Weld Overlay Hardness Testing: Methods, Standards, and Industrial Applications

Weld overlay hardness testing is the principal acceptance method for verifying that hardfacing and corrosion-resistant surfacing deposits meet specified wear, erosion, and cracking-resistance requirements. This article explains the applicable ISO, ASTM, EN, and GB/T hardness testing standards, specimen preparation requirements, cross-sectional hardness profiling techniques, and typical acceptance criteria for overlay deposits used in oil and gas, mining, power generation, and heavy machinery. It serves as a practical reference for welding engineers, quality inspectors, and procurement specialists specifying or evaluating overlay deposits in accredited third-party laboratories.

Technical Overview

Weld overlay, also known as surfacing, hardfacing, or weld cladding, is a fabrication process in which one or more layers of wear-resistant or corrosion-resistant alloy are deposited onto a base metal using arc, plasma transferred arc (PTA), submerged arc, or laser welding processes. Because the functional performance of the deposit, including abrasion resistance, erosion resistance, and resistance to hydrogen-assisted cracking, is closely correlated with its hardness, hardness testing is the most widely specified acceptance criterion for overlay work. Testing is typically performed on welding procedure qualification coupons, production test plates, or finished components.

Overlay hardness testing differs from routine bulk-material hardness testing in several important respects. The deposit is relatively thin, chemically heterogeneous due to dilution with the substrate, and metallurgically graded across the fusion line. A complete evaluation therefore addresses not only the average surface hardness but also the hardness distribution through the deposit thickness, the dilution zone, the heat-affected zone (HAZ), and the unaffected base metal. The governing method standards include ISO 6506 and ASTM E10 for Brinell testing, ISO 6507 and ASTM E92 for Vickers testing, ISO 6508 and ASTM E18 for Rockwell testing, and ASTM E384 for microindentation testing, together with their Chinese national equivalents GB/T 231.1, GB/T 4340.1, and GB/T 230.1. Overlay-specific filler metal classification and qualification requirements are covered by AWS A5.13 and AWS A5.21, while maximum hardness limits for sour-service environments are defined in ISO 15156.

• Verify that the as-deposited overlay meets the minimum hardness specified for wear or erosion resistance
• Confirm that corrosion-resistant overlays remain below maximum hardness limits to avoid cracking susceptibility
• Characterize dilution and hardness gradients across the deposit, fusion line, and HAZ
• Determine the effective depth of the wear-resistant layer after finishing operations
• Support welding procedure qualification and ongoing production quality control

Testing Samples

Procedure Qualification and Production Test Coupons

Test coupons are prepared using the same welding consumable, process parameters, and layer build-up sequence as production overlays. These coupons are sectioned perpendicular to the welding direction to produce metallographic cross-sections suitable for through-thickness hardness traverses, and they represent the most common sample type submitted to accredited third-party laboratories.

Corrosion-Resistant Overlay Samples

Stainless steel (308L, 309L, 347) and nickel-based (Alloy 625-type, C-276-type) overlays applied to carbon steel or low-alloy steel pressure parts are tested to verify both chemical integrity and compliance with maximum hardness restrictions, particularly for components operating in wet hydrogen sulfide (H2S) service governed by ISO 15156.

Hardfacing Samples on Wear Components

Deposits of iron-based chromium-carbide alloys, cobalt-chromium alloys, and nickel-based hardfacing alloys on components such as valve seats, drill pipe tool joints, mill rolls, and crusher elements are tested to confirm the specified minimum hardness, typically in the range of 40 to 65 HRC.

Metallographic Cross-Section Mounts

For thin deposits or detailed evaluation, sections are mounted, ground, polished, and etched (commonly with nital) to reveal the deposit structure, carbide distribution, fusion line, and HAZ, enabling precise microhardness placement by zone.

Testing Items

The scope of overlay hardness testing is defined by the product specification, the welding procedure specification (WPS), and the applicable qualification standard. Typical test items include:

• Surface Rockwell hardness (HRC for hard deposits, HRB for softer corrosion-resistant overlays)
• Brinell hardness (HBW) for thicker deposits and as-welded surfaces
• Macro-Vickers hardness (HV10, HV30) on prepared cross-sections
• Micro-Vickers hardness traverses (HV0.2, HV0.5, HV1) through deposit, dilution zone, HAZ, and base metal
• Through-thickness hardness distribution profiles and hardness gradient curves
• Effective wear-resistant layer depth after finishing allowance
• Verification of maximum hardness for sour-service overlays (typically 22 HRC or 248 HV per ISO 15156)
• HAZ hardness checks on the base metal side to detect hard, crack-susceptible microstructures
• Hardness uniformity assessment across single-pass, multi-pass, and overlapped bead regions

Testing Methods

Rockwell Hardness Testing

Rockwell testing per ISO 6508, ASTM E18, and GB/T 230.1 is the standard method for hardfacing deposits above approximately 20 HRC. The HRC scale is preferred for iron-based and cobalt-based hardfacings, while the HRB scale suits softer austenitic overlays. Because the Rockwell indentation is shallow, the deposit must be sufficiently thick so that the indentation depth does not exceed one tenth of the layer thickness; otherwise the substrate will artificially lower the reading. As a practical rule, HRC testing requires a sound deposit of at least 0.6 to 1.0 mm after grinding, and the surface should be flat, clean, and free of scale.

