Papers by Author: Jens Gibmeier

Abstract: 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. MnCo_1.9Fe_0.1O₄ (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.

Abstract: Welding using low transformation temperature (LTT) filler materials is an innovative approach to mitigate detrimental welding residual stresses without cost-intensive post weld-treatments [1, 2]. Due to the local generation of compressive residual stresses in the weld line by means of a delayed martensite transformation a significant enhancement of the cold cracking resistance of highly stressed welded components can be expected. For the effective usage of these materials a deeper understanding of the microstructural evolution inside the weld material is necessary to determine the complex processes that cause the residual stress formation during welding. Solid-state phase transformation kinetics and the evolution of strain in LTT weld filler materials are monitored in-situ at the instrument ID15A@ESRF in Grenoble. The transferability to real components is implemented by using a realistic MAG welding process under consideration of structural restraint. During welding of multilayer joints, the phase transformation and phase specific strain evolution of each individual layer is investigated in transmission geometry by means of energy-dispersive X-ray diffraction EDXRD using high energy synchrotron radiation with a counting rate of 2.5 Hz. The measurement results of a 10% Cr / 10% Ni LTT weld filler are compared to data monitored for the conventional weld filler material G89. The in-situ data clearly indicate a strong effect on the local strain evolution and the formation of compressive strain. This results from the restraint volume expansion during the postponed austenite to martensite transformation of the LTT weld filler, which counteracts the thermal shrinkage. In contrast, for the conventional weld filler material the thermal contraction strains lead to tensile residual strain during welding. Furthermore, the results of in-situ observation during welding show that the transformation kinetic is dependent on the welding sequence.

Abstract: 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.

Abstract: The effect of defined preloading in the tensile and compressive regime on the near surface (residual) stress distributions, which result from laser surface hardening, is systematically studied in-situ, i.e. under the applied preload and after unloading. Samples made of steel grade AISI 4140 are defined surface hardened by means of a high-power diode laser (HPDL) system during uniaxial compressive elastic loading at-300 MPa as well as during uniaxial elastic tensile loading at 300 MPa using a custom designed 4-point-bending device, which can be mounted on an X-ray diffractometer. The results of X-ray stress analysis were compared to data derived for a sample state unaffected by any preload. Without external loading compressive residual stresses are induced inside the process zone that are balanced by tensile residual stresses outside this zone. The investigations show that external loading in the tensile and compressive regime has a strong impact on the resulting lateral residual stress distribution in loading direction. The results further indicate that undesirable tensile residual stresses outside the process zone can even be suppressed by using a defined appropriate preloading in the tensile regime.

Abstract: The incremental hole-drilling method is a widely used technique to determine residual stress depth profiles in technical components. Its application is limited in respect to the components geometry, for instance the components thickness. In this paper, a direct correction of the measured strain relaxations is proposed to consider the impact of deviant geometries, here the component thickness, on the residual stress evaluation that moreover, allows the application of commercially available evaluation software. The herein proposed approach is based on finite element simulation of the incremental hole drilling. The simulated strain relaxations for thin metal sheets are evaluated with an algorithm as used in commercially available evaluation software (i) for uncorrected data as well as (ii) for strain data corrected by the proposed correction procedure. It is shown that the correction approach leads to a significant improvement of the measurement accuracy. Further, by means of the approach residual stress depth profiles in thin metal sheets can be as usual determined using commercial evaluation software for the incremental hole-drilling method regardless of the algorithm used, i.e. differential or integral.

Abstract: The tool and workpiece surface layer states of the tribosystem uncoated WC-Co cutting tools vs. normalised SAE 1045 workpiece material are studied in detail for a dry metal cutting process. Within the system the cutting parameters (cutting speed, feed rate, cutting depth) determine the wear state of the cutting tool and the resulting surface layer state (residual stress) in the workpiece. As the built-up edge can be used as a possible wear protecting layer [1] the influence of built-up edge and wear behaviour of the cutting tool was examined with respect to the workpiece surface layer state for knowledge based metal cutting processing. Small compressive stresses (-60-80 MPa) are induced in the surface layer, that are nearly homogeneous for the highest built-up edge, which lead to the lowest tool wear in combination with lowest cutting temperature.

