Weld Overlay of Sealing Surfaces on Bellows Globe Valve Bodies: Process Technology and Qualification Analysis
1. Definition and Technical Principles
The weld overlay of sealing surfaces on bellows globe valve bodies refers to the application of a metallurgically bonded overlay layer—typically composed of hardfacing alloys, stainless steel transition layers, or corrosion-resistant alloys—onto the machined seat surfaces, plug faces, and other sealing interfaces of bellows globe valves. This process is executed primarily through Gas Tungsten Arc Welding (GTAW/TIG) and/or Gas Metal Arc Welding (GMAW/MIG) techniques, with the objective of enhancing the tribological performance, corrosion resistance, and sealing integrity of the valve body.
Bellows globe valves are high-integrity pressure-containing devices widely deployed in petroleum refining, petrochemical processing, LNG facilities, and hazardous service environments. The bellows mechanism provides a zero-emission seal for the stem, while the globe-style internal architecture delivers precise flow control. The sealing surfaces—comprising the valve seat, plug (or disc), and any auxiliary sealing lands—are the critical functional interfaces that must withstand cyclic compression, thermal cycling, and aggressive process media. Weld overlay on these surfaces ensures that the tribological pair achieves the required surface hardness, chemical compatibility, and dimensional stability throughout the valve's operational life.
The fundamental metallurgical principle involves achieving a sound dilution-controlled weld metal composition through careful selection of filler alloys, base metal preparation, and multi-pass welding sequences. The overlay must exhibit full metallurgical fusion to the substrate while maintaining the mechanical and chemical properties specified by the applicable material standard. In practice, this requires managing heat input, interpass temperature, and solidification microstructure to prevent cracking, porosity, and excessive dilution of the overlay alloy's beneficial alloying elements.
2. Category and Business Positioning
Within the three principal technology routes of Cladding Technology Shanxi Co., Ltd., the weld overlay of bellows globe valve sealing surfaces falls squarely under the TIG/MIG Weld Overlay business line. This route encompasses precision overlay welding for pressure parts, valves, flanges, heat exchanger tubes, and other components where dimensional accuracy, metallurgical soundness, and surface quality are paramount.
The hydraulic explosive bonding and explosion welding routes, while powerful for producing clad plate and pipe at industrial scale, are not directly applicable to valve body sealing surface overlay due to the complex three-dimensional geometry, tight dimensional tolerances, and small feature sizes inherent to valve manufacturing. However, the qualification methodology, NDT protocols, and quality management frameworks developed across all three routes are synergistically transferable, reinforcing the company's overall technical credibility and audit readiness.
This specific process research entry represents a focused capability development initiative. It demonstrates the company's ability to extend its cladding expertise from large-scale plate and pipe products into the specialized domain of pressure equipment components—specifically valves—which represent a high-value, high-margin segment of the industrial equipment supply chain. By mastering the overlay of sealing surfaces on bellows globe valves, the company positions itself as a qualified supplier to valve manufacturers, EPC contractors, and end-users in the oil, gas, and chemical sectors.
3. Technical Purpose and Value
The technical purpose of weld overlay on bellows globe valve sealing surfaces encompasses several interrelated objectives:
- Wear Resistance Enhancement: Globe valve seats and plugs are subjected to repeated contact and sliding during operation. Overlaying with hardfacing alloys (e.g., Co-Cr-W based Stellite-type alloys, or Ni-based alloys) significantly increases surface hardness—often to HRC 40–60—reducing wear rates and extending service intervals.
- Corrosion Resistance: In aggressive process environments (chloride-containing media, acidic streams, high-temperature sulfidation), overlaying with austenitic stainless steels (e.g., 309L, 316L, 321) or nickel-based alloys (e.g., Inconel 625, Hastelloy C-276) provides a chemically inert barrier that protects the base carbon or low-alloy steel valve body.
- Sealing Integrity: Precise control of overlay thickness, surface roughness (typically Ra ≤ 1.6 μm after machining), and dimensional accuracy ensures that the mating surfaces achieve the required seat contact pressure and leak-tightness per API 598 or ISO 15848 testing protocols.
- Repair and Restoration: Overlay welding enables the restoration of worn or damaged valve seats and plugs, extending the service life of expensive bellows globe valves and reducing lifecycle costs for end-users.
The business value of this capability is multifaceted. It enables the company to offer a value-added service to valve OEMs who may lack in-house welding expertise for exotic alloy overlays. It also supports the company's broader strategy of building a comprehensive qualification portfolio that covers both large-format cladding products and precision pressure part components.
