Study on Mechanical Properties, Microstructure Evolution, and Fire Resistance Mechanism of 0.4Mo Refractory Steel at Different Temperatures

Herein, the mechanical properties of 0.4Mo refractory steel bars at various temperatures are studied. The yield strength of the steel bar at 600 °C and below for 15 minutes is more than 2/3 (286 MPa) of the yield strength at room temperature, and the yield strength at 600 °C is 296 MPa. The strength of the test steel decreased as the temperature increased. The microstructure of tempered and quenched samples was observed by optical microscope and scanning electron microscope. The initial microstructure of the experimental steel consisted of ferrite and granular bainite. As the temperature increased, the bainite gradually dissolved. At 700 °C, the ferrite grains coarsened, and the granular bainite became finer and more dispersed. The original microstructure contained a high density of dislocations, which became entangled under stress, preventing the failure of the steel bars. Molybdenum carbides hindered the migration and annihilation of dislocations at high temperatures and served as nucleation sites to promote grain refinement. High‐temperature confocal microscopy observed the changes in the microstructure and second‐phase particles from room temperature to 1300 °C, with a heating rate of 60 °C/s. The austenitizing temperature of the experimental steel was ≈950 °C, and the precipitated phase gradually grew and dissolved as the temperature increased.