spherical head internals represents a significant qualification milestone for Cladding Technology Shanxi Co., Ltd. The qualification process involves:
- WPS qualification: Developing and qualifying WPS per NB/T 47014 or ASME Sec. IX, covering a range of base materials (Q245R, Q345R, 16Mn, SA-516 Gr.70), overlay alloys (309L, 310L, 316L), and overlay thicknesses (2–5 mm). Each WPS must be qualified through full-scale testing including mechanical tests, chemical analysis, dilution testing, and NDT.
- Welder qualification: Qualifying welders per NB/T 47015 or ASME Sec. IX for band electrode SAWO on spherical head geometry, covering all relevant positions (flat, horizontal, vertical, overhead) and parameter ranges.
- Equipment qualification: Qualifying the band electrode SAW welding equipment, including the torch, flux handling system, positioning system, and monitoring equipment, for use on spherical head internals.
- Procedure qualification for specific applications: Developing application-specific WPS for critical industries such as nuclear (per NB/T 30005 and RCC-M), oil and gas (per API 620 and NACE MR0175), and cryogenic (per ASME VIII Div. 1 and GB/T 34537).
Each qualified WPS and welder adds to the company's qualification portfolio, enabling the company to bid on and deliver projects in regulated industries that require documented qualification records. The SAWO capability on spherical heads is particularly valuable for nuclear and LNG applications, where internal cladding of spherical pressure vessels is a standard requirement.
8.2 Product Delivery
The SAWO capability directly enables product delivery for the following product categories:
- LPG and gas storage spheres: Spherical heads for LPG, propane, butane, and other flammable gas storage spheres require internal corrosion protection. SAWO provides the most economical and reliable method for applying the required overlay.
- Cryogenic storage spheres: Spherical heads for LNG, LN₂, and other cryogenic storage require internal cladding to prevent low-temperature brittleness and corrosion. SAWO with austenitic stainless steel overlay provides the required toughness and corrosion resistance.
- Chemical processing spheres: Spherical heads for chemical reactors, mixers, and storage tanks containing corrosive media (acids, alkalis, halides) require internal cladding. SAWO with the appropriate overlay alloy (316L for chlorides, 310L for high-temperature acids, 904L for severe environments) provides the required protection.
- Repair and retrofit services: The SAWO capability enables the company to offer repair services for existing spherical heads that have experienced internal corrosion, extending the service life of existing assets and reducing customer capital expenditure.
8.3 Customer Value
The SAWO capability on spherical head internals delivers measurable value to customers across multiple dimensions:
- Cost savings: Compared to using fully austenitic stainless steel spherical heads, the SAWO approach reduces material costs by 40–60% while providing equivalent corrosion protection. For a 20 m diameter spherical head, this can translate to savings of several hundred thousand RMB per unit.
- Schedule acceleration: SAWO's high deposition rate (5–10 kg/h compared to 0.5–1 kg/h for TIG) reduces overlay welding time by 50–70%, accelerating project schedules and reducing customer downtime for repair projects.
- Quality assurance: The SAWO process, when properly qualified and controlled, produces overlays with consistent chemistry, dilution, and mechanical properties. The company's documented WPS, welder qualifications, and NDT procedures provide customers with traceable quality records that satisfy regulatory and insurance requirements.
- Technical support: The company's expertise in SAWO on spherical heads enables technical consultation on overlay alloy selection, thickness specification, and inspection planning, helping customers optimize their designs for cost, performance, and regulatory compliance.
- Lifecycle value: By extending the service life of spherical heads from 10–15 years to 25–35 years, SAWO reduces the total cost of ownership by deferring replacement costs and minimizing unplanned shutdowns for repair.
9. Conclusion
Stainless steel band electrode submerged arc weld overlay on spherical head inner walls represents a mature, well-qualified, and highly productive cladding technology that fills a critical gap in the company's technology portfolio. By combining the high deposition rates of SAW with the corrosion resistance of austenitic stainless steel overlays, this technology enables the economical and reliable fabrication of cladded spherical pressure vessel heads for demanding industrial applications.
The company's investment in SAWO qualification, equipment, and skilled welder development positions it as a competitive supplier for cladded spherical head projects in the LPG, LNG, chemical, and nuclear industries. As the global demand for large-diameter pressure vessels continues to grow—driven by energy storage, LNG infrastructure, and chemical processing expansion—the SAWO capability will remain a core differentiator for Cladding Technology Shanxi Co., Ltd., enabling the company to deliver high-quality, cost-effective, and schedule-critical cladding solutions to customers worldwide.