Microstructure and Wear Resistance of Overlay Welding Electrodes for Centrifugal Fans

Literature Overview

This paper by Xu Xiangyang from North China Electric Power University, published in China Electric Power (2001, Vol. 34, No. 5, pp. 4-6), investigates three commonly used wear-resistant overlay welding electrodes for centrifugal fans in power plant boilers. The study focuses on erosion wear conditions and examines how overlay layer properties vary with impingement angle. The research is particularly relevant to engineers working on boiler air preheater fan blade maintenance and life-extension programs in thermal power stations.

Core Technical Findings

The three electrodes studied were Cr-W-V type, Cr-Mo-Si type, and high-chromium cast iron type. The key finding is that under erosion wear conditions, the Cr-W-V electrode demonstrated superior wear resistance compared to both the Cr-Mo-Si and high-chromium cast iron electrodes. This ranking is consistent with the general principle that hardening phase density and matrix toughness together determine erosion resistance, rather than hardness alone.

The paper presents a critical observation that the wear resistance of the overlay layer varies significantly with impingement angle. This angular dependence is identified as the primary cause of non-uniform wear patterns observed on fan blades in service. Blades experience varying angles of particle impact depending on their position relative to the airflow stream, which explains why some regions wear out before others even though the same overlay material was applied uniformly.

Another important conclusion is that overlay layer hardness does not have a simple or direct correspondence with wear resistance. The author explicitly states that hardness should not be used as the sole or primary basis for overlay electrode design and selection. This is a valuable reminder for engineers who tend to rely on hardness specifications as a shortcut for performance prediction.

Interpretation of Technical Points

Erosion Wear Mechanism

Erosion wear on fan blades is fundamentally different from sliding or abrasive wear. Particles in the flue gas stream impact the blade surface at varying angles and velocities, causing material removal through plastic deformation and micro-cracking rather than simple abrasion. The Cr-W-V system likely performs better because the combined hardening effect of chromium carbides, tungsten carbides, and vanadium carbides provides a heterogeneous microstructure that resists both cutting and impact-type material removal.

Angular Dependence of Wear

The variation of wear rate with impingement angle follows a well-known trend in erosion research: for ductile materials, wear rate typically peaks at low angles (15-30 degrees), while for brittle materials, it peaks at higher angles (60-90 degrees). Overlay layers with different microstructural characteristics will exhibit different angular sensitivity. For fan blade applications, where the leading edge experiences near-normal impact and the trailing edge experiences grazing impact, a single overlay material may not be optimal across the entire blade surface.

Hardness-Wear Resistance Decoupling

The finding that hardness is not a reliable predictor of erosion resistance is consistent with the understanding that erosion involves both material removal and material displacement. A very hard but brittle overlay may chip and spall under impact, while a slightly softer but tougher overlay may deform and shed particles without significant material loss. This has direct implications for quality control procedures, where relying solely on hardness testing for acceptance criteria may lead to premature failure in service.

Engineering Practice Implications

For engineers managing fan blade maintenance programs, several practical recommendations emerge from this study. First, overlay material selection should be based on erosion test data representative of actual service conditions rather than on hardness specifications alone. Second, the non-uniform wear pattern caused by angular variation suggests that periodic blade profiling and selective re-overlay of high-wear zones may be more effective than uniform re-overlay of the entire blade. Third, the Cr-W-V system, while more expensive, may provide significantly longer service intervals in high-wear environments such as coal-fired boiler forced draft fans.

Key Questions and Reflections

The study raises an important question about the representativeness of laboratory erosion tests for actual fan blade service conditions. Laboratory erosion tests typically use monodisperse particles at controlled velocities, whereas actual flue gas contains a wide range of particle sizes, shapes, and velocities with additional chemical attack. The actual service life of overlay layers in power plants is often shorter than predicted by laboratory tests, suggesting that additional degradation mechanisms such as thermal cycling, oxidation, and thermal fatigue cracking are not adequately captured in simple erosion tests.

From my experience in power plant engineering, I have observed that overlay layers on fan blades often fail at the interface with the base metal rather than through the overlay itself. This interfacial failure mode is related to thermal expansion mismatch between the overlay and the base material, which creates tensile stresses at the interface during thermal cycling. The paper does not address this failure mode, which represents a significant gap in the research. Future studies should incorporate thermal cycling and thermal fatigue into the evaluation of overlay welding electrodes for fan blade applications.

The finding that hardness is not a reliable predictor of erosion resistance should prompt a reevaluation of current quality control practices in the industry. Many specifications still use minimum hardness requirements as the primary acceptance criterion for overlay welding. While hardness is an easily measurable and reproducible property, it should be supplemented with more representative testing such as erosion testing, impact testing, and thermal cycling testing to ensure reliable service performance.

Study Insights and Implications

This paper, though published in 2001, remains highly relevant to today's engineering practice. The fundamental principles of erosion wear mechanism and overlay material selection have not changed, even as testing methods and analytical techniques have advanced. The emphasis on the inadequacy of hardness as a sole criterion for overlay selection is particularly important in an industry that continues to rely heavily on hardness specifications. Engineers should advocate for more comprehensive testing protocols that include erosion testing, thermal cycling, and interfacial strength evaluation to ensure that overlay welding solutions provide reliable and durable protection in demanding service environments.