Restoration Of Ductility Research Articles

Multiscale evaluation of asphalt binder rejuvenation dosing and efficacy

The final composition of asphalt binder in an asphalt mix can vary depending if it is a neat, virgin binder, or a blend of neat binder with reclaimed asphalt pavement (RAP), modifiers, and/or rejuvenators. As recycling of asphalt concrete becomes more commonplace worldwide, so too must be the use of chemical rejuvenators which can restore the original properties of the associated asphalt binder, contributing to the performance and therefore sustainability aspect of this material. In this context, this work aimed to evaluate the influence of rejuvenation in different proportions of aged and unaged binder, as well as the best way to dose rejuvenators, taking into consideration aspects that go beyond the Superpave Performance Grading characterization of binders. In the first phase of this study, when rejuvenator contents based on low PG recovery were proportionally added to different aged/unaged binder blends, it was observed that restoration of ductility and strength of the rejuvenated aged binder was limited. However, when an additional cycle of aging was applied to simulate the new service life of recycled material, the rejuvenated recycled material generally performed better than unrejuvenated recycled material. This long term effect can better differentiate the performance of different rejuvenators. Afterwards, the dosing procedure was investigated using an extensive range of dosages. An optimal dosage determination approach was developed and verified by means of the rejuvenation of aged asphalt mixtures from the rheological, mechanical, and chemical perspectives. Finally, the differences between laboratory-aged binder and laboratory-aged mixtures when rejuvenators are used was investigated, and it was determined that the mechanisms of rejuvenation at the two scales are not completely alike. Key findings also include the importance of re-aging asphalt binder blends before testing when a rejuvenator is used and the dominance of unaged binder within binder blends with limited replacement with recycled asphalt. Overall, this work opens the door to future studies of rejuvenation of asphalt using different approaches than those which have been conventionally used, including consideration of amount of replaced binder, using nonlinear viscoelastic characterization, fundamental chemical studies, and the method of addition based on mixture versus binder scale studies.

Microstructure and texture evolutions in FeCrAl cladding tube during pilger processing

The microstructure of FeCrAl cladding tubes depends on the fabricating process history. In this study, the microstructural characteristics of wrought FeCrAl alloys during industrial pilger processing into thin-walled tubes were investigated. The hot extruded tube showed ∼100 μm equiaxed grains with weak α∗-fiber in {h11}<1/h12> texture, while pilger rolling process change the microstructure to fragmented and elongated grains along the rolling direction. The pilgered textures could be predicted with the VPSC model. The inter-pass annealing at 800–850 °C for 1 h results in recovery and recrystallization of the ferric matrix and restoration of ductility. The final finished tube shows fine recrystallized grains (∼11 μm) with dominant γ-fiber in three dimensions. Pilger rolling enhanced α-fiber while annealing reduced α-fiber and enhanced γ-fiber. Microstructural evolution in the Laves precipitates followed the sequence of faceted needle-like → spherical → faceted ellipsoidal. Thermomechanical processing resulted in cladding tubes with an area fraction of ∼5% and a number density of 5 × 10−11 m−2 in Laves precipitates, which is half that of the first-pilgered tube. Laves precipitates pin the grain boundaries to control the microstructure and prevent grain coarsening.

Restoration of ductility in hydrogen embrittled dual-phase (DP 780) steel by the electric pulse treatment

High-strength steel components operating in a hydrogen environment are more susceptible to hydrogen embrittlement. De-hydrogenation of these components could prolong their service life, encouraging the industrial use of high-strength steel. In this study, hydrogen (H)-induced damage recovery and restoration of ductility are realized in dual-phase (DP 780) steel by the electric pulse treatment (EPT). The experimental results indicate that a complete restoration of ductility and H-induced damage recovery could be achieved with suitable electro-pulsing parameters in DP 780 steel. A centerline crack is observed in the fractographic examination of each hydrogenated specimen after the uniaxial tensile test. After EPT, post-necking ductility improves, and the centerline crack disappears from the fractured surface. The in-situ digital image correlation (DIC) technique is utilized during the uniaxial tensile test to measure the local strain evolution. Both uniaxial tensile necking and fracture strains are restored after EPT of hydrogenated specimens.

Hot ductility behavior of a low carbon advanced high strength steel (AHSS) microalloyed with boron

The current study analyses the influence of boron addition on the hot ductility of a low carbon advanced high strength NiCrVCu steel. For this purpose hot tensile tests were carried out at different temperatures (650, 750, 800, 900 and 1000 °C) at a constant true strain rate of 0.001 s −1. Experimental results showed a substantial improvement in hot ductility for the low carbon advanced high strength steel when microalloyed with boron compared with that without boron addition. Nevertheless, both steels showed poor ductility when tested at the lowest temperatures (650, 750 and 800 °C), and such behavior is associated to the precipitation of vanadium carbides/nitrides and inclusions, particularly MnS and CuS particles. The fracture mode of the low carbon advanced high strength steel microalloyed with boron seems to be more ductile than the steel without boron addition. Furthermore, the fracture surfaces of specimens tested at temperatures showing the highest ductility (900 and 1000 °C) indicate that the fracture mode is a result of ductile failure, while in the region of poor ductility the fracture mode is of the ductile–brittle type failure. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism which in turn causes a hot ductility loss. Likewise, dynamic recrystallization (DRX) which always results in restoration of ductility only occurs in the range from 900 to 1000 °C. Results are discussed in terms of boron segregation towards austenitic grain boundaries and second phase particles precipitation during plastic deformation and cooling.