Making the grade

News and developments in the world of corrosion resistant alloys through the eyes of Corrotherm International

Mechanical Properties and Applications of INCONEL Alloy 617

18 August 2021

Mechanical Properties and Applications of INCONEL Alloy 617

Versatile, adaptable and exceptionally resistant, INCONEL alloy 617 is an excellent choice for a wide spectrum of applications. This nickel-chromium-cobalt-molybdenum alloy reacts to heat by rapidly forming an oxide layer that protects it from corrosion and oxidation at temperatures up to 1150ºC (2100ºF), while also retaining its strength.

That’s why INCONEL alloy 617 is a popular choice for use in extreme, high-temperature and high-stress environments and those involving corrosive substances. Common fields of application include power generation (nuclear and fossil fuel), nitric acid production, petrochemical processing and molybdenum refining.

The advantages of INCONEL alloy 617 can be summed up in three words: strength, stability, and resistance. Highly resistant to reducing media, alloy 617 displays impressive tensile and yield strength across a vast range of operating temperatures. Its composition and treatment help to prevent it from deforming under strain. This lengthens its lifespan compared to other commonly used materials and makes it a safe and cost-effective option. The alloy can be fabricated using several standard technologies, so it’s easily adaptable to a range of potential applications.

Read on to discover more about these three core qualities of INCONEL alloy 617.

Tensile properties and fatigue strength

INCONEL alloy 617 is ideally suited for operating at very high temperatures. Tests show that it retains around 80% of its tensile strength even at 600ºC (1112ºF). This makes it far less likely to fracture under stress than many other materials when exposed to extreme heat. Its capacity to endure strain also remains high, since its yield strength only declines by around 20% above the 600ºC threshold.

This retention of yield strength is particularly important in terms of fatigue performance. Applications such as power generation and petrochemical processing mean that crucial components are subjected to intervals of high strain (cyclic loading), with a large number of cycles. The yield strength of a particular component effectively determines the threshold at which it will fail. INCONEL alloy 617’s high yield strength means that its resistance to fatigue does not significantly decrease at high temperatures. Because of this, a component made from alloy 617 will have a longer operative life and perform better in challenging conditions.

Stability of properties

INCONEL alloy 617 is unusual in combining a high degree of strength with stability of properties. When exposed to very high temperatures, a metal will usually become stronger and less malleable (ductile). Typically, as its ductility decreases, it will also become more brittle. Alloy 617 also gains in strength and loses ductility as the temperature rises, but it does not become more brittle in the process. This is one of the factors that help to prevent it from fracturing in extremely hot environments.

The stability and strength of alloy 617 are also enhanced by its corrosion and oxidation resistance, which remain consistent across a large temperature range. The formation of the oxide layer in reaction to heat takes place so quickly that there is no opportunity for the material to be adversely affected. All this makes INCONEL alloy 617 a secure, useful and reliable option for long-term use in high-temperature, high-stress conditions.

Because INCONEL alloy 617 is an exceptionally strong material, moulding and shaping it requires a high degree of force. However, it’s easy to fabricate using conventional machining, welding and forming procedures. When welding alloy 617, using the right filler will ensure that the material preserves both its strength and its resistance to corrosion. For shielded metal-arc welding, INCONEL Welding Electrode 117 should be used; INCONEL Filler Metal 617 is appropriate for gas-metal-arc and gas-tungsten-arc welding.

LCF considerations

Low-cycle fatigue (LCF) is a major consideration when using metal components at high temperatures. This is when the component is subject to repeated high stress at long intervals, which is typical for the fields in which INCONEL alloy 617 is most often used. In response to this stress, the metal will deform over time, shortening its useful life and eventually leading to failure. Because it occurs gradually over a long period, this type of damage is referred to as “creep”. Creep is especially hazardous in a system where each component is required to have very precise measurements. Even a slight degree of deformity can severely limit the system’s lifespan.

Metals such as steel and aluminium tend to be prone to creep when exposed to stress at high temperatures. However, INCONEL alloy 617 was developed especially to resist creep at a wide temperature range. The alloy is typically used in solution-annealed forms: heat-treated at temperatures specifically calculated to achieve the best possible creep resistance, in combination with improved stress rupture properties. The key to this balance is achieving a grain size that is coarse enough to provide protection against creep, but fine enough to avoid decreasing the material’s strength. The range of acceptable ASTM grain sizes is 3 to 6, with 4 or 5 being the optimal size.

A practical, cost-effective choice

INCONEL alloy 617’s combination of creep and stress-rupture resistance, metallurgical stability, and resistance to corrosion and oxidation all make it an outstanding choice for key applications in high-temperature environments. Its good formability and ease of fabrication mean that it is a practical and cost-effective option, especially where the operative lifespan of a system would otherwise be limited by low-cycle fatigue.

The composition of INCONEL alloy 617 includes nickel (minimum 44.5%) and chromium (20–24%), both of which give the alloy its resistance to corrosion and oxidation. Oxidation resistance is also enhanced by a small proportion of aluminium (0.8–1.5%). Alloy 617’s strength comes from cobalt (10–15%) and molybdenum (8–10%). Should even higher performance be required, INCONEL alloy 625 has a similar composition to 617 with the addition of niobium, which enhances the alloy’s strength without the need for specific heat treatment..

To find out more about alloy 617 and all the available INCONEL alloys, you can visit our Grades section. You can also contact us to discuss your requirements with a member of our sales team.

Published in Corrotherm News