I. Altenberger

Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304

Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling pressures, respectively. The resulting near surface properties were characterized by cross sectioning transmission electron microscopy (TEM), residual stress and phase analysis as well as interference line half-width and microhardness measurements. Cyclic deformation curves were obtained by hysteresis measurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peening and deep rolling lead to a complex near surface microstructure, consisting of nanocrystalline regions, deformation bands and strain induced martensitic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation behaviour: Plastic strain amplitudes and cyclic creep were drastically decreased by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remarkably, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell structures were only formed in greater depths whereas the nanocrystalline layer remained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation.

Cyclic deformation and near surface microstructures of normalized shot peened steel SAE 1045

Cylindrical specimens of the normalized plain carbon steel SAE 1045 were shot peened and cyclically deformed under stress control. A special cross-sectioning technique was applied to prepare near surface regions for transmission electron microscopy. The resulting properties of these regions were characterized by X-ray diffraction and microhardness measurements. The cyclic deformation curves are affected characteristically by dislocation arrangements induced by shot peening. The stability of these dislocation arrangements and the depth profile obtained by residual stress and interference line half-width value measurements depend strongly on the stress amplitude. High stress amplitudes lead to the formation of dislocation cell structures in the shot peened layers, accompanied by the total disappearance of compressive stresses and the decrease of interference line half-width values down to the initial level. Thus, a clear correlation between stress relaxation, cyclic deformation behaviour and microstructural alterations in shot peened regions is revealed. Experiments carried out on shot peened hollow specimens indicate that the cyclic deformation behaviour is not merely a function of dislocation arrangement and density but also of the thickness of strain hardened near surface material in relation to the diameter of the specimen.

Residual Stress Relaxation and Cyclic Deformation Behavior of Deep Rolled AlMg4.5Mn (AA5083) at Elevated Temperatures

The cyclic deformation behavior of deep rolled and polished aluminium wrought alloy AlMg4,5Mn in the temperature range 20-300°C has been investigated. Results of quasistatic tension and compression tests of untreated specimens in the temperature range 20-300°C are presented. To characterize the fatigue behavior for stress-controlled tests as a function of test temperature, s-n curves, cyclic deformations curves and mean strains as a function of number of cycles are given. The residual stress- and work hardening states near the surface of deep rolled aluminium alloy AlMg4.5Mn before and after fatigue tests were investigated by X-ray diffraction methods. The investigated AlMn4.5Mn aluminium alloy shows cyclic hardening until fracture at all stress amplitudes in stress-controlled fatigue tests at 25-150°C. With increasing temperature the deformation behavior shifts from cyclic hardening to cyclic softening. Below a certain stress amplitude at a given temperature deep rolling led to a reduction of the plastic strain amplitude as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM-values. This reduction in plastic strain amplitude is associated with enhanced fatigue lives. The effectiveness of deep rolling is governed by the cyclic and thermal stability of nearsurface work hardening rather than macroscopic compressive residual stresses. Since nearsurface work hardening is known to retard crack initiation, deep rolling is also effective in temperature- and stress ranges where macroscopic compressive residual stresses have relaxed almost completely, but where near-surface work hardening prevails. Above certain stress amplitudes and temperatures, deep rolling has no beneficial effect on the fatigue behavior of AlMg4.5Mn. This is a consequence of instable near-surface microstructures, especially instable near-surface work hardening.

Residual Stress State and Cyclic Deformation Behaviour of Deep Rolled and Laser-Shock Peened AISI 304 Stainless Steel at Elevated Temperatures

