Characterization of the temperature-activated secondary hardening effect of fire-resistant steels during fire simulations

Abstract

Enhancing the high temperature mechanical performance of structural steels during the event of a fire is a complex task due to the strong economic restrictions in the civil construction field. The development of ''Fire-Resistant'' steels imposes practical limitations, since these have to be at least as cheap, tough and weldable as the commonly used structural steels, while simultaneously retaining two-thirds of the room temperature yield strength at 600 °C. The addition of microalloying elements, such as niobium and boron, under small additions of carbon and molybdenum, has proven to be a successful methodology to produce Fire-Resistant steels, though not as cheap. Additionally, the development of non-conventional thermomechanical processing routes is a promising approach to further increase the high temperature performance. A secondary hardening mechanism can be used as an ''in situ'' temperature-activated protection during a fire, due to the precipitation of nanometric niobium and boron carbides and pure molybdenum clusters. Such nanometric precipitation can be enhanced by increasing the dislocation density of the steel, since dislocations act as sites for preferential nucleation. Therefore, controlled amounts of hard microconstituents, such as bainite, transforming along short isothermal times, and aided by plastic strain, can potentially result in the increase of the secondary hardening effect. Likewise, by using short isothermal bainitic transformation times, a higher availability of niobium, boron, carbon and molybdenum in solid solution can be induced, thus increasing the secondary hardening potential. This proposal aims to conduct carefully designed thermomechanical processing cycles in order to produce microstructures which can later benefit from a temperature-activated secondary hardening protection mechanism during the event of a fire. Advanced characterization techniques such as Atom Probe Tomography and Transmission Electron Microscopy will be used to quantify the availability of the microalloying elements in solid solution, as well as the size and distribution of the nanosized secondary precipitates before and after fire-simulation tests.