Relaxation Of Compressive Residual Stress Research Articles

Quantitative evaluation of residual stress and microstructural effects on the surface hardness of machined Ti-6Al-4 V alloy with microscopic characterization techniques

The surface hardness of machined Ti-6Al-4 V components has a significant influence on the service performance and fatigue life. It’s acknowledged that the hardness value of machined surface is dependent on microstructure and residual stress, however, the quantification of their influence levels on surface hardness is still an unresolved issue, which limits the potential for further enhancing the mechanical property of machined parts. Furthermore, it’s difficult for current studies to fully reveal the mechanism of hardness fluctuation due to release of residual stress. To address these gaps, this study presents a novel method for quantitatively evaluating residual stress and microstructural effects on the machined surface hardness using microscopic characterization techniques. The microstructures of machined Ti-6Al-4 V workpiece were characterized by electron backscatter diffraction (EBSD), and the microstructural hardening effects were analyzed. The residual stresses release and evaluation were performed by the Focused Ion Beam-Digital Image Correlation (FIB-DIC) ring core method, then the surface hardening originated from residual stress and microstructural effects can be assessed by nano-indentation measurements. The results show that the surface hardness increase induced by the combined effects of compressive residual stress and microstructure, where the microstructural hardening can be strengthening by obtaining smaller grain size and high proportion low grain boundary, and the higher magnitudes of compressive residual stress can also enhance the hardness increase. The surface softening induced by compressive residual stress relaxation with no obvious microstructural alterations was observed and the maximum softening reached 14.54% in this study. The findings are beneficial for understanding the mechanical property strengthening mechanism of the machined parts.

Revisit of bake hardening mechanism: Influence of baking on tensile properties of press hardening steels

Paint baking is a process in automotive industries to accelerate the curing of paint and increase yield strength of steel panel for improving the dent resistance due to the formation of well-known Cottrell's atmosphere. However, our careful study on bake hardening mechanism of three press hardening steels (PHS) indicates a different explanation. We developed two novel Nb/Si alloyed steels with superior tensile properties when compared with the commercial grade of 22MnB5. It was found that the paint baking at 170 °C led to the significant increase of yield strength but the decreases of both uniform and total elongation for all the three PHSs. According to the neutron diffraction results, both the baking and the tensile straining until the elastic-plastic transition strain (εep) led to the decrease of full width at half maximum (FWHM), indicating that both should release the residual stress (RS) that was generated during martensite transformation; whilst the internal friction examinations reveal no presence of Snoke peak and no influence of the baking on Snoek-Kê-Köster peak. Based on these results, the kinetics simulation on forming Cottrell's atmosphere and the relevant microstructural characterization results, we can safely conclude that the strengthening caused by the baking is not ascribed to the change of Cottrell atmosphere, carbide precipitates or retained austenite (RA) grains during the baking although commonly believed, but just due to the release of RS. Moreover, the relaxation of compressive RS in martensite and the transformation of RA grains to martensite are the main contributor to the strain hardening of invented PHSs before εep. Therefore, the baking treatment decreases both strain hardening capability and ductility of studied PHSs due to the release of the RS.

Thermal relaxation of compressive residual stresses in surface gradient nanostructure of TC11 titanium alloy

The thermal relaxation behaviour of compressive residual stress (CRS) in the gradient nanostructure of TC11 titanium alloy treated with ultrasonic surface rolling process (USRP) was investigated by X-ray residual stress test, transmission electron microscope (TEM) and high-resolution TEM (HRTEM). The Zener-Wert-Avrami (ZWA) model was successfully used to simulate the CRS relaxation behaviour in nanocrystalline and non-nanocrystalline regions at different temperatures. The results show that the relaxation of CRS is thorough and fast at higher temperature (650 ºC). There are two distinct relaxation stages at lower temperatures, and the relaxation rate in the first stage is much faster, dominated by the annihilation of lattice defects through short-range diffusion. The relaxation in the second stage is slow and dominated by dislocation rearrangement. In contrast to the situation at low temperatures, the nanocrystals appear to be a negative factor in stabilizing the CRS at high temperatures, as the unstable sub-grain boundaries act as sources of stress relaxation and the multiple grain boundaries act as high-speed diffusion channels and accelerate the annihilation of dislocations.

