Residual Stress Relaxation Research Articles

Residual stress release and corresponding microstructural changes in high-energy impact-modified GH4169 after aging at 425 °C and 650 °C

The nickel-based superalloy is a critical material in aero-engines, where enhanced durability and resistance to stress relaxation are essential. This study evaluates the effectiveness of a high-energy composite modification technique, applied through dual surface friction methods, to induce residual stress and examines the stability of this stress under medium-temperature aging conditions. GH4169 was selected and a foundational study was conducted to quantify residual stress relaxation under medium-temperature aging treatment. The surface strengthening method used was a high-energy composite modification known for its significant post-treatment effects. The specific temperatures for medium-temperature aging treatment were 425 °C and 650 °C. The test results of the residual stress field confirmed the phenomenon of residual stress relaxation under the influence of temperature. Correspondingly, the different degrees of stress relaxation caused by varying aging treatment conditions are closely related to the microstructural changes within the material. Areas with a higher number of annealing twins exhibited a more pronounced degree of residual stress relaxation. By elucidating the link between stress relaxation and microstructural changes, this research offers a theoretical foundation for optimizing surface-strengthening processes and setting optimal operational temperatures for advanced structural materials.

Behavior, failure analysis, and effectiveness of mechanical stress improvement process in residual stress relaxations in butt-welded austenitic piping using a numerical simulation approach

Behavior, failure analysis, and effectiveness of mechanical stress improvement process in residual stress relaxations in butt-welded austenitic piping using a numerical simulation approach

Manganese Partitioning and its Effect on Residual Stress

The residual stress in low‐carbon high‐strength steel is not effectively relaxed during alloy carbide precipitation but is fully relaxed during manganese partitioning. The specific microscopic mechanisms responsible for this observed phenomenon have yet to be fully elucidated. This study employs first‐principles calculations in conjunction with experiments to demonstrate that alloy carbide precipitation leads to the formation of dense dislocation tangles that hinder carbon diffusion and carbide precipitation. Conversely, manganese partitioning aids carbon diffusion by altering dislocation morphology while also causing plastic deformation. The simultaneous occurrence of partitioning plasticity and precipitation plasticity during manganese partitioning results in a more significant relaxation of residual stress compared to alloy carbides precipitation: The plasticity coefficient for alloy carbide precipitation is 1.303 × 10−5, while the plasticity coefficient for manganese partition is 2.691 × 10−5. During alloy carbide precipitation, the elastic strain energy decreases to 43.48% of its initial value, whereas it can be further reduced to 25.29% after manganese partitioning.

Effect of high-energy radiation on the relaxation of residual stresses in polycarbonate and epoxy resin by stress optics

Residual stresses are an important factor in many polymers because they affect the components strength and potential warpage. Polymers exposed to γ-ray radiation are no exception. The two polymers investigated (bisphenol-A based polycarbonate and epoxy resin), are potentially irradiated either as part of a sterilization process or during later application as part of composite or insulation material. Both polymers show a photoelastic effect that is used in this research to study the effect of 200 kGy and 500 kGy on the relaxation behavior of epoxy and polycarbonate, respectively. The photoelastic sensitivity for pristine and irradiated samples was found to decrease by irradiation treatment. In addition, EP and PC relaxed by 36 and 27% respectively, due to irradiation treatment. Furthermore, epoxy resin shows some artifacts in the stress optical images, which are likely related to curing and are not related to residual stresses.

Estimating the fatigue limits for quenched and tempered steel rods treated with multifunction cavitation considering residual stress, hardness, and surface pits

AbstractIn this study, multifunction cavitation (MFC) was performed on low‐alloy steel (AISI 4140 steel) rods with different hardnesses to increase their fatigue limit. It was found that a high compressive residual stress was generated on the surface of steel rods by MFC and that the magnitude of the compressive residual stress tended to increase with increasing specimen hardness, which resulted in a higher fatigue limit. However, fatigue cracks are known to be initiated from the pits and red rust that form on the surface during MFC treatment in water. Furthermore, relaxation of the compressive residual stress was also investigated during the fatigue test to elucidate the mechanism for improving the fatigue properties. The results showed that the fatigue limit for MFC‐treated steel rods was accurately estimated by considering residual stress relaxation, hardness, and pit formation. Validation of the fatigue limit estimation was also conducted through comparison with Murakami's equation.

