Mechanical Properties of ASTM A572 Steel After Fire Exposure

ASTM A572 steel was subjected to two cooling methods: air cooling (slow cooling rate) and water cooling (rapid cooling rate).
 
Tensile tests were conducted on uniaxial notched specimens at temperature intervals of 100 °C within the range of 500–1000 °C. The test results indicated the following:
 
At a temperature of 600 °C, the mechanical properties of the ASTM A572 steel showed no significant variation between the two cooling methods.
 
At temperatures above 600 °C, air cooling reduced both the yield strength and the ultimate tensile strength of the ASTM A572 steel, while simultaneously enhancing its ductility.
 
When water-cooled from temperatures exceeding 600 °C, the formation of a martensitic phase resulted in an increase in the ultimate tensile strength of the ASTM A572 steel, accompanied by a reduction in its ductility.
 
Under both air-cooled and water-cooled conditions, a high stress triaxiality ratio led to a decrease in the ductility of the ASTM A572 steel, while simultaneously increasing its yield strength and ultimate tensile strength.
 
The synergistic effect of high stress triaxiality and the formation of the martensitic phase caused the yield strength and ultimate tensile strength of ASTM A572 steel—when water-cooled from temperatures above 800 °C—to increase significantly by 158% and 172%, respectively; conversely, the combination of high stress triaxiality and water cooling resulted in a ductility loss of up to 89% in the ASTM A572 steel cooled from temperatures above 800 °C.