Inconel is a brand name for a family of nickel-based superalloys. Inconel alloy 600 is 72% nickel, 16% chromium, and 8% iron. Other forms of inconel exist, each with slightly different additions. e.g. Inconel alloy 750 has a small percentage of titanium and aluminium added for hardenability (600 is not hardenable by heat treatment).
Inconel is highly oxidation and corrosion resistant, even at very high temperatures, and retains a high mechanical strength under these conditions as well. Thus, it is often used in extreme conditions, such as aircraft engine parts, turbocharger turbine wheels, chemical processing and pressure vessels. Inconel 600 & 800 are also used in the pressure tubes of CANDU nuclear reactors.
Other versions of Inconel resist acid and other aggressive conditions such as Ni-20Cr-16Mo-4W Inconel alloy 686.
Inconel alloy 718
(UNS N07718/W.Nr. 2.4668) is a high-strength, corrosion-resistant nickel chromium material used at -253°C to 700°C (-423°F to 1300°F). Typical composition limits are shown in Table 1.
The age-hardenable alloy can be readily fabricated, even into complex parts. Its welding characteristics, especially its resistance to postweld cracking, are outstanding.
The ease and economy with which INCONEL alloy 718 can be fabricated, combined with good tensile, fatigue, creep, and rupture strength, have resulted in its use in a wide range of applications. Examples of these are components for liquid fueled rockets, rings, casings and various formed sheet metal parts for aircraft and land-based gas turbine engines, and cryogenic tankage. It is also used for fasteners and instrumentation parts.
|Table 1 - Limiting Chemical Compositions, %|
|Nickel (plus Cobalt)||50.00-55.00|
|Niobium (plus Tantalum)||4.75-5.50|
INCONEL alloy 718 has excellent corrosion resistance to many media. This resistance, which is similar to that of other nickel-chromium alloys, is a function of its composition. Nickel contributes to corrosion resistance in many inorganic and organic, other than strongly oxidizing, compounds throughout wide ranges of acidity and alkalinity.
It also is useful in combating chloride-ion stress-corrosion cracking. Chromium imparts an ability to withstand attack by oxidizing media and sulfur compounds. Molybdenum is known to contribute to resistance to pitting in many media.
Heat treating and mechanical properties
For most applications, INCONEL alloy 718 is specified as: solution annealed and precipitation hardened (precipitation hardening, age hardening, and precipitation heat treatment are synonymous terms).
Alloy 718 is hardened by the precipitation of secondary phases (e.g. gamma prime and gamma double-prime) into the metal matrix. The precipitation of these nickel- (aluminum, titanium, niobium) phases is induced by heat treating in the temperature range of 1100 to 1500°F. For this metallurgical reaction to properly take place, the aging constituents (aluminum, titanium, niobium) must be in solution (dissolved in the matrix); if they are precipitated as some other phase or are combined in some other form, they will not precipitate correctly and the full strength of the alloy with not be realized. To perform this function, the material must first besolution heat treated (solution annealed is a synonymous term).
Two heat treatments are generally utilized for INCONEL alloy 718:
- 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.
- (This is the optimum heat treatment for alloy 718 where a combination of rupture life, notch rupture life and rupture ductility is of greatest concern. The highest room-temperature tensile and yield strengths are also associated with this treatment. In addition, because of the fine grain developed, it produces the highest fatigue strength.)
- 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.
- (This is the treatment preferred in tensile-limited applications because it produces the best transverse ductility in heavy sections, impact strength, and low-temperature notch tensile strength. However, this treatment has a tendency to produce notch brittleness in stress rupture.)
If the material is to be machined, formed, or welded, it typically is purchased in the mill annealed or stress relieved condition. The material is then fabricated in its most malleable condition. After fabrication, it can be heat treated as required per the applicable specification.
Inconel® is a registered trademark of Special Metals Corporation.