Severe Shot Peening Research Articles

Effect of shot peening coverage on hydrogen embrittlement of a ferrite-pearlite steel

Effect of a wide range of shot peening (SP) coverage on hydrogen diffusion and hydrogen embrittlement (HE) of a steel consisting of ferrite and pearlite was investigated. Conventional SP (CSP) reduces diffusivity and HE because of increase of dislocation trapping sites. Although severe SP (SSP), which uses higher coverages than CSP, increases the area of ferrite grain boundaries as a result of grain refinement, it cannot suppress further the diffusion and HE, as it cannot induce a further evident increase in strong hydrogen trapping sites such as dislocations. However, when coverage is increased to so high that causes the breaking and refinement of pearlite phase, diffusion and HE is suppressed further, because the dispersed distribution of ultrafine pearlite particles within ferrite matrix provides more strong interface trapping sites. SP-induced change of trapping sites plays a more critical role in HE and diffusion, in comparison with induced residual compressive stresses.

Experimental Analysis on the Material Properties of A356.0 Aluminum Alloy Surface Nanostructured by Severe Shot Peening

The effects of severe shot-peening process and formation of a nanostructured surface layer on mechanical properties of A356.0 alloy were investigated in this paper. X-ray diffraction analyses revealed that the average size of near-surface grains in severe shot-peened specimens is 75.8 nm. Three types of disk-shaped specimens, non-treated, conventionally shot-peened, and severely shot-peened were subjected to pin-on-disk wear test in the dry condition, in different loading and sliding speeds. Shot-peening process increases both hardness and roughness of the surface, and these two factors have, respectively, positive and negative effects on wear resistance. However, because of high-density grain boundaries in severe shot-peened specimens, the effect of increased hardness overcomes the effect of increased roughness. As a result, the wear resistance of severely shot-peened specimens is higher than the others. Wear tracks were characterized by scanning electron microscopy. By changing the normal load from 5 to 10 N, the wear regime alters from mild to severe. Morphology of the specimens illustrated that the predominant wear mechanism in the mild regime and the severe regime are, respectively, oxidation and abrasive. Experimental results also showed that severe shot-peening procedure improves hardness, yield strength, and fracture toughness up to 52, 16, and 49%, respectively.

Effect of gradient microstructures on strengthening and toughening of AZ31

A designed gradient microstructure has the potential to enhance mechanical performance of materials. Severe shot peening was used to develop gradient microstructure in twin-roll cast AZ31. The resulting microstructure has four layers including recrystallized ultra-fine grains, a transient layer, deformed coarse grains containing a high fraction of tension twins, and the as-received structure in the metal interior (coarse grains). Each layer has a distinct feature to control yield/ultimate strength and ductility. Hence, various combinations of them lead to a unique performance. This study establishes a relationship between the microstructure, the hardening mechanism, and the tensile behavior of the gradient structure. A method was established to estimate the yield strength of the designed structure based on each layer hardness via a modified rule of mixtures. Tensile and microhardness measurements along with microstructures quantified by electron backscatter diffraction and optical microscopy were utilized to identify strengthening and toughening mechanisms. The optimized structure had a remarkable enhancement in both yield and ultimate tensile strength.

Influence of severe double shot peening on microstructure properties of Ti 6Al-4V and Titanium Grade 2 dissimilar joints using laser beam welding

The present investigation deals with the effect of Severe Double Shot Peening (SDSP) of Peening pre-weldment and Post peening weldment on microstructure properties of Ti 6Al-4V and Titanium Grade 2 dissimilar joint made by laser beam welding. Laser beam welding was performed for butt joint configuration. Full depth of penetration was obtained for the laser beam of 1200 W, welding speeds of 250 mm/min and focus distance of 8 mm configuration. Microstructure studies at weld zone and heat affected zone revealed the presence of more amounts of α martensite and B phases. Tensile study displayed that all the failures occurred at the weld zone in both Peening pre-weldment and Post peening weldment of SDSP. X-rays Diffraction analysis showed the slight presence of residual stress in the Post peening weldment of SDSP. Average hardness value of both dissimilar material reached 285 HV. While comparing the hardness at heat affected zone and welded zone, the hardness of welded zone gradually decreased owing to the failure of conversion of α phase into β1/β2 phase. SEM image exhibited the depth of Peening pre-weldment and Post peening weldment weldments of 8 µm and 10 µm respectively which led to the enhancement of hardness value. Further, the coarse grain boundaries, O-phases and α/α′ phases were observed in the heat affected zone. Fracture morphology of tensile samples revealed the inner minor cracks and voids at fusion zone. Moreover, the formation of Cr2N and CrN was not observed in both welded and heat affected zone that improved the better tensile and bending strength.

