Jérémy Epp

In situ X-Ray Diffraction Analysis of Carbon Partitioning During Quenching of Low Carbon Steel

In situ X-ray diffraction investigations of phase transformations during quenching of low carbon steel were performed at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) at beamline ID11. A dynamic stabilization of the retained austenite during cooling below martensite start was identified, resulting in an amount of retained austenite of approximately 4xa0vol pct. The reason for this dynamic stabilization is a carbon partitioning occurring directly during quenching from martensite (and a small amount of bainite) into retained austenite. A carbon content above 0.5xa0mass pct was determined in the retained austenite, while the nominal carbon content of the steel was 0.2xa0mass pct. The martensitic transformation kinetic was compared with the models of Koistinen-Marburger and a modification proposed by Wildau. The analysis revealed that the Koistinen-Marburger equation does not provide reliable kinetic modeling for the described experiments, while the modification of Wildau well describes the transformation kinetic.

Combined Neutron and X-Ray Diffraction Analysis for the Characterization of a Case Hardened Disc

The present work has been executed within the framework of the collaborative research center on Distortion Engineering (SFB 570) in order to evaluate the residual stress state of a disc after carburizing and quenching as well as to validate a simulation procedure. The combined use of X-ray and neutron diffraction analysis provided information about the residual stress states in the whole cross section. However, the stress free lattice spacing d0 for the neutron diffraction experiments is problematic and induces systematic uncertainties in the results and the application of a force balance condition to recalculate d0 might be a solution for improving the reliability of the results. Comparison of experimental results with simulation showed that an overall satisfying agreement is reached but discrepancies are still present.

In situ X-Ray Diffraction Investigation of Surface Modifications in a Deep Rolling Process under Static Condition

