Comparison between surface and near-surface residual stress measurement techniques using a standard four-point-bend specimen

Background: Determination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties. Objective: This study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI). Methods: These measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties. Results: The XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation. Conclusion: The ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method

Recrystallization mechanisms and associated microstructure evolution during billet conversion of a gamma-gamma' nickel based superalloy

A partially recrystallized sample of the Ni-based superalloy AD730 was taken from an intermediate stage of the ingot to billet conversion process and isothermally forged in a single stroke compression test at a sub-solvus temperature (1080 °C). The as-received material had a heterogeneous microstructure, containing a mixture of coarse and much finer recrystallized grains as well as unrecrystallized ones, and also heterogeneous γ′ precipitation. The recrystallization mechanisms occurring dynamically in the different grain populations were investigated via electron backscatter diffraction (EBSD). It was found that local microstructure could affect the operative recrystallization mechanism, with different mechanisms seen in the deformed and recrystallized regions, owing to their different precipitate distributions. Within a single deformed grain, three apparently distinct dynamic recrystallization (DRX) mechanisms were identified. The interaction of recrystallization with precipitates plays a central role in DRX. In certain cases precipitates may stimulate discontinuous DRX by providing recrystallization nuclei, alternatively they may impede and limit the growth of recrystallized grains, or in other cases still they promote continuous recrystallization

Evolution of microstructure and crystallographic texture during dissimilar friction stir welding of duplex stainless steel to low carbon-manganese structural steel

Electron backscattered diffraction (EBSD) was used to analyze the evolution of microstructure and crystallographic texture during friction stir welding of dissimilar type 2205 duplex stainless steel (DSS) to type S275 low carbon-manganese structural steel. The results of microstructural analyses show that the temperature in the center of stirred zone reached temperatures between Ac 1 and Ac 3 during welding, resulting in a minor ferrite-to-austenite phase transformation in the S275 steel, and no changes in the fractions of ferrite and austenite in the DSS. Temperatures in the thermomechanically affected and shoulder-affected zones of both materials, in particular toward the root of the weld, did not exceed the Ac 1 of S275 steel. The shear generated by the friction between the material and the rotating probe occurred in austenitic/ferritic phase field of the S275 and DSS. In the former, the transformed austenite regions of the microstructure were transformed to acicular ferrite, on cooling, while the dual-phase austenitic/ferritic structure of the latter was retained. Studying the development of crystallographic textures with regard to shear flow lines generated by the probe tool showed the dominance of simple shear components across the whole weld in both materials. The ferrite texture in S275 steel was dominated by D 1, D 2, E, E¯ , and F, where the fraction of acicular ferrite formed on cooling showed a negligible deviation from the texture for the ideal shear texture components of bcc metals. The ferrite texture in DSS was dominated by D 1, D 2, I, I¯ , and F, and that of austenite was dominated by the A, A¯ , B, and B¯ of the ideal shear texture components for bcc and fcc metals, respectively. While D 1, D 2, and F components of the ideal shear texture are common between the ferrite in S275 steel and that of dual-phase DSS, the preferential partitioning of strain into the ferrite phase of DSS led to the development of I and I¯ components in DSS, as opposed to E and E¯ in the S275 steel. The formations of fine and ultrafine equiaxed grains were observed in different regions of both materials that are believed to be due to strain-induced continuous dynamic recrystallization (CDRX) in ferrite of both DSS and S275 steel, and discontinuous dynamic recrystallization (DDRX) in austenite phase of DSS

Microstructural evolution of an interface region in a nickel-based superalloy joint produced by direct energy deposition

Microstructure analysis of additively manufactured (AM) materials is an important step in understanding the interrelationship between mechanical properties and materials performance. Literature on the effect of a laser-based AM process parameters on the microstructure in the substrate-deposit interface is limited. The interface region, the adjoining area of substrate and deposit, is characterized by the presence of the fusion zone (FZ) and heat affected zone (HAZ) experiencing rapid thermal gyrations resulting in thermal induced transformations. Inconel 718 was utilized as a work material for both the substrate and deposit. Three blocks of Inconel 718 material were deposited by Direct Energy Deposition (DED) using three different laser powers, 550W, 750W and 950W, respectively. A coupled thermo-mechanical transient approach was utilized to correlate temperature history to the evolution of microstructure. Thermal history of the deposition process was monitored with the thermocouples installed inside the substrate material. Interface region of the blocks were analyzed with Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) including electron backscattered diffraction (EBSD) technique. Laser power was found to influence the dissolution of intermetallic precipitated phases in the substrate and grain growth in the interface region. Microstructure and thermal history data were utilized to draw conclusive comparisons between the investigated process parameters

Evolution of γ′ precipitation during the early stages of industrial forging of a nickel-based superalloy

A sample of the Ni based superalloy AD730 was heat treated at a supersolvus temperature (1160 °C) then slowly cooled through the solvus temperature (1110 °C) at 10 °C/hr down to 1080 °C, i.e., a rate representative of the cooling conditions of an industrial scale billet undergoing controlled cooling. The γ′ precipitate distribution which forms during this cooling was investigated, and a mix of continuous and discontinuous precipitation was found. The discontinuous γ′ precipitates were imaged using 3D tomography, and were shown to present very different sizes, morphologies and aspect ratios when observed in different 2D imaging planes. The interaction between different populations of γ′ precipitate and recrystallization was investigated, and it was found that the discontinuous precipitates present more of a barrier to recrystallization than the continuous ones. This has been explained based on the different inter-precipitate spacing observed for the two populations. In addition to these γ′ precipitates which form during slow cooling, a fine and dense distribution of approximately spherical γ′ precipitates was found to form dynamically, during subsequent subsolvus forging, within unrecrystallized grains