Zhuojin Pipe Fitting Co., LtdCO2 Surfacing Welding of Drilling Pump Valve Box Sealing Surface
Literature Overview
This 1993 paper by Gu Wei and colleagues from Harbin Institute of Technology documents the application of CO2 gas metal arc surfacing welding to repair worn sealing surfaces on 35CrMo steel drilling pump valve boxes. The work addresses a critical operational problem in oilfield drilling equipment, where valve box sealing surfaces undergo severe abrasive and corrosive wear under high-pressure, high-temperature conditions involving CO2-containing formation fluids. The authors demonstrate that using 1.2 mm H08Mn2Si wire with short-circuit transfer CO2 welding can produce dense, wear-resistant surfacing layers suitable for production use. This represents an early but practical contribution to the field of overlay welding for oilfield equipment repair in China.
Core Technical Content
The fundamental challenge addressed is the repair of sealing surfaces on 35CrMo steel valve boxes used in triplex plunger pumps for drilling operations. These surfaces are subjected to cyclic pressure loading, sliding contact with plunger packing, and exposure to corrosive drilling fluids. The authors selected CO2 shielded metal arc surfacing as the repair method, with specific attention to wire selection, transfer mode, and weld metal quality.
Process Parameters and Wire Selection
| Parameter | Specification | Rationale |
|---|---|---|
| Base material | 35CrMo steel | High-strength alloy steel used for pump valve boxes |
| Welding wire | H08Mn2Si, 1.2 mm | Low-carbon manganese-silicon wire, good ductility and low hydrogen |
| Shielding gas | CO2 | Low cost, good penetration, suitable for surfacing |
| Transfer mode | Short-circuit (short-circuit arc) | Low spatter, low heat input, good for thin overlay layers |
| Deposition rate | Moderate | Multiple passes for controlled dilution |
The choice of H08Mn2Si wire is notable because it is a low-carbon, low-alloy wire that provides good ductility in the weld metal, minimizing the risk of cracking in the thick 35CrMo base material which is prone to hydrogen-induced cold cracking. The short-circuit transfer mode limits heat input per pass, reducing the risk of distortion and controlling dilution from the base metal into the overlay layer.
Weld Metal Quality Assessment
The authors report that the resulting surfacing metal exhibits:
- Dense microstructure without porosity or slag inclusions
- Adequate hardness for wear resistance under valve box operating conditions
- Good bonding strength with the 35CrMo base metal
- Acceptable dilution levels when using multi-pass techniques
The density of the weld metal is particularly important because porosity in a sealing surface would lead to immediate failure under the high pressures encountered in drilling pump operations, typically exceeding 35 MPa.
Engineering Practice Insights
From my perspective, this work illustrates several important principles that remain relevant in modern overlay welding practice. First, the selection of a low-hydrogen, low-carbon wire for surfacing on high-strength alloy steels like 35CrMo is a sound metallurgical decision. The 35CrMo steel, with its typical composition of 0.32-0.40% C, 0.8-1.1% Cr, and 0.15-0.30% Mo, has a hardenability that makes it susceptible to cold cracking if hydrogen levels are not controlled. The H08Mn2Si wire, with its low carbon content (0.08%) and absence of hardening elements, dilutes the carbon and alloy content in the fusion zone, reducing the hardness and cracking susceptibility of the weld metal.
Second, the use of short-circuit transfer CO2 welding for surfacing represents a practical compromise. While CO2 welding is known for high spatter and deep penetration, the short-circuit mode mitigates these issues. The current range typically used for 1.2 mm wire in short-circuit mode is 100-180 A, with voltage of 16-22 V. This provides sufficient deposition while maintaining good control over the weld bead profile, which is essential for creating a smooth sealing surface.
Third, the multi-pass approach implied by the need for controlled dilution is a standard practice in overlay welding. The first pass (tack pass or dilution pass) is typically deposited with lower current to minimize fusion with the base metal, and subsequent passes are built up to the required thickness. This technique is critical when surfacing on dissimilar materials or when the base metal composition must not significantly contaminate the overlay.
FMEA Analysis of Potential Failure Modes
Applying a Failure Mode and Effects Analysis approach to this surfacing application reveals several critical considerations:
| Failure Mode | Cause | Effect | Mitigation |
|---|---|---|---|
| Cold cracking in weld metal | Hydrogen from moisture in CO2 or base metal contamination | Delayed cracking leading to seal failure | Preheat base metal to 150-200°C, ensure dry shielding gas, use low-hydrogen practices |
| Porosity in surfacing layer | Gas pickup from contaminated base metal or wire | Reduced density, seal leakage under pressure | Thorough surface preparation, flux-cored wire if necessary, proper gas flow rate |
| Excessive dilution | High heat input, single-pass application | Overlay hardness too low for wear resistance | Multi-pass technique, controlled current and travel speed |
| Hot cracking | High carbon content in fusion zone | Cracking during cooling | Wire selection with low carbon, preheat to reduce cooling rate |
| Distortion of valve box | Excessive thermal input | Dimensional tolerance loss, misalignment | Balanced welding sequence, backing plate, low heat input |
Study Insights and Implications
This 1993 paper, while modest in scope, reflects an important era in Chinese welding engineering where practical solutions were developed for critical oilfield equipment using readily available materials and processes. The CO2 surfacing approach demonstrated here was likely chosen for its economic viability in a production environment, as CO2 is significantly cheaper than argon-based shielding gases. The success of this method in production, as reported by the authors, validates the technical approach.
However, from a modern perspective, several questions arise. The paper does not provide detailed mechanical property data such as hardness profiles, tensile strength of the weld metal, or fatigue performance of the surfacing layer under cyclic loading. In contemporary practice, one would expect to see hardness measurements across the weld cross-section, metallographic examination of the microstructure including carbide morphology, and possibly wear testing using standardized methods such as the pin-on-disk or block-on-ring tester. Additionally, the paper does not discuss the residual stress state in the surfacing layer, which is critical for the fatigue life of the repaired component under the cyclic pressure loading of pump operation.
The work also does not address the corrosion resistance of the H08Mn2Si surfacing layer in CO2-containing drilling fluids, which may contain chlorides and other aggressive species. In modern oilfield practice, the combination of wear and corrosion (erosion-corrosion) is often the dominant failure mechanism, and the surfacing material should ideally provide resistance to both.
Summary
The CO2 surfacing welding approach documented in this paper represents a practical, cost-effective solution for repairing worn sealing surfaces on 35CrMo drilling pump valve boxes. The selection of H08Mn2Si wire with short-circuit transfer CO2 welding demonstrates sound engineering judgment in balancing metallurgical compatibility, process control, and economic considerations. While the paper lacks the depth of mechanical characterization expected in modern literature, its core technical approach remains valid and applicable. The key lesson is that successful overlay welding requires careful attention to wire selection, transfer mode, heat input control, and multi-pass technique to achieve dense, wear-resistant deposits on high-strength alloy steels. Engineers working on similar repairs today should build upon this foundation by incorporating modern non-destructive testing, residual stress measurement, and comprehensive mechanical property evaluation to ensure long-term reliability.