Controlling the grain size of steels is an effective way for tailoring their mechanical properties, such as yield strength, impact toughness, and ductility. In this study, a new industrial ...thermomechanical treatment was applied to a low-alloyed TRIP-assisted bainitic steel 13MnSiCr7 to achieve a substantial microstructural refinement. In this way the average grain size of the new micro-bainitic steel was decreased from ∼25 μm to ∼5 μm. Fatigue tests were carried out in order to investigate the influence of this new thermomechanical treatment on crack propagation behavior. Besides electron backscatter diffraction (EBSD), vibrating sample magnetometry (VSM), and high-energy synchrotron X-ray diffraction (HEXRD) were used to study the microstructure in the vicinity of the fatigue crack tip. The applicability of each method for detecting the martensitic transformation is discussed. In addition, the contribution of the martensitic transformation to fracture toughness was assessed on the basis of the results obtained by HEXRD.
•We have produced an ultrafine-grained micro-bainitic TRIP-assisted steel.•TRIP effect occurs despite of significant austenite grain refinement of the new steel.•The higher residual austenite content improves the resistance to crack growth.
The high luminosity LHC (HL-LHC) project is aimed at studying and implementing the necessary changes in the LHC to increase its luminosity by a factor of five. Among the magnets that will be upgraded ...are the 16 superconducting low-β quadrupoles placed around the two high luminosity interaction regions (ATLAS and CMS experiments). In the current baseline scenario, these quadrupole magnets will have to generate a gradient of 140 T/m in a coil aperture of 150 mm. The resulting conductor peak field of more than 12 T will require the use of Nb 3 Sn superconducting coils. We present in this paper the HL-LHC low-β quadrupole design, based on the experience gathered by the US LARP program, and, in particular, we describe the support structure components to pre-load the coils, withstand the electro-magnetic forces, provide alignment and LHe containment, and integrate the cold mass in the LHC IRs.
Powder-in-tube (PIT) Nb3Sn wires are competing with Restacked-Rod-Process (RRP ) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors ...have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP and a lower non-Cu Jc (12 T, 4.2 K) (2500-2700 A mm−2 versus 2900-3000 A mm−2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ∼25% is made up of ∼1-2 m sized grains (typically ∼10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-Tc grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with Tc <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer Jc and high specific grain boundary pinning force, QGB. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance.
Nowadays there is a great deal of interest in the scientific community in developing next-generation accelerator magnets based on high-Jc Nb3Sn Rutherford cables. Inside a cable the wires are ...subjected to the combined effect of axial and transverse load. Since Nb3Sn is a strain sensitive material, electromechanical characterization of cables is essential for magnet design. Testing a full-size Rutherford cable is an extremely complex and involved task. For this reason special Walters springs have been developed at the University of Geneva to test single wires under longitudinal and transverse load. In this work we analyze three PIT wires under transverse compressive load. To better understand the experimental results, a finite element model was developed. This model enabled better understanding of the mechanical behavior of the three samples and investigation of the mechanisms that determine wire performance degradation upon loading.
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