Post‐fire Mechanical Behaviors and the Mechanism of their Influence on a 690 MPa Grade High‐Strength Steel for High‐Rise Buildings

Abstract

Herein, the residual mechanical behaviors of a 690 MPa grade high‐strength steel after exposure to fire for an extended duration are investigated, and the factors affecting their variation based on the theories of metallurgy are explored. The corresponding tensile properties and microstructure tests are conducted on this steel soaked at 300–900 °C for 3 h and then cooled in air. The results show that there is negligible influence of temperature on residual mechanical properties at 600 °C or lower. It can be considered that the critical fire temperature is 600 °C, beyond which the strength decreases sharply. This can be attributed to the formation of ferrite and grain coarsening. However, at 900 °C, its residual strength can still maintain approximately half of that at room temperature. It is related to the contribution from fine nanoscaled precipitates of 10–40 nm in size. Compared with steels having the same nominal yield strength, the dependence of residual mechanical properties on elevated temperatures for this steel varies due to the difference in microstructure characteristics. New predicted equations are proposed for this novel structural steel to guide on better prediction of residual mechanical properties in steel structure systems.

Effect of Microstructure on Mechanical Properties of X65MS Welded Pipe Joint

“…The mean size of nanoscale precipitates at BM and WM is ≈6 nm and 11 nm, and the fine precipitates are uniformly distributed within the grains and play a precipitation strengthening role, which is an important reason for significantly improving the strength of steel. [ 39 ]…”

“…The mean size of nanoscale precipitates at BM and WM is %6 nm and 11 nm, and the fine precipitates are uniformly distributed within the grains and play a precipitation strengthening role, which is an important reason for significantly improving the strength of steel. [39] Due to the impact of the welding thermal cycle, dissimilar microstructures are formed near the welded joint, leading to significant variations in hardness in each area. The microhardness of the full thickness welded joint is uniform, and the microhardness of the joint is higher than BM, conforming to the strength standards for pipeline steel welded joint.…”