Hardfacing wear-resistant plate armor is a bimetallic composite produced by welding chromium carbide or tungsten carbide overlay deposits onto a ductile steel substrate, delivering exceptional abrasion resistance in mining, cement, power generation, and bulk material handling equipment. Professional testing and inspection of these clad plates verifies surface hardness, carbide microstructure, overlay thickness, bond integrity, and abrasive wear performance against recognized ISO, ASTM, EN, and GB standards. This article provides a comprehensive technical guide to the sample types, testing items, analytical methods, and quality acceptance criteria used in hardfacing wear plate armor evaluation.
Technical Overview
Hardfacing wear-resistant plate armor, also known as clad wear plate or chromium carbide overlay (CCO) plate, is manufactured by depositing multiple layers of hardfacing alloy onto a common structural steel substrate such as Q235B, Q355B, or S235JR using open-arc or submerged-arc welding with flux-cored wires. The result is a composite product that combines an extremely hard, carbide-rich working surface with a tough, weldable, and formable base plate. Typical configurations include 3+3, 4+4, 6+4, and 6+6 mm structures, where the first figure denotes substrate thickness and the second denotes overlay thickness.
The performance of hardfaced armor depends on several metallurgical variables that can only be confirmed through systematic laboratory and field testing: the volume fraction and morphology of primary M7C3 chromium carbides, the degree of weld dilution at the bond line, through-thickness hardness distribution, and the integrity of the fusion interface. Excessive dilution lowers surface hardness, while poor bonding or improper crack patterns can lead to spalling and premature liner failure. Consequently, manufacturers, end users, and accredited third-party laboratories rely on standardized test programs to qualify both incoming plate and in-service armor.
- Working surface hardness is typically specified between 55 and 65 HRC for chromium carbide grades.
- Relief cracks perpendicular to the bead direction are an intentional design feature that manages residual welding stress.
- Service life improvements of 5 to 25 times that of plain carbon steel are commonly achieved in low-stress abrasive environments.
- Weld overlay consumables are classified under EN 14700 and AWS A5.13/A5.21, which define hardness and property requirements for surfacing deposits.
Testing Samples
Chromium Carbide Overlay (CCO) Plates
The most widely tested sample type consists of a high-chromium, high-carbon deposit (typically 25-30% Cr with additions of Mo, Nb, V, or W) on a low-carbon steel substrate. Coupons are usually cut by plasma or waterjet from production plates in dimensions such as 100 mm by 100 mm for metallography, 25 mm by 75 mm flat specimens for abrasive wear testing, and full-thickness sections for bend and shear evaluation. Cutting methods that avoid additional heat input into the overlay are preferred so that the as-welded microstructure is preserved.
Tungsten Carbide Overlay Plates
These armor plates contain macroscopic cast tungsten carbide or sintered carbide particles embedded in a nickel, iron, or cobalt matrix, and are used for extreme abrasion combined with moderate impact, such as dredge cutter heads and drilling tools. Testing focuses on particle distribution, particle crushing resistance, matrix hardness, and particle-to-matrix retention after wear exposure.
Complex Alloy and Multi-Layer Overlay Plates
Multi-carbide grades containing niobium, vanadium, molybdenum, and boron, as well as multi-layer deposits with transition layers, require additional evaluation of hardness gradients between layers and verification that successive passes do not excessively re-dilute the previously deposited hardfacing alloy.
Fabricated Wear Armor Components
Finished protective components, including chute liners, hopper panels, fan impeller blades, wear bars, pipe segments, bucket side cutters, and screen decks, are tested to confirm that post-weld fabrication such as rolling, bending, plasma cutting, and plug welding has not degraded the overlay or introduced delamination at the bond line.
Testing Items
A complete qualification program for hardfacing wear plate armor covers the following test items, grouped by property category:
- Chemical composition of the overlay deposit and base substrate, including C, Si, Mn, Cr, Mo, Nb, V, W, B, S, and P.
- Weld dilution rate at the bond line, typically required to remain below approximately 10-15% for optimal surface hardness.
- Surface macrohardness, with acceptance values commonly specified as an average of 58-65 HRC for CCO grades.
