If you need parts to withstand high temperatures, oxidation and corrosive chemicals, consider investment casting them in Inconel. Inconel is a nickel-based superalloy that resists oxidation and retains its strength at temperatures over 1,000° F. It’s produced in a number of different compositions with some being investment cast more than others.

This blog post takes a look at Inconel. It discusses the various types and compositions and explains the benefits of making Inconel parts by the investment casting process.

An Introduction to Inconel and Superalloys

Inconel is a trade name owned by Special Metals Corporation. If you specify a part should be made from Inconel you’re asking for one of their products. Other companies make similar superalloys, but they can’t put the Inconel name on them.

“Superalloy” is used to describe a metal that retains its strength as it gets close to its melting point. Non-superalloys like stainless steel become progressively weaker as they get hotter but a super alloy has its full strength when it’s at 90% of its melting temperature.

Inconel has a particular characteristic that makes it extremely useful for oxidizing environments: as it gets hot it forms a passivating oxide layer on the surface. This resists the oxidation that quickly shortens the life of parts made from other alloys.

Superalloys divide into three types, those that are nickel-based, those that are cobalt-based, and a smaller group that are iron-based. Inconel is around 60% nickel, 18% chromium and 19% iron, plus trace amounts of cobalt, molybdenum, aluminum and titanium. The exact proportions are what differentiates the various types of Inconel, and the equivalent superalloys.

Why Investment Cast Inconel?

Hardness, toughness and high temperature strength make nickel-based superalloys extremely difficult to machine. Investment casting avoids this problem by producing accurate parts that are near-net shape. While the processes used for making wax patterns and ceramic cores are complicated, investment castings need little final machining. This makes investment casting cost-effective for more complex parts required in higher volumes.

In the early days of superalloys, parts were forged rather than cast. However, the development of vacuum investment casting eliminated the porosity problems that plagued early attempts at casting. Today almost all investment casting of superalloys is performed under vacuum.

Correct Heat Treatment is Essential

Superalloys like Inconel get their strength from the way in which the alloy crystalizes. The post-casting heat treatment process is an integral part of this.

The problem with superalloys is that the many elements making up the alloy solidify at different temperatures. This can lead to them separating out. Heat treatment addresses this by re-homogenizing the alloy.

Heat treatment involves solution annealing by soaking at around 1,800° F, after which the alloy is water quenched. This may be followed by precipitation hardening which involves a series of soaks at progressively lower temperatures. Precipitation-hardened grades of Inconel tend to have the highest high temperature strength and corrosion resistance.

Types of Inconel for Investment Casting

More than 20 types of nickel-based superalloys are sold under the Inconel name. Those most often investment cast are 600, 625, 713 and 718. However, other types such as 617 and 690 can also be cast this way. (Note that 600 and 625 are not suitable for precipitation hardening.)

For projects needing a specific type of Inconel, consult a foundry specialist before committing to using the investment casting process.

For alternatives to Inconel, one indication is when alloys are described by the same number. For example, Inconel 718 is often described more generally as “Alloy 718”. In addition, Haynes 230, Rene 41 and MAR-M200 (from Martin Marietta,) are similar in composition.

Superalloy Selection Advice for Your Project

Nickel-based superalloys like Inconel offer exceptional high temperature strength. Most also resist oxidation and chemical attack, making them suitable for a host of chemical processing and oil industry “down hole” applications. However, this strength makes them very difficult to machine.

Some types of Inconel are suitable for forging. For complex geometries though, the investment casting process is a better alternative. Its high precision minimizes the amount of machining needed, which has the added benefit of keeping total manufacturing costs competitive.

Impro is experienced with investment casting Inconel and other superalloys. For advice on the most suitable type of metal for your application, contact us.