Residual Stress Depth Distribution 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.

Manufacture of Defined Residual Stress Distributions in the Friction-Spinning Process: Driven Tool and Subsequent Flow-Forming

Friction-spinning as an innovative incremental forming process enables large degrees of deformation in the field of tube and sheet metal forming due to a self-induced heat generation in the forming zone. This paper presents a new tool and process design with a driven tool for the targeted adjustment of residual stress distributions in the friction-spinning process. Locally adapted residual stress depth distributions are intended to improve the functionality of the friction-spinning workpieces, e.g. by delaying failure or triggering it in a defined way. The new process designs with the driven tool and a subsequent flow-forming operation are investigated regarding the influence on the residual stress depth distributions compared to those of standard friction-spinning process. Residual stress depth distributions are measured with the incremental hole-drilling method. The workpieces (tubular part with a flange) are manufactured using heat-treatable 3.3206 (EN-AW 6060 T6) tubular profiles. It is shown that the residual stress depth distributions change significantly due to the new process designs, which offers new potentials for the targeted adjustment of residual stresses that serve to improve the workpiece properties.

Manufacture of Defined Residual Stress Distributions in the Friction-Spinning Process: Investigations and Run-to-Run Predictive Control

Friction-spinning as an innovative incremental forming process enables high degrees of deformation in the field of tube and sheet metal forming due to self-induced heat generation in the forming area. The complex thermomechanical conditions generate non-uniform residual stress distributions. In order to specifically adjust these residual stress distributions, the influence of different process parameters on residual stress distributions in flanges formed by the friction-spinning of tubes is investigated using the design of experiments (DoE) method. The feed rate with an effect of −156 MPa/mm is the dominating control parameter for residual stress depth distribution in steel flange forming, whereas the rotation speed of the workpiece with an effect of 18 MPa/mm dominates the gradient of residual stress generation in the aluminium flange-forming process. A run-to-run predictive control system for the specific adjustment of residual stress distributions is proposed and validated. The predictive model provides an initial solution in the form of a parameter set, and the controlled feedback iteratively approaches the target value with new parameter sets recalculated on the basis of the deviation of the previous run. Residual stress measurements are carried out using the hole-drilling method and X-ray diffraction by the cosα-method.

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.

Influence of residual stress depth distribution on lifecycle behaviour of AISI4140

Abstract Surface integrity has a major influence on the fatigue behaviour of metallic components. Using deep rolling, the residual stress state can be influenced to a high degree under consideration of the contact stresses between workpiece and tool. This can be used to tailor the residual stress state to improve the fatigue and lifecycle behaviour of metallic componetens. To combine the existing knowledge about the relationship between mechanical load during deep rolling and the residual stress state and lifecycle experiments, a previously developed model is applied to quenched and tempered AISI4140 components to generate parts with similar surface residual stresses but different depth distributions. The parts are stressed using rotating bending tests and the influence of the residual stress state on the residual stress relaxation and lifespan is evaluated. The results show that it is possible to influence the residual stress state to a targeted profile by the adaption of the machining parameters. The performed deep rolling experiments result in two cases: Both parameter variations result in similar compressive surface residual residual stresses. Parameter set A (high pressure, low overlap) generates a higher residual stress penetration depth and a higher roughness, while set B (low pressure, high overlap) generates a lower penetration depth and a smoother surface. The rotating bending tests result in a higher lifetime and a lower residual stress relaxation for parameter set A.

Optimization of the stream finishing process for mechanical surface treatment by numerical and experimental process analysis

The stream finishing process represents an efficient mass finishing process capable in mechanical surface modification. In order to generate a deeper understanding of the cause-effect relationships, normal forces, material removal and surface topography were analyzed and correlated for varied process parameters of disc-shaped AISI 4140 specimens. Local resolution of tangential velocities of the particles and normal forces on the workpiece’s surface were simulated using the discrete element method for defined process parameter configurations and were correlated with experimental results. A deep process understanding is accomplished enabling the process design for efficient surface smoothing and improved residual stress depth distribution.

