Wrought weldable nickel-chromium alloy Inconel 718 is fit for service at moderate temperature about 1300oF. However not as strong as other super alloys that service from 1500oF to 1800oF, its combination of good mechanical and fabrication characteristics at high and low temperature has earned it an essential role in various aerospace equipments. It is used for good cryogenic characteristics in cryogenic tank for rockets, its short time strength at temperatures to 1200oF has allowed its use in liquid-fueled rocket engines, its creep rupture characteristics at temperatures up to 1300oF have enabled to be used in fabricated components of different aircraft turbine engines.

The chemical composition and heat processing of Inconel 718 act together in developing the required properties. In specific, it has been noticed that columbium, aluminium, titanium and carbon concentration have essential effects.

The combination of low aluminium concentration, high annealing temperature and low aging temperature is good with respect to fatigue life and short transverse characteristics however is not required when suitable long time stress rupture properties are required. If aluminium concentration is not low, 1750oF anneal in combination with a 1325oF to 1350oF aging treatments seem to be suitable. Increasing columbium and tantalum content about 2 – 6% results into increase in yield strength. With annealing, an increase in tensile and yield strengths were also noticed that symbolizes solution strengthening.

High strength in Inconel 718 is developed by high temperature annealing treatment by lower temperature aging treatment. The specific annealing and aging temperature and times and rates of cooling from these limits have been constantly altered over the past five years.

Annealing temperatures up to 1750oF were recommended, users should not use annealing temperatures above 1800oF. Enhanced mechanical characteristics could be received by adjusting the aging treatment. Double aging procedure are found to be advantageous. For suitable tensile characteristics, annealing at 1700oF to 1800oF was preferred, however for the maximum stress rupture characteristics, annealing temperature should be 1900oF.

Inconel alloy 718 is mainly used in the wrought form. Nevertheless, specific features of the cast structure can be retained in the wrought structures and in these cases, have a strong effect on the mechanical properties of the wrought products.

Alloy 718 bar has a microstructure of wrought nickel-base alloys. Aging of annealed wrought structure at temperatures up to 1300oF to 1400oF precipitates the gamma prime corresponding to Ni3(Cb, Mo, Ti) or Ni3(Cb, Mo, Al, Ti). Overaging for about 30 hours at 1400oF causes precipitation that develop considerable darkening of grains and some precipitations at the grain boundaries. This darkened microstructure is feasibly a sign of the Ni3Cb precipitation.

The double aging treatments are:

1325oF for 8 hour, furnace cooling at a rate of 20o per hour to 1150oF, air cool

1325oF for 8 hour, furnace cool at a rate of 100o per hour to 1150oF. Retained at 1150oF for 8 hour then air cooled.

Gamma prime phase is essential in strengthening of Inconel 718 bar that necessitates the double aging treatment for higher strength than single aging. Precipitate gamma prime as much as possible for the maximum strength.