Wear Mechanism and Anti-Wear Overlay Welding of Exhaust Fan Impellers

Literature Overview

The paper by Peng Zhenwei, Liu Yongsheng, Wang Li, and Wang Zongyu, published in the Journal of Jiamusi University (Natural Science Edition) in 1999, investigates the wear mechanism of exhaust fan impellers in power plant applications and presents a practical overlay welding solution using tungsten carbide electrodes. The authors identified the wear mechanism as犁沟状 (ploughing-type) wear caused by quartz and feldspar particles in low-quality coal, and demonstrated that overlay welding with Tiedui 707 (a tungsten carbide electrode) effectively mitigates the wear problem.

Technical Context: Exhaust Fan Systems in Power Plants

System Description

Exhaust fans (induced draft fans) are critical components in coal-fired power plants, responsible for:

Drawing flue gases from the furnace through the boiler system
Overcoming pressure losses in the boiler, economizer, air preheater, and ductwork
Maintaining the required negative pressure in the furnace for proper combustion

The impeller (fan wheel) is the most wear-critical component because it:

Rotates at high speed (typically 1,000–3,000 RPM depending on diameter)
Is directly exposed to the abrasive flue gas stream
Operates continuously for extended periods (often 8,000+ hours between overhauls)
Handles gas temperatures of 120–180°C at the fan inlet

Wear Conditions

The wear environment for exhaust fan impellers is characterized by:

Parameter	Typical Range	Influence on Wear
Gas velocity at impeller	15–35 m/s	Higher velocity = more severe impact
Particle concentration	1–10 g/m³	Higher concentration = more particles
Particle size	5–100 μm	Larger particles = deeper grooves
Particle hardness	7–9 Mohs (quartz)	Harder particles = more abrasive
Gas temperature	120–180°C	Elevated temperature = softer base material
Operating hours	8,000–20,000+	Cumulative wear damage

Wear Mechanism Analysis

Identification of Wear Type

The authors identified the wear pattern as犁沟状 (ploughing-type) wear, characterized by:

Parallel grooves on the impeller surface following the gas flow direction
Groove depth proportional to particle hardness and impact energy
Material removal primarily by micro-ploughing and micro-cutting mechanisms
Evidence of particle embedding followed by dragging across the surface

This wear morphology is characteristic of three-body abrasion in a gas-solid two-phase flow, where hard particles are carried by the gas stream and impact the impeller surface at various angles.

Wear Mechanism Classification

Wear Mechanism	Dominant Condition	Evidence
Micro-ploughing	Hard particles, low impact angle	Parallel grooves, material displacement
Micro-cutting	Hard particles, moderate impact angle	Material removal, chip formation
Micro-gouging	Hard particles, high impact angle	Deep grooves, material tearing
Impact fatigue	Repeated particle impacts	Surface cracking, spalling

Root Cause Analysis

The wear is primarily caused by:

Quartz (SiO₂) particles: Hardness of 7 Mohs, angular morphology, high abrasive potential
Feldspar particles: Hardness of 6 Mohs, less abrasive than quartz but still significant
Low-quality coal: Higher ash content means more mineral particles in the flue gas
Poor combustion efficiency: Incomplete combustion generates more particulate matter

Wear Rate Estimation

For typical power plant conditions, the wear rate of uncoated carbon steel impellers is approximately:

Leading edge: 0.05–0.15 mm/1,000 hours (most severe due to direct particle impact)
Blade surface: 0.02–0.05 mm/1,000 hours
Trailing edge: 0.01–0.03 mm/1,000 hours
Hub: 0.005–0.01 mm/1,000 hours

These rates translate to 40–120 mm of material loss over a typical 8,000–20,000 hour service period, which is unacceptable for precision-machined impeller surfaces.

Overlay Welding Solution

Material Selection: Tiedui 707 Electrode

The Tiedui 707 electrode is a tungsten carbide-based hardfacing electrode with the following characteristics:

Property	Specification
Electrode type	SMAW hardfacing electrode
Bonding flux	Iron-based (for good fusion)
WC content	40–60% (typical)
Dilution rate	30–50% (typical for iron-bonded)
As-welded hardness	60–70 HRC (400–700 HV)
Tempered hardness (400°C, 2h)	55–65 HRC (350–600 HV)
Wear resistance	5–10× carbon steel
Crack resistance	Moderate (requires proper technique)

Welding Procedure

The specific welding procedure recommended in the paper includes:

Parameter	Specification
Electrode diameter	3.2 mm or 4.0 mm
Welding current	120–180 A (3.2 mm), 180–250 A (4.0 mm)
Arc voltage	22–28 V
Travel speed	200–400 mm/min
Preheating temperature	100–150°C
Interpass temperature	< 200°C
Number of passes	2–3 (for adequate thickness)
Post-weld treatment	Stress relief at 400°C for 2 hours

Overlay Strategy

The overlay welding strategy for impellers should consider:

Selective overlay: Apply overlay only to wear-critical areas (leading edges, blade surfaces)
Multi-pass deposition: Build up thickness gradually to control dilution and stress
Weld sequence: Start from the hub and work outward, or follow a pattern that minimizes distortion
Post-weld machining: Machine the overlay surface to restore aerodynamic profile

Hardness Optimization

The as-welded hardness of tungsten carbide overlays is very high but brittle. Tempering at 400°C for 2 hours:

Reduces hardness slightly (from ~70 HRC to ~60 HRC)
Significantly improves toughness and crack resistance
Dissolves some brittle intermetallic phases
Reduces residual stress

This tempering treatment is essential for service in a dynamic environment where the impeller experiences centrifugal loading and vibration.

Quality Control and Inspection

Pre-Welding Requirements

Check Item	Acceptance Criteria	Method
Surface preparation	Clean, free of rust and scale	Visual + grit blasting
Base metal hardness	< 30 HRC	Rockwell hardness test