CLADDING TECHNOLOGY SHANXI CO., LTDNickel-Based Overlay Welding Process for Valve Body Sealing Surfaces
Literature Overview
This study addresses the challenging requirements for nickel-based overlay welding on valve body sealing surfaces, which must provide excellent corrosion resistance, wear resistance, and dimensional accuracy in aggressive process media environments. Valve sealing surfaces are subjected to cyclic compression, sliding contact, and exposure to corrosive fluids (acids, chlorides, hydrocarbons), making the overlay layer a critical component for long-term reliability.
Core Technical Content
Material Selection for Valve Sealing Surfaces
| Application | Overlay Material | Key Properties | Typical Application |
|---|---|---|---|
| Acid service | Hastelloy C-276 | Mo 14–16%, W 2.5–3.5% | Sulfuric acid, hydrochloric acid |
| General corrosion | Inconel 625 | Ni ≥ 55%, Mo 8–10% | Chloride-containing media |
| High temperature | Inconel 617 | Al 5–6%, Ti 1–1.5% | High-temperature oxidation |
| Wear + corrosion | Stellite 6 | Cr 21–25%, Co 50–65% | Sliding contact in corrosive media |
| Severe wear | Stellite 21 | Cr 29–33%, Co 55–65% | High-pressure sliding contact |
Process Selection and Parameters
The selection of overlay process depends on the valve size, sealing surface geometry, and required surface finish:
| Process | Applicable Valve Size | Surface Finish | Dilution | Efficiency | Cost |
|---|---|---|---|---|---|
| GTAW (TIG) | Small (< DN50) | Ra 0.2–0.8 μm | 5–15% | Low | Medium |
| GMAW | Medium (DN50–DN200) | Ra 0.8–1.6 μm | 10–25% | Medium | Low |
| FCAW | Medium-large (DN50–DN500) | Ra 1.6–3.2 μm | 15–30% | High | Low |
| PTA | All sizes | Ra 0.2–0.8 μm | 2–10% | High | High |
| Laser cladding | Small-medium | Ra 0.1–0.4 μm | 1–5% | Medium | High |
| Hot-wire TIG | Medium-large | Ra 0.4–1.0 μm | 5–15% | High | Medium |
Recommended Process Parameters for GTAW Overlay
For small valve bodies where surface finish and low dilution are critical:
| Parameter | Value | Notes |
|---|---|---|
| Current | 120–200 A | DCEN for tungsten electrode |
| Voltage | 12–18 V | Arc length 1–2 mm |
| Travel speed | 30–80 mm/min | Slower for better fusion |
| Wire feed speed | 1.0–2.5 m/min | Match to travel speed |
| Shielding gas | 100% Ar or Ar/He mix | 5–15 L/min |
| Preheating | 100–150°C | Reduce residual stress |
| Layer thickness | 0.5–2.0 mm per pass | Multi-pass for total thickness |
| Total overlay thickness | 2–5 mm | Minimum for corrosion allowance |
Microstructure and Performance Requirements
| Property | Requirement | Test Method |
|---|---|---|
| Hardness | 200–350 HV (Ni-based); 400–500 HV (Co-based) | GB/T 231.1 |
| Tensile strength | ≥ 450 MPa | GB/T 228.1 |
| Elongation | ≥ 20% | GB/T 228.1 |
| Intergranular corrosion | No attack after 120 h | GB/T 4334 |
| Pitting resistance | PREN ≥ 32 (C-276); ≥ 25 (625) | ASTM G48 |
| Crevice corrosion | No attack after 24 h | ASTM G48 Method C |
| Surface finish | Ra ≤ 0.8 μm (after machining) | GB/T 1031 |
Defect Analysis and Countermeasures
| Defect | Cause | Detection | Countermeasure |
|---|---|---|---|
| Tungsten inclusion | Contaminated electrode, arc instability | RT, MT | Use clean electrode, stable arc |
| Porosity | Gas entrapment, moisture | RT, UT | Dry flux/wire, proper shielding |
| Lack of fusion | Insufficient heat input, poor technique | UT, MT | Increase current, improve technique |
| Cracking | High carbon equivalent, hydrogen | MT, PT | Preheat, use low-hydrogen consumables |
| Excessive dilution | Too high heat input, too fast travel | Chemical analysis | Reduce heat input, optimize parameters |
| Surface roughness | Poor technique, excessive spatter | Visual, surface roughness | Optimize parameters, use proper shielding |
Engineering Practice Considerations
Pre-Weld Preparation
- Surface cleaning: Remove all scale, rust, and contaminants by grinding to bare metal, followed by acetone cleaning.
- Substrate analysis: Verify base metal composition to select appropriate overlay material and welding parameters.
- Fit-up: Ensure proper alignment and fit-up of valve body components to minimize welding distortion.
- Preheating: Apply uniform preheating at 100–150°C to reduce thermal gradient and residual stress.
Post-Weld Treatment
- Stress relief: Heat treatment at 600–700°C for 1–2 hours to reduce residual stresses.
- Machining: Machine the overlay surface to achieve required dimensional accuracy and surface finish (Ra ≤ 0.8 μm).
- Passivation: Chemical passivation with nitric acid or citric acid to remove free iron contamination and activate the passive film.
- Inspection: Complete NDT (VT, MT, PT) and performance testing (hardness, corrosion) before assembly.
Quality Control Strategy
A comprehensive quality control strategy should follow the PDCA (Plan-Do-Check-Act) cycle:
- Plan: Develop and qualify the welding procedure (WPQ) per NB/T 47014 or ASME IX, including all required tests.
- Do: Execute welding with trained personnel, proper equipment, and controlled environment.
- Check: Perform NDT (VT, MT, PT, UT, RT as applicable) and mechanical testing per the qualified procedure.
- Act: Document results, identify non-conformances, and implement corrective actions for continuous improvement.
Key Reflections and Insights
The nickel-based overlay welding of valve body sealing surfaces is a specialized application that requires careful attention to both metallurgical and mechanical considerations. The overlay layer must provide corrosion resistance in aggressive media while maintaining dimensional accuracy for proper valve sealing function. The balance between these competing requirements is achieved through careful material selection, process optimization, and post-weld machining.
A critical insight from practical experience is that the passivation step after machining is often overlooked but is essential for achieving the full corrosion resistance of the nickel-based overlay. Free iron contamination from machining can significantly reduce the pitting and crevice corrosion resistance, particularly in chloride-containing environments. Proper passivation with nitric acid (20–30% by weight, 60–80°C, 10–30 minutes) or citric acid (10–20% by weight, 60–70°C, 20–40 minutes) is mandatory.
From a standards perspective, valve body fabrication must comply with the applicable pressure vessel or piping standard (GB/T 12221, GB/T 12224, ASME B16.34, or API 6D), and the overlay welding must be qualified per NB/T 47014 or ASME IX. The overlay layer must meet the material specification requirements (ASTM B366, B409, B564, or equivalent) and pass all required corrosion and mechanical tests before the valve is released for service.