Surface layer nanocrystallization of carbon steels subjected to severe shot peening: Analysis and optimization

Severe shot peening (SSP) process is widely used for surface nanocrysallization of a bulk material that demonstrates excellent mechanical properties compared with its coarse-grained equivalents. In this study, a plastically deformed surface was produced with nanostructured grains on different materials of AISI 1045, 1050, and 1060 carbon steels by means of SSP. Shot peening was applied with a wide range of Almen intensities and coverages. Optical microscopy, scanning electron microscopy, field emission scanning electron microscopy, high resolution transmission electron microscope observations, and X-ray diffraction analysis were employed to analyze the mechanism of grain refinement experimentally as well as the surface roughness and residual stress measurements. Afterwards, different shot peening treatments were used to develop a novel alternative approach based on artificial neural network (ANN) for modelling as well as parametric and sensitivity analysis of grain refinement and surface roughness. The experimental results were utilized to implement the ANN. The modelling results indicated that the neural network-based approach can be used to effectively analyze nanocrystallization and roughness variations of the shot peened carbon steels.

Investigation of nanostructured surface layer of severe shot peened AISI 1045 steel via response surface methodology

Shot peening (SP) and also severe shot peening (SSP) provide high level compressive residual stress on a certain thickness just beneath the surface. By exposing severe plastic deformation (SPD) via SSP, the nanocrystallization is formed without any chemical alteration and the structure is to be hardened by fully mechanized process. The difference among SP, SSP and repeening (RP) is only related with the selection of the input parameters. Most of the input parameters combination constructs the Almen intensity which is the most powerful condition to be made the decision on the final shot peening of real parts. AISI 1045 medium carbon steel is selected for the optimization of input parameters (shot size, peening duration and air pressure at constant coverage-100%) by investigating the output responses (residual stress, hardness) using response surface methodology. Optimum SP parameters are introduced by response optimizer and the model is verified by the confirmation tests.

Effect of severe shot peening on the fatigue life of the laser-cladded Inconel 718 specimens

This paper presents the influence of the severe shot peening process on the fatigue life of the laser-cladded Inconel 718 specimens which can be employed during refurbishment of components enduring high mechanical cyclic loads such as gas turbine components. In order to quantitatively evaluate the destructive effect of laser cladding on the fatigue endurance, fatigue tests are first performed on the as-received and laser-cladded specimens. Then, microstructural analysis by scanning electron microscope (SEM) is carried out to identify the root causes of the drawback of laser cladding. Moreover, by employing a comprehensive laser-cladding FE analysis considering cyclic plastic material modeling, the induced residual stress distribution is determined. To decrease the detrimental effects of laser cladding on the microstructure and residual stresses, the specimens are next subjected to severe shot peening treatment, and then, extra sets of fatigue tests are conducted on the specimens. The conducted experiments show that the laser cladding can cause the deterioration of the fatigue life of the specimens at about 40%, and the damaged fatigue life can be almost restored by the severe shot peening. The experiments also indicate that the clad-toe zone is the weakest area concerning fatigue issues in laser-cladded specimens with or without shot peening post-process. Hence, the clad-toe zone needs some special attention.

Obtaining tailored surface characteristics by combining shot peening and electropolishing on 316L stainless steel

Abstract Shot peening is a well-established surface treatment commonly used to improve mechanical properties of material's surfaces. Studies have shown notable enhancement of fatigue strength, wear, scratch and corrosion resistance and fretting properties as a result of an intensive shot peening treatment, the so-called severe shot peening. Nevertheless, this process has some drawbacks including quite high surface roughness and possible embedment of shot residuals at the treated surface, which can have adverse effects depending on the final application. In this paper, electropolishing has been evaluated as a cleaning post-treatment for shot peened surfaces of 316L stainless steel. For this purpose, electropolishing voltage has been varied with the aim of obtaining clean and smoother surfaces without sacrificing the mechanical improvements produced by shot peening. The treated surfaces have been characterized considering surface roughness, morphology, wettability, residual stresses and microhardness evolution after various shot peening treatments combined with electropolishing using different parameter set-ups to identify the most promising combinations.

