Forging Astroloy Supersolidus-Sintered Preforms: Necklace Structure Achievement
Primary turbine disk requirements are high creep-rupture properties at rim operating conditions and a good low-cyle fatigue behaviour at hub operating ones. The maximum operating temperature does not exceed 760°C (at blade attachments), the bulk of the material being at about 650°C.
Duplex structures such as the “necklace” one exhibit a good compromise between these two major mechanical aspects; good creeprupture properties are due to large warm-worked grains, and a high resistance to crack initiation and propagation results from small recrystallized grains of the duplex structure.
This kind of structure can be obtained by forging supersolidussintered preforms, just as with conventionally HIP’ed or cast preforms. Forging temperatures and reductions were investigated and ultimately chosen such that a “necklace” structure was formed. Two batches of Astroloy powders were used (differing mainly by their carbon content, 420 p.p.m. against 260 p.p.m., and their particle zize distribution). The recovery and recrystallization mechanisms are described and illustrated in relation with deformation conditions and heat treatments. Structures and related mechanical behaviour have been characterized. The “necklace” material compares well with conventional direct HIP’ed for forged materials.
Therefore, the combinaison of liquid phase sintering and forging leading to a “necklace” structure is well-adapted to the realization of high-performance materials.
KeywordsStress Intensity Factor Fatigue Crack Growth Duplex Structure Fatigue Crack Growth Behaviour Test Fatigue Crack Growth
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