Study on mechanical properties of butt-welded Q345 steel after high temperature cooling
The mechanical properties of steel will change significantly due to different fire temperatures.In particular,the residual mechanical properties of steel structure connection joints after fire play a vital role in the integrity of steel structures.The mechanical properties of butt-welded Q345 structural steel after simulated fire were studied in depth.The fracture characteristics of butt-welded Q345 steel after high temperature cooling at 20～900℃were observed,and the key parameters such as stress-strain curve and mechanical properties were obtained.Meanwhile,the fracture morphology and metallographic changes of the specimens were observed by instruments and equipment,and the reasons for the changes of macroscopic mechanical properties were explored from the microstructure.The results show that 600℃is the critical temperature for the change of mechanical properties of the specimen.When the fire temperature does not exceed 600℃,the change of yield strength and ultimate strength is small and does not exceed 9%.When the temperature exceeds 600℃,the yield strength and ultimate strength gradually decrease with the increase of temperature,which are 69%and 84%of the initial value at 900℃,respectively.In addition,there are a large number of dimples with different shapes and sizes on the fracture surface of the specimens after high temperature.The microstructure of the weld at room temperature is mostly composed of ultra-low carbon bainite and acicular ferrite,while the base metal near the heat affected zone of the weld is composed of ferrite and bainite.When the temperature is not more than 500℃,the microstructure of the base metal has no obvious change.After 900℃,the specimen is mainly composed of ferrite and partial cementite with larger crystal size.
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