Effect of creep strain on mechanical behaviour of ultra-high strength (Grade 1200) steel subject to cooling phase of a fire

This paper evaluates the mechanical behaviour of Grade 1200 ultra-high strength steel (UHSS) tube under a multi-phase loading scenario including fire and creep. To achieve this, two sets of experiments are performed on standard dog-bone specimens taken from UHSS tubes. In the first set of experiments, termed Creep-Heat-up tests, the specimens are axially loaded to a specific stress level, f_s. Whilst the axial load is maintained, the samples are heated to elevated temperatures of up to 700 °C and a tensile test is performed on them at elevated temperature. The second set of experiments, which are the main focus of this study, are termed Creep-Cooling tests and are similar to the first set except that the specimens are tested to failure after being cooled to room temperature. The axial load in this set of tests is maintained during both heat-up and cooling phases of a fire. The stress-strain curves, the creep strain the specimens experience due to the sustained axial load during fire, and the residual strength of the test specimens are discussed. In order to investigate the effect of steel grade, Grade 800 high strength steel (HSS) and Grade 350 mild steel (MS) specimens are also tested and the results are compared. Finally, the microstructural origin of accelerated softening in UHSS due to creep strain is discussed.