Overview
ISO 4491-3:2025 - "Metallic powders - Determination of oxygen content by reduction methods - Part 3: Hydrogen-reducible oxygen" - specifies a laboratory method to determine the hydrogen-reducible oxygen content in metallic powders containing 0.05 % to 3 % oxygen (mass percentage). The method applies to unalloyed, partially or fully alloyed metal powders and to carbide + binder mixtures. It excludes powders that contain lubricants or organic binders, and is intended to be used together with ISO 760 (Karl Fischer method) and ISO 4491-1 (general guidelines).
Key topics and technical requirements
- Principle: Dry a weighed test portion, reduce metal oxides in a stream of dry hydrogen, absorb the water produced in anhydrous methanol and quantify by Karl Fischer titration (visual or electrometric end‑point).
- Applicability: Unalloyed, alloyed and hardmetal/binder mixtures; can be extended to carbon-containing powders using a nickel catalytic converter that converts CO/CO2 to methane + water at 380 °C.
- Test portion masses (guidance):
- 0.05–0.5 % O → 5 g sample
- 0.5–2.0 % O → 2 g sample
- 2.0–3.0 % O → 1 g sample
- Reduction temperatures: Specified per metal (examples from the standard):
- Copper: 850 ±15 °C
- Iron and steel: 1 100 ±20 °C
- Nickel, cobalt, tungsten: 1 000 ±20 °C
- Tin: 425 ±10 °C
- Hardmetal mixtures: 1 000 ±20 °C
- Apparatus: Hydrogen and N2/Ar supply with purifiers, gas selection and drying units, gas‑tight quartz/refractory reduction tube with furnaces, boats, titration flask with electrodes, burette, and optional nickel catalytic converter. Typical gas flows and conditioning steps are defined in the procedure.
- Reagents & safety: Use analytical‑grade reagents. Karl Fischer reagent contains iodine, SO2, pyridine and methanol - handle with care and follow safety instructions. Do not stop hydrogen flow while the hot tube remains hot.
Practical applications and users
- Quality control and R&D laboratories in powder metallurgy, hardmetal and additive manufacturing powder supply chains.
- Producers of metal powders (unalloyed or alloyed) and carbide + binder systems who need reliable oxygen measurement for process control and specification compliance.
- Test houses and material certification labs that require standardized, repeatable oxygen determinations for acceptance testing, troubleshooting or material development.
Related standards
- ISO 760 - Determination of water - Karl Fischer method (general method)
- ISO 4491-1:2023 - Metallic powders - Determination of oxygen content by reduction methods - Part 1: General guidelines
- Other parts of the ISO 4491 series (see ISO website for the full list)
Keywords: ISO 4491-3:2025, metallic powders, oxygen determination, hydrogen-reducible oxygen, Karl Fischer titration, powder metallurgy, catalytic converter, reduction method, laboratory standard.