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Inconel vs Stainless Steel

When manufacturing a new part, one important question is what metal should you use? Although the answer might be easy, such as choosing a plain carbon steel because it’s cheap, other applications may not have such clear cut answers. When deciding on a material to use and a method to produce that part, a few questions need to be answered:

(1) What are the conditions this part is going to be in?

(2) What kind of strength do I need?

(3) What price point do I need this part to be at?

If a part needs to have superior strength and corrosion resistance at high temperatures, Inconel alloys are the best for those applications. On the other hand, a part for handling medical supplies may be better suited for an alloy of stainless steel. The rest of this post documents what kind of other applications you can use for these two metals!

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Inconels are a class of nickel-chrome-based super alloys characterized by high corrosion resistance, oxidation resistance, strength at high temperatures, and creep resistance. Inconel is able to withstand elevated temperatures and extremely corrosive environments due to two factors. One reason is because of the formation of the intermetallic compound Ni3Nb in the gamma double prime (ɣ’’) phase. This intermetallic phase acts as a ‘glue’ on the grain boundaries. This ‘glue’ prevents the grains from increasing in size when heated to high temperatures (for the most part, smaller grains means a high strength and larger grains lends itself to a higher ductility). Second, Inconel has a high corrosion resistance due to the passivation layer that forms when heated. Both of these factors make Inconel ideal alloys for applications from hot section gas turbines to deep sea well drilling. So what is the downside of these alloys? They are expensive, hard to machine, and hard to work.

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Stainless Steel

Stainless steels are characterized by a chromium content higher than 10.5% by weight. Stainless steels have a higher corrosion resistance than traditional steels. There are five classes of stainless steels: Austenitic, Ferritic, Martensitic, Duplex, and Precipitation Hardenable stainless steels. These stainless steels are named after the microstructure that forms in those alloys. Duplex stainless steel is a mix of austenitic and ferritic microstructures, and precipitation hardening means that precipitates can form to increase the strength of those class of stainless steels. The strength and the corrosion resistance of the stainless steel is dependant on these microstructures. For example, austenitic stainless steels are going to be the most ductile class of stainless steel, but also the most corrosion resistant. On the other hand, the martensitic class has typically much higher strength, but less corrosion resistance. Austenitic stainless steels have a larger more equiaxed grain structure which is what gives it these properties compared to the smaller lathe structure found in martensitic stainless steels. Stainless steel is a diverse group of alloys that can be found in everything from kitchen cutlery to structural building applications to biomedical and food preparation applications.

What’s the difference?

For one thing, Inconel is much more expensive than stainless steel alloys. Inconel also has much better corrosion resistance and strength at high temperatures. However, at lower temperatures, steels like 17-4PH stainless will have a higher strength than Inconel. When deciding between the two different alloys, the main deciding factor is cost, temperature, and strength. If a lower cost, but relatively strong part is required, stainless steel is probably the way to go. If an air-critical component is required with high temperature needs, then Inconel is a much better alloy for that application.

Why 3D print it?

Since Inconel is very expensive and hard to machine, it can be much easier to 3D print it. 3D printing decreases the amount of waste material, labor costs, tooling costs, and decreasing lead time, which in turn can decrease the cost of the part. Stainless steel, on the other hand, depends a lot on the application, current cost of the part and machinability. 3D printing both Inconel and stainless steel with the Markforged Metal X is a great solution for many applications including those in the oil and gas industry in our Inconel 625 material or replacing legacy parts with our 17-4 PH Stainless Steel. 3D printing these parts can reduce cost, material waste, and lead time which is important in high-cost alloys like Inconel 625 or lead time reduction with impossible legacy parts.

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