The Handbook indicates solution treatment at 980 C (1800 F), followed by aging at 720 C (1325 F) followed by aging at 620 C (1150 F). What is the basis for two aging treatments? What is the reason for 720 C first and then 620 C instead of 620 C followed by 720 C? TTT diagram of 718 is attached.
The two step aging heat treatment renders Alloy 718 the maximum strength and good stress-rupture ductility at 650 -705 C.
The ordered BCT γ” and ordered fcc-γ’ precipitates form at 720 °C heat treatment in addition to M6C. During the 620 °C aging, predominantly γ’ precipitates form at the γ” denuded zone. The strengthening of 718 is predominantly determined by precipitation of γ”. The formation kinetics of γ” is very sluggish and as noted from the TTT diagram it will take more than 1000 h if aged at 620 °C. The second step 620 °C aging treatment gives resistance to high temperature notch sensitivity, as γ’ precipitates forms in the γ” denuded zones and strengthen those regions. In addition to the aging treatments, solution treatment(s) also play a critical role in determining the creep-rupture strength of Alloy 718.
If the 620 °C aging was carried out first, then the spherical γ’ precipitates of size less than 25 nm would form. There may not be γ” precipitates. At 720 °C aging step, these γ’ precipitates would grow to a larger size (~60 nm?), and also act as nucleation site for γ”. In effect, there may not be very uniform distribution of the secondary phase precipitates that would impart high strength combined with good high temperature notch ductility.
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