CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding of Valve Disc Sealing Surfaces During Warm Rough Turning
Literature Overview
Control valves are essential components in process piping systems, regulating the flow of fluids under varying pressure and temperature conditions. The sealing surfaces of valve discs are subjected to high-velocity fluid erosion, cavitation, and corrosion, necessitating the application of hardfacing overlay layers to extend service life. This literature discusses the overlay welding technology applied to valve disc sealing surfaces, with a particular focus on the warm rough turning approach to achieve the required surface geometry and finish after overlay deposition.
The challenge addressed here is the integration of overlay welding and machining operations to produce a sealing surface that meets both the hardness requirements for erosion resistance and the surface finish requirements for valve sealing integrity. The "warm rough turning" approach involves machining the overlay layer at an elevated temperature, which reduces the cutting force and improves the surface finish while maintaining the overlay hardness.
Core Technical Analysis
Overlay Material Selection for Valve Disc Sealing Surfaces
The selection of overlay material for valve disc sealing surfaces depends on the service conditions, including the fluid type, temperature, pressure, and flow velocity. The following table summarizes the common material systems:
| Overlay Material | Hardness (HRC) | Application | Typical Valve Type |
|---|---|---|---|
| Stellite 6 (Co-Cr-W) | 42-48 | High-temperature, high-pressure | Steam valves, hot oil valves |
| Stellite 21 (Co-Cr) | 38-42 | Moderate temperature | Water valves, moderate service |
| 316L Stainless Steel | 22-28 (annealed) | Corrosive, low-temperature | Chemical process valves |
| Inconel 625 | 32-38 (solution treated) | High-temperature, corrosive | Steam, hot gas valves |
| 17-4PH Stainless Steel | 42-46 (precipitation hardened) | High-strength, moderate corrosion | Hydraulic valves |
For high-temperature steam valve applications, Stellite 6 is the preferred overlay material due to its excellent wear resistance, thermal stability, and resistance to erosion by high-velocity steam. The overlay hardness of 42-48 HRC provides superior resistance to cavitation erosion and high-temperature oxidation compared to stainless steel alternatives.
Warm Rough Turning Process
The warm rough turning process is a specialized machining technique applied to overlay layers that have been deposited by welding. The process involves heating the valve disc to a temperature of 200-350°C before rough turning the overlay layer to the near-final dimension. This elevated temperature reduces the hardness of the overlay material by 10-20%, significantly reducing the cutting force and tool wear while allowing for larger depth of cut and higher feed rates.
| Process Parameter | Cold Turning | Warm Rough Turning (200-350°C) |
|---|---|---|
| Cutting force | High | Reduced by 30-50% |
| Tool life | Short | Extended by 2-3x |
| Surface roughness (Ra) | 3.2-6.3 μm | 1.6-3.2 μm |
| Depth of cut | 0.1-0.3 mm | 0.3-0.8 mm |
| Feed rate | 0.05-0.1 mm/rev | 0.1-0.2 mm/rev |
| MRR | Low | 2-3x higher |
The warm rough turning process is particularly advantageous for overlay layers containing hard carbides, such as Stellite alloys, which are difficult to machine at room temperature. The elevated temperature softens the matrix while the carbides remain relatively unaffected, allowing the carbides to be cut rather than ground, which is more efficient and produces a better surface finish.
Overlay Welding Process and Parameters
The overlay welding of valve disc sealing surfaces is typically performed using gas tungsten arc welding (GTAW) or plasma arc welding (PAW) to achieve precise control over the weld bead geometry and dilution rate. The process parameters are optimized to ensure complete bonding to the base metal while minimizing dilution of the overlay alloy.
| Parameter | GTAW | PAW |
|---|---|---|
| Current | 80-150 A | 100-200 A |
| Travel speed | 50-100 mm/min | 100-200 mm/min |
| Shielding gas | Argon | Argon |
| Wire/powder feed | φ1.6-2.4 mm wire | Powder, 0.05-0.15 mm |
| Dilution rate | 10-20% | 5-10% |
| Layer thickness | 0.5-1.0 mm/pass | 0.2-0.5 mm/pass |
The valve disc is typically oriented with the sealing surface facing upward during welding, and the overlay is applied in a spiral or circular pattern to ensure uniform coverage. The number of passes is determined by the required overlay thickness, which is typically 1.5-3.0 mm for valve disc sealing surfaces.
Quality Control and Defect Prevention
| Defect | Cause | Prevention |
|---|---|---|
| Lack of fusion | Insufficient current or excessive travel speed | Increase current; reduce travel speed |
| Cracking | High residual stress; incompatible base metal | Preheat base metal; use compatible filler |
| Surface porosity | Contaminated filler or shielding gas | Clean filler; ensure gas purity |
| Hardness variation | Uneven dilution | Monitor dilution rate; adjust parameters |
| Geometric inaccuracy | Distortion during welding | Use balanced welding pattern; fixture support |
The dimensional accuracy of the sealing surface after overlay and machining is critical to valve performance. The sealing surface must be flat to within 0.01 mm over the sealing diameter, with a surface roughness of Ra 0.2-0.4 μm for critical applications. The warm rough turning process, followed by a cold finishing pass, achieves this level of accuracy.
Engineering Practice Integration
In manufacturing practice, the overlay welding of valve disc sealing surfaces is performed during the valve body fabrication stage, before the disc is assembled into the valve body. The disc is machined to a near-final dimension, leaving an allowance of 2-3 mm for the overlay layer. After overlay welding, the disc is heat treated to relieve stresses and to achieve the required hardness in the overlay layer.
The warm rough turning operation is performed after the overlay layer has been deposited and heat treated, but before the final finishing operation. The disc is heated to 200-350°C using an induction heater or resistance heater, and the overlay layer is rough turned to within 0.1-0.2 mm of the final dimension. A final cold finishing pass is then applied to achieve the required surface finish and dimensional accuracy.
A notable challenge is the control of thermal distortion during the warm rough turning process. The heating and cooling of the valve disc can cause warping, particularly for thin-walled or large-diameter discs. This is mitigated by using a controlled heating rate (less than 100°C/hour) and by supporting the disc on a rigid fixture during machining.
Key Reflections
The warm rough turning approach to machining valve disc overlay layers represents a practical solution to the challenge of machining hardfacing alloys that are difficult to machine at room temperature. The elevated temperature reduces the cutting force and tool wear while improving the surface finish, resulting in a more efficient and cost-effective machining process.
A key insight is that the warm rough turning process must be carefully controlled to avoid exceeding the temperature at which the overlay material begins to soften significantly. For Stellite alloys, the temperature should be kept below 400°C to maintain the carbide structure and hardness. Exceeding this temperature can result in excessive softening and a reduction in the erosion resistance of the overlay layer.
The integration of overlay welding and warm rough turning into a unified manufacturing process requires careful planning and coordination. The process sequence must ensure that the overlay layer is deposited with adequate thickness, heat treated to the correct hardness, and then machined to the required geometry and finish without introducing new defects or dimensional deviations.
Summary
The overlay welding of valve disc sealing surfaces combined with warm rough turning machining is a well-established manufacturing approach that produces high-quality sealing surfaces with excellent wear resistance and dimensional accuracy. The warm rough turning process offers significant advantages in terms of cutting efficiency and surface finish quality, but requires careful temperature control to maintain the overlay material properties. Engineers should approach this application with a systematic understanding of the metallurgical and machining interactions, and should validate the process through rigorous testing and inspection before production implementation.