Microstructural Characteristics and Sliding Wear Performance of High-Chromium Cladding Wear-Resistant Plate for Scraper Conveyor Middle Trough

Literature Overview

This study investigates the microstructural evolution and sliding wear behavior of high-chromium alloy cladding plates designed specifically for scraper conveyor middle trough applications. The high-chromium overlay (typically 20–30% Cr with 2–4% C) forms a complex carbide matrix that provides exceptional resistance to abrasive sliding wear. The literature combines metallographic analysis, hardness mapping, and tribological testing to establish structure-property relationships and guide material selection for this demanding application.

Material Composition and Classification

High-chromium cladding alloys for wear applications fall into several categories based on chromium and carbon content:

Alloy Type	Cr (%)	C (%)	Primary Carbide	Hardness (HRC)	Application
High-Cr, Low-C	20–25	1.0–1.5	M₇C₃	50–55	Moderate abrasion, good toughness
High-Cr, Medium-C	20–28	1.5–2.5	M₇C₃ + M₂₃C₆	55–62	Severe abrasion, balanced properties
High-Cr, High-C	25–30	2.5–4.0	M₂₃C₆ + M₆C	60–68	Extreme abrasion, reduced toughness
Ultra-high-Cr	28–35	3.0–5.0	M₆C dominant	65–72	Maximum hardness, brittle

For scraper conveyor applications, the High-Cr Medium-C category (20–28% Cr, 1.5–2.5% C) is typically specified as it provides the optimal balance between wear resistance and impact toughness required for the dynamic loading conditions.

Microstructural Analysis

The microstructure of the high-chromium cladding layer consists of:

Matrix Phase

Martensite: The primary matrix phase, formed during rapid cooling after welding. The high carbon and chromium content stabilize the martensitic structure, achieving hardness of 55–65 HRC in the matrix regions.
Retained austenite: Typically 5–15% retained austenite, which contributes to toughness and can transform to martensite during impact loading, providing work-hardening capability.

Carbide Phases

M₇C₃ carbides: Form as a network along grain boundaries, with hardness of 1400–1600 HV. These plate-like carbides provide primary abrasion resistance.
M₂₃C₆ carbides: Appear as angular particles 5–20 μm in size, with hardness of 1500–1800 HV. These contribute to the overall layer hardness.
Primary carbides: In high-carbon compositions, primary M₆C carbides form as large (20–50 μm) angular particles with hardness exceeding 2000 HV, but reduce toughness significantly.

Grain Structure

Grain size typically 20–50 μm in the overlay layer
Columnar grain structure in the weld direction, transitioning to equiaxed grains near the fusion line
Grain boundary carbide network provides crack resistance through crack deflection

Sliding Wear Mechanisms

The study identifies three primary wear mechanisms operating in scraper conveyor service:

Wear Mechanism	Dominant Condition	Microstructural Response	Wear Rate
Abrasive (two-body)	Hard particles in material	Carbide ploughing and matrix cutting	0.5–2.0 mg/1000 cycles
Abrasive (three-body)	Free rolling particles	Rolling and ploughing	1.0–3.5 mg/1000 cycles
Adhesive	High contact pressure	Material transfer and tearing	0.8–2.5 mg/1000 cycles
Fatigue	Cyclic loading	Surface cracking and spalling	0.3–1.5 mg/1000 cycles

The high-chromium cladding layer resists abrasive wear primarily through:

Carbide hardness exceeding the abrasive particle hardness (quartz: 7 Mohs, ~1200 HV)
Carbide network preventing crack propagation through the matrix
Martensitic matrix providing adequate toughness to resist chipping
Work-hardening capability of retained austenite under impact

Wear Test Results and Structure-Property Relationships

The study conducted standardized wear testing (ASTM G99 dry sliding, ASTM G65 pin-on-disk) and established the following relationships:

