Through-thickness Microstructure Research Articles

Effect of through-thickness microstructure inhomogeneity on mechanical properties and strain hardening behavior in heavy-wall X70 pipeline steels

The ever-increasing demand for oil & gas exploration from land to subsea necessitates using higher-strength and high-ductility heavy-wall thickness pipes, which may increase the microstructural inhomogeneity along the steel plate sections, ultimately impacting the mechanical properties. In this work, the differences in microstructure and mechanical properties between the through-thickness layers of a heavy-wall ferrite/bainite (F/B) dual-phase (DP) X70 pipeline steel plate were studied. The results showed that the size of the ferrite grain and the volume fraction of bainite increased from the surface layer to the mid-thickness layer of the DP X70 pipeline steel plate. The surface and mid-thickness layers had a lower uniform elongation and higher strength and hardness. In comparison, the quarter layer had higher uniform elongation and lower strength and hardness. The refined grain size and the strong {001}<110> texture were responsible for enhanced in strength and hardness. The higher γ-fiber texture at the quarter layer favored the higher plasticity. The analysis results of strain-hardening behaviors based on the modified C-J model showed that the mid-thickness layer had a higher strain-hardening capacity at the beginning of the plastic strain, leading to deformation instability and reduced material plasticity. The surface layer maintained a high hardening exponent even at high plastic strains, which facilitated the deformation properties of the material. The results provide a basis for evaluating the influence of microstructural factors on the strength and plasticity of heavy-wall X70 pipeline steel and the coordination mechanism of micro deformation in DP steels.

Through-thickness microstructure and wear resistance of plasma transferred arc Stellite 6 cladding: Effect of substrate

Through-thickness microstructure and wear resistance of plasma transferred arc Stellite 6 cladding: Effect of substrate

Multiscale modeling of laminated thin-shell structures with Direct FE[formula omitted

Shell finite element is computationally efficient and thereby favored in large-scale structural analysis. With the application of advanced materials, understanding the relation between micro-heterogeneities and structural behavior is important. To this end, we propose a multiscale method in the current contribution for modeling thin-walled fiber reinforced composite laminates with Mindlin–Reissner shell theory and Direct FE2. By exploiting the length scales in layered structure together with a shell formulation degenerated from continuum solid element, both in-plane and through-thickness periodicity can be assumed. A single-fiber unit cell attached to each Gauss point is employed as a representative volume element (RVE) rather than a through-thickness microstructure. In this way, the scale transitions are naturally dealt with using a first-order homogenization framework, in which the downscaling of transverse shear strain, commonly troublesome for the through-thickness RVE, is straightforward. The other merit inherited from Direct FE2 is that the two-scale problem is formulated in a single finite element analysis wherein the macro and micro DOFs are coupled based on the downscaling kinematics. It avoids the upscaling of the macroscopic stresses and tangents in the conventional nested FE2 scheme, which is challenging in the case of complex microstructural response. The developed method is then verified through several benchmarking examples, including a flat plate and two curve shells. Isotropic materials and composite laminates with homogeneous and heterogeneous RVEs have been examined. Two examples with plasticity and cohesive cracks are further presented to showcase its potential in modeling damage propagation in composite structures.

Cleavage fracture micromechanisms in thick-section quenched and tempered S690 high-strength steels

For structural assessment and optimal design of thick-section high-strength steels in applications under harsh service conditions, it is essential to understand the cleavage fracture micromechanisms. In this study, we assess the effects of through-thickness microstructure of an 80-mm-thick quenched and tempered S690 high-strength steel, notch orientation, and crack tip constraint in cleavage nucleation and propagation via sub-sized crack tip opening displacement (CTOD) testing at −100 °C. The notch was placed parallel and perpendicular to the rolling direction, and the crack tip constraint was analysed by varying the a/W ratio: 0.5, 0.25, and 0.1. The notch orientation does not play a role, and the material is considered isotropic in-plane. Nb-rich inclusions were observed to act as the weak microstructural link in the steel, triggering fracture in specimens with the lowest CTOD values. While shallow-cracked specimens from the top section present larger critical CTOD values than deep-cracked ones due to stress relief ahead of the crack tip, the constraint does not have a significant influence in the middle due to the very detrimental microstructure in the presence of Nb-rich inclusions. Some specimens show areas of intergranular fracture due to the combined effect of C, Cr, Mn, Ni, and P segregation along with precipitation of Nb-rich inclusions clusters on the grain boundaries. Several crack deflections at high-angle grain boundaries were observed where the neighbouring sub-structure has different Bain axes.