Brinell Hardness Testing

Brinell testing per ISO 6506, ASTM E10, and GB/T 231.1 uses a large carbide ball (commonly 10 mm at 3000 kgf) that averages over coarse structures, porosity, and undulating weld ripples. It is suitable only for thick, multi-layer deposits where the indentation is well removed from the fusion line, and it is often applied to large components such as roll sleeves and wear plates where local averaging is desired.

Vickers and Micro-Vickers Hardness Testing

Vickers testing per ISO 6507, ASTM E92, ASTM E384, and GB/T 4340.1 is the preferred method for cross-sectional evaluation. Macro loads (HV10 to HV30) map the overall hardness profile, while micro loads (HV0.2 to HV1) resolve individual beads, the dilution zone, the fusion line, and the HAZ. A typical traverse places a row of indentations from the deposit surface toward the base metal at defined increments, with center-to-center spacing of at least three indentation diagonals and an edge distance of at least 2.5 diagonals to avoid interaction effects. The resulting profile directly quantifies dilution, which typically reduces deposit hardness near the fusion line.

Portable and In-Situ Methods

For finished components that cannot be sectioned, portable Rockwell testers, ultrasonic contact impedance (UCI) instruments per ASTM A1038, and Leeb rebound testers per ASTM A956 provide indicative surface hardness values on flat, rigid areas. Results from portable instruments should be correlated with laboratory bench methods, and conversions between hardness scales must be treated as approximate in accordance with ASTM E140 and ISO 18265.

Specimen Preparation and Test Planning

Accurate overlay hardness measurement depends on specimen preparation. Sectioning must avoid burning or excessive heating, mounting must support the deposit edge, and grinding and polishing must proceed through fine grits to eliminate work-hardened or deformed surface layers. Etching reveals the zones so that indentations can be positioned deliberately. A typical test plan requires a minimum of five valid readings per zone, discards readings that violate spacing or thickness rules, and reports results as individual values together with averages per zone.

Applications

Weld overlay hardness testing is applied across industries wherever surfacing deposits protect components from wear, erosion, high-temperature degradation, or corrosion:

• Oil and gas: gate valve seats and discs, ball valves, and wellhead components with cobalt-chromium hardfacing, with hardness verification for ISO 15156 sour-service compliance
• Downhole drilling: hardbanding on drill pipe tool joints and stabilizers, where controlled surface hardness protects casing while resisting abrasion
• Mining and aggregates: crusher hammers, jaw plates, roll crushers, slurry pump impellers, and bucket teeth with chromium-carbide overlays
• Steel and metals processing: continuous caster rolls, hot and cold mill rolls, guides, and shear blades
• Power generation: boiler tube corrosion-resistant cladding, turbine and valve internals, and coal pulverizer components
• Chemical and petrochemical: reactor and pressure vessel corrosion-resistant cladding, heat exchanger tube sheets, and flange sealing faces
• Maintenance and repair: reclamation of worn shafts, track links, and dies, where hardness verification confirms that restored surfaces match original performance

FAQ

Which hardness scale should be specified for a hardfacing overlay?

HRC is the default scale for deposits intended to resist abrasion and erosion, since most hardfacings fall between 40 and 65 HRC. For thin deposits below about 1 mm, or for softer corrosion-resistant overlays, Vickers scales (HV5 to HV30) are more appropriate because they impose smaller, better-controlled indentations and can be applied to polished cross-sections.

How thick must the overlay be for reliable hardness testing?

As a general rule, the indentation depth must not exceed one tenth of the deposit thickness beneath the indentation. For HRC this implies a minimum sound deposit of roughly 0.6 to 1.0 mm; for HBW 10/3000, several millimeters of deposit are required. When the layer is thinner, testing must shift to micro-Vickers methods on a mounted cross-section.

Why does hardness vary through the deposit thickness?

Dilution with the substrate lowers the alloying content, and therefore the hardness, of the first pass near the fusion line. Successive passes, carbide precipitation, bead overlap patterns, and post-weld heat treatment further modify the distribution. This is precisely why specifications call for through-thickness hardness traverses rather than a single surface reading.

Can hardness be tested on finished components without cutting a sample?

Yes. Portable Rockwell, UCI per ASTM A1038, and Leeb per ASTM A956 instruments allow on-site verification on suitable surfaces. These methods provide indicative results, and where contractual acceptance is required, they should be correlated with laboratory Rockwell or Vickers measurements on representative coupons.

What acceptance criteria are commonly applied?

Wear-resistant overlays are usually held to a specified minimum hardness, for example 58 HRC for chromium-carbide deposits in abrasive service. Corrosion-resistant overlays and their HAZs in sour service are held to a maximum of 22 HRC (approximately 248 HV) per ISO 15156. The governing values always come from the purchaser specification, the WPS qualification requirements, or the applicable AWS classification.

How should results be reported?

A compliant test report identifies the standard used, test scale and force, specimen preparation, indentation locations by zone, all individual readings, per-zone averages, minimum and maximum values, any invalid readings and their cause, and a statement of conformity against the specified acceptance criteria.