Manufacturing Process Analysis and Optimization of Inner Wall Weld Overlay Polished Heads (Dished Ends)
1. Definition and Technical Principles
Inner wall weld overlay polished heads refer to dished ends (torispherical, semi-elliptical, or hemispherical heads) used in pressure vessels and process equipment, where a corrosion-resistant alloy layer is deposited on the internal surface through arc welding processes (TIG or MIG), followed by precision mechanical polishing to achieve a smooth, defect-free surface finish suitable for fluid contact. This composite manufacturing technique combines the structural integrity of a carbon steel or low-alloy steel base head with the corrosion resistance of a noble alloy overlay, providing an economical alternative to full-alloy heads in aggressive chemical environments.
The fundamental principle relies on the metallurgical bonding between the overlay alloy and the base metal substrate through arc fusion welding. The overlay material—typically austenitic stainless steels (304L, 316L, 321), duplex stainless steels (2205), nickel-based alloys (Inconel 625, Hastelloy C-276), or copper alloys—is deposited in multiple passes to achieve the required thickness and dilution control. Subsequent polishing removes surface defects, oxidation, and residual stress concentrations, producing a finish typically in the range of Ra 0.2–0.8 μm depending on service requirements.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG weld overlay technology route of Cladding Technology Shanxi Co., Ltd., representing a specialized application of multi-pass overlay welding on formed pressure vessel components. Unlike flat plate cladding, the manufacturing of overlay heads introduces unique challenges related to:
- Curved surface geometry: Maintaining consistent bead spacing and penetration on non-planar, three-dimensional surfaces
- Thermal distortion control: Preventing deformation of the formed head during multi-pass welding
- Post-weld finishing: Achieving uniform surface quality on complex contours through manual or semi-automated polishing
- Quality verification: Performing NDT on curved surfaces where probe coupling and signal interpretation differ from flat specimens
Within the company's product portfolio, overlay heads serve as critical components for pressure vessel fabrication, connecting to flanged nozzles and cylindrical shells that share the same overlay specification. This positions the capability as a complete solution provider for lined pressure vessels rather than a standalone component manufacturer.
3. Technical Purpose and Value
3.1 Economic Value
The primary economic driver for inner wall overlay heads is the significant cost reduction compared to full-alloy construction. A carbon steel head with a 3–5 mm overlay of 316L stainless steel may cost 30–50% less than a solid 316L head of equivalent thickness, while delivering comparable corrosion performance for the specified service life. For nickel alloy applications (Inconel 625, Hastelloy), the cost savings can exceed 60%.
3.2 Engineering Value
- Corrosion protection: Provides a continuous, metallurgically bonded barrier against process media
- Mechanical integrity: Retains the superior mechanical properties and cost efficiency of carbon steel base material
- Weldability compatibility: Allows the vessel shell to be joined using standard carbon steel welding procedures while the overlay handles chemical attack
- Surface quality: Polished finish eliminates crevice corrosion initiation sites and facilitates cleaning in pharmaceutical/food service
3.3 Qualification and Certification Value
The process analysis and optimization documented in this capability entry directly supports WPS/PQR qualification under ASME Section IX and NB/T 47014, establishing traceable manufacturing procedures that enable:
- Customer qualification audits and factory acceptance testing
- Regulatory compliance for pressure vessel certification (TSG 21 in China, ASME U Stamp internationally)
- Repeatable manufacturing across production batches
4. Key Process and Implementation Points
4.1 Process Flow Overview
- Base head forming: Cold or hot stamping of carbon steel plate (Q345R, 16MnR, or equivalent per GB 150/ASME SA-516)
- Pre-weld preparation: Surface cleaning, edge beveling, and thermal preheating
- Transition layer deposition: Single or multi-pass 309L/309L+316L overlay to control dilution
- Overlay layer deposition: Multi-pass application of the final corrosion-resistant alloy
- Post-weld heat treatment (if required): Solution annealing or stress relief per material specification
- Mechanical polishing: Grinding and polishing to achieve target surface finish
- Non-destructive testing: PT/MT/UT verification of overlay integrity