4. Key Process and Implementation Points
4.1 Base Metal Preparation
Proper base metal preparation is the foundation of a successful overlay weld. The sealing surface area must be ground or machined to remove all mill scale, rust, oxide, and prior coatings. The surface should be prepared to a minimum of 2B finish or better. A chamfer or groove may be machined around the perimeter of the overlay area to provide a mechanical anchor and reduce the risk of edge cracking. The surface must be cleaned with solvent (e.g., acetone or MEK) within one hour of welding to remove any organic contamination.
Preheating requirements depend on the base material. For carbon steel valve bodies (e.g., WCB, WC6), a preheat of 100–150°C is typical to reduce hydrogen-induced cracking susceptibility. For low-alloy steels (e.g., F22, F91), preheat temperatures of 200–300°C may be necessary. The preheat temperature must be measured at the weld zone using a calibrated contact pyrometer or infrared thermocouple.
4.2 Filler Metal Selection
The selection of filler metal is governed by the service environment, the base material composition, and the required overlay properties. The following table summarizes common filler metal selections for bellows globe valve sealing surface overlay:
| Service Condition | Base Material | Transition Layer Filler | Overlay Filler | Standards Reference |
|---|---|---|---|---|
| General hydrocarbon service | WCB / WC6 | E309L (AWS A5.4 / A5.18) | E309L or E316L | AWS A5.4, AWS A5.18 |
| Chloride-containing / acidic | WC6 / F22 | E309L | E316L or Inconel 625 (ERNiCrMo-3) | AWS A5.4, ASTM B335 |
| High-temperature wear | WCB / WC6 | E309L | Stellite 6 (Co-Cr-W) or Ni-60 | ASTM B447, AWS A5.15 |
| Low-temperature / cryogenic | LCCB / LCC6 | E309L | E309L or E310L | AWS A5.4 |
| High-pressure steam | F91 / F92 | Special Cr-Mo filler | Special Cr-Mo or 321 SS | ASME SA-335, AWS A5.4 |
4.3 Welding Parameters
Typical TIG welding parameters for sealing surface overlay on bellows globe valve bodies are as follows:
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 80–160 A | Depends on plate thickness and filler wire diameter |
| Welding voltage | 10–18 V | AC or DCEN depending on filler type |
| Travel speed | 3–8 cm/min | Slower speeds for better penetration and dilution control |
| Shielding gas | Argon (99.99%) or Ar + 5% O₂ | Oxygen addition improves wetting on stainless overlays |
| Gas flow rate | 10–15 L/min | Back purge required for thick sections |
| Wire diameter | 1.0–2.4 mm (φ) | 1.0–1.6 mm for thin sections; 2.0–2.4 mm for thick |
| Interpass temperature | ≤ 150°C (carbon steel) / ≤ 100°C (stainless) | Critical for preventing cracking and maintaining overlay composition |
| Heat input | 0.5–2.5 kJ/mm | Controlled to minimize dilution and HAZ effects |
4.4 Multi-Pass Welding Sequence
A typical overlay sequence on a bellows globe valve seat surface involves the following steps:
- Root pass: A single-pass TIG weld applied along the perimeter groove or chamfer to establish the initial metallurgical bond between the base metal and the overlay system. This pass typically uses a transition filler (e.g., E309L) to bridge the compositional gap between the carbon/low-alloy steel base and the austenitic overlay.
- Build-up passes: Two to four subsequent passes are applied to build the overlay to the required thickness (typically 3–6 mm for seat surfaces, depending on the valve size and service conditions). Each pass is deposited with a controlled overlap of approximately 50% of the previous pass width to ensure full fusion and uniform composition.
- Cap pass: The final pass is deposited with the primary overlay filler (e.g., E316L, Inconel 625, or Stellite 6) to achieve the target surface composition and hardness. This pass may be deposited with a slightly reduced current to minimize dilution from the underlying layers.
4.5 Post-Weld Machining and Surface Treatment
After overlay welding, the sealing surface is machined to the final dimensional profile and surface finish. The machining allowance must be sufficient to remove all weld surface irregularities while leaving a minimum overlay thickness as specified in the drawing or WPS. Typical final surface roughness for seat surfaces is Ra 0.4–1.6 μm, depending on the valve class and sealing requirement.
Post-weld heat treatment (PWHT) may be required depending on the base material and applicable code requirements. For carbon steel and low-alloy steel valve bodies, PWHT per ASME Section VIII Div. 1 or Div. 2 requirements is typically performed at 590–650°C for a time proportional to wall thickness. For stainless steel or nickel-based overlays, PWHT is generally not performed to avoid sensitization or precipitation hardening that could degrade corrosion resistance.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
The following standards govern the design, fabrication, welding, and testing of bellows globe valves with weld overlay sealing surfaces:
- ASME BPV Code Section VIII, Division 1 and Division 2: Governs the design, fabrication, and examination of pressure vessels and components, including valve bodies. Div. 2 provides more rigorous requirements for fitness-for-service and fracture mechanics-based design.
