High Compressive Residual Stress Research Articles

Influence of Shot Peening Parameters on Residual Stresses in Flake and Vermicular Cast Irons

Test samples of grey and compacted graphite cast irons with pearlitic matrix were shot-peened to different surface conditions using twelve different combinations of shot size, peening intensity and peening coverage percentage. Relatively high surface compressive residual stresses varying between 245 to 565 MPa were observed and the compressive residual stresses reached a depth between 280 µm and 770 µm. Within the range of peening parameters used, the compacted graphite cast iron with its vermicular graphite showed a somewhat better response to the same shot-peening treatment than the grey cast iron containing flake graphite, giving a larger peening affected zone with higher compressive residual stresses. For both the cast irons, an increase in peening coverage percentage, shot size or peening intensity led often to a lower surface compressive stress. However, peening using a higher intensity greatly increased the degree and extent of plastic deformation and therefore increased the magnitude and penetration depth of the subsurface compressive residual stresses, while the effect of increasing shot size also depends on the peening intensity. On the other hand, measurements on the grey cast iron samples showed that the peening coverage has little effect on the depth profile of residual stress.

Influence of surface modifications on pitting corrosion behavior of nickel-base alloy 718. Part 1: Effect of machine hammer peening

Abstract The effect of surface modifications induced by machine hammer peening on pitting corrosion behavior of nickel-base alloy 718 in a 3.5 wt.% NaCl solution is investigated. Severe work hardening and high compressive residual stress are generated with surface smoothing and microstructure evolution in terms of formation of nano-grains and nano-twins in the near surface region after machine hammer peening. Electrochemical tests results show that machine hammer peening has a beneficial influence on the corrosion resistance, indicated by a significant increase of the critical pitting potential (+134 mV) accompanied with lower corrosion current density and higher polarization resistance.

The influence on gear surface properties using shot peening with a bimodal media size distribution

New technologies developed for automotive engines also have highlighted the transmission torque capacity as restriction factor in the development of enhanced vehicle dynamics performance. Generally, the restriction is defined by the lifetime of the first speed gear. At the end of the gear manufacturing process, shot peening is used to induce compressive residual stresses and further increase fatigue life. This research verified the influence of introducing a bimodal media size distribution into the shot peening process. The proposal is supported by the independent effects of each media class. A higher compressive residual stress can be obtained from larger spheres. And the use of a lower diameter media class improves the surface homogeneity. The bimodal distribution was defined with a probabilistic approach over plasticity and contact stress theories. The experimental validation scope includes a topography analysis and residual stress profile measurements. The results showed mixtures combining up to an increase of 30.9% in the compressive residual stresses without jeopardising the surface quality. The collected data supported the expectation for improving the gear lifetime. It also represented the validation of a new peening process. An improvement in the product properties can be achieved in comparison to the parts produced using conventional shot peening; additionally, a lower process lead time is required in comparison to that for dual peening.

Effect of laser shock processing on stress corrosion cracking behaviour of AZ31 magnesium alloy at slow strain rate

An investigation was performed to evaluate the effect of laser shock processing (LSP) on AZ31 magnesium alloy using the slow strain rate tensile test. Microstructure revealed that a fine grained structure was obtained by LSP. X-ray diffraction showed that high compressive residual stress occurred at the surface after LSP. Meanwhile, the tensile strength of the LSP samples was improved. The stress corrosion cracking behaviour was examined, including both the base material and the LSP samples in air and 3·5%NaCl solution respectively. Test results demonstrated that the AZ31 Mg alloy after LSP exhibits higher stress corrosion cracking resistance than the base material.

Effects of Laser Shock Peening on TC11 Titanium Alloy with Different Impacts

The blade, made of TC11 titanium alloy, is prone to result in fatigue failure in the formidable environment in aero-engine. So a higher performance request of the material is brought forward. In this paper, laser shock peening(LSP) as a solution is applied to TC11 titanium alloy and microstructure, residual stress and microhardness with and without LSP were examined and compared via transmission electron microscope(TEM), X ray diffraction(XRD)and microhardness tester. The TEM results indicate that a great high density of dislocations are generated and evolve into the dislocation wall, sub-boundary and grain boundary. The nanocrystallites are formed and become smaller and more uniform with greater impacts. A high compressive residual stress above -540MPa is introduced with an increasing plastically affected layer with different impacts. The microhardness test result shows that LSP can obviously increase the hardness by 20 percent or so, and the affected depth increases with the impact from 600μm to 1200μm.

Mechanical and Metallurgical Evaluation of Carburized, Conventionally and Intensively Quenched Steels

Steels subjected to carburizing, quenching, and tempering are widely used for components that require hardness and superficial mechanical resistance together with good core toughness. Intensive quenching is a method that includes advantages including crack prevention, increased mechanical resistance, and improvement in fatigue performance when subjected to very fast (intensive) cooling. However, achieving these advantages requires the formation of sufficiently high surface compressive residual stresses and fine grains at the core of steel components. If the cooling rate is sufficiently high after intensive quenching, then low-hardenability, killed plain carbon steels may be used instead of higher-cost, low alloy steels because compressive residual stresses are formed at the surface of steel parts. The objective of this study was to compare between carburized non-killed AISI 1020 steel samples, which were not modified by Al that were subsequently conventionally and also intensively quenched to determine the effect of quenching on achieving the necessary formation of fine grain size. For comparison, carburized AISI 8620 steel test specimens were conventionally quenched. After quenching, all test specimens were characterized by metallurgical and mechanical analyses. The results of this study showed that when the two quenching methods were compared for carburized non-killed AISI 1020 steel, intensive quenching method was found to be superior with respect to mechanical and metallurgical properties. When comparing the different steels, it was found that intensively quenched, non-killed, AISI 1020 steel yielded grain sizes which were three times greater than those obtained with conventionally quenched, carburized AISI 8620 steel. Therefore, the benefits of intensive quenching were negated. These results show that plain carbon steels must be modified by Al to make fine grains if intensively quenched plain-carbon steel is to replace alloyed AISI 8620 steel.

Very hard ZrC thin films grown by pulsed laser deposition

Abstract Thin ZrC films were grown on (1 0 0) Si substrates at temperatures from 30 to 500 °C by the pulsed laser deposition technique. Auger electron spectroscopy investigations found that films contained oxygen concentration below 2.0 at%, while X-ray photoelectron spectroscopy investigations showed that oxygen is bonded in an oxy-carbide type of compound. The films’ mass densities, estimated from X-ray reflectivity curve simulations, and crystallinity improved with the increase of the substrate temperature. Williamson–Hall plots and residual-stress measurements using the modified sin 2 ψ method for grazing incidence X-ray diffraction showed that the deposited films are nanostructured, with crystallite sizes from 6 to 20 nm, under high micro-stress and compressive residual stress. Nanoindentation investigations found hardness values above 40 GPa for the ZrC films deposited at substrate temperatures higher than 300 °C. The high density of the deposited films and the nm-size crystallites are the key factors for achieving such high hardness values.

Mechanical Characteristics of Electrolytes Assessed with Resonant Ultrasound Spectroscopy

AbstractElastic properties and residual stresses are critical for the assessment of the reliability of ceramics components. The thin electrolyte layer of anode supported SOFCs is under a state of high compressive residual stress. In the present study, the elastic properties of this layer as well as the residual stress distribution induced in the anode‐supported planar SOFC cell were examined by using the resonant ultrasound spectroscopy (RUS). A modal analysis by finite element simulation demonstrated that the RUS has a great potential to determine these mechanical characteristics. The Young's modulus can be determined from the natural frequencies of the layered sample whilst the residual stress can be determined using the sample with a semi‐hole. Estimates of the Poisson's ratio are possible. In the experiment, various individual resonant oscillations were successfully identified using the present RUS system. Hence, by comparison of experimental results and simulations, the elastic properties as well as the residual stress were obtained. Although the proposed method is still experimentally challenging, it could be developed to become a powerful tool to simultaneously determine residual stresses and elastic constants of thin‐layered systems.

Zr-based metallic glass thin film coating for fatigue-properties improvement of 7075-T6 aluminum alloy

In order to improve the fatigue life of high strength aluminum alloy, a protection coating with 200nm Zr-based metallic glass thin film (MGTF) coupled with 50nm titanium thin film buffer layer was coated on the 7075-T6 aluminum alloy by DC sputtering. The results of four-point-bending fatigue test revealed that the fatigue life of 7075-T6 aluminum alloy with Zr-based MGTF could be improved by 30 times at a stress level of 250MPa. The improvement of Zr-based MGTF coated samples in fatigue limit was 235MPa (56.7% increase) in comparison with 150MPa for the uncoated samples. The superior mechanical properties of Zr-based MGTF, such as high strength and good bending ductility, lower surface roughness, good adhesion between the film and the substrate, and high compressive residual stress are the key factors to improve the fatigue resistance of the coated aluminum alloy.

Superior surface integrity by sustainable dry hard milling and impact on fatigue

Dry milling is a sustainable finishing process in mold/die manufacturing. However, the progression of tool wear causes great concern on inferior surface integrity and reduced part life. This study focuses on the relationships among process conditions, surface integrity, and fatigue in dry milling of hardened H13 steel. Superior surface integrity including good surface finish, highly compressive residual stress, free-of white layer, and work-hardening can be produced up to flank wear at least 0.2mm. Furthermore, fatigue life of the machined surfaces in finish conditions reaches over 1 million cycles under the synergistic effect of high compressive residual stress and work-hardening.

OS2104 高圧縮残留応力を有する軸受鋼の水素脆化挙動の考察

OS2104 高圧縮残留応力を有する軸受鋼の水素脆化挙動の考察

A Study on Grinding Parameter Optimization of Reaction Bonded Silicon Carbide

In order to enhance the efficiency and the surface smooth degree of the RBSiC grinding, a three factors two levels full factorial design was utilized to optimize the process. Combined with the effects of grinding parameters on surface roughness, the grit cut depth analysis was employed to choose the appropriate grinding parameters. The strength reliability and the residual stresses of the RBSiC ground using the optimized parameters were investigated. The results show that comparing to the polished RBSiC the ground ones have higher compressive residual stress, lower crack scatter and similar average bending strength.

Ti-6Al-4V合金の回転曲げ疲労特性に及ぼすレーザピーニング処理の影響

In order to investigate the effects of laser peening on high cycle fatigue properties of (α+β) type Ti-6Al-4V alloy, rotating bending fatigue tests were carried out. The laser peening conditions were a spot size of 0.8mm in diameter, a pulse energy of 200mJ and an irradiation pulse density of 38 pulse/mm2. The fatigue properties were not improved with these conditions of laser peening ; even though the material was hardened to some extent and high level compressive residual stresses were induced in the near-surface layer. Because the reason that the fatigue properties were not improved was considered the notch effect of surface roughness introduced by laser peening, further fatigue tests were performed with laser-peened specimens followed by surface polishing. However, the fatigue lives of polished specimens were shorter than those of unpolished specimens, unexpectedly. Considering the peening effects such as surface roughness, hardness and residual stress, the reasons for arriving at the unexpected results would be the influence of the strain-induced transformation of β phase into α' martensite and a larger yield strength in tension than that in compression due to the HCP structure of α phase of titanium alloy.

Estimation of the Yield Stress of Stainless Steel from the Vickers Hardness Taking Account of the Residual Stress

In this paper, a method that uses the Vickers hardness to estimate the yield stress of a metallic material with taking account of residual stress is proposed. Although the yield stress of bulk metal can be evaluated by a tensile test, it cannot be applied to local yield stress varied by surface modification methods, such as the peening technique which introduces high compressive residual stress at the surface. Therefore, to evaluate the local yield stress employing a relatively easy way, the Vickers hardness test was conducted in this paper. Since the Vickers hardness depends on both the residual stress and the yield stress, the relationship between the residual stress and the Vickers hardness was experimentally examined. It was concluded that the yield stress of the surface treated by several peening techniques can be estimated from the Vickers hardness once this has been corrected for residual stress.

Improvement of Fatigue Strength of Ti–6Al–4V Alloy by Hybrid Surface Treatment Composed of Plasma Nitriding and Fine-Particle Bombarding

This study was conducted to improve the fatigue strength of Ti­6Al­4V alloy by hybrid surface treatment composed of plasma nitriding and fine-particle bombarding (FPB). Plasma nitriding to form a hardened layer was performed at relatively low temperature to suppress the growth of the microstructure of the substrate as far as possible. Subsequent FPB eliminated the outermost brittle compound layer leading to a reduction in fatigue strength, and also introduced compressive residual stress at the same time. Hybrid surface treatment formed a hardened layer that can improve wear resistance without marked deterioration of the mechanical properties. Since the formed layer possessed high compressive residual stress and relatively high hardness, initiation of fatigue cracks from the surface was strongly suppressed and the crack initiation site was located in the substrate under the formed layer. As a result, the fatigue strength improved by 47% and the fatigue ratio (fatigue strength/tensile strength) reached a very high level of 91%. [doi:10.2320/matertrans.M2013228]

Study on the Influence of Ultrasound Impacting to the Properties of 16 Mn Welded Joints

In this article, the technology of ultrasound impacting is mentioned. The toe of 16Mn welded joints are treated by the ultrasound impacting, and the fatigue, hardness and residual compressive stress of the 16Mn welded joints are compared after treated by ultrasound impacting. The results shows that, the ultrasound impacting bring high residual compressive stress on the surface of the 16Mn welded joints, besides, the treatment of ultrasound impacting can improve the fatigue and hardness of the 16Mn welded joints, and when the impacting electric current is 1.5A, and the impacting time is 10 min, the fatigue and hardness of the sample is the best.

Steels of optimal chemical composition combined with intensive quenching

Manufacturing steels of optimal chemical composition, combined with intensive quenching, is an important step to save essential energy and make the environment cleaner. Historically, alloy and high alloy steels are hardened in oils or high concentration polymers to prevent crack formation during quenching. However, very slow cooling in oils requires more alloy elements to provide the needed surface hardness and hardenability. To extract expensive alloy elements from the ore one needs additional energy, plus, metallurgical processes are harmful to the environment. In this paper an idea on manufacturing and use steels of optimal chemical composition is discussed. Such steels produce an optimal hardened layer and optimal residual stress distribution in machine components after intensive quenching. Compressive residual stresses and high cooling rate within the martensite range result in additional strengthening of material. Both high compressive residual stresses at the surface of steel parts and additional strengthening (superstrengthening of the material) compensate for the decrease in alloy elements in the steel. Decreasing alloy elements saves essential energy and decreases environment pollution. The intensive quenching increases productivity and service life of steel parts. These important problems are widely discussed in the paper. Appropriate calculations are made and practical examples are provided.

Enhanced fatigue durability of Al–6 Mg alloy by applying ultrasonic impact peening: Effects of surface hardening and reinforcement with AlCuFe quasicrystalline particles

Composite layer reinforced with quasicrystalline (QC) Al63Cu25Fe12 particles was fabricated on the surface of Al–6Mg alloy specimens by ultrasonic impact peening (UIP). Stress-controlled fatigue response of the specimens was studied and compared with those for the annealed and UIP-treated specimens. The notch effect of the UIP induced surface roughness calculated in terms of the stress concentration factor does not exceed∼10%. XRD, OM and SEM analyses were used to characterize formed surface layers and fatigue fracture surfaces. Surface composite layer of 40–50μm thick contains the homogeneously dispersed QC particles (the volume fraction Vf∼0.15) trapped by high compressive residual stresses. The layer demonstrates almost triple increase in microhardness comparing to that for the annealed alloy and twice exceeding of that for the UIP-treated specimen. Superior fatigue endurance of Al–6Mg alloy after the UIP process and the UIP-induced formation of the composite layer is explained by sub-surface fatigue cracks’ initiation promoted with high compressive residual stresses and tight interfacial bonding of QC reinforcement and the matrix alloy. The improved fatigue behavior of the UIP-treated specimens in both the low cycle and high cycle regimes can be ensured by combination of the following favorable characteristics: (i) sufficiently high ductility and resistance to fatigue damage and crack growth in the core parent material along with (ii) superior fatigue strength supported by high microhardness and compressive stresses in the surface layer, which contains fine grained matrix and/or uniformly distributed and tightly bonded QC reinforcements.

Fatigue behavior of notched steel specimens with nanocrystallized surface obtained by severe shot peening

Among severe plastic deformation methods that result in surface nanocrystallization, shot peening has proved to be a promising technique. Application of severe air blast shot peening results in surface nanocrystallization, affects a thick layer of material with high compressive residual stresses but at the same time produces rather high surface roughness. In this study notched specimens with a stress concentration factor common in many structural components have been subjected to severe shot peening process. The mentioned treatment uses peening parameters essentially different from conventional ones. Roughness and X-ray diffraction residual stress measurements as well as microscopy observations have been carried out on the treated specimens. Room temperature rotating bending fatigue tests are performed to evaluate the effect of the treatment on specimens’ fatigue strength. Fracture surfaces have been then observed by scanning electron microscopy. The results indicate a very significant fatigue strength improvement for severely shot peened specimens in spite of their very high surface roughness.

Ultrafine-grained textured surface layer on Zr–1%Nb alloy produced by ultrasonic impact peening for enhanced corrosion resistance

An ultrafine-grained (UFG) surface layer was produced on Zr–1%Nb alloy using severe plastic deformation process induced by ultrasonic impact peening (UIP). XRD analysis and TEM observations allow establishing the links between the microstructure formed at different extents of the effective strain ē and superficial microhardness and corrosion resistance. In the topmost surface layer about 10μm thick, average grain size diminishes down to approx. 100nm after the UIP process for 4min (ē≈0.86). The mechanism of the grain subdivision is discussed on the basis of TEM observations of microstructure in surface layer at different strain extents with taking into account the dynamic recrystallization process facilitated by deformation induced heating. XRD analysis reveals high compressive residual stresses and strong basal texture. Two features of the surface layer formed after the UIP process, viz. UFG structure and strong basal texture, are considered to play the major roles in essential increase in microhardness (from 1 to 2.35GPa) and corrosion resistance of the Zr-1%Nb alloy in saline solution. Decreased surface roughness and large compressive residual stresses also promote higher corrosion resistance.