Showing papers on "Peening published in 2008"

Characterization of ultrasonically peened and laser-shock peened surface layers of AISI 321 stainless steel

Abstract: The effects of ultrasonic impact peening (UIP) and laser-shock peening (LSP) without protective and confining media on microstructure, phase composition, microhardness and residual stresses in near-surface layers of an austenitic stainless steel AISI 321 are studied. An X-ray diffraction analysis shows both significant lines broadening and formation of strain-induced e- and α-martensite after UIP with additional peaks found near austenite ones in the low-angle part after LSP supposedly due to formation of a dislocation-cell structure in the surface layer. TEM studies demonstrate that a nano-grain structure containing either only austenitic grains with e-martensite (at strains up to 0.42) or both austenite and α-martensite grains (at higher strains) can form in the surface layer after UIP. Highly tangled and dense dislocation arrangements and even cell structures in fully austenitic grains are revealed both at the surface after LSP and in the layer at a depth of 80 μm after UIP. UIP is found to produce a sub-surface layer 10 times thicker and about 1.4 times harder than that formed by LSP. A mechanism of formation of the dislocation-cell structure in such steels (with a low stacking fault energy) is discussed. A nucleation process of α-martensite is discussed with respect to strain, strain rate, local heating and mechanical energy accumulated/applied to the surface layer under conditions of UIP and the LSP and compared to literature data for different loading schemes.

About the role of residual stresses and surface work hardening on fatigue ΔKth of a nitrided and shot peened low-alloy steel

Abstract: Nitriding and shot peening are surface treatments widely used to improve the fatigue strength of mechanical and structural components. Both treatments enhance the mechanical properties of the surface layer of material: nitriding mainly by means of chemical transformations and formation of a very hard case, shot peening mainly due to compressive residual stress field in the sub-surface layer of material. The combined application of nitriding and shot peening has not been adequately investigated in literature and the known data do not allow to define a general criterion to assess if and when shot peening application after nitriding can be useful. In this paper the effect of nitriding plus shot peening on fatigue strength of a low-alloy steel is investigated by means of experimental tests carried out on specimens containing a micro-hole, acting as a pre-crack. To analyse the role played by shot peening induced residual stresses, a series of specimens was heat-treated after shot peening, being the aim the partial removal of residual stresses without strongly modifying the mechanical properties of the surface layer of material. After a critical discussion of the results in terms of residual stress, micro-hardness trend and fatigue strength, an original fracture mechanics based approach is proposed to predict the threshold value of the stress intensity factor of nitrided and shot peened steels. The results are in good agreement with the experimental evidence.

Effects of peening on mechanical properties in friction stir welded 2195 aluminum alloy joints

Abstract: The effects of surface treatment techniques like laser and shot peening on the mechanical properties were investigated for friction stir welded 2195 aluminum alloy joints. The loading in the tensile specimens was applied in a direction perpendicular to the weld direction. The peening effects on the local mechanical properties through the different regions of the weld were characterized using a digital image correlation technique assuming an iso-stress condition. This assumption implies that the stress is uniform over the cross-section and is equal to the average stress. The surface strain and average stress were used giving an average stress–strain curve over the region of interest. The extension of the iso-stress assumption to calculate local stress–strain curves in surface treated regions is a novel approach and will help to understand and improve the local behavior at various regions across the weld resulting in a sound welding process. The surface and through-thickness residual stresses were also assessed using the X-ray diffraction and the contour methods. The laser peened samples displayed approximately 60% increase in the yield strength of the material. In contrast, shot peening exhibited only modest improvement to the tensile properties when compared to the unpeened FSW specimens. The result that laser peening is superior to shot peening because of the depth of penetration is original since this superiority has not been presented before regarding mechanical properties performance.

