Zhuojin Pipe Fitting Co., LtdCO2 Gas-Shielded Surfacing Welding in Equipment Repair Applications
Literature Overview
The paper by Gao Mingdong and Zhang Jiajun (2017), published in China Pulp & Paper, documents the practical application of CO2 gas-shielded flux-cored wire surfacing welding in the repair of pulp and paper industry equipment. Authored by engineers from Jiangsu Weitai High-Tech Welding Co., Ltd., this work bridges the gap between consumable manufacturing and field repair practice. The company specializes in high-performance flux-cored wires, wear-resistant flux-cored wires, and composite wear-resistant plates, providing both online and offline repair services.
The specific equipment addressed includes hydraulic pulpers, impeller blades, screen bottom knives, and screw conveyors—all of which are critical components in pulp and paper manufacturing that experience severe abrasive and corrosive wear.
Consumable Specifications
The paper details the specific flux-cored wire products used in these repair applications:
| Product Code | Classification | Base Composition | Typical Application |
|---|---|---|---|
| VTA308L(Q) | Stainless steel FCAW | 308L equivalent | Corrosive-abrasive environments |
| VTA309L(Q) | Stainless steel FCAW | 309L equivalent | Dissimilar metal surfacing |
| VTA316L(Q) | Stainless steel FCAW | 316L equivalent | High corrosion resistance |
| VTE711 | Carbon steel FCAW | High-carbon steel | Heavy abrasion |
| VTE711Ni | Carbon steel FCAW | Ni-enhanced high-carbon | Abrasion + thermal shock |
The "(Q)" designation indicates a quick-hardening or specialized formulation optimized for surfacing applications. The 309L-based wire (VTA309L) is particularly useful for surfacing carbon steel substrates with stainless steel overlay, as the higher Cr and Ni content compensates for dilution from the base metal.
Application Scenarios and Repair Strategy
The paper describes several distinct repair scenarios in pulp and paper equipment:
Hydraulic Pulpers
Hydraulic pulpers experience combined abrasive and corrosive wear from the pulp slurry containing wood fibers, chemical additives, and abrasive particles. The repair strategy typically involves:
- Removal of severely worn material by grinding
- Surface preparation to a sound substrate
- Multi-pass surfacing with VTA308L(Q) or VTA316L(Q) for corrosion-resistant overlay
- Final pass with a harder consumable for wear resistance
Impeller Blades
Impeller blades in hydraulic pulpers experience erosion from high-velocity pulp flow containing abrasive particles. The repair approach involves:
- Assessment of remaining blade thickness
- Surfacing with VTE711 or VTE711Ni to restore blade profile
- The Ni-enhanced VTE711Ni provides better resistance to thermal shock from cyclic temperature variations
Screen Bottom Knives
Screen bottom knives experience severe sliding abrasion against the screen surface. These are typically repaired with:
- VTE711 for maximum hardness and abrasion resistance
- Careful control of bead geometry to maintain knife edge profile
Screw Conveyors
Screw conveyors handling abrasive pulp stock experience wear on both the flights and the barrel. Repair involves:
- Surfacing of worn flight edges with appropriate consumable
- Selection based on whether the wear is primarily abrasive (VTE711) or corrosive-abrasive (VTA308L/Q)
Process Considerations for CO2 FCAW Surfacing
CO2 gas-shielded flux-cored wire welding offers several advantages for surfacing repair applications:
| Advantage | Explanation |
|---|---|
| High deposition rate | Flux-cored wires deposit 2-3x faster than solid wire |
| Good penetration | Adequate fusion with base metal for metallurgical bonding |
| Reduced spatter | Flux protects the arc and molten pool |
| Lower cost | CO2 shielding gas is inexpensive and readily available |
| Versatile positioning | Suitable for all weld positions |
However, several process challenges must be managed:
- Dilution control: CO2 FCAW typically exhibits higher dilution than TIG or plasma arc methods. Multi-pass deposition is often necessary to achieve the desired surface composition.
- Porosity risk: Inadequate shielding or surface contamination can lead to gas porosity. Pre-weld cleaning and adequate gas flow are essential.
- Cracking susceptibility: High-carbon surfacing alloys (VTE711 series) are susceptible to hot cracking. Preheating and controlled interpass temperature are critical.
- Hardness uniformity: Achieving consistent hardness across the surfacing layer requires careful control of welding parameters and pass sequencing.
Engineering Practice Implications
For maintenance engineers in the pulp and paper industry, this work provides practical guidance on consumable selection and repair strategy:
- Consumable selection matrix: Match the consumable to the wear mechanism—corrosive-abrasive environments require stainless steel consumables (VTA308L/Q, VTA316L/Q), while primarily abrasive environments can use carbon steel consumables (VTE711, VTE711Ni).
- Online vs. offline repair: The paper distinguishes between online repair (performed while equipment is partially operational) and offline repair (complete equipment shutdown). Online repair requires consumables with lower cracking susceptibility and faster deposition rates.
- Cost-effectiveness: CO2 FCAW surfacing is significantly more cost-effective than replacing entire components, particularly for large impellers and screw conveyors where replacement costs are prohibitive.
- Preventive maintenance: Regular surfacing repairs at planned intervals can extend component life significantly compared to reactive repair after failure.
Key Questions and Reflections
A practical question that arises is the long-term adhesion and fatigue life of the surfacing layer. In cyclic loading applications such as impeller blades, the surfacing layer must withstand repeated stress cycles without delamination. The metallurgical bond between the surfacing layer and the base metal is critical, and the dilution zone composition must be carefully managed.
Another consideration is the interaction between the surfacing layer and the process environment. In pulp and paper applications, the surfacing layer is exposed to a complex chemical environment containing acids, alkalis, and organic compounds. The corrosion resistance of the surfacing alloy must be adequate not only for the initial service period but also after prolonged exposure to the process chemicals.
Study Insights and Reference Value
This paper, while primarily a practical application report, provides valuable insight into the real-world challenges of surfacing repair in the pulp and paper industry. The emphasis on consumable selection based on wear mechanism is a fundamental principle that applies across all industries. For maintenance engineers, the work reinforces the importance of understanding the specific wear environment and selecting the appropriate consumable rather than defaulting to a single hardfacing alloy for all applications. The paper also highlights the value of having a reliable consumable supplier with technical support capability, which is often as important as the consumable itself in achieving successful repair outcomes.