Overview

Whenever you talk about nickel alloys or superalloys, you may have heard of an alloy called Inconel. In fact, it is one of the most commonly used superalloys. If this alloy is to be used in your project, then you definitely need to have some understanding of its properties.

In this article, we will introduce what is Inconel alloy, and let you have an overall understanding of Inconel alloy from three aspects: chemical composition, mechanical properties and physical properties.

What is Inconel?

Inconel is a general term for a series of alloys developed by Special Metals. It has excellent high temperature resistance and corrosion resistance. According to different chemical composition ratios, Inconel alloys are divided into many different grades. The earliest grade Inconel X-750 can be traced back to 1943.

Are you wondering: why is Inconel divided into so many grades? Can't one grade be used instead of another? Why are these grades classified as Inconel and not others?

These questions can be summarized with one answer: Inconel alloys include a series of alloys with similar properties, but these alloys have their own advantages and disadvantages in different properties according to different needs.

Next, let's talk about the common points of Inconel alloys.

Almost all Inconel alloys are nickel-chromium alloys. In other words, Inconel alloy is an alloy formed by adding a large amount of chromium to nickel. Below are some typical Inconel alloys. You can see that they all have nickel and chromium in common. (Hold the mouse over the corresponding elements, you can see their specific composition percentages)

Nickel is the base composition of Inconel alloys. Its ability to maintain a stable austenite structure at any temperature is the guarantee of high temperature resistance of all Inconel alloys.

Chromium is also important to Inconel. It can form a protective film on the surface of the alloy. This protective film protects the alloy from corrosive substances.

In summary, nickel and chromium are the sources of high temperature and corrosion resistance for Inconel alloys.

Below, we will introduce the differences of different Inconel alloy grades from three aspects.

Chemical Composition of Inconel

Nickel and Chromium

As mentioned above, nickel and chromium determine the high temperature resistance and corrosion resistance of Inconel alloy respectively. Well, the first difference between the different Inconel alloys is the ratio of nickel to chromium. This ratio determines whether the alloy pays more attention to high temperature resistance or corrosion resistance.

For example, Inconel 600 has a nickel content of 72%, which is significantly greater than Inconel 690's 58%. Therefore, the high temperature performance of Inconel 600 is better than that of Inconel 690. In contrast, Inconel 690's 28% chromium content is greater than Inconel 600's 14%. Therefore, Inconel 690 has better corrosion resistance than Inconel 600.

Molybdenum

We know that chromium has excellent corrosion resistance. However, it is only resistant to some kind of corrosive substances. Molybdenum is a good complementary element to corrosion that chromium cannot resist. Adding chromium and molybdenum at the same time can make the alloy resist almost all corrosion, which increases the overall corrosion resistance of the alloy. In addition, molybdenum can also increase the strength of the alloy.

Some nickel-chromium-molybdenum alloys exist in Inconel's grades. Among them, the most typical is Inconel 625. Its corrosion resistance in complex environments is better than Inconel 600.

Niobium, Cobalt and Tungsten

Chromium and molybdenum increase corrosion resistance. The addition of niobium, cobalt and tungsten improves the properties of the alloy.

The niobium in Inconel 625 increases the strength of the alloy through a solid solution strengthening mechanism.

Inconel 617 is a nickel-chromium-cobalt-molybdenum alloy. The addition of cobalt increases the strength of the alloy in another way. Compared to Inconel 625, Inconel 617 can maintain strength at higher temperatures.

Inconel 686 is a nickel-chromium-molybdenum-tungsten alloy. The addition of tungsten not only improves the strength of the alloy, but also gives the alloy excellent pitting corrosion resistance.

Mechanical Properties of Inconel

Inconel alloys are divided into solid solution strengthened alloys and precipitation strengthened alloys according to the different microstructures. Among them, the strength of precipitation strengthened alloys is much higher than that of solid solution strengthened alloys. This is one of the reasons why Inconel needs to be divided into various grades. If your project requires materials with very high strength, precipitation strengthened alloys should be preferred. While the grades of solid solution strengthened alloys have a variety of choices to meet your other needs.

Below, we will introduce the classification of these two alloys respectively.

Solid Solution Strengthened Inconel Alloy

Solid solution strengthened alloy is characterized by the fact that its microstructure contains only one crystal phase. Therefore, it is a single-phase alloy.

In fact, the classification of alloys follows a naming rule. Among the many grades of Inconel. Alloys whose first digit is 6 are solid solution strengthened alloys. Such as: Inconel 600, Inconel 625.

The strength of such alloys depends on their chemical composition. If the alloy contains elements with obvious solid solution effects such as molybdenum, tungsten, niobium, and cobalt, its strength will be higher. On this basis, if you want to further increase the strength, you can only achieve work hardening by cold working.

The following table shows the mechanical properties of solid solution strengthened Inconel alloys:

Precipitation Strengthened Inconel Alloy

As you may have guessed, the alloys with the first digit 7 are precipitation strengthened Inconel alloys. Typical alloys are Inconel 718, Inconel X-750.

Unlike solid solution alloys, one or two additional strengthening phases are present in its microstructure. Therefore, it is a multiphase alloy.

The key elements for precipitation strengthening alloys are aluminum, titanium, niobium and tantalum. They can form precipitation strengthening phases. You can observe that all precipitation strengthened alloys contain at least one of these four elements.

Precipitation strengthened alloys can be further aging treated to greatly increase the strength of the alloy after work hardening.

The following table shows the mechanical properties of precipitation strengthened Inconel alloys:

Physical Properties of Inconel

For Inconel alloys, the physical property of greatest concern is density. The chemical composition of different Inconel alloys varies greatly, so they have different densities.

Density is an important physical quantity for calculating alloy weight. If the weight is not calculated according to the density of the specific grade, the result will be very different from the actual weight.

You can use the following tools to quickly calculate the weight of different Inconel alloys:

Nickel Alloy Weight Calculator (Monel, Inconel, Incoloy, Hastelloy)

FAQ

Further Reading

Quickly Understand What are the Main Inconel Types

Summary of Density and Melting Point of Monel, Inconel, Incoloy, Hastelloy Alloys

What is Solid Solution? Why do Nickel Alloy / Superalloy need Solution Treatment?

What is Precipitation Strengthening & Aging Treatment. Which Alloys need Aging Treatment?

Conclusion

Inconel alloy belongs to nickel-chromium alloy. These alloys are suitable for use in high temperature corrosive environments. Different elements are added to the alloy to form a variety of different Inconel grades. They focus on different performance.

We provide high quality Inconel alloys for your selection. Please contact us for any needs.