Dew point is the single most critical parameter in MIM sintering atmosphere quality — and one that is often overlooked until a batch of parts comes out discolored or below density specification.
Why dew point matters:In a hydrogen sintering atmosphere, the purpose of hydrogen is to reduce surface oxides on metal powder particles, enabling particle-to-particle diffusion. Water vapor (H₂O) is the byproduct of this reduction reaction. If the atmosphere contains too much moisture (high dew point), the reduction reaction stalls:
FeO + H₂ → Fe + H₂O (reduction) H₂O + Fe → FeO + H₂ (reversal if too much moisture)
Dew point targets by material:| Atmosphere | Critical Dew Point | Consequence Above Threshold |
|---|---|---|
| 100% H₂ (stainless steel) | < -40°C (< 120 ppm H₂O) | Surface oxidation, reduced density, discoloration |
| 75% H₂ / 25% N₂ (low-alloy steel) | < -35°C (< 200 ppm) | Incomplete oxide reduction, lower mechanical properties |
| Argon (titanium) | < -50°C (< 40 ppm) | Oxygen pickup from moisture — embrittlement |
| Vacuum | N/A (pressure-based) | Partial pressure of water vapor < 10⁻⁵ mbar |
- Continuous measurement: Chilled mirror hygrometer is the gold standard (±0.2°C accuracy). Capacitive sensors are less expensive but drift and require frequent calibration
- Monitoring frequency: Continuous for production furnaces. Alarm at -35°C for H₂ atmospheres. Automatic shutdown at -30°C
- Causes of high dew point: Leaking water-cooled furnace sections, insufficient gas purging after furnace maintenance, saturated desiccant dryers, or a leaking heat exchanger
For hydrogen-atmosphere MIM sintering of stainless steels, the dew point must remain below -40°C. Every 10°C increase in dew point (e.g., from -40°C to -30°C) increases the water vapor content by approximately 3x, significantly reducing the reducing power of the atmosphere. If the dew point reaches -20°C, the atmosphere is effectively oxidizing — parts will come out discolored and below density.
If you see pink, blue, or grey discoloration on sintered 316L parts (rather than the normal matte silver/grey), check the dew point first — it is the most likely cause.