Residual Stress Depth Distribution Research Articles

Effect of Boriding Time on Microstructure and Residual Stresses in Borided Highly Alloyed X210CR12 Steel

Boriding of highly alloyed steels done with the aim of increasing their wear resistance faces several issues connected with the microstructure of the base material and restraints during the diffusion of boron. The aim of the performed analyses was to ascertain whether significant increase of boriding time can enhance the surface hardness, contribute to creation of more compact microstructure and even lead to beneficial state of residual stresses in the borided layer. Using combination of X-ray diffraction and electro-chemical polishing, residual stress depth distributions in few tens of micrometres thick borided layers were obtained.

Residual Stress Depth Distribution after Piezo Peening of Quenched and Tempered AISI 4140

Piezo peening is a new alternative mechanical surface treatment process. Thereby a piezo actuator with an indenter is causing mechanical deformation of the surface area by multiple impacts in a defined way. First results for quenched and tempered AISI 4140 show a great potential: large surface compressive residual stresses of up to -1200 MPa could be generated. In order to obtain a process understanding different process parameters are systematically changed. After the mechanical surface treatments the residual stress and full width at half maximum (FWHM) depth distribution were measured using X-ray diffraction technique. By varying the process parameters path distance, feed rate and amplitude several combinations of residual stress and FWHM depth distributions were generated. The dependency of surface residual stresses, penetration depth, FWHM on the process parameters is analyzed.

An Almost User-Independent Evaluation Formalism to Determine Arbitrary Residual Stress Depth Distributions with the Hole-Drilling Method

The incremental hole-drilling method is the method of choice to determine residual stress depth distributions with limited costs and minor destruction of the investigated component. With a spatial resolution of commonly two millimeters in diameter and one millimeter in depth especially the effects of frequently used surface treatments like e.g. shot peening or deep rolling can be reliably detected if the in depth residual stress gradients are relatively smooth. Nevertheless up to now the quantitative accuracy of the method is poor for residual stress analyses close to the materials surface up to depths of approximately 0.2 mm and in the case of steep in-depth residual stress gradients or oscillating residual stress depth distributions. In this paper, residual stress depth distributions of a broad range introduced by mechanical surface-treatments in flat specimens were analyzed with the hole-drilling method and compared with the results measured by X-ray diffraction as the reference. It comes out, that arbitrary residual stress depth distributions can be successfully determined with a modified differential evaluation formalism. For this purpose, often neglected well known weak points of the hole-drilling method were considered and improved, e.g. hole geometry, numerical calibration and data conditioning. Especially, the proposed strategy of data conditioning results in an almost user-independent evaluation formalism.

Corrosion and Fatigue of AL-Alloys AA359.0 and AA6060 in Different Surface Treatment States

The consequences of near surface materials properties and residual stress states produced by specific manufacturing operations on damage evolution during corrosion fatigue of the Al-base alloys AA359.0 (German grade G-AlSi9Cu3) and AA6060 (German grade AlMgSi0.5) were systematically investigated. Specimens were processed applying mechanical surface treatments like shot peening or deep rolling and investigated in comparison with turned states. Surface topographies as well as near surface work hardening states and residual stress depth distributions were analyzed. Tension-compression fatigue tests were carried out under laboratory air as well as under salt spray test conditions or in salt solution. Crack formation and crack propagation was studied and characteristic examples are presented. The influence of the mechanical surface treatments on the electrochemical behavior was also investigated. To assess the consequences of near surface materials properties on the corrosion fatigue behavior, their stability during fatigue, in particular the relaxation of residual stress distributions introduced by mechanical surface treatments, was taken into account.

Mechanical Surface Treatment Technologies for Improving HCF Strength and Surface Roughness of Blisk-Rotors

The manufacturing of electron beam welded blade integrated disk (blisk) rotors sets new demands on mechanical surface treatment technologies. High durability and high efficiency of the blades are in general strict requirements for the component and have ideally to be increased by a mechanical surface treatment. The high complex 3D shape of the blades and the need to treat the blades on the rotor caused by a post weld heat treatment are additional challenges to solve. Limited clearance between the blades and low space between the individual blisk’s increase as well the requirements on the technology. Conventional technologies reach their limits and have to be improved. This paper gives an insight into the capabilities of different mechanical surface treatments regarding the treatment of blades on blisk-rotors. Compared with the benchmark shot peening, surface roughness, residual stress depth distribution and high cycle fatigue (HCF) are investigated on deep rolled and vibropeened specimen. Assets and drawbacks are shown and discussed.

