Overview
ISO 7778:2014 - "Through-thickness characteristics for steel products" - defines criteria and test procedures for measuring through-thickness ductility of steel by reduction of area (Z) in the thickness direction (perpendicular to the surface). It applies to fully killed steels supplied as plates, wide flats, or sections with thicknesses from 15 mm to 400 mm and with specified minimum yield or proof strength ≤ 960 MPa. The standard supplements product delivery standards by providing acceptance criteria, sampling and test-piece requirements, tensile-test procedures and assessment rules.
Key topics and requirements
- Through-thickness classes: Z15, Z25, Z35 with minimum average and minimum individual reduction-of-area values:
- Z15 - average ≥ 15%, individual ≥ 10%
- Z25 - average ≥ 25%, individual ≥ 15%
- Z35 - average ≥ 35%, individual ≥ 25%
- Scope limits: applies to fully killed steel, thickness 15–400 mm; application outside these limits is by agreement when ordering.
- Sampling and test units:
- Three tensile test pieces are taken from the thickest product of the test unit (centre or 1/4 width for continuous cast material).
- Test-unit definitions vary with sulfur content and manufacturer options (rolled unit, heat unit, mass-limited unit).
- Ultrasonic testing (ISO 17577) may be mandatory for test units in certain cases.
- Test-piece geometry:
- Cylindrical pieces; diameter depends on product thickness (e.g., d = 6 mm or 10 mm for thinner products, 10 mm for thicker).
- Effective gauge length L ≥ 1.5 × diameter; welding of extension pieces permitted if heat-affected zones are excluded from the effective length.
- Tensile testing and assessment:
- Tensile tests performed per ISO 6892-1.
- Assessment uses sets of three tests: average must meet class minimum; one individual may fall below average if above the individual minimum.
- Retest and rejection rules are specified when initial results fail.
Applications and users
ISO 7778 is used by:
- Steel manufacturers and mills for specifying and certifying through-thickness ductility of plates and sections.
- Quality, inspection, and testing laboratories performing tensile and ultrasonic tests.
- Structural engineers, fabricators and welders who need assurance of through-thickness properties where components are loaded perpendicular to plate surfaces (e.g., heavy structural members and welded assemblies).
- Procurement and compliance teams specifying material acceptance criteria in contracts and technical data sheets.
Related standards
- ISO 377 - sampling and preparation of test pieces
- ISO 404 - general technical delivery and sequential assessment methods
- ISO 6892-1 - tensile testing at room temperature
- ISO 17577 - ultrasonic testing for flat steel products
- Bibliography references noted: ASTM, EN, JIS ultrasonic testing standards
Keywords: ISO 7778, through-thickness, reduction of area, steel products, tensile test, fully killed steel, plates, sections, ultrasonic testing.