- Final inspection and documentation
4.2 Critical Welding Parameters
| Parameter | Typical Range (TIG) | Typical Range (MIG) | Notes |
|---|---|---|---|
| Welding Current | 120–220 A | 100–180 A | Adjusted for head curvature and pass number |
| Arc Voltage | 14–18 V | 16–22 V | Lower for transition layer to minimize dilution |
| Travel Speed | 50–100 mm/min | 200–400 mm/min | Slower on high-curvature zones |
| Wire Diameter | 1.6–2.4 mm (electrode) | 1.0–1.2 mm | 309L for transition; 316L/625 for overlay |
| Shielding Gas | Ar or Ar+2%O₂ | Ar or Ar+2%CO₂ | Pure Ar preferred for Ni-alloys |
| Preheat Temperature | 50–150°C | 50–150°C | Higher for thicker base or low-temperature service |
| Interpass Temperature | ≤150°C (SS); ≤100°C (Ni-alloy) | ≤150°C (SS); ≤100°C (Ni-alloy) | Critical for avoiding sensitization and cracking |
| Overlay Thickness | 3.0–6.0 mm (typical) | 2.5–5.0 mm | Determined by corrosion allowance and design life |
4.3 Optimization Strategies Identified Through Process Analysis
4.3.1 Weld Sequence Optimization for Curved Surfaces
The process analysis emphasizes the critical importance of weld sequence planning on curved head surfaces. A symmetrical, balanced welding sequence is essential to minimize thermal distortion. The recommended approach involves:
- Dividing the head surface into sectors (typically 8–12 sectors for torispherical heads)
- Welding in opposing pairs simultaneously or in rapid succession to balance thermal input
- Starting from the knuckle radius and progressing toward the crown, or vice versa based on distortion analysis
- Using back-plate support or internal clamping rings to resist hoop stress during welding
4.3.2 Dilution Control Through Transition Layer Design
Dilution is the primary metallurgical challenge in overlay welding on carbon steel. The optimized process employs:
- First pass (transition): 309L or 309Cb with high current and low travel speed to create a wide, shallow bead with controlled dilution (target: 20–30% base metal dilution)
- Second pass (buffer): 316L or matching alloy at moderate parameters
- Final passes (overlay): Final alloy at reduced current and optimized travel speed to minimize further dilution
| Overlay Material | Recommended Transition | Target Dilution (%) | Minimum Overlay Thickness |
|---|---|---|---|
| 304L / 316L | 309L (1 pass) | ≤30% | 3.0 mm |
| 2205 Duplex | 309L (1–2 passes) | ≤25% | 4.0 mm |
| Inconel 625 | 309L + 625 (2 passes) | ≤15% | 5.0 mm |
| Hastelloy C-276 | 309L + C-276 (2 passes) | ≤15% | 5.0 mm |
4.3.3 Polishing Process Optimization
The polishing stage is critical for achieving the required surface quality and is often the most labor-intensive step. Key optimization points include:
- Coarse grinding (80–120 grit): Remove weld bead profile, slag inclusions, and surface irregularities
- Medium grinding (180–240 grit): Refine surface and prepare for fine finishing
- Fine grinding (320–600 grit): Achieve near-final surface texture
- Polishing (800–2000 grit or paste): Final finish to target Ra value
- Use of flexible polishing tools: Drum mandrels and flexible backing plates conform to head curvature
- Directional consistency: All polishing passes in one direction to avoid cross-grain defects
4.3.4 Thermal Management on Thick-Walled Heads
For heads with base thickness exceeding 25 mm, the thermal mass creates unique challenges:
- Higher preheat temperatures (up to 200°C) to reduce cooling rate and prevent martensite formation in transition zones
- Reduced interpass temperature monitoring using infrared thermography
- Consideration of post-weld stress relief (PWSR) at 550–620°C for 2 hours per 25 mm thickness
5. Applicable Standards and Acceptance Criteria
5.1 Design and Fabrication Standards
- GB/T 150 (Pressure Vessels): General requirements for pressure vessel design and fabrication in China
- ASME BPV Code Section VIII, Division 1: Constructional rules for pressure vessels
- ASME BPV Code Section II, Part D: Material specifications for overlay alloys
- NB/T 47014: Qualification rules for welding procedures in pressure vessel fabrication
- GB/T 12467: Welding procedure specification requirements
5.2 Welding Procedure Standards
- ASME Section IX: Qualification of welding, brazing, and fusion-bonding procedures
- ISO 15614-1: Qualification testing of welding procedures for metallic materials
- GB/T 985.1: Welding procedure specification preparation and approval
- NB/T 47015: Technical specification for welders in pressure vessel fabrication
5.3 NDT and Acceptance Standards
| NDT Method | Standard Reference | Acceptance Criteria | Application in Overlay Heads |
|---|---|---|---|
