ISO 15351:1999

Overview

ISO 15351:1999 describes a routine thermal conductimetric method for the determination of nitrogen in steel and iron. The method measures nitrogen released as N2 after high‑temperature fusion of a test portion in an inert gas stream (helium). It is applicable to nitrogen levels from 0.002 % (m/m) to 0.6 % (m/m) and is intended for laboratories performing routine composition analysis and quality control on ferrous materials.

Key topics and requirements

• Principle: Fusion of a weighed test portion in a single‑use graphite crucible under high‑purity helium; nitrogen is carried as N2 and measured by a thermal conductivity detector after separation from other gases.
• Scope: Applicable for steels and irons with nitrogen between 0.002–0.6 % (m/m).
• Calibration: Use certified reference materials (CRMs) with matrix match; calibration sets recommended for low and higher ranges:
  • Low range: ~0.002, 0.01, 0.03, 0.05, 0.10 % N
  • Higher range: ~0.10, 0.30, 0.50 % N
• Reagents and gas purity: High‑purity helium (total impurities ≤ 0.0005 % m/m); oxidation/catalyst (CuO or Pt) and purification media such as magnesium perchlorate (or anhydrous CaSO4) and NaOH (ascarite) for gas cleanup.
• Apparatus: Resistive electrode (graphite crucible) furnace, helium flow control and purifying unit, glass‑wool filters; follow manufacturer instructions for instrument operation.
• Quality checks:
  • Blank tests (duplicate) after degassing crucible; mean blank and difference between blanks must not exceed 10 μg N.
  • Stabilize instrument and perform trial analyses after reagent/filter changes.
• Precision: Inter‑laboratory trials (23 labs) provide repeatability and reproducibility data; limits vary with nitrogen content (document contains numerical precision results).
• Safety and sampling: Use crucible tongs and appropriate protective measures when fusing; sample handling and preparation per ISO 14284.

Applications and users

ISO 15351 is used by:

• Metallurgical and chemical analysis laboratories performing nitrogen determination in steel and iron
• Steel producers, foundries and heat‑treatment facilities for quality control
• Certification and materials testing labs establishing composition for specification compliance
• R&D teams developing alloys where nitrogen affects mechanical properties and corrosion resistance

Practical uses include process control, acceptance testing, alloy verification, and traceability of nitrogen values via CRM‑based calibration.

Related standards

• ISO 10720 - thermal conductimetric methods for nitrogen (related techniques)
• ISO 10702 - titrimetric method after distillation for nitrogen
• ISO 14284 - sampling and preparation for chemical composition analysis
• ISO Guides 31 & 35 - guidance on reference and certified reference materials

Keywords: ISO 15351:1999, nitrogen determination, steel and iron, thermal conductimetric method, fusion under inert gas, helium, graphite crucible, CRM, quality control.