- Through-thickness hardness profile from the wear surface to the substrate to confirm an adequate hardened layer depth.
- Microhardness of the carbide phase (often 1200-1700 HV0.1) and the eutectic matrix (typically 600-850 HV0.1).
- Overlay thickness and thickness uniformity across the plate, verified at multiple grid positions.
- Microstructural evaluation, including primary M7C3 carbide volume fraction (typically 30-50%), carbide size, orientation, and distribution, heat-affected zone characteristics, and bond line fusion quality.
- Abrasive wear resistance expressed as volume loss and relative wear ratio compared with reference substrate steel.
- Bond shear strength and bend delamination resistance between the overlay and the substrate.
- Surface crack pattern evaluation, including crack spacing, transverse orientation, and confirmation that cracks do not propagate through the interface into the base metal.
- Dimensional and geometric inspection, covering length, width, total thickness tolerance, flatness, and camber.
- Surface defect inspection for porosity, slag entrapment, lack of fusion, spatter, and overlay repair quality.
- Substrate mechanical properties, including tensile strength, yield strength, and elongation, and where required, Charpy impact toughness.
Testing Methods
Chemical Composition Analysis
Spark-discharge optical emission spectrometry (OES) is performed on a ground and polished area of the overlay in accordance with ASTM E415 and the corresponding GB/T 223 series for reference analysis. Carbon and sulfur are verified by combustion infrared absorption per ASTM E1019, while X-ray fluorescence screening per ASTM E1621 provides rapid verification of alloying elements. Glow-discharge OES per ISO 14707 can additionally characterize composition gradients near the bond line, supporting dilution calculations.
Hardness Testing
Bench Rockwell testing per ASTM E18 and ISO 6508-1 (equivalently GB/T 230.1) provides the primary surface hardness measurement on ground patches of the overlay. Brinell testing per ASTM E10 and ISO 6506-1 is applied to the substrate. Vickers microindentation per ASTM E384 and ISO 6507-1 (GB/T 4340.1) is used for hardness traverses and for individual phase hardness of carbides and matrix. For on-site acceptance of fabricated armor, portable Leeb rebound testers per ASTM A956 and ultrasonic contact impedance instruments are employed, with results correlated to bench hardness on reference coupons.
Metallographic Examination
Cross-sections are prepared per ASTM E3 and etched per ASTM E407 to reveal the overlay, heat-affected zone, and substrate. Quantitative image analysis and manual point counting per ASTM E562 determine the volume fraction of primary carbides, while grain size assessment per ASTM E112 characterizes the substrate and heat-affected zone. Metallography is also the definitive method for measuring actual overlay thickness, penetration depth, dilution zone width, and detecting micro-porosity or interfacial defects.
Abrasive Wear Testing
The dry sand and rubber wheel abrasion test per ASTM G65 (Procedures A and B) is the industry-standard method for ranking hardfaced armor, reporting mass or volume loss after a defined wheel revolution count. Wet sand rubber wheel testing per ASTM G105 and the steel-wheel abrasive test per ASTM B611 are used where slurry or high-stress gouging conditions prevail. Results are expressed as volume loss in cubic millimeters and as a relative wear resistance ratio against a reference mild steel specimen tested under identical conditions.
Mechanical and Bond Integrity Testing
Bend and delamination testing per ASTM E290 and ISO 7438 (GB/T 232.1) subjects full-thickness strips to controlled bending to confirm that the overlay remains adherent without separation. Dedicated shear fixtures quantify interfacial bond strength. Substrate tensile properties are determined per ASTM E8/E8M or ISO 6892-1 (GB/T 228.1), and Charpy V-notch impact testing per ASTM E23 and ISO 148-1 verifies base plate toughness where low-temperature service or impact loading is expected. Consumable classification tests, including deposit hardness requirements, follow EN 14700 and AWS A5.21 procedures.
Dimensional and Visual Inspection
Total thickness is measured with ultrasonic thickness gauges and micrometers at a defined grid, flatness is checked with straight edges and laser scanning, and visual inspection of the weld deposit follows the principles of ISO 17637. Crack pattern acceptability is assessed against manufacturer or purchaser specifications, with particular attention to crack spacing, freedom from longitudinal connected cracking, and the absence of cracks crossing the fusion boundary.
