Phase Specific Residual Stresses Research Articles

Residual Stress Engineering for Wire Drawing of Austenitic Stainless Steel X5CrNi18-10 by Variation in Die Geometries-Effect of Drawing Speed and Process Temperature.

As a result of conventional wire-forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent forming processes such as bending operations. High tensile residual stresses typically occur in the near-surface region of the wires and can limit further application and processability of the semi-finished products. This paper presents an approach for tailoring the residual stress distribution by modifying the forming process, especially with regard to the die geometry and the influence of the drawing velocity as well as the wire temperature. The aim is to mitigate the near-surface tensile residual stresses induced by the drawing process. Preliminary studies have shown that modifications in the forming zone of the dies have a significant impact on the plastic strain and deformation direction, and the approach can be applied to effectively reduce the process-induced near-surface residual stress distributions without affecting the diameter of the product geometry. In this first approach, the process variant using three different drawing die geometries was established for the metastable austenitic stainless steel X5CrNi18-10 (1.4301) using slow (20 mm/s) and fast (2000 mm/s) drawing velocities. The residual stress depth distributions were determined by means of incremental hole drilling. Complementary X-ray stress analysis was carried out to analyze the phase-specific residual stresses since strain-induced martensitic transformations occurred close to the surface as a consequence of the shear deformation and the frictional loading. This paper describes the setup of the drawing tools as well as the results of the experimental tests.

Determination of the through-thickness residual stress in thick duplex stainless steel welded plate by wavelength-dependent neutron diffraction method

In this study, a wavelength-dependent neutron diffraction method used to measure the residual stress in thick duplex stainless steel (DSS) welded plate was proposed, which was validated by contour method. By choosing appropriate wavelength, the penetration capability of neutron was improved. The neutron with appropriate wavelengths was then applied to investigate the through-thickness volume fraction, macroscopic and phase-specific residual stress in a 55 mm thick DSS welded plate. The results showed that a wavelength at 2.395 Å had more superiorities than the conventionally-used wavelength at 1.593 Å in the measurement of longitudinal and transverse stresses of a thick DSS plate. In general, high tensile stresses were generated in austenite while high compressive stresses were found in ferrite. For elastic mismatch residual stresses, their maximum values in austenite and minimum values in ferrite were located near the mid-thickness region. Reasons for the formation and characteristics of the phase-specific stresses were also discussed. Besides, large tensile macroscopic stresses were found near the surface whilst significant compressive stresses were found to generate in the mid-thickness of weld metal.

Residual stresses in deep-drawn cups made of duplex stainless steel X2CrNiN23-4

Residual stress development in deep drawing processes is investigated based on cylindrical cups made of duplex stainless steel sheet. Using a two-scale approach combining finite element modelling with a mean field homogenization scheme the macro residual stresses as well as the phase-specific micro residual stresses regarding the phases ferrite and austenite are calculated for steel X2CrNiN23‑4 for various drawing depths. The simulation approach allows for the numerical efficient prediction of the macro and phase-specific micro residual stress in every integration point of the entire component. The simulation results are validated by means of X‑ray diffraction residual stress analysis applied to a deep-drawn cup manufactured using corresponding process parameters. The results clearly indicate that the fast simulation approach is well suited for the numerical prediction of residual stresses induced by deep drawing for the two-phase duplex steel; the numerical results are in good agreement with the experimental data. Regarding the investigated process, a significant influence of the drawing depth, in particular on the evolution of the residual stress distribution in drawing direction, is observed. Considering the appropriate phase-specific strain hardening, the two-scale approach is also well suited for the prediction of phase specific residual stresses on the component level.

Neutron and X-ray Diffraction Analysis of Macro and Phase-Specific Micro Residual Stresses in Deep Rolled Duplex Stainless Steels.

