Overlay Welding Electrodes for Centrifugal Fan Blades: Microstructure and Wear Resistance

Literature Overview

This paper by Xu Xiangyang from North China Electric Power University, published in Electric Power in 2001, investigates the wear resistance of three types of overlay welding electrodes commonly used for repairing centrifugal fan blades in power station boiler systems. The study addresses a critical practical problem in power plant maintenance: the selection of appropriate overlay welding consumables for fan blade repair under erosive wear conditions.

Erosive Wear Mechanism in Fan Blades

Centrifugal fan blades in power station boilers operate in a harsh environment characterized by high temperatures, abrasive fly ash particles, and cyclic thermal loading. The primary wear mechanism is erosive wear, where abrasive particles impact the blade surface at various angles, causing material removal through cutting, ploughing, and micro-fracture. The wear rate is highly dependent on the impact angle of the abrasive particles, which varies across the blade surface due to the complex flow patterns in the fan housing.

The authors identified that the non-uniform wear pattern observed in service is primarily due to the variation in impact angle across the blade surface. Regions where the impact angle is close to 30 to 45 degrees experience the most severe wear, as this is the angle at which cutting and ploughing mechanisms are most effective. Regions where the impact angle approaches 90 degrees experience less severe wear, as the abrasive particles tend to bounce off the surface rather than cutting into it.

Electrode Comparison and Performance Evaluation

Three types of overlay welding electrodes were evaluated:

The results were somewhat counterintuitive: the Cr-W-V electrode exhibited the best wear resistance despite having lower hardness than the high chromium cast iron electrode. This finding challenges the common assumption that higher hardness always correlates with better wear resistance. The Cr-W-V alloy achieves superior wear resistance through a combination of mechanisms: the formation of fine, hard carbides (WC, VC) that provide abrasion resistance, and the presence of a tough matrix that prevents micro-crack initiation and propagation.

The high chromium cast iron electrode, despite its high hardness, exhibited poor wear resistance under erosive conditions. This is attributed to the brittleness of the high chromium cast iron microstructure, which is prone to micro-cracking under impact loading. Once micro-cracks initiate, they propagate rapidly, leading to spalling and accelerated material loss. The Cr-Mo-Si electrode performed moderately, with wear resistance intermediate between the other two types.

Impact Angle Effect and Wear Pattern Analysis

The authors demonstrated that the wear resistance of each electrode type varied significantly with impact angle. The Cr-W-V electrode maintained relatively consistent wear resistance across a wide range of impact angles, while the high chromium cast iron electrode showed a pronounced peak in wear rate at intermediate angles. This finding has important implications for fan blade repair design: the electrode selection should be based on the expected impact angle distribution across the blade surface, not simply on the maximum hardness of the overlay layer.

For practical applications, the authors recommended that the overlay layer be applied with a thickness of 3 to 5 mm, which provides adequate wear protection without excessive weight addition. The overlay should be applied in multiple passes to ensure good fusion and uniform microstructure. A post-weld tempering treatment at 550 to 600 °C for 1 to 2 hours is recommended to relieve residual stresses and improve the toughness of the overlay layer.

Engineering Practice Recommendations

Based on the findings of this study, the following recommendations are made for centrifugal fan blade repair in power station applications:

1. The Cr-W-V electrode should be the first choice for blade repair, as it provides the best combination of wear resistance and toughness under erosive wear conditions.
2. The overlay layer thickness should be selected based on the expected service life, with a minimum of 3 mm for severe conditions.
3. The welding sequence should be designed to minimize residual stresses, with the first pass applied to the region of highest expected impact angle.
4. Post-weld tempering is essential to prevent cracking and to optimize the microstructure for wear resistance.
5. The hardness of the overlay layer should not be used as the sole criterion for electrode selection, as wear resistance under erosive conditions depends on both hardness and toughness.

Study Insights

The most important insight from this study is that the relationship between hardness and wear resistance is not straightforward, particularly under erosive wear conditions. Engineers who rely solely on hardness as a selection criterion for overlay welding electrodes may make suboptimal choices that result in premature failure. A comprehensive evaluation that considers the microstructure, toughness, and impact angle dependence of wear resistance is essential for selecting the appropriate electrode for a given application. This study provides a valuable framework for such evaluations and should be referenced by any engineer involved in the repair of wear-damaged rotating machinery components.