Investigation of Through-Thickness Residual Stress, Microstructure and Texture in Radial Forged High-Strength Alloy Steel Tubes

Gradient variations of through-thickness residual stress, microstructure and texture greatly affect the performance of cold radial forged tubes. In this work, the through-thickness distribution of residual stress was measured based on the Debye ring. The microstructure was characterized with the electron backscattering diffraction technique. The texture was measured by the X-ray diffractometer. The influence of microstructure and texture on the strength and anisotropy of forged tubes with different thickness reductions was analyzed. The results show that the residual stress varies gradually from compressive to tensile from the outer to inner surface. The microhardness of the outer surface is lower than the inner. The dislocation density and low-angle grain boundary fraction are the smallest in the one-third thickness. The dislocation density and low-angle grain boundary fraction increase gradually from the one-third thickness to the inner surface. The main texture components of the forged tube include {111}&lt;110&gt;, {001}&lt;110&gt; and {114}&lt;110&gt;. Texture {111}&lt;110&gt; deflects gradually toward {114}&lt;110&gt;, {112}&lt;110&gt; and {110}&lt;110&gt; from the external tube to the internal tube. The gradient variation of strength mainly resulted from the difference of the dislocation density. The difference of strength along the radial direction is reduced with a larger thickness reduction. This work has important significance for improving the performance of high-strength alloy steel tubes processed by cold radial forging.

A comprehensive quantitative characterisation of the multiphase microstructure of a thick-section high strength steel

The through-thickness heterogeneous microstructure of thick-section high strength steels is responsible for the significant scatter of properties along the thickness. In this study, in order to identify the critical microstructural features in the fracture behaviour and allow for design optimisation and prediction of structural failure, the through-thickness microstructure of thick-section steels was extensively characterised and quantified. For this purpose, samples were extracted from the top quarter and middle thickness positions, and a combination of techniques including chemical composition analysis, dilatometry, and microscopy was used. The hardness variation through the thickness was analysed via micro-Vickers measurements and the local hardness variation in the middle section was studied via nanoindentation. The middle section presented larger prior austenite grain (PAG) sizes and larger sizes and area fraction of inclusions than the top section. Additionally, cubic inclusions were observed distributed as clusters in the middle, sometimes decorating PAG boundaries. Defects associated with the cubic inclusions or the interface between the matrix and the circular and cubic inclusions were observed in the mid-thickness. Moreover, the middle section presented long interfaces with the most significant hardness gradients due to the presence of hard centreline segregation bands. Hence, the microstructural and nanoindentation analyses indicated the middle section as the most likely area to have the lowest fracture toughness and, therefore, the most unfavourable section for fracture performance of the investigated S690QL high strength steel. The detrimental effect of the middle section was confirmed via CTOD tests where the middle presents lower fracture toughness than the top section.

Through-Thickness Microstructure Characterization in a Centrifugally Cast Austenitic Stainless Steel Nuclear Reactor Primary Loop Pipe Using Time-of-Flight Neutron Diffraction

The US code of Federal Regulations mandates regular inspection of centrifugally cast austenitic stainless steel pipe, commonly used in primary cooling loops in light-water nuclear power plants. These pipes typically have a wall thickness of ~8 cm. Unfortunately, inspection using conventional ultrasonic techniques is not reliable as the microstructure strongly attenuates ultrasonic waves. Work is ongoing to simulate the behavior of acoustic waves in this microstructure and ultimately develop an acoustic inspection method for reactor inspections. In order to account for elastic anisotropy in the material, the texture in the steel was measured as a function of radial distance though the pipe wall. Experiments were conducted on two 10 × 12.7 × 80 mm radial sections of a cast pipe using neutron diffraction scans of 2 mm slices using the HIPPO time-of-flight neutron diffractometer at the Los Alamos Neutron Science Center (LANSCE, Los Alamos, NM, USA). Strong textures dominated by a small number of austenite grains with their (100) direction aligned in the radial direction of the pipe were observed. ODF analysis indicated that up to 70% of the probed volume was occupied by just three single-grain orientations, consistent with grain sizes of almost 1 cm. Texture and phase fraction of both ferrite and austenite phases were measured along the length of the samples. These results will inform the development of a more robust diagnostic tool for regular inspection of this material.

