CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Selection of Cladding Materials for Valve Sealing Surfaces
Literature Overview
The technical work authored by Su Zhidong in 2000 from the Shenyang Valve Research Institute addresses the critical engineering problem of selecting appropriate cladding (overlay welding) materials for valve sealing surfaces. Valve sealing surfaces are among the most demanding applications for weld overlay technology, requiring simultaneous resistance to erosion, galling, cavitation, and corrosion under high-velocity fluid flow conditions. This publication represents a systematic approach to material selection methodology for this specific application domain.
Technical Requirements for Valve Sealing Surface Cladding
Valve sealing surfaces face unique and often combined degradation mechanisms that must be addressed through careful material selection:
- Erosion resistance: High-velocity fluid flow, particularly with solid particle-laden fluids, requires overlay materials with high hardness and good toughness.
- Galling resistance: Under reciprocating sliding contact, materials must resist cold welding and adhesive wear.
- Cavitation resistance: In water and low-viscosity liquid service, the overlay must withstand micro-jet impingement from collapsing vapor bubbles.
- Corrosion resistance: Chemical compatibility with the process fluid is essential, particularly in chemical processing and marine applications.
- Hardness matching: The overlay hardness must be compatible with the mating seat surface to prevent accelerated wear of either surface.
Material Selection Framework
The study proposes a systematic selection approach based on service conditions:
| Service Condition | Recommended Overlay Material | Typical Hardness (HRC) | Key Property |
|---|---|---|---|
| Water service (low cavitation) | Stellite 6 (Co-Cr-W) | 40-45 | Cavitation resistance |
| Water service (severe cavitation) | Stellite 21 (Co-Cr) | 45-50 | Superior cavitation resistance |
| Oil/gas service (abrasive) | Alloy 15 (Ni-Cr-Mo) | 45-50 | Erosion-corrosion resistance |
| Slurry service | Alloy 17 (Ni-Cr-C) | 45-50 | Abrasive wear resistance |
| Chemical service (non-abrasive) | Alloy 5 (Ni-Fe-Cr) | 35-40 | General corrosion resistance |
| High-temperature gas | Alloy 35 (Co-Cr) | 40-45 | Thermal stability |
| Seawater service | Alloy 7 (Ni-Fe-Cr) | 35-40 | Chloride pitting resistance |
| Cryogenic service | Alloy 31 (Ni) | 25-30 | Low-temperature toughness |
Process Considerations for Valve Cladding
The application of overlay materials to valve sealing surfaces requires special attention to process parameters due to the geometric constraints and functional requirements:
- Surface preparation: The base metal surface must be machined to within 0.1 mm of final dimensions, leaving a minimum of 1.5-2.0 mm of overlay material to be removed by subsequent machining.
- Preheating: Typically 250°C to 400°C for carbon steel and low-alloy steel valve bodies, depending on the base material and overlay alloy being applied.
- Interpass temperature: Must not exceed 400°C for cobalt-based alloys to prevent grain growth and degradation of wear properties.
- Post-weld treatment: Stellite-based overlays may be solution-treated at 1010°C to 1120°C followed by air cooling to optimize carbide distribution.
- Machining allowance: Minimum 2 mm of overlay material should be provided for post-weld machining to achieve the required surface finish (typically Ra 0.4-0.8 μm for sealing surfaces).
Quality Control and Inspection
The inspection regime for valve sealing surface cladding is critical to ensuring long-term service reliability:
- Visual inspection: All overlay surfaces must be free of cracks, undercut, and excessive porosity.
- Magnetic particle testing (MT): Required for ferromagnetic base materials to detect surface and near-surface discontinuities.
- Hardness testing: Overlay hardness must be verified at multiple locations to confirm proper material application and adequate build-up.
- Bond strength testing: Peel tests or bend tests per ASTM G125 or equivalent to verify overlay-to-base adhesion.
- Chemical analysis: Spectroscopic verification of overlay composition to confirm correct material application.
Engineering Practice Insights
The study emphasizes that material selection for valve cladding must consider not only the overlay material properties but also the interaction between the overlay and the mating surface material. A common engineering error is to apply a very hard overlay (e.g., Stellite 6, HRC 40-45) to one sealing surface while the mating surface remains in soft carbon steel (HRC 15-20), which results in accelerated wear of the softer surface. The optimal approach is to apply compatible overlay materials to both sealing surfaces or to ensure adequate hardness differential.
Furthermore, the study highlights that the cladding process itself can introduce defects that compromise valve performance. Incomplete fusion at the overlay-base interface, lack of fusion between overlay passes, and hot cracking in cobalt-based alloys are the most common defects encountered in production. Process qualification per NB/T 47014 or ASME IX is essential to ensure consistent quality.
Summary
The systematic approach to valve sealing surface cladding material selection presented in this work provides a valuable reference framework for engineers specifying overlay materials for valve applications. The key takeaway is that there is no single "best" overlay material; rather, the optimal choice depends on the specific combination of service conditions, mating surface materials, and maintenance philosophy. This holistic approach to material selection should be integrated into valve design specifications and procurement documents to ensure reliable long-term performance.