Study Of Properties And Microstructure Transformation Of Welded Joint Of BWELDY960Q Steel

BWELDY960Q steel with high strength and good toughness is high strength low alloy(HSLA) steel to use for welded structure, high strength is favorable for reducing weight of welded structure in order to save materials. HSLA steels have been widely used in the field of engineering machinery,mine,port and hydropower etc., the use of HSLA steel with good weld ability can promote the engineering structure to large scale, light weight and efficiency development. In engineering practical application, HSLA steel welding is the key problem for actual application, because cold crack, toughness deterioration, softening and other issues of HSLA welded joint have appeared on the difference welding conditions. In addition, it demands that the properties of HSLA welded joint are constantly improved. HSLA steel must fufill the requirements of high strength, and toughness of weld metal and heat affected zone(HAZ) is improved to prevent crack formation in the process of welding. In this paper, the changes of HAZ microstructure and properties of BWELDY960 Q steel under single and double thermal cycle are studied by using weld thermal simulation technique to find properties weak parts. Microstructure and properties of welded joint are researched by using metal active gas welding(MAG) to determine optimal welding parameters, comparative analysis of microstructure and properties in HAZ is also investigated by using thermal simulation and practical welding. At the same time, effect of vibration process on microstructure and properties of welded joint is studied by changing frenquency and amplitude on the basis of conventional welding.The results of simulated HAZ experience of single thermal cycle test show that coarse grained heat affect zone(CGHAZ) at the peak temperatures 1320℃ and 1200℃ appears embrittlement, and intercritical heat affect zone at 800℃ is the same as CGHAZ. The toughness of Coarse grained heat affect zone is decreased by 82.17% of the base metal in single thermal cycle test to cause serious embrittlement. It is also found that the toughness loss of intercritical heat affect zone is 46.53% compared with that of the base metal, the degree of embrittlement is second only to coarse grained heat affected zone. When welding heat input is between 10kJ/cm and 40 k J/cm, the toughness decreases with the increase of heat input, the toughness of coarse grained heat affected zone no longer continue to reduce at he heat input of more than 30kJ/cm.The results of simulated reheat coarse grained heat affected zone(RCGHAZ) experience of difference peack temperature in double thermal cycle test show that the toughness of reheat coarse grained heat affected zone is improved at different degree. The toughness loss of untransformation and intercritical reheat coarse grained heat affected zone is 73.26% and 67.32% respectively at the peak temperature 1200℃ and 800℃ compared with that of the base metal, embrittlement is serious and the phenomenon of structure heredity exists in these regions.The results of conventional metal active gas welding(MAG) show that the optimum match between toughness and strength of welded joint is obtained as the welding current is 220 A, the tensile strength of welded joint is 872 MPa, cantraction area and elongation percentage reach the maximum of 41% and 12.5% respectively, impact energy of weld metal and fusion zone is 60 J and 34J(-20℃) separately. Vibratory welding based on conventional welding is applied, the welding current, vibration frenquency and amplitude parameters are changed accordingly in the vibration welding process, vibratory welding parameters of the optimal properties of welded joint can be obtained by using the orthogonal test. Tensile strength is 883 MPa and impact energy are up to 64J(-20℃) when the welding current is 220 A, vibration frenquency and amplitude is 40 Hz and 0.05 mm respectively, the properties of BWELDY960 Q steel welded joint achieves the optimum.The properties of welded joint also change with the variation of vibration parameters, when the welding current is 220 A. The properties firstly increase and subsequently reduce as the increase of vibration frequency and amplitude, tensile strength is from 790 Mpa to 883 MPa, elongation is frome 9% to 14% and impact energy is the range from 50 J to 64 J. Meanwhile, impact toughness of weld metal at low temperature can be improved under vibratory welding, ductile-brittle transition temperature of weld metal is-73.70℃ and-75.02℃ by using metal active gas welding and vibratory welding.Through the comparison analysis of HAZ microstructure and properties of thermal simulation and actual welding, the austenite grain size of heat affected zone in the thermal simulation condition is larger than that of the corresponding region of actual welding. The hardness of coarse and fine grain heat affected zone decreases, but hardness of the intercritical heat affected zone increases slightly compared with actual welding. Although HAZ microstructure and properties of thermal simulation and actual welding exists the difference, the change of HAZ microstructure and properties can be revealed by using thermal simulation technology.