The effects of laser peening on high-cycle fatigue in 7085-T7651 aluminum alloy

Abstract: This study reports on the high-cycle fatigue performance of 7085-T7651 aluminum treated with combinations of laser peening and anodization Stress-life fatigue data were generated in a high humidity environment using smooth (Kt = 1) specimens tested at a stress ratio R = 01 and a range of maximum stress Tests were performed in the as-machined (untreated), anodized, laser peened, and laser peened then anodized conditions The results show that laser-peened specimens exhibited significantly improved fatigue performance, with non-anodized specimens having a factor of 5–14 improvement in fatigue life (depending on stress level) and anodized specimens having a factor of 8–9 improvement in fatigue life Without laser peening, anodized specimens had reduced fatigue life at all stress levels With laser peening, anodized and non-anodized specimens had similar lives at high stress (life

In situ stress evolution during magnetron sputtering of transition metal nitride thin films

Abstract: Stress evolution during reactive magnetron sputtering of TiN, ZrN, and TiZrN layers was studied using real-time wafer curvature measurements. The presence of stress gradients is revealed, as the result of two kinetically competing stress generation mechanisms: atomic peening effect, inducing compressive stress, and void formation, leading to a tensile stress regime predominant at higher film thickness. No stress relaxation is detected during growth interrupt in both regimes. A change from compressive to tensile stress is evidenced with increasing film thickness, Ti content, sputtering pressure, and decreasing bias voltage.

Residual Stresses Distribution through Thick HVOF Sprayed Inconel 718 Coatings

Abstract: Residual stress buildup in thick thermal spray coatings is a property of concern. The adhesion of these coatings to the substrate is influenced by residual stresses that are generated during the coating deposition process. In the HVOF spray process, significant peening stresses are generated during the impact of semimolten particles on the substrate. The combination of these peening stresses together with quenching and thermal mismatch stresses that arise after deposition can be of significant importance. Both numerical method, i.e., Finite Element Method (FEM), and experimental methods, i.e., the Modified Layer Removal Method (MLRM) and Neutron Diffraction, to calculate peening and quenching stresses have been utilized in this work. The investigation was performed on thick Inconel 718 coatings on Inconel 718 substrates. Combined, these numerical and experimental techniques yield a deeper understanding of residual stress formation in the HVOF process and thus a tool for process optimization. The relationship between the stress state and deposit/substrate thickness ratio is given particular interest.

Improving the fatigue strength of the elements of a steel belt for CVT by cavitation shotless peening

Abstract: The elements of steel belts used for continuously variable transmission (CVT) are subjected to a bending load during operation. The weakest portion of the elements is at the root of the ‘‘neck’’ which works into metallic rings. In order to reduce the stress concentration, the root of the neck is rounded and the shape of element is optimized. Nevertheless, if the fatigue strength of the elements can be improved, the steel belt can be applied to larger engines. Although conventional shot peening is one way of enhancing the fatigue strength, it is very difficult for shot to reach into deep and narrow regions. Recently, a peening method using the impact produced as cavitation bubbles collapse has been developed [1–9]. This method is called ‘‘cavitation shotless peening (CSP)’’, as shot are not required [3–6, 8]. CSP can peen the surface even through deep narrow cavities, as the bubbles can reach these parts and collapse where peening is required. In the present article, improvement of the fatigue strength of the elements of a CVT metallic belt by CSP was demonstrated experimentally. Elements were treated with different processing times and evaluated by a fatigue test to find the optimum processing time. In order to evaluate the peening effect by CSP, the residual stress was measured. Note that this is the first report published on the improvement made in the fatigue strength of a part with regions that cannot be hit directly by shot. Cavitation shotless peening was applied to the element using cavitating jet apparatus, the details of which can be found in references [3–6, 8]. The jet was injected into the neck region through grooves in the elements, which were stacked and held together, and scanned perpendicularly over the elements, as shown in Fig. 1. The processing time per unit length, tp, is defined by the number of scans n and the scanning speed v; tp 1⁄4 n v ð1Þ

Residual-stress distribution in shot-peened metallic-glass plate

Abstract: A plate of bulk metallic glass Zr55Ti5Al10Cu20Ni10, shot-peened on both faces, has been studied by synchrotron X-ray diffraction in transmission. The ellipticity in the first diffraction halo changes throughout the plate thickness. The elastic strain in the plate interior is measured and the compressive stress profile in the peened surfaces is estimated. Structural anisotropy and dilatation induced by plastic deformation preclude direct measurement of the elastic strain in the peened layers. Plastic deformation induces softening, but changes in the elastic properties remain undetermined.

