Nowadays several technologies rely on sintering to transform porous, fragile parts into sturdy, fully dense components: from powder pressing to metal injection moulding, moving to binder jetting additive manufacturing and metal FDM (Fused Deposition Modelling).

Stainless-steel components represent a large part of the market for sintered parts; they can be produced using any of the technologies mentioned above and have a wide variety of applications such as automotive, biomedical industries, mechanical and fashion.

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

In this article we are going to discuss how sintering parameters, and especially the sintering atmosphere, may affect the quality achievable from sintered stainless-steel parts.
We’ll analyse the three gas options, and we’ll see that in some circumstances there are interesting solutions that can fit your needs. Read on!

Introduction to Vacuum Sintering

The atmosphere plays an essential role in the successful outcome of the sintering process, for this reason the sintering atmosphere must be carefully selected in relation to the material and the final application.

Sintering under vacuum (which is in fact a reducing atmosphere) has several advantages:

• Bright parts after sintering.
• Lack of oxidation and/or atmosphere contamination.
• Precise setting and monitoring of the process parameters.

Some material must be sintered under vacuum with pressure ranging between 10^-2 millibars and 10^-4 millibars. Those are, for example, the best condition for sintering extremely reactive materials, such as titanium.

However, most of the sintered materials requires atmospheres enriched with inert gases, which is also the case for stainless steels.

The starting condition is always a cold furnace that has reached the proper vacuum level, which is then backfilled (partial pressure or over-pressure) with inert gas. This leads to the following benefits:

• The depletion of alloying elements can be avoided (i.e. chromium and manganese).
• The elimination of residual binder is eased by a continuous flow of gas, which is constantly pumped out of the furnace chamber.
• The oxygen content on the parts can be controlled by exploiting the reducing capacities of the process gases used and/or of the vacuum itself.
• The carbon content of the processed parts can be controlled.

Process gas for vacuum sintering of stainless steel

The gases used as a protective atmosphere inside a vacuum furnace are most commonly:

• Nitrogen: it is an inert gas and the less expensive among the process gas, for this reason it is widely used for sintering applications.
• Argon: it is more expensive than the nitrogen therefore it is chosen as an inert gas mostly when the processed material is nitrogen sensitive.
• Hydrogen: it is the most expensive among the listed gases and has safeties implications for both the furnace and the installation site of the furnace itself. However, hydrogen is an extremely reducing agent.

If we focus on the sintering of stainless steels, all the of the above-mentioned atmospheres are viable choices.

Let's briefly delve into the topic, highlighting the pros and cons of the 3 process gases.

Nitrogen

Nitrogen is soluble in the steel matrix and act as a solid solution strengthening in austenitic stainless steels.

Nitrogen can form nitrides at high temperature.
Focusing on stainless steel, chromium nitrides precipitation can affect the corrosion resistance of the part by forming sensitized regions that act as corrosion initiator.

For this reason, high cooling rates are often adopted after sintering in nitrogen to minimize the phenomena.

For some stainless steels, nitriding during the sintering process is a requirement to obtain the desired properties and microstructure. This is, for example, the case of the nickel free stainless steel X15CrMnMoN17-11-3 (Catamold ® PANACEA) that is usually sintered using a high partial pressure of nitrogen of around 700 mbar.

Argon

Usually, pure Argon is not an optimal for processing stainless steel. In fact, Argon is not soluble in the steel matrix, and may generate porosity due to the gas trapped inside the part.

Hydrogen

Hydrogen is widely used for the sintering of stainless steel due to its ability of reducing oxides, thus helping to obtain clean parts.

Hydrogen also plays an important role in the carbon control of the parts by removing the residual carbon left by the binder itself at the end of the binder burnout (since binders typically used in powder metallurgy are carbon based).
Hydrogen can be used in vacuum furnaces with both partial pressure (0,1 – 10 mbar) and with slightly over-pressure (backfilling with approx. 1.1 bar hydrogen).

However vacuum furnaces operating with hydrogen require additional safety measures.
For this reason, specific design solutions (such as double seals on all the furnace flanges) and software safeties are adopted.

Despite the increased degree of complexity of the equipment and the higher process costs, vacuum furnaces operating with hydrogen over-pressure bring several advantages:

• Oxygen cannot enter in the furnace under any circumstance since the furnace is pressurized.
• There are more reactive hydrogen molecules available in the furnace compared to partial pressure atmosphere, therefore maximizing the reductive power.

Hydrogen mixtures, a valuable alternative

Under some circumstances, the use of inert gas (nitrogen or argon) and hydrogen mixtures can be a good trade-off.
In fact, they retain some of the reducing capability specific of pure hydrogen atmosphere while lowering the operating and investment costs.

Moreover, mixtures of inert gases with low hydrogen percentage (>5.5 mol% hydrogen in nitrogen and >3 mol% hydrogen in argon) can be used without implementing the safety measures that are required with hydrogen over-pressure.

Argon-based mixtures are typically preferred to avoid chromium nitrides precipitation during cooling when high cooling rates cannot be achieved, while Nitrogen ones are used for all the other cases.

Conclusions

All the gas alternatives discussed in this article are viable choices for the sintering of the stainless-steel parts, however a thorough analysis is crucial to achieve the desired properties.

Choosing the right sintering atmosphere for your process may help you get the best trade-off between results and operating costs.

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