Determination of Oxygen and Nitrogen in Ferroalloys

Importance of the Topic

Determining oxygen and nitrogen in ferroalloys is essential for quality control in steel and cast iron production. Excess oxygen promotes oxide formation and carbon loss during solidification, while elevated nitrogen reduces ductility, especially at high temperatures. Accurate measurement of these elements helps steelmakers optimize alloy composition, prevent defects, and ensure mechanical performance.

Objectives and Scope of the Study

This application note describes a method for simultaneous quantification of oxygen and nitrogen in ferroalloy samples using the ON736 impulse furnace system. The study aims to demonstrate method setup, calibration, analytical parameters, and performance characteristics for common ferroalloys under helium or argon carrier gas.

Methodology and Instrumentation

The procedure involves combusting a weighed sample in a graphite crucible within an impulse resistance furnace. Oxygen reacts with the graphite crucible to form CO, which is oxidized to CO₂ and measured by non-dispersive infrared detection. Nitrogen evolves as N₂ and is quantified by thermal conductivity detection. Key steps include:

• Sample preparation: homogeneous powders or granules analyzed as received.
• Fluxing: ~0.4 g nibbled nickel flux added to ~0.10 – 0.14 g sample in a nickel capsule to promote analyte release.
• Blank and calibration: perform at least three blank replicates and three calibration replicates using LECO Certified Reference Materials for oxygen (titanium pins) and nitrogen (steel pins).
• Carrier gas: helium or argon streams controlled via mass flow controller to sweep evolved gases through a heated reagent pack (CO to CO₂, H₂ to H₂O), NDIR cell, water trap, and finally the thermal conductivity cell.

Used Instrumentation

• ON736 Impulse Furnace Analyzer
• Graphite crucibles (782-720) and graphite powder (501-073)
• Nickel capsules (502-822) and nibbled nickel flux (501-598)
• Mass flow controller, heated reagent cartridges
• Non-dispersive infrared (NDIR) detector for CO₂
• Thermal conductivity (TC) detector for N₂

Main Results and Discussion

Typical precision and accuracy were evaluated for several ferroalloys under helium and argon carriers. Representative findings include:

• Ferrochromium (0.017 % N certified): oxygen averaged 0.128 % (s=0.009 %) under helium and 0.137 % (s=0.015 %) under argon; nitrogen averaged 0.016 % (s<0.001 %) with both gases.
• High-carbon ferrochromium (0.031 % N certified): oxygen precision was within 0.003–0.005 % (s), nitrogen precision <0.001 % (s).
• Ferromanganese (informational 0.041 % N): consistent oxygen and nitrogen recoveries across matrices, with relative standard deviations around 4 % for oxygen and 2–3 % for nitrogen.

Results demonstrate linear calibration through the origin, robust blank correction, and acceptable repeatability for production-level alloy analyses. Helium and argon carriers yield comparable precision, although power and timing parameters require adjustment.

Benefits and Practical Applications

• Simultaneous O/N analysis reduces instrument time and sample handling.
• Automated sample drop enhances throughput and repeatability.
• The method applies to diverse ferroalloy types, supporting QA/QC in steelmaking, alloy development, and raw material evaluation.

Future Trends and Applications

Advances may include integration of multi-element detection, refined furnace control algorithms, and alternative carrier gases to further improve sensitivity and matrix tolerance. Coupling with digital workflows and predictive maintenance can enhance laboratory efficiency and data traceability.

Conclusion

The ON736 impulse furnace method offers a reliable, high-throughput approach for oxygen and nitrogen determination in ferroalloys. With validated precision, linear calibrations, and adaptable parameters for helium or argon carriers, this technique supports stringent quality control requirements in metallurgical operations.