Effect of Fine Particle Peening Treatment prior to Nitriding on Fatigue Properties of AISI 4135 Steel

Abstract: In this study, a new hybrid surface modification process, fine particle peening (FPP) treatment prior to nitriding, was proposed. In order to clarify the effects of FPP treatment prior to nitriding on the fatigue strength of notched AISI 4135 steel with a stress concentration factor Kt of 2.36, fatigue tests were conducted at room temperature using a rotational bending fatigue testing machine. Hardness and residual stress distributions were measured in order to characterize the surface-modified layer. As a result, the surface hardness of the FPP-treated specimen before nitriding was higher than that of the nitrided specimen. The surface microstructures of treated specimens were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystal structure of the compound layer of the FPP-treated specimen before nitriding was different to that of the nitrided specimen. The compound layer of the FPP-treated specimen was dense. This suggests that FPP treatment prior to nitriding is very effective for improvement of the fatigue strength of steel.

Fatigue Assessment and Lefm Analysis of Cruciform Joints Fabricated with Different Welding Processes

Abstract: In this study fatigue testing and defect assessment were carried out on specimens welded with robotic and manual welding using flux cored (FCAW) and metal cored (MCAW) filler materials in order to study the effect of the welding method on the fatigue strength and weld quality. Thirteen different batches were investigated of which two was shot peened before fatigue testing. The local weld geometry was measured for all the specimens before testing. The specimens welded with flux cored weld wire showed the best fatigue strength, small defects and low residual stresses. Large scatter in the fatigue data is observed, especially when manual welding is employed. The few largest defects were removed by the shot peening process, although small defects survived. This led to a smaller scatter in fatigue live for the shot peened specimens. Linear elastic fracture mechanics, LEFM, was employed for analysis of the fatigue test results. The fatigue life predictions using a 2D LEFM FE-model for simulating a continuous cold lap defect along the weld toe showed a qualitative agreement with the fatigue test results. The 2D analysis showed that a continuous cold lap defect should be no more than 0.5 mm deep in order to comply with the requirement of fatigue lives for normal weld quality according to the IIW design rules. For larger defects (> 0.8 mm) an increased toe radius will have a small effect on the fatigue strength. A 3D LEFM analysis of crack growth from a spatter-induced cold lap defect was also carried out. This showed similar trends in crack growth compared to the 2D analysis of a continuous cold lap, although the spatter-induced cold lap defect (semi-elliptical) had a longer fatigue life (x2.7), and hence is less dangerous from a fatigue point of view.

Residual stress and microstructure of as-deposited and annealed, sputtered yttria-stabilized zirconia thin films

Abstract: The microstructure and residual stress of sputter-deposited yttria-stabilized zirconia (YSZ) films are presented as a function of thickness (5–1000 nm), deposition pressure (5–100 mTorr), and post-deposition temperature. The as-deposited residual stress of YSZ ranges from −1.4 GPa to 100 MPa with variations in sputtering conditions. Transitions from compressive to tensile stress are identified with variations in working pressure and film thickness. The origins and variations in as-deposited stress are determined to be from tensile stress due to grain coalescence/growth, and compressive stresses are due to forward sputtering/“atomic peening” of target atoms. The evolution of residual stress with post-deposition annealing shows a tensile stress hysteresis of up to 1 GPa for films deposited at low working pressures. This hysteresis is believed to be due to crystallization and the diffusive relief of compressive stresses initially generated by atomic peening during deposition. Discussion and evaluation of other common residual stress mechanisms are presented throughout.

Effect of shot peening parameters on microhardness of AISI 1045 and 316L material: an analysis using design of experiment

Abstract: Shot peening is widely used to improve the fatigue properties of components and structures. Residual stresses, surface roughness, and work hardening are the main beneficial effects induced in the surface layer from shot peening, which depend on the correct choice of the peening parameters. In this investigation, experiments were designed using the full factorial design of experiment (DOE) technique and an air blast type of shot peening machine. Effects of process parameters such as pressure, shot size, stand-off distance, and exposure time on surface microhardness for AISI 1045 and 316L materials were investigated. An ANOVA was carried out to identify the significant peening parameters. In the case of 316L material, the maximum surface hardness was found to be in the range of 450–824 Hv, whereas it was found to be in the range of 314–360 Hv for AISI 1045. A critical assessment was made so as to understand the variation of microhardness in the direction of peening. Empirical equations between the peening parameters and the surface microhardness for both materials were developed, which are useful in predicting the surface microhardness. It is believed that this technique could prove beneficial in industries for reduction of performance variation and cost and to increase productivity.

The Effects of Laser Peening and Shot Peening on Mechanical Properties in Friction Stir Welded 7075-T7351 Aluminum

Abstract: Peening techniques like laser peening and shot peening were used to modify the surface of friction stir welded 7075-T7351 Aluminum Alloy specimens. The tensile coupons were machined such as the loading was applied in a direction perpendicular to the weld direction. The peening effects on the global and local mechanical properties through the different regions of the weld were characterized and assessed. The surface hardness levels resulting from various peening techniques were also investigated for both sides of the welds. Shot peening resulted in an increase to surface hardness levels, but no improvement was noticed on the mechanical properties. In contrast, mechanical properties were improved by laser peening when compared to the unpeened material.

Plastic Deformation in Silicon Crystal Induced by Heat-Assisted Laser Shock Peening

Abstract: The response of solid to shock compression has been an interesting topic for more than a century. The present work is the first attempt to experimentally show that plastic deformation can be generated in brittle materials by a heat-assisted laser shock peening process, using silicon crystal as a sample material. Strong dislocation activity and large compressive residual stress are induced by this process. The dislocation structure is characterized with transmission electron microscopy and electron backscattered diffraction. The residual stress is measured using Raman scattering. This work presents a fundamental base for the application of laser shock peening in brittle materials to generate large compressive residual stress and plastic deformation for better mechanical properties, such as fatigue life and fracture toughness.

Effectiveness of shot peening in suppressing fatigue cracking at non-metallic inclusions in Udimet® 720

Abstract: The fatigue lives of modern powder metallurgy disk alloys can be reduced over an order of magnitude by cracking at inherent non-metallic inclusions. The objective of this work was to study the effectiveness of shot peening in suppressing LCF crack initiation and growth at surface non-metallic inclusions. Inclusions were carefully introduced at elevated levels during powder metallurgy processing of the nickel-base disk superalloy Udimet ® 720. Multiple strain-controlled fatigue tests were then performed on machined specimens with and without shot peened test sections at 427 and 650 °C. The low cycle fatigue lives and failure initiation sites varied as functions of inclusion content, shot peening, and fatigue conditions. A large majority of the failures in as-machined specimens with the introduced inclusions occurred at cracks initiating from inclusions intersecting the specimen surface. These inclusions reduced fatigue life by up to 100×, when compared to lives of material without inclusions residing at specimen surface. Large inclusions produced the greatest reductions in life for tests at low strain ranges and high strain ratios. Shot peening improved life in many cases by reducing the most severe effects of inclusions.

Peening Device for Peening Welds Inside Steel Submarine Pipes, Process for Producing Steel Submarine Pipes Using Such a Device, and Submarine Connection Pipe

Abstract: The present application describes a peening device useful for peening regions of annular welds (6) inside submarine pipes, comprising at least one peening tool (5), said device comprising at least one hammer actuated so as undergo reciprocating radial peening translations against the internal surface of the pipe (2i) and/or of said weld (6), in which said peening tool (5) comprises a single hammer, and said hammer is pivotally mounted so as to be able to vary its angle of inclination to the surface to be peened, and said device includes a carriage (3) capable of moving in axial longitudinal translation (XX) inside a pipe and supporting means (4a, 4b) for moving said peening tool (5) in longitudinal and radial translation and in rotation relative to said carriage (3); and said hammer cooperates with actuating means undergoing reciprocating radial peening translations, by the use of electromagnetic energy and reciprocating movement inside a solenoid coil preferably furthermore cooperating with a spring.