Determination of lowest carbon concentrations in steel ‐ a challenge to the steelworks laboratory

A working group of the chemists’ committee of Verein Deutscher Eisenhüttenleute, VDEh (The German Iron and Steel Institute), was engaged in the elaboration of suitable procedures for the reliable determination of carbon concentrations <50 μg/g in steel, which can be routinely applied in the steelworks laboratory. Besides the shape of the sample and the kind of its preparation, the influences of quantity and purity of the combustion additives, and of quality and pretreatment of the crucibles on the analytical result were studied. Furthermore, the execution of the combustion analysis with commercially available instruments was optimized. Round‐robin tests on nine steel samples with carbon concentrations between 1 and 40 μg/g showed that with the optimized procedure concentrations >5 μg/g C can be successfully determined; for accurate results at lower concentrations most careful execution and a most recent type of carbon analyzer are required. The calibration of the instruments with micro‐samples of carbonaceous compounds of exactly known stoichiometry (primary standards) was also subject of investigation, in order to meet the requirements of traceability to SI units for the certification of standard reference materials.