Surface Effects on Low Cycle Fatigue Behavior in IN718 Alloy

Abstract

Superalloys used in gas turbine engine rotating components require superior low cycle fatigue (LCF) properties, but often, the intrinsic LCF capability of the rotor, as determined by laboratory specimen tests, is limited by presence of fine particles such as carbides, borides and ceramic inclusions, as well as, pores, voids, and large grains in the alloy. Further, manufacturing processes such as turning, milling, and broaching can limit LCF life if they introduce surface damage and/or tensile residual stress. In IN718, prior studies have shown that fatigue initiation from MC carbides is promoted at lower test temperature and high stress, and the LCF life is significantly reduced. The present study investigates the effect of machining that can introduce carbide damage on LCF life, and a few post-machine processing methods for LCF recovery. The IN718 material used in this study came from a gas turbine disk forging that was directly aged without the conventional solution heat-treatment. The microstructure was fine-grained with an average grain size of 11 μm. Smooth and notched LCF properties were evaluated with machined and polished surfaces at test temperature and stress that promoted carbide initiation of fatigue cracks. Majority of the tests were performed at temperature of 560 K and at stresses of 1102 and 1240 MPa for smooth, and nominal stresses of 595 and 650 MPa for notched specimens. The min to max stress ratio (R) was 0.05. The cycles to failure were analyzed in terms of machining effect on surface carbides, and the fatigue initiation sites were studied by scanning electron microscopy (SEM). Smooth LCF specimens included asmachined (lathe-turned) and polished (600 grit SiC) surfaces and the notched LCF specimens included single-point bored (SPB) and media-polished notch surfaces. Lathe-turning and SPB cracked or damaged surface and subsurface carbides, considerably impacting the LCF life by factors of 3-15X (depending on test conditions). For a smooth lathe-turned surface, shot peening was effective in recovering LCF life by 5-10X at minimum over the as-machined surface. Some conventional post-machine processes for holes, such as mechanical honing, extrude hone and jig grinding and their influence on LCF life were studied as well.