Metal Injection Molding (MIM) technology makes it possible to manufacture stainless steel parts with high precision and the advantage of serial production. However, when sintering is carried out—especially in graphite furnaces—the choice of atmosphere plays a critical role in the density, mechanical properties and corrosion resistance of the resulting parts.
In this article, I will discuss the advantages and risks of the atmospheres used in the sintering process of 316L feedstock (H₂, N₂, Ar, vacuum and forming gas), and in particular why hydrogen is preferred.
316L Stainless Steel and the MIM Process
This feedstock, optimized for 316L stainless steel, enables parts to be obtained that preserve their high-density and stainless properties after the injection → debinding → sintering steps.
There are three critical stages in the process:
- Green part: powder + binder mixture
- Brown part: after debinding, porous structure
- Sintered part: diffusion bonding of metal particles, increase in density
Atmosphere Selection: Hydrogen or Nitrogen?
Hydrogen (H₂):
- It is a strong reducing gas.
- It dissolves oxide layers and cleans surfaces.
- It provides high densification and mechanical strength.
- With moisture control (dew point < -40 °C), carbon diffusion is minimized.
Nitrogen (N₂):
- It has no reducing capability.
- There is a risk of nitridation (CrN formation) at high temperature.
- This can weaken the chromium oxide layer, which is the fundamental mechanism of stainless steel's corrosion resistance.
Argon (Ar) and Vacuum:
- It provides inert protection and limits the effects of oxygen.
- However, the cost is high and oxide removal is not as effective as with hydrogen.
Forming Gas (5% H₂ / 95% N₂)
- It partially benefits from the advantages of hydrogen.
- It has a more limited effect compared with pure H₂, but it is preferred in some facilities for reasons of safety and cost.
Special Risks in Graphite Furnaces
Graphite furnaces bring with them the risk of carburization due to carbon diffusion. This can lead to an increase in hardness at the part surface but a loss of ductility.
Solution: cleaning the furnace, controlling the hydrogen dew point, and using Al₂O₃ support surfaces.
Atmosphere: 100% dry and clean H₂, or Ar as an inert alternative.
Quality Control and Testing
- Density measurement (Archimedes)
- Microstructure examinations (SEM/optical)
- O, C, N analyses (LECO)
- Mechanical tests (tensile, hardness, fatigue)
- Corrosion tests (pitting, salt spray)
Validation with test bar specimens is recommended for every new batch or process change.
Conclusion and Recommendations
For 316L to be sintered as reliably as possible in graphite furnaces:
- Pure, dry hydrogen is the most suitable atmosphere.
- The use of nitrogen is risky due to nitridation and oxide removal issues.
- As alternatives, vacuum, argon or forming gas can be considered; however, optimization is essential.
If hydrogen use is possible at your facility, you can achieve the best results in terms of density, mechanical performance and corrosion resistance.
References and Sources
Forward-AM / Ultrafuse 316L User Guidelines (sinter in pure H2 or Ar; typical cycle). Source URL: https://forward-am.com/wp-content/uploads/2021/04/UserGuidelines_2021_03_29.pdf
Forward-AM / Ultrafuse process instructions (example cycle: RT-600-1380°C, dewpoint < -40°C). Source URL: https://igo3d.com/mediafiles/Sonstiges/BASF/Ultrafuse/ForwardAM_BASF_Process_Instructions_Ultrafuse316L.pdf
BASF Catamold® brochure and product pages. Source URL: https://www.basf.com/dam/jcr:0b6b5087-e328-30b3-9693-ff090696ca79/basf/www/jp/documents/ja/products-and-industories/chemicals/Catamold_Brochure_E.pdf
TAV Vacuum Furnaces — stainless steels vacuum sintering (hydrogen reduces oxides). Source URL: https://www.tav-vacuumfurnaces.com/blog/62/en/stainless-steels-vacuum-sintering
MDPI Processes — simultaneous sintering and surface nitriding of 316L (nitrogen effects). Source URL: https://www.mdpi.com/2227-9717/8/3/297
Papers and PIM International articles on MIM processing and atmospheres. Source URL: https://www.pim-international.com/wp-content/uploads/sites/2/2016/04/PIM-International-June-2012-SP.pdf