Experimental Study On Mechanical Properties Of Butt Welded Connections At High Temperature

In modern steel structural engineering,the reliability of connected quality and connection forms play a crucial role in realizing steel structural safety,which causes the steel structures to resist the earthquake and other disaster with good properties.Therefore,the study on the mechanical properties of butt welds at high temperature could supply the design theories for steel structural fire safety and for analyzing the fire behavior of steel structure buildings in fire.And more,the results could provide some parameters for a mass simulation analysis of steel structures by using finite element methods.Hence,it's of great significance for the development of the steel structural fire resistance theory.This study designed Q345B?Q420B butt welding specimens and some butt welding defect specimens.By using steady-state tests and transient-state tests,the high temperature transition rule of mechanical properties of butt welds was studied.The results included the stress-strain relationship,elastic modulus,yield strength,ultimate strength and other mechanical properties at different temperatures.Additional,metallographic analysis was carried out on the base metal zones,heat affected zones(HAZ)and fusion zones(FZ)of the Q345 B butt welds,which were to present the reason of fracture at a micro level.Based on the stress-strain curve and the temperature-strain curve of the butt welding specimens of Q345 B and Q420 B respectively,the steady-state tests and transient-state tests were adopted to analyze the high-temperature transformation law of the mechanical properties of the butt welds.Where the elastic modulus,yield strength,ultimate strength and high-temperature reduction coefficient were compared with the existing standards.The results showed that with the increase of temperature,the fracture location of the butt welds of the two different metallic specimens is transferred from the base metal zone to the HAZ because the weldments are more sensitive to high temperature..Nevertheless,there is a risk of brittle fracture in FZ when the butt welds were at higher temperature,such as above 700?.The reduction coefficient of elastic modulus and yield strength in current codes are not applicable to design the butt welds in fire.The metallographic structure of the butt welds of Q345 B after high temperature was analyzed.At above 400 ?,the granular carbides in HAZ and micro cracks between article acicular ferrite will reduce the strength,hardness and so oncomprehensive mechanical properties of the bainite.In that,the acicular ferrite also causes the specimens fracturing in the HAZ within 400 ? or higher.It can be found that the ferrite transformation was the main reason of neckless fracture in weld zone at800 C.Based on the fracture patterns and stress-strain curves of the specimens with different defective area ratios at different temperatures,the influence of the defects on the mechanical properties of the butt weld was analyzed by the steady-state tests.Brittle fracture of FZ will occur when the proportion of defect area is 36% under the condition of full temperature from 20? ~600?.As for the use of the concept of "suitable for using" of welding defects,the influence of defect area ratios should be considered as well as the influence of the defects inducing brittle fracture of components and thus causing structural instability at ambient.With the increase of defect area ratios and temperature,the yield strength,ultimate strength and other mechanical properties of butt weld would degrade seriously.The relevant parameters and their reduction coefficients obtained from the above tests could be used for the fire resistant design of butt weldments and the evaluation of steel structural safety in fire.