The deep rolling process is widely used as a finishing step, improving the surface properties through cold working and creation of residual strains. In the present investigations, in situ X-ray diffraction experiments were performed with a self-built deep rolling device at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The measurements were performed with a cylindrical roller-tool on steel samples at beamline ID11 with a monochromatic beam of 50 × 50 μm in transmission. Several properties could be investigated based on the diffraction data. In particular, 2D-strain maps were determined in the range of several millimeters around the deep rolling tool. Based on the data collected during loading and after unloading, knowledge about the transient state leading to the resulting remaining property modifications like residual strains and plastic deformation were generated. Introduction The new Collaborative Research Centre “Process Signature” of the University of Aachen and Bremen concentrates on the study of process-independent surface modification mechanisms in order to achieve predictive manufacturing processes. After the process, a given volume near the surface shows property modifications including microstructural changes, increased hardness and generation of compressive residual strains. The internal strain field created during the process can be predicted by FEM simulations as well as contact mechanics theory depending on the applied load and contact geometry. However, an experimental verification of the real strain fields during the process does not exist up to now. The characterization of material modifications in mechanical processes is often constrained to ex situ analysis of the end state and a theoretical evaluation of the external factors that lead to the observed modifications. This approach often shows deviations in simulations and theoretical concepts applied to the process, because of unknown parameters. From external factors, however, a specific internal material load is found in the surface near region and in the region below, which directly influences the materials response to the applied mechanical load. Measurements of the internal material load and of the material modifications during the process show several problems depending on the utilized method. Optical methods with digital speckle photography allow measurements of surface strain fields but no direct analysis of the volume state as can be seen in the works of Johnson [1]. Another method is the integration of sensors inside the material, as done by Tausendfreund et al. [2]. The bondings between the resistive thin film and the material as well as the whole integration however introduce errors in the propagation of the strain field, shown by Dumstorff and Lang [3]. One further method for in situ process analysis is based on the use of high energy synchrotron radiation, which can penetrate steel samples up to a depth of several millimeters or even centimeters and give diffraction patterns from the illuminated crystalline structure along the beam path with sufficient intensity, like already used by Uhlmann et al. [4] for the analysis of an Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 431-436 doi: http://dx.doi.org/10.21741/9781945291173-73 432 orthogonal cutting process. The method makes the measurement of a strain state during mechanical loading possible and gives additional information about texture, grain size, phase and specific elastic strain depending on the analysis as shown by Liss et al. [5]. In the present study monochromatic synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) has been used to analyze the strain field and further material modifications of samples in transmission geometry during and after static loading with a deep rolling tool cylinder, to determine internal material load contribution and resulting residual strain state in situ respectively. Experimental methods Materials. The experiments were performed on quenched and tempered state of the 42CrMo4 (AISI 4140H) material. The chemical composition is given in Table 1. Table 1 Chemical composition in wt.% of steel AISI 4140H (EN 42CrMo4) in the samples Steel AISI Heat treatment C [%] Cr [%] Mo [%] Mn [%] Si [%] S [%] Hardness [HRC] 42CrMo4 (Q+T) 4140H 850°C/2h Quenched to 60°C (Oil)/ Tempered at 400°C/4h 0.43 1.09 0.25 0.74 0.26 <0.001 47 ± 2 Experimental deep rolling device. For the experiments performed at the ESRF on beamline ID11EH1 a frame was constructed from aluminium profiles, which made the installation of an industrial deep rolling system with the full pressure range of up to 400 bar, provided through an external hydraulic fluid system, possible. Application of pressure was done in a top-down way and the sample positioning and movement was provided through a linear stage. Since the tool head is fixed in respect to the sample stage and the material state is homogenous isotropic, gauge volumes at different positions in respect to the contact point can be scanned during processing. The basic setup, a close up view of the sample with the deep rolling tool and the schematic layout are shown in Fig. 1. Fig. 1a. Deep rolling setup frame mounted in ID11 b. Deep rolling tool on sample during process c. Schematic view of geometry Experimental and measurement parameters. The experiment was carried out at beamline ID11 on experimental hutch 1 at the ESRF with high energy monochromatic radiation. The parameters for the measurement with the detector and the slit system settings are shown in Table 2. Table 2 Experimental parameters for the measurements at ID11 EH1 Photon Energy Beam size [μm] Detector Measurement time [s/ meas.] 2Theta range 100 [keV] 0.0123984 [nm] 50 x50 FReLoN 2D CCD (2048x2048 pixel) 0.2s (+1.0s for axis movement) Complete diffraction rings 0 10° The detector was positioned at a distance of 273 mm from the sample behind the frame and the diffraction rings were measured in transmission geometry. A photon energy of 100 keV was used to c. a. b. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 431-436 doi: http://dx.doi.org/10.21741/9781945291173-73 433 allow steel samples of sufficient width to be measured in transmission so that mechanical stability during the processing in the setup is ensured, while small beam dimensions are best suited for the spatial resolution of the strain field, which has a high gradients below the contact point in this geometry. Sample geometry was approximately cuboid with a 20 mm by 70 mm height and length, where the initial thickness of 3 mm from the wire-cut electrical discharge machining was further reduced by 100 μm in the experimentally scanned area by electrolytic surface removal, giving a final sample thickness of 2.8 mm width . The deep rolling tool for the static loading had a cylindric geometry with diameter of 13 mm and a length of 15 mm, housed inside a brass tool head, and was machined from tungsten carbide (WC) material. It was applied at a pressure of 300 bar, corresponding to a force of 3100±100 Newton along the width of the sample. Methodical approach. With these parameters measurements of the 2D internal material load field around the deep rolling contact point were possible. Scans of the strains during static loading and the residual strain state after unloading were performed. As an example, the standard field in a static test consisted of a matrix of points with up to 50μm spatial resolution in the region near the contact point, with maximum grouping of points at the center along the yand z-axis, which is shown in Fig. 2. A full scanning field consists of 946 points in these measurements, where a range of 7.8 mm was scanned in horizontal direction and a zone of 4.15 mm depth from the surface of the sample was analyzed in vertical direction. The movement of the setup frame on the diffractometer table for the different positions increases the total time for each point with 0.2 s measuring time + 1.0 s moving time to a mean value of 1.2 s/ point, giving an effective measurement frequency of 0.78 Hz. Fig. 2 Full-Field point distribution in scan range below contact point Data analysis. Information from the gauge volume of 50 μm x 50 μm x 3 mm = 0.0075 mm3 at each measurement point contains the information about the strain state, phase content and other parameters of the microstructural condition of the diffracting crystallites. Because of geometric and material parameters, uniform loading and deformation of the material by the rolling tool is achieved in the region of the beam path, which is along the x-direction. εyy,zz hkk = �dyy,zz hkk − d0� d0 ⁄ . (1) Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 431-436 doi: http://dx.doi.org/10.21741/9781945291173-73 434 Strain evaluation along orthogonal directions relies on caking, meaning azimuthal integration of a part of the diffraction ring that corresponds to the strain directions, as is shown in Fig.3. Using Eq. 1 the strain in the direction corresponding to the azimuthal section can be determined by using a d0value of the unstrained material state, which was determined from a scan of the region before contact and compared with values from the edge points during and after loading. The strain evaluation is obtained from the α{211} reflex of the tempered martensite structure, as seen in the azimuthal integration in Fig. 4. This reflex is used because of its position around diffraction angle 6.05 deg with an intensity compared to the α{110} of I{211} I{110} ⁄ = 0.25, achieving a mean error of ∆ε= 35 μstrains and ∆FFHH= 0.0007 deg. Further analysis based on determination of lattice spacing from all available reflexes were not done yet but are planned in the next steps. Fig. 3 Diffraction rings from detector readout Fig. 4 Integrated diffraction pattern with cak

