High Compressive Residual Stress Research Articles

Fatigue behaviour of notched U75V-steel treated by bearing ultrasonic peening

ABSTRACTThe notched U75V steel specimen was treated by a special way called bearing ultrasonic peening on the base of traditional ultrasonic peening. The bearing ultrasonic peening means that ultrasonic peening was applied after fatigue static load was imposed. Severe plastic deformation and work hardened layer were produced in the notched surface. The effect of compressive residual stress induced by ultrasonic peening treatment on fatigue properties was deeply analysed. Results showed that the depth of work hardened layer is around 90 μm and the hardness value of the treated surface is about 1.7 times of base material. Fatigue strength corresponding to 107 cycles of as-notched specimens treated by traditional ultrasonic peening and bearing ultrasonic peening was improved by 71.9 and 81.8%, respectively. The lasting high compressive residual stress during fatigue tests is the main reason of fatigue life improvement for as-notched specimens treated by bearing ultrasonic peening.

Comparison in surface integrity and fatigue performance for hardened steel ball-end milled with different milling speeds

This paper aims at comparing the surface integrity and fatigue performance of hardened AISI D2 steel ball-end milled with different milling speeds. Results show that the milling speed has a significant effect on surface topography, surface roughness, microhardness and residual stress. The fatigue life for samples with hard milled surface was raised at least 83% compared with that of polished samples. This is ascribed to the high surface residual compressive stress and the enhanced microhardness induced by the hard milling process. The maximum difference in fatigue life is about 12.2% for samples milled with spindle speed N ranging from 3000 r/min to 8000 r/min. Samples milled with a low or high spindle speed have a longer fatigue life than those samples milled with a middle spindle speed condition (N=6000 r/min). Moreover, excessive surface strain hardening will increase surface brittleness which is detrimental to fatigue resistance. The increase of spindle speed can decrease the degree of strain hardening to some extent. Based on this, selecting a relatively high spindle speed can obtain a satisfying combination of surface residual stress and microhardness during the ball-end milling process of hardened AISI D2 steel, which is feasible and prospective from high fatigue performance of ball-end milled surface and the machining efficiency point of view.

Characteristics of Residual Stresses Generated by Induction Heating on Steel Plates

This paper presents experimental and numerical investigations on the characteristics of residual stresses generated by induction heating (IH) on 12 mm thick steel plates. IH at 250 °C and 350 °C provided high tensile residual stresses in the heating field but high compressive stresses away from the heating field. The double heating case generated higher compressive residual stresses—around 200 MPa—than the single heating case because of the superposition of the compressive residual stresses. It will be expected to improve the fatigue performance of welded joints when IH is applied for repair work on existing steel structural members susceptible to fatigue damage. Numerical simulation models for predicting residual stresses by IH were proposed by adopting the body heat flux input and the surface heat flux input. They will be beneficial for identifying the optimum heating conditions, such as the target temperature and the heating field, for applying IH to the actual repair work of the steel structural members.

Silicon carbonitride thin films deposited by reactive high power impulse magnetron sputtering

Amorphous silicon carbonitride thin films for biomedical applications were deposited in an industrial coating unit from a silicon target in different argon/nitrogen/acetylene mixtures by reactive high power impulse magnetron sputtering (rHiPIMS). The effects of acetylene (C2H2) flow rate, substrate temperature, substrate bias voltage, and HiPIMS pulse frequency on the film properties were investigated. Low C2H2 flow rates (<10sccm) resulted in silicon nitride-like film properties, seen from a dense morphology when viewed in cross-sectional scanning electron microscopy, a hardness up to ∼22GPa as measured by nanoindentation, and Si-N bonds dominating over Si-C bonds in X-ray photoelectron spectroscopy core-level spectra. Higher C2H2 flows resulted in increasingly amorphous carbon-like film properties, with a granular appearance of the film morphology, mass densities below 2g/cm3 as measured by X-ray reflectivity, and a hardness down to 4.5GPa. Increasing substrate temperatures and bias voltages resulted in slightly higher film hardnesses and higher compressive residual stresses. The film H/E ratio showed a maximum at film carbon contents ranging between 15 and 30at.% and at elevated substrate temperatures from 340 °C to 520 °C.