Non-Destructive Testing
Ultrasonic examination per EN 10160 and ISO 16809 from the substrate side detects lack of fusion and disbonding at the clad interface. Magnetic particle testing per ISO 9934 and ASTM E709, applied to the substrate and weld-attached areas, reveals fabrication-induced cracks, while liquid penetrant testing per ISO 3452 and ASTM E165 is used on non-magnetic nickel-matrix overlays. NDT is routinely specified by accredited third-party laboratories for fabricated liner packages before shipment and for in-service armor surveys.
Applications
Hardfacing wear plate armor is deployed wherever abrasive bulk materials contact steel surfaces, and testing programs are tailored to the dominant wear mechanism of each application:
- Mining and mineral processing: chute linings, hopper interiors, screen panels, conveyor transfer points, and crusher component liners.
- Cement and building materials: fan impeller and blade armor, raw mill and cement mill liners, cyclone protection, and clinker cooler walls.
- Power generation: coal conveyor chutes, pulverizer internals, ash handling piping, and boiler soot blower shielding.
- Steel and metallurgical plants: sinter machine screens, slag handling equipment, charging chute armor, and ladle and caster section protection.
- Dredging and marine engineering: cutter head teeth, suction tube lining, and pipeline bends exposed to sand and gravel slurries.
- Construction and aggregate equipment: excavator bucket liners, side cutters, crusher cheek plates, and asphalt mixer paddles.
- Agriculture and recycling: tillage tool armor, glass cullet handling liners, and shredder wear packages.
In addition to product qualification, periodic in-service inspection combining ultrasonic thickness mapping and visual crack assessment allows maintenance teams to forecast remaining liner life and schedule armor replacement before substrate exposure occurs.
FAQ
What surface hardness should a qualified chromium carbide overlay plate deliver?
A well-qualified CCO plate should average 58-65 HRC on the wear surface, with individual readings rarely below 55 HRC. Hardness is measured on ground patches at multiple locations because bead ripple and micro-cracks can distort single-spot readings. Microhardness testing supplements macrohardness by confirming that the carbide phase itself reaches 1200 HV0.1 or higher.
Are the cracks on the overlay surface considered defects?
No. Controlled, regularly spaced transverse relief cracks are an intentional metallurgical feature of chromium carbide overlays. They relax residual welding stresses within the brittle hardfacing layer and stop at the ductile bond line. What testing must exclude are longitudinal connected cracks, cracks propagating into the substrate, and spalling, which indicate improper welding parameters or excessive dilution.
How is dilution measured and why does it matter?
Dilution is quantified chemically by comparing the overlay composition with the nominal wire composition, metallographically by measuring the melted substrate fraction in the cross-section, and mechanically through the hardness profile. Excessive dilution reduces carbon and chromium concentration in the deposit, suppresses primary carbide formation, and lowers wear life; qualified plate is therefore verified to keep dilution, particularly in the first pass near the bond line, within specified limits.
Which wear test best predicts field performance?
ASTM G65 dry sand rubber wheel testing is the most widely referenced screening method for low-stress scratching abrasion, which dominates in chutes and hoppers. For high-stress grinding abrasion in crushers and mills, ASTM B611 or ASTM G105 provides a closer simulation. Because no single laboratory test reproduces all field conditions, results should always be compared as relative rankings against a reference material tested in the same run.
Can hardfaced armor plates be cut, bent, and welded during fabrication?
Yes, within defined limits. Plasma or laser cutting is used for shaping, cold rolling to generous radii is possible with the overlay in compression, and attachment is performed by plug or fillet welding to the substrate side. Post-fabrication bend delamination testing and ultrasonic bond scanning verify that forming stresses have not separated the overlay from the substrate.
Who should perform the testing?
Manufacturer self-inspection covers routine production control, while buyers in critical applications typically require certificates from accredited third-party laboratories operating under ISO/IEC 17025 quality systems, with all methods referenced to ISO, ASTM, EN, or GB standards to ensure traceable and comparable results.