CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Salt Circulation Pump Impellers in Brine Service
Overview of the Source Material
This 1996 publication by Wang Qilong from the Equipment Department of Anhui Dingyuan Salt Mine provides a practical case study on the repair of salt circulation pump impellers using the weld overlay method. The paper appears in China Well Mine Salt and addresses a specific industrial challenge: the severe erosion and corrosion damage experienced by pump impellers in brine circulation systems used in salt mining operations. The publication is particularly valuable because it documents a real-world repair solution developed and implemented in a Chinese salt mine, offering practical insights that are directly applicable to similar industrial scenarios.
Core Technical Content
Problem Analysis
Salt circulation pumps in brine mining operations are subjected to a highly aggressive service environment characterized by:
- High chloride ion concentration in the circulating brine solution
- Continuous mechanical abrasion from suspended salt particles and mineral impurities
- Electrochemical corrosion accelerated by the presence of chlorides
- Thermal cycling effects from variations in brine temperature
The combined effect of erosion and corrosion (erosion-corrosion) leads to rapid material degradation of pump impellers, resulting in reduced pump efficiency, increased energy consumption, and frequent unplanned shutdowns for impeller replacement. The original impeller material, typically a carbon steel or low-alloy steel, lacks the necessary resistance to this aggressive environment.
Weld Overlay Repair Methodology
The repair approach described in this literature involves the application of a corrosion-resistant and wear-resistant overlay layer on the impeller surface using arc welding techniques. The key steps in the repair process are as follows:
- Surface preparation: Removal of existing corrosion products, scale, and damaged material through grinding, wire brushing, or sandblasting to achieve a clean, sound substrate surface.
- Substrate assessment: Visual and magnetic particle examination to identify and repair any pre-existing cracks or defects.
- Overlay welding: Application of multiple passes of a suitable overlay alloy using shielded metal arc welding (SMAW) or gas metal arc welding (GMAW).
- Post-weld treatment: Grinding of the overlay surface to achieve the required dimensional accuracy and surface finish.
- Quality inspection: Visual examination, dimensional verification, and functional testing of the repaired impeller.
Selection of Overlay Material
The selection of the overlay material is critical to the success of the repair. Based on the service conditions described in the literature, the following materials are suitable for salt circulation pump impeller repair:
| Overlay Material | Composition (wt%) | Hardness (HV) | Corrosion Resistance | Wear Resistance |
|---|---|---|---|---|
| 304L Stainless Steel | Cr: 18-20, Ni: 8-10.5 | 200-250 | Excellent | Moderate |
| 316L Stainless Steel | Cr: 16-18, Ni: 10-14, Mo: 2-3 | 200-250 | Excellent | Moderate |
| Stellite 6 | Co: balance, Cr: 21-25, W: 8.5-9.5 | 400-500 | Good | Excellent |
| Ni-Cr-Mo Alloy | Ni: balance, Cr: 25-30, Mo: 1-2 | 250-300 | Excellent | Good |
For salt brine service, a 316L stainless steel overlay is typically recommended as a balance between corrosion resistance and cost-effectiveness. For more severe erosion-corrosion conditions, a Stellite 6 overlay provides superior performance but at a higher material cost.
Process Parameters and Technical Considerations
Welding Process Parameters
The following welding parameters are recommended for the overlay repair of salt circulation pump impellers:
| Parameter | SMAW | GMAW |
|---|---|---|
| Wire diameter | 2.5–3.2 mm | 1.0–1.2 mm |
| Current | 80–120 A | 100–180 A |
| Voltage | 20–25 V | 18–22 V |
| Travel speed | 50–100 mm/min | 80–150 mm/min |
| Shielding gas | — | Ar + 5% CO₂ |
| Interpass temperature | < 150 °C | < 150 °C |
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking | High拘束 stress, hydrogen embrittlement | Preheat substrate, control interpass temperature, use low-hydrogen electrodes |
| Porosity | Surface contamination, inadequate shielding | Thorough surface preparation, ensure gas coverage |
| Insufficient fusion | Excessive travel speed, low current | Optimize welding parameters, ensure proper torch angle |
| Dilution | Excessive heat input, thin overlay passes | Use low-heat-input parameters, apply thin overlay layers |
| Warping | Thermal distortion from welding | Use back-step welding sequence, employ clamping fixtures |
Engineering Practice Implications
Repair Economics and Decision Making
A critical aspect of the weld overlay repair approach is the economic evaluation compared to complete impeller replacement. The following factors should be considered in the decision-making process:
- Material cost: The cost of the overlay material and consumables is typically 20-40% of the cost of a new impeller.
- Labor cost: The welding, grinding, and inspection labor represents a significant portion of the repair cost.
- Downtime cost: The time required for repair versus replacement affects production losses.
- Service life extension: The expected remaining life of the repaired impeller determines the cost-effectiveness of the repair.
A typical decision framework is to proceed with weld overlay repair when the remaining life of the impeller exceeds 12 months and the repair cost is less than 60% of the replacement cost.
Quality Assurance Protocol
The following quality assurance protocol is recommended for weld overlay repairs of pump impellers:
- Pre-repair inspection: Document the extent of damage, measure remaining wall thickness, and assess the condition of the base metal.
- Repair process documentation: Record all welding parameters, consumable lot numbers, and operator qualifications.
- Post-repair inspection: Conduct visual examination, dimensional verification, and hardness testing of the overlay layer.
- Functional testing: Perform a hydrostatic test or leak test to verify the integrity of the repaired impeller.
- Post-repair monitoring: Implement a condition monitoring program to track the performance of the repaired impeller during service.
Key Questions and Reflections
One important question that arises from this case study is the long-term reliability of weld overlay repairs in dynamic pump impeller applications. Unlike static components, impellers are subjected to centrifugal forces, vibration, and fluid dynamic loading during operation. These dynamic loads can potentially initiate fatigue cracks at the overlay-base metal interface if the bond strength is insufficient or if there are residual stresses from the welding process.
The literature does not provide extensive data on the long-term performance of the repaired impellers, which is a limitation for engineering decision-making. In practice, it is recommended to conduct periodic inspections of the overlay layer during service to monitor for any signs of delamination, cracking, or excessive wear. A practical approach is to perform a visual inspection and thickness measurement every 6 months during the first year of service after repair.
Another reflection is the potential for improving the repair process through the use of advanced welding techniques such as laser cladding or plasma transferred arc (PTA) cladding. These techniques offer lower heat input, reduced dilution, and better control over the overlay layer composition and thickness. However, the equipment cost and technical requirements may not be justified for routine pump impeller repairs in smaller operations.
Study Insights and Implications
This literature provides a practical and accessible solution to a common industrial problem: the erosion-corrosion damage of pump impellers in salt brine service. The weld overlay repair approach offers a cost-effective alternative to complete impeller replacement, with the potential to extend the service life of the impeller by 2-3 times compared to the original material. The key to successful repair lies in the careful selection of the overlay material, the optimization of welding parameters, and the implementation of a rigorous quality assurance protocol.
The case study also highlights the importance of practical engineering solutions that can be implemented with relatively simple equipment and techniques. Not all industrial repairs require advanced technologies; often, well-executed conventional welding techniques, when applied with proper process control and quality assurance, can deliver excellent results. Engineers should always evaluate the most appropriate solution for each specific application, considering the balance between technical performance, cost, and practicality.