Room-temperature low-cycle fatigue and fracture behaviour of asymmetrically rolled high-strength 7050 aluminium alloy plates

The asymmetrical rolling (ASR) process with shear deformation is considered as a promising technology to adjust/improve through-thickness microstructure homogeneity and integrated properties of high-strength aluminium alloy plates. But the advantages may come with caveats that are the subject of our research. In this paper, the room-temperature low cycle fatigue properties and fracture behaviour of the ASR-ed AA7050 aluminium alloy plates are compared with the symmetrical rolling (SR) one. It is shown that after either type of rolling the plates exhibit similar low-cycle fatigue lives but the SR-ed one displays a better cyclic deformation ability and slightly higher fatigue lives at high strain amplitudes. It is demonstrated that the severe surface-localized deformation contributes to the formation of slip relief on the surface and subsequently initiates micro-cracks that are propagated via transgranular and/or intergranular fracture modes along with obvious fatigue striations. Recrystallised grains with coarse grain boundary precipitates and wide precipitate-free zones near the upper/bottom layers as well as numerous and larger secondary particles in the ASR-ed plates may cause early crack initiation for a short crack initiation life. However, the SR-ed plate with more frequent subgrains near surface layers, numerous fine subgrains and less indissoluble particles could possess better crack initiation/propagation resistance and cyclic loading behaviour. Fine subgrains with higher microhardness/strength can facilitate passing of dislocations or slip bands into adjacent grains so as to delay crack propagation such as via energy-intensified transgranular fracture for extending fatigue life. Properly balancing the through-thickness strain/deformation distribution and the formation of recrystallization/indissoluble particles via implementing a feasible ASR process becomes a critical issue to achieve fracture-resistant microstructures for high-strength aluminium alloy plates. The underlying causes/mechanisms regarding the differences of microstructures and mechanical behaviour are revealed and discussed based on modelling through-thickness temperature/strain distribution and detailed microstructure characterization.

Through-Thickness Microstructure and Strain Distribution in Steel Sheets Rolled in a Large-Diameter Rolling Process

The rolling condition for fabricating a low-carbon niobium-microalloyed steel sheet with an ultrafine-grained (UFG) structure was examined through rolling experiments and finite element analysis. A large-diameter rolling process was proposed to create a UFG structure. The rolling was conducted near the transformation point, Ar3, from austenite to ferrite. The Ar3 was measured at the surface and the center of the sheet. First, the through-thickness microstructure and equivalent strain distribution in a 1-pass rolled sheet 2.0 mm thick were examined. In the rolling experiments, the embedded pin method was employed to understand through-thickness deformation. The magnitude of the equivalent strain to obtain a UFG structure was estimated to be 2.0. Based on these results, the fabrication of a 2 mm UFG steel sheet by 3-pass rolling for an initial thickness of 14.5 mm was attempted by the proposed large-diameter rolling process.

Pass number dependence of through-thickness microstructure homogeneity in tantalum sheets under the change of strain path

Microstructure and crystallographic texture are the key factors that determine the properties of sputtering target used in integrated circuit fabrication. Multi-pass clock-rolling has been used recently to produce Ta sputtering targets. However, the effect of strain per pass, or the number of passes to achieve desired thickness, in clock-rolling on the homogeneity of final microstructure has not been investigated yet. In this study, deformation and recrystallization behaviors of Ta sheets during 8-pass and 16-pass clock-rolling as well as subsequent annealing are investigated. X-ray diffraction reveal that 16-pass sample has relatively homogeneous {111} ⟨uvw⟩ {〈111〉//normal direction (ND)} and {100} ⟨uvw⟩ {〈100〉//ND} fibers along its thickness, whereas the distribution of texture in 8-pass sample is uneven. X-ray line profile analysis show that the distribution of stored energy is also more homogeneous in the 16-pass sample. Electron backscatter diffraction and transmission electron microscopy reveal numerous microshear bands and well-arranged microbands predominantly occurring in {111} deformed matrix. These prevail in the center of the 8-pass sample, whereas grain subdivision within {100} and {111} matrix in the 16-pass sample is more homogeneous through thickness. Average grain orientation spread (GOSaverage) within the {111} matrix in the center of the 8-pass sample is significantly higher than that at the surface and quarter-thickness, indicating enhanced grain subdivision. Schmid factor and Taylor model analysis demonstrate that a greater probability of activation of the primary slip system only in the 8-pass sample compared to larger number of active slip systems in the 16-pass sample. Upon annealing, uniform nucleation combined with a slower grain growth rate results in a finer and more uniform grain size in the 16-pass sample. By contrast, severe gradient in through-thickness recrystallization microstructure forms in the 8-pass sample. This is attributed to higher stored energy and preferential nucleation sites in the center leading to faster recrystallization compared to the surface. Overall, the increase of pass number improves the homogeneity of through-thickness recrystallization microstructure in Ta sheets under the change of strain path.