Evolution of structure and residual stress of a fcc/hex-AlCrN multi-layered system upon thermal loading revealed by cross-sectional X-ray nano-diffraction

Understanding the influence of process conditions and coating architecture on the microstructure and residual stress state of multi-layered coatings is essential for the development of novel thermally and mechanically stable coatings and requires advanced depth resolving characterization techniques. In this work, an arc-evaporated multi-layered coating, consisting of alternating Al70Cr30N and Al90Cr10N sublayers with an individual layer thickness between 120 nm and 380 nm, was investigated. The as-deposited state of the multi-layered coating and the state after vacuum annealing at 1000°C for 30 min was studied along its cross-section by synchrotron X-ray nano-diffraction using a beam with a diameter of 50 nm. The results revealed sublayers with alternating cubic and hexagonal phase, causing repeated interruption of the grain growth at the interfaces. The in-plane residual stress depth distribution across the coating thickness could be tuned in a wide range between pronounced compressive and slight tensile stress by combining the effects of the coating architecture and the modulated incident particle energy controlled by the substrate bias voltage ranging from −30 V to −250 V. This resulted in an oscillatory stress profile fluctuating between −2 GPa and −4.5 GPa or pronounced stress gradients with values between −4 GPa and 0.5 GPa. Finally, the decomposition routes of the metastable cubic Al70Cr30N phase could be controlled by the Al90Cr10N sublayers which acted as nucleation sites and governed the texture of the decomposition products as Cr2N. The results demonstrate that the cross sectional combinatorial approach, relying on a sophisticated multi-layer architecture combining various materials synthesized under tailored conditions, allowed for resolving structural variations and stress profiles in the individual layers within the complex architecture and pioneers the path for knowledge-based development of multi-layered coatings with predefined microstructure and a dedicated stress design.

Experimental and numerical analysis of residual stress in carbon-stabilized expanded austenite

Expanded austenite obtained by gaseous carburizing of stainless steel was investigated with X-ray diffraction to determine composition-depth and residual stress-depth distributions. Avoiding ghost stress effects in the analysis of X-ray diffraction data, the obtained composition- and stress-depth profiles are in excellent quantitative agreement with those obtained with other techniques. The residual stress-depth profile was attempted calculated from the composition-depth profile assuming elastic-plastic accommodation of the lattice expansion. In the model, composition-dependence of Young's modulus, yield stress and work hardening exponent were considered. Excellent quantitative agreement was achieved between the experimental and numerical residual stress-depth profiles.

Experimental and numerical analyses of residual stresses induced by tube drawing

Lightweight constructions are used to fulfil the ever-increasing demands regarding fuel efficiency and carbon dioxide emission in transportation industries. In order to reduce weight, technical components made of solid materials are often replaced by tubular structures. Under service conditions, the components are frequently exposed to cyclic loads. Hence, residual stresses that are induced by manufacturing processes can have a significant impact on service life. In this work, the focus is on tube manufacturing processes, precisely cold tube sinking and fixed plug drawing. Both processes induce characteristic residual stress states, which are important to assess the mechanical integrity and load-carrying capacity of tubular components during service. The aim of this article is to examine the residual stress depth distribution for medium-carbon steel tubes manufactured by cold tube sinking and fixed plug drawing. The residual stresses are measured by means of the Sachs method and the hole-drilling method, respectively. The measured results are compared to finite element simulations of the tube drawing process. It is shown that the residual stress obtained with the different experimental methods and the numerical simulations are consistent. Furthermore, it is shown that the residual stresses can be significantly reduced when a plug is used in the drawing process.

