Overview
ISO 4935:1989 - "Steel and iron - Determination of sulfur content - Infrared absorption method after combustion in an induction furnace" specifies a standardized laboratory method to measure sulfur in steel and iron. The method applies to sulfur concentrations from 0.002 % to 0.10 % (m/m). It defines the principle, required reagents and apparatus, sampling and test-portion preparation, measurement and calibration procedures, expression of results and reporting. Informative annexes cover interlaboratory precision data (Annex A/B) and features of induction furnaces and IR sulfur analysers (Annex C).
Key topics and requirements
- Analytical principle: Combustion of a weighed test portion with a sulfur-free accelerator in a high-frequency induction furnace in a pure oxygen stream; conversion of sulfur to SO2 and measurement by infrared (IR) absorption.
- Scope: Applicable range 0.002–0.10 % S (m/m).
- Reagents & materials: High-purity oxygen (≥99.5 %), sulfur-free accelerator (e.g., tungsten), magnesium perchlorate dehydration agent, inert ceramic trap, pure low-sulfur iron for blanks, suitable solvents (acetone), and certified sulfur standard solutions.
- Apparatus: High-frequency induction furnace with oxygen supply and purifying unit, IR gas analyser, flowmeter, tin capsules, ceramic crucibles, micropipettes (50 µl/100 µl) and class A volumetric glassware.
- Sampling & test portions: Sampling per ISO 377; typical test portion masses indicated (dependent on expected sulfur level and instrument).
- Calibration & blanking: Multi-point calibration using certified sulfur standard solutions deposited in tin capsules and dried, construction of calibration graph, and routine blank tests (duplicate blanks). The standard sets contamination limits (mean blank ≤ 0.005 mg S; difference between blanks ≤ 0.003 mg S).
- Precision: Results of a 22-laboratory trial are summarized; precision (repeatability and reproducibility) varies with sulfur level and is presented graphically in Annex B.
- Reporting: Requires method reference, sample identification, numeric results (expressed as % m/m), unusual features and any non-standard operations.
Applications and users
- Practical applications:
- Quality control in steel and iron production (castings, bars, sheet).
- Incoming material inspection and supplier certification.
- Failure analysis, R&D and process optimization where sulfur influences mechanical properties and corrosion behavior.
- Regulatory compliance and contract testing where trace sulfur levels are critical.
- Typical users:
- Metallurgical and chemical laboratories, QA/QC technicians, process metallurgists, instrument manufacturers (induction furnaces and IR analysers), and standards bodies.
Related standards
- ISO 377 (sampling and sample preparation for steel)
- ISO 5725 (precision of test methods - repeatability and reproducibility)
- Laboratory glassware standards referenced for volumetrics (ISO 385‑1, ISO 648, ISO 1042)
Keywords: ISO 4935:1989, sulfur determination, infrared absorption, induction furnace, steel and iron analysis, sulfur content, IR sulfur analyser, calibration, laboratory method, precision data.
