Zhuojin Pipe Fitting Co., LtdApplication of Plasma Surfacing in Valve Manufacturing
Literature Overview
This paper, authored by Zu Yujie from Muling Power Station Valve Factory and published in Applied Science and Technology in 1999, presents a practical engineering study on the application of plasma arc surfacing (cladding) technology in the manufacture of power station valves. The work addresses a long-standing challenge in valve production: how to achieve wear-resistant, corrosion-resistant, and erosion-resistant surfaces on valve internals without resorting to expensive alloy materials for the entire component. The author demonstrates that plasma surfacing offers a viable, economical, and technically sound alternative to conventional casting or solid-state hardening approaches.
Core Technical Content
The fundamental principle behind plasma surfacing involves generating a high-temperature plasma arc using a constricted arc in a gas flow, typically argon or a mixture of argon with hydrogen or nitrogen. The plasma temperature can reach 10,000 to 30,000 K, which is sufficient to melt surfacing powders or wires and deposit them onto the base metal surface with controlled dilution rates. For valve manufacturing applications, the typical parameters include a plasma current of 100 to 200 A, arc voltage of 20 to 30 V, and gas flow rates of 10 to 20 L/min for the shielding gas and 1 to 5 L/min for the stabilizing gas.
The key advantage of plasma surfacing over conventional arc welding for valve components lies in the extremely low dilution rate, typically between 5% and 15%, compared to 30% to 50% for submerged arc or shielded metal arc welding. This low dilution ensures that the as-deposited layer retains the inherent properties of the surfacing alloy, whether it is a cobalt-based Stellite alloy, a nickel-based alloy, or a chromium-carbide composite. For valve seats and valve plugs that experience severe erosion from high-velocity fluid or steam, this property retention is critical.
Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Deposition Quality |
|---|---|---|
| Plasma current | 100–200 A | Higher current increases deposition rate but may increase dilution |
| Arc voltage | 20–30 V | Controls arc length and penetration depth |
| Travel speed | 50–200 mm/min | Faster speed reduces heat input and dilution |
| Powder feed rate | 100–400 g/min | Must match current to maintain stable arc |
| Shielding gas flow | 10–20 L/min | Prevents oxidation of molten pool |
| Preheating temperature | 150–300 °C | Reduces cracking tendency in high-alloy deposits |
The paper emphasizes that the selection of surfacing material must be matched to the specific service condition of the valve. For steam valves operating at temperatures above 500 °C, cobalt-based alloys such as Stellite 6 provide excellent resistance to thermal fatigue and erosion. For water service valves, chromium-carbide composite powders offer superior cavitation resistance. The author also discusses the importance of surface preparation prior to surfacing, including grinding to remove scale and oxidation, and the use of appropriate joint design to ensure adequate overlap between successive weld passes.
Engineering Practice Integration
In the context of power station valve manufacturing, the application of plasma surfacing transforms the economic calculation of valve production. Instead of machining valve seats and plugs from expensive alloy castings, manufacturers can use conventional carbon steel or low-alloy steel forgings and apply a thin layer of hardfacing alloy via plasma surfacing. This approach reduces material costs by 40% to 60% while achieving equivalent or superior surface performance. The deposited layer thickness is typically 1 to 3 mm, which is sufficient for the expected service life and can be reworked during maintenance.
A critical engineering consideration discussed in the paper is the residual stress management during multi-pass surfacing. When building up a thick deposit on a valve seat, the sequential heating and cooling of each pass creates significant residual tensile stresses in the earlier layers. The author recommends a post-deposition stress relief treatment at 750 to 850 °C for cobalt-based alloys and 900 to 1000 °C for nickel-based alloys, held for a period proportional to the section thickness. Without proper stress relief, cracking can occur during subsequent machining or during service, leading to catastrophic valve failure.
The paper also touches upon quality control methods for verifying surfacing quality. Visual inspection is the first step, followed by magnetic particle testing or liquid penetrant testing for surface defects. Hardness testing, typically using Vickers or Rockwell C scales, verifies that the deposited layer meets the specified hardness requirements, generally 40 to 50 HRC for cobalt-based alloys and 35 to 45 HRC for nickel-based alloys. Metallographic examination of cross-sections provides the most comprehensive assessment, revealing the dilution rate, microstructure, and any subsurface defects such as lack of fusion or internal porosity.
Study Insights and Reflections
Reading this paper, I am struck by how the fundamentals of plasma surfacing established in the late 1990s remain relevant today. The physics of plasma arc generation, the thermodynamics of powder melting and deposition, and the metallurgical considerations of dilution and residual stress have not fundamentally changed. What has evolved is the precision of parameter control, the availability of advanced surfacing alloys, and the integration of robotic systems for consistent multi-pass deposition. However, the core engineering principles — matching the surfacing alloy to the service environment, controlling dilution through process parameter optimization, and managing residual stresses through preheating and post-heat treatment — remain as valid today as they were in 1999.
One aspect that particularly resonates with my own engineering experience is the emphasis on the practical limitations of the technology. The paper acknowledges that plasma surfacing requires skilled operators, careful surface preparation, and rigorous quality control. It is not a plug-and-play solution that eliminates the need for engineering judgment. The interplay between travel speed, powder feed rate, and arc parameters requires empirical calibration for each specific application. This reminds us that even advanced welding technologies demand a deep understanding of the underlying metallurgy and process physics.
The reference value of this paper lies in its clear demonstration of how a relatively mature welding technology can be adapted to solve specific manufacturing challenges in the power generation industry. For engineers working on valve design and manufacturing today, this paper serves as a reminder that the selection of surface engineering technology should be driven by service requirements and economic considerations, not by technological novelty. Plasma surfacing remains a highly effective tool in the valve manufacturer's arsenal, and its principles continue to inform the development of newer surface engineering techniques.