[ Innovation ]
[ Innovation ]
Figure 4 . The heart of the CMS experiment . Image courtesy of CERN .
As part of its HL-LHC upgrade programme , in 2026 major elements of the CMS detectors will be changed in order to operate at high luminosity . The high-granularity calorimeter ( HGCal ) is one of the major upgrades of CMS . It will feature an extremely high readout and granularity and will allow to cope with the harsh radiation environment during HL-LHC operation . The HGCal includes a so called “ hadronic compartment ” made out of austenitic stainless steel absorbers . HGCal will operate in the strong CMS magnetic field of 4 T . Therefore , all used materials shall be non-magnetic or at least , feature a very feeble , controlled and predictable magnetic permeability . The total weight of the stainless steel plates needed for HGCal absorber layers is around 564 tonnes . The relative magnetic permeability in the bulk plate material has been specified not to exceed 1.05 , in order to limit the axial forces attracting the absorber plates to the centre of the CMS superconducting solenoid . The focal ways to avoid this value to be
exceeded is a stringent control of delta-ferrite content and guaranteeing sufficient stability against martensitic transformations . This is paramount , taking into account that due to the quantity involved , cost effectiveness and easy machinability a 304L grade was selected . “ In this constraining framework , two precautions were taken : selecting a steelmaking process based on ingot casting , to limit as much as possible residual delta-ferrite issued from primary solidification and difficult to revert in the subsequent hot rolling process , and specifying a high alloy 304L ( 1.4306 ) to reduce the risk of spontaneous and strain induced martensitic transformations ”, says Katie Elizabeth Buchanan , staff materials engineer at the CERN Materials section . The material was produced by ArcelorMittal Industeel with an optimised well-balanced composition and is presently being machined in Pakistan . Due to the stringent requirements of magnetic permeability , the magnetic properties of each plate had to be extensively checked , not only at the surface but also at mid-thickness in several positions around the edges ( Fig . 5 ).
Advanced inspection “ The relationship between the averaged magnetic properties measured on the faces and on
Inspection at the supplier ’ s premises of the stainless steel absorber plates witnessed by CERN .
26 Stainless Steel World March 2024 www . stainless-steel-world . net