Shot-peened Material Research Articles

High-resolution digital image correlation study of the strain localization during loading of a shot-peened RR1000 nickel-based superalloy

The improved crack nucleation resistance of shot-peened materials has been usually studied from the point of view of the residual stresses. However, experimental limitations have made difficult to quantify strain localization during further deformation. In this study, we have used high-resolution digital image correlation (HRDIC) and electron backscatter diffraction (EBSD) in a large area covering shot-peened material and bulk to study quantitatively the strain distribution at the submicron scale. We found that, at the earlier deformation steps (e < 2.5%), the slip intensity and distribution at the shot-peened region differ from that observed at the bulk. In the former, well-spaced, sharp and homogeneously distributed slip bands are observed. On the other hand, in the shot-peened region, slip is less evident, and a more homogeneous diffuse deformation is observed along the majority of the grains. However, this region also contains a small number of slip bands that accumulate high strains, mainly along Σ3 annealing twin boundaries. Additionally, the border between both regions, at a depth with no residual stress, present the minimum strain values. During further deformation (e > 2.5%), the strain distributions become similar along the material simultaneously to the residual stresses relaxation. A further analysis of the HRDIC maps allowed for the first time to quantify strain heterogeneity. This study reveals the importance of the local stress state and the strain history on the strain localization during loading of peened materials.

Influence of Shot Peening on the Isothermal Fatigue Behavior of the Gamma Titanium Aluminide Ti-48Al-2Cr-2Nb at 750 °C

One possibility to improve the fatigue life and strength of metallic materials is shot peening. However, at elevated temperatures, the induced residual stresses may relax. To investigate the influence of shot peening on high-temperature fatigue behavior, isothermal fatigue tests were conducted on shot-peened and untreated samples of gamma TiAl 48-2-2 at 750 °C in air. The shot-peened material was characterized using EBSD, microhardness, and residual stress analyses. Shot peening leads to a significant increase in surface hardness and high compressive residual stresses near the surface. Both effects may have a positive influence on lifetime. However, it also leads to surface notches and tensile residual stresses in the bulk material with a negative impact on cyclic lifetime. During fully reversed uniaxial tension-compression fatigue tests (R = −1) at a stress amplitude of 260 MPa, the positive effects dominate, and the fatigue lifetime increases. At a lower stress amplitude of 230 MPa, the negative effect of internal tensile residual stresses dominates, and the lifetime decreases. Shot peening leads to a transition from surface to volume crack initiation if the surface is not damaged by the shots.

Quantitative Representation of Mechanical Behavior of the Surface Hardening Layer in Shot-Peened Nickel-Based Superalloy.

Surface hardening treatment can usually introduce severe grain distortion with a large gradient in the surface layer. It results in mechanical properties being difficult to accurately determine through macroscopic tests due to the non-uniformity of the shot-peened material. In this study, the mechanical behavior of uniformly pre-deformed nickel-based superalloy IN718 was investigated with monotonic tensile tests and instrumented indentation tests. For the shot-peened material, the hardness distribution of the surface hardening layer after shot peening was identified through the instrumented indentation method. According to the stress–strain results of pre-deformed materials, Ramberg–Osgood model parameters could be presented with plastic deformation. Assuming the power-law relationship between hardness and plastic deformation, the plastic deformation distribution along the depth of the surface hardening layer was clarified. Based on the results, a method to identify the stress–strain relationships of hardened material at different depths was established. Finally, the finite-element simulations of the instrumented indentation test considered residual stress and strain hardening were built to verify the method presented herein. The results show that the solution to evaluate the mechanical properties of hardening layer materials in the microscopic zone is feasible, which can provide the foundation for the failure analysis of shot-peened materials.

Influence of the surface roughness on the fatigue properties in ausferritic ductile irons (ADI)

Heat treatment of cast ductile iron (DI) to ausferritic ductile iron (ADI) is known to increase fatigue properties. However, the surface roughness of the cast material is also of significant importance. In this investigation, test rods with seven different surface qualities were cast from the same melt i.e. with same chemical composition. The surfaces of the test rods were varied by a number of parameters; grain size of the moulding sand, coated or non-coated mould surfaces, as-cast or machined and polished, shot peened or not. In addition, a reference material in conventional DI was cast and tested. All eight series were subjected to high-cycle fatigue bending tests. The results show that surface defects, such as micro porosity and minor inclusions drastically decrease the fatigue properties. For some ADI materials the stress amplitude limit was actually lower compared to the non-heat treated DI. The machined, polished and shot-peened material demonstrated the best fatigue properties, which is as expected.

The effect of shot peening on the microstructure and properties of a near-alpha titanium alloy following high temperature exposure

The effect of shot peening on the microstructure, oxygen ingress and high-cycle fatigue properties of a near-α titanium alloy is investigated. Microstructural characterization of shot-peened material is performed through light microscopy, scanning electron microscopy and electron backscatter diffraction. Oxygen uptake during thermal exposure is analytically determined using secondary ion mass spectrometry. Shot peening leads to an increase in subsurface oxygen content following thermal exposure in air and the increased levels of oxygen content are accompanied by the precipitation of silicides along (or in close proximity to) the mechanical twin boundaries and in regions of high dislocation density. Preliminary fatigue data obtained through a staircase loading method show that a decrease in room temperature fatigue performance occurs in shot-peened material following prolonged thermal exposure in air at 650°C. The observed phenomena suggest that shot peening may be deleterious to the mechanical integrity of titanium alloy components that are subject to high temperature exposure in oxidizing environments.

ショットピーニング特性に及ぼす機械的性質と残留オーステナイトの影響

Shot peening is used for the material which are high carbon steel such as carburized steel and spring steel as a surface enhance method to increase fatigue strength. The compressive residual stress and work hardening induced by shot peening are important shot peening characteristics to increase fatigue strength. The compressive residual stress and work hardening induced by shot peening are strongly influenced by material property such as mechanical property and retained austenite, and to investigate influence of material property on shot peening characteristics is very important. In this study, we carried out shot peening to chrome steel and chrome molybdenum steel which changed retained austenite volume and mechanical property, and we investigated relationship between shot peening characteristics and material property. As a result, we gained the following conclusions.1) It was found that materials with a higher rate of change of retained austenite content are more easily hardened by shot-peening because of a larger amount of deformation-induced martensite and work hardening.2) When there was almost no change in retained austenite content, the maximum compressive residual stress was about 60% of the 0.2% proof stress. A higher rate of change of retained austenite content tended to result in increased maximum compressive residual stress compared to the 0.2% proof stress. This is probably due to the increased 0.2% proof stress of the shot-peened material.

Prediction of fatigue crack growth and residual stress relaxations in shot-peened material

The fatigue crack growth, incorporating effects of shot peening, is investigated using two-dimensional finite element analysis. The cohesive zone model, which takes into account the damage evolution under cyclic load, is adopted in the finite element analysis to simulate the potential fracture surfaces. The effects of shot peening on the fatigue crack growth and the relaxation of residual stress under cyclic load are studied. It is shown that the residual stress retards the fatigue crack propagation and the retardation effect depends not only on the shot peening intensity but also on the cyclic load amplitude. The residual stress relaxes nonlinearly as the load cycles increase.

OS0711 微粒子衝突処理を施した純チタンの表面組織および疲労特性(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

This study was conducted to investigate influence of Fine-Particle Bombarding (FPB) on the microstructure near the surface and fatigue properties of pure titanium. For comparison, a shot-peened material was used. The results showed that surface hardness and compressive residual stress of the FPBed material were higher than those of the shot-peened material. Furthermore, its microstructure near the surface was greatly fined through dynamic recrystallization and nano-crystallized. As a result, the fatigue strength of the FPBed material was improved more than that of the shot-peened material because propagation of intemal cracks to the surface was effectively suppressed by the formed layer.

Surface grain boundary engineering of shot-peened type 304 stainless steel

The effect of thermal annealing on shot-peened Type 304 stainless steel has been examined using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The objective was to evaluate the potential for surface property control by grain boundary engineering. The near surface microstructure of shot-peened material showed a gradual change of the grain boundary character distribution with depth. Twin (Σ3) and higher order twin grain boundaries (Σ9, Σ27) identified closer to the shot-peened surface had significant deviations from their optimum misorientation. The subsequent application of annealing treatments caused depth-dependent changes of the near surface microstructure, with variations in grain size, low Σ CSL grain boundary populations and their deviation from optimum misorientation. Microstructure developments were dependent on the applied heat treatment, with the near surface microstructures showing similarities to microstructures obtained through bulk thermo-mechanical processing. Shot peening, followed by annealing, may therefore be used to control the near surface microstructure of components.

Experimental Analysis on the Fatigue Threshold Behaviour of a Nitrided and Shot Peened Steel

The effectiveness of the application of shot peening after nitirding for improving the fatigue strength of high-performance steel components has not been adequately treated in literature, and it is not clear if shot peening can really and drastically increase mechanical performances of nitirded elements. In this paper an approach to assess the influence of shot peening on nitrided components is presented. It is based on fracture mechanics concepts. Smooth specimens with a micro-hole acting as a pre-crack were fatigue tested. The values of micro-hardness and of the residual stresses were measured and related to the experimental results. By elaborating the results it was possible to determine the threshold values of the stress intensity factors of the nitrided and shot peened material. A formula to predict the value of Kth of shot peened and nitrided steels is proposed.

On the Fatigue Crack Growth Behavior of a Nitrided and Shot Peened Steel

In this paper the fatigue crack growth properties of a nitrided shot-peened steel is dealt with: different peening intensities were considered and the resulting residual stresses measured by means of an X-ray diffractometer. Rotating bending fatigue tests were executed on specimens with a blind micro-hole, acting as a pre-existent crack. The results allowed determining the threshold of propagation of the nitrided and of nitrided and shot peened material. It was also possible to relate the crack initiation point with the applied and the residual stresses.

Effect of Chemical Composition to &lt;I&gt;φ&lt;/I&gt; splitting in 2&lt;I&gt;θ&lt;/I&gt;-sin&lt;I&gt;φ&lt;/I&gt;&lt;sup&gt;2&lt;/sup&gt; Diagram on Residual Stress Measurement of Shot Peening material

High durability is required for automotive engine valve springs. To ensure the performance of springs, it is necessary to evaluate and measure the residual stress distribution on surface and inner direction of the spring. Theoretically, it is used to be treated that the residual stress of a shot-peened material surface is uniform. However, the shape of a coil spring causes various peening angles corresponding to the surface location, so the uneven shot peening angles may leads to a non-uniform residual stress on the coil spring surface. On the other hand, an analysis method of non-liner 2θ-sin2 φdiagram (or φ-splitting analysis) is well known to directionally deformed material such as ground, planed or rolled materials. Recently, an φ-splitting analysis has been reported for the shot-peened materials. In this study, the residual stress distribution of slant shot-peened spring material was measured and, φ-splitting (non-linear 2θ-sin2 φdiagram) was studied in the view of chemical compositions and microstructures. As a results, the extent of φ-splitting was mainly affected by the distribution of carbide particles, not the kind of carbide particles itself.

ショットピーニング加工を施した（α＋γ）二相ステンレス鋼の疲労強度に及ぼす影響因子に関する研究

A shot peening process is generally utilized as one of the surface treatment processes to improve the fatigue life and the fatigue strength of cyclically loaded components. This improvement is achieved by inducing compressive residual stresses and work hardening effects in areas close to the surface. But the influence of the shot peening process on fatigue strength is not clarified enough. It's significant technologically and industrially to clarify the influence factor of shot peening on the fatigue strength. Former study of author confirmed that main effect on the fatigue strength of the SUS304 steel hitted by controlled steel ball is the hardening of austenite structure itself due to the shot peening. In this study, the SUS329J1 steel having both austenite and ferrite phases was also investigated from the viewpoint of the work hardening of both structures. An effect of the residual stress on fatigue limit is not observed at all. As a result, we found that the work hardening of the SUS329J1 steel by shot peening was attributed to both the strain-induced martensitic transformation and the hardening of austenite and ferrite phase themselves. Especially, the ferrite phase influences the Vickers hardness distribution of shot-peened material, and the hardness increment of the austenite phase is larger than other phase and mainly due to the fine structure.

The Structural Integrity of Cast Aluminium Automotive Components Subjected to Fatigue Loads

In moves towards lighter, more energy-efficient vehicles, the automotive industry is showing interest in space-frame aluminium structures for car bodies. One concept design features the use of hollow extrusions which slot over suitably shaped cast nodes, which will be subjected in service to bending stresses and to shear stresses. Given the presence of bending stresses and the sensitivity of fatigue life to defects, particular attention has been paid in the present research to the surface defects produced by casting and to effects of polishing or shot-peening the as-cast surface. The alloy studied is LM25 TF and two main casting processes have been employed: sand-casting and gravity-die-casting. For sand-castings, the defects of significance were found to be surface hollows and pores, but fatigue cracks in the die-cast material were associated mainly with entrained aluminium oxide films. Surface machining removes the surface hollows but leaves pores and films unaffected, The fatigue S-N curves can be derived from a consideration of fatigue-crack growth-rate data and distributions of initial defect size, with an allowance for an initiation contribution. The calculations are made on a probabilistic basis. The paper also discusses results obtained on shot-peened material, the residual stresses generated, and ways in which these may be incorporated into crack-growth models.

Analyse de la relaxation des contraintes résiduelles de grenaillage sous une sollicitation de flexion plane répétée

Special attention is paid in this paper to investigate the stress release behaviour of residual stresses introduced by shot peening.The testpieces are sheets of SAE 4135 steel. After shot peening, fluctuating 4-point bending tests are carried out at ranges of alternating stress amplitudes from 225 to 375 MPa. X-Ray dijfractometry techniques are applied to residual stresses and line broadening analysis. About 40 in depth residual stress profiles and in depth FWHM profiles are presented.Detailed analysis of the obtained data can provide new information on the characteristics of the residual stress release and on the properties of the shot-peened material.