Both Inconel 625 and Inconel 718 are superalloys with excellent comprehensive properties.

The main difference between them is that the performance of Inconel 625 is more towards corrosion resistance, while the performance of Inconel 718 is more towards high strength.

This article will compare alloy 625 and alloy 718 from all aspects, which will give you a clearer understanding of these two alloys.

From the analysis of the composition of the two alloys, the purity of Inconel 718 is higher than that of Inconel 625, which also makes Inconel 718 a better foundation in terms of strength.

Silicon

The silicon content has a certain effect on the instantaneous and durable properties of the alloy.

The silicon content of alloy 625 and alloy 718 is controlled below 1%. Within this range, the silicon content has no significant effect on the instantaneous properties of the alloy.

However, the silicon content has a very significant effect on the durability of the alloy. This makes Inconel 718 with lower silicon content even better in terms of alloy durability.

Manganese

The manganese content has no significant effect on the instantaneous properties of the alloy, but it also has a significant effect on the durability of the alloy.

The manganese content control of Inconel 718 is more stringent, which further improves the durability of the alloy.

In addition, a small amount of manganese is also necessary for alloy 718, which can improve the welding performance of alloy 718.

Inconel 625 has much less iron content than Inconel 718, which makes it have more nickel, chromium and molybdenum content.

Nickel itself has good corrosion resistance, and chromium is the most important corrosion resistance element in superalloys. Therefore, the corrosion resistance of alloy 625 is better than that of alloy 718.

The molybdenum element can well supplement the anti-reduction ability of alloy and improve the comprehensive corrosion resistance of alloy. This further opened the gap between the corrosion resistance of 625 alloy and 718 alloy.

Aluminum and Titanium

An essential difference between Inconel 625 and Inconel 718 is the way the alloy is strengthened.

Inconel 625 is a solid solution strengthened alloy, which is mainly strengthened by solid solution. Most of the aluminum and titanium in the alloy will dissolve into the alloy matrix (γ-austenite). This has a limited strengthening effect on the alloy.

In contrast, Inconel 718 is a precipitation strengthened alloy. During the production process, aluminum and titanium will precipitate from the matrix to form a γ' phase. This greatly improves the strength of the alloy.

This essential difference makes alloy 718 much stronger than alloy 625.

Niobium and Tantalum

Niobium and tantalum still play a solid solution strengthening role in alloy 625. Because the atomic radii of niobium and tantalum are very large, their solid solution strengthening effect on 625 alloy is greater than that of aluminum and titanium.

Niobium and tantalum also play a precipitation strengthening effect in Inconel 718. But unlike aluminum and titanium, part of niobium and tantalum enter the γ matrix for solid solution strengthening, and part of it enters the γ' phase to increase the effect of precipitation strengthening, while more niobium and tantalum can form a γ'' precipitation strengthening phase. The strengthening effect of this precipitation strengthening phase at medium and low temperature is even more obvious than that of the γ' phase. However, at high temperatures, the effect of the γ'' phase will be significantly weakened.

Comparison of Mechanical Properties

The following is a comparison of the mechanical properties of Inconel 625 and Inconel 718, you can clearly see the gap between them.

As mentioned above, Inconel 625 is a solid solution strengthened alloy. Therefore, the standard heat treatment method of 625 alloy is solution treatment, and alloy 625 after solution treatment can achieve the best performance state.

Inconel 718 is a precipitation strengthened alloy, so after solution treatment, aging treatment is generally carried out. The purpose is to fully precipitate the precipitation strengthening phase and optimize the strength of the alloy.

Inconel 718 has two heat treatment methods:

• • Solution anneal at 1700-1850°F followed by rapid cooling, usually in water, plus precipitation hardening at 1325°F for 8 hours, furnace cool to 1150°F, hold at 1150°F for a total aging time of 18 hours, followed by air cooling.
• • Solution anneal at 1900-1950°F followed by rapid cooling, usually in water, plus precipitation hardening at 1400°F for 10 hours, furnace cool to 1200°F, hold at 1200°F for a total aging time of 20 hours, followed by air cooling.

The properties of the alloys obtained by the two heat treatment methods are different, and can be selected according to the actual application. The default heat treatment method is the first.

Inconel 625 is a solid solution strengthened alloy, the biggest feature is excellent corrosion resistance.

Inconel 718 is a precipitation strengthened alloy, the biggest feature is very high strength.

The two alloys have their own functions and cannot replace each other.

AEETHER can supply Inconel 625 and Inconel 718 products, and will strictly carry out solution treatment or aging treatment according to the standard. If you have material purchase requirements, you can send the inquiry to our email: