Microstructures and mechanical properties after heating of reinforcing 500 MPa class weldable steels produced by various processes (Tempcore, microalloyed with vanadium and work-hardened)

Abstract

The load bearing capacity of reinforced concrete structures after fire can be assessed if the modified mechanical properties of the building materials, designated here as ‘residual’, are known. In this respect, the residual properties of reinforcing steels produced (a) by the Tempcore process; (b) microalloying with vanadium; and (c) work-hardening, all falling into grade FeB500S, were selected for investigation, aiming to represent a wide range of steels currently available for the construction industry in Europe. For both the microalloyed with vanadium and for the work-hardened reinforcing steels typical diameters were investigated. For the Tempcore-type reinforcing steels, due to their heterogeneous structure, the effect of size and composition have been also examined. Specimens of the above types of steel were heated at temperatures ranging from 200 °C to 800 °C for 1 h and then cooled in air to room temperature. Thereafter, their residual properties were assessed by hardness measurements, tensile and Charpy-V impact tests. Metallographic analysis was used to correlate mechanical properties to microstructural characteristics. From the above investigation it was concluded that the Tempcore steels presented the more stable behavior up to temperatures of 500 °C, while the microalloyed steel, although it presented very satisfactory tensile properties, displayed low impact toughness due to coarsening of vanadium carbides. The work-hardened steel showed a continuous drop of its properties from approximately 250 °C and suffered from brittleness in the vicinity of 200 °C due to strain aging phenomena.