Hardness testing of steel: HBW, HV and reporting

Hardness testing measures a material's resistance to local indentation. Steel MTCs and project specifications commonly use Brinell (HBW) or Vickers (HV) values, but those scales and test conditions should not be mixed casually.

Brinell hardness — HBW

The Brinell method presses a hardmetal ball indenter into the prepared surface under a specified test force and determines hardness from the resulting indentation.

ISO 6506-1:2014 is the current published ISO test-method standard at the verification date.

Brinell leaves a relatively large indentation and is well suited to bulk hardness assessment on many steels, castings, forgings and plate-type products.

A Brinell result should include enough notation to identify the test conditions — not merely a bare number such as 220.

Vickers hardness — HV

The Vickers method uses a diamond pyramid indenter and calculates hardness from the measured indentation diagonals. ISO 6507-1:2023 covers the method across different test-force ranges.

Because Vickers can use smaller indentations, it is especially useful for:

• weld/HAZ hardness traverses;
• thin sections;
• local microstructural zones;
• cases where a large Brinell indentation is undesirable.

Notation such as HV10 or HV5 identifies the Vickers scale together with the nominal test force convention. The exact reporting format should follow the applicable standard.

HBW and HV are not interchangeable numbers

A hardness conversion table can provide an approximate relationship between scales for specified material groups. ISO 18265 gives principles and conversion tables for this purpose.

But conversion is not the same as direct measurement. ISO explicitly limits the direct applicability of converted values to the relevant material basis.

If a purchase specification says max 250 HV10, do not assume that a nearby HBW value automatically proves compliance unless the governing specification allows conversion.

Does hardness testing require cutting the pipe or plate?

Not always.

A bulk hardness test can often be made directly on an accessible, suitably prepared product surface. Portable Brinell equipment also exists.

A coupon or cross-section becomes necessary where the required test location is not accessible on the finished surface or where the specification requires a profile through specific zones, such as:

• weld metal;
• fusion line;
• HAZ;
• through-thickness positions;
• a prescribed distance from a surface.

So the question is not simply “Does hardness destroy the pipe?” but where does the governing procedure require the indentation to be made?

Hardness on the MTC

Whether hardness must appear on an MTC depends on the material/product standard, grade and project supplementary requirements.

Do not assume that every API, EN or offshore grade requires a reported hardness value. Conversely, if a project specification imposes a maximum hardness for sour service, weldability or fabrication control, the absence of the required result can be a real documentation gap.

See Common MTC red flags and inconsistencies.

Parent material vs weld hardness

These are different inspection objectives.

Parent-material hardness can be used as a general material/property check. Weld hardness surveys often use Vickers traverses through weld metal, HAZ and base metal to identify local hard zones associated with welding thermal cycles.

A single Brinell result on the pipe body cannot replace a required weld/HAZ hardness traverse.

What to check in a hardness requirement

A complete requirement should identify:

• hardness method/scale: HBW, HV, etc.;
• test force/condition where relevant;
• maximum/minimum acceptance value;
• test location;
• number/frequency of measurements;
• parent material or weld/HAZ scope;
• whether conversions are permitted;
• reporting/certificate requirement.

Procurement takeaway

A hardness number without a scale and test condition is incomplete. Check method, load, location, frequency and acceptance limit, and avoid using converted values as a substitute for specified direct testing unless the governing requirement explicitly permits it.

References and verification sources

• ISO 6506-1:2014 — Brinell hardness test
• ISO 6507-1:2023 — Vickers hardness test
• ISO 18265:2013 — Conversion of hardness values