Relaxation of residual stress in welded plates during long life fatigue loading

The presence of residual stress in welded components can have a significant impact on fatigue response. Residual stresses were assessed in thick welded A36 cantilever specimens (T-specimens) at the fatigue critical area. Testing involved subjecting the T-specimens to varying bending stresses and load cycles, leading to different levels of local plasticity at the weldtoe. Extensive X-ray diffraction (XRD) measurements were conducted to analyze the residual stress distribution near the weldtoe. Residual stress states were evaluated in both as-welded specimens and those subjected to fatigue loading with strain amplitudes up to 0.001 at the weldtoe stress concentration zone. The findings suggest that fatigue loading can induce changes in the welding-induced residual stress field, ranging from moderate to significant alterations. Therefore, fatigue life predictions should consider the potential evolution of residual stresses due to cyclic loading, including the effects of cyclic stress relaxation in fatigue life calculations.

Strengthening Ti3SiC2/Cu brazed joint assisted with cold spray additive manufacturing: Lower brazing temperature through interdiffusion and graded reinforcement for stress relaxation

Graded composite fillers exhibit unique advantages in alleviating residual stress in ceramic/metal brazed joints. However, traditional methods for preparing graded composite fillers are often complex and may deplete their strengthening phases. In this study, cold spray additive manufacturing was employed to fabricate yttria-stabilized zirconia (YSZ)-Ti25Zr25Ni25Cu25-Ag graded composite coatings on Cu substrates to serve as fillers. The graded composite coatings and the resultant Ti3SiC2/Cu brazed joints were systematically characterized to elucidate the deposition mechanism of the coatings and its residual stress-relaxed mechanism. The bondings between particles in the coatings, based on their plastic deformation ability, were classified into noncrystalline weak bonds and homogeneous strong bonds. Both bondings facilitated the elemental interdiffusion between filler and Cu substrate during the heating process, which contributed to the formation of Ag-Cu eutectic. This significantly reduced the required brazing temperature and its induced residual stress. Furthermore, the solid phase transformation of graded-distributed YSZ within the brazing seam from tetragonal to monoclinic achieved a smooth thermal transition and releasing residual stress at the same time. Consequently, the shear strength of the brazed joint reached to the highest value of 110.23 ± 3.45 MPa, which was Much higher than the previously reported values. This study introduces a novel method for residual stress relaxation assisted with cold spray technology and broadens its application in the brazing field.

Development of a Flow Rule Based on a Unified Plasticity Model for 13Cr-4Ni Low-Carbon Martensitic Stainless Steel Subject to Post-Weld Heat Treatment

This study aims to develop a flow rule for evaluating the relaxation and redistribution of residual stresses during the post-weld heat treatment (PWHT) of hydroelectric runners made from low-carbon martensitic stainless steel (13Cr-4Ni composition). During the PWHT, austenite reforms in the filler metal and surrounding areas of the base metal near welded joints. The evolving inelastic strain rate with reformed austenite led to defining two distinct flow rules in the pure martensitic (α′) and austenitic (γ) phases. A linear rule of mixture was then applied to assess global effective stress based on the inelastic strain rate and current austenite fraction during the PWHT. A unified constitutive model incorporating drag stress and back stress, evolving with creep and plastic deformation mechanisms during the PWHT, described the stress–strain behavior. To validate this analysis, a third flow rule was determined in the 18% tempered austenitic microstructure, compared with the rule of mixture’s effective stress contribution from each phase on the inelastic strain rate. Isothermal constant strain rate tests in stabilized crystalline microstructures evaluated constants specific to their respective flow rules. This study demonstrates the stability of reformed austenite at elevated temperatures during slow cooling and its significant influence on the mechanical properties of 13Cr-4Ni steels. The effectiveness of estimating yield stress using the rule of mixture based on individual phase behaviors is also confirmed.

Residual stress distribution and relaxation in U-rib full penetration welds by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) as an effective method for alleviating welding residual stresses (WRS) is used to improve the fatigue performance of welded structures. However, its application to single- and double-sided U-rib welds in orthotropic decks has been limited. To address this, an experimental and numerical investigation on WRS and UIT was carried out for different deck thicknesses. A 3D thermo-elastic-plastic approach was employed to model the temperature and stress fields by the ABAQUS software. Additionally, a dynamic explicit calculation method was developed to simulate the UIT process, and its accuracy was verified by comparing the actual and numerical grooves and WRS after UIT. Ultrasonic stress measurement was employed to determine the distribution and magnitude of WRS before and after UIT. The results demonstrate that most relative errors of residual stress numerical and test results under different stress paths after UIT are within 10 %, confirming the reliability and effectiveness of this method. For single-sided U-rib welds after UIT, the reduction in WRS at the weld seam ranges from approximately 25.6 % to 87.4 %, while for double-sided U-rib welds, the reduction varies from about 22.2 % to 65.5 %. The impact of UIT on longitudinal residual stresses is more significant compared to its effect on transverse residual stresses. However, UIT results in a relatively smaller reduction in WRS away from the weld seam. Furthermore, it is observed that altering the deck thickness results in different disparities in WRS outcomes after UIT.