Deep rolling and laser-shock peening are known to induce several favourable effects to enhance the fatigue behaviour of metastable austenitic stainless steels at room temperature. In particular, the formation of high dislocation densities, strain-induced martensite and nanocrystalline regions serve to inhibit or retard fatigue crack initiation, whereas deep „cases“ of compressive residual stresses are known to reduce fatigue crack growth. During elevated temperature service, such as in power plants or gas turbines, these near-surface properties gradually become instable, depending on the nature of the surface treatment induced microstructures and the thermomechanical loading conditions. It is the objective of this work to clarify the role of different near-surface microstructures on the thermomechanical stability of macroand microstresses in mechanically surface treated AISI 304. For this purpose a classical and a relatively novel surface treatment method are compared to each other in terms of their impact on fatigue behavior at elevated temperatures. Both treatments induce distinctly different near-surface microstructures with different thermal stability. The results emphasize the importance of microstresses for fatigue life enhancement and demonstrate difficulties associated with the prediction of fatigue life by using macrostress data. Introduction Mechanical surface treatments are known to induce several beneficial effects into metallic surfaces [1,2]. The basic principles of mechanical surface treatments are well known: A localised elasticplastic deformation in near-surface regions leads to the formation of compressive residual stresses, alterations of roughness and severe microstructural alterations, rendering the thus strengthened near-surface regions more resistant against fatigue crack initiation and propagation [3,4]. These beneficial effects prevail only if the surface treatment induced near-surface compressive residual stresses or work hardening remain stable during mechanical loading or exposure to elevated temperatures. Deep rolling can significantly improve the fatigue behaviour of metallic materials owing to surface smoothening, near surface strain hardening, compressive residual stresses and in some cases nanocrystallisation [5]. In comparison to deep rolling, laser-shock peening does not induce any surface nanocrystallisation, but can lead to similar fatigue lives [6]. In this study, we therefore compare the residual stress stability and fatigue behaviour of a commonly used austenitic stainless steel under stress-controlled isothermal fatigue conditions in the temperature range 25-600°C in three different surface states, namely deep rolled, laser-shock peened and turned. Materials Science Forum Online: 2005-07-15 ISSN: 1662-9752, Vols. 490-491, pp 376-383 doi:10.4028/www.scientific.net/MSF.490-491.376

Residual Stress State and Fatigue Behaviour of Laser Shock Peened Titanium Alloys

Laser shock peening is a very effective mechanical surface treatment to enhance the fatigue behaviour of highly stressed components. In this work the effect of different laser shock peening conditions on the residual stress depth profile and fatigue behaviour without any sacrificial coating layer is investigated for two high strength titanium alloys, Ti-6Al-4V and Timetal LCB. The results show that the optimization of peening conditions is crucial to obtain excellent fatigue properties. Especially, power density, spot size and coverage severely influence the residual stress profile of laser shock peened Ti-6Al-4V and Timetal LCB specimens. For both alloys, subsurface as well as surface compressive residual stress peaks can be obtained by varying the peening conditions. In general, Timetal LCB exhibits steeper stress gradients than Ti-6Al-4V for identical peening conditions. The main parameters affecting the fatigue life are near-surface cold work and compressive residual stresses.

Residual Stress Stability in High Temperature Fatigued Mechanically Surface Treated Metallic Materials

Different classes of metallic materials (aluminum alloys, steels, titanium alloys) were mechanically surface treated by deep rolling and laser shock peening and isothermally fatigued at elevated temperature under stress control. The fatigue tests were interrupted after different numbers of cycles for several stress amplitudes and residual stresses and FWHM-values were measured by X-ray diffraction methods at the surface and as a function of depth. The results summarize the response of the surface treatment induced residual stress profiles to thermomechanical loading conditions in the High Cycle Fatigue (HCF)- as well as in the Low Cycle Fatigue (LCF) regime. The effects of stress amplitude, plastic strain amplitude, temperature and frequency are addressed in detail and discussed. The results indicate that residual stress relaxation during high temperature fatigue can be predicted for sufficiently simplified loading conditions and that thermal and mechanical effects can be separated from each other. A plastic strain based approach appears to be most suitable to describe residual stress relaxation. Frequency effects were found to be not very pronounced in the frequency range investigated.

High-Temperature Fatigue of Deep Rolled Aluminium Alloy AA6110-T6

The precipitation-hardened aluminium wrought alloy AA6110-T6 (Al-Mg-Si-Cu) was mechanically surface treated (deep rolled) at room temperature. The cyclic deformation behavior and s/n-curves of deep rolled AA6110-T6 have been investigated by stress-controlled fatigue tests at room and elevated temperatures up to 250°C and compared to the polished condition as a reference. The effect of deep rolling on fatigue lifetime under high-loading and/or elevatedtemperature conditions will be discussed. The stability of near-surface residual stresses as well as work-hardening states (FWHM-values) was investigated by X-ray diffraction methods. Residual stress- and FWHM-depth-profiles before and after fatigue tests at elevated temperature are presented. It was found that the investigated AA6110-T6 aluminium alloy shows cyclic softening during stress controlled fatigue tests at room and elevated temperatures. Below a certain stress amplitude at a given temperature, deep rolling can enhance the fatigue lifetime of AA6110-T6 as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM values. From the s/n data of deep rolled and polished AA6110-T6, an effective boundary line for the deep rolling treatment in a stress amplitude-temperature diagram can be established.