Accelerated biodegradation and improved mechanical performance of pure iron through surface grain refinement

Pure iron and its biocompatible and biodegradable alloys have a high potential to be used for temporary load bearing medical implants. Nevertheless, the formation of passive iron oxide and hydroxide layers, which lead to a considerably low degradation rate at the physiological environment, has highly restricted their application. Herein we used numerical and experimental methods to evaluate the effect of severe shot peening, as a scalable mechanical surface treatment, on adjusting the performance of pure iron for biomedical applications. The developed numerical model was used to identify the range of peening parameters that would promote grain refinement on the pure iron surface. Experimental tests were then performed to analyze the gradient structure and the characteristics of the interface free surface layer created on peened samples. The results indicated that severe shot peening could notably increase the surface roughness and wettability, induce remarkable surface deformation and grain refinement, enhance surface hardness and generate high in-depth compressive residual stresses. The increased surface roughness besides the high concentration of micro cracks and dislocation density in the grain refined top layer promoted pure iron’s degradation in the biologically simulated environment. Statement of SignificanceBiodegradable metallic materials with resorbable degradation products have a high potential to be used for temporary implants such as screws, pins, staples, etc. They can eliminate the need for implant retrieval surgery after the damaged tissue is healed, and result in reduced patient suffering besides lowered hospitalization costs.Pure iron is biodegradable and is an essential nutrient in human body; however, its application as biomedical implant is highly restricted by its slow degradation rate in physiological environment. We applied a scalable surface treatment able to induce grain refinement and increase surface roughness. This treatment enhances mechanical performance of pure iron and accelerates its degradation rate, paving the way for its broader applications for biomedical implants.

Efficiency Analysis of Shot Peening Parameters on Variations of Hardness, Grain Size and Residual Stress via Taguchi Approach

In the present study, the main purpose is to detect the most key factors of shot peening (SP) process on the microhardness, grain size, and residual stress of AISI 1060 high carbon steel. The specimens were treated using various types of SP process namely conventional shot peening and severe shot peening. Several experiments were performed to study metallurgical and mechanical properties of AISI 1060 steel. Almen intensity and surface coverage from one side and microhardness, grain size, and residual stress from the other side were considered as input and output parameters for the design of experiment methodology, respectively. The $$L_{18} (2^{1} \;and\;3^{1} )$$ mixed level of Taguchi orthogonal array design was used to study all cases. The test results were investigated by signal-to-noise ratio formula. It was identified that the surface coverage is the most key factors for shot peening process considering the affected depth. Also, the effect of this parameter on the microhardness, grain size and residual stress was obtained approximately 68, 89 and 57%, respectively. Eventually, the results obtained from all Taguchi sensitivity analysis indicated that it would be better to adjust the surface coverage factor in comparison with Almen intensity factor in order to create the surface compressive residual stress on the material and consequently to increase the fatigue lifetime of component using shot peening treatment.

Effects of severe plastic deformation on pre-osteoblast cell behavior and proliferation on AISI 304 and Ti-6Al-4V metallic substrates

In this study, titanium alloy (Ti-6Al-4V) and austenitic stainless steel (AISI 304) biomedical alloys were subjected to surface severe plastic deformation (SSPD) via severe shot peening (SSP) with the conditions of A28-30 Almen intensity. SSP is widely accepted as much more effective than the conventional surface modification techniques since it forms a nano-grain layer with large number of dislocations and grain boundaries. The SSP treatment in this study was led to a very thin rough layer in Ti-6Al-4V titanium alloy compared to that of AISI 304. The thicker layer of AISI 304 was created by twin-twin intersections and a martensite structure transformations. SSP treatment was resulted in a severe plastically deformed material surface and led to an increase in full width half maximum (FWHM) that expresses grain size reduction below nanometer regime. Also, SSP treatment increased the surface roughness of materials by forming pits and bumps and led to deterioration on surface topography. Besides physical and microstructural innovations, SSP treatment was improved the surface mechanical properties of the metals. On the other hand, it was noted that the effects of the alteration of surface topography and structural innovations (nano-grain crystal-deformation induced layer) were led to improved cellular behavior and increased cell proliferation regardless of material type.

