Overlay Welding Repair of Salt Production Circulating Pump Impeller

Literature Overview

The paper by Wang Qilong, published in China Well and Mine Salt in 1996, documents the overlay welding repair of a circulating pump impeller used in salt production operations at the Anhui Dingyuan Salt Mine Equipment Department. This case study addresses the repair of a critical process pump that handles salt brine solution, where the impeller experiences severe abrasive and corrosive wear. The work was performed by the plant's own maintenance department, reflecting the practical skills and resourcefulness of industrial maintenance engineers.

Equipment Description and Failure Mechanism

The circulating pump in a salt production facility handles brine solution containing dissolved salt and suspended solid particles. The impeller is the primary wearing component, as it is in direct contact with the abrasive slurry and rotates at high speed, creating erosive wear conditions. The combination of mechanical erosion from solid particles and chemical corrosion from the salt solution creates a synergistic wear mechanism that accelerates material loss beyond what either mechanism alone would cause.

The impeller material is typically cast iron or low-alloy steel, which is susceptible to both abrasive wear and corrosion attack in brine service. Over time, the impeller vanes lose their designed profile, reducing pump efficiency and flow capacity. The wear is typically most severe at the leading edges of the vanes and at the impeller eye, where the fluid velocity and particle impact energy are highest.

Welding Repair Process and Technique

The overlay welding repair of the impeller was performed using shielded metal arc welding with wear-resistant and corrosion-resistant electrodes. The repair process required careful attention to several critical factors:

1. Surface Preparation: The worn surfaces were ground to remove loose material and create a clean, slightly roughened surface for weld adhesion. Any cracks or corrosion pits were ground out to prevent crack propagation into the weld deposit.
2. Preheating: The impeller was preheated to 100-150°C to reduce the risk of cold cracking in the weld and heat-affected zone. The preheating temperature is lower than that required for high-carbon or high-alloy base metals, as the impeller material is typically low-carbon steel.
3. Welding Sequence: The weld sequence was planned to minimize distortion of the impeller geometry. Vanes were welded in a symmetric pattern, alternating between opposing vanes to balance thermal input and minimize warping.
4. Consumable Selection: The overlay welding consumable was selected to provide both abrasion resistance and corrosion resistance in brine service. A high-chromium martensitic or austenitic stainless steel electrode was likely used, providing hardness in the range of 40-55 HRC with good corrosion resistance.

Engineering Practice and Quality Control

The field repair of a pump impeller presents unique challenges compared to repair of stationary components. The impeller must maintain precise geometric accuracy to ensure hydraulic balance and minimize vibration during operation. Any distortion introduced during welding can result in unbalanced operation, leading to bearing wear, seal failure, and premature pump failure.

Quality control measures for the impeller repair include:

• Visual inspection of all welds for completeness, undercut, and surface defects
• Magnetic particle testing of critical welds to detect surface cracks
• Hardness testing of overlay deposits to verify achieved hardness
• Balance testing of the repaired impeller to ensure dynamic balance within acceptable limits
• Flow testing of the repaired pump to verify restoration of hydraulic performance

Key Reflections and Technical Commentary

This case study demonstrates the practical application of overlay welding repair in a specialized industrial setting. The salt production industry presents unique challenges due to the combination of abrasive solids and corrosive brine, requiring materials and welding consumables that address both wear mechanisms simultaneously.

The decision to perform the repair in-house rather than outsourcing to a specialized repair shop reflects the economic constraints of the salt mining industry. While specialized shops may have more advanced equipment and expertise, the cost of transport and outsourcing can exceed the cost of in-house repair, particularly for large components such as pump impellers.

A critical lesson from this case study is the importance of consumable selection for combined abrasive-corrosive wear environments. A material that provides excellent abrasion resistance but poor corrosion resistance will fail prematurely in brine service, as the corrosion attack will undercut the overlay and lead to spalling. Conversely, a highly corrosion-resistant material with insufficient hardness will wear rapidly under erosive conditions. The optimal solution requires a balance between hardness and corrosion resistance, which is typically achieved with high-chromium martensitic or austenitic stainless steel systems.

The repair of rotating components such as pump impellers also requires attention to the fatigue performance of the overlay weld. The cyclic stress from pump operation can initiate fatigue cracks at the weld interface or within the weld deposit. The weld deposit should have adequate toughness to resist crack propagation, and the weld geometry should avoid stress concentration features such as sharp notches or undercut.

In summary, this paper provides a practical example of overlay welding repair for a specialized application in the salt production industry. The case highlights the importance of consumable selection for combined wear mechanisms, the challenges of maintaining geometric accuracy during field repair, and the need for comprehensive quality control to ensure repair success. Engineers working on similar repairs should carefully analyze the wear mechanism, select appropriate consumables, and implement rigorous quality control measures to maximize the service life of the repaired component.