- ASME Section IX: Governs the qualification of welding procedures (WPS/PQR) and welder performance qualification (WPQ). All overlay welding procedures must be qualified per Section IX, Part QW.
- ASME Section II, Part D: Specifies filler metal requirements (e.g., AWS A5.4 for stainless steel electrodes, AWS A5.15 for nickel-cobalt base alloys).
- API 6D: Specification for pipeline valves, including bellows-sealed globe valves. Defines performance testing, material requirements, and inspection criteria.
- API 598: Valve inspection and testing standard. Defines pressure tightness testing, leak rate classification, and seat-to-seat differential pressure testing.
- ISO 15848: International standard for fugitive emissions from industrial valves. Defines leak rate classes (Class A, B, C) for bellows-sealed valves.
- GB/T 12221: Chinese national standard for industrial valve general technical conditions.
- GB/T 12224: Chinese national standard for industrial valve testing and inspection.
- NB/T 47013: Chinese national standard for non-destructive testing of pressure equipment.
- ASTM E165 / ASTM E171: Standards for magnetic particle testing (MT) and dye penetrant testing (PT), respectively.
- ASTM E230: Standard practice for radiographic testing of welds.
5.2 Acceptance Criteria
Acceptance criteria for weld overlay on bellows globe valve sealing surfaces are typically defined in the applicable WPS and product specification. Key acceptance parameters include:
- Visual inspection (VT): No cracks, undercut, excessive spatter, or surface irregularities. Weld profile must be smooth and continuous.
- Magnetic particle testing (MT): Per ASTM E165 or NB/T 47013.3. Acceptance per ASME Section V, Article 7, with no indications classified as rejectable. Linear indications > 1/16 inch (1.6 mm) are typically rejected.
- Dye penetrant testing (PT): Per ASTM E165 or NB/T 47013.5. Used for non-ferromagnetic overlay materials (stainless steel, nickel alloys). No cracks or linear indications permitted.
- Radiographic testing (RT): Per ASTM E230 or NB/T 47013.2. Required for full-penetration overlay welds or where specified by the WPS. Acceptance per ASME Section V, Article 2, typically at Level T-2.
- Hardness testing: Overlay surface hardness must meet the specification (e.g., HRC 40–55 for Stellite 6, HB 180–250 for 316L). Base metal hardness must not be adversely affected beyond the specified HAZ width.
- Chemical analysis: Overlay composition verified by optical emission spectrometry (OES) or XRF. Dilution must be within the acceptable range (typically ≤ 25% for critical overlays).
- Dimensional inspection: Overlay thickness, surface flatness, and geometric tolerances verified per the valve body drawing. Typical flatness tolerance is ≤ 0.05 mm over the sealing surface diameter.
6. Common Risks and Controls
The following table summarizes the principal technical risks associated with weld overlay on bellows globe valve sealing surfaces and the corresponding mitigation measures:
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Cracking (hot/cold) | Excessive heat input, high hydrogen content, low ductility of overlay | Seal failure, leakage, valve rejection | Control heat input, use low-hydrogen filler, preheat base metal, maintain interpass temperature |
| Excessive dilution | High current, slow travel speed, large weld pool | Overlay composition out of specification, reduced corrosion/wear resistance | Reduce current, increase travel speed, use multi-pass build-up with transition layers |
| Porosity | Contaminated surface, inadequate shielding gas coverage | Reduced overlay integrity, potential leak path | Rigorous surface cleaning, adequate gas flow, back purge for thick sections |
| Surface roughness exceedance | Uncontrolled welding parameters, improper post-weld machining | Poor seal contact, accelerated wear | Controlled welding parameters, precision grinding/machining post-overlay |
| Dimensional distortion | Uneven heat input, asymmetric weld sequence | Valve body geometry deviation, assembly interference | Symmetric welding sequence, fixture clamping, stress-relief machining |
| Intergranular corrosion | Chromium carbide precipitation in sensitized HAZ | Reduced corrosion resistance, premature failure | Use of low-carbon fillers (309L/316L), minimize HAZ time in sensitization range |
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
The weld overlay of bellows globe valve sealing surfaces is a core application within the TIG/MIG weld overlay technology route. This route is characterized by precision control of heat input, filler metal composition, and weld geometry, making it ideal for the complex, small-feature geometries of valve bodies. The company's TIG/MIG capabilities extend to:
- Overlay of seat surfaces, plug faces, and stem sealing surfaces on bellows globe valves for refinery, chemical, and LNG applications.