Plasma nitriding of gradient structured AISI 304 at low temperature: Shot peening as a catalyst treatment

This study involves the capability of severe shot peening (SSP) as a catalyst intake for plasma nitriding process and the probability of reducing the requirement of thermal energy individually on the diffusion of interstitial atoms. To this end, combination of mechanical-thermal energy is run with pure thermal plasma assisted energy. Therefore, SSP is exposed to AISI 304 austenitic stainless steel as a former treatment and gradient structured surface (nanograined zone, ultrafine grain martensite-twin intersections zone and twin densed zone) is created. Then, plasma nitriding at 4000C-4h and 4750C-2h temperature-duration conditions. The condition of 4750C-2h provides the requirements of diffusion individually however former treatment SSP increases the diffusion layer effectively. In contrast, 4000C-4h condition is not adequate for diffusing atoms and white layer formation on the surface. The support of SSP as a mechanical energy triggers the process and white layer and the interstitial atoms diffusion take place influentially.

Non-Destructive Evaluation of Steel Surfaces after Severe Plastic Deformation via the Barkhausen Noise Technique

This paper reports about the non-destructive evaluation of surfaces after severe shot peening via the Barkhausen noise technique. Residuals stresses and the corresponding Almen intensity, as well as microstructure alterations, are correlated with the Barkhausen noise signal and its extracted features. It was found that residual stresses as well as the Barkhausen noise exhibit a valuable anisotropy. For this reason, the relationship between the Barkhausen noise and stress state is more complicated. On the other hand, the near-the-surface layer exhibits a remarkable deformation induced softening, expressed in terms of the microhardness and the corresponding crystalline size. Such an effect explains the progressive increase of the Barkhausen noise emission along with the shot-peening time. Therefore, the Barkhausen noise can be considered as a promising technique capable of distinguishing between the variable regimes of severe shoot peening.

Effects of shot peening parameters on gradient microstructure and mechanical properties of TRC AZ31

Microstructural variation along the depth of twin-roll cast sheet AZ31 Mg alloy was achieved by applying severe plastic deformation via intensified shot peening on the surface of the work-piece. Mechanical incompatibility between different layers from the ultra-fine crystallized structure on the peened surface to the un-deformed layers on the specimen interior introduces enhanced mechanical properties. AZ31 has shown distinct sensitivities to various parameters of the shot peening procedure such as pressure, shot size, and processing time. Thus, shot peening with shot sizes from less than 0.4 mm (conventional) to 3.18 mm for different processing times and under various pressures were performed to characterize their roles in controlling the microstructure and mechanical properties. Microstructural analysis revealed a direct relation between the thickness of the ultra-fine grained layer and severe shot peening parameters whereas each of them has a distinct effect on grain size. Electron backscatter diffraction data were used to define three distinct deformed layers wherein grain size and texture were changed during the process. Furthermore, microhardness tests demonstrated how pressure and shot size control fine grains at the surface. Tensile test results revealed that the best mechanical properties were obtained by maximum shot size and pressure at minimum processing time. Adding post-annealing treatment to the severely shot peened specimens enhanced ductility without a noticeable loss of ultimate strength. Consequently, this process has improved strength and ductility simultaneously.

Effect of severe shot peening on corrosion behavior of AZ31 and AZ91 magnesium alloys

Severe shot peening (SP) was performed on the AZ31 and AZ91 Mg alloys to obtain a nanostructured surface layer. The microstructure, micro-hardness and corrosion behavior of AZ31 and AZ91 Mg alloys after SP treatment were studied using transmission electron microscopy, micro-hardness tester and corrosion tests, respectively. Results showed that SP treatment produced a plastic deformation layer with α-Mg grains at the nanometer scale in the surface of AZ31 and AZ91 Mg alloys. After SP treatment, the micro-hardness in the near surface region displayed an obvious increase for two alloys, owing to the combination of the grain refinement and the work-hardening. The corrosion resistance of AZ31 alloy was improved by SP treatment, because a relatively compact passive film formed rapidly on the nanostructured surface. Unfortunately, SP treatment played a minor role in enhancing the corrosion resistance of AZ91 alloy, because it had little effect on size and distribution of β-phases.

Numerical study of grain refinement induced by severe shot peening

Three-dimensional finite element models of single shot impact and multiple shot impacts are developed to study grain refinement induced by severe shot peening. A multiscale constitutive model coupling the microscopic dislocation density and the macroscopic plasticity theory is proposed and implemented into the finite element models to characterize the formation and evolution of dislocation cell structure. The random distribution of the indentations under probability control is achieved by parametric modeling to simulate multiple shot impacts with respect to 100% peening coverage. Severe shot peening of AISI 4340 steel under various coverages is simulated and the cell sizes predicted by the finite element models are in good agreement with the experimental data. Parametric analysis of severe shot peening is carried out by using different size shots with the same kinetic energy. The numerical results indicate that: (1) the larger shot causes a larger depth of grain refinement, (2) the smaller shot is beneficial for grain refinement of the peened surface and subsurface, and (3) the shot oblique impact results in a finer dislocation cell structure. Moreover, the interactive effect of multiple shot impacts can effectively improve grain refinement of near surface materials.

Severe shot peening of AISI 321 with 1 000 % and 1 300 % coverages: A comparative study on the surface nanocrystallization, phase transformation, sub-surface microcracks, and microhardness

AbstractIn this study, AISI 321 austenitic stainless steel samples were surface treated using severe shot peening (SSP) with 1 000 % and 1 300 % coverages. Microstructural features including the grain size, phase transformation, and formation of sub-surface microcracks were investigated at the rough top surface and about 40 μm depth (top surface after grinding and removal of initial rough surface layer created by SSP). In addition, microhardness variations were thoroughly analyzed in-depth. Experimental results demonstrated that for both 1 000 % and 1 300 % coverages, the microstructures of top surface and 40 μm depth are respectively composed of equiaxed nano-grains and lamella-shaped cells; however, enhanced imparted strain in the case of 1 300 % coverage leads to the formation of considerable amounts of strain-induced martensite (α′) phase in the surface layers and consequently, due to strength increase and lack of deformability, some microcracks are created in the sub-surface layers (up to 20 μm depth).

Roles of surface coverage increase and re-peening on properties of AISI 1045 carbon steel in conventional and severe shot peening processes

Surface coverage as one of the most effective parameters of shot peening process has substantial influence on the functionality of the peened component. The aim of this study is to investigate the effects of surface coverage increase on the properties of treated specimens experimentally before and after re-shot peening. Different shot peening treatments from conventional to severe; were performed on the AISI 1045 steel. Microstructural observations and XRD measurements were applied to characterize the circumstance of microstructure changes. In order to investigate the mechanical properties, microhardness profiles were achieved, surface roughness was evaluated and the residual stresses distribution were measured using XRD method. The results indicate that severe shot peening processes that have high coverages are more effective techniques to obtain nanostructured surface layer and superior mechanical properties and moreover re-shot peening has influential effects to decrease the surface roughness.

A Study on the Microstructural Evolution of a Low Alloy Steel by Different Shot Peening Treatments

Recent studies have shown that severe shot peening can be categorized as a severe plastic deformation surface treatment that is able to strongly modify the microstructure of the surface layer of materials, by both increasing the dislocation density and introducing a large number of defects that define new grain boundaries and form ultrafine structure. In this work, conventional shot peening and severe shot peening treatments were applied to 39NiCrMo3 steel samples. The samples were characterized in terms of microstructure, surface roughness, microhardness, residual stresses, and surface work-hardening as a function of surface coverage. Particular attention was focused on the analysis of the microstructure to assess the evolution of grain size from the surface to the inner material to capture the gradient microstructure. Severe shot peening proved to cause a more remarkable improvement of the general mechanical characteristics compared to conventional shot peening; more significant improvement was associated with the microstructural alteration induced by the treatment. Our datas provide a detailed verification of the relationship between shot peening treatment parameters and the microstructure evolution from the treated surface to the core material.

Insights into formation of gradient nanostructured (GNS) layer and deformation induced martensite in AISI 316 stainless steel subjected to severe shot peening

Severe peening is a well-accepted top-down approach to engender surface nanocrystallization in austenitic stainless steels. In the present study, AISI 316 grade austenitic stainless steel is subjected to severe peening through air blast shot peening technique. Study is aimed at analyzing the microstructural features of the peened layer and deformation induced martensite through transmission electron microscopy technique. Gradient nanostructured (GNS) layer formed as a result of high strain rate, multi-directional deformation during severe peening found to extend to about 500 μm from the surface. Nucleation of deformation induced martensite is not limited to shear band intersections as affirmed by the published literature related to severe peening. It is observed to nucleate at multiple locations in the austenite matrix. Martensite units thus formed, coalesce with each other to form continuous layer of lath martensite layer at about 15–20 μm from the surface. Upon further deformation, lath morphology transforms to dislocation cell-type; resulting in fine martensite crystallites at the topmost layer of the peened surface.