NEW WAYS OF DESIGNING SUPER STRONG MATERIALS BASED ON USE OF ALLOYED LOW HARDENABILITY STEELS

Currently, metallurgists are trying to improve mechanical properties of steel by increasing alloy elements in it that require slow cooling in oils or high concentration of water polymers solutions. The paper discusses opposite way in increasing service life of machine components by cardinal decrease alloy elements in steel and providing intensive cooling in locally agitated plain water performed by moving sprayers. The proposed new approach is based on two fundamental phenomena which include creation of high surface compressive residual stresses after quenching and obtaining super - strengthened material in surface layers after intensive cooling. These two factors compensate decrease alloy elements in steel. The paper provides methodology of calculation for achieving maximal effect in residual surface compressive stress formation and super- strengthening of material depending on martensite finish temperature of steel. Effectiveness and benefits are considerable due to saving alloy elements, energy and increasing service life of products. A team of Ukrainian leading specialists is organized to design appropriate software for governing and optimizing hardening processes with the aim of achieving above benefits and make environment clean. New steel and technology of hardening is based on UA Patents No. 109577 and No. 114174

Surface integrity and corrosion performance of biomedical magnesium-calcium alloy processed by hybrid dry cutting-finish burnishing.

Biodegradable magnesium-calcium (MgCa) alloy is a very attractive orthopedic biomaterial compared to permanent metallic alloys. However, the critical issue is that MgCa alloy corrodes too fast in the human organism. Compared to dry cutting, the synergistic dry cutting-finish burnishing can significantly improve corrosion performance of MgCa0.8 (wt%) alloy by producing a superior surface integrity including good surface finish, high compressive hook-shaped residual stress profile, extended strain hardening in subsurface, and little change of grain size. A FEA model was developed to understand the plastic deformation of MgCa materials during burnishing process. The measured polarization curves, surface micrographs, and element distributions of the corroded surfaces by burnishing show an increasing and uniform corrosion resistance to simulated body fluid.

Assessment of Mechanical Properties, Residual Stresses and Diffusible Hydrogen of Longitudinal Weld in Electric Water Heater Tanks

The core technique of electric storage water heaters is the manufacture of the tanks, and the welding technique is the key technology of the tank manufacture. A company producing water heaters received feedback from its after-sales that a large quantity of water leakage from the tanks was found in the market. A follow-up on the manufacturing site by the researchers showed that the position of the water leakage defects mostly happens in the welds of the tanks. Series of experiments were conducted to investigate the mechanical properties, residual stress, and diffusible hydrogen content of the longitudinal weld of the tank. Results of the mechanical properties showed that both the micro-hardness and the tensile strength of the welds are higher than the base metal. There are higher residual compressive stresses in the weld center and the adjacent-weld zone. Vertical residual compressive stress decreases first and then increases (149 MPa–28 MPa–134 MPa) from the weld ends to the middle, whilst the transversal stress decreases continuously (332 MPa~240 MPa) from the weld center to the heat affected zone. After enameling, the longitudinal welds’ peak micro-hardness of coarse grained region reduces by 30.9 HV1, and the average micro-hardness of the HAZ reduces by 5 HV1. The average vertical residual compressive stress difference of the same test points reduces from 270.6 MPa to 20.4 MPa. It proves that enameling can improve the mechanical property of the longitudinal welds.

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.

Nanoindentation data analysis of loading curve performed on DLC thin films: Effect of residual stress on the elasto-plastic properties

The present work is a contribution to the understanding of the mechanical behavior of DLC thin films through nanoindentation tests. DLC films of different thicknesses deposited by the PECVD process on a silicon substrate contain high residual compressive stresses when they are very thin and the stresses become relatively low and more relaxed as the film thickens. These different levels of residual stress influence the values of hardness (H) and Young's modulus (E) obtained when probing the film-substrate system by nanoindentation. It is observed that the DLC layers exhibit different mechanical behaviors even when they are deposited under the same conditions. It is proposed that the compressive stress induces structural modifications resulting in modifying the elasto-plastic properties of each thin film-substrate system. Data analysis of the loading curve can provide information on the elasto-plastic properties of DLC thin films, particularly the stiffness (S) and Er2/H, as a function of residual compressive stresses. The structural changes induced by residual stresses were probed by using Raman spectroscopy and correlated to the mechanical properties.