Study of stability of microstructure and residual strain after thermal loading of plasma sprayed YSZ by through surface neutron scanning

Yttria stabilized zirconia (YSZ) is often applied as thermal barrier coating on metal parts as e.g. turbine blades made of nickel base super alloys. The coating process in combination with the preconditioning of the substrate material induces characteristic residual stress distributions in the coating system consisting of topcoat, bondcoat and the substrate material. Knowledge about the residual stress depth distribution in the coating and at the interfaces down to the substrate material is essential for the assessment of the mechanical integrity and the reliability of the coating. In this regard the stability of the microstructure and the residual stresses is of particular interest; hence this forms the scope of our investigations. Yttria (8 wt.%) stabilized zirconia with a NiCoCrAlY bondcoat was deposited by atmospheric plasma spraying (APS) at different spray conditions on a nickel base super alloy substrate material. The coatings were subjected to different heat-treatment processes, i.e. static aging and cyclic thermal loadings. Through surface scanning using neutron diffraction was carried out for the as sprayed condition and for the thermally loaded samples. Based on the measured diffraction data the stability of the microstructure (phases) and the residual strain/stresses through the depths of the coating system were assessed.

Residual Stress Depth Distributions for Atmospheric Plasma Sprayed MnCo<sub>1.9</sub>Fe<sub>0.1</sub>O<sub>4</sub> Spinel Layers on Crofer Steel Substrate

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the use of ferritic stainless steel can lead to degradation in cell performance due to chromium migration into the cells at the cathode side [1]. Application of a coating on the ferritic stainless steel interconnect is one option to prevent Cr outward migration through the coating. MnCo1.9Fe0.1O4 (in the following designated as MCF) spinels act as a diffusion barrier and retain high conductivity during operation [2]. Knowledge about the residual stress depth distribution throughout the complete APS coating system is important and can help to optimize the coating process. This implicitly requires reliable residual stress analysis in the coating, the interface region and in the substrate.For residual stress analysis on these specific layered systems diffraction based analysis methods (XRD) using laboratory X-ray sources can only by applied at the very surface. For larger depths sublayer removal is necessary to gain reliable residual stress data. The established method for sublayer removal is electrochemical etching, which fails, since the spinel layer is inert. However, a mechanical layer removal will affect the local residual stress distribution.As an alternative, mechanical residual stress analyses techniques can be applied. Recently, we established an approach to analyse residual stress depth distributions in thick film systems by means of the incremental hole drilling method [5, 6]. In this project, we refined our approach for the application on MCF coatings with a layer thickness between 60 – 125 μm.

Incremental Hole Drilling for Residual Stress Analysis of Thin Walled Components with Regard to Plasticity Effects

The incremental hole-drilling method is widely used in residual stress depth distribution analysis. However, two specific difficulties with the generalization of the incremental method exist, including the consideration of the sample thickness and residual stress states close to the local material’s yield strength. The stress concentration effect of the hole can lead to plastic deformation in the vicinity of the hole, which results in an overestimation of residual stresses. Typically, the effect of the component’s thickness and the plasticity effects are analyzed separately and correction approaches are proposed. In the current paper, we analyze the combined effects of plasticity and thickness on residual stress analysis using the incremental hole-drilling method. A systematic study was performed on steel samples with (i) isotropic and (ii) anisotropic elastic and elasto-plastic material behavior with varying thicknesses ranging between 1 mm and 4 mm. Electronic speckle pattern interferometry (ESPI) hole-drilling experiments were conducted on beam samples loaded using a 4-point bending fixture. Finite element simulations were conducted to gain insight into the effects of incremental hole-drilling. The results indicate that reducing the component’s thickness increases the plastic deformation in the vicinity of the hole and results in significant stress deviations. Thin components bend during hole-drilling as a result of the loss of stiffness, which amplifies the plasticity effect.

