Tensile Strength and Hardness: Conversion Approaches and Practical Solutions

Tensile Strength and Hardness: Conversion Approaches and Practical Solutions

Tensile strength (Rm) is the maximum tensile stress a material withstands before fracture, given in N/mm². For grey cast iron (GJL), it ranges under DIN EN 1561 from 100 N/mm² (EN-GJL-100) to 350 N/mm² (EN-GJL-350); for ductile iron (GJS), it ranges under DIN EN 1563 from 350 N/mm² (EN-GJS-350-22) to 900 N/mm² (EN-GJS-900-2).^[3] The figure in the grade name is the minimum tensile strength for separately cast test samples. No reliable formula converts hardness to tensile strength for either material.

Basics of the Relationship Between Hardness and Tensile Strength

Between tensile strength (Rm) and hardness measurements there is an empirical relationship. Both properties depend on the microstructure of the material. For grey cast iron and ductile iron, this has not produced a reliable formula; the standard grade tables below take its place.

Hardness Ranges by Cast Material per Mill Standard

Grey Cast Iron / Lamellar Graphite Iron (GJL)

A conversion formula from hardness to tensile strength is not reliable for GJL. Hardness ranges of selected grades per mill standard, measured on the separately cast test sample Ø 30 mm; values change at other wall thicknesses:^[1]

Grade	Rm [N/mm²]	HB
EN-GJL-250	250	180 to 255
EN-GJL-350	350	200 to 275

Some grey cast iron variants, such as Meehanite cast iron, fall within this GJL family, for which no reliable hardness-to-tensile-strength formula exists either.

Ductile Iron / Nodular Graphite Iron (GJS)

The same applies to GJS: no formula, but hardness ranges of selected grades per mill standard, also measured on the separately cast test sample Ø 30 mm; values change at other wall thicknesses:^[2]

Grade	Rm [N/mm²]	HB
EN-GJS-400-15	400	130 to 190
EN-GJS-500-7	500	170 to 230
EN-GJS-600-3	600	190 to 270

Ferritic microstructures of this kind are often produced through a heat treatment process, such as the one used for annealed cast iron.

Practical Application of the Conversion

When Is a Conversion Useful?

Fast quality control: hardness measurements are less destructive and quicker to carry out than tensile tests
Series production: spot-checking of larger batches
Retrospective assessment: for components that are already installed

This is particularly relevant for wear-resistant grades such as Ni-hard cast iron, where hardness is a primary quality indicator and is checked frequently.

Limits of the Conversion to Consider

There is no conversion formula; only empirical values from the mill standard
For safety-critical components, always carry out direct tensile tests

Practical Solutions for Foundry Operations

Developing Plant-Specific Correlation Curves

Carry out parallel measurements for typical material/alloy combinations
Create your own conversion tables for your specific casting processes
Update this data regularly whenever processes change

Digital Solutions for Quality Assurance

Implement automated measurement procedures in your production line
Use software to evaluate your plant-specific correlation curves and for statistical analysis
Integrate the measurement data into your QM system for trend analysis

Conclusion

Hardness testing often replaces the tensile test for quality control in foundry work. For grey cast iron and ductile iron, the standard grade table provides the reliable values; no conversion formula exists. For precise results, it is advisable to develop plant-specific correlation curves based on your own measurement data.

Frequently Asked Questions

How is tensile strength estimated from Brinell hardness for ductile iron?

There is no reliable formula for ductile iron (GJS). Use the standard grade table above instead: EN-GJS-400-15 has HB 130 to 190, EN-GJS-500-7 has HB 170 to 230, and EN-GJS-600-3 has HB 190 to 270.

Why are hardness-to-tensile-strength conversions used in foundries?

Because hardness testing is faster and less destructive than tensile testing, it is well suited for fast quality control, spot-checking batches in series production, and assessing components that are already installed.

What is the typical accuracy of these conversion formulas?

For GJL and GJS, no formula reaches useful accuracy; use the standard grade table instead. Safety-critical components should always be verified with a direct tensile test.

Related Standards

Tensile testing of metallic materials, including cast materials, is generally carried out in accordance with DIN EN ISO 6892-1. Brinell hardness testing is standardized in DIN EN ISO 6506-1.

References

Kemptener Eisengiesserei Adam Hönig AG: KE mill standard per DIN EN 1561, Grey Cast Iron / Lamellar Graphite Iron (GJL), hardness ranges of standard grades EN-GJL-250 and EN-GJL-350.
Kemptener Eisengiesserei Adam Hönig AG: KE mill standard per DIN EN 1563, Ductile Iron / Nodular Graphite Iron (GJS), hardness ranges of standard grades EN-GJS-400-15 to EN-GJS-600-3, dated 21 Nov 2006.
Grade list per EN 1561:2012 Table 1 (grey cast iron, EN-GJL-100 to EN-GJL-350) plus guss.de, BDG trade portal, for EN 1563 (ductile iron, strength grades EN-GJS-350-22 to EN-GJS-900-2), retrieved 27 Aug 2026. Secondary source, confirms the scope of the grade designation, not the point values in the tables above.