How does hardness-to-tensile conversion work according to DIN EN ISO 18265?
The conversion builds on the positive correlation between hardness and tensile strength described in DIN EN ISO 18265. Hardness expresses the resistance against the penetration of a harder indenter. Tensile strength is the maximum axial stress that a specimen can sustain before fracture. Under defined conditions both properties correlate, enabling fast and quasi non-destructive estimates of tensile strength from hardness measurements, for example in field checks or part-based evaluations.
Materials covered
The standard provides reliable correlations mainly for the following steels:
- Unalloyed steels
- Low alloy steels
- Cold-work steels
- Tool steel 1.1243
- Steel castings
The conversion is especially helpful when tensile specimens cannot be machined from a part, e.g. field inspections, very small components or when profiling strength along a component. Keep in mind that the correlation exhibits scatter. Round-robin trials by VDEh reported ±25 HV10 or about ±85 MPa (based on ~700 data points) when converting hardness to tensile strength.
Selecting the correct table for alloy and heat treatment is critical because the correlations are empirical and material-specific. Mixing up tables or heat-treatment regions results in higher deviations of the converted tensile strength.
Note: for a concrete project we can help select the relevant table or curve from DIN EN ISO 18265 and quantify the scatter for your material state.