Experimental analyses of depth distributions of phase-specific residual stresses after deep rolling were carried out by means of laboratory X-ray diffraction and neutron diffraction for the two duplex steels X2CrNiMoN22-5-3 and X3CrNiMoN27-5-2, which differ significantly in their ferrite to austenite ratios. The aim of the investigation was to elucidate to which extent comparable results can be achieved with the destructive and the non-destructive approach and how the process induced phase-specific micro residual stresses influence the determination of the phase- and {hkl}-specific reference value d0, required for evaluation of neutron strain scanning experiments. A further focus of the work was the applicability of correction approaches that were developed originally for single-phase materials for accounting for spurious strains during through surface neutron scanning experiments on coarse two-phase materials. The depth distributions of macro residual stresses were separated from the phase-specific micro residual stresses. In this regard, complementary residual stress analysis was carried out by means of incremental hole drilling. The results indicate that meaningful macro residual stress depth distributions can be determined non-destructively by means of neutron diffraction for depths starting at about 150–200 µm. Furthermore, it was shown that the correction of the instrumental surface effects, which are intrinsic for surface neutron strain scanning, through neutron ray-tracing simulation is applicable to multiphase materials and yields reliable results. However, phase-specific micro residual stresses determined by means of neutron diffraction show significant deviations to data determined by means of lab X-ray stress analysis according to the well-known sin2ψ-method.

Numerical characterization of residual stresses in a four-point-bending experiment of textured duplex stainless steel

The resulting shapes in production processes of metal components are strongly influenced by deformation induced residual stresses. Dual-phase steels are commonly used for industrial application of, e.g., forged or deep-drawn structural parts. This is due to their ability to handle high plastic deformations, while retaining desired stiffness for the products. In order to influence the resulting shape as well as component characteristics positively it is important to predict the distribution of phase-specific residual stresses which occur on the microscale of the material. In this contribution a comparative study is presented, where two approaches for the numerical simulation of residual stresses are applied. On the one hand a numerically efficient mean field theory is used to estimate on the grain level the total strain, the plastic strains and the eigenstrains based on macroscopic stress, strain and stiffness data. An alternative ansatz relies on a Taylor approximation for the grain level strains. Both approaches are applied to the corrosion-resistant duplex steel X2CrNiMoN22-5-3 (1.4462), which consists of a ferritic and an austenitic phase with the same volume fraction. Mean field and Taylor approximation strategies are implemented for usage in three dimensional solid finite element analysis and a geometrically exact Euler–Bernoulli beam for the simulation of a four-point-bending test. The predicted residual stresses are compared to experimental data from bending experiments for the phase-specific residual stresses/strains which have been determined by neutron diffraction over the bending height of the specimen.

Phase-Specific Strain Hardening and Load Partitioning of Cold Rolled Duplex Stainless Steel X2CrNiN23-4

Multi-phase materials often times consist of constituents with high contrasts in phase-specific mechanical properties. Here, even after homogeneous plastic deformation phase-specific residual stresses develop that may affect the components behaviour in service. For numerical simulation of phase-specific residual stresses, knowledge of the particular phase-specific strain hardening behaviour is essential. In this study, the strain hardening of ferrite and austenite in cold rolled duplex stainless steel of type X2CrNiN23-4 is investigated. By means of X-ray diffraction, the phase-specific load partitioning and residual stress evolution are analysed for uniaxial load application in three directions within the sheets plane, taking into account the sheet metals phase specific anisotropy. In order to assess the necessity for experimental determination of anisotropic phase specific behaviour, the strain hardening parameters, derived from only one loading direction, are implemented in a mean-field approach for prediction of phase-specific stresses. A simplified simulation approach is applied that only considers macroscopic plastic anisotropy and results are compared to experimental findings. For all investigated loading directions, it was observed that austenite is the high-strength phase. This load partitioning behaviour was confirmed by the evolution of phase-specific residual stresses as a result of uniaxial elasto-plastic loading. With the simplified and fast numerical approach, satisfying results for prediction of anisotropic phase-specific (residual) stresses are obtained.

Prediction of residual stresses of second kind in deep drawing using an incremental two-scale material model

ABSTRACT For modern engineering applications, the prediction of residual stresses is a starting-point for the subsequent optimisation of the design with regard to the process induced stresses on the macro and the grain scale. In this paper, a new approach for the numerically efficient prediction of phase-specific residual stresses (residual stresses of second kind) in two-phase materials is presented. The proposed model allows for a two-scale simulation of complex forming processes, solely based on the phase-specific constitutive equations. This enables the calculation of the average stresses and strains in each phase during the entire process and the prediction of the macroscopic material response. The numerical results are compared to experimental diffraction-based data of a bending bar and a deep drawn cup and are aligned better compared to existing models.