Parameter	Correlation	Wear Performance
Total carbide volume fraction	Positive	Higher volume → lower wear rate
Carbide hardness	Positive	Harder carbides → lower wear rate
Matrix hardness	Positive (up to 65 HRC)	Higher matrix hardness → lower wear rate
Retained austenite content	Optimal at 8–12%	Too low → brittle; too high → soft
Grain size	Negative	Finer grains → lower wear rate
Carbide distribution uniformity	Positive	Uniform → consistent wear; clustered → localized failure

The optimal microstructure for scraper conveyor applications features:

30–45% carbide volume fraction
Carbide size distribution: 60% between 5–15 μm, 30% between 15–30 μm, 10% larger
Matrix hardness: 58–62 HRC
Retained austenite: 8–12%
Grain size: 25–40 μm

Defect Analysis and Quality Control

Defect	Microstructural Indicator	Impact on Performance	Prevention
Excessive carbide coarsening	Carbides > 40 μm	Reduced toughness, chipping	Control cooling rate; limit interpass temperature
Carbide clustering	Localized carbide-rich regions	Uneven wear, premature failure	Ensure uniform powder composition and mixing
Cracking in overlay	Cracks along grain boundaries	Reduced wear life, material loss	Preheat base metal; use appropriate filler composition
Excessive retained austenite	> 20% austenite	Reduced hardness, increased wear	Ensure adequate carbon content for martensite transformation
Dilution from base metal	Reduced Cr content in overlay	Lower hardness, poor corrosion resistance	Control first pass parameters; use backing material

Engineering Application Guidelines

For scraper conveyor middle trough applications, the following recommendations emerge from the study:

Material selection: Specify high-chromium alloy with 22–26% Cr and 2.0–2.5% C for optimal balance of hardness, toughness, and wear resistance in typical coal handling applications.
Layer thickness: Minimum 6 mm overlay thickness is recommended to accommodate wear life of 12–18 months before re-cladding is required. Thinner layers risk exposing base metal prematurely.
Surface preparation: Base metal must be machined or ground to within 0.5 mm of final contour before cladding. Surface roughness should be Ra 12.5–25 μm to promote wetting without excessive dilution.
Welding procedure: Multi-pass welding with controlled interpass temperature (200–300°C) produces the finest grain structure and most uniform carbide distribution. Single-pass welding produces coarser microstructure with reduced wear life.
Post-weld treatment: Stress relief at 550–600°C for 2 hours reduces residual stresses without significantly affecting hardness. Avoid temperatures above 650°C which cause carbide coarsening and hardness reduction.
Inspection criteria: Acceptable overlay should show:

Hardness ≥ 58 HRC uniformly across the layer
No cracks > 0.3 mm in visual or MT inspection
Carbide volume fraction 30–50% in metallographic examination
Bond strength ≥ 25 MPa in shear testing

Study Insights and Conclusions

The literature provides comprehensive insight into the microstructural factors governing sliding wear performance of high-chromium cladding overlays. The key finding is that wear resistance is not simply a function of overall hardness but depends critically on the carbide morphology, distribution, and volume fraction within the matrix. A layer with 60 HRC hardness but poorly distributed coarse carbides will perform worse than a layer with 58 HRC but fine, uniformly distributed carbides.

The study also highlights the importance of considering the complete microstructural picture—including retained austenite content, grain size, and carbide network connectivity—when evaluating cladding performance. Engineers should not rely solely on hardness measurements for quality assessment but should incorporate metallographic examination and, where possible, tribological testing to verify that the microstructure meets the requirements for the specific service application.

For scraper conveyor applications specifically, the dynamic loading conditions (impact, vibration, cyclic stress) make toughness equally important as hardness. The optimal material specification should target a minimum Charpy V-notch impact energy of 20 J at 20°C while maintaining hardness above 58 HRC. This dual requirement guides both material selection and welding procedure design, ensuring that the cladding layer provides reliable long-term service without brittle failure under impact loading.