Internal stress on adhesion of hard coatings synthesized by multi-arc ion plating

The performance of PVD hard coatings is strongly affected by the level of residual stress as it may cause the coatings to detach from the substrate and thus shorten the tool's life. In this study, the residual stress on adhesion of hard coating synthesized by multi-arc ion plating (MAIP) has been investigated. The results indicate that the residual stress of hard coatings has a strong dependence on processing conditions, which is shown by increasing bias voltage and/or decreasing working pressure. Low critical load is assessed for thick coating indicating high interfacial stress, yet the average residual stress obtained from substrate curvature is decreased with increasing thickness. By using finite element modeling (FEM), it is found that high compressive residual stress induces large shear stress at the coating/substrate interface. This implies that the accumulated compressive residual stress is more appropriate than the average one in characterizing the effect of residual stress on coating's adhesion strength. In addition, the effects of residual stress on coating's bonding strength are clearly revealed by using cyclic spherical indentation (CSI) test.

Torsional Fatigue Strength of Newly Developed Case Hardening TRIP-Aided Steel

The torsional fatigue strength of newly developed case hardening steel, i.e., transformation-induced plasticity-aided martensitic steel subjected to vacuum carburizing followed by fine particle peening, was investigated for the fabrication of downsized precision gears with high torque capacity and wear resistance. The surface-hardened layer properties—i.e., high Vickers hardness, high compressive residual stress, and a large amount of retained austenite—considerably increased the torsional fatigue limits of vacuum-carburized and fine particle peened TM and JIS-SNCM420 steels, although the notch-sensitivity to fatigue was increased. The relation between torsional and rotational bending fatigue limits for the smooth specimens was found to be between the maximum principal stress and the minimum shear strain energy criterions. On the other hand, this relation for the notched specimens was represented through the maximum principal stress criterion.

Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening

The aim of this paper was to utilize combined laser shock processing (LSP) and shot peening (SP) to study the grain refinement mechanism and mechanical properties modification of β forging Ti17 alloy. Firstly, LSP experiments were conducted on the surface of Ti17 alloy by a YAG laser system and square spots of 8% overlapping rate. LSP process parameters were pulse width of 15ns, pulse energy of 30J and spot size of 4mm×4mm. Secondly, SP experiments were carried on the surface of LSPed Ti17 alloy. ASH230 steel shots were used to SP with a diameter of 0.3mm and an intensity of 0.3mmA (A-type Almen stripe). Lastly, the microstructures and mechanical properties in the surface layer of Ti17 alloy were investigated by surface topography and roughness, micro-hardness, X-ray diffraction (XRD) analysis, residual stress, scanning electron microscopy (SEM) and transmission electron microscope (TEM). Results showed that surface topography and surface roughness amplitude were increased by combined LSP and SP. The amplitude and depth of the micro-hardness in the surface layer were also significantly improved. No new phase was formed after combined LSP and SP. High amplitude compressive residual stress with −613.5MPa was induced in the surface with combined LSP and SP. The smaller phase sizes in β phase and more obvious crystal defects (deformed twining and dislocations in α phase and dislocations in β phase) were generated by combined LSP and SP. Grain refinement mechanisms were attributed to high density dislocations in α and β phases and multidirectional twin intersections in α phase.

