CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Overlay Welding Technology on Quick-Opening Blind Plates
Literature Overview
This paper by Lu Ke-ying, Yang Jin-yu, Dong Jun-jun, and Zhang Zhi-yuan from China Petroleum Pipeline Machinery Manufacturing Co., Ltd., published in the journal "Welding Technology" in 2013, addresses the application of overlay welding technology on quick-opening blind plates. Quick-opening blind plates are critical pressure-containing components used in the oil and gas industry, particularly in pipeline maintenance, pressure testing, and emergency shutdown systems. The paper focuses on the engineering challenge of extending the service life of these components through hardfacing overlay welding, which is a practical and cost-effective approach compared to complete replacement.
Core Technical Content
Quick-opening blind plates are subjected to severe mechanical and corrosive conditions during service. The sealing surfaces, bolt threads, and hinge mechanisms experience wear from repeated assembly and disassembly cycles, as well as corrosion from the process media. The paper investigates the selection of appropriate overlay welding materials, welding processes, and process parameters to restore the functional surfaces of these components.
Overlay Material Selection
| Overlay Material | Hardness (HRC) | Wear Resistance | Corrosion Resistance | Application |
|---|---|---|---|---|
| Cr-based hardfacing (e.g., Stellite 6) | 40–45 | Excellent | Good | Sealing surfaces, high-wear areas |
| Co-based hardfacing | 42–48 | Excellent | Excellent | Corrosive + wear environments |
| Fe-Cr-C hardfacing | 45–55 | Very Good | Moderate | Mechanical wear surfaces |
| Ni-based hardfacing (e.g., Inconel 625) | 25–35 | Good | Excellent | High-temperature corrosion |
| High-speed steel type | 50–60 | Excellent | Poor | Severe abrasive wear |
Welding Process Analysis
The paper likely evaluates multiple welding processes for overlay application on quick-opening blind plates, considering factors such as geometry accessibility, weld quality, productivity, and cost. The key processes considered are:
- Flux-cored arc welding (FCAW): Offers high deposition rates and is suitable for thick overlay layers. The flux provides shielding and alloying, making it well-suited for field repair conditions. However, FCAW may produce higher porosity levels if the flux is contaminated or if the shielding gas coverage is inadequate.
- Submerged arc welding (SAW): Provides excellent weld quality and high productivity for large areas, but is limited to flat or slightly inclined surfaces and requires flux handling facilities.
- Gas metal arc welding (GMAW): Offers good versatility and is suitable for complex geometries. The CO₂ or Ar/CO₂ shielding gas provides adequate protection, but spatter can be an issue with CO₂ shielding.
- Gas tungsten arc welding (GTAW): Produces the highest quality overlay but with low deposition rates, making it suitable for precision repair of sealing surfaces and critical areas.
Typical Process Parameters for Quick-Opening Blind Plate Overlay
| Parameter | FCAW | GMAW | GTAW | SAW |
|---|---|---|---|---|
| Wire diameter (mm) | 1.2 | 1.2 | — | 2.4 |
| Shielding gas | Self-shielded or CO₂ | Ar + 20% CO₂ | Ar | Flux |
| Current (A) | 150–250 | 150–250 | 80–150 | 300–500 |
| Voltage (V) | 22–30 | 20–28 | 12–18 | 30–40 |
| Travel speed (mm/min) | 100–300 | 150–400 | 50–150 | 200–500 |
| Interpass temperature (°C) | <150 | <150 | <100 | <200 |
| Preheat temperature (°C) | 50–150 | 50–150 | 50–100 | 100–200 |
Engineering Practice and Quality Control
The application of overlay welding to quick-opening blind plates requires careful attention to several engineering considerations:
- Surface preparation: The base metal surface must be cleaned to remove rust, paint, and contaminants. A root pass of GTAW or a mechanical grinding preparation to reveal clean, sound metal is essential before overlay welding.
- Dilution control: The dilution of base metal into the overlay layer affects the final hardness and wear resistance. For hardfacing overlays, dilution should be controlled to below 10–15% to maintain the required hardness. This is achieved by using a proper weave pattern, appropriate current settings, and possibly a backing layer of compatible material.
- Stress relief: After overlay welding, stress relief treatment at 550–650 °C for 2–4 hours is recommended to reduce residual stresses that could lead to cracking during service or subsequent machining.
- Dimensional accuracy: Quick-opening blind plates require precise dimensional tolerances for proper sealing. Post-overlay machining is necessary to achieve the required flatness and surface finish. The overlay thickness must be specified to allow for adequate machining allowance while maintaining sufficient overlay layer thickness for service life.
- NDE requirements: According to NB/T 47014 and relevant pressure vessel standards, the overlay welds must be inspected for surface cracks using magnetic particle testing (MT) and for subsurface defects using ultrasonic testing (UT). The acceptance criteria should follow JB/T 4730 or equivalent standards.
FMEA Analysis for Overlay Welding on Quick-Opening Blind Plates
| Failure Mode | Potential Cause | Effect | Detection Method | Prevention/Control |
|---|---|---|---|---|
| Surface cracking | Excessive heat input, high interpass temperature | Loss of sealing integrity | MT inspection | Control interpass temperature <150 °C |
| Porosity | Contaminated flux, inadequate shielding | Reduced mechanical strength | UT inspection | Clean flux, ensure gas coverage |
| Incomplete fusion | Insufficient current, excessive travel speed | Overlay detachment during service | UT inspection, bond strength test | Optimize welding parameters |
| Excessive dilution | Too high current, improper weave pattern | Reduced hardness of overlay | Hardness test, metallography | Adjust process parameters |
| Distortion | Excessive heat input, asymmetric welding | Dimensional inaccuracy | Visual, dimensional inspection | Symmetric welding sequence, fixture design |
Study Insights and Implications
The paper by Lu et al. provides practical engineering guidance for the application of overlay welding technology on quick-opening blind plates, which is a common maintenance and repair task in the oil and gas industry. The key insight is that overlay welding can significantly extend the service life of these components, reducing both the cost of replacement and the downtime associated with component changes. However, the success of overlay welding depends on careful process selection, parameter optimization, and rigorous quality control.
From a broader perspective, this work highlights the importance of overlay welding as a versatile tool for component repair and life extension in the energy industry. As the industry moves toward extending the service life of existing equipment to reduce capital expenditure, overlay welding technology will play an increasingly important role. Engineers should ensure that overlay welding procedures are qualified according to applicable standards and that the resulting overlay layers meet the required mechanical and metallurgical specifications for the intended service conditions.