Estimation of residual stress relaxation in low alloy steel with different hardness during fatigue by in situ X-ray measurement

In situ X-ray stress measurements were conducted to investigate the relaxation of compressive residual stress induced by fine particle peening during fatigue. To quantitatively assess the stability of the compressive residual stress, a concept of relaxation threshold stress was proposed. The relaxation threshold stress, which indicates the stability of the compressive residual stress, showed a direct correlation with specimen hardness. If the combined value of the compressive residual stress and compressive applied stress surpass the relaxation threshold stress, then the compressive residual stress relaxes. The relaxation threshold stress can be used to estimate the relaxation that occurs during fatigue.

Verification of stress transfer mechanism in fatigue crack growth under overload

Cyclic plastic finite element modeling was carried out to investigate effect of overload on fatigue crack growth under blunting and no-blunting mechanisms at different overload ratios. The stress transfer mechanism was verified and was found more obvious in the case of crack blunting though its effect was time-limited. The level of crack closure varied periodically due to gradual relaxation of compressive residual stress during crack propagation. Both the overload plastic zone and the overload affected zone were linearly correlated with overload ratio, and an overload ratio value of 1.1 was the best choice for studying overload effect.

Hot salt corrosion behavior of Ti–6Al–4V alloy treated with different surface deformation strengthening processes

In this study, the effects of shot peening (SP) and ultrasonic surface rolling process (USRP) on the high-temperature oxidation and hot salt corrosion (HSC) behavior of the Ti–6Al–4V alloy were systematically investigated. The results showed that the high-temperature oxidation resistance of this titanium alloy can be significantly improved by the above two treatments. However, the hot salt corrosion resistance of Ti–6Al–4V alloy cannot be improved by SP due to surface roughness increasement, the surface defects and the complete relaxation of compressive residual stress (CRS) at high temperature. On the contrary, the hot salt corrosion resistance of USRP treated specimen was remarkably improved because USRP treatment significantly reduced the surface roughness and introduced deep CRS field with good thermal stability. In addition, the refined surface microstructure promoted the formation of a protective oxide film, which was also beneficial for inhibiting hot salt corrosion.

Improving the high cycle fatigue property of Ti6Al4V ELI alloy by optimizing the surface integrity through electric pulse combined with ultrasonic surface rolling process

To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy, the electric pulse has been introduced into the ultrasonic surface rolling process (USRP), which is called electric pulse-assisted ultrasonic surface rolling process (EUSRP). With the help of “electroplasticity” of the electric pulse, the thickness of the surface gradient deformation layer was about three times of the USRP specimens by adjusting the pulse current level. However, the surface hardness decreases due to the continuous effect of the pulse current and the “skin effect” during treatment. It is worth noting that the higher the applied pulse current, the more severe the softening. This paradox causes the fatigue performance of EUSRP specimens lower than that of USRP specimens. To break this paradox, the EUSRP treatment is followed by a USRP treatment. The EUSRP-2 (with a pulse current of 200 A) +USRP specimens exhibit excellent surface hardness, a gradient deformation layer thickness of about 400 µm, low surface roughness and high compressive residual compressive stress. Besides, the hardening mechanisms of the different surface strengthening specimens have been quantitatively analyzed in combination with microstructure analysis. The fatigue life of Ti6Al4V ELI alloy can be improved by about 25 times at 780 MPa using the EUSRP-2+USRP treatment, the main reason for the highest fatigue life is the deepest surface gradient layer and the deepest crack initiation site. The fatigue limit of the EUSRP-2+USRP specimens is not the highest because too much surface hardening causes compressive residual stress relaxation during cycling and the beneficial effect of compressive residual stress is eliminated.

Fatigue crack initiation and growth behavior within varying notch geometries in the low‐cycle fatigue regime for FV566 turbine blade material

AbstractPlain bend bars made from FV566 martensitic stainless steel were extracted from the root of ex‐service power plant turbine blades and several industry‐relevant notch geometries were introduced. Some of the samples were shot peened. The notched bend bars were loaded plastically in the low‐cycle fatigue regime and finite element (FE) modeling carried out to investigate the effects of changing notch geometry, combined with shot peening, on fatigue behaviors such as crack initiation, short crack growth, and coalescence. Shot peening damaged the notch surface, accelerating initiation behaviors, but had a lifetime‐extending effect by retarding short crack growth in all tested notch geometries. At a total strain range higher than 1.2%, the lifetime extension benefit from shot peening was diminished due to compressive residual stress relaxation in the notch stress field. Notch geometry (and the associated varying constraint levels and stress/strain gradients) was found to have no notable difference on fatigue life when tested at identical notch‐root strain ranges.

