Influence Of Cooling Regime On Mechanical Properties Of Welded Q345 Structural Steel

Welding is a common method to connect structural steel into a whole component.It is widely used in the construction of steel structures because of its advantages of not weakening the section and convenient operation.At the same time,welding quality and reliability has become an important key to steel structure design and construction.However,steel structure buildings may suffer from disasters such as fires during the whole life cycle.After fire,some steel members in the steel structure can continue to be used after testing,identification and safety assessment.Replacing or removing the members with insufficient residual bearing capacity and serious deformation can effectively avoid the economic waste caused by the total replacement.Therefore,this paper takes Q345 steel which has been widely used in steel structure as the object,and studies the mechanical properties of butt weld and cross joint Q345 steel after different fire extinguishing methods from high temperature cooling,so as to provide suggestions for safety assessment and reliability identification after fire.The main work and results of this paper are as follows:(1)Experimental study on mechanical properties of welded Q345 steel after fire.According to the fire temperature of the steel structure,the monotonic tensile test was carried out on the butt weld and cross joint Q345 steel after natural cooling,immersion cooling and foam extinguishing agent cooling.Firstly,in the high temperature test,the specimens were heated to the specified temperature and then extinguished in different ways to obtain the pretreated specimens.Secondly,the tensile test of the specimen was carried out to obtain the stress-strain curve and related mechanical performance parameters.Then,the mechanical properties parameters are analyzed and compared with the research of relevant scholars,and their performance differences,advantages and disadvantages are summarized.Finally,the prediction formula of residual mechanical properties under two welding methods is fitted.(2)Study on the influence of welding methods on the residual mechanical properties of Q345 steel after fire.The concept of weld strength coefficient is introduced to study the weakening or strengthening of welded joints by welding materials,welding methods and other factors,and the mechanical properties of welded joints after different fire extinguishing methods are comprehensively analyzed.Through comparative analysis,the influence of different fire temperatures and fire extinguishing methods on the yield strength and ultimate strength of butt weld and cross joint Q345 steel was studied.(3)The effects of fire temperature and fire extinguishing methods on the fracture mode and microstructure of welded Q345 steel were studied.According to the fracture of tensile specimen and the specimen after high temperature,the changes of micro fracture morphology,metallographic structure and hardness value of weld with temperature and different fire extinguishing methods were studied,and the mechanism of mechanical property change and fracture mode was revealed microscopically.