CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Technology for Valve Sealing Surfaces
Literature Overview and Industry Context
This 1989 publication by Gao Qingbao and Wang Dequan addresses one of the most demanding applications of weld overlay technology: the restoration and hardening of valve sealing surfaces in industrial valves. Valve sealing surfaces are subjected to extreme combined loading conditions including high contact pressure, sliding friction, cavitation erosion, and aggressive chemical media. Failure of the sealing surface leads to valve leakage, which can result in process shutdowns, environmental contamination, and safety hazards. Weld overlay provides an economical and effective means to restore worn sealing surfaces or to enhance the performance of new valve components through the deposition of specialized overlay materials.
The publication emerged during a period when Chinese valve manufacturing was transitioning from basic carbon steel valves to more sophisticated alloy and overlay-enhanced valves for the petroleum, chemical, and power industries. The technical challenge was to develop reliable overlay procedures that could produce sealing surfaces with precise geometry, smooth surface finish, and long service life under demanding operating conditions.
Core Technical Content and Process Analysis
The publication covers multiple aspects of valve sealing surface overlay, including material selection, welding process selection, procedure development, quality control, and performance verification. The key technical elements are discussed below.
Overlay Material Selection
The selection of overlay material is dictated by the service conditions, including medium type, temperature, pressure, and flow velocity. The following table summarizes typical overlay materials used for valve sealing surfaces.
| Application Condition | Overlay Material | Hardness (HB) | Key Properties |
|---|---|---|---|
| Clean water, low pressure | Stellite 6 (CoCr) | 300-400 | Wear and corrosion resistance |
| Carbon steel valve body, general service | 1Cr13 (410) | 250-300 | Moderate corrosion resistance |
| Acidic media | Hastelloy C-276 | 200-250 | Excellent corrosion resistance |
| High-temperature steam | Inconel 625 | 250-300 | Creep and oxidation resistance |
| Slurry service | Carbide-based (CrC, WC) | 500-700 | Abrasion resistance |
| Cryogenic service | 304/316 stainless | 200-250 | Low-temperature toughness |
Welding Process Selection for Valve Sealing Surfaces
The choice of welding process depends on the valve size, geometry, and required surface quality. The following processes are commonly employed:
| Process | Valve Size Applicability | Surface Finish (Ra) | Penetration Control | Production Rate |
|---|---|---|---|---|
| GTAW (TIG) | Small to medium | 0.4-1.6 μm | Excellent | Low |
| GMAW (MIG) | Medium to large | 1.6-3.2 μm | Good | Medium |
| Oxy-fuel (flame) | Large flat surfaces | 3.2-6.3 μm | Moderate | High |
| Plasma arc (PTA) | Medium to large | 0.4-1.6 μm | Excellent | High |
| Laser cladding | Small precision surfaces | 0.2-0.8 μm | Excellent | Medium |
For precision valve sealing surfaces, GTAW is the preferred process because it provides excellent control over heat input, penetration depth, and surface finish. The low dilution rate of GTAW (typically 5-15%) ensures that the overlay material retains its designed properties. For larger valve bodies or seat rings, plasma arc welding or GMAW may be more economical while still achieving acceptable surface quality.
Multi-Pass Overlay Strategy
A typical multi-pass overlay strategy for valve sealing surfaces involves the following sequence:
- Surface preparation: The sealing surface is ground to remove scale, rust, and existing worn material. A uniform groove with a depth of 1.5-3.0 mm and an included angle of 60-90 degrees is prepared. The groove edges are chamfered to prevent stress concentration.
- First pass (tack/bonding pass): A thin pass (0.5-1.0 mm) of compatible filler material is deposited to establish metallurgical bonding with the substrate. This pass uses slightly higher heat input to ensure adequate fusion.
- Intermediate passes: Additional passes (1.0-2.0 mm each) are deposited to build up the required overlay thickness. Each pass is inspected for defects before the next pass is applied.
- Final finishing pass: A thin final pass (0.5-1.0 mm) is deposited with reduced heat input to minimize surface roughness and residual stress. This pass is critical for achieving the required surface finish for sealing performance.
