CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Transferred Arc Cladding Application in Valve Manufacturing Study Note
Literature Overview
This 1999 publication by Zu Yujie from Muling Power Station Valve Factory, published in the journal "Applied Science and Technology," documents the practical application of plasma transferred arc (PTA) cladding technology in valve manufacturing. The adoption of PTA cladding represents a significant technological upgrade from traditional arc welding processes, offering superior control over dilution, deposit quality, and surface finish — all critical parameters for valve seal surfaces and pressure-containing components.
Technical Advantages of PTA Cladding
Plasma transferred arc cladding offers several distinct advantages over conventional arc welding processes for valve manufacturing:
| Performance Parameter | PTA Cladding | GMAW Overlay | SAW Overlay |
|---|---|---|---|
| Dilution rate | 5–15% | 20–40% | 30–60% |
| Surface roughness (Ra) | 0.5–2.0 μm | 3.0–10.0 μm | 5.0–15.0 μm |
| Deposit thickness per pass | 0.1–0.5 mm | 0.5–2.0 mm | 1.0–3.0 mm |
| Heat input (kJ/mm) | 0.5–2.0 | 2.0–5.0 | 5.0–15.0 |
| Microstructure refinement | Excellent | Moderate | Coarse |
| Surface oxidation | Minimal | Moderate | Heavy (flux) |
The low dilution rate of PTA is the most significant advantage for valve manufacturing. For overlaying stainless steel or nickel-based alloys on carbon steel valve bodies, the low dilution ensures that the overlay composition closely matches the filler metal composition, providing the intended corrosion resistance and mechanical properties.
Application in Valve Manufacturing
Typical Valve Applications
| Valve Component | Overlay Material | Service Condition | PTA Advantage |
|---|---|---|---|
| Globe valve trim | 316L / Inconel 625 | Corrosive media | Low dilution maintains alloy composition |
| Gate valve seat | Stellite 6 / 21 | Abrasive slurry | Hard, wear-resistant surface |
| Ball valve seat | 316 / Monel 400 | High-pressure corrosive | Smooth surface, low porosity |
| Butterfly valve disc | 316L / Duplex 2205 | Marine/chemical | Excellent surface finish for sealing |
| Control valve plug | Inconel 625 / Hastelloy C276 | High-temp corrosive | Fine microstructure, low HAZ softening |
PTA Process Parameters for Valve Cladding
| Parameter | Range | Notes |
|---|---|---|
| Plasma current (A) | 100–400 | Depends on deposit thickness required |
| Arc voltage (V) | 15–30 | Related to arc length and transfer mode |
| Travel speed (mm/min) | 50–200 | Higher speed = thinner deposit |
| Powder feed rate (g/min) | 50–200 | Must match travel speed and current |
| Shielding gas flow (L/min) | 10–20 | Ar or Ar + He mixture |
| Powder-to-wire ratio | 1:1 to 3:1 | Adjusts deposit composition |
| Arc length (mm) | 3–6 | Must be controlled for stability |
| Powder particle size | 45–75 μm | Uniformity critical for stable transfer |
The powder-to-wire ratio is a unique parameter in PTA that allows composition adjustment without changing the base wire. For example, using a 316L wire with 316L powder produces a deposit with composition very close to 316L, while adjusting the ratio can shift the composition toward higher or lower alloy content.
Process Development and Quality Assurance
Process Qualification Requirements
For pressure-containing valve applications, the PTA cladding process must be qualified per the relevant code:
| Code/Standard | Qualification Requirement | Test Coupons |
|---|---|---|
| ASME VIII Div.1 | WPQ per ASME IX | Flat and groove welds |
| ASME VIII Div.2 | WPS qualification | Qualification per Sec. V |
| GB/T 150 | WPS qualification | Flat and groove welds |
| NB/T 47014 | WPS qualification | Flat, groove, and overlay |
| API 934 | Overlay qualification | Peel, shear, and hardness tests |
Non-Destructive Testing Requirements
| Inspection Method | Application | Acceptance Criteria |
|---|---|---|
| Visual (VT) | Surface quality, porosity | No visible defects > 0.5 mm |
| Magnetic particle (MT) | Surface cracks | No linear indications |
| Penetrant (PT) | Surface discontinuities | No indications > 1.0 mm |
| Ultrasonic (UT) | Internal defects | Per relevant code |
| Radiographic (RT) | Volumetric defects | Per relevant code |
Engineering Case Study
A typical application scenario involves the overlay of Inconel 625 on a carbon steel control valve body operating in a high-temperature, high-chloride environment (e.g., desalination plant seawater service):
- Surface preparation — Grind the overlay area to bare metal, extend 10 mm beyond the final overlay boundary.
- Base layer — Apply 0.5 mm Inconel 625 PTA overlay with 150 A, 200 mm/min travel speed.
- Build-up passes — Apply 2–3 additional passes to achieve total thickness of 1.5–2.0 mm.
- Post-weld treatment — Stress-relieve at 550°C for 2 hours in protective atmosphere.
- Inspection — MT for surface cracks, UT for interface bonding, hardness traverse.
- Machining — Machine trim to final dimensions, Ra 0.4 μm surface finish.
The resulting overlay provides excellent resistance to chloride stress corrosion cracking (SCC) and pitting corrosion, extending valve service life from 6 months to over 5 years in aggressive seawater service.
Key Technical Challenges
Dilution Control
The most critical challenge in PTA cladding for valve applications is maintaining dilution below 15%. High dilution from the carbon steel base material introduces ferrite and reduces the corrosion resistance of the overlay. Control measures include:
- Using low current (100–200 A) for thin deposits
- High travel speed (150–200 mm/min)
- Pre-depositing a low-dilution base layer before building up thickness
- Using wire + powder combination to dilute the base metal contribution
Thermal Distortion
Valve bodies are precision components where dimensional accuracy is critical. The thermal input from PTA cladding, while lower than conventional arc welding, can still cause distortion in thin-walled or complex geometries. Mitigation strategies include:
- Sequential cladding in a symmetric pattern to balance thermal input
- Fixturing the valve body in a rigid fixture during cladding
- Limiting total heat input by using multiple thin passes rather than few thick passes
- Post-weld stress relief at 550°C (for austenitic overlays) to relieve residual stresses
Study Insights and Implications
The adoption of PTA cladding in valve manufacturing represents a paradigm shift from "welding" to "surface engineering." The low dilution, fine microstructure, and superior surface finish achievable with PTA enable overlay materials to perform at their full potential — something impossible with high-dilution conventional processes.
The study's practical value lies in demonstrating that PTA technology, once considered too expensive for routine production, can be economically justified for high-value valve components where failure consequences are severe. The cost of a failed control valve in a power plant or chemical process can exceed $50,000 in unplanned shutdown costs alone, making the premium for PTA-clad trim a trivial fraction of the risk mitigation value.
The methodology established in this study — systematic process parameter optimization, rigorous quality assurance, and economic justification — provides a template for introducing advanced cladding technologies into traditional manufacturing environments. The key lesson is that technology adoption requires not only technical competence but also organizational commitment to quality and willingness to invest in operator training and equipment maintenance.