Nondestructive separation of residual stress and composition gradients in thin films by angle- and energy-dispersive X-ray diffraction. I. Theoretical concepts

Different X-ray measurement and data evaluation concepts are presented, which allow for residual stress analysis in thin films with pronounced gradients in chemical composition. These gradients lead to a variation in the strain-free lattice parameter a 0 with respect to the film thickness and superimpose the lattice strain induced by the film's inherent stresses. Non-consideration of a 0(z) gradients is shown to lead to considerable errors (`ghost stresses') in the residual stress depth profiles. With the simulated example of a TiC x N1−x film with a pronounced carbon gradient, the first part of this series introduces four approaches, which permit the separation of residual stress and composition depth distributions at different levels of approximation. They are based on lattice spacing depth profile measurements performed in either the sin2ψ mode or the scattering vector mode, or in combinations of these two scanning modes. Depending on the approach used for separating the residual stress and composition gradients, angle- or energy-dispersive diffraction has to be applied, employing monochromatic X-ray sources available in the laboratory or either white high-energy synchrotron radiation or the Bremsstrahlung of conventional X-ray tubes, respectively. The methods introduced here assume a biaxial residual stress state within the film. For a triaxial residual stress state with σ33 ≠ 0, a separation of stress and composition gradients is not straightforward, because an a 0(z) gradient cannot be distinguished from the hydrostatic part of the stress tensor.

Local Residual Stress Depth Distribution in the Inner Gearing of a Case Hardened Sliding Collar

With respect to residual stress analysis the inner gearing of an automotive sliding collar is a component with a challenging, complex geometry. The accessibility of the tooth root does not exist for conventional measuring approaches. However, the process steps like e.g. broaching and case hardening induce characteristic residual stress distributions, which must be known for the valuation of the mechanical integrity of the parts. For lab X-ray stress analysis approaches according to the sin2ψ-method [1] the ring like structure must be sectioned, which affects the process induced residual stress state. The tooth root is rather small, which further leads to shadowing effects during tilting of the sample. Standard mechanical approaches like incremental hole drilling can be excluded due to the narrow tooth root. Local neutron diffraction residual stress analysis in the tooth root by means of neutron through surface strain scanning at the STRESS-SPEC instrument at the research reactor FRM II, Garching (Germany) was successfully carried out for the inner gearing. A measuring and evaluation strategy is proposed, where special attention is paid to the compensation of the surface effect due to the incomplete immersion of the nominal gauge volume during through surface scanning and to the local variation of the D0-value as a consequence of the case hardening process.

Diffraction analysis of strongly inhomogeneous residual stress depth distributions by modification of the stress scanning method. II. Experimental implementation

The modified stress scanning method [Meixner, Fuss, Klaus & Genzel (2015).J. Appl. Cryst.48, 1451–1461] is experimentally implemented for the analysis of near-surface residual stress depth distributions that are strongly inhomogeneous. The suggested procedure is validated by analyzing the very steep in-plane residual stress depth profile of a shot-peened Al2O3ceramic specimen and comparing the results with those that were obtained by well established X-ray diffraction-based gradient methods. In addition, the evaluation formalism is adapted to the depth-dependent determination of the residual stresses inside of multilayer thin-film systems. The applicability for this purpose is demonstrated by investigating the residual stress depth distribution within the individual sublayers of a multilayered coating that consists of alternating Al2O3and TiCN thin films. In this connection, the specific diffraction geometry that was used for the implementation of the stress scanning method at the energy-dispersive materials science beamline EDDI@BESSYII is presented, and experimental issues as well as limitations of the method are discussed.

Diffraction analysis of strongly inhomogeneous residual stress depth distributions by modification of the stress scanning method. I. Theoretical concept

Exploiting the advantages of energy-dispersive synchrotron diffraction, a method for the determination of strongly inhomogeneous residual stress depth gradients is developed, which is an enhancement of the stress scanning technique. For this purpose, simulations on the basis of a very steep residual stress depth profile are performed, and it is shown that conventional real space evaluation approaches fail, because they do not take into account the variation of the residual stresses within the gauge volume. Therefore, a concept facilitating the deconvolution of the diffraction signal by considering the effect of the gauge volume geometry as well as the influence of the material absorption on the average information depth is proposed. It is demonstrated that data evaluation requires a three-dimensional least-squares fit procedure in this case. Furthermore, possible aberrations and their impact on the analysis of the residual stresses by applying the `modified stress scanning' method are treated theoretically.

