Carbon content in MIM parts can shift during processing — and even small changes can significantly affect mechanical properties, especially for high-carbon materials like 420 stainless steel.
How carbon changes during MIM processing:| Process Step | Carbon Change | Mechanism | Most Affected Materials |
|---|---|---|---|
| Debinding (incomplete) | +0.05-0.20% | Residual binder carbonizes during sintering, adding carbon | All materials — especially problematic when debinding is incomplete |
| Sintering in H₂ with moisture | -0.05-0.30% | Water vapor reacts with carbon: C + H₂O → CO + H₂ | High-carbon materials (420 SS, tool steels) |
| Sintering in vacuum with carbon furnace elements | +0.02-0.10% | Carbon pickup from graphite heating elements | Materials with tight carbon tolerances (316L) |
| Sintering near oil or hydrocarbon contamination | +0.10-0.50% | Pyrolysis of hydrocarbons deposits carbon on part surface | All materials — visible as black surface discoloration |
| Material | Carbon Spec | Effect of Too Much Carbon | Effect of Too Little Carbon |
|---|---|---|---|
| 316L | < 0.03% | Sensitization — reduced corrosion resistance | — (already near zero) |
| 17-4PH | < 0.07% | Reduced ductility, carbide precipitation | Reduced age-hardening response |
| 420 SS | 0.20-0.40% | Excessive hardness, brittleness | Loss of hardenability — cannot reach 48-55 HRC |
- Verify debinding weight loss (90-95% target) to prevent residual binder carbonization
- Control sintering atmosphere dew point below -40°C to minimize decarburization
- Use molybdenum or ceramic hot zones (not graphite) for carbon-sensitive materials
- Test carbon content per batch using LECO combustion analysis
MIM 316L must have carbon below 0.03%. If carbon exceeds this limit during processing (typically from incomplete debinding), chromium carbides form at grain boundaries during sintering, depleting the surrounding matrix of chromium and causing intergranular corrosion. This is why debinding completeness is verified by weight loss before sintering — and why 316L parts with black streaks should be investigated for carbon contamination.