Experimental Study On Mechanical Properties Of Welds At High Temperature

Welds are the main connecting method of the steel structure building owing to the good airtightness and not weaken the section.With the development of steel structures for high-rise and long-span space,the probability of fire also increases,resulting in the thread to welded steel structures.This puts forward some new requirements for fire protection design of steel structure.Therefore,the importance of high temperature mechanical properties of materials is self-evident.In fire,the fire resistance of the welded joints is important to the building safety and roubness.High temperature mechanical properties of pure welds and fillet welds can provide a theoretical basis for fire protection design of steel structure and numerical simulation of the welded structure subjected to fire.In this study,welds are manufactured by Q235 as base metal,E4303 as welding material and Q345 B as base metal,E5015 and ER50-6 as welding materials.Transverse fillet welds,longitudinal fillet welds and cruciform joints are manufactured by Q345 B steel and E5015 welding material.The stress-strain relationship,elastic modulus,yield strengths and ultimate strengths as well as their reduction factors of welds,transverse fillet welds,longitudinal fillet welds and cruciform joints provided by steady-state test,were compared with the current standards;the metallographic analysis of E4303 welds and E5015 welds at typical temperatures was carried out,and the research results as follows:With rising temperature,the elastic modulus and yield strengths of the pure welds gradually decrease;the ultimate strength first increases and then decreases;at300 ℃,the ultimate strengths of welds reaches the maximum due to the blue embrittlement phenomenon;the high-temperature degradation trend of the mechanical properties of the transverse fillet welds,the longitudinal fillet welds,and the cruciform joints are similar.At 300 ℃,the ultimate strengths of welds reaches the maximum due to the release of the residual stress of the fillet welds.The reduction of elastic modulus and yield strengths of steel at elevated temperatures in the current standards is not fully applicable to the pure welds and fillet welds in fire resistant design,so it is necessary to modify them in an engineering application.The phase organization of E4303 welds and E5015 welds are ferrite + pearlite under typical temperature conditions,but the mechanical properties of materials will be changed when various phase organization exist in different forms and proportions.At 20 ℃ and 400 ℃,E4303 welds and E5015 welds are composed of ferrite andpearlite with uneven grain size.At 600 ℃,the phase organization of both welds is further uniformly refined,and carbides are precipitated on the specimens.The content of ferrite increases and that of pearlite decreases.Therefore,the ultimate strengths of the pure welds decreased gradually.At 800 ℃,E4303 welds and E5015 welds is not fully austenitized,and the phase organization of the welds is austenite and high temperature ferrite.The elastic modulus of transverse fillet welds and the cruciform joints is obviously higher than that of E5015 welds at different temperatures,while that of longitudinal fillet welds is obviously lower than that of E5015 welds,which is caused by the anisotropy of welds,and the stress mode of welds has a great influence on its elastic modulus.The ultimate strengths of transverse fillet welds and the cruciform joints are considerably higher than that of E5015 welds.On the one hand,it may be due to the difference of proportion and grain size of phase organization,on the other hand,there are uncontrollable penetration at the root of fillet weld.In this paper,the penetration of fillet welds is conservatively treated as zero;the ultimate strengths of longitudinal fillet weld is significantly lower than that of E5015 welds.The stress distribution of the longitudinal fillet welds along the length direction of the weld is uneven.The stress of the welds is largeat at the ends,and the stress is small at the middle.Therefore,when the stress at the ends of welds exceeds its ultimate strengths,the end of the longitudinal fillet welds will be damaged,resulting in the cracks,which will penetrate the whole weld then the longitudinal fillet welds lost of its bearing capacity.The average angle of the actual failure surface of the transverse fillet weld is 18°,and that of the cruciform joint is 19.6°;the average angle of the longitudinal fillet weld is 45°,which is the same as the theoretical value specified in the codes.At ambient temperature,the ratio of the ultimate strength of the transverse fillet welds to the longitudinal fillet welds with the same fillet size and length is 1.625.With increasing temperature,the ratio shows tendency to ascend;the ultimate strengths of the transverse fillet welds is 1.028 times as much as the cruciform joints.At other temperatures,the ratio of ultimate strength of transverse fillet welds to the cruciform joints is larger than that at normal temperature.The mechanical properties of the welds and fillet welds obtained in this study can be used for fire resistant design of steel structure and numerical simulation of fillet welds subjected to fire.