A hardness test determines the hardness of a material by pressing a hard indenter into the metal surface under a defined load and measuring the size or depth of the resulting indentation. It is widely used because it is fast and leaves only a small mark on the part, and hardness also gives an indication of strength. FKT NDT provides hardness testing for parent materials and welded joints.
What Does a Hardness Test Measure?
The metal under the indenter deforms plastically and a permanent indentation remains on the surface. The hardness value is a number related to the size of that indentation. Some methods measure its diameter or diagonals, others its depth. The most common methods are:
- Vickers (HV): a square-based diamond pyramid; the diagonals of the indentation are measured. The preferred method for hardness surveys across welds.
- Brinell (HB / HBW): a hard-metal ball; the indentation diameter is measured. Suitable for coarse-grained materials and castings.
- Rockwell (HRC, HRB etc.): the indentation depth is measured. Fast, and common on heat-treated parts.
Where Is It Used?
- Welding procedure qualification (WPQR): a hardness traverse across the weld metal, heat-affected zone (HAZ) and parent metal checks for excessive hardening. High hardness can indicate a risk of cold cracking.
- Heat treatment control: verifying the result of hardening, tempering or stress relieving.
- Material acceptance and sorting: a quick check that material matches the expected grade.
- Failure investigation: assessing worn or damaged components.
In weld qualification, hardness is usually measured on the macro examination specimen and assessed together with the tensile test and Charpy impact test results.
Standards
Hardness testing is commonly applied according to:
- ISO 6507-1: Metallic materials – Vickers hardness test
- ISO 6506-1: Metallic materials – Brinell hardness test
- ISO 6508-1: Metallic materials – Rockwell hardness test
- ISO 9015-1: Hardness testing of arc-welded joints
- ASTM E92, ASTM E10 and ASTM E18: Vickers, Brinell and Rockwell testing to American standards
- ISO 15614-1: when hardness testing is required in procedure qualification, and the maximum permitted values
How the Test Is Performed
- Surface preparation: the test surface is ground or polished to the finish the method requires. For welds, the cross-section is etched to reveal the weld zones.
- Locating test points: on welds, measurements are made along lines crossing the parent metal, HAZ and weld metal.
- Measurement: the indenter is applied at the specified load for a set dwell time and the indentation is measured.
- Evaluation: values are compared with the limits in the specification or applicable standard.
- Reporting: the report gives the method and load (for example HV10), the position of each test point, the individual hardness values and the result.
FAQ
What does HV10 mean?
HV10 is a Vickers hardness measurement made with a 10 kgf (about 98 N) load. It is the load level commonly used in welding procedure qualification.
Can hardness values be converted between scales?
Approximate conversion tables exist between HV, HB and HRC, but conversions depend on the material and are only approximate. Testing should use the method the specification calls for.
Is hardness testing non-destructive?
It leaves a small indentation, so in many cases the part can stay in service. A hardness traverse across a weld section is destructive, however, because the specimen is cut from the joint.
Why is hardness measured on welds?
Rapid cooling during welding can form hard, brittle structures in the HAZ. Hardness testing reveals this risk and shows whether measures such as preheating are adequate.
To arrange hardness testing, contact us with the material, method and standard, and we will prepare a quotation.