CLADDING TECHNOLOGY SHANXI CO., LTDCO2 Shielded Arc Overlay Welding on Drilling Pump Valve Box Sealing Surfaces
Literature Overview
This 1993 publication by Gu Wei, Hu Pimin, Gu Genfu, Wang Tianwei, and Wang Zongxia from Harbin Institute of Technology and Weihai Technical School addresses a highly practical problem in petroleum drilling equipment: the deterioration of valve box sealing surfaces in drilling pumps due to abrasive and corrosive downhole fluids. The work was published in the journal "Materials Science and Technology" (材料科学与工艺) and represents an early systematic investigation into applying CO₂ shielded arc welding (GMAW-C) for restoring sealing surfaces in drilling pump valve boxes. Given that drilling pumps operate under extreme cyclic loading, high pressure, and aggressive fluid environments containing sand, cuttings, and corrosive components, the sealing surface integrity directly determines pump reliability and operational safety.
Core Technical Problem and Material Selection
The valve box in a drilling pump is subjected to a combination of mechanical fatigue, abrasive wear from solid particles in the drilling fluid, and chemical corrosion from chlorides, sulfides, and acidic components. Conventional carbon steel or low-alloy steel valve box materials experience rapid surface degradation, leading to seal failure, fluid leakage, and ultimately pump breakdown. The research group evaluated several overlay materials suitable for CO₂ arc welding, focusing on materials that would provide both hardness for abrasion resistance and sufficient corrosion resistance for the harsh drilling fluid environment.
Candidate Overlay Materials and Their Characteristics
| Material Type | Typical Composition | Hardness (HRC) | Corrosion Resistance | Weldability in CO₂ |
|---|---|---|---|---|
| Cr-Mo cast iron | 3-5% Cr, 1-2% Mo | 50-58 | Moderate | Good |
| High-Cr stainless | 20-25% Cr, 2-3% Ni | 40-50 | Excellent | Fair (crack prone) |
| Hardfacing alloy (Co-based) | Co, Cr, W, C | 55-65 | Excellent | Poor in CO₂ |
| Maraging steel | 8% Ni, 4% Co, 4% Mo | 45-55 (after aging) | Good | Good |
| High-silicon cast iron | 15-20% Si, 3-5% Cr | 50-60 | Excellent (acid) | Moderate |
The CO₂ shielding gas presents unique challenges for overlay welding. The high oxygen activity of CO₂ promotes oxidation of the weld metal, increases spatter, and can lead to increased dilution from the base metal. For sealing surface applications, controlling the dilution ratio is critical because excessive base metal dilution reduces the hardness and corrosion resistance of the overlay layer. The authors likely investigated multi-layer welding sequences to minimize dilution in subsequent layers, ensuring that the final sealing surface layer achieves the target composition and properties.
Process Parameters and Welding Strategy
CO₂ shielded arc welding is a cost-effective and widely available process, making it attractive for field repair applications in drilling operations. However, the process parameters must be carefully optimized for overlay welding to achieve a sound, crack-free, and properly bonded overlay layer.
Typical Process Windows for CO₂ Overlay Welding
| Parameter | Range | Notes |
|---|---|---|
| Wire diameter | 1.2 - 1.6 mm | 1.2 mm preferred for thin overlay |
| Current | 180 - 280 A | Depends on wire diameter and travel speed |
| Voltage | 22 - 28 V | Short-circuit transfer regime |
| Travel speed | 80 - 150 mm/min | Slower for better wetting and lower dilution |
| Shielding gas flow | 12 - 20 L/min | Higher flow needed due to CO₂ density |
| Preheat temperature | 100 - 200°C | For carbon steel base metals |
| Interpass temperature | < 250°C | Control to limit HAZ hardness |
The welding sequence for a valve box sealing surface typically involves a transition layer followed by multiple overlay layers. The transition layer serves to dilute the carbon and alloy content of the base metal and to reduce the risk of cracking in subsequent layers. For CO₂ welding, a low-carbon stainless steel or austenitic stainless steel transition layer is often employed before applying the final hardfacing or corrosion-resistant overlay layers.
Dilution Control
The dilution ratio in overlay welding is defined as the mass percentage of base metal incorporated into the overlay weld metal. For the first layer, dilution can be as high as 50-70%, while subsequent layers typically show 10-30% dilution. The authors would have measured the actual composition of the overlay layer using optical emission spectrometry (OES) or chemical analysis to verify that the dilution was within acceptable limits for the target properties.
Defect Analysis and Countermeasures
CO₂ overlay welding introduces several characteristic defects that must be addressed:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Porosity | CO₂ decomposition, hydrogen pickup | Increase gas flow, clean base metal, use dry flux |
| Cracking (hot) | Sulfur and phosphorus in base metal | Preheat, use low-sulfur consumables, add sulfur scavengers |
| Cracking (cold) | High HAZ hardness, hydrogen embrittlement | Post-weld heat treatment (PWHT), preheat |
| Excessive dilution | High heat input, deep penetration | Reduce current, increase travel speed, use smaller wire |
| Undercut | Travel speed too high, improper gun angle | Reduce travel speed, adjust stick-out |
| Spatter | CO₂ arc characteristics | Increase stand-off distance, add anti-spatter agent |
The hydrogen-induced cracking (HIC) risk is particularly relevant in this application because drilling fluids often contain hydrogen sulfide (H₂S), which can cause sulfide stress corrosion cracking (SSC) in the overlay layer if the microstructure is not properly controlled. The authors likely performed hydrogen permeation tests or slow strain rate tests to evaluate the susceptibility of the overlay weld metal to HIC and SSC.
Engineering Practice and Field Application
The drilling pump valve box is a critical component that, when failed, can cause catastrophic pump failure during drilling operations, leading to lost circulation, well control issues, and significant economic losses. The application of CO₂ overlay welding for sealing surface restoration offers several advantages over replacement of the entire valve box:
- Cost reduction: Overlay welding repair costs are typically 20-40% of the cost of a new valve box.
- Downtime reduction: Field repair can be completed in hours rather than days required for replacement.
- Material optimization: The overlay layer can be tailored to the specific drilling fluid composition encountered at the well site.
However, the repair must meet strict quality requirements. The overlay layer thickness should be uniform within ±0.2 mm to ensure proper seal engagement. Surface roughness after grinding should not exceed Ra 1.6 μm for the sealing surface. Non-destructive testing (NDT) using magnetic particle testing (MT) or penetrant testing (PT) should be performed to detect surface cracks and porosity.
Study Insights and Reflections
This 1993 publication is notable for its practical orientation and its focus on a cost-effective welding process (CO₂ GMAW) for a critical industrial application. The research demonstrates that even relatively simple welding processes can be effectively applied to demanding overlay welding tasks when process parameters are carefully controlled and the metallurgical interactions are well understood. The emphasis on dilution control, transition layer design, and post-weld heat treatment reflects a mature understanding of overlay welding metallurgy that remains relevant today.
One area for further development would be the extension of this approach to include post-weld grinding and machining procedures to achieve the precise surface geometry and roughness required for sealing applications. The integration of overlay welding with precision machining is essential for achieving the dimensional tolerances (typically IT6-IT7) required for pump valve box sealing surfaces.