Influence of artificial cooling methods on post-fire mechanical properties of Q355 structural steel

The Chinese National Standard (GB/T 1591-2018) requires a replacement of Q345 steel to Q355 steel from February 1, 2019. As a new structural steel, the post-fire mechanical properties, regarded as a crucial indicator to assess structural damage and reusability after fire, need to be studied. Hence, a comprehensive experimental investigation on the influence of two artificial cooling methods on post-fire mechanical properties of Q355 steel is reported in this study. The specimens were first heated to target temperatures of 200 degrees C, 400 degrees C, 500 degrees C, 600 degrees C, 700 degrees C, 800 degrees C and 900 degrees C to simulate various fire conditions and then cooled by fire-extinguishing foam cooling and water cooling. Uniaxial tensile tests were performed on these fire-affected specimens to extract the stress-strain curves and associated post-fire mechanical properties. Additionally, fracture behavior of Q355 steel cooled by the two artificial cooling methods was also discussed from macroscopic and microscopic fracture morphology. For the two artificial cooling methods, it is explicitly found that the loss of mechanical properties of Q355 steel is negligible after exposure to temperatures below 600 degrees C. However, both cooling methods and elevated temperatures have significant effects on post-fire mechanical properties of Q355 steel after the exposure temperature exceeds 600 degrees C. In case of fire-extinguishing foam cooling, a dramatic decrease in yield strength and ultimate strength is observed, while the ductility is gradually increased with increasing temperature. In contrast, a sharp increase in yield strength and ultimate strength can be observed clearly, while the ductility is sharply reduced with increasing temperature. After exposure to the temperature of 900 degrees C, Q355 steel cooled by fire-extinguishing foam is able to retain 79% of its original ultimate strength, while Q355 steel cooled by water is able to gain 131% of its original ultimate strength in the as-received state. At last, new equations are developed to predict post-fire mechanical properties of Q355 steel for fire damage evaluation. (C) 2020 Elsevier Ltd. All rights reserved.