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Introduce

Inconel 718 is an age-hardenable Alloy-chromium alloy that combines corrosion resistance with high strength and good fabricability. It has high creep-rupture strength at temperatures up to 700 ºC. Its excellent relaxation resistance contributes to its application in springs.

Application

The alloy is used in jet engine and high-speed airframe parts such as wheels, buckets and spacers, and high-temperature bolts and fasteners. Inconel 718 is also used in the oil and gas drilling and production industries due to its high strength and resistance to chlorides, stress corrosion and sulfide stress cracking. Within these industries the alloy has been used for valves, pump shafts and wellhead components.

Heat-Treat Process

Inconel® 718 is typically purchased in the mill- or solution-annealed condition. In some instances, it has a stress-relief operation performed on it prior to fabrication and heat treatment. The solution anneal is followed by a precipitation (age) hardening step. Precipitation of secondary phases (e.g., gamma prime and gamma double-prime) into the metal matrix hardens the material. The precipitation of nickel-aluminum, nickel-titanium and nickel-niobium phases is triggered by aging in the temperature range of 600-815˚C (1100-1500˚F). The key to the heat-treatment process is to be sure these age-hardening constituents are fully in solution at high temperature (i.e., fully dissolved in the matrix), otherwise precipitation will not result in full strength.Two heat treatments are common (although slight temperature variations of these recipes are often employed).Solution anneal at 925-1010˚C (1700-1850˚F) followed by rapid cooling (usually in water). This is followed by precipitation hardening at 720˚C (1325˚F) for 8 hours, furnace cooling to 620˚C (1150˚F) and holding for 10 hours followed by air cooling.
Solution anneal at 1040-1065˚C (1900-1950˚F) followed by rapid cooling (usually in water). This is followed by precipitation hardening at 760˚C (1400˚F) for 10 hours, furnace cool to 650˚C (1200˚F) and holding for 10 hours followed by air cooling.Solution heat treatments are typically done in low-dew-point argon or vacuum furnaces, often with all-metal hot zones to avoid discoloration. The effect of annealing for 30 minutes on the grain size (Fig. 2) of sheet varies with temperature. Also, aging response of Inconel® 718 is rather slow in comparison with that of aluminum-titanium-hardened alloys. Thus, in most sizes, the alloy can be heated and cooled through the aging temperature range at normal speeds yet retain softness and ductility. The effect of aging time and temperature on the hardness of annealed sheet (Fig. 3) reflects this.

Standards

718 Sheet, Coil and Plate: UNS N07718, AMS 5596, ASTM B 670, ASME SB 670, B50TF14,

718 Round Bar: UNS N07718, AMS 5662, ASTM B 637, B50TF15, B50809D (Power Generation)

other:AMS 5662, AMS 5663, AMS 5832, AMS 5962, ASTM B637, GE B14H89, ISO 15156-3, (NACE MR 0175)

Key Features

Good creep rupture strength at high temperatures

Higher strength than Inconel X-750

Better mechanical properties at lower temperatures than Nimonic 90 & Inconel X-750

Age hardenable

High temperature dynamic applications

Chemical Composition

Mechanical Properties