Stainless Steel Weld Overlay on Pressure Vessel Heads: Experimental Research and Process Qualification

1. Definition and Technical Principles

Stainless steel weld overlay on the inner wall of pressure vessel heads (dished ends, torispherical heads, ellipsoidal heads, and hemispherical heads) is a specialized surface engineering technique that deposits a corrosion-resistant alloy layer onto the interior surface of carbon steel or low-alloy steel pressure vessel heads. This process creates a metallurgically bonded composite structure in which the base material provides structural strength while the overlay layer provides resistance to aggressive process media.

The fundamental principle relies on the dilution-controlled deposition of austenitic or duplex stainless steel filler metal onto a carbon steel substrate through controlled heat input. The weld metal must achieve sufficient dilution control to maintain corrosion resistance while ensuring adequate metallurgical bond strength with the base material. Unlike hydraulic explosive bonding or explosion welding, weld overlay on heads is a fusion welding process that requires precise thermal management due to the curved geometry, thin-wall sections, and often limited accessibility of the inner head surface.

The experimental research documented in this capability entry focuses on systematic parameter optimization—including heat input control, interpass temperature management, filler metal selection, weld sequence planning, and dilution measurement—to establish reliable welding procedures for head overlay applications that meet regulatory and performance requirements.

2. Category and Business Positioning

This capability falls squarely within the TIG/MIG Weld Overlay technology route, which represents one of the company's three core technology platforms for producing clad and overlay products. Within this route, head overlay is a high-difficulty sub-application because:

In terms of business positioning, head overlay capability enables the company to deliver complete clad pressure vessel components rather than merely clad plate or pipe, significantly increasing value-add and customer integration. This positions the company as a full-spectrum cladding solutions provider capable of handling complex vessel fabrication programs in the oil, chemical, pharmaceutical, and food processing industries.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

3.2 Value Contribution to Qualification Building

The experimental research on head overlay serves as foundational work for:

4. Key Process and Implementation Points

4.1 Base Material Preparation

4.2 Filler Metal Selection Matrix

Application Environment Recommended Filler Standard Reference Typical Overlay Thickness
General corrosion, mild acids 308L (E308L-16 / ER308L) GB/T 983, AWS A5.4 3–5 mm (2–3 passes)
Chloride-containing media 316L (E316L-16 / ER316L) GB/T 983, AWS A5.4 4–6 mm (2–3 passes)
High-temperature oxidation 310S / 309L transition GB/T 983, AWS A5.4 5–8 mm (3–4 passes)
High-strength corrosion resistance 2205 Duplex (ENiCrMo-3) ISO 14343, AWS A5.14 4–6 mm (2–3 passes)
Transition layer (CrMo base) 309L (E309L-16) GB/T 983, AWS A5.4 3 mm (1–2 passes)

4.3 Weld Sequence Strategy for Heads

The welding sequence for head overlay is critical to managing distortion and ensuring uniform coverage. The recommended approach follows these principles:

  1. Radial segmentation: Divide the head into 6–12 radial sectors, welding each sector sequentially from the knuckle (crown) toward the straight edge or vice versa
  2. Alternating direction: Alternate between adjacent sectors to balance thermal input and minimize residual stress concentration
  3. Multi-pass build-up: For overlay thicknesses exceeding 2 mm, employ multiple passes with interpass grinding to control dilution
  4. Final dress-up pass: A final 1.5–2 mm pass with the lowest dilution filler to ensure surface composition meets corrosion resistance requirements

4.4 Critical Process Parameters

Parameter Typical Range (TIG) Typical Range (MIG) Control Objective
Heat Input 0.8–1.5 kJ/mm 1.0–2.0 kJ/mm Minimize dilution, prevent base metal cracking
Interpass Temperature ≤150°C (≤200°C for austenitic) ≤200°C Prevent grain growth, control dilution
Shielding Gas Flow 8–12 L/min (Ar) 15–25 L/min (Ar/CO₂ mix) Prevent oxidation, ensure clean weld
Travel Speed 200–400 mm/min 300–600 mm/min Control penetration depth
Weld Width/Height Ratio 1.5–2.5:1 2.0–3.0:1 Ensure adequate bond strength
Backing Gas Ar, 5–8 L/min Ar, 8–12 L/min Prevent backside oxidation

