Two-dimensional mapping of residual stresses in a thick dissimilar weld using contour method, deep hole drilling, and neutron diffraction

Residual stress variations were determined through the thickness of a 70-mm-thick ferritic–austenitic dissimilar steel weld using contour method, deep hole drilling, and neutron diffraction. The result shows that significant tensile stresses were distributed distinctly along the interface between ferritic and austenitic phases. The band of the large tensile stresses was about 8 mm wide and the magnitude reached 400 MPa, which is approaching 100 % of the yield strength of the base metal, near the top surface (about 15 % of the depth). It is attributed to the large difference (5.8 × 10−6 1/°C) of the thermal expansion coefficient between ferritic and austenitic steels of the interface. The microstructure analysis elucidates that the martensitic phase prevailed near the interface and results in microhardness increases

Synergetic strengthening of layered steel sheet investigated using an in situ neutron diffraction tensile test

Synergetic strengthening induced by plastic strain incompatibility at the interface, and the resulting extra geometrically necessary dislocations (GNDs) generated during plastic deformation, were investigated to understand the origin of extra strength in heterogeneous structured (HS) materials. The mechanism of extra GND generation in twinning-induced plasticity (TWIP)-interstitial free (IF) steel layered sheet was quantitatively analyzed by conducting in situ neutron scattering tensile test. Load partitioning due to the different mechanical properties between the TWIP-steel core and IF-steel sheath at the TWIP/IF interface was observed during the in situ tensile testing. Because of the plastic strain incompatibility from load partitioning, extra GNDs are generated and saturate during tensile deformation. The extra GNDs can be correlated with the back-stress evolution of the HS materials, which contributes to the strength of layered materials. Because of the back-stress evolution caused by load partitioning, the strength of TWIP-IF layered steel is higher than the strength estimated by the rule-of-mixtures. This finding offers a mechanism by which extra GNDs are generated during load partitioning and shows how they contribute to the mechanical properties of HS materials.11Ysciescopu

\u3cem\u3eSevere Plastic Deformation using Friction Stir Processing, and the Characterization of Microstructure and Mechanical Behavior using Neutron Diffraction\u3c/em\u3e

Friction-stir welding (FSW) is a solid-state joining process, which utilizes a cylindrical rotating tool consisting of a concentric threaded tool pin and tool shoulder. The strong metallurgical bonding during the FSW is accomplished through: (1) the severe plastic deformation caused by the rotation of the tool pin that plunges into the material and travels along the joining line; and (2) the frictional heat generated mainly from the pressing tool shoulder. Recently, a number of variations of the FSW have been applied to modify the microstructure, for example, grain refinements and homogenization of precipitate particles, namely friction-stir processing (FSP). Applications of the FSP/FSW are widespread for the transportation industries. The microstructure and mechanical behavior of light-weight materials subjected to the FSW/FSP are being studied extensively. However, separating the effect of the frictional heat and severe plastic deformation on the residual stress and texture has been a standing problem for the fundamental understanding of FSW/FSP. The fundamental issues are: i) the heat- and plastic- deformation-induced internal stresses that may be detrimental to the integrity and performance of components; ii) the frictional heating that causes a microstructural softening due to the dissolution or growth of the precipitates in precipitation-hardenable Al alloys during the process; and iii) the crystallographic texture can be significantly altered from the original texture, which could affect the physical and mechanical properties. The understanding of the influences of the de-convoluted sources (e.g. frictional heat, severe plastic deformation, or their combination) on the residual stress, microstructural softening, and texture variations during FSW can be used for a physics-based optimization of the processing parameters and new tool designs. Furthermore, the analyses and characterization of the natural aging behavior and the aging kinetics can be practically applied to the predictions of mechanical behavior and material selection for the FSW/FSP. Finally, the experimental results can be useful to develop more accurate computational simulations

Analysis of Strain Hardening in B2 Dual Phase Low-density Steel by in-situ Neutron Diffraction

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Neutron diffraction and finite element analysis of the residual stress distribution of copper processed by equal-channel angular pressing

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The effect of initial stress induced during the steel manufacturing process on the welding residual stress in multi-pass butt welding