Deformation-induced phase transition and nanotwins in SS 304 steel during cryogenic laser shock peening without coating

The influence of Laser shock peening without coating at a cryogenic temperature (CLPwC) on SS 304 alloy was investigated. Three different laser energies, such as 150 mJ, 300 mJ, and 450 mJ were chosen to investigate the effects of CLPwC in SS 304 steel. The impact of CLPwC on the residual stress, surface roughness, hardness, phase transformation and microstructural changes was studied. The XRD studies show the peak broadening and strain-induced martensite (γ→α′) of 36.3% after CLPwC at 9 GW cm−2. The work-hardening effect was observed up to 300–400 μm depth from the peened surface. The microhardness could effectively increase to 309.43 HV at 9 GW cm−2 when it compared to unpeened hardness (192.1 HV). Relaxation of compressive residual stress at cryogenic temperature was distinguished, and the maximum compressive stress of −138 MPa was obtained at 50 μm from the surface. Microstructural studies were carried out by different techniques such as EBSD and TEM. An increase in low angle grain boundary fraction (43%) and grain size reduction was observed using EBSD after CLPwC. The TEM investigations revealed deformation twins (width of 6–70 nm), shear bands, stacking faults, and martensitic phase. Strain induced martensitic phase is witnessed in XRD, EBSD, TEM studies.

In-situ warm shot peening on Ti-6Al-4V alloy: Effects of temperature on fatigue life, residual stress, microstructure and mechanical properties

Shot peening (SP) is a matured surface enhancement technique to improve fatigue strength and fatigue lives of cyclically loaded metallic components at room temperature. In order to achieve enhanced beneficial effects beyond SP’s conventional performance, SP has been attempted at elevated temperatures or under tensile stress loading on spring steel with some success. In this work, SP on α/β Ti-6Al-4V alloys has been carried out when they were heated at elevated temperatures up to 400 °C. The effects of such in-situ warm SP on fatigue life of Ti-6Al-4V at different temperatures have been evaluated and correlated with respective development in compressive residual stress distribution, microstructure and hardness. The optimum in-situ warm SP condition was achieved at 100 °C after a trade-off between compressive residual stress relaxation and enhanced grain refinement, both of which are affected by thermal softening but to different degrees.

Long Term Stability Evaluation of Stress Improvement Effect on Ni-Based Alloy Welds by Various Residual Stress Improvement Methods

Stress corrosion cracking (SCC) is one of serious aging degradation problems for the Alloy 600 components of pressurized water reactors (PWRs). In order to prevent SCC, various methods such as water jet peening (WJP), laser peening (LP), surface polishing have been used to introduce compressive stresses at the surfaces of the PWR components. However, it has been reported that such compressive residual stress introduced by these methods might be relaxed during the practical operation, because of high temperature environment. In this study, the hardness reduction behavior of the Alloy 600 processed by LP, Buff and WJP in the thermal aging process has been investigated to estimate the stability of the residual stress improving effect by each method, based on the fact that there is a correlation between the compressive residual stress relaxation and the decrease of hardness. The behavior of the residual stress relaxation in the processed materials in the high temperature environment has been discussed with kinetic analysis.

Fatigue limit estimation for carburized steels considering surface texture and relaxation of compressive residual stress

Fatigue limit estimation for carburized steels considering surface texture and relaxation of compressive residual stress

Evaluation of the Compressive Residual Stress Relaxation Behavior by <i>in situ</i> X-ray Stress Measurement

Evaluation of the Compressive Residual Stress Relaxation Behavior by <i>in situ</i> X-ray Stress Measurement

Influence of pulse frequency on microstructure and mechanical properties of Al-Ti-V-Cu-N coatings deposited by HIPIMS

