Neutronographic Residual Stress Analysis for Materials With Depth Gradients of the Strain Free Lattice Parameter d0{{{d}}}_{0} for the Example of a Case-Hardened Steel 20MnCr5

In the present work, ring-shaped samples made from steel 20MnCr5 were low-pressure carburized (LPC) and subsequently hardened by gas quenching (case-hardened). This results in a near-surface gradient in chemical composition, microstructure- and hardness distribution, as well as a three-dimensional residual stress (RS) distribution, which was investigated by neutron diffraction. Near-surface RSs in the ferrite-/martensite- and austenite phase are additionally determined by X-ray diffraction. It is shown that the chemical gradient has an influence on the chosen ${{{d}}}_{0}$ strategy and how such a reference sample should be extracted. If near-surface RS values are to be determined by neutron diffraction, the pseudo-strain effect must be taken into account. For this purpose, a suitable approach using the ‘‘open source’’ software SIMRES and STRESSFIT is also presented. By combining neutron and X-ray diffraction data, a complete RS distribution over the whole sample can be obtained

Effects of finish turning on an austenitic weld investigated using diffraction methods

Arc welding generally introduces undesired local residual stress states on engineering components hindering high-quality performance in service. Common procedures to reduce the tensile residual stresses are post-heat treatments or mechanical surface treatments like hammering or shot-peening. Assessments of residual stress profiles of post-weld treatments underneath the weld surface are essential, especially in high safety exigency systems like pressure vessels or piping at power plants. In this study, neutron diffraction is used to determine the stress profile after finish milling of an austenitic steel weld in order to verify a chained finite element simulation predicting the final residual stress fields including milling and welding contributions. Non-destructive measurements with spatial resolutions of less than 0.2 mm within the first 1 mm from the surface were mandatory to confirm the finite element simulations of the coarse-grained austenitic material. In the data analysis procedure, the obtained near-surface data have been corrected for spurious strain effects whenever the gauge volume was partially immersed in the sample. Moreover, constraining the surface data to values obtained by x-ray diffraction and data deconvolution within the gauge volume enabled access of the steep residual stress profile within the first 1 mm

Neutron surface residual stress scanning using optimisation of a Si bent perfect crystal monochromator for minimising spurious strains

For non destructive stress analysis of surface treated steel samples the application of laboratory X rays or high energy synchrotron radiation in reflection mode covers the region from some micrometers up to a depth of about 150 200 amp; 956;m. To access depth regions deeper than 200 amp; 956;m the incremental layer removal technique in combination with the repeated application of X ray stress analysis for the newly generated surfaces can be used. However, this procedure is destructive, laborious and furthermore, it has to be checked whether corrections have to be applied due to stress relaxation. By using neutron radiation penetration depths generally up to several millimetres can be achieved non destructively. However neutron measurements are critical at the surface. When scanning a sample surface, aberration peak shifts caused by so called spurious strains arise due to the fact that the gauge volume defined by the primary and secondary optics is partially outside of the sample. These aberration peak shifts can be of the same order of magnitude as the peak shifts related to residual strains. In this exemplary study it will be demonstrated that, by optimising the bending radius of a Si 400 monochromator, the spurious surface strains can be strongly reduced when compared to the values obtained with a traditional Ge 311 mosaic monochromator, even when the gauge volume is mainly out of the surface. The objective of the experiments is to find the optimal monochromator settings for the Si 400 monochromator at the STRESS SPEC instrument at the research reactor FRM II, Munich, Germany. For the parametric studies a stress free steel sample of the fine grained construction steel, S690QL was used. The optimised conditions for the Si 400 monochromator that resulted from the systematic studies were applied to a shot peened plate of steel SAE 4140. The residual stress distribution is analysed by means of through surface strain scanning. The residual stress gradient obtained is in very good agreement with the well characterised residual stress depth profile obtained within a round robin test in the scope of the BRITE EURAM project ENSPED European Network of Surface and Prestress Engineering and Design . The results indicated that surface residual stress profiles can be measured with neutrons up to 200 amp; 956;m underneath the surface without time consuming and laborious surface effect corrections