A residual stress generated in the steel structure is broadly categorized into initial residual stress during manufacturing steel material, welding residual stress caused by welding, and heat treatment residual stress by heat treatment. Initial residual stresses induced during the manufacturing process is combined with welding residual stress or heat treatment residual stress, and remained as a final residual stress. Because such final residual stress affects the safety and strength of the structure, it is of utmost importance to measure or predict the magnitude of residual stress, and to apply this point on the design of the structure. In this study, the initial residual stress of steel structures having thicknesses of 25 mm and 70 mm during manufacturing was measured in order to investigate initial residual stress (hereinafter, referred to as initial stress). In addition, thermal elastic plastic FEM analysis was performed with this initial condition, and the effect of initial stress on the welding residual stress was investigated. Further, the reliability of the FE analysis result, considering the initial stress and welding residual stress for the steel structures having two thicknesses, was validated by comparing it with the measured results. In the vicinity of the weld joint, the initial stress is released and finally controlled by the weld residual stress. On the other hand, the farther away from the weld joint, the greater the influence of the initial stress. The range in which the initial stress affects the weld residual stress was not changed by the initial stress. However, in the region where the initial stress occurs in the compressive stress, the magnitude of the weld residual compressive stress varies with the compression or tension of the initial stress. The effect of initial stress on the maximum compression residual stress was far larger when initial stress was considered in case of a thickness of 25 mm with a value of 180 MPa, while in case of thickness at 70 mm, it was 200 MPa. The increase in compressive residual stress is almost the same as the initial stress. However, if initial stress was tensile, there was no significant change in the maximum compression residual stress. Keywords: Initial stress, Thermal elastic plastic FE analysis, Welding residual stress, Manufacturing steel process, Multi-pass butt weldin

Welding Residual Stress Distributions in the Thickness Direction under Constraints Using Neutron Diffraction and Contour Methods

Using high-strength steel for offshore structures with a yield stress of 500 MPa, this study evaluated the distribution characteristics of welding residual stress in the thickness direction under the influence of constrained conditions during welding using cutting, neutron diffraction, and contour methods. Welding residual stress inevitably occurs during welding and impacts fracture stability in structures. As high-strength steel better reflects the effects of phase transformation, the behavior of welding residual stress is known to differ from that of general steel. This study fabricated fully constrained and unconstrained specimens and evaluated them under identical conditions to evaluate welding residual stress according to the influence of constraints on high-strength steel welds. The results indicated that the maximum tensile residual stress of the fully constrained specimen occurred in the first layer of the weld joint, while the maximum tensile residual stress of the fully unconstrained specimen occurred in the last layer of the weld joint. Additionally, the welding residual stress of the fully constrained specimen was larger. Although some errors occurred in the residual stress values in the thickness direction depending on the measurement method, both methods applied in this study exhibited nearly identical distributions. Meanwhile, a maximum angular deformation of about 6° occurred in the fully unconstrained specimen, and we considered that the residual stress decreased owing to the occurrence of angular deformation. The maximum welding residual stress is generally the degree of yield stress. When residual stress greater than the yield stress occurs, it changes to the plastic range and appears in the form of angular, longitudinal, and lateral deformation. Under the fully unconstrained condition, reduced residual stress is considered to appear in the form of angular deformation. The welding tensile residual stress decreased to around 42% in the unconstrained specimen compared to the constrained specimen, and at that time, angular deformation of approximately 6° occurred. Therefore, it is estimated that an angular distortion of about 2.4° occurs as the stress of 100 MPa decreases

X-ray and neutron diffraction measurements of dislocation density and subgrain size in a friction stir welded aluminum alloy

The dislocation density and subgrain size were determined in the base material and friction-stir welds of 6061-T6 aluminum alloy. High-resolution X-ray diffraction measurement was performed in the base material. The result of the line profile analysis of the X-ray diffraction peak shows that the dislocation density is about 4.5 x 10{sup 14} m{sup 02} and the subgrain size is about 200 nm. Meanwhile, neutron diffraction measurements have been performed to observe the diffraction peaks during friction-stir welding (FSW). The deep penetration capability of the neutron enables us to measure the peaks from the midplane of the Al plate underneath the tool shoulder of the friction-stir welds. The peak broadening analysis result using the Williamson-Hall method shows the dislocation density of about 3.2 x 10{sup 15} m{sup -2} and subgrain size of about 160 nm. The significant increase of the dislocation density is likely due to the severe plastic deformation during FSW. This study provides an insight into understanding the transient behavior of the microstructure under severe thermomechanical deformation