Investigation of the Influence Factors on Distortion in Induction-Hardened Steel Shafts Manufactured from Cold-Drawn Rod

In this study, the distortion of steel shafts was investigated before and after induction hardening. Several essential influencing factors in the manufacturing process chain regarding cold drawing, cutting method, notches on the shafts, and induction hardening were analyzed by design of experiment (DoE). Further necessary examinations of microstructures, hardness profile, segregation of chemical composition, and residual stress state were conducted for understanding the distortion behavior. The results of the statistical analysis of the DoE showed that the drawing process is the most important factor influencing distortion. The surface hardening depth of induction hardening is the second main factor. The relationship between inhomogeneities in the work pieces and the distortion was finally discussed.

Zerstörungsfreie Eigenspannungsanalyse von Stahlwellen nach unterschiedlichen Prozessschritten vom Drahtziehen zum Induktionshärten

Kurzfassung Eine Prozesskette für die Herstellung von Maschinenbauteilen besteht im Allgemeinen aus mehreren Prozessschritten, wie z. B. Stranggießen, Warmwalzen, Glühen, Richten, Drahtziehen, Richten und Polieren, spanende Bearbeitung, Induktionshärten und Schleifen. Verzug wird häufig nach der finalen Wärmebehandlung beobachtet. Verantwortlich dafür ist nicht nur das Härten, sondern auch die Vorgeschichte der Prozesskette. In der vorliegenden Arbeit wurden einige wesentliche Schritte einer Prozesskette experimentell untersucht und die Eigenspannungsverteilungen als wesentliche Verzugspotenzialträger mit Neutronenstrahlung über den gesamten Querschnitt der Zylinder zerstörungsfrei analysiert. Die Ergebnisse zeigen, dass nach dem Richten der entrollten Drähte unsymmetrische Eigenspannungsverteilungen über den Querschnitt vorliegen. Das Kaltdrahtziehen führt zu hohen Zugeigenspannungen an der Oberfläche in radialer und tangentialer Richtung, während im Kern vor allem Druckeigenspannungen in axialer Richtung vorliegen. Der anschließende Schritt – Schrägwalzen-Richten (SWR) – führt zu einer Reduktion der Eigenspannungen. Das finale induktive Härten verändert die Eigenspannungszustände in Abhängigkeit der Prozessbedingungen wieder sehr stark. Die Einflüsse der untersuchten Prozessschritte auf die Eigenspannungszustände werden schließlich in Verbindung mit den auftretenden Verzügen diskutiert.

S45 Procedures for Nondestructive RS-Measurements of Inner Surfaces of Ball Bearing Components

Machining induced residual stress states contribute to the distortion of machined components during heat treatments. Thus, ideally a complete characterization of the components residual stress state is required. Magnetic and micromagnetic analysis of residual stresses can represent an important gain of time if compared to X-ray diffraction. Another important advantage is the access of inner faces of components like ball bearing rings. This study presents a comparison of these two methods for the characterization of outer and inner surfaces of ball bearing rings. A good calibration of micromagnetic measurements with X-ray diffraction data is necessary. Reliable results then can be achieved and the residual stress states of rings can be characterized. Effects of clamping systems and feed on the residual stress levels and distribution will be discussed, as well as complex interactions. An implementation of the presented micromagnetic method for non-destructive control in large manufacturing series could be possible.

