Controlling hot tearing in superalloys used in turbine disks operating above 800°C

Abstract

To meet the requirements of high-performance aero-engines, the alloying degrees and γ′ phase mass fractions of newly wrought superalloys used in the aero-engines are maintained at high values, leading to elevated hot tearing susceptibility of the alloy. The hot cracking behavior of a large-diameter (ϕ350 mm) GH4151 superalloy with an Al + Ti + Nb content exceeding 10wt% and the γ′ phase content exceeding 50% was investigated via experiments and simulations. The solidification cracking and strain-age cracking (SAC) indices of different deformed superalloys were also evaluated. The results showed that the crack type of the GH4151 alloy was hot tearing. The cracks propagated intergranularly along high-angle grain boundaries. The hot tearing was due to excessive Mo, Nb, and Ti segregation in dendrites, creating a wide solidification range, and the successive formation of metastable carbide (MC), η, (γ + γ′) eutectic, Laves, and M₂B phases during the last stage of solidification, hindering liquid phase replenishment and forming pore defects. The pores produced cracks under the influence of thermal and phase-transformation stresses. B and Zr in an alloy can significantly reduce its solidification range. The critical solidification range for hot cracking in B and Zr-free superalloys was 230°C, while their strain-age crack criterion was 0.0008. The residual stress and microstructure of the alloys could be significantly improved through mold preheating and annealing, finally leading to a crack-free ingot.

Keywords

high γ′ phase content / hot tearing / liquid film / residual stress / annealing treatment

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