Shaping of Ceramics Using Residual Stresses

Shot peening is a common procedure used to improve the static and cyclic strength of metal components and for forming of thin walled components. The underlying mechanisms are localized plastic deformation, work hardening and the introduction of compressive stresses into the near-surface region. During the last decade we have been establishing damage-free shot peening processes for brittle materials such as ceramics. Based on these results we are now developing processes for peen-forming of ceramic components. This paper describes the first successful experiments aimed at shaping ceramic specimens using shot peening. Strips of different thicknesses, made of silicon nitride ceramic, were shot-peened using different shot sizes, peening pressures and coverage. The residual stress-depth distributions were determined using X-ray diffraction. Based on the experimentally determined stress states, the curvatures of the strips were calculated analytically and using Finite Element calculations (FEM). The results of the curvature measurements and calculations agree well.

Improved Calibration Coefficients for the Hole-Drilling Method Considering the Influence of the Poisson Ratio

For residual stress analyses with the incremental hole-drilling method adequate evaluation formalisms in order to transform the measured strains into stresses are required. The Integral Method is the most important theory used for analyses of nonlinear residual stress depth distributions. It is based on a calibration, which is carried out by Finite-Element-Analyses. For the sake of simplicity the used numerical models often represent an ideally cylindrical hole and ideal elastic material behavior with fixed elastic constants. The adaption of the calibration coefficients from the numerical simulations to the real experimental state is often performed by simple correction factors or is ignored completely. The following investigation highlights the influence of the Poisson ratio on the calibration coefficients for the most commonly used strain gage rosettes geometries, which are standardized by ASTME837-08. It comes out that the application of existing simple correction factors is only valid within a small range and better approximations can be obtained by alternative strategies.

Evaluation of the High-Speed Drilling Technique for the Incremental Hole-Drilling Method

The incremental hole-drilling method is frequently used for residual stress depth distribution analyses, due to its fast and economical experimental execution. Depending on the planned use of the component, the drilled hole that is made to measure the residual stress can often be repaired or ignored if it does not affect the intended use of the part. Nevertheless an important experimental issue and assumption is the introduction of an ideal cylindrical hole into the component without additional plastic deformation. Although high-speed drilling is well established the consequences of the resulting hole geometries compared to ideal assumptions are not well known. Therefore, a detailed comparison between different bits and drilling techniques was carried out and is discussed in this paper in order to detect the best experimental conditions and to find out reasons especially for the lack of accuracy of the hole-drilling method for the first increments close to the specimens surface. It comes out that the orbital drilling with common used six-blade bits results in the best compromise of an ideal cylindrical hole and centricity to the center of the strain gage rosette. In the case of conventional drilling the hole geometry differs from the ideal one if six-blade bits were used due to the influence of chamfers at the cutting edges and a non 180° plane end face and also in the case of a two-blade bit due to a non 180° plane end face and the tendency to more eccentric holes. Diamond bits cannot be recommended under all tested conditions due to their geometrical undefined shape.

Influence of shot peening and laser ablation on residual stress state and phase composition of cemented carbide cutting inserts

Substrate residual stresses in coated carbide tools are a result of pre-coating and coating processes. One prominent PVD pre-coating process is shot peening, that in addition to the necessary surface pre-treatment, induces compressive residual stress into the substrate's subsurface. Shot peening processes performed with different parameters and their influence on residual stresses are investigated and evaluated. A novel method for shaping carbide cutting tools is given by laser ablation. This process gives rise to heating of the specimen's subsurface. Hence, it can be expected, that tensile residual stresses are created. Laser ablation processes with different parameters have been performed and their influence on residual stress depth distribution has been determined.

Experimental and Numerical Strategies to Consider Hole Eccentricity for Residual Stress Measurement with the Hole Drilling Method

Abstract For the analysis of residual stress depth distributions the semi-destructive incremental hole drilling method is the process of choice for quick and cost-efficient measurements. Nevertheless, the experimental execution has to be carried out carefully. Minor deviations of the ideal case, for example incorrect depth setting or eccentricity of the drilled hole to the center of the applied strain gage rosette can result in large errors or invalid results. In this paper two different strategies are presented, in order to take eccentricity into consideration. The first strategy requires a specific strain gage rosette consisting of eight single strain gages arranged at 45° angle to each other. The second strategy works with standard strain gage rosettes, but with calibration functions modified for the eccentric case. Both strategies are able to reveal and to correct the consequences of eccentricity on the results of stress analyses. In order to improve the quantitative accuracy of residual stress measurements the latter strategy should be preferred.

