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.

Influence of residual stresses on the fatigue crack growth from surface anomalies in a nickel-based superalloy

“…These results are consistent with the effect of compressive residual stresses introduced by surface treatment like shot peening and characterized in the literature [2,[6][7][8][9][10]. The presence of compressive residual stresses under the anomalies generates a beneficial effect on the fatigue performance of the material.…”

“…In some cases, their introduction result from processing and/or machining operation as recently discussed and analysed by Bhowal et al for Direct Aged (DA) 718 alloy [2]. Many studies [2][3][4][5] were undertaken to determine the effect of machining parameters on the surface integrity and the creation of residual stresses on nickel-based superalloy. Authors indicate that the use of a worn tool or abusive machining conditions results in poor surface integrity and contributes to create harmful residual stresses.…”

“…The studies dedicated to the behaviour of this type of surface anomaly in low cycle fatigue are scarce in literature. Some studies [11,12] evaluate the presence of hardening and residual stresses around scratches, but little is known concerning their influence on fatigue life [2]. Other authors have investigated the harmfulness of foreign object impact damage [13][14][15], which can be assimilated to a high impact energy dent.…”

Surface integrity after post processing of EDM processed Inconel 718 shaft

“…This is similar to results reported by others showing a high compressive stress level. For example, Bhowal et al reported of compressive stresses after shot peening of Inconel 718 of approximately − 1200 MPa which improved the fatigue strength 5-10 times the one of a machined surface with tensile residual stresses [36]. Such high compressive residual stresses are desirable from a fatigue perspective and were further reported by Chen et al who showed that shot peening of Inconel 718 greatly influences the surface performance and increases the fatigue limits by introducing high compressive residual stresses [27].…”

The Effect of the Conditions of Shot Peening the Inconel 718 Nickel Alloy on the Geometrical Structure of the Surface

“…During the finishing treatment of nickel alloys, the physical properties of the surface layer are changed. As a result of the conducted shot peening of the Inconel 718 alloy, compressive stresses are deposited on the surface layer [19,20], which translates into the increased fatigue strength [1,3]. The stress depth and its value depends on the machining time, material of the balls and the distance at which the workpiece is located from the nozzle [19].…”