Tailoring the microstructure and mechanical properties of laser directed energy deposited Inconel 718 alloy via thermostatic substrate heating

The effects of different thermostatic substrate heating (TSH) temperatures on the stress state, microstructures, and mechanical properties of laser directed energy deposited (LDEDed) Inconel 718 alloy were systematically investigated. The results showed that the optimized TSH temperature (200 °C) transformed the original tensile residual stresses into compressive residual stresses, improved microstructure uniformity, and enhanced the comprehensive mechanical properties of the LDEDed Inconel 718 alloy. When the TSH temperature was 200 °C, the ultimate tensile strength (UTS) and yield strength (YS) of the specimen perpendicular to the building direction (0°) did not change significantly, while the elongation (EL) value was improved by 35.2 %. It was attributed to reduced dislocation density in conjunction with the relaxation of residual stresses. In contrast, the UTS and YS in the specimen parallel to the building direction (90°) increased by 20.4 % and 34.9 %, respectively, whereas the EL values decreased slightly. It was due to the combined effects of the massively precipitated finer and more homogeneous γ'' phase and grain refinement. Finally, the strengthening mechanism of TSH on the microstructures and mechanical properties of the LDEDed Inconel 718 alloy were systematically revealed.

Local relaxation of residual stress in high-strength steel welded joints treated by HFMI

This research studies the influence of high-peak loads on local relaxation of residual stress and fatigue damage in high-strength steel welded joints treated by high-frequency mechanical impact (HFMI) treatment. The joint behavior is simulated with elastic–plastic finite element analyses that account for the combined effect of geometry, residual stress, and material properties. This simulation uses two treated geometry models: with or without surface roughness on HFMI groove, and two material properties: S690QL and AH36 structural steels. The results show that surface roughness and load history, including high-peak loads, significantly influence fatigue response. It is revealed that the model neglecting the surface roughness cannot represent the amount of residual stress change and fatigue damage at less than 100 µm depth from the surface. In addition, the local yield strength in the HFMI-treated zone affects the plasticity behavior near the surface imperfection under the high-peak loads, which provides comparatively different fatigue damage between S690QL and AH36 in some cases. As a result, this study provides the further understanding needed to develop a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

Fatigue life prediction model for shot-peened laser powder bed fused 304L steel considering residual stress relaxation and defect distribution

Shot peening (SP) is a post-treatment technology that enhances the surface smoothness, mechanical properties, and fatigue performance at the surface layer of metal materials produced through laser powder bed fusion (LPBF). Therefore, accurate life prediction is crucial to effectively design and evaluate the SP process and enhance fatigue life. This study employs a cost-effective and efficient analytical method to investigate relaxation in residual stress (RS). The method calculates the interaction between RS in SP and external loading and combines it with a life prediction model. In addition, a defect-based parameter area is employed to characterize the impact of the effective defect dimensions on the fatigue life of LPBF materials. The effects of compressive residual stress (CRS) introduced by SP and process-induced internal defects on the fatigue life of LPBF material are investigated via a modified Goodman‐Basquin model. The fatigue life of 304L stainless steel (SS) prepared by LPBF after SP is predicted, and good consistency between measurement and prediction is observed.

Reducing part deformation of isotactic polypropylene specimens fabricated with powder bed fusion technique through controlling crystallization behaviors

AbstractSerious shrinkage and warpage are obstacles to the development of ideal isotactic polypropylene (iPP) materials for polymer‐based powder bed fusion (PBF) technique. In this work, the variations of the dimensional accuracy of the PBF‐printed iPP parts were investigated with various printing parameter and nucleating agent. The iPP parts printed at a scanning speed of 700 mm/s exhibit smaller extents of shrinkage and curling than those at lower speeds, due to a lower degree of crystallinity. Interestingly, iPP blended with the α‐ or β‐nucleating agent demonstrates more serious part deformation with respect to neat iPP. Especially, α‐nucleating agent tends to trigger the most severe curling under the investigated printing parameters. Based on X‐ray diffraction and differential scanning calorimetry (DSC) results, both neat iPP and α‐iPP parts crystallize into α‐crystal, while β‐iPP parts display the coexistence of β‐ and α‐crystals. And, the difference of the crystallinity is less than 3% in three specimens. This suggests that both crystallinity and crystalline structure are not the main reasons for the shrinkage and warpage in this case. Instead, the severe part deformation of the α‐iPP parts is assigned to the narrower sintering window as well as the higher onset crystallization temperature of α‐iPP, which hinder relaxation of residual stresses. This work provides insights into the part deformation mechanism for PBF‐processed polymer materials.