Residual stresses under quasi-static and cyclic loading in shot peened Inconel 718

Abstract The residual stress state induced by shot peening should be taken into account in the dimensioning of turbine components. Understanding the changes in the residual stress state caused by the application of quasi-static and cyclic loads is a prerequisite. In order to describe the residual stress state after quasi-static loading, several different shot peened Inconel 718 specimens were loaded isothermally up to specific tensile loadings. To analyze the residual stress state after cyclic loading, isothermal low cycle fatigue tests were performed. These tests were stopped after a defined number of cycles. Finally, after the specimens had been subjected to different loads, the surface residual stresses and — for special loadings — the residual stress depth distributions were determined experimentally by using X-ray diffraction. The surface — core model was adapted so that the complete residual stress depth distribution after quasi-static and cyclic loading can now be described.

Changes in the axial residual stresses in AISI 1045 steel bars resulting from a combined drawing process chain

The present paper investigates the redistribution of residual stresses resulting from manufacturing steps in the production of cold-drawn AISI 1045 steel bars. The main steps in this process chain are the pre-straightening of coiled material, shot-blasting, cold-drawing, cutting, and final straightening with crossed rolls. Drawing and final straightening parameters were changed to evaluate their influence on the residual stress distributions as measured by X-ray diffraction and hole-drilling techniques after the different manufacturing steps. It was found that, depending on the manufacturing parameters, the pre-straightened material generates non-homogeneity of residual stresses which can remain even after the last manufacturing steps. The angle between the crossed rolls in the final straightening has a high influence on the homogeneity of the surface residual stresses. Both the drawing angle and the angle between the crossed rolls significantly affect the residual stress depth distribution.

Residual Stresses in Multilayer Welds with Different Martensitic Transformation Temperatures Analyzed by High-Energy Synchrotron Diffraction

Low Transformation Temperature (LTT) alloys were developed in order to control the residual stress development by the martensitic phase transformation already during cooling of the weld metal. The positive effect of such LTT alloys on the mitigation of detrimental tensile residual stresses during welding has already been confirmed on the basis of individual laboratory tests. Within the current project it was experimentally investigated whether the phase transformation mechanisms are effective under increased restraint due to multi-pass welding of thicker specimens. The local residual stress depth distribution was analyzed non-destructively for V-type welds processed by arc welding using energy dispersive synchrotron X-ray diffraction (EDXRD). The use of high energy (20 keV to 150 keV) EDXRD allowed for the evaluation of diffraction spectra containing information of all contributing phases. As the investigated LTT alloy contains retained austenite after welding, this phase was also considered for stress analysis. The results show in particular how the constraining effect of increased thickness of the welded plates and additional deposited weld metal influences the level of the residual stresses in near weld surface areas. While the longitudinal residual stresses were reduced in general, in the transition zone from the weld to the heat-affected zone (HAZ) compressive residual stresses were found.

Effect of different production phases on residual stress field in double-layer cast rolls

The aim of this investigation was to determine the effect of different production operations and parameters on residual stress level in double-layer cast rolls. Using a hole-drilling method, residual stress depth distribution in surface layer was measured around and along the body of the double-layer cast roll after casting, heat treatment, turning and grinding. Furthermore, using different turning and grinding parameters (speed, feed), and heat treatment parameters (temperature, time), influence of these parameters on residual stress level and distribution was determined. Results of the investigation show that the conditions in the casting pit have an influence on the residual stress field generated in double-layer cast rolls, with surfaces exposed to faster cooling rates in general showing higher residual stress values. Very critical phase in rolls production is coarse grinding, which if not carried out properly will generate very high tensile stresses in the roll surface and cause surface cracking or even roll fracture. Heat treatment on the other hand results in uniform compressive residual stress field in the roll's surface. However, reduction in tempering time and especially increase in hardening temperature lead to increased residual stress level in the roll surface. Final machining of the roll surface further influences residual stress level in the roll. Increase in turning speed of up to 40% results in compressive residual stress increase of up to 60%, while less than 30% increase in feeding rate gives almost 100% increase in residual stress level.

