Papers by Keyword: Shot Peening

Abstract: The commercial pure Mo sheet was shot peened to increase high-temperature mechanical and thermal resistance. Shot peenining was conducted on the surface of cold-rolled Mo sheet with 0.4MPa of shot pressure. The hardness of bcc Mo sheet was increased with increase of shot peening time. Surface hardenss is gradually increased to 120s at the 0.4MPa pressure, but the profiles become almost flat at the prolonged time. The grains were deformed and work hardened in the surface layer. The surface roughness was also increased with peening time. The grain size of shot-peened Mo sheet was smaller than that of cold-rolled Mo sheet in the all recrystallization temperature range. The reason for this could be a larger density of nucleation sites caused by the higher surface deformation of shot-peened Mo sheet. Mo₂C carbide phase was analyzed on the surface of recrystallized Mo sheet at the secondary recrystallization temperature range. It was considered that molybdenum carbide was formed due to the evaporation of graphite heating element in the hot-zone furnace. From this study, shot peening of Mo sheet could be a good cold work hardening method to improve high-temperature mechanical and thermal resistance properties.

Abstract: The surface of 304 austenitic stainless steel sheet specimen was processed by three different shot peening pressure. The thickness of grain refinement layer and refined degree were characterized by SEM, and surface hardness was tested by microhardness tester. The stress corrosion susceptibility of the specimens treated by 0.25,0.30 and 0.40 MPa shot peening pressure was compared through the slow strain rate tensile tests in 5% sodium chloride solution. The results showed that he grain refinement was caused by shot peening treatment in the surface layer and hardness increased; with the rise of shot peening pressure, the thickness of grain refinement layer increased, the surface hardness heightened and the stress corrosion susceptibility index decreased.

Abstract: Surface conditions are of significant importance for the fatigue performance of rotating components for aircraft and automotive applications. Compared to the electropolished reference state the fatigue performance of gamma titanium aluminides at ambient temperature can be significantly improved by shot peening, while the fatigue strength after roller burnishing is hardly affected. Fatigue strength improvements after shot peening are mainly caused by cyclically stable residual compressive stresses in the near-surface region. Notably, also conventional turning operations generate high dislocation densities and residual compressive stresses in the near-surface region resulting in fatigue strength improvements similar to shot peening. Surface strengthening by shot peening or machining leads to subsurface fatigue crack nucleation and overcompensates the detrimental influence of the high surface roughness on fatigue. However, for temperatures above 650 °C, residual compressive stresses induced by shot peening and machining quickly relax. This indicates that, at elevated temperatures, surface roughness and dislocation strengthening become increasingly important for the fatigue performance of components.

Abstract: The formation of an Fe-Al intermetallic compound film on carbon steel by shot lining and heat treatment was investigated. In the experiment, a centrifugal-type peening machine with an electrical heater was employed. The shot medium was high-carbon cast steel. The substrate was a commercial carbon steel, and the sheet was commercially available pure aluminum. The shot lining process of carbon steel with an aluminum sheet was carried out at 300 °C in air using a peening machine. Heat treatment was performed at diffusion temperatures from 600 to 1050 °C in vacuum. The lined substrates exhibited a harder layer of Al-rich intermetallics, such as Fe₂Al₅, in the diffusion temperature range of 650 to 900 °C. When the temperature of the lined substrates was more than 930 °C, the surface was covered with thicker and highly anticorrosive layers of Fe-rich intermetallics, such as FeAl and Fe₃Al. We found that the present method could be used for the formation of functional films on carbon steel.

Abstract: The corrosion is one kind of major damage mechanisms of airplane structure and is also one of the primary causes resulting in flight failure and disaster .The aim of this paper is to analyze the effect of corrosion time on denudation depth, mass loss and the total depth of corrosion damage. Meanwhile, the three parameters have significant influence on residual fatigue life of aluminum alloy, the residual fatigue life of the aluminum alloy is of great difference between shot peening and without shot peening aluminum alloy. Then residual fatigue life tests are carried on Instron 8801 for the shot peening and without shot peening aluminum alloy. Based on contrast analysis of the experimental results, valuable conclusions are obtained: with the increase of the denudation depth, mass loss and the total depth of corrosion damage, fatigue life reduce continuously respectively; Fatigue life decreases rapidly when the denudation depth is in the range of 0.12～0.17mm.

Abstract: The pre-corrosion damage tests of LC4 aluminum alloy with the shot peening surface treatment and un-shot peening surface treatment are performed under different time levels. The residual strength has been studied by test after corrosion damage. An analysis procedure is developed using 3D reconstruction solid model and finite element analysis to predict residual strength, The relationship of static intensity and the corrosion damage are founded. An effective engineering technique for predicting residual strength of corroded structure is established on the base of this studying work.

Abstract: The effect of slight surface REM finishing, that is a chemically accelerated polishing process, in order to reduce surface roughness after shot peening has been studied on different series of specimens shot peened by varied peening parameters. Room temperature rotating bending fatigue tests have been performed on the shot peened series and the series subjected to shot peening and successive REM finishing process. The results represent how REM finishing affects the final fatigue behavior of differently treated specimens in two ways: decreasing surface roughness and removing a thin layer of material with compressive residual stresses.

Abstract: Shot peening is a mechanical treatment usually performed on metals for improving the mechanical properties of the surface layers. The application of such treatment to sintered steels causes a local surface densification and the birth of residual compression stresses. In this paper, the effect of different shot peening cycles, selected after a preliminary optimization, has been investigated focusing especially on the fatigue resistance measured by bending tests and on the surface residual stresses measured by XRD technique. Two high performances PM steels were considered: the former was sintered starting from a diffusion bonded powder, whereas the latter is based on a pre-alloyed powder. Two different nominal densities were considered, 6.9g/cm³ and 7.1g/cm³.

Abstract: The paper contains the main results of an investigation of an effect of shot peening on residual fatigue life of specimens made of an aircraft V-95 Al-alloy (equivalent to 7075 alloy) containing physically short fatigue cracks. In the first part, an adaptation and verification of direct current potential drop method for detection and measurement of short crack growth is described and discussed. The specific properties of the material and quite large dimensions of flat specimens with side necking with a low stress concentration factor had to be considered when the position of electrodes was specified. The specimen type and dimensions were proposed considering the investigation of shot peening effects. Physically short fatigue cracks of the length from 0.2 mm to more than 3 mm, most of them between 0.8 – 1.5 mm, were prepared under high cycle fatigue loading of the constant nominal stress amplitude ±160 MPa. Specimens with existing short fatigue cracks were shot peened using two different groups of parameters. Crack growth characteristics after shot peening were measured and compared with crack growth in specimens without shot peening. Retardation of crack growth was significant particularly for cracks shorter than 2 mm. Results are also discussed form the viewpoint of fracture mechanics approach. For the specific stress amplitude, evaluated results enable to estimate threshold length of defects, which after the application of shot peening will be reliably arrested.

Abstract: In general, shot peening is a surface treatment that improves the performance of engineering components, since application of surface compressive stresses reduces the tensile component of stress. In the shot peening process, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2 mm. More recently, a new type of microshot has been developed to enhance the peening effect. The diameter of the new spheres is in the range from 0.02 to 0.15 mm. The effect of microshot peening on the residual stress of spring steel was investigated. The projective method of the microshot was of the compressed air type. The microshot of 0.1 mm diameter was high-carbon cast steel and cemented carbide, and the workpiece used was the commercially spring steel JIS-SUP10. The effect that process variables such as shot speed and peening time have primarily on residual stress was studied. The surface layer of the workpieces was sufficiently deformed by microshot peening. The residual stress was observed near the surface. At a large number of cycles to fracture, microshot peening can more effectively enhance the fatigue strength. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for spring steel.