Phase-specific residual stresses induced by deep drawing of lean duplex steel: measurement vs. simulation

The final geometry and fatigue behavior of deep drawn components in service is strongly influenced by deformation-induced residual stresses. For multi-phase materials, besides macro residual stresses (first kind), phase-specific residual stresses (second kind) occur on the microscale of the material. In order to influence the component characteristics positively it is important to predict the distribution of the residual stresses on both scales. A two-scale simulation and measurement approach is presented which allows for an efficient determination and validation of the phase-specific residual stresses. Finite-element simulations are performed to predict the deformation-induced macro residual stresses. A numerically efficient mean-field homogenization is used to estimate the total strain, the plastic strain and the eigenstrain on the grain level based on macroscopic stress, strain and stiffness data. The simulated residual stresses are compared to experimental data. Macro residual stresses are determined by means of incremental hole drilling method, whereas phase-specific residual stresses are analyzed with use of X-ray diffraction according to the $$\sin ^2\psi$$ method. The simulation and measurement approaches are applied to a representative deep-drawing process for the lean duplex stainless steel $$\mathrm {X2CrNiN23{-4}}$$ , which consists of a ferritic and an austenitic phase both with the same volume fraction. The results indicate that the proposed two-scale simulation approach is well suited for the prediction of phase-specific residual stresses after a deep drawing process of lean duplex steel.

The New Materials Science Diffractometer RSD at CIAE

China Advanced Research Reactor (CARR) is a 60 MW tank-in-pool inverse neutron trap type reactor at CIAE. In order to use the non-destructive analytical techniques for phase specific residual stresses research, the new materials science diffractometer RSD was designed at CARR, which is one of the first commissioned instruments to meet the demand of industrial applications. The system comprises two types of monochromator, mosaic flat copper (220) and perfect double focusing silicon (311). The take-off angle of monochromator could change continuously from 41 to 109 degree. Therefore the neutron wavelength varies from 0.895 and 2.666 angstrom. The detector is a one dimension position sensitive detector. Various affiliated equipments enable it to do texture and tensional experiments. The best resolution could reach 0.2% using double focusing Si (311) monochromator. When RSD is available for routine operation, some research work in the field of nuclear industrial and aviatic engineering materials will be carried out using the beam of CARR.

Analysis of stress gradients in physical vapour deposition multilayers by X-ray diffraction at fixed depth intervals

The objective of this article is to develop and apply a model for the design and evaluation of X-ray diffraction experiments to measure phase-specific residual stress profiles in multilayer systems. Using synchrotron radiation and angle-dispersive diffraction, the stress measurements are performed on the basis of the sin2ψ method. Instead of the traditional Ω or χ mode, the experiments are carried out by a simultaneous variation of the goniometer angles χ, Ω and φGto ensure that the penetration and information depth and the measuring direction φ remain unchanged when the polar angle ψ is varied. The applicability of this measuring and evaluation strategy is demonstrated by the example of a multilayer system consisting of Ti and TiAlN layers, alternately deposited on a steel substrate by means of physical vapour deposition.

Phase Identification and Internal Stress Analysis of Steamside Oxides on Plant Exposed Superheater Tubes

During long-term, high-temperature exposure of superheater tubes in thermal power plants, various oxides are formed on the inner side (steamside) of the tubes, and oxide spallation is a serious problem for the power plant industry. Most often, oxidation in a steam atmosphere is investigated in laboratory experiments just mimicking the actual conditions in the power plant for simplified samples. On real plant-exposed superheater tubes, the steamside oxides are solely investigated microscopically. The feasibility of X-ray diffraction for the characterization of steamside oxidation on real plant-exposed superheater tubes was proven in the current work; the challenges for depth-resolved phase analysis and phase-specific residual stress analysis at the inner side of the tubes with concave surface curvature are discussed. Essential differences between the steamside oxides formed on two different steels typically applied for superheaters, ferritic-martensitic X20CrMoV12-1 and lean austenitic stainless steel TP347H, respectively, are revealed by X-ray diffraction.