Effect of strain path change on the through-thickness microstructure during tantalum rolling

High purity tantalum was respectively processed by unidirectional rolling (UR) and clock rolling (CR), and the through-thickness microstructures were investigated by multiple characterization techniques including electron backscatter diffraction (EBSD), Vickers hardness (HV) and X-ray diffraction (XRD). Results show that the through-thickness stored energy distribution in CR specimens is more homogeneous than in UR specimens due to uniform texture distribution. {111} grains possess larger Schmid factors and the corresponding Schmid factor difference ratio than {100} grains, indicating the activation of uniserial slipping in {111} grains, which leads to inhomogeneous deformation and higher stored energy. Besides, X-ray line profile analysis (XLPA) suggests that the stored energy of {111} grains increases successively from the surface to center layer, regardless of strain paths, due to the influence of redundant friction on the surface layers. While the occurrence of multiple slipping in {100} grains leads to homogeneous deformation and lower stored energy.

Through-thickness microstructure and mechanical properties in stationary shoulder friction stir processed AA7075

ABSTRACTStationary shoulder friction stir processing of high strength thick AA7075 aluminium alloy demonstrated tiny gradient in the microstructure refinement across the thickness, which was attributed to the less heat input and small temperature gradient by the stationary shoulder. Stationary shoulder produced a very smooth surface finish with a little amount of flash throughout the processing area. Probe-dominated stir zone (SZ) resulted in uniform grain refinement as well as hardness distribution across the SZ thickness. The ductility in SZ increased by 48% in comparison to the unprocessed material without costing tensile strength. This tensile behaviour trend maintained throughout the thickness in SZ.

Effects of asymmetrical rolling on through-thickness microstructure and texture of body-centered cubic (BCC) tantalum

In conventional symmetric rolling (SR), the microstructure and texture inhomogeneities along the thickness are intractable problems for body-centered-cubic (BCC) metals with high stacking fault energy. In this study, high-purity tantalum was selected as a representative case, and a new asymmetrical rolling (ASR) technique was developed to alleviate this inhomogeneity. Specimens were rolled to 80% thickness reduction using 90-degree cross SR and 90-degree cross ASR; two different rolling speed ratios, namely, 1.1 and 1.2, were adopted in ASR. The through-thickness texture and microstructure were characterized via X-ray diffraction and electron backscatter diffraction. Results showed that the ASR deformed specimens presented more homogeneous through-thickness microstructure and texture than SR-deformed specimens possibly due to the highly uniform accumulative shearing strain distribution introduced by ASR. Therefore, textural and microstructural uniformity increased with increasing rolling speed ratio in ASR.

Through-thickness microstructure and mechanical properties of electron beam welded 20 mm thick AISI 316L austenitic stainless steel

Through thickness microstructure and mechanical properties of defect-free electron beam welded 20mm thick AISI 316L austenitic stainless steel are investigated as a function of beam power. The weld microstructure characterised by columnar and equiaxed dendritic ferrite in an austenite matrix. The dendritic structure was finer at the bottom of the weld zone. A microstructural boundary called “Parting” was seen along the weld centreline. Tensile tests, using digital image correlation technique, demonstrated that the highest strain concentrated in the FZ. The bottom section of the weld metal showed yield strength of about 14–52MPa higher than the top section. The ultimate tensile strength in the bottom of the weld was also about 4% higher than the top. The final fracture was detected in the parting region. It was observed from the EBSD scan that the grains in the weld zone contained a weak orientation and showed high Schmid factor intensity with interception between some strong grains and soft grains at the weld centerline boundary. This explains the high weld ductility and the failure to happen in the parting region.

Microstructure and Mechanical Properties of Fe-18Mn-18Cr-0.5N Austenitic Nonmagnetic Stainless Steel in Asymmetric Hot Rolling

Asymmetric hot rolling (ASHR) with a mismatch speed ratio of 1.15 in a single pass was applied to Fe-18Mn-18Cr-0.5N steel and was compared with symmetric hot rolling (SHR). The results indicated that a through-thickness microstructure gradient was formed in the plate due to the shear strain (0.36) introduced by ASHR. A fine-grained layer with the average size of ~3 μm was achieved at the top surface of ASHR plate, while numerous elongated grains with a few recrystallized grains were presented at the center layer. The texture was distributed randomly at the top surface of ASHR plate, and a weaker intensity of typical hot-rolled texture in austenitic steel was obtained at the center layer of ASHR plate compared to SHR plate. An excellent combination of microhardness, strength and ductility was obtained in the ASHR plate, which was attributed to gradient microstructure induced by ASHR.