In-situ-Untersuchung von Randschichten während des Gasnitrierens mittels Röntgendiffraktometrie und photothermischer Radiometrie*

Abstract The aim of most applications of nitriding treatments at steel components is to obtain a compact compound layer and/or a deep diffusion layer. The possibility of a survey of the nitriding treatment by analyzing directly the component´s surface state during the nitriding process is particularly interesting, since it allows a process monitoring and control based on the actual nitriding result. In the present study, two measurement methods were developed and combined with the aim of direct surface state analysis during a nitriding treatment: the in-situ X-ray diffraction method and the photothermal radiometry. An experimental setup including a miniature nitriding furnace was developed in order to allow the combined application of both methods during a nitriding process under controlled atmosphere. In the present work, results of combined in-situ measurements on the steel AISI 4140 regarding the nitride layer formation during nitriding process as well as the nitride layer change during the following denitriding of the layer in nitrogen gas are presented and discussed. The investigations show that the photothermal radiometry is sensitive to the changing surface properties due to growing compound layers and when porous layers are generated. This method has a high potential for implementation in industrial nitriding furnaces, but for this, further development for quantitative evaluations of the measurements will be required.

Investigation of Deformation Induced Martensitic Transformation during Incremental Forming of 304 Stainless Steel Wires

Wires with 1 mm initial diameter have been reduced between 10 and 64 percent at different temperatures and strain rates by infeed rotary swaging, which is an incremental cold forming process mainly used for rods and pipes. The volume fraction of martensite in the deformed wires has been determined by X-Ray diffraction and by magnetic induction for different processing parameters. Measurements show that for already small percentage of reduction, martensite is present in the wires and its amount changes with the strain rate and temperature. While for smaller strain rates at room temperature the formation of martensite is promoted, it is restrained for higher strain rates and higher temperatures. Results also reveal that the martensite distribution in the sample is inhomogeneous. Further investigations have been made to analyze the microstructure by optical microscopy and to determine mechanical properties by tensile testing.

Einfluss der Oberflächenfertigung und des Nitrierens auf den Eigenspannungszustand des warmfesten Stahls X38CrMoV5-3

Kurzfassung Um den hohen Beanspruchungen bei der Warmmassivumformung zu begegnen, werden die Oberflächen und Randbereiche der eingesetzten Werkzeuge nitriert. Durch das Nitrieren entstehen hohe Druckeigenspannungen in Oberflächennähe. Aber auch die Fertigungsbedingungen führen zu Eigenspannungen im Randbereich, die bei dem warmfesten Stahl X38CrMoV5-3 während des Nitrierens nur teilweise abgebaut werden können. In der vorliegenden Arbeit wurden Eigenspannungen an unterschiedlich gefertigten Proben gemessen, um die sich ergebenen Spannungen aus der Fertigung und der Nitrierbehandlung zu ermitteln. Die fertigungsbedingten Druck- und Zugeigenspannungen werden beim Nitrieren des Warmarbeitsstahls nicht abgebaut. Nach dem Nitrieren wurden Druckeigenspannungen von bis zu −1000 MPa gemessen.

Verzugskompensation von Wälzlagerringen durch Festwalzen

Kurzfassung In der Herstellung dünnwandiger Bauteile wie z. B. von Wälzlagerringen treten an verschiedenen Prozessschritten unerwünschte Maβ- und Formänderungen auf. Die Korrektur des Verzugs bedingt erhöhte Aufmaβe bei der kostenintensiven Hartbearbeitung durch Schleifen oder Honen. Die Arbeiten des Sonderforschungsbereichs 570 „Distortion Engineering“ zielen sowohl auf eine prozessübergreifende Betrachtung des Verzugs als auch auf die Verzugsbeherrschung in einzelnen Prozessschritten ab. In den hier vorgestellten Arbeiten wird der neue Ansatz verfolgt, einspannungsbedingte Biegelastspannungen durch Festwalzen in der Randschicht dünnwandiger Ringe zu speichern und somit auftretende Formabweichungen zu kompensieren. Es wird gezeigt, dass eine gewünschte Formänderung durch Variation von Einspannlage, Festwalzkraft sowie Spannkraft reproduzierbar erzielt werden kann und demnach ein hohes Potenzial zur Verzugskompensation besteht.