Zhuojin Pipe Fitting Co., LtdHardfacing of Copper Sealing Surfaces on Valve Bodies
Literature Overview
This 1990 paper published in Valves (No. 4, pp. 15–16) by Deng Hongli addresses the hardfacing of copper alloy sealing surfaces on valve bodies. Valve bodies are critical components in fluid control systems, where the sealing surface between the valve body and the valve seat (or plug) determines the leak-tightness of the valve. For certain applications, copper alloy sealing surfaces are required to provide superior sealing performance, corrosion resistance, and compatibility with specific media.
Service Requirements Analysis
Why Copper Alloy Sealing Surfaces?
Copper alloy sealing surfaces are specified for several critical reasons:
| Requirement | Copper Alloy Advantage | Alternative Material |
|---|---|---|
| Sealing performance | Soft, conformable, excellent sealing | Hard materials may leak |
| Corrosion resistance | Good resistance to water, steam, mild acids | Carbon steel corrodes rapidly |
| Galling resistance | Low friction coefficient, prevents galling | Hard-on-hard contacts gall |
| Compatibility | Compatible with many media | Some materials react with media |
| Thermal conductivity | High, prevents hot spots | Low-conductivity materials overheat |
Common copper alloys used for valve sealing surfaces include:
- Aluminum bronze (Cu-Al-Fe-Ni): Good strength, excellent corrosion resistance
- Nickel silver (Cu-Ni-Zn): Good machinability, moderate corrosion resistance
- Bronze (Cu-Sn): Good wear resistance, moderate corrosion resistance
- Cupronickel (Cu-Ni): Excellent corrosion resistance, good for marine applications
Challenges of Hardfacing Copper on Steel
Hardfacing copper alloy on steel valve bodies presents several metallurgical challenges:
- High melting point difference: Copper melts at 1085°C, while steel melts at 1370–1500°C. This creates a large thermal gradient during welding.
- Low diffusivity of copper in iron: Copper does not readily dissolve in iron, leading to potential interfacial reactions and brittle intermetallic formation.
- Thermal expansion mismatch: Copper has a higher coefficient of thermal expansion than steel, leading to residual stresses during cooling.
- Oxidation sensitivity: Copper readily forms oxides at welding temperatures, which can lead to porosity and poor fusion.
- Dilution control: Excessive dilution with the steel base material reduces the copper content in the deposit, compromising its sealing and corrosion properties.
Hardfacing Process Design
Material Selection
The hardfacing material selection must consider:
- Copper content: Sufficient to provide the required sealing and corrosion properties (typically > 70% Cu in the deposit)
- Alloying elements: Al, Ni, Sn, or Zn to improve specific properties
- Weldability: Good fusion with the steel base material
- Crack resistance: Ability to accommodate thermal expansion mismatch
- Post-weld machinability: The deposit must be machinable to achieve the required surface finish
Based on these requirements, the following hardfacing materials were considered:
| Material Type | Composition (Approx.) | Hardness (HB) | Sealing Performance | Suitability |
|---|---|---|---|---|
| Aluminum bronze | Cu-10Al-5Fe-5Ni | 150–200 | Excellent | Good for high-pressure valves |
| Nickel silver | Cu-18Ni-20Zn | 120–150 | Good | Good for general applications |
| Bronze | Cu-10Sn | 130–170 | Good | Good for moderate pressure |
| Cupronickel | Cu-30Ni | 100–130 | Excellent | Good for marine applications |
Process Selection
Given the geometry of valve bodies (complex internal cavities, curved surfaces, thin sections), the following welding processes were evaluated:
| Process | Advantages | Disadvantages | Suitability |
|---|---|---|---|
| SMAW | Simple, portable, good for complex geometries | Lower deposition rate, higher operator dependence | Good for field repair |
| GTAW | Precise control, low dilution, good for thin sections | Lower deposition rate, requires skilled operator | Good for critical sealing surfaces |
| OAW (Oxy-Acetylene) | Simple, no power required | Lower quality, higher dilution | Limited use |
| Laser cladding | Precise, low dilution, high quality | Requires specialized equipment | Premium option |
For valve body sealing surfaces, GTAW was selected as the primary process due to its precision and low dilution characteristics. SMAW was used for thicker deposits where precision was less critical.
Process Parameters
GTAW Parameters
| Parameter | Value | Notes |
|---|---|---|
| Tungsten electrode | 2.4–3.2 mm, ceriated | DCEN |
| Filler wire | ERNiCuAl (or equivalent copper alloy) | Match deposit composition |
| Current | 100–150 A | DCEN |
| Shielding gas | Argon (99.99%) | 15–20 L/min |
| Travel speed | 100–150 mm/min | Ensure good fusion, low dilution |
| Preheat temperature | 100–150°C | Reduce cracking tendency |
| Interpass temperature | < 200°C | Control cooling rate |
| Number of passes | 2–3 | Build up to required thickness |
SMAW Parameters
| Parameter | Value | Notes |
|---|---|---|
| Electrode type | E-CuAl (or equivalent) | Low-hydrogen, copper-alloy |
| Electrode diameter | 3.2 mm | For general areas |
| Current | 100–140 A | DCEN |