Through-Thickness Deformation Gradient in a Part-Pilgered Zirconium Tube: Experimental Measurements and Numerical Validation

Through-thickness microstructure development and residual stress evolution were explored in a part-pilgered Zircaloy-4 tube. Clear gradients in microstructure and residual stress were experimentally established at different locations. Such locations were considered along the axial length and also across the wall thickness. These were naturally subjected to different von Mises effective strains and were reflected on strain/microstructural gradients. The deformation gradients were also simulated with a three-dimensional elastoplastic finite element model. The model used both isotropic and anisotropic yielding. Though deformation gradients were similar with both yielding criteria, the anisotropic yielding provided a better match with experimental residual stress gradients.

Effects of Cooling Paths on Through-Thickness Microstructure and Mechanical Properties of Heavy Gauge X80 Pipeline Steel

The effects of various cooling paths on uniformity of through-thickness microstructure and mechanical properties of X80 pipeline steel of 22.0 mm in thickness were studied. The finite difference method was employed to calculate the temperature field during cooling. It was confirmed by the experimental result and temperature field calculation that the optimizing process was achieved by the ultra-fast cooling with medium cooling capacity (cooling rate of ~23 K/s) followed by ultimate cooling capacity (cooling rate of ~50 K/s). After optimization, the experimental steel displayed much uniform microstructure and the deviation of through-thickness hardness was controlled within 20 HV. In addition, the yield strength, tensile strength and elongation of the experimental steel were 621, 728 MPa and 21.5%, respectively, meeting the requirements of the API standard for X80 pipeline steels.

Advances in compact manufacturing for shape and performance controllability of large-scale components-a review

Research on compact manufacturing technology for shape and performance controllability of metallic components can realize the simplification and high-reliability of manufacturing process on the premise of satisfying the requirement of macro/micro-structure. It is not only the key paths in improving performance, saving material and energy, and green manufacturing of components used in major equipments, but also the challenging subjects in frontiers of advanced plastic forming. To provide a novel horizon for the manufacturing in the critical components is significant. Focused on the high-performance large-scale components such as bearing rings, flanges, railway wheels, thick-walled pipes, etc, the conventional processes and their developing situations are summarized. The existing problems including multi-pass heating, wasting material and energy, high cost and high-emission are discussed, and the present study unable to meet the manufacturing in high-quality components is also pointed out. Thus, the new techniques related to casting-rolling compound precise forming of rings, compact manufacturing for duplex-metal composite rings, compact manufacturing for railway wheels, and casting-extruding continuous forming of thick-walled pipes are introduced in detail, respectively. The corresponding research contents, such as casting ring blank, hot ring rolling, near solid-state pressure forming, hot extruding, are elaborated. Some findings in through-thickness microstructure evolution and mechanical properties are also presented. The components produced by the new techniques are mainly characterized by fine and homogeneous grains. Moreover, the possible directions for further development of those techniques are suggested. Finally, the key scientific problems are first proposed. All of these results and conclusions have reference value and guiding significance for the integrated control of shape and performance in advanced compact manufacturing.

Influence of asymmetric hot rolling on through-thickness microstructure gradient of Fe–20Mn–4Al–0.3C non-magnetic steel

Through-thickness microstructure gradient as a function of asymmetric hot rolling (ASHR) process parameters was investigated for a high manganese non-magnetic steel. Shear strain distribution was calculated using a two-dimensional finite element method. According to the simulation results, the surface layer of ASHR plate undergoes the highest total shear deformation which leads to the formation of a fine-grained surface layer under conditions of large rolling reduction and relatively high temperature. In the central layer is still the partially recrystallized microstructure. With velocity ratio increasing, the depth of fine-grained surface layer approaches 1/4 plate thickness, while the recrystallization proportion for the central layer is also enhanced due to the reinforced extra shear deformation. By appropriate parameter optimization, the austenitic grain size is finally refined to ~5µm for the surface layer and ~9µm for the center, such that the tensile property is improved.

Through-thickness texture in clock-rolled tantalum plate

135° clock rolling, which denotes that the rolling direction clockwise rotates 135° after per pass, was utilized to develop a desired cold-rolled texture distribution through the thickness of Ta plate. Meanwhile, the influence of starting texture and microstructure was also investigated in detail. Texture from surface to center of the initial and rolled Ta plates was characterized by X-ray diffraction (XRD). In order to explore the correlation between initial microstructure and the evolution of rolling texture, through-thickness microstructure of the initial Ta plate was observed by optical microscope (OM). Results showed that 135° clock rolling was effective to mitigate the through-thickness texture gradients often existing in cold-rolled tantalum plate. It was found, furthermore, that texture and microstructure prior to rolling play a particular role in the formation of rolling texture.