Thin film stress and microstructure analysis by energy-dispersive diffraction

The most important advantage of energy dispersive (ED) diffraction compared with angle dispersive methods is that the former provides complete diffraction patterns in fixed but arbitrarily selectable scattering directions. Furthermore, in experiments that are carried out in reflection geometry, the different photon energies E(hkl) of the diffraction lines in an ED diffraction pattern can be taken as an additional parameter to analyze depth gradients of structural properties in the materials near surface region. For data evaluation advantageous use can be made of whole pattern methods such as the Rietveld method, which allows for line profile analysis to study size and strain broadening [1] or for the refinement of models that describe the residual stress depth distribution [2]. Concerning polycrystalline thin films, the features of ED diffraction mentioned above can be applied to study residual stresses, texture and the microstructure either in ex-situ experiments or in-situ to monitor, for example, the chemical reaction pathway during film growth [3]. The main objective of this talk is to demonstrate that (contrary to a widespread opinion) high energy synchrotron radiation and thin film analysis may fit together. The corresponding experiments were performed on the materials science beamline EDDI at BESSY II which is one of the very few instruments worldwide that is especially dedicated to ED diffraction. On the basis of selected examples it will be shown that specially tailored experimental setups allow for residual stress depth profiling even in thin films and multilayer coatings as well as for fast in situ studies of film stress and microstructure evolution during film growth.

Residual Stresses in Multi-Pass Butt-Welded Tubular Joints

In this study, the mechanics of the residual stress development in multi-pass butt-welded joints of ferritic-pearlitic steel pipes are investigated. Therefore, the effects of the welding process and its parameters, as well as the impact of the preheating temperature on the residual stress state are studied by determining the temperature profile during welding and performing X-ray diffraction measurements on the pipes inner and outer surfaces. Additionally, residual stress depth distributions are determined using the incremental hole-drilling method. The residual stress distributions can be explained by phase transformations and thermal shrinkage, the latter being the governing mechanism in the tubular geometry of the welds.

Combining XRD with Hole-Drilling Method in Residual Stress Gradient Analysis of Laser Hardened C45 Steel

LLocalized laser hardening of steel surfaces performed with optimal parameters not only hardens the surface layer, but is also capable of generating advantageous hook-like shape of residual stress depth distribution. Upon combining two most common methods of XRD and hole-drilling, the residual stresses up to the depth of 3 mm were analysed for two widths of high-powered diode laser beam. Narrower laser beam generates wider area with compressive stresses and the diffraction technique as compared with hole-drilling method always indicates higher values of compressive residual stress maxima.

The influence of plasma sprayed multilayers of Cr2O3 and Ni10wt%Al on fatigue resistance

The influence of layering sequence of plasma sprayed composite coatings on the fatigue life was studied. Plain, grit-blasted, and plasma sprayed specimens with one to three coating layers were examined. The layered coatings were composed of alternating sequence of ceramic (Cr2O3) and metallic (Ni10wt%Al) deposits. Deflection controlled resonance symmetrical bending fatigue tests of flat specimens were performed. The deformation amplitude was 1.3 millistrain in the crack initiation site and loading frequency was around 80Hz. The significance of the effect of surface treatment on fatigue properties was examined statistically using Wilcoxon test. From the obtained data, the effect of individual layers was deduced. In order to explain the observed fatigue behavior, the fractographic analysis was performed and the coating–substrate composite was characterized. Strain hardening profile in the substrate was estimated by microhardness measurement and electron back scatter diffraction (EBSD) analysis. Residual stress depth distribution in the substrate was measured using neutron diffraction. The elemental and phase compositions were analyzed using energy dispersive microanalysis (EDX) and X-ray diffraction (XRD). The explanation of the fatigue behavior of tested specimens consistent with the obtained results was formulated.