Overview
ISO 4968:2022 - "Steel - Macrographic examination by sulphur print (Baumann method)" specifies a contact-printing procedure to reveal sulphur-bearing inclusions and segregation in steels and cast irons. The method uses photographic paper (or flat film) impregnated with an acid reagent to convert metal sulphides into hydrogen sulphide, which darkens the silver-sensitive emulsion. ISO 4968:2022 applies to steels with sulphur content up to 0.40 wt % and provides updated guidance on reagents, surface preparation, printing procedure and result classification.
Key Topics
- Scope & limitations: Qualitative macrographic test; not a direct measure of sulphur concentration. Applicable to steels with S ≤ 0.40% and to cast irons.
- Principle: Local release of H2S from sulphides darkens silver halide emulsion forming a visible print of inclusion distribution.
- Test materials:
- Photographic paper or flat film (thin matt, gelatin-bromide enlarging papers recommended).
- Reagents: sulphuric, acetic or citric acid solutions with concentrations selected by steel sulphur content (see Table 1 in the standard).
- Fixing solution: commercial fixer or 15–20% sodium thiosulphate solution.
- Surface preparation & machining:
- Clean, minimally cold-worked, ground surface (recommended surface roughness Rz/Ra guidance).
- Avoid mirror finishes (paper slippage) and contamination (clean with alcohol/acetone).
- Procedure parameters:
- Soak photographic paper in reagent (≈5 min), apply damp paper to test surface, remove bubbles, maintain firm contact.
- Contact time typically 30 s to 10 min, depending on reagent and sulphur content.
- Wash, fix, rinse and dry the print; usually only one usable print per ground surface unless regrinding.
- Result reporting: Include steel grade, cast number, surface position, reagent types/concentrations, and test result.
- Classification of prints: Positive, negative, central, dispersed and linear segregation patterns (informative Annex A).
- Caveats: Print darkness can be affected by steel chemistry (e.g., Ti > 0.1% can mask sulphides), surface condition and paper sensitivity.
Applications
- Quality control in steel mills and foundries to map sulphide inclusions and chemical segregation.
- Incoming inspection for bars, billets and forgings to identify regions for further testing or sampling.
- Forensic metallography to reveal porosity, cracks or segregation-related defects.
- Process optimization (melting, casting, rolling) by monitoring inclusion distribution.
Users include metallurgists, QC engineers, inspection laboratories, foundries and steel fabricators seeking a low-cost, visual method to assess macro-inhomogeneities.
Related Standards (if applicable)
- ISO 4968:2022 contains no normative references; interpretation and acceptance criteria are normally defined in product standards or by special agreement between parties. For implementation, coordinate test parameters and acceptance criteria with applicable product or contractual standards.
