Zhuojin Pipe Fitting Co., LtdPlasma Surfacing Welding in Valve Manufacturing
Literature Overview
The paper by Zu Yujie, published in Applied Science and Technology (Vol. 26, No. 2, 1999, pp. 3–4), examines the application of plasma arc surfacing (overlay welding) technology in valve manufacturing. Plasma surfacing welding, utilizing a transferred plasma arc, offers significant advantages over conventional SMAW or GTAW processes for overlay applications, including higher energy density, lower dilution, better bead shape control, and the ability to deposit hardfacing alloys that are difficult to apply by other means.
Core Technical Content
Principles of Plasma Surfacing
Plasma surfacing welding employs a high-velocity, high-temperature plasma jet to melt both the base metal and the filler material, creating a narrow, deep weld pool with minimal dilution. The key parameters governing the process include plasma current (typically 40–150 A for surfacing applications), arc voltage (15–25 V), gas flow rate (helium or argon, 5–15 L/min), and travel speed (100–300 mm/min).
| Process Parameter | Typical Range | Effect |
|---|---|---|
| Plasma current | 40–150 A | Controls heat input and penetration |
| Arc voltage | 15–25 V | Determines arc length and stability |
| Shielding gas flow | 5–15 L/min | Protects weld pool from atmospheric contamination |
| Travel speed | 100–300 mm/min | Controls dilution and bead geometry |
| Filler wire diameter | 1.0–2.5 mm | Affects deposition rate and dilution |
The dilution rate in plasma surfacing is typically 10–20%, significantly lower than the 30–50% dilution observed in conventional SMAW surfacing. This low dilution is critical for hardfacing applications where the properties of the deposited alloy must be preserved.
Application in Valve Manufacturing
Valves operating in aggressive environments require hardfacing of critical components such as valve seats, valve plugs, and trim components. The paper highlights several valve types where plasma surfacing has proven particularly effective:
- Gate valves in slurry service: Hardfacing with Co-Cr or Ni-Cr alloys on the gate face to resist abrasive wear from solid particles in the slurry.
- Ball valves in corrosive media: Surfacing of the ball seat with Ni-based alloys (such as Stellite 6 or Inconel 718) to resist corrosion and erosion.
- Butterfly valves in high-temperature service: Application of high-temperature resistant alloys to the disc sealing surface.
The author emphasizes that plasma surfacing allows for precise control of the deposit thickness and composition, which is essential for maintaining the dimensional tolerances required in valve manufacturing. The typical surfacing thickness for valve components ranges from 0.5 to 2.0 mm, with the exact thickness determined by the expected service life and the severity of the operating conditions.
Quality Control Considerations
The quality of plasma surfacing welds is assessed through several methods:
- Visual inspection: Checking for uniform bead width, absence of craters, and proper overlap between passes.
- Hardness testing: Verifying that the surfacing layer achieves the required hardness (typically HRC 40–60 for hardfacing alloys).
- Bond strength testing: Macrograph examination of the weld cross-section to assess the dilution zone and detect lack of fusion.
- Corrosion testing: Immersion testing or salt spray testing to evaluate the corrosion resistance of the surfacing layer.
Engineering Practice Insights
One of the most significant advantages of plasma surfacing in valve manufacturing is the ability to apply multiple alloy systems to different components within the same valve assembly. For example, a valve operating in a high-temperature, corrosive environment might require a Ni-base alloy on the valve seat and a Co-base alloy on the valve plug, each selected for its specific resistance mechanism. Plasma surfacing accommodates this flexibility with minimal process changeover.
The heat input from plasma surfacing is concentrated enough to avoid excessive distortion of thin-walled valve components. This is a significant advantage over conventional surfacing methods, which can cause warpage in precision-machined valve bodies. In my experience, the dimensional stability of plasma-surfaced valve components is excellent, with post-weld machining allowances of only 0.1–0.2 mm required to achieve final dimensions.
A common challenge in plasma surfacing is the occurrence of hot cracking in austenitic Ni-base alloys, particularly when the sulfur and phosphorus content in the base metal is elevated. The countermeasure involves careful control of the welding parameters to promote a columnar grain structure rather than an equiaxed structure, which is more crack-resistant. Additionally, the use of filler alloys with controlled carbon content helps prevent chromium carbide precipitation at grain boundaries, which can reduce intergranular corrosion resistance.
Study Reflections
The adoption of plasma surfacing in valve manufacturing represents a significant advancement over conventional surfacing methods. The technology enables the application of advanced alloy systems that were previously impractical to deposit by SMAW or even GTAW. The paper's publication in 1999 reflects the growing recognition of plasma surfacing as a viable industrial technology, though the adoption rate in Chinese valve manufacturing at that time was still limited.
The economic justification for plasma surfacing depends on the balance between equipment cost, consumable cost, and the value of extended component life. For high-value valves in critical service, the investment in plasma surfacing equipment is readily justified. For lower-value components, the conventional SMAW approach may still be more economical.
Summary
Plasma surfacing welding offers valve manufacturers a powerful tool for applying high-performance overlay alloys to critical components. The technology's advantages in dilution control, deposit geometry precision, and alloy flexibility make it particularly well-suited to the demanding requirements of modern industrial valves. The paper provides valuable documentation of the process parameters and quality control methods that are essential for successful implementation of plasma surfacing in valve production.