Sintering/Additive Manufacturing

Fig. 1. Detail of the metallic chamber with a circular section

Giorgio Valsecchi – TAV VACUUM FURNACES SPA; Caravaggio, ITALY

Several technologies currently rely on sintering to transform porous, fragile parts into sturdy, fully dense components – from press and sintering to metal injection molding and binder-jetting additive manufacturing to metal FDM.

Stainless steel components represent a large part of the market for sintered parts. They can be produced using any of the aforementioned technologies and have a wide variety of applications, such as automotive, biomedical industries, chemical processing and fashion.

Some of the most widespread stainless steels used for sintering are 304L, 316L, 440, 410 and 17-4 PH. They are chosen for their mechanical properties coupled with their exceptional corrosion resistance.

This article discusses how sintering parameters, especially the atmosphere, can be optimized on vacuum furnaces to obtain the best-possible quality from sintered stainless steel parts.

Some material can be sintered directly under vacuum with pressure ranging between 10^-2 millibar and 10^-4 millibar. Those are the best conditions for sintering reactive materials (such as titanium).

For most materials, however, a small pressure of gas is used during the sintering cycle. This is also the case for stainless steels.

Filling the furnace chamber with a partial pressure of gas has multiple functions:

• Avoiding the depletion of alloying elements (e.g., chromium and manganese)

• Facilitating the elimination of evaporated binder residual through a continuous flow of gas that is pumped out of the furnace chamber

• Controlling the oxygen content on the workpiece through oxide reduction

• Controlling the carbon content on the workpiece

Introduction to Vacuum Sintering

The atmosphere plays an essential role in the successful outcome of the sintering process. For that reason, the sintering atmosphere must be carefully selected in relation to the material. Sintering under vacuum (which is, in fact, a lack of atmosphere) has several advantages:

• Bright parts

• Lack of oxidation or atmosphere contamination

• Close monitoring of the process parameters

Process Gas for Vacuum Sintering of Stainless Steel

The most common gases used as a protective atmosphere inside vacuum furnaces are nitrogen, argon and hydrogen.

• Nitrogen is an inert gas and the less-expensive process gas. For that reason, it is widely used for sintering.

• Argon is costlier than nitrogen. Therefore, it is chosen as an inert gas when the workpiece material is nitrogen-sensitive.

• Hydrogen has a high price, and it can form an explosive mixture with oxygen. Hydrogen is a reducing agent.

Concerning the sintering of stainless steels, all of the aforementioned atmospheres are viable choices.

Nitrogen is soluble in the steel matrix and acts as solid-solution strengthening in austenitic stainless steels. For some stainless steels, nitriding during the sintering process is a requirement to obtain the desired properties and microstructure. This is the case for nickel-free stainless steel X15CrMnMoN17-11-3 (Catamold^® PANACEA), which is usually sintered using high partial pressure of nitrogen around 700 mbar.

Nitrogen, however, can form nitrides at high temperature. In stainless steel, in particular, chromium-nitride precipitation can compromise the corrosion resistance of the part by forming sensitized regions that act as a corrosion initiator. For that reason, high cooling rates are often adopted after sintering in nitrogen to minimize the phenomena.

Pure argon is usually not an optimal solution for stainless steel. Argon, in fact, is not soluble in the steel matrix, causing the formation of porosity due to the gas trapped inside the part.

Hydrogen is often chosen for stainless steel sintering for its ability to reduce oxides, which helps to obtain clean parts. Hydrogen also plays a part in the carbon control of the parts by removing residual carbon left from the binder at the end of the binder burnout (since binders typically used in powder metallurgy are carbon-based).

Vacuum furnaces operating with hydrogen, however, require additional safety measures since it can form an explosive mixture in the presence of oxygen when enclosed in a confined space. For that reason, specific precautions (such as double gaskets) are adopted.

Vacuum furnaces can be backfilled with hydrogen with both low gas partial pressure or at pressure above atmospheric (e.g., backfilling at 1.1 bar hydrogen).

Despite the higher complexity of the equipment and gas consumption, vacuum furnaces operating with hydrogen gas over-pressure have some advantages:

• Oxygen cannot enter the furnace under any circumstance since it is pressurized.

• Compared to a low partial pressure of gas, there are more-reactive hydrogen molecules available in the furnace atmosphere.

• When operating in over-pressure, there is the possibility of burning debinding products instead of using a cooled condenser.

Using a hydrogen-enriched mixture of inert gases (nitrogen or argon) can be a good trade-off since they retain some of the reducing capability of a pure-hydrogen atmosphere while lowering the cost.

Moreover, mixtures of inert gases with low hydrogen percentage (>5.5 molecular % hydrogen in nitrogen and >3 molecular % hydrogen in argon) can be used without the concern for additional safety measures related to pure hydrogen. In that respect, argon-based mixtures are usually preferred when high cooling rates cannot be achieved to avoid chromium nitride precipitation during cooling.

Conclusion

Even if all of the possibilities discussed here are practical choices for stainless steel sintering, careful selection is crucial to maximize the desired part properties without adding unnecessary costs. In the end, choosing the right sintering atmosphere for your vacuum furnace will help you get the best possible results with lower expenses.

For more information:  Contact Giorgio Valsecchi, R&D engineer, TAV VACUUM FURNACES SPA TAV VACUUM FURNACES SPA, located on Via dell’industria 11- 24043 Caravaggio (BG) – ITALY; e-mail: info@tav-vacuumfurnaces.com; web: www.tav-vacuumfurnaces.com.  

All images courtesy TAV VACUUM FURNACES.

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