<i>In Situ</i> Observation of Phase Transformations during Welding of Low Transformation Temperature Filler Material

Tensile residual stresses introduced by conventional welding processes diminish the crack resistance and the fatigue lifetime of welded components. In order to generate beneficial compressive residual stresses at the surface of a welded component, various post-weld treatment procedures are available, like shot peening, hammering, etc. These post-weld treatments are, however time and cost extensive. An attractive alternative is to generate compressive stresses over the complete weld joint in the course of the welding procedure by means of so-called Low Transformation Temperature (LTT) filler materials. The volume change induced by the transformation affects the residual stresses in the weld and its vicinity. LTT fillers exhibit a relatively low transformation temperature and a positive volume change, resulting in compressive residual stresses in the weld area. In-situ measurements of diffraction profiles during real welding experiments using Gas Tungsten Arc (GTA)-welding process were realized successfully for the first time. Transformation temperatures during heating and subsequent cooling of LTT welding material could be assessed by means of energy dispersive diffraction using high energy synchrotron radiation. The results show that the temperature of martensite start (Ms) is strongly dependent on the content of alloying elements. In addition the results indicate that different phase transformation temperatures are present depending on the welding depth. Additional determination of residual stresses allowed it to pull together time and temperature resolved phase transformations and the resulting phase specific residual stresses. It was shown, that for the evaluation of the residual stress state of LTT welds the coexisting martensitic and austenitic phases have to be taken into account when describing the global stress condition of the respective material in detail.

Texture and Strain Experiments at OPAL

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. The determination of global texture and the local variation of texture for example by inhomogeneous deformation are very important due to the coherence between the texture and the physical and mechanical properties of materials.

Creep Analysis of an Isotropic Rotating Al—SiCp Composite Disc taking into Account the Phase-specific Thermal Residual Stress

Analysis of steady-state creep in a rotating disc made of Al—SiCp composite has been carried out using isotropic Hoffman yield criterion and the results obtained are compared with those using von Mises yield criterion ignoring difference in yield stresses. The material parameters characterizing difference in yield stresses have been used from experimental results of other studies. The stress and strain rate distributions developed due to rotation have been calculated and compared for both the criteria. It is observed that the stress distribution is not too much affected due to presence of phase specific thermal residual stress. The presence of residual stress has contributed to an increase in the tangential strain rate particularly in the region near the outer radius of the disc, as compared to the tangential strain rate in the disc without residual stress. The radial strain rate, which is compressive, changes significantly due to the presence of residual stress and even becomes tensile in the middle of the disc. It is thus concluded that the presence of residual stress significantly affects the creep in an isotropic rotating disc.

Determination of Residual Stresses in Low Transformation Temperature (LTT -) Weld Metals using X-ray and High Energy Synchrotron Radiation

Crack and fatigue resistance are relevant evaluation criteria for welded joints and are decreased by tensile residual stresses resulting from the welding and cooling process, while compressive residual stresses can have a positive influence on the characteristics mentioned. In order to generate compressive residual stresses, a set of post weld treatment procedures is available, like shot peening, hammering, etc. These procedures have the disadvantage that they are time and cost extensive and have to be applied after welding. As another point, such technologies can only produce compressive stresses at the top surface, i.e. can only contribute to the reduction of the risk of cracks initiated at the surface, like fatigue cracks. A chance to generate compressive stresses over the complete weld joint during the welding procedure is offered by the so-called Low Transformation Temperature (LTT -) filler wires. Compared to conventional wires, these materials show lower phase transformation temperatures, which can work against cooling-related tensile stresses, resulting from respective shrinkage restraint. In consequence, distinct compressive residual stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially applicable to high-strength steel welding. Welds produced with different LTT — filler wires have shown different levels and distributions of the resulting residual stresses depending on the specific transformation temperature. The transformation temperatures are determined by temperature measurement. Classical X-ray diffraction as well as diffraction methods using high energy synchrotron radiation have been used for residual stress analysis. By means of high energy synchrotron diffraction in reflection mode residual stress depth gradients can be determined non-destructively. The phase selective nature of the diffraction measurements enables the simultaneous determination of the phase specific residual stresses of all contributing crystalline phases within one experiment. The application of white beam diffraction implies recording of a multitude of diffraction lines within the energy range of the provided energy spectrum of the white beam. By this means phase specific residual stress depth distributions up to distances of 150 μm below the surface can be analysed for steel using the energy dispersive set-up of the HMI-beamline EDDI at the Bessy site, Berlin, providing an energy range between 20–150 keV. As a side effect quantitative phase analysis can be carried out using white energy dispersive diffraction e.g. the determination of the content of retained austenite in the weld.