Identifying multiple synergistic factors on the susceptibility to stress relaxation cracking in variously heat-treated weldments

The 347H austenitic stainless steel has been widely used for pressure vessels and pipeline (PVP) applications due to its excellent creep and corrosion resistance, which fit ideally to the harsh conditions in petrochemical industries, fossil fuel or nuclear power plants, and modern energy storages. However, a failure mode has been commonly observed with cracks emerging at the heat affected zone (HAZ) of weldments during post-weld heat treatment (PWHT) or under intermediate to high temperature service conditions. This phenomenon is termed as Stress Relaxation Cracking (SRC) since the purpose of PWHT is to relieve the welding-induced residual stress fields, or as Stress Age Cracking (SAC) if failure happens during service. A leading literature explanation of this failure suggests that the residual stress relaxation and the precipitation dissolution and/or re-precipitation occur in the same temperature range, which can lead to locally high strains and thus to crack at the grain boundaries. Since in situ spatial measurements of residual stress fields, microstructural evolution, and failure processes are nearly infeasible, this work recourses to a micromechanical finite element framework that models the high temperature failure as the nucleation and growth of grain boundary cavities, whereas various parameters such as thermomechanical loading history and its evolution, the competition of grain-interior dislocation creep and grain-boundary diffusion in failure lifetime, and microstructural heterogeneities (such as the precipitate free zone near grain boundaries) can be quantitatively incorporated. It can be concluded from these microstructure-explicit simulations that an accurate knowledge of residual stress evolution and a carefully calibrated set of material constitutive parameters are the essential prerequisites for lifetime predictions. The understanding of individual governing factors also leads to a mechanistic interpretation of the observed SRC susceptibility C-curves. These results suggest that the criticality of residual stress evolution, but not the precipitation-induced local strains, be the leading factor for SRC.

In-situ synchrotron X-ray diffraction investigation of martensite decomposition in Laser Powder Bed Fusion (L-PBF) processed Ti–6Al–4V

Laser powder bed fusion (L-PBF) processed Ti–6Al–4V components present two substantial challenges: the presence of tensile residual stress and brittleness of the as-fabricated component due to the formation of the martensite (α′) phase. Industrially, post-heat treatment is performed to improve the quasi-static mechanical properties of the Ti–6Al–4V component through martensite (α′) decomposition and residual stress relaxation. However, detailed insights into the martensite decomposition and stress relaxation mechanisms are still lacking. To address this, in-situ high-energy synchrotron x-ray diffraction (HEXRD) was employed to simultaneously track: (I) martensite (α′) decomposition into a mixture of hexagonal α and body-centered cubic β phases, (II) residual stress relaxation process during heating. Rietveld refinement of 2D diffraction patterns recorded over a temperature range spanning from 25 to 1000°C revealed a three-stage evolution in the α′ unit cell parameters. Unit cell parameter analysis combined with the phase fraction analysis based on the diffraction patterns and the chemical composition profiles predicted by Thermocalc simulation gave detailed insight into the martensite (α′) decomposition process. In Stage 1 (until 375°C), the α′ phase unit cell exhibited linear expansion without indications of any phase transformation or stress relaxation. Upon commencement of stage II at 375°C, an increased expansion rate of the martensite (α′) unit cell was observed, attributed to the enhanced outward diffusion of vanadium (V), succeeded by β phase nucleation at 575°C. Simultaneously, during stage II, residual stress relaxation was observed at both macro and micro scale, culminating in a stress-free state achieved at approximately 750–800°C. Lastly, α-to-β phase transformation was observed in stage 3. The thermal evolution of the hexagonal unit cell dimensions was compared between martensitic and mill-annealed α+β microstructures. This comparison unraveled the link between the initial microstructure and the thermal expansion behavior of the hexagonal α′ and α phase.

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.