Residual Stress in PVD-Coated Carbide Cutting Inserts - Applications of the sin<sup>2</sup>ψ and the Scattering Vector Method

Premature collapse in terms of cohesive damage of PVD-coated carbide cutting tools often results in a time and cost consuming immediate interrupt of the cutting process. It is assumed that the residual stress state of the composite coating – substrate in combination with external loads during tool use is responsible for cohesive damage. The X-ray diffraction methods sin2 and scattering vector are applied for determination of the residual stress depth distribution in the coating and the substrate’s subsurface. Investigations of the residual stress state of commercial PVD-coated carbide cutting tools are presented. It is determined to what extent the single process steps during tool manufacturing are responsible for the final residual stress state of the PVD-coated tool. Furthermore the meaning of the PVD-coating process for the substrate’s residual stress state is investigated. Moreover, possibilities of controlling the residual stress state of the substrate by changing process variables of selected process steps are analyzed.

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.

Influence of the Residual Stress State on Cohesive Damage of PVD‐Coated Carbide Cutting Tools

Cohesive damage of PVD-coated carbide cutting tools always means an unscheduled production downtime and in many cases a partial damage of the workpiece. The cause for this kind of tool breakdown is ascribed by a model to a disadvantageous residual stress state of substrate and coating. The presented work observes the tool's residual stress states after single process steps during production by application of the sin2ψ method and the scattering vector method for the determination of residual stress depth distributions in order to verify the model. The obtained results prove the assumptions of the model.

Residual Stress Gradients in PVD-Coated Carbide Cutting Tools

PVD-coated cutting tools show a typical kind of failure in use: cohesive damage, which is believed to be a result of the residual stress state of substrate and coating. As the sin2ψ-technique does not give satisfactory information on near surface residual stress trends of coated tools the scattering vector method was applied to determine residual stress depth distributions of coating and substrate. The results are presented and an attempt for an interpretation is given.

Determination of Real Space Residual Stress Distributions σ<sub>ij</sub>(z) of Surface Treated Materials with Diffraction Methods Part I: Angle-Dispersive Approach

A measuring and evaluation strategy for the determination of residual stress depth distributions σ ij (z) in real space within different depths beneath the samples surface is introduced. The proposed method allows the analysis of residual stresses within a specific gauge volume using the well-known sin 2 ψ-method. Hence it allows a real 'stress scanning' in real space and will be applicable to laboratory X-ray sources as well as middle and high energy synchrotron radiation, consequently holding for different information depths. In contrast to the hitherto applied strain scanning methods the basic idea of the herein proposed procedure considers the use of fixed beam limiting apertures being inserted in the primary and the secondary beam path. In the project, which is a cooperation between the Institute for Materials Engineering (University of Kassel) and the Hahn-Meitner-Institute Berlin, it is distinguished between an angle-dispersive and an energy-dispersive approach. For the angle-dispersive approach (soft X-rays) the gauge element is defined by using beam limiting masks coupled to the samples surface. The geometries of the masks are tailored with respect to the X-ray wavelength and the samples tilt relative to the beam path. For the energy dispersive approach, the gauge, volume is defined by a pair of slits, coupled with the sample. Its geometry depends on the ψ tilt of the sample and the alignment allows a translation relative to the sample in order to realize the depth scan (see also Part II [1]). In part I of the contribution an outline of the entire scope of the project is given. In addition detailed information on the handling of the angle-dispersive approach will be presented.

Round Robin Test on the Determination of Residual Stress Depth Distributions by X-ray Diffraction

A round robin test on the determination of residual stress depth profiles of shot peened surfaces by X-ray diffraction was carried out within the scope of the BRITE-EURAM project ENSPED (European Network of Surface and Prestress Engineering and Design). 12 ENSPED-partners (8 industrial partners and 4 research institutes) participated in the round robin test. The aim of the test was to ensure the capability of the different partners to determine a residual stress depth distribution properly. Two batches of specimens were manufactured of a heat treatable steel, german grade 42 CrMo 4, - a normalized and a quenched and tempered state. The specimens were shot peened afterwards. In addition to the residual stress values determined by the partners, the raw-data of the measurements were evaluated by one of the research institutes as a reference using a common evaluation software. This evaluation was performed with respect to the individually chosen measurement set-up and the respective measurement parameters. Results will be presented by ensuring the anonymity of the different partners.