NET TG1: Residual stress analysis on a single bead weld on a steel plate using neutron diffraction at the new engineering instrument ‘STRESS-SPEC’

Non-destructive analysis of phase specific residual stresses and textures is only possible by diffraction methods. The new neutron diffractometer STRESS-SPEC was installed at FRM-II in Munich, Germany, for this purpose. It is designed to be equally applied to texture and residual stress analyses by virtue of its flexible configuration. The results of measurements on a single bead on plate weld within the framework of the European Network on Neutron Techniques Standardisation for Structural Integrity (NET) are presented here, in order to demonstrate the reliability and accuracy of the instrument by comparing previous results from other neutron diffraction facilities by using a Bayesian statistical approach.

Surface and bulk residual stresses in Li 2O · 2SiO 2 glass–ceramics

In the present work surface and bulk residual stresses generated in partially crystallized Li 2O · 2SiO 2 glass–ceramics are analyzed after different heat treatments. The phase specific residual stresses in the crystalline Li 2Si 2O 5-phase are evaluated for the first time in the near-surface zone and the bulk of the samples using both medium and high energy synchrotron radiation. The results reveal that in the crystals within the bulk of the samples micro residual stresses generated by the thermal anisotropy of the isolated individual crystallites depend on the crystallographic direction. In contrast, the residual stress state in the near-surface zone is isotropic due to the superposition of thermal residual stresses in and around the crystals of the near-surface area. Residual stress calculations using a modified Selsing’s model yield a good estimate of the anisotropic residual stresses in the bulk crystallites, whereas the isotropic residual stress state in the crystallized surface layer can be described by an elastic stress model for thin films.

The New Materials Science Diffractometer STRESS-SPEC at FRM-II

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. In order to cater for the development of these analytical techniques the new Materials Science Diffractometer STRESS-SPEC at FRM-II is designed to be equally applied to texture and residual stress analyses by virtue of its flexible configuration. The system compromises a highly flexible monochromator setup using three different monochromators: Ge (511), bent silicon (400) and pyrolitic graphite (PG). This range of monochromators and the possibility to vary the take-off angles from 2θM = 35º to 110º allows wavelength adjustment such that measurements can be performed around a scattering angle of 2θS ~ 90º. This is important in order to optimise neutron flux and resolution, especially for stress analysis on components, since the gauge volume element in that case is cubic and large vertical divergences due to focusing monochromators do not affect the spatial resolution. The instrument is now available for routine operation and here we will present details of recent experiments and instrument performance.

Relaxation of Phase Specific Elastic Strains/stresses during Deformation of Al/Si-MMCs at Elevated Temperatures

Thermal and mechanical loading induces phase specific strains/stresses in metal matrix composites (MMCs). At elevated temperature part of the phase specific stresses relax. The relaxation of the phase specific residual stresses is determined by in situ experiments using white high-energy synchrotron radiation (HESR). They reveal that time-resolved strain measurements are possible and that short-time phenomena can be accessed using white HESR. The elastic strain relaxation behavior is similar for all lattice planes accessed. The influence of the temperature on the characteristic relaxation time is determined.

Non‐destructive Investigation of AlSi7Mg/SiC70p MMCs by means of Acoustic Scanning Microscopy, X‐Ray and Neutron Diffraction

AbstractParticulate silicon carbide reinforced aluminium matrix composites (Al‐SiC) have a high potential for advanced engineering materials beeing developed for use in traction, automotive, aviation and space applications. Especially high volume composites with up to 70 vol.% reinforcement of silicon carbide particles in a cast AlSi metal matrix (AlSi7Mg/SiC70p) show most interesting characteristics for application in high power electronics (IGBTs). To characterize such MMCs respectively reasons for failure, different non‐destructive material investigations were carried out among them Acoustic Scanning Microscopy, X‐Ray and Neutron Diffraction. Acoustic Scanning Microscopy delivers information about lateral density differences respectively material inhomogeneities integrated over sample thickness. Diffraction methods are used for a non‐destructive evaluation of absolute and phase specific residual stresses, whereas X‐Ray Diffraction provides near surface information (several tens of micrometers). High Energy Synchrotron and Neutron Diffraction on the contrary provide bulk information of several millimeters up to several centimeters. Important for the understanding of the deformation behaviour and dimensional stability is the understanding of the residual stress state and inhomogeneities in the composite and its change under operating conditions respectively under dynamic thermal load.