Effect of titanium interlayer configurations on residual stresses and performance of titanium/titanium nitride multilayer coatings

Multilayer coatings provide an effective measure to control and optimise of residual stresses, enhance coating adhesion, fracture toughness, wear and corrosion resistance of coating system. In this study, we designed and produced bi-layer and four-layer titanium/titanium nitride magnetron sputtered coating configurations by altering the position of the interlayer. The in-plane residual stresses in different coating configurations are measured by multiple peak grazing incidence X-ray diffraction techniques. Subsequently, the effect of different compressive residual stresses on adhesion, mechanical properties, wear and corrosion resistance of coatings are investigated. Results show that the position of interlayer in multilayer coating configuration has a great impact on relaxation of residual stresses. The residual stresses are relieved by 26% and 46% by lowering the position of titanium interlayer from 800 nm at the top to 1000 nm and 1200 nm, respectively. The nanoindentation hardness is decreased by 11% and 13%, respectively. The wear rate of multilayer coating having interlayer position at 800 and 1000 nm from the top is ≈ 23% lower compared to bilayer however it is increased by ≈ 50% when interlayer coatings position changed to 1200 nm from the top. The interposition of interlayer in multilayer coating configurations decreased the corrosion current density by ≈ 50%. All multilayer coatings including higher residual stresses show better adhesion to the substrate compared to bilayer coatings which shows severe delamination.

Residual Stress Relaxation in the Laser Welded Structure after Low-Cycle Fatigue and Fatigue Life: Numerical Analysis and Neutron Diffraction Experiment

The residual stress relaxation behaviour in low-cycle fatigue brings uncertainty to accurately predict fatigue life. Therefore, establishing the residual stress relaxation model for the welded structure is critical. In this paper, the residual stress is simulated through Abaqus finite element software (6.14). The residual stress relaxation model related to the magnitude of cyclic loading and the number of cycles is proposed. Furthermore, the residual stress relaxation model is applied to predict low-cycle fatigue life. Finally, the simulation results are validated by experimental data obtained using the reliable neutron diffraction method, and a good agreement is observed.

Ag Surface Outflow Kinetics in Ag/AlN Nanomultilayers

The heat‐induced microstructure evolution of nanomultilayers (NMLs) with metallic components is widely studied due to its critical importance in ensuring the reliable application of these nanomaterials. The thermal degradation of NMLs is controlled by mass transport of one of the NML components from the NML volume to its surface. The outflow occurs at temperatures well below the melting point of metal, offering new opportunities for the engineering of innovative NML‐based brazing fillers. A combined experimental‐modeling approach for the quantitative analysis of silver (Ag) outflow kinetics in Ag/AlN NMLs is presented. The method is based on in situ monitoring of the X‐ray diffraction intensity evolution of Ag acquired from the near‐surface region of NML (grazing incidence geometry) upon annealing in the temperature range of 230–425 °C (atmosphere of N2). The experimental data are compared to model predictions of Ag diffusion. It is assumed that Ag diffusion is driven by the relaxation of residual stresses in Ag nanolayers. Due to the multilayer geometry, the surface outflow kinetics are mainly defined by the kinetics of Ag diffusion along Ag/AlN interphase boundaries. The parameters defining the Ag surface outflow kinetics, i.e., diffusion coefficients and activation energy for diffusion, are derived.

Role of the martensitic microstructure in the stabilized residual stresses under cyclic loading and in the fatigue behavior of two steels

The influence of martensitic microstructure on the evolution of residual stresses during cyclic loading has been investigated using X-ray diffraction, both in martensite and in retained austenite. Different microstructures containing retained austenite fractions of 25% and 40% were obtained by vacuum carburizing applied to the 14NiCr11 steel. Additionally, other microstructures, such as quenched or tempered martensite of the 42CrMo4 steel were also studied.The results indicate that, for the same hardness and similar stabilized residual stresses in retained austenite, the greater the longitudinal residual stresses in quenched martensite, the greater the improvement in fatigue strength.From initial biaxial isotropic residual stresses, cyclic loading close to the endurance limit created anisotropic residual stress relaxation in quenched martensite and retained austenite. After cyclic loading, the longitudinal residual stresses were higher than the transverse residual stresses, regardless of the type of loading (bending in the case of 14NiCr11 carburized steel or combined bending and torsion in the case of the 42CrMo4 steel). The high level of longitudinal residual stresses after cyclic loading was correlated with the high retained austenite transformation to martensite in the case of 14NiCr11 carburized steel. The anisotropy of the stabilized residual stresses (after a few 10th of cycles) is due to the metastability of quenched martensite under cyclic loading.