Hardness mapping permits quantification of the properties of materials over microstructurally significant lengths. A technique has been developed whereby hardness maps can be generated to account for specific weld geometry and further refined using an adaptive approach. Once a preliminary map is produced, subsequent indents are placed in high hardness gradient locations to decrease interpolation distances between indentation sites. The method is demonstrated with three test cases: an Al-7010 friction stir weld, an Alloy 600/82 (NeT TG6) weld and an SA508-4N/Alloy 82/316LN dissimilar metal weld. The results show that the method has identified and resolved high regions with elevated hardness gradients. This provides the ability to resolve weld regions rapidly across large areas minimising indent counts.
This paper describes in detail two neutron diffraction residual stress measurements, performed on the ENGIN-X neutron scattering instrument at the ISIS facility in the UK and on the SALSA instrument at the Institut Laue–Langevin in Grenoble, France. The measurements were conducted as part of the NeT Task Group 6 (TG6) international measurement round robin on an Alloy 600/82 multi-pass weldment – a slot in an Alloy 600 plate filled with three Alloy 82 weld beads, simulating a repair weld. This alloy/weld combination is considered challenging to measure, due to the large grain size and texture in the weld, and large gradients in the stress-free lattice parameter between the parent and weld metal. The basic principles of the neutron diffraction technique are introduced and issues affecting the reliability of residual stress characterization are highlighted. Two different analysis strategies are used for estimation of residual stresses from the raw data. Chemical composition studies are used to measure the mixing of parent and weld metal and highlight the steep lattice parameter gradients that arise as a consequence. The inferred residual stresses are then compared with three sets of measurements performed on the same plate by other NeT partners on E3 at the HZB in Berlin, STRESS-SPEC at the FRM II in Munich and KOWARI in Sydney. A robust Bayesian estimation average is calculated from the combined five-instrument data set, allowing reliable best estimates of the residual stress distribution in the vicinity of the weldment. The systematic uncertainties associated with the residual stress measurements are determined separately in the weld and parent materials, and compared with those in the NeT TG4 benchmark. This is a three-pass slot-welded plate fabricated from American Iron and Steel Institute AISI 316L(N) austenitic stainless steel, and is normally considered less challenging to measure using diffraction techniques than all nickel welds. The uncertainties in the stress measurements by neutron diffraction for these two weldments seem to be comparable.
A round robin benchmark weldment, denoted TG6, from the European Network on Neutron Techniques Standardization for Structural Integrity (NeT), offers the ability to test the technique of residual stress determination using neutron diffraction to an extreme. This test component comprises a 200 mm by 150 mm by 12 mm rectangular base plate made from Inconel 600 with three passes of Alloy 82 weld metal deposited in a slot of length 76 mm. This not only has large grain issues in the weld region but a large interplanar spacing variation caused by high strain gradients and a change in material composition. Because of the high absorption of neutrons in the Inconel 600 within the gauge volume, there is a large difference between the instrumental gauge volume IGV (which is the volume of space defined by the neutron beam paths through the defining apertures, taking into account properties of the beam) compared with the sampled gauge volume SGV (which is the intersection of the IGV with the sample that takes into account the absorption of the neutrons). The relative shift in position of the center of gravity of measurement (when the gauge volume is fully immersed in the sample) has been observed to be significant, for example, ∼0.3 mm, using 1 by 1 by 5 mm3 defining optics (where 5 mm is the gauge volume height) and increases approximately proportionally with larger gauge volumes dimensions in the horizontal plane. The magnitude of this shift depends upon many parameters. Measuring the sample and rotating 180° and measuring again, taking the average is another way one can eliminate this absorption effect. This novel approach is compared with three neutrons instruments on the same sample.
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