Alloy 718 Forging Development for Large Land-Based Gas Turbines

Abstract

Alloy 718 forgings have been used extensively in gas turbines for aircraft engines for decades. However their application to land-based power generation gas turbines has been limited to the size of the forgings which can be produced by available forging equipment. GE's recently developed heavy duty gas turbines now require forgings which must be produced from 915 mm diameter ingots which exceed 15,000 kg. The initial hurdle was to melt these large ingots successfully without segregation defects. This was achieved and the results were discussed in a previous conference. This paper describes the unique challenges to converting these ingots into the largest Alloy 718 forgings ever produced. The development work associated with processing these ingots into a uniform fine grain microstructure with the required mechanical properties will be described. The importance of process modeling for developing a process to achieve the structure and property goals will also be discussed.

Closed Die Hammer Forging of Inconel 718

“…Therefore, it can be assumed that "necklace" recrystallisation observed at the top of the dome may be attributed to lower deformation and die chilling. However, it was reported that such limited or partial recrystallised structure does not affect the mechanical properties (tensile and fatigue) [3]. The midportion of the dome had slightly elongated grains with the remains of some large recovered grains as shown in Figure 6(b).…”

“…In the present study, closed die hammer forging was employed due to nonavailability of large press. Even though published work is available on development of large forgings, it is mostly confined to processing using press [3][4][5]. There are no reports on production of IN-718 hemispherical domes by closed die hammer forging or press forging in open literature.…”