Effect of laser shock peening on the high-temperature fatigue performance of 7075 aluminum alloy

7075 aluminum alloy was subjected to intensive laser shock peening (LSP), and the effect of LSP on the microstructure and high-temperature fatigue properties of the alloy at various elevated temperatures was investigated. Microstructural characterization of the laser-shock-peened (LSPed) material was performed with scanning electron microscopy (SEM) and transmission electron microscopy. The LSPed sample exhibited an improved high-temperature fatigue performance. Its fatigue life increased by 110% at 150°C. Grain refinement, work hardening, and precipitates were detected through SEM. After LSP, significant changes in surface morphology in three stages of high-temperature fatigue were examined. Results suggested that the highly dense dislocation structure and high compressive residual stress induced by LSP significantly improved the high-temperature fatigue performance of the 7075 aluminum alloy.

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.

Effects of laser shock processing on corrosion resistance of AISI 304 stainless steel in acid chloride solution

The effects of laser shock processing (LSP) on corrosion behavior of AISI 304 stainless steel in acid chloride solution have been investigated using electrochemical impedance spectroscopy (EIS). Surface modifications including microstructure, residual stress, micro-hardness and phase transformation are characterized. Results show that LSP can promote the formation of deformation twins and deformation induced martensite, produce grain refinement, generate high magnitude compressive residual stress and increase surface micro-hardness. EIS results show that charge transfer resistance of one layer LSP treated sample is always higher than those of double layer LSP treated sample and non-treated sample in both acid chloride solutions, which implies a higher corrosion resistance for one layer LSP due to the formation of the compact passive film with less defects. Grain refinement and compressive residual stress induced by LSP can enhance the corrosion resistance of AISI 304 stainless steel, but the deformation induced martensite generated during LSP decreases the corrosion resistance. Corrosion resistance of AISI 304 stainless steel in acid chloride solution depends on the combined effects of these factors.

Effect of Residual Stress and Microstructures on 316 Stainless Steel Treated by LSP

In order to improve the stress corrosion resistance of 316 stainless steel, a new technology was proposed and studied. The 316 stainless steel sample was treated by laser shock processing (LSP). The residual stress and microstructures of 316 stainless steel with and without LSP were measured and compared by the methods of X-ray, transmission electron microscopy (TEM) and Electron Back-ScatteredDiffraction (EBSD), and the strengthening mechanism was discussed. It showed that the high residual compressive stress introduced by laser shock processing was about-112 MPa. The TEM and EBSD results showed that severe plastic deformation and nanocrystals layer were formed by LSP, and the orientation of the grains had evident rotation in the process of plastic deformation. These helped to enhance the stress corrosion resistance of 316 stainless steel.

Isolating the effect of residual stresses on coating wear by a mechanical stress relaxation technique

Residual stresses play an essential role for the tribological performance of sputtered coatings since compressive residual stresses yield positive effects on wear resistance. As deposition induced residual stresses are always related to the coating microstructure, a combination of different effects is typically assessed if coatings are validated by tribological testing. Therefore, the real effect of residual stresses on the tribological performance of coatings is not entirely clear. The study aims to examine this issue by isolating the residual stress effect from the microstructural impact by mechanical relaxation of the coating after the deposition process. For this purpose, MoS2 coated substrates are mechanically pre-stretched during the coating process and relaxed again afterwards in order to reduce the coating's macroscopic residual stresses. Other samples, which are not pre-stretched but coated inside the same process with identical distance to the target, serve as a reference material. The proper function of the relaxation technique is demonstrated by applying a recently developed method for residual stress measurement, namely the “Focused Ion Beam – Digital Image Correlation” method. Both samples, the relaxed and as-deposited one, were subjected to a tribological pin-on-disk test. The results show that at high compressive residual stress levels, only mild wear at the upper surface inside the contact area occurs. In contrast, at low compressive residual stress levels, wear results from an abrasive material removal inside the contact track but most significantly from additional coating detachments outside the contact area. The observations can be related, firstly, to a superimposition of harmful bending stresses and beneficial compressive residual stresses beneath the contact area and, secondly, to a stress relaxation effect taking place only inside the tribological contact zone. In summary, owing to the developed stress relaxation technique, a microstructurally independent effect of residual stresses on wear resistance of sputtered MoS2-coatings has been proved.

