Overview - ISO 3908:2009 (Hardmetals, Insoluble / Free Carbon, Gravimetric)
ISO 3908:2009 specifies a gravimetric laboratory method to determine the mass fraction of insoluble (free) carbon in carbides and hardmetals. The procedure applies to carbides of hafnium, molybdenum, niobium, tantalum, titanium, vanadium, tungsten and zirconium, their mixtures with binder metals, and all grades of pre‑sintered or sintered hardmetals. The method covers samples with free carbon in the range 0.02 % to 0.5 % (mass fraction).
Key topics and technical requirements
- Principle: chemical decomposition of carbide followed by collection and gravimetric determination of insoluble carbon (combustion to CO2).
- Scope limits: applicable to specified carbides and hardmetals, free of lubricant; measurement range 0.02–0.5 %.
- Sample preparation: crush to powder passing a 180 µm sieve; perform analysis on two or three test portions (approx. 2.5 g each).
- Reagents: analytical‑grade nitric acid and hydrofluoric acid (specified concentrations/preparations).
- Apparatus: standard lab glassware plus platinum dish (200 ml), ceramic or refractory filter device (Gooch crucible), vacuum filtration, and an oxygen combustion furnace (~1 200 °C tube temperature).
- Procedure highlights: attack with 75 ml HNO3 and ~10 ml HF on a steam bath until dissolution; filter and wash residue thoroughly; dry filter at 110 °C; burn residue in oxygen and measure CO2 mass.
- Calculation: mass fraction of carbon (%) = (m1 − m0) × 27.29 / m
where m = test portion mass; m1 = CO2 mass from sample combustion; m0 = CO2 mass from blank; 27.29 = CO2→C conversion × 100.
- Quality control / tolerances: allowed deviations between replicate determinations:
- 0.02–0.10 %: ±0.02 % (two determinations) or ±0.03 % (three)
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0.10–0.50 %: ±0.04 % (two) or ±0.05 % (three)
- Report arithmetic mean rounded to 0.01 %.
- Safety note: HF and concentrated HNO3 are highly hazardous - use a fume cupboard and appropriate PPE.
Applications and users
ISO 3908 is used by:
- Hardmetal and cemented carbide manufacturers for quality control of free carbon content.
- Powder metallurgy and materials testing laboratories performing compositional analysis.
- R&D labs optimizing sintering parameters and binder/carbide ratios.
- Failure‑analysis and process control teams monitoring carbon‑related defects.
Accurate free carbon measurement supports control of microstructure and performance (e.g., binder phase stability and wear behavior) in hardmetals.
Related standards
- ISO 3907:2009 - referenced normative method for determination of total carbon (gravimetric), which complements ISO 3908 when full carbon balance is needed.
Keywords: ISO 3908, hardmetals, free carbon, insoluble carbon, gravimetric method, carbides, cemented carbide, quality control, powder metallurgy, HF safety.