As an important parameter of HIPIMS, pulse frequency has significant influence on the microstructure and mechanical properties of the deposited coatings, especially for the multi-component coatings deposited by using a spliced target with different metal sputtering yields. In this study, a single Al67Ti33-V-Cu spliced target was designed to prepare Al-Ti-V-Cu-N coatings by using high power impulse magnetron sputtering (HIPIMS). The results showed that the peak target current density decreased from 0.75 to 0.24 A∙cm−2 as the pulse frequency increased, along with the microstructure transferred from dense structure to coarse column structure. The pulse frequency has significant influence on chemical compositions of Al-Ti-V-Cu-N coatings, especially for Cu content increasing from 6.2 to 11.7 at.%. All the coatings exhibited a single solid-solution phase of Ti-Al-V-N, and the preferred orientation changed from (111) to (220) when the pulse frequency increased above 200 Hz. The decrease in peak target current density at high pulse frequencies resulted in a sharp decrease in the coating hardness from 35.2 to 16.4 GPa, whereas the relaxation of compressive residual stress contributed to an improvement in adhesion strength from 43.3 to 79.6 N.

Analysis on the Fatigue Properties of Shot-Peened Al-Si-Mg Alloy and Its Fatigue Life Prediction

Ceramic micro-shot peening (CMSP) and steel micro-shot peening (SMSP) were utilized to investigate the effect of micro-shot peening (MSP) on the high-cycle fatigue properties of Al-7Si-0.3Mg casting aluminum alloy in a previous study. However, the improvement effects of CMSP and SMSP on the fatigue strength (at 5 × 107 cycles) were only 33% because the depth of harden layers was only 20 and 55 μm while the depth of compressive residual stress affected layers was only 37 and 68 μm. In this study, conventional shot peening (CSP) was utilized, and the results were compared with those of MSP, with the expectation that CSP would provide a greater improvement in the fatigue strength. The affected surface layers of the shot-peened specimens were characterized using surface morphology, microhardness, and residual stress analyses. In addition, the effect of CSP on the fatigue strength at 5 × 107 cycles was investigated using a rotating bending fatigue test (R = − 1). An investigation of the extensive surface compressive residual stress relaxation process for the three different shot-peened specimens during cyclic loading was conducted using x-ray diffraction. In addition, the initiation sites for fatigue cracks on the fracture surface were observed using scanning electron microscopy. Furthermore, the fatigue life of the samples with the internal casting defect failure mode was predicted using linear elastic fracture mechanics, while that for samples with the surface crack initiation failure mode was predicted using the modified Morrow model considering the residual stress.

Effects of High-Temperature Deep Rolling on Fatigue, Work Hardening, and Residual Stress Relaxation of Martensitic Stainless Steel AISI 420

The deep-rolling, mechanical surface treatment was modified by applying heat during operation on the martensitic stainless steel AISI 420. Fatigue performance of the high-temperature deep-rolled condition was investigated and compared with the non-surface-treated and room-temperature deep-rolled conditions. Thermal and mechanical relaxation behaviors of residual stresses and work-hardening states were investigated. Annealing processes were carried out at a temperature range of 300-600 °C, with soaking times between 0.1 and 104 min for thermal relaxation. Fatigue tests were carried out at given stress amplitudes of 517-600 MPa, with the different number of cycles for the mechanical relaxation. The residual stresses and work-hardening states were determined using x-ray diffraction with the sin2Ψ method. It was found that the thermal relaxation can be described with the Zener–Wert–Avrami function. The relaxation mechanism was changed from the volume diffusion of the room-temperature deep-rolled condition to the dislocation-cored diffusion of the high-temperature deep-rolled condition. The mechanical relaxation of compressive residual stresses was higher than that of the work-hardening states. The residual stresses of the high-temperature deep-rolled condition are more stable than those of the room-temperature deep-rolled condition under cyclic loading.

Role of ZrO2 oxide layer on the fretting wear resistance of a nuclear fuel rod

The ZrO2 oxide layers of Zr-based fuel claddings have been intensively studied to unveil their oxidation mechanisms in high temperature pressurized water. Nevertheless, there has been insufficient research on their mechanical properties, which are key factors determining the resistance to grid-to-rod fretting damage in normal Pressurized Water Reactor (PWR) operation. An experimental approach was applied to examine the tribological behavior of time-dependent oxide layers on both Zr cladding and grid, which were prepared in simulated PWR conditions for up to 360 days. It was found that the wear rate of pre-oxidized Zr cladding suddenly dropped with increasing oxide thickness of both the cladding and grid. The increase of surface roughness with oxide growth on the Zr-based grid could result in a rapid increase of wear damage by third-body abrasion. The well-developed columnar structure of the ZrO2 oxide layer could have a detrimental effect on the resistance to plastic deformation due to the enlarged grain size and relaxation of compressive residual stress by tetragonal to monoclinic ZrO2 transformation and crack formation. Consequently, ZrO2 oxide layers formed on fuel cladding and spacer grid under high temperature pressurized water show sufficient ductility to accommodate plastic deformation, which results in enhanced fretting wear resistance.

