Materials Verification · HT
Hardness Testing
One controlled dent, a window into strength.
Hardness testing measures a material’s resistance to permanent indentation — pressing a defined indenter with a defined force and measuring the result. Because hardness correlates with tensile strength and reflects heat treatment and welding history, a controlled dent a fraction of a millimetre across reads out material condition on the spot.
The explainer
Watch how it works.
Schematic · loops automatically
1Prepare the test point
The surface is ground smooth and clean — roughness and scale corrupt small impressions and rebound readings alike.
2Apply a defined force
The indenter — diamond pyramid, carbide ball or impact body — meets the surface under controlled load or impact energy.
3The material yields — measurably
A precise impression forms; its size or depth, or the rebound energy lost, quantifies the resistance.
4Number on the right scale
The reading is expressed as HV, HB, HRC or HL, with standardised conversions where comparison is needed.
5Interpret the metallurgy
Values are mapped across weld, HAZ and parent — confirming procedure limits, heat treatment or suspected degradation.
The physics
All the classical scales share one idea: push a hard, precisely-shaped indenter into the surface with a known force, and measure the material’s resistance. Vickers (ISO 6507) presses a diamond pyramid and measures the diagonal of the impression; Brinell (ISO 6506) presses a carbide ball and measures the crater; Rockwell (ISO 6508) reads the depth of penetration directly. Smaller impression for the same force means harder material.
Hardness matters because it is a proxy with teeth. For steels, tensile strength tracks hardness closely enough for standardised conversion tables; welding procedure qualifications set maximum hardness limits in the heat-affected zone because hard microstructures are brittle and crack-susceptible — a hardness traverse across weld, HAZ and parent metal is a metallurgical health check in a row of numbers.
On site, two portable families dominate. Leeb rebound (ISO 16859) fires a tungsten-carbide impact body at the surface and compares rebound to impact velocity — energy lost to plastic deformation reads as softness. UCI (ultrasonic contact impedance) presses a vibrating Vickers diamond and reads the frequency shift as the contact stiffens. Each has its geometry and mass constraints, which is why method selection is part of the service, not an afterthought.
Where it earns its keep
Typical applications
- Weld procedure qualification hardness traverses
- In-service verification after repairs and heat treatment
- HAZ surveys on pressure plant and structural welds
- Incoming material checks alongside PMI
Standards & schemes
| Designation | Covers |
|---|---|
| ISO 6507 / 6506 / 6508 | Vickers, Brinell and Rockwell hardness testing |
| ISO 16859 | Leeb rebound hardness testing |
| ISO 9015 / ISO 22826 context | Hardness testing of welded joints |
Hardness surveys are performed by trained technicians with calibrated instruments and reference blocks, laboratory work under Metlab’s ISO/IEC 17025 accredited scope.
Why do welding procedures set hardness limits?
Because rapid weld cooling can create hard, brittle microstructures in the heat-affected zone that crack in service — especially in sour or hydrogen environments. A maximum hardness (commonly in the HV10 scale) is a proxy limit on those microstructures, and the traverse proves the procedure respects it.
How accurate are portable hardness testers?
Very usable when matched to the job: Leeb on heavy sections, UCI on welds and finer geometry, both verified on reference blocks. Where the number carries contractual weight, laboratory Vickers or Brinell on prepared specimens remains the referee method.
Can hardness tell me the steel’s tensile strength?
Approximately — standardised conversion tables map hardness to tensile strength for steels, good enough for screening and condition assessment. It never replaces a tensile test where the specification demands measured properties; it tells you where tensile testing is worth the cut.
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