Overview

Iron-based superalloy is a kind of superalloy. It is characterized by adding other elements on the basis of iron for alloying. Corresponding to iron-based superalloys are nickel-based superalloys and cobalt-based superalloys. In this article, we will introduce the characteristics of iron-based superalloys from multiple dimensions.

What is Iron-based Superalloy

Iron-based superalloys generally refer to superalloys based on iron. Since the price of iron is lower than that of nickel and cobalt, the price of iron-based superalloys is also relatively low. Although there is no clear regulation, it is generally believed that superalloys with iron content greater than 20% can be called iron-based superalloys.

Iron element has the property of allotropy, it can only maintain austenite structure in a certain temperature range. Austenite can maintain the high plasticity of the alloy and is a necessary structure for superalloys. In order to stabilize the austenitic structure of the alloy, usually about 25% nickel is added to the iron-based alloy. When the nickel content is greater than 40%, the alloy is also called an iron-nickel-based superalloy.

Generally, about 15% chromium is added to iron-based superalloys. Iron is very easily oxidized at high temperatures. The chromium element in the alloy can form an oxide film on the metal surface. It protects the interior of the alloy from further oxidation.

In summary, iron-based alloys are often iron-nickel-chromium alloys. Common iron-based superalloys are:

Fe-15Cr-25Ni / Fe-20Cr-25Ni type alloys:
Incoloy A-286 (UNS S66286), Incoloy 926 (UNS N08926)

Fe-15Cr-35Ni / Fe-20Cr-35Ni type alloys:
Incoloy 800 (UNS N08800), Incoloy 020 (UNS N08020)

Fe-15Cr-45Ni / Fe-20Cr-45Ni type alloys:
Incoloy 825 (UNS N08825), Incoloy 925 (UNS N09925), Incoloy 945 (UNS N09945)

Iron-based superalloys is divided into solid solution strengthened iron-based alloys and precipitation strengthened iron-based alloys according to the strengthening method. We will introduce the performance characteristics of these two iron-based alloys in detail later.

Operating Temperature

Iron-based superalloys are suitable for use at moderate temperatures. All of them can be used for a long time at a temperature of 600~800°C. Among them, the precipitation strengthened iron-based alloy can maintain good permanent strength in this temperature range.

Solid Solution Strengthened Iron-Based Alloys

The main principle of solid solution strengthening is to use the atomic radius difference between the strengthening element and the basic element to expand the lattice to achieve strengthening. The principle of solid solution strengthening is introduced in detail below:

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

Chemical Composition

Iron-based superalloys often use molybdenum for solid solution strengthening. Molybdenum atoms are much larger than iron atoms. This can significantly improve the solid solution strengthening effect. In addition, there are also a small number of iron-based superalloys that use tungsten to achieve solid solution strengthening.

Mechanical Property

Iron-based superalloys can maintain good mechanical properties below 900 °C. They are suitable for parts such as enclosures or containers that are not subject to excessive stress. Among these components, they are durable in high temperature. Compared with nickel-based alloys, iron-based superalloys have poorer high-temperature oxidation resistance and structural stability.

Precipitation Strengthened Iron-based Alloy

The principle of precipitation strengthening is to allow the precipitation strengthening elements in the alloy to precipitate to form a precipitate phase to achieve the effect of hindering the displacement of the alloy crystal. The precipitation phase can be divided into γ' phase and γ" phase. These two strengthening phases act on superalloys at different temperatures respectively. The following article introduces the principle of precipitation strengthening in detail:

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

Chemical Composition

The aluminum and titanium added to the iron-based superalloy will precipitate out of the matrix to produce γ' phase. Unlike nickel-based superalloys, more titanium is added to iron-based alloys to achieve precipitation strengthening.

Compared with nickel-based superalloys, iron-based superalloys have much less precipitation strengthening phases. The proportion of precipitation strengthening phase in nickel-based superalloys can reach up to 65%, while that of iron-based superalloys is often less than 20%.

In precipitation strengthened iron-based superalloys, the γ′ phase becomes unstable when the titanium content exceeds twice the aluminum content. In this case, if it is used at high temperature for a long time, the γ' phase will transform into the η phase. At this time, the mechanical properties of the material will deteriorate. However, this phenomenon can also be exploited to improve the mechanical properties of alloys. The η phase can also be used to refine the alloy grains through the fine grain forging process. At this time, the precipitation strengthening phase is distributed in the alloy in a finer form. This is beneficial to increase the mechanical properties of the alloy.

In some alloys (such as Inconel 706), niobium is also added to form a γ" phase. The γ" phase will strengthen the alloy below 650°C. When the temperature rises above 650°C, γ" will transform into δ phase. At this time, the mechanical properties of the alloy will deteriorate.

The amount of aluminum content determines who is dominant in the γ' phase and the γ" phase. The higher the aluminum content, the more γ' phase and the less γ" phase.

Mechanical Property

Precipitation strengthened iron-based superalloys have excellent mechanical properties below 750°C. Compared with nickel-based superalloys, the number of precipitated phases in iron-based superalloys is less. This makes it more ductile than nickel-based superalloys. The iron-based superalloy has its unique grain refinement process. Its high temperature strength is also not lower than that of nickel-based alloys. Therefore, in general, the medium-temperature mechanical properties of iron-based superalloys are better than those of nickel-based superalloys. It is often used in aero-engine parts that are subject to long-term stress.

Conclusion

Iron-based superalloys are alloys formed by adding elements such as nickel and chromium to the basis of iron. It is less expensive and not as corrosion resistant as other superalloys. Through a unique grain refinement process, precipitation strengthened iron-based superalloys can achieve excellent mechanical properties.

We produce high-performance iron-based superalloys, and we can also customize alloys with different grain sizes according to your requirements. If you have needs, please contact us.