4.5 Dilution Control and Measurement

Dilution is the single most critical quality parameter in stainless steel weld overlay. The experimental research establishes:

5. Applicable Standards and Acceptance Criteria

5.1 Design and Material Standards

5.2 Welding Procedure and Qualification Standards

5.3 Inspection and Acceptance Standards

5.4 Acceptance Criteria Summary

Inspection Type Acceptance Level Standard Reference
Visual Inspection (VT) No cracks, porosity >1 mm, undercut >0.5 mm NB/T 47013.1 / ISO 17637
Magnetic Particle Testing (MT) No linear indications; round indications ≤2 mm NB/T 47013.4 / ASTM E709
Penetrant Testing (PT) No cracks, no linear indications NB/T 47013.5 / ASTM E165
Hardness Testing Overlay ≤ 250 HV; HAZ ≤ 300 HV (per base material) NB/T 47013.8 / ISO 6507
Dilution Analysis ≤30% (general); ≤20% (chloride); ≤10% (critical) Project specification / NB/T 25102
Macrograph Examination Full penetration, no lack of fusion, uniform grain GB/T 1954 / ASTM E3

6. Common Risks and Controls

6.1 Technical Risks

Risk Category Description Mitigation Strategy
Cracking Cold cracking in HAZ due to high carbon equivalent base material; hot cracking in weld metal due to sulfur/phosphorus segregation Preheat control, low heat input, use of low-sulfur filler metals (E308L, E316L), post-weld heat treatment if required
Excessive Dilution Base metal alloying elements dilute into overlay, reducing corrosion resistance below specification Multi-pass strategy with transition layer, reduced heat input, interpass grinding, dilution monitoring per pass
Distortion Welding-induced deformation exceeding dimensional tolerances for head geometry Back-step welding sequence, fixture design with clamping, controlled heat input, post-weld straightening if necessary
Porosity Gas inclusion from inadequate shielding or contaminated surfaces Enhanced gas flow, wind protection, surface decontamination, backing gas on thin sections
Intergranular Corrosion Sensitization of austenitic overlay at 450–850°C during welding or subsequent heat treatment Use of low-carbon fillers (308L, 316L), stable grades (321, 347) for high-temperature service, controlled interpass temperature
Lack of Fusion Incomplete bonding between overlay and base material, especially at weld toe on curved surfaces Adequate root preparation, proper travel speed, consistent gun angle, adequate current settings

6.2 Quality System Controls

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay (Primary Application)

Head overlay is the flagship application within the TIG/MIG weld overlay route. Typical scenarios include:

7.2 Hydraulic Explosive Bonding (Complementary Application)

While hydraulic explosive bonding (waterjet-assisted explosive bonding) is primarily used for flat plate and pipe cladding, the experimental knowledge from head overlay research contributes to:

7.3 Explosion Welding (Complementary Application)

Explosion welding produces clad plate that can be formed into heads. The head overlay research contributes to:

8. Contribution to Customer Value and Qualification Building

8.1 Qualification Building Impact

The experimental research on head overlay directly supports the company's qualification portfolio in the following ways:

8.2 Product Delivery Enhancement

8.3 Customer Value Proposition

"The ability to deliver fully overlaid pressure vessel heads as a single qualified component eliminates the need for customers to coordinate between vessel fabricators and overlay specialists, reducing interface risk, ensuring metallurgical compatibility across all welds, and providing single-point accountability for the complete pressure boundary."

9. Conclusion and Forward Development

The experimental research on stainless steel weld overlay for pressure vessel heads represents a critical capability that bridges the gap between clad material production and complete pressure vessel component delivery. By systematically establishing qualified procedures, dilution control methodologies, and quality assurance protocols, this work enables the company to offer high-value-added overlay services for the most geometrically complex pressure vessel components.

Future development directions include: