Large Compressive Residual Stress Research Articles

Numerical analysis of stress-induced and concentration-dependent carbon diffusion in low-temperature surface carburisation of 316L stainless steel

A kinetic model based on stress-induced and concentration-dependent carbon diffusion was developed for simulating the carbon concentration-depth profile of carburised austenitic stainless steel. The model considers that the carbon diffusivity is dependent of carbon concentration and the stress induced by diffusion of the dissolved carbon atoms can affect the diffusion behaviour in turn. The results show that in carburised 316L stainless steel, the calculated carbon concentration-depth profile is in good agreement with experimental results, which indicates that the stress and concentration-dependent diffusivity play important roles in carbon diffusion. As a result of carburisation, large compressive residual stress is generated and gradiently distributes in the carburised layer, meanwhile, the diffusion of carbon atoms can be accelerated by stress. Although, the compressive residual stress is not the dominant reason for total carbon diffusivity increases significantly with increasing carbon concentration, as the next driving force it cannot be ignored during low-temperature surface carburisation.

Numerical Analysis of Shot Peening with Incident Angle

Shot peening has been used to improve the fatigue strength of metal structural components because it can produce large compressive residual stress near the surface. The distribution of compressive residual stress is strongly dependent on shot peening conditions such as the material of the shot, the shot diameter, the shot velocity, the incident angle, and the peening time (coverage). In this study, a finite element model including multiple shots and the specimen was developed in order to investigate the influence of the incident angle on the compressive residual stress and the depth of its location from the surface in high-strength aluminum alloy. The residual stress and its distribution were analyzed when multiple shots were randomly impacted on the surface of the specimen. The analyzed residual stress distribution using the kinematic material model agreed very well with experimental ones at the incident angles of 90° and 30°. It can be concluded that the depth of the compressive residual stress is proportional to the velocity component perpendicular to the surface.

Effect of chemical dilution and the number of weld layers on residual stresses in a multi-pass low-transformation-temperature weld

Influences of chemical dilution and the number of weld layers on residual stresses in a multi-pass low-transformation-temperature (LTT) weld were investigated by finite element modelling and neutron diffraction. A coupled thermal-metallurgical-mechanical (TMM) model that took into account the chemical dilution effect was developed to simulate the complex LTT welding phenomena. The model was strictly validated by comparing the predictions with experimental measurements and the results found good agreement between them. The results showed that a transformation strain caused by LTT martensitic transformation introduced large compressive residual stresses (−500 MPa) into the weld zone and the quantities were closely related to the chemical dilution between welding layers. Simulation results revealed that the chemical dilution helped mitigate the large localized tensile residual stresses by modulating the phase transformation process and a single LTT layer was sufficient to generate high compression near the weld surface (~2.5 mm), which suggests a great importance for repair welding.

A micromechanical model to study the closure stress effect on fatigue life of Ti6Al4V subjected to laser shock peening

With the wide application of titanium alloy in various engineering fields, the service life of Ti alloy product has drawn much more attention. It’s been proven that the laser shock peening (LSP) is an effective technology to improve the fatigue performance of titanium alloy by introducing a large and deep compressive residual stress. In this paper, a microstructural fracture mechanics model incorporating the crack closure stress effect is presented to investigate the reinforcement mechanism of laser shock peened (LSPed) titanium alloy. Firstly, a rigid-plastic simplified model is utilized to describe the residual stress characteristics of LSPed Ti6Al4V. Then the extended Navarro-Rios model incorporating crack closure stress effect is established to systematically analyze the crack propagation behavior. Finally, the effect of crack closure stress on fatigue life can be accurately predicted. Result shows that the crack propagation processing is a behavior reflecting the deceleration and acceleration of the crack growth rate. A higher threshold stress is required for LSPed Ti6Al4V to promote the propagation of fatigue crack. The closure stress effect reduces the barrier stress and critical crack length in each grain leading to the highly raised fatigue life of LSPed Ti6Al4V. Besides, the closure stress effect has a greater improvement in fatigue performance for a sample with a shorter crack length and thus can be neglected for a long crack.

Effect of tailored martensitic transformation in a thick weld: Residual stresses mitigation, heterogeneous microstructure, and mechanical properties

The residual stresses, microstructural heterogeneities, and mechanical properties were extensively examined in a 25 mm thick weld consisting of two regions which respectively undergo phase transformation at relatively low (100 °C) temperature (LTT) and high (670 °C) temperature (HTT) during cooling. Due to the LTT martensitic transformation, which features a transformation strain of ~4000 με, large compressive residual stress (−510 MPa) was generated as confirmed by neutron diffraction and contour method. Meanwhile, significant heterogeneities were observed in terms of the chemical composition, yield strength, and microhardness across the interface between the LTT and HTT regions. The dependency of martensitic transformation starting temperature (Ms) on chemical composition is empirically formulated, and it reveals that the dependency of Ms on the Ni and Cr compositions becomes stronger when the temperature is lower than 200 °C. The maximum change of residual stresses (σxmax) exponentially decreases as Ms decreases in the manner of σxmax = 97.5 exp(Ms/227)-649. The fracture behavior was found highly dependent upon the volume fraction of retained austenite influenced by Ms. While a ductile fracture mode was found in the cellular LTT region containing relatively higher amount of retained austenite (~10%), the interface shows transgranular brittle fracture features and sub-cracking due to the relatively small amount of retained austenite and the predominant martensite constituent.

A Numerical and Experimental Study of Distribution of the Residual Stress on the Shot Peened Low Alloy Steel

Shot peening is a cold working process, which is used to enhance the properties of materials, especially the fatigue life as it induces large compressive residual stresses in the subsurface of materials. In this paper, the effect of the shot peening process on the topography of the shot peened surface and the distribution of the residual stresses in the subsurface of the material was systematically investigated. A technique to estimate the shot peening coverage was employed using a finite element model which was further developed using experimental results for increased accuracy. The comparison between the numerical and experimental studies gives a good agreement of the distribution of the residual stresses in the subsurface of the shot peened material. The shot peening pressure and media size are two main factors affecting on the presence of compressive residual stresses in the subsurface of the material.

Effect of ultrasonic nanocrystal surface modification on elevated temperature residual stress, microstructure, and fatigue behavior of ATI 718Plus alloy

Ultrasonic Nanocrystal Surface Modification (UNSM) is a mechanical surface treatment that induces large compressive residual stresses and near-surface microstructural changes in the material by using repetitive impacts at ultrasonic frequencies with a WC tip. In this study, we investigate the use of UNSM to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650 °C. UNSM induced severe surface plastic deformation which led to nano-sized crystallites, twins, and high dislocation density in the near surface regions, coupled with an increase in surface hardness and high magnitude of compressive residual stresses. This change in the near-surface microstructure remained stable at elevated temperatures. In addition, UNSM retained ∼−680 MPa near-surface residual stress (56% of its initial residual stress) even after 140 h exposure to 650 °C. The retained near-surface residual stresses and the stable microstructure from the UNSM increased the yield strength by ∼11% (∼105 MPa) and endurance limit by ∼8% (∼70 MPa) in corresponding tests at 650 °C. This improvement in fatigue life was attributed to near-surface microstructure changes, material hardening and high compressive residual stress induced by UNSM. The estimated crack growth rates were ∼60% lower for UNSM-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of UNSM in improving the fatigue life of 718Plus at 650 °C.

Implementation of advanced characterisation techniques for assessment of grinding effects on the surface integrity of WC–Co cemented carbides

ABSTRACTGrinding is a key step on the manufacturing process of WC–Co cemented carbides (hardmetals). In this work, an investigation of grinding effects on the surface integrity of hardmetals is conducted. It is done by combining diverse advanced characterisation techniques: X-ray diffraction, field emission-scanning electron microscopy, electron back scatter diffraction, focused ion beam – 3D tomography and transmission electron microscopy. The study is carried out in a fine-grained WC–Co grade. Besides ground state, polished surface finish condition is assessed for comparison purposes. It is evidenced that grinding induces significant alterations: 3D tomography illustrates microcracking exists down to 2.5 μm depth with a highly anisotropic distribution at the subsurface, large compressive residual stresses extending until subsurface levels of about 12 μm, and phase transformation of binder from the original fcc phase into the hcp one, as well as severe plastic deformation observed within the binder at the surface level.Invited keynote papers from EuroPM2017, Milan.

Surface integrity and its influence on fatigue life when turning nickel alloy GH4169

Fatigue life of a machined part strongly depends on its surface condition. This research investigated the machining induced surface integrity after turning the nickel based superalloy GH4169 standard fatigue test specimens by using different cutting parameters. Surface roughness, residual stress and the metamorphic layer influenced by applying different cutting parameters were studied. The specimens’ fatigue lifes were obtained by high frequency fatigue test. The fracture and crack initiation analysis, the mechanism of cutting parameters influences on the fatigue life have been investigated as well. The results showed that using appropriate cutting parameters can lead to an optimal surface with the proper surface integrity indexes when turning by using the coated carbide tools. Finally, he high frequency fatigue tests with 0.1 stress ratio have been carried out. The results proved that the specimens which surface was with larger compressive residual stress and thinner metamorphic layer have stronger abilities to resist fatigue crack initiation.

Effect of Ultrasonic Nanocrystal Surface Modification on residual stress, microstructure and fatigue behavior of ATI 718Plus alloy

Ultrasonic Nanocrystal Surface Modification (UNSM) is a mechanical surface treatment that induces large compressive residual stresses and near-surface microstructural changes in the material using repetitive impacts at ultrasonic frequencies with a WC tip. In this study, we investigate the use of UNSM to improve the fatigue life of ATI 718 Plus (718Plus). UNSM induced severe surface plastic deformation in 718Plus, which led to nano-sized crystallites, twins, and high dislocation density in the near surface regions, coupled with an increase in surface hardness by 2.3GPa and high magnitude of compressive residual stresses. These changes increased the endurance limit by ~ 13% (~ 100MPa) in room temperature fatigue tests. This improvement in fatigue life was attributed to near-surface microstructural changes, material hardening and high compressive residual stress induced by UNSM. The crack propagation rates were 66% lower for UNSM-treated 718Plus as compared with untreated material. The residual stress relaxation after cyclic loading indicates the effectiveness of UNSM in improving the fatigue life of 718Plus.

The Properties and Residual Stress of Argon arc Cladding Metal by Low Temperature Martensitic Transformation Powder

Martensitic Fe-based metal was achieved by argon arc surface cladding using self-developed low-temperature phase transformation (LTT) powders. Residual stress (RS), retained austenite (RA) content, hardness and wear resistance of the cladding layer under different powder compositions were tested. Meanwhile, the phase composition, microstructure and wear mechanism of the cladding metal were also analyzed. The results show that under suitable process parameters, the cladding layer and the substrate are metallurgical bonding without crack and porosity. The microstructure of the cladding layer is mainly lath martensite with less RA distributed on. Due to the volume expansion during phase transformation from austenite to martensite at about 200°C degree, large compressive RS is produced within the cladding metals, which has effectively compensated the tensile RS result from thermal shrinkage. The compressive RS reaches maximum of -361 MPa, and the average RA in the cladding layer is 10.85% for powder with composition of 10% Cr and 8% Ni. The hardness of the cladding layer is 2.6 times higher than that of the matrix material, which could reach up to 557.2 HV. The wear resistance of the surface cladding layer is nearly 57 times higher than that of the base material, which has the most excellent comprehensive performance.

Neutron diffraction residual stress analysis during fatigue crack growth retardation of stainless steel

After tensile overloading during fatigue crack growth, retardation of the crack growth rate was significant. Neutron diffraction was employed to examine the evolution of crack-tip residual stress fields during constant-amplitude cyclic loading and during fatigue crack growth following the overload. It was found that the tensile overload induces larger compressive residual stress and zone size near the crack tip in the crack-growth and crack-opening direction. For the maximum crack growth retardation, the largest compressive residual stresses were measured in the region between an overloading point and the current propagating crack tip, for all three of the orthogonal directions. Such large compressive residual stresses in the crack-wake region are thought to reduce the crack tip driving force, thereby retarding the crack propagation rate significantly. Residual stress mapping was performed to examine the effect of the fatigue stress state on the residual stresses in the three different regions from the centerline to the surface, along the through-thickness direction in the compact-tension specimen. Much larger compressive residual stresses were measured at the surface than at mid-thickness. It is thought that larger compressive residual stresses at the surface result from the plane stress condition, resulting in a slower fatigue crack growth rate at the surface of the specimen.

Effect of laser shock peening on elevated temperature residual stress, microstructure and fatigue behavior of ATI 718Plus alloy

Laser Shock Peening (LSP) is a mechanical surface treatment that induces large compressive residual stresses and microstructural changes in the material by using repetitive shocks from laser pulses. In this study, we investigate the use of LSP to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650°C. LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses and changes in the near-surface microstructure which caused high surface hardening. This change in the near-surface microstructure was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of near-surface nanoscale sub-grains/crystallites that remained stable at elevated temperatures. In addition, LSP retained ∼−470MPa residual stress (68% of its initial residual stress) even after 140h exposure to 650°C. The retained residual stresses and the stable microstructure from the LSP increased the yield strength by ∼14% (∼140MPa) and endurance limit by ∼10% (∼90MPa) in corresponding tests at 650°C. This improvement in fatigue life was attributed to near-surface microstructure, hardening and high compressive residual stress. The estimated crack growth rates were 72% lower for LSP-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of LSP in improving the fatigue life of 718Plus at 650°C.

Residual stress and electrical properties of Pb(Zr0.52,Ti0.48)O3 thin films RF sputtered directly on Cu foils

AbstractPolycrystalline Pb(Zr0.52Ti0.48)O3 (PZT) thin films between 250 and 1000 nm thick were deposited on Cu foils via RF magnetron sputtering. Samples were crystallized ex situ between 550°C and 750°C in a low oxygen partial pressure atmosphere, pO2, in order to avoid oxidation of the substrate. These were compared to films made on more common Pt/TiOx/SiO2/Si substrates also crystallized under low pO2 conditions. The mismatch of the coefficients of thermal expansion for Cu and PZT caused large compressive residual stresses to develop in the films, whereas films on Pt‐Si experienced more moderate tensile stresses. Stress was measured using the sin2ψ method. In addition to mechanical implications, i.e., film cracking and delamination, the effect of residual stress on electrical properties is discussed. Dielectric constants of PZT were lower on Cu than on Pt/TiOx/SiO2/Si. This could be due either to a dead layer effect or to the residual stress imposed by the substrate. The remanent polarizations for films on Cu were between 18 and 41 μC/cm2, while coercive fields were between 37 and 54kV/cm. Rayleigh analysis was used to describe the role of defects affecting domain wall mobility, as they act as pinning centers and decrease the extrinsic polarization response.

Improving fatigue strength of welded 1300 MPa yield strength steel using HFMI treatment or LTT fillers

Abstract Fatigue improvement techniques are widely used to increase fatigue strength of welded high strength steels. In this paper high frequency mechanical impact (HFMI) and a Low Transformation Temperature (LTT) filler material were employed to investigate the effect on fatigue strength of welded 1300 MPa yield strength steel. Fatigue testing was done under fully reversed, constant amplitude bending load on T-joint samples. Fatigue strength of LTT welds was the same as for welds produced using a conventional filler material. However, HFMI treatment increased the mean fatigue strength of conventional welds about 26% and of LTT welds about 13%. Similar distributions of residual stresses and almost the same weld toe radii were observed for welds produced using LTT and conventional consumables. HFMI increased the weld toe radius slightly and produced a more uniform geometry along the treated weld toes. Relatively large compressive residual stresses, adjacent to the weld toe were produced and the surface hardness was increased in the treated region for conventional welds after HFMI. For this specific combination of weld geometry, steel strength and loading conditions HFMI treatment gave higher fatigue strength than LTT consumables.

In situ direct observation of toughening in isotropic nanocomposites of alumina ceramic and multiwall carbon nanotubes

Toughening of brittle ceramics by multiwall carbon nanotube (MWCNT) reinforcements is an attractive approach. However, in situ direct observation of crack propagation and direct toughness-curve (T-curve) measurements using sharp starter-cracks in well-controlled ceramic/MWCNTs nanocomposites are lacking. Here, fully-dense, isotropic Al2O3/10%MWCNTs nanocomposites with individually dispersed acid-treated MWCNTs are fabricated using colloidal processing and spark plasma sintering (SPS). The MWCNTs are primarily embedded within Al2O3 grain boundaries, and appear to be clamped by a large compressive residual stress of the order of −3.7 GPa. In situ-direct and ex situ characterization of crack propagation is performed, observations from which provide insights into the exceptional crack-bridging toughening mechanisms in these nanocomposites. It is revealed that random MWCNTs bridge the crack by ‘unfurling’ (or disentangling), aligning normal to the crack plane, and pulling out. It is confirmed that the MWCNTs pullout occurs primarily between the weakly bonded nested layers (‘sword-sheath’), accompanied by some helical unraveling. The T-curve (T(Δc)) measured using four-point bending on Al2O3/10%MWCNTs nanocomposite specimens containing sharp starter-cracks of various lengths (Δc) shows modest toughening, with the steady-state toughness reaching 4.6 MPa m0.5. Crack profile of a stable equilibrium crack in the Al2O3/10%MWCNTs nanocomposite is measured, and it is used to calculate the bridging-stress constitutive relation. This relation is used to calculate the T-curve independently, and it is found to compare favorably with the experimentally measured T-curve. The implications of this new understanding of toughening in isotropic ceramic/MWCNTs nanocomposites are discussed.

Influence of surface integrity on the fatigue behaviour of a hot-forged and shot-peened C70 steel component

Hot-forging is a process commonly used in the manufacturing of automobile parts such as connecting rods. Most hot-forged components are shot-blasted after forging in order to clean off the forging scale. The shot-blasting process is akin to shot-peening and greatly affects the surface integrity of the components by introducing hardening and residual stresses. This study focuses on the influence of the surface integrity on the fatigue behaviour of a hot-forged C70 steel connecting rod. Specimens were machined out of the connecting rods in order to perform fatigue testing on the forged surface. Several surfaces states, including shot-peened surfaces, were studied in order to quantify the influence of the surface integrity.A thorough characterisation of the surface integrity of the specimens was first performed. The hardness, residual stresses and microstructure gradients were analysed, and the surface of each specimen was scanned using a profilometer. The specimens show complex networks of surface defects introduced during forging, and the shot-blasting process introduces important surface residual stress and microstructure gradients. High cycle fatigue tests in plane bending were then performed on the specimens, with the surface scans helping to identify the critical defect on which crack initiation occurred. The fatigue results, presented in the form of a Kitagawa diagram, are analysed in order to determine which surface integrity parameters are the most influential on fatigue behaviour. The forging defects have an important negative effect on fatigue strength. After shot-blasting, the fatigue strength increases considerably because of the large compressive residual stresses introduced by the shot-blasting process.

ショットピーニングを施した軸受鋼の異物混入潤滑下における転がり疲労挙動評価

In this study, the effect of shot peening on rolling contact fatigue life of bearing steel was investigated under contaminated lubrication. Shot peening was carried out on bearing steel SUJ2 specimens. Peened specimens were polished to remove the dimples generated by shot peening. The distribution of Vickers hardness and residual stress of the specimens were measured before and after the shot peening. The resistance to indentations was compared by examining the profile of the Vickers indentations. Then, rolling contact fatigue tests were carried out under contaminated lubrication. The material hardness was improved up to about 100 HV on the surface to a depth of 300 μm, and compressive residual stress was generated up to 1546 MPa at a depth of 25 μm due to the shot peening. In addition to the width and the depth of the Vickers indentation, the ridge height which causes the stress concentration around the indentation, was reduced by 17% by the shot peening. The rolling contact fatigue life associated with a 50% reliability (L50) was improved 62 % by the shot peening. However, the improvement of rolling contact fatigue life associated with a 90% reliability (L10) was only 26 % because of the increase in dispersion of the fatigue lives due to surface cracks created by the shot peening. Based on the present result, the increase of surface hardness and generating the large compressive residual stress near the surface by shot peening was found to be effective in increasing the rolling contact fatigue life in contaminated lubrication.

In-situ TOF neutron diffraction studies of cyclic softening in superelasticity of a NiFeGaCo shape memory alloy

Real-time in-situ neutron diffraction was conducted during uniaxial cycling compression of a Ni49.3Fe18Ga27Co5.7 shape memory alloy to explore the mechanism on its superelasticity at room temperature, which was manifested by the almost recoverable large strains and the apparent cyclic softening. Based on the Rietveld refinements, the real-time evolution of volume fraction of martensite was in-situ monitored, indicating the incremental amount of residual martensite with increasing load cycles. Real-time changes in intensities and lattice strains of {hkl} reflections for individual phase were obtained through fitting individual peaks, which reveal the quantitative information on phase transformation kinetics as a function of grain orientation and stress/strain partitioning. Moreover, a large compressive residual stress was evidenced in the parent phase, which should be balanced by the residual martensite after the second unloading cycle. The large compressive residual stress found in the parent austenite phase may account for the cyclic effect on critical stress required for triggering the martensitic transformation in the subsequent loading.

Numerical analysis of stress-induced and concentration-dependent carbon diffusion in low-temperature surface carburisation of 316L stainless steel

A kinetic model based on stress-induced and concentration-dependent carbon diffusion was developed for simulating the carbon concentration-depth profile of carburised austenitic stainless steel. The model considers that the carbon diffusivity is dependent of carbon concentration and the stress induced by diffusion of the dissolved carbon atoms can affect the diffusion behaviour in turn. The results show that in carburised 316L stainless steel, the calculated carbon concentration-depth profile is in good agreement with experimental results, which indicates that the stress and concentration-dependent diffusivity play important roles in carbon diffusion. As a result of carburisation, large compressive residual stress is generated and gradiently distributes in the carburised layer, meanwhile, the diffusion of carbon atoms can be accelerated by stress. Although, the compressive residual stress is not the dominant reason for total carbon diffusivity increases significantly with increasing carbon concentration, as the next driving force it cannot be ignored during low-temperature surface carburisation.