- Overlay of corrosion-resistant layers on valve body internal cavities (e.g., Inconel 625 or Hastelloy C-276 overlay for sour service per NACE MR0175).
- Overlay of wear-resistant layers on globe valve trim components (plugs, seats, cages) for high-cycle operation.
- Repair overlay of worn or damaged valve seats to restore dimensional accuracy and sealing performance.
7.2 Hydraulic Explosive Bonding Route (Supporting Application)
While hydraulic explosive bonding is primarily used for producing clad plate and pipe at industrial scale, it plays an indirect but important supporting role in the valve overlay business. Specifically:
- Hydraulically bonded clad plate (e.g., 316L/SAE 1010 or Inconel 625/SAE 1045) can be used as the base material for valve body forgings or castings, providing a corrosion-resistant substrate that reduces the need for extensive overlay welding.
- The company's expertise in hydraulic bonding provides a metallurgical understanding of interface bonding mechanisms that is transferable to the fusion bonding achieved in overlay welding.
- Hydraulic bonding can be used to produce clad sheet for valve bonnets, covers, and other auxiliary components that may require corrosion resistance but not the precision of overlay welding.
7.3 Explosion Welding Route (Qualification Synergy)
Explosion welding, while not directly applicable to valve body overlay, contributes to the company's overall qualification and certification portfolio in the following ways:
- The NDT protocols (MT, PT, RT, UT) developed and validated for explosion weld interface inspection are directly transferable to overlay weld inspection, reinforcing the company's NDT competency and auditor confidence.
- Explosion welding qualification per ASTM A795 or ASME Section IX provides evidence of the company's capability to produce sound metallurgical bonds between dissimilar metals—a fundamental principle shared with overlay welding.
- The quality management system (QMS) developed for explosion welding production (ISO 9001, ASME "U" stamp, etc.) provides the governance framework within which overlay welding activities are executed and audited.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This process research entry represents a critical step in building the company's qualification portfolio for pressure part welding. Key qualification outcomes include:
- WPS/PQR Qualification: Development and qualification of welding procedure specifications per ASME Section IX for overlay welding on bellows globe valve bodies. Each WPS is qualified through a Procedure Qualification Record (PQR) that documents the weld parameters, test results (mechanical, metallurgical, NDT), and acceptance criteria.
- WPQ Qualification: Welder performance qualification per ASME Section IX, Part QW, ensuring that operators are certified for the specific overlay welding configurations used on valve bodies.
- ASME "U" Stamp: The company's ASME "U" stamp authorization for pressure part fabrication is reinforced by demonstrating competence in overlay welding on pressure-containing valve bodies.
- API 6D Monogram: Qualification for API 6D monogrammed valve manufacturing requires demonstrated capability in overlay welding of sealing surfaces, including WPS qualification, welder certification, and NDT competency.
8.2 Product Delivery
The process research directly enables the company to deliver qualified, code-compliant bellows globe valve overlay services. The technical knowledge gained through this research translates into:
- Shorter production lead times due to optimized welding parameters and reduced rework rates.
- Higher first-pass yield through validated WPS and trained welders.
- Ability to handle a broader range of overlay specifications (from basic 309L/316L to exotic Inconel 625 and Stellite 6) without extensive requalification.
- Capability to support emergency repair and restoration of in-service valves, reducing unplanned shutdown time for end-users.
8.3 Customer Value
The customer value proposition of this capability is significant across multiple dimensions:
- Extended Asset Life: Overlay of sealing surfaces with wear-resistant or corrosion-resistant alloys extends the service life of bellows globe valves by 3–5 times compared to uncladded surfaces, reducing replacement frequency and lifecycle costs.
- Reduced Emissions: High-integrity overlay sealing surfaces contribute to achieving ISO 15848 Class A fugitive emissions performance, supporting customers' environmental compliance and carbon reduction goals.
- Operational Reliability: Precise overlay geometry and surface finish ensure consistent seat sealing performance, reducing the risk of in-service leakage and unplanned shutdowns.
- Cost Efficiency: Overlay repair of worn valve seats is typically 60–80% more cost-effective than replacement of the entire valve body, providing significant savings for asset owners.
9. Conclusion
The process research on weld overlay of sealing surfaces on bellows globe valve bodies represents a strategically significant capability development for Cladding Technology Shanxi Co., Ltd. It extends the company's core TIG/MIG weld overlay expertise into the high-value pressure equipment components segment, reinforcing the company's qualification portfolio, expanding its product delivery scope, and creating tangible customer value through extended asset life, reduced emissions, and improved operational reliability. The technical rigor, standards compliance, and quality management frameworks established through this research are directly transferable across the company's three technology routes, creating a synergistic qualification ecosystem that enhances the company's competitive position in the industrial cladding and overlay welding market.