Critical Quality Control Parameters
| Parameter | Acceptance Criteria | Inspection Method |
|---|---|---|
| Surface roughness (Ra) | ≤ 1.6 μm (typical), ≤ 0.4 μm (precision) | Surface profilometer |
| Hardness uniformity | ± 30 HB variation across surface | Microhardness tester |
| Overlay thickness | Within ± 0.5 mm of nominal | Ultrasonic thickness gauge |
| Surface defects | No cracks, porosity, or inclusions | Visual + dye penetrant (PT) |
| Undercut | Maximum 0.5 mm depth | Visual + profilometer |
| Dilution rate | ≤ 20% for first pass, ≤ 10% for final pass | Metallographic analysis |
Engineering Practice and Case Studies
The publication describes several successful applications of valve sealing surface overlay in industrial settings. One notable case involved the overlay of Stellite 6 on the sealing surfaces of a high-pressure gate valve used in a petroleum refining application. The valve was subjected to sour gas service (H₂S-containing) at temperatures up to 200°C and pressures up to 15 MPa. The original carbon steel sealing surfaces exhibited severe galling and wear after only 500 hours of operation. After overlaying with Stellite 6 using a multi-pass GTAW procedure, the service life was extended to over 10,000 hours, representing a 20-fold improvement.
Another application involved the overlay of 316 stainless steel on the seats of a butterfly valve used in a chemical processing plant handling dilute sulfuric acid. The overlay provided both corrosion resistance and improved sealing performance, eliminating the need for frequent valve replacement.
The authors also discuss the importance of post-weld machining in achieving the required sealing surface geometry. After overlay welding, the surface is typically machined to the final dimensions and surface finish. For precision valve applications, the machining allowance after welding should be 1.0-2.0 mm to allow for removal of the rough weld surface and any residual stresses in the near-surface region.
Key Technical Challenges and Solutions
Prevention of Cracking
Cracking in valve sealing surface overlays can occur due to several mechanisms:
- Hot cracking: Caused by low-melting-point phases in Co-Cr or Ni-based overlay materials. Mitigated by controlling sulfur and phosphorus content in filler materials and maintaining appropriate cooling rates.
- Cold cracking (hydrogen-induced): Occurs in high-strength martensitic overlay materials such as 1Cr13. Prevented by preheating to 200-300°C, rapid post-weld cooling to avoid the embrittlement temperature range (200-400°C), and post-weld tempering.
- Stress cracking: Resulting from residual stresses in the overlay. Mitigated by stress-relief annealing or controlled welding sequences that minimize thermal gradients.
Surface Finish Achievement
Achieving a smooth sealing surface after welding is challenging because weld beads inherently produce a rough surface. The following strategies are employed:
- Using thin, closely-spaced beads with minimal overlap to minimize surface irregularities.
- Applying the final pass with reduced current and slower travel speed to produce a flatter bead profile.
- Using a backing plate or backing rod to prevent burn-through and control the back-side geometry.
- Employing post-weld machining or grinding to achieve the final surface finish.
Study Insights and Implications
This publication provides valuable insights into the practical application of weld overlay technology for valve sealing surfaces, reflecting the engineering challenges and solutions developed during a period of rapid industrialization. The systematic approach to material selection, process development, and quality control demonstrates a mature understanding of overlay welding technology.
For contemporary practice, several implications emerge from this work. First, the emphasis on multi-pass overlay strategies with controlled dilution remains fundamental to achieving reliable sealing surface performance. Second, the importance of post-weld machining cannot be overstated, as the final sealing performance is determined by the machined surface rather than the as-welded surface. Third, the material selection framework presented in this work remains largely applicable today, with modern overlay materials offering enhanced properties but following similar selection principles.
Modern engineers should complement the empirical approaches described in this publication with advanced techniques such as computational fluid dynamics (CFD) for predicting erosion patterns on valve surfaces, and in-situ monitoring systems for real-time assessment of valve condition. The fundamental metallurgical principles, however, remain unchanged: the overlay material must be metallurgically compatible with the substrate, the welding process must be controlled to minimize defects, and the final surface must meet the geometric and finish requirements for the intended sealing function.