Influence of cyclic loading on the relaxation behavior of compressive residual stress induced by UIT

Fatigue strength of welded joints is improved by inducing compressive residual stress at weld toes. The ultrasonic impact treatment (UIT) is regarded as an effective method for inducing compressive residual stress at weld toes. However, excessive loads that cause plastic deformation at weld toes relieve the induced compressive residual stress. In addition, stress fluctuation affects the relaxation of residual stress even it is smaller than the yield strength. The relaxation of compressive residual stress decreases the fatigue strength improvement. Therefore, in order to evaluate the fatigue strength of welded joints enhanced by UIT, it is necessary to quantify the relaxation of local residual stresses at weld toes under cyclic loading. In this study, we measured a residual stress at the UIT groove using the X-ray diffraction method with a small-diameter collimator and an irradiation angle that was corrected based on the shape effect of the UIT groove, and we investigated its relaxation behavior under cyclic loading. In addition, we estimated the fatigue test results of the out-of-plane gusset joints improved by UIT using the modified Goodman diagram considering the relaxation of residual stress.

Effect of low-temperature surface hardening by carburization on the fatigue behavior of AISI 316L austenitic stainless steel

The influence of low-temperature gaseous carburization on the fatigue behavior of AISI 316 L austenitic stainless steel was investigated. Tension-compression axial fatigue tests were performed under ambient conditions on untreated and carburized AISI 316 L. The results show that the carburized AISI 316 L has a 22% higher endurance limit compared to untreated AISI 316 L. Fractography investigations with scanning electron microscope (SEM) reveal that for the untreated AISI 316 L fatigue cracks initiate at the surface regardless of the applied stress level. For the carburized AISI 316 L fatigue cracks initiate at the surface for relatively high-level stresses; for relatively low-level stresses fatigue cracks initiate at inclusions beyond the carburized case. After carburization, the ductility in the outmost 10 μm of the carburized case has significantly reduced, leading to micro-crack occurrence during fatigue tests and associated relaxation of compressive residual stress in this region. Beyond this surface-adjacent region, no evident stress relaxation occurs due to the enhanced yield strength of the carburized case. The enhanced fatigue performance is mainly ascribed to the compressive residual compressive stress profile introduced by the carbon-concentration profile over the case. Moreover, solid solution strengthening by interstitially dissolved carbon contributes to improve the fatigue performance.

Microstructure-mechanical property correlation in shot peened and vibro-peened Ni-based superalloy

In this experimental research, shot peening (SP) and vibro-peening (VP) processes were employed to modify the surface and sub-surface of a Ni-based superalloy Udimet®720Li. The effect of these treatments on the surface topography, microstructure evolution, and mechanical properties was investigated using instrumented tests. The compressive residual stresses and plastic strain generated through surface treatment were characterized using X-ray diffraction (XRD). The depth of plastically strained region was estimated using a quantitative analysis of grain orientation spread (GOS) data from electron back-scattered diffraction (EBSD) technique. It was observed that the magnitude of compressive residual stresses induced by VP is relatively low compared to SP, but the converse is true for depth of influence (DOI) for the same peening intensity. After thermal exposure at 650 °C (∼0.4Tm) for 5 h, the relaxation of compressive residual stresses and plastic strain was less pronounced in vibro-peening when compared to shot peening representing an improved thermal stability of the former process. Furthermore, 5.24 GPa surface micro-hardness of the untreated sample (O) was increased by ∼30% and ∼25.2% following SP and VP respectively. This resultant micro-hardness enhancement is due to grain refinement, plastic strain effects, and the introduction of low angle grain boundaries (LAGBs). Finally, a positive correlation among microstructural features and mechanical properties was fitted from the experimental data.