| Visual Inspection (VT) | GB/T 3323 / ASME Section V Art. 6 | No surface defects >1 mm | 100% of overlay surface |
| Penetrant Testing (PT) | GB/T 18851 / ASME Section V Art. 7 | No linear indications; round spots ≤2 mm | 100% of overlay surface |
| Magnetic Particle Testing (MT) | GB/T 26951 / ASME Section V Art. 8 | No cracks or linear indications | 100% of ferromagnetic base/overlay boundary |
| Ultrasonic Testing (UT) | GB/T 11345 / ASME Section V Art. 4 | Level II acceptance per ASME XII-112 | Overlay thickness and bond quality verification |
| Hardness Testing | GB/T 230 / ASTM E92 | Within specified range per material spec | Overlay surface hardness verification |
| Corrosion Testing | NACE TM0169 / ASTM G48 | No intergranular corrosion | ASTM A262 Practice E (if required) |
5.4 Surface Finish Acceptance
- Pharmaceutical/Food service: Ra ≤ 0.4 μm (electropolished equivalent), per 3-A Sanitary Standards
- General chemical service: Ra ≤ 0.8 μm, per API 570 inspection guidelines
- High-pressure cryogenic service: Ra ≤ 0.8 μm with no visible scratches, per ASME Section VIII
5.5 Overlay Qualification Standards
- ASME Section VIII, Division 1, UW-25: Overlay welds for corrosion resistance
- ASME Section IX, QW-451: Qualification for overlay welding
- ISO 14230: Welding — Overlay welding
- EN 15614-1: Qualification testing for overlay welding procedures
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Hot cracking in overlay | High sulfur/phosphorus in base; excessive interpass temp | Use 309L transition; control interpass ≤150°C; low-sulfur consumables |
| Intergranular corrosion (sensitization) | Prolonged exposure in 450–850°C range during welding | Low-carbon grades (304L/316L); minimize heat input; post-weld annealing if needed |
| Excessive dilution | High current, slow travel speed, thin first pass | Optimized transition layer; multiple thin passes; verify by chemical analysis |
| Hydrogen-induced cracking (HIC) | Hydrogen entrapment in high-strength base | Preheat; low-hydrogen consumables; post-weld baking for susceptible materials |
| Porosity | Contaminated base surface; inadequate shielding | Thorough surface preparation; back purging with Ar; proper gas flow |
6.2 Geometric and Distortion Risks
- Head ovality distortion: Controlled through symmetrical weld sequence, back-plate support, and limited heat input per pass
- Knuckle radius deformation: Monitored by post-weld dimensional inspection against GB/T 150 dimensional tolerances
- Overlay thickness variation: Addressed through welder skill qualification and in-process thickness monitoring
6.3 Surface Quality Risks
- Polishing-induced defects: Over-grinding that reduces overlay thickness below minimum specification; controlled by marking minimum material boundaries and in-process thickness gauging
- Residual grinding marks: Inconsistent polishing direction or grit progression; addressed through documented polishing procedures
- Contamination during finishing: Cross-contamination from ferrous abrasives; controlled by dedicated non-ferrous polishing tools and cleaning protocols
6.4 Quality Assurance Controls
- Welder qualification: Per NB/T 47015 / ASME Section IX, with specific qualification for overlay welding on curved surfaces
- WPS/PQR qualification: Validated on production-representative specimens including curved surface mock-ups
- In-process inspection: Layer-by-layer visual and thickness monitoring during multi-pass overlay
- Final verification: Full-surface PT/MT, UT thickness mapping, and surface finish measurement
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This is the primary and most mature application of inner wall overlay polished heads. Key applications include:
- Chemical process vessels: Reactors, digesters, and storage tanks handling sulfuric acid, hydrochloric acid, and mixed chemical solutions—overlay with 316L or Hastelloy C-276
- Pharmaceutical equipment: Stainless steel-lined heads requiring polished surfaces per GMP requirements—overlay with 316L, polished to Ra ≤ 0.4 μm
- Food and beverage processing: Fermentation tanks and heat exchangers—304L/316L overlay with sanitary polish
- Power generation: Condensers and heat recovery steam generators (HRSG)—Inconel 625 overlay for flue gas environments
- Oil and gas: Separator drums and process vessels—overlay with duplex 2205 for chloride-containing environments
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding is primarily employed for flat plate and pipe cladding, the technology analysis of overlay heads provides complementary knowledge for the following scenarios:
- Hybrid vessel construction: Where the cylindrical shell uses hydraulic explosive bonded clad plate but the dished heads require weld overlay due to the impracticality of explosive bonding on curved geometries
