CLADDING TECHNOLOGY SHANXI CO., LTDStudy Notes on Valve Sealing Surface Weld Overlay Technology
Overview of Valve Sealing Surface Overlay Requirements
Valve sealing surfaces are among the most critical functional components in fluid control systems across petroleum, chemical, power generation, and pharmaceutical industries. The sealing face must simultaneously satisfy demands for corrosion resistance, wear resistance, thermal stability, and dimensional accuracy. In practice, the base material of a valve body—often carbon steel or low-alloy steel—is selected for structural strength and cost efficiency, while the sealing surface requires a hard, corrosion-resistant overlay to resist cavitation erosion, galling, and chemical attack from the process medium. This dual requirement is precisely what makes weld overlay technology indispensable in valve manufacturing.
The study of valve sealing surface overlay technology reveals that the choice of overlay process is dictated by the geometry of the sealing face, the required overlay thickness (typically 0.5 to 3.0 mm), the tolerance for dilution from the base metal, and the post-weld machining allowance. Common processes include submerged arc welding (SAW), plasma transferred arc (PTA) welding, gas tungsten arc welding (GTAW) with hot wire, and laser cladding. Each process offers a different balance between dilution control, deposition rate, and surface quality.
Key Technical Parameters and Process Selection
The following table summarizes the typical process parameters and applicable scenarios for valve sealing surface overlay:
| Process | Typical Current (A) | Travel Speed (mm/min) | Dilution (%) | Overlay Thickness (mm) | Surface Roughness (Ra, μm) | Typical Application |
|---|---|---|---|---|---|---|
| SAW | 300–500 | 100–300 | 20–40 | 1.5–4.0 | 25–50 | Large body valves, gate valves |
| PTA | 80–200 | 200–800 | 5–15 | 0.3–1.5 | 5–15 | Precision ball valves, butterfly valves |
| GTAW (Hot Wire) | 60–150 | 150–500 | 10–25 | 0.5–2.0 | 8–20 | Medium-size globe valves |
| Laser Cladding | 2–6 kW | 300–1000 | 3–10 | 0.2–1.0 | 3–10 | High-precision seat rings, plug valves |
From a materials perspective, common overlay consumables include Stellite 6 (Co-Cr-W), Inconel 625, 316L stainless steel, and tungsten carbide-cobalt cermets. The selection depends on the service medium: Stellite 6 is preferred for high-temperature and abrasive slurry services, while Inconel 625 is selected for high-temperature hydrogen or sulfur-containing environments. For cryogenic service, austenitic stainless steel overlays are commonly specified to avoid hydrogen embrittlement and maintain toughness at low temperatures.
Dilution Control and Microstructure Considerations
Dilution is arguably the most critical quality parameter in valve sealing surface overlay. Excessive dilution introduces carbon and alloying elements from the base steel into the overlay, which can lead to the formation of brittle intermetallic phases, reduced corrosion resistance, and premature hardening that causes cracking during post-weld machining. The study emphasizes that dilution must be controlled below 15% for cobalt-based overlays and below 20% for nickel-based overlays to ensure the overlay retains its intended chemical composition and mechanical properties.
The microstructure of a well-executed Stellite 6 overlay typically consists of M6C and M23C6 carbides dispersed in an austenitic matrix. The carbide content and morphology are strongly influenced by cooling rate and dilution level. Rapid cooling from PTA or laser cladding promotes finer carbide distribution and higher hardness (typically 40–50 HRC), while slower cooling from SAW may produce coarser carbides and slightly lower hardness. Post-weld stress relief at 800–850°C for 2 hours is sometimes employed to reduce residual stresses, but care must be taken not to exceed the solidus temperature of the overlay or to anneal the carbides excessively.
Defect Analysis and Quality Assurance
Common defects in valve sealing surface overlay include porosity, lack of fusion, cracking (hot and cold), and excessive undercut. Porosity is often associated with inadequate gas shielding or contaminated base metal surfaces. Lack of fusion typically results from insufficient heat input or excessive travel speed. Cracking in cobalt-based overlays is relatively rare but can occur when dilution exceeds 30%, leading to martensitic transformation during cooling.
Quality assurance procedures for valve sealing surfaces include visual inspection (VT), magnetic particle inspection (MT) for surface and near-surface defects, and dye penetrant inspection (PT) for surface cracks. Hardness testing is performed on the overlay surface after machining, and the measured hardness must fall within the specified range (e.g., 38–50 HRC for Stellite 6). Dimensional inspection using optical comparators or coordinate measuring machines (CMM) verifies that the sealing face geometry meets the required flatness and concentricity tolerances, typically within 2 μm for precision valves.
Engineering Practice Insights
In engineering practice, the most common challenge is achieving consistent overlay quality on complex valve geometries where access for welding is limited. For ball valves, the spherical sealing surface requires either a specialized robotic welding system or manual TIG welding with careful technique. The study highlights that preheating the base metal to 200–300°C is often necessary to reduce thermal stress and prevent cracking, particularly for thick-walled valve bodies made of low-alloy steel.
A practical case from a refinery project involved the overlay of Inconel 625 on the seating surfaces of 12-inch gate valves operating in a high-temperature hydrogen service. The selected process was PTA welding with a three-pass approach: a first pass to establish a low-dilution bond layer, a second pass for bulk deposition, and a third pass for surface finishing. The resulting overlay achieved a dilution level of 12%, a hardness of 28–32 HRC, and passed 100% MT inspection. The valves were subsequently stress-relieved at 850°C and hydrostatically tested at 1.5 times the design pressure. This case demonstrates that careful process planning, including weld procedure qualification per ASME IX or NB/T 47014, is essential for reliable results.
Summary
The study of valve sealing surface weld overlay technology underscores that successful overlay depends on the integrated control of process parameters, material selection, dilution management, and post-weld quality assurance. The choice of process must be matched to the valve geometry, service conditions, and required overlay properties. Dilution control is the single most important factor determining overlay performance, and modern processes such as PTA and laser cladding offer superior dilution control compared to conventional SAW. Engineers must also pay close attention to residual stress management, post-weld machining allowances, and rigorous non-destructive testing to ensure the long-term reliability of valve sealing surfaces in demanding industrial applications.