Weld Hardness Testing: Methods and Standards (ISO 9015, ISO 15614)
How to survey parent metal, heat-affected zone and weld hardness with the Vickers method.
Weld hardness testing measures the hardness across a welded joint, namely parent metal, heat-affected zone (HAZ) and weld metal, almost always with the Vickers method (HV5 or HV10) on a polished macro cross-section. It is used to detect excessive HAZ hardness, which signals a risk of cracking and brittleness, and to qualify welding procedures. For the wider method context, see the Hardness Testing Academy.
Why test the hardness of welds?
The thermal cycle of welding changes the microstructure of the heat-affected zone, which can become hard and brittle and prone to hydrogen-induced cracking. A hardness survey verifies that the joint stays below the maximum hardness allowed by the specification, confirming a sound, ductile weld.
How is weld hardness measured?
A transverse macro-section of the joint is cut, mounted, ground, polished and lightly etched to reveal the weld, HAZ and parent metal. Rows of Vickers indentations (typically HV10, sometimes HV5) are placed across these zones, near the surfaces (cap and root) where the hardest microstructures usually form. The maximum value found is compared to the limit in the standard. See the Vickers hardness testing guide for the method details.
Where should the indentations be placed?
Indentation rows follow the layout defined in ISO 9015-1: lines of measurement crossing the parent metal, both heat-affected zones and the weld metal, with additional readings close to the fusion line and in the coarse-grained HAZ where peak hardness occurs. Spacing must respect the minimum distance between indentations to avoid interaction.
Step-by-step: how to perform a weld hardness test
This practical procedure describes a Vickers hardness survey of a welded joint, from sectioning the weld to issuing the report, in accordance with ISO 9015-1 and the limits set by ISO 15614-1.
- Section the joint. Cut a transverse cross-section through the weld with an abrasive cut-off machine and plenty of coolant, keeping the cut perpendicular to the weld axis. Avoid heat that could alter the microstructure.
- Mount the sample. Embed the section in resin with an edge-retention compound, so the cap and root edges stay flat and are not rounded during preparation.
- Grind and polish. Grind with progressively finer silicon carbide papers, then polish to a mirror finish so the indentations will have sharp, measurable corners.
- Etch to reveal the zones. Apply a suitable etchant (for example Nital on ferritic steels) to reveal the weld metal, the fusion line, the heat-affected zone and the parent metal, so you can position the indentation rows correctly.
- Light re-polish for automatic reading. After etching, give the surface a very short, light re-polish. This reduces the relief and darkening left by the etchant, so an automatic Vickers system can reliably detect and measure the indentation diagonals, while the zone boundaries stay visible enough to place the rows.
- Verify the tester. Verify the Vickers tester at the test load with a certified test block and confirm calibration traceability.
- Set the parameters. Select the load (HV10 for a standard macro survey per ISO 9015-1, or HV5 and microhardness for a narrow or thin heat-affected zone) and set the dwell time.
- Define the indentation lines. Lay out rows across the parent metal, both heat-affected zones and the weld metal per ISO 9015-1, near the cap and the root, with extra points in the coarse-grained HAZ next to the fusion line where peak hardness forms.
- Run the indentation rows. Make the indentations respecting the minimum spacing between them and from the edge required by ISO 6507. The HAZ is narrow, so position each point carefully. See the Vickers hardness testing guide.
- Measure and record. Measure both diagonals of each indentation, automatically or manually, and record hardness versus position across the joint.
- Evaluate against the limit. Identify the maximum hardness, usually in the coarse-grained HAZ, and compare it to the limit in the specification (for many ferritic steels about 350 to 380 HV10 per ISO 15614-1).
- Issue the report. Record the method and standard, the load, the limit, the hardness values by zone, the maximum value, the tester identification and traceability, the operator and the date, together with the indentation map.
Best practices for weld hardness testing
- Etch to see the zones, then lightly re-polish so the automatic Vickers reader can still measure the indentations clearly.
- Concentrate readings in the coarse-grained HAZ next to the fusion line, where the peak hardness usually occurs.
- Keep the section exactly transverse and perpendicular, so the indentations are not distorted.
- Control the etching time to avoid over-etching, which roughens the surface and hampers automatic reading.
- Use HV10 for standard macro surveys, and switch to HV5 or microhardness for very narrow heat-affected zones or thin sections.
- Photograph the etched macro before re-polishing, to document the zone positions for the report.
- Use that macro to set the indentation coordinates, then re-polish so the automatic reader works reliably.
- If auto-reading fails in dark etched areas, increase the illumination or give a second light re-polish.
- Label the rows (cap, middle, root) consistently so the report is easy to trace.
- Look for the hardest point just inside the fusion line on the HAZ side.
Which standards apply to weld hardness testing?
ISO 9015-1 covers hardness testing on arc-welded joints (macrohardness, Vickers), ISO 9015-2 covers microhardness testing, and ISO 15614-1 (welding procedure qualification) sets the maximum permitted hardness values. For many ferritic steels a common limit is 380 HV10 (non heat-treated) or 350 HV10 (heat-treated). The Vickers method itself follows ISO 6507 and ASTM E92.
Frequently asked questions
Which hardness scale is used for welds?
Vickers (HV), usually HV10 for macrohardness surveys and HV5 or lower for thin sections, because it gives a continuous scale and small indentations that resolve the narrow heat-affected zone.
What is the maximum allowable weld hardness?
It depends on the material and standard; ISO 15614-1 commonly limits ferritic steels to about 350 to 380 HV10. Always follow the applicable specification.
Why is HAZ hardness important?
A hard heat-affected zone indicates brittle microstructures that raise the risk of cracking, so keeping HAZ hardness below the limit confirms a safe, ductile joint.
Verify your Vickers tester with traceable, ILAC-MRA certified reference blocks.