- Transition zone design: Understanding dilution and metallurgical compatibility from overlay technology informs the design of weld joints between explosive-bonded shells and overlay heads
- Repair and retrofit: When existing vessels with hydraulic explosive bonded cladding require head replacement, the overlay head process provides a compatible repair methodology
7.3 Explosion Welding Route
Explosion welding (explosive cladding) is primarily a flat plate and pipe technology, but the process analysis of overlay heads contributes to the following integrated applications:
- Complete vessel solutions: Where the vessel shell is explosion-welded clad plate and the heads are manufactured with weld overlay, ensuring metallurgical compatibility at the shell-head joint
- Material selection alignment: The dilution control knowledge from overlay welding guides the selection of explosion-welded cladding materials that can be field-welded to overlay heads
- Technical consulting: The company's expertise in overlay head manufacturing enables comprehensive material and process recommendations for customers evaluating different cladding approaches
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR database expansion: The documented process analysis and optimization directly contributes to the company's qualification records under ASME Section IX and NB/T 47014, enabling acceptance of a broader range of customer orders
- Welder skill development: The systematic approach to curved surface overlay welding establishes training protocols that elevate welder competency
- Factory accreditation: Demonstrated capability in overlay head manufacturing supports factory certification for pressure vessel fabrication under TSG 21 (China) and ASME U Stamp
8.2 Product Delivery Enhancement
- Reduced rework rates: Optimized weld sequences and parameter control minimize distortion and metallurgical defects, reducing rework by an estimated 40–60%
- Shorter lead times: Standardized procedures and trained personnel reduce manufacturing cycle time for overlay heads
- Consistent quality: Documented processes ensure repeatable results across production batches and multiple production lines
8.3 Customer Value Creation
- Cost optimization: Providing customers with overlay head solutions at 30–60% cost savings versus full-alloy alternatives without compromising service life
- Technical consultation: Leveraging deep process knowledge to recommend optimal overlay thickness, material selection, and surface finish for specific service conditions
- Complete solution capability: Offering integrated vessel solutions combining overlay heads with clad shells (via any of the three technology routes), reducing customer interface complexity
- Extended service life: Properly executed overlay with controlled dilution and high-quality polishing delivers corrosion performance approaching that of solid alloy construction
9. Continuous Improvement and Future Development
The process analysis and optimization framework described in this capability entry establishes a foundation for ongoing improvement through:
- Automated welding integration: Development of robotic TIG/MIG systems with contour-following capabilities for consistent overlay on complex head geometries
- Real-time monitoring: Implementation of in-situ sensor systems (acoustic emission, infrared thermography, arc voltage monitoring) for real-time weld quality feedback
- Advanced materials: Extension of overlay capabilities to newer alloy systems including Alloy 625, Alloy 718, and advanced duplex/super-duplex grades
- Digital documentation: Integration of manufacturing data with digital twin models for predictive quality assessment and traceability
10. Conclusion
The manufacturing process analysis and optimization of inner wall weld overlay polished heads represents a core competency of Cladding Technology Shanxi Co., Ltd. within its TIG/MIG weld overlay technology route. This capability addresses a critical market need for economical, corrosion-resistant pressure vessel heads while maintaining full metallurgical and surface quality integrity. Through systematic process optimization—encompassing weld sequence design, dilution control, thermal management, and precision finishing—the company delivers qualified, reliable products that support customer qualification requirements, ensure regulatory compliance, and provide significant economic value across chemical, pharmaceutical, power generation, and oil and gas industries. The knowledge gained from this process analysis also strengthens the company's integrated capability across all three technology routes, enabling comprehensive cladding solutions for complex process equipment.