What does a weld hardness profile show?
Different regions of a joint experience different thermal cycles during welding. Weld metal solidifies from the melt. The adjacent heat-affected zone, or HAZ, remains solid, but its microstructure can change as a result of the heat. Depending on the material and its condition, harder or softer regions can form.
A single hardness measurement describes the region it samples. A series across the cross-section shows how hardness changes between base metal, HAZ and weld metal. This helps compare process variants, heat treatments or specimens with unexpected behaviour. A hardness profile is particularly useful when investigating hardening, softening or uneven transitions.
How do we plan the measurements?
The testing question determines the measurement plan. Meaningful comparisons require documented test conditions and positions. We therefore agree which zones and which depths within the weld cross-section should be investigated.
- Identify the specimen: record the material, initial condition, welding process and any post-weld heat treatment.
- Prepare the cross-section: choose the sampling position, orientation and preparation so that the relevant regions are accessible and indentations can be evaluated.
- Define the measurement path: include base metal, both transitions and weld metal; plan additional rows where needed.
- Select force and spacing: consider indentation size, specimen geometry and applicable requirements together.
- Locate each result: document the measurement position, hardness scale and test conditions alongside the hardness values.
Vickers and microhardness testing are the main methods for these profiles. Lower test forces can sample smaller regions, but place greater demands on surface preparation and measurement. They are therefore not automatically the best choice for every task.
Confirmed service scope: Testawell offers Vickers testing including HV 0.3 and HV 10, together with hardness profiles on prepared cross-sections. The appropriate force and spacing are selected from the applicable standard, material, zone width and test objective.
Which standard applies to the welded joint?
Weld hardness testing covers different applications. The relevant standard depends on factors including the welding process and weld width:
| Standard | Scope to consider |
|---|
| ISO 9015-1:2001 | Hardness testing on cross-sections of arc-welded joints. Austenitic stainless steels are outside its scope. |
| ISO 9015-2:2016 | Microhardness testing where hardness changes sharply. Very narrow laser and electron-beam welds, and resistance spot, projection and seam welds are excluded. |
| ISO 22826:2005 | Vickers or Knoop hardness testing of narrow laser and electron-beam welded joints. Applicability to wider hybrid laser/arc welds requires separate assessment. |
This table outlines the standards. The specific procedure, edition and any acceptance limit are agreed for the job. Reference to a standard does not indicate laboratory accreditation; Testawell does not hold its own accreditation.
When can hardness be used to assess strength?
A hardness profile can reveal where the material condition changes. Under suitable conditions, hardness can also be used to estimate tensile strength. ISO 18265:2013 describes conversions and their limitations: a relationship cannot be transferred regardless of material and condition.
A weld can contain several different microstructures. One conversion relationship may therefore not apply equally well to every zone. A hardness value also provides neither a complete local flow curve nor a universally valid determination of yield strength. Toughness, freedom from cracks and service life cannot be established from hardness alone.
Southern and co-authors (2023) compare hardness conversions with indentation profile analysis for several metallic materials. For a practical testing task, the first step is to define the information needed and then choose the appropriate method.
When additional local flow curves are needed
Local flow curves help explain when a zone starts to deform permanently and how it strengthens during further deformation. For a suitable specimen, we can complement hardness profiling with mechanical indentation testing. In this method, optical 3D measurement captures the residual indentation shapes for inverse analysis. This provides model-based plastic flow curves and tensile-equivalent properties.
The volume of material mechanically affected by the test must fit the width of the region being investigated. A fine measurement grid alone does not guarantee that every HAZ subzone can be resolved separately. We assess the useful data that can be obtained from the material, geometry and testing objective.
Our page on weld mechanical testing with local flow curves explains the process. The study of local strength in S460M welded joints provides a specific research example.
What specimen is needed, and what results will you receive?
To plan the work, we need the material designation and condition, welding process, dimensions, and a sketch or photograph of the joint. Please also describe the issue or decision behind your enquiry and any specified test procedure. Agree the sampling position with us before cutting the specimen.
We agree a reporting scope that includes a hardness profile with measurement positions, hardness values and test conditions. This makes it possible to locate individual results within the cross-section and compare variants. Any local flow curves or additional properties are defined as a separate part of the agreed scope.
Effort depends on preparation, the number and location of measurements, repeats and the required evaluation. The quotation and schedule therefore define these items for the actual task. The report documents specimen identification, measurement positions, method, test force, results and the agreed technical assessment.
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Common questions about weld hardness testing
What is the maximum permitted hardness of a weld?
There is no single limit for all welded joints. The material, condition, application and agreed test or product specification determine the relevant requirements. This basis and the appropriate test force are established before evaluating the results.
Is microhardness suitable for every narrow heat-affected zone?
Suitability must be assessed for the actual specimen. Indentation size, required spacing and preparation determine whether a zone can be measured separately. Small spacing between measurement points alone does not establish sufficient spatial resolution.
Does a hardness profile also provide a stress-strain curve?
A hardness profile shows the spatial distribution of hardness. It does not contain a complete flow curve. For a suitable specimen, additional indentation testing with inverse analysis can provide local plastic flow curves and tensile-equivalent properties.