Abstract: The effect of processing atmosphere on the microstructure and residual stresses are studied for laser surface hardening on steel samples of grade AISI 4140. Samples were hardened in air, vacuum and inert gas atmosphere (Helium) by means of a stationary laser beam. A high-power diode laser (HPDL) system was used in combination with a custom-designed process chamber. Residual stress distributions in lateral and in depth direction were analysed after laser processing by means of X-ray diffraction according to the well known sin² - method. X-ray residual stress analyses were supplemented by microscopic investigations of the local microstructure. The results indicate a widening of the compressive stressed region in lateral as well as in depth direction by surface hardening in inert gas atmosphere compared to laser surface hardening in air or vacuum atmosphere. This is due to the local heating flux distribution during the laser assisted heat treatment which is strongly affected by the processing atmosphere an leads to an extension of the hardening zone when using helium as inert gas.

Abstract: Internal load transfer in an interpenetrating metal/ceramic composite has been studied in this work using energy dispersive synchrotron X-ray diffraction. One of the samples was loaded in tension and the other one in compression. In each case, the sample was first loaded into the elastic-plastic regime, unloaded to zero stress, and reloaded beyond the prior maximum stress. Results show that at stress amounts greater than 100 MPa aluminum deforms plastically and the load is transferred to alumina and silicon. Unloading and reloading typically show reverse plastic deformation, Bauschinger effect and strain hardening in aluminum.

Abstract: Welding residual stress is of major concern for structural integrity assessment in industrial components. Shear and volume strains resulting from the austenite-martensite-transformation affect the development of residual stress during welding. Controlling the phase transformation allows adjustment of the welding residual stress. Low transformation temperature (LTT) weld filler materials exhibiting reduced MS-temperatures allow postponing the phase transformation. The associated strain arising from the delayed transformation compensates for the thermal contraction strains and as such may reduce tensile or even introduce compressive residual stress. In this article we discuss the tri-axial residual stress distribution in 15 mm S690Q steel plates joined with LTT filler materials with 10 wt% Cr and a Ni-content that varies from 8 to 12 wt%. Using complementary synchrotron X-ray and neutron diffraction stress analysis the macroscopic residual stress was derived from the phase specific lattice strain and phase fraction of martensite and retained austenite, respectively. The local phase specific unstrained lattice parameters were determined using stress relieved combs. The investigation revealed increasing phase fraction of retained austenite with increasing Ni-content. Further, independent of the Ni-content in each weld in the fusion zone, significant compressive residual stresses were found in the longitudinal direction, which are balanced by tensile residual stresses in the heat affected zone (HAZ). In the weld transverse and normal direction the stress distribution is qualitatively similar but less in magnitude. The increased amount of retained austenite reduces the compressive stress arising from shear and volume strains during the delayed phase transformation and therefore no significant increase in compression was observed for decreasing M_S-temperatures.

Abstract: Precise determination of diffraction peak positions is of particular importance for the evaluation of residual strains. Neutrons are commonly used to probe residual strains from material volumes in depths of several millimetres under the sample surface. However, neutron strain analyses are critical for the near surface region. When scanning close to a sample surface, aberration peak shifts arise, which can be of the same order as the peak shifts related to residual strains [1]. Series of Monte Carlo (M.C.) simulations using the software package RESTRAX/SIMRES [2] were carried out to simulate the peak shift as a function of gauge volume depth, monochromator curvature and other instrumental parameters, which can be used to quickly optimise the experimental setup for direct measuring residual strains near the sample surface at an arbitrary surface orientation. The M.C. simulations were compared and agree very well with the experimental data, not only for a stress free steel sample but as well for a deep rolled steel sample, measured at the STRESS-SPEC diffractometer at the research reactor FRM II, Garching (Germany).