Introduction

Surface enhancement is widely used to improve the fatigue life of components by creating a surface layer of compressive residual stress that delays fatigue crack initiation and retards small crack propagation. However, the benefits of surface enhancement are lost if the compressive layer relaxes at the operating temperature of the component. Surface enhancement methods producing minimal cold work are shown to produce the most thermally stable compression.
The residual stress and cold work distributions developed in IN718 by shot peening, gravity peening, laser shock peening (LSP) and low plasticity burnishing (LPB) are compared. Thermal relaxation at temperatures ranging from 525C to 670C is correlated to the degree of cold working, independent of the method of surface enhancement. Highly cold worked (>15%) shot peened surfaces relax to half the initial level of compression in only minutes at all temperatures investigated. The rapid initial relaxation is shown to be virtually independent of either time or temperature from 525C to 670C. High cycle fatigue (HCF) performance after elevated temperature exposure is compared for surfaces treated by LPB and conventional (8A intensity) shot peening.
Since the introduction of shot peening, the HCF life of components has been improved by “surface enhancement” to induce a surface layer of compressive residual stress. The compressive layer resists both crack initiation and small crack propagation, and the subsurface residual stress has long been correlated with improved HCF strength.[1-2] However, if the residual compression relaxes during service, the fatigue benefits are lost. Risk of relaxation prevents designers from “taking credit” for the benefits of surface enhancement. Of the three mechanisms for residual stress relaxation: thermal, overload, and cyclic, only thermal relaxation is significant for nickel base superalloys in HCF limited applications. This paper summarizes research of the thermal stability of surface enhancement methods applied to the nickel base superalloy, IN718.

Surface Enhancement Methods

All mechanical surface enhancement methods create a layer of compressive residual stress by tensile deformation of the surface, but differ in the magnitude and form of the resulting residual stress and cold work (plastic deformation) distributions produced. The residual stress and cold work distributions produced by conventional shot peening, gravity peening, laser shock peening (LSP), and low plasticity burnishing (LPB) of IN718 are compared in Figure 1. Surface compression is comparable, but the compressive depths differ by an order of magnitude. Cold working ranges from 40% at the shot peened surface to a few percent for LPB.
The depth of the compressive layer and the degree of cold working produced by shot peening depend upon the peening parameters: shot size, velocity, coverage, and impingement angle. Shot impact stretches the surface in tension, leaving a compressive dimple. Because shot impacts are random, peening results in multiple overlapping impacts [3] and a surface highly cold worked from 10% to 50%.[4] Cold work is accumulative, and increases with coverage or repeated shot peening applications. The depth and degree of cold working increase with peening intensity. Surface compression may decrease during shot peening of work hardening alloys as the yield strength of the surface increases with continued cold working.
Gravity peening utilizes the same mechanism as shot peening, but employs fewer impacts of larger shot, producing less cold work and improved surface finish. Compression comparable to shot peening is achieved with 5 to 10% cold work.
Conventional roller or ball burnishing and “deep rolling” tools are pressed into the surface of the work piece with sufficient force to deform the near surface layers with multiple passes, often under increasing load, to improve surface finish and cold work the surface for improved strength. Fatigue enhancement is attributed to improved finish, the development of a compressive surface layer, and the increased yield strength of the cold worked surface.[5-24] X-ray diffraction line broadening and micro-hardness studies of deep rolling reveal cold work even greater than shot peening.[25-27]
Laser shock peening (LSP) [28] has been applied to a variety of alloys including titanium, nickel superalloys, and steels.[29] LSP produces a compressive layer of comparable magnitude to shot peening, but much deeper with less cold work. Single shock LSP can produce high surface compression with less than 1% cold work, affording excellent thermal stability in IN718.[30] However, multiple laser shock cycles are required to produce compression to depths of 1mm (3X in Figure 1), accumulating cold work to 5-7%.[31]
Low plasticity burnishing (LPB) produces a deep layer of high compression, comparable to LSP, but with improved surface finish, lower cost, and minimal cold work [32], typically an order of magnitude lower than deep rolling and conventional burnishing. The material, heat treatment, processing and testing details have been described in detail previously. [33]

Thermal Relaxation

Cold work from surface enhancement has been correlated with both the speed and magnitude of thermal relaxation of surface compression.[30] The rate and amount of stress relaxation for 10A, 200% shot peening and gravity peening, differing primarily in the amount of cold work developed, are shown in Figures 2 and 3 for exposures to 525C from 10 to 2000 minutes. Research in steel confirms that shot peening surface compression initially relaxes very rapidly, and then predictably with time and temperature.[34]  Highly cold worked shot peened IN718 surfaces can relax to less than 50% of the initial value in only minutes at even low engine operating temperatures.[30]
The fraction of compression remaining after thermal exposure to temperatures from 525C to 670C for times ranging from 10 to 2000 minutes is plotted for shot peened IN718 in Figure 4. The relaxation of a variety of different surface enhancement methods after just 10 min. at 670C is plotted vs. cold work in Figure 5. The compression remaining after thermal exposure for any of the surface treatment methods is nearly independent of the temperature (for the 525C to 670C range examined), and only slightly dependent upon the time of exposure. Relaxation depends primarily upon the initial cold work and dislocation density. Dislocation annihilation appears to play a roll above a threshold level associated with nominally 3 to 5% cold work. Surface compression created with minimal cold work is clearly more stable at high temperatures. The data in Figures 4 and 5 indicates a cold work threshold for relaxation in IN718 to the order of 3% to 5%.
Residual stress and cold work distributions for shot peening and LPB exposed to 525C and 600C for 100h are shown in Figure 6. Compression at the shot peened surface relaxes nearly completely at either temperature, while the lightly worked LPB samples are stable. Only the subsurface maximum compression level is reduced to the alloy yield strength at the exposure temperature.

Conclusions

The compressive layer induced by shot peening of IN718 relaxes very rapidly at moderate turbine engine temperatures. Regardless of the surface treatment method, thermal relaxation depends strongly on the degree of cold work and dislocation density induced. Rapid initial relaxation is nearly independent of exposure temperature or time, and primarily a function of cold work above a threshold of a few percent. Surface enhancement methods such as LSP and LPB that produce minimal cold work, offer the greatest resistance to thermal relaxation.

References

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[32] U.S. Patent 5,826,453 (Oct. 1998), other patents pending.
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