Microstructure and Properties of Plasma-Nitrided Fe-Based Superalloy Fe-25Ni-15Cr

Fe-based superalloy Fe-25Ni-15Cr was plasma nitrided at a low temperature of 723 K (450 °C). The nitrided layer was characterized by optical microscopy (OPM) and scanning electron microscopy (SEM) and X-ray diffraction (XRD) through stepwise mechanical polishing and transmission electron microscopy (TEM). The results indicated that the double expanded austenite (γ N1 and γ N2) was developed on the nitrided surface. Energy-dispersive X-ray spectrum (EDS) revealed that separate expanded austenite layers with distinctly different nitrogen contents occurred: high (18.98 to 11.49 at. pct) in the surface layer and low (5.87 to 5.32 at. pct) in the subsurface. XRD analysis indicated that large lattice expansion and distortion relative to the untreated austenite of an idea face-centered-cubic (fcc) structure occurred on the γ N1, but low expansion and less distortion on the γ N2. No obvious lattice distortion on the γ N1 was determined by calculating its electron diffraction pattern (EDP), except for detectable lattice expansion. Inconformity between XRD and EDP results suggested that the high compressive residual stress in the γ N1 was mainly responsible for the lattice distortion of the γ N1. TEM indicated that the γ N1 layer exhibited the monotonous contrast characteristic of an amorphous phase contrast to some extent, and corresponding EDP showed a strong diffuse scattering effect. It was suggested that the pre-precipitation took place in the γ N1 in the form of strongly bonded Cr-N clusters or pairs. Decomposition of the γ N1 into CrN and γ occurred at the grain boundaries, and the orientation of both phases remained cubic and cubic relationship, i.e., the planes and the directions with identical Miller indices in both phases were parallel. The nitrided surface was found to have significantly improved wear resistance. Further, the nitrided surface showed no adverse effect in the corrosion resistance but slight improvement in the 3.5 pct NaCl solution.

Increasing fracture strength in bulk metallic glasses using ultrasonic nanocrystal surface modification

Microstructure inhomogeneity is imparted into the surface of a Vit1b Zr-based bulk metallic glass (BMG) using ultrasonic nanocrystal surface modification (UNSM). As a result, compressive residual stresses with a maximum magnitude of 1130 MPa were induced in the near-surface region. Substantial improvements in both fracture strain and strength were observed for the treated specimen in three-point bending tests. The microstructure inhomogeneity and free volume generated by severe plastic deformation resulted in increased shear band density during three-point bending tests, which is evidenced by the vein-like pattern observed on the fracture surface of the treated specimen. Moreover, the enrichment of shear bands can cause the interaction between shear bands and can thus obstruct their propagation, leading to work-hardening behavior. High magnitude compressive residual stresses are also believed to impede and slow down the propagation of the shear bands. The synergistic effect of induced inhomogeneity, increased free volume and compressive residual stresses improves the fracture stress and strain of the UNSM-treated BMG.

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

The effects of Laser Shock Peening (LSP) on the microstructure, residual stress, hardness, strength, and fatigue life of ATI 718 Plus (718Plus) alloy was investigated and the results are reported. Microstructure before and after LSP and after mechanical testing was characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD)/orientation imaging microscopy (OIM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses (∼−725MPa) and changes in the near-surface microstructure. This change was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of a few near-surface sub-grains, which, in turn, caused high surface hardening (∼8.3GPa). In addition, a 16% increase in the yield strength (∼175MPa) and around 15% increase in the endurance limit (∼110MPa) occurred in corresponding tests at room temperature. The improvement in fatigue life was due to the shielding and resistance to crack initiation that the LSP treatment provided to the material through the near-surface microstructure, hardening and high compressive residual stress. This shielding also hindered crack propagation, lowering its rate to a third compared with baseline. Results of studies of stress relaxation with cycles showed how the LSP shield was affected by cycling and, in turn, the improvement in fatigue life of 718Plus. The results have demonstrated that LSP is a powerful surface engineering technique that can improve mechanical properties and fatigue performance of many important metallic materials.