Abstract: In this paper,the HAZ structure of the 3.25% silicon transformer steel joint welded by MIG process as well as the correlation between the structure and the brittle tendency of the HAZ metal has been studied.The structure of the HAZ can be divided into three types:(1)the intergranular cementite precipitates;(2) the precipitateswhich possess a characteristic distribution of Widmanstätten morphology in the ferrite grain interior;(3)the acicular constituent,a M-A type structure after complex (α→γ→α) transformation during the continuous cooling down from welding tmperature.The acicular constituent is not so sensitive to brittle fracture under strain as compared with the strengthened ferrite grain base,although some martensite might be present.In fact,no microcracking nucleates in it or propagatse along it under the bending strain,but intergranular cracking can be first nucleated somewhere at the grain boundary and consequently induces a transgranular propagation of cleavage fracture in the neighbouring grains.However,the nucleated intergranular cracking has only a limited and localized effect on the macro-brittle fracturing of the coarse grain regionin HAZ,and for the overall effect,it is the strengthening of the ferrite which plays the maior role.The overall effect is so pronounced that whenever the precipitate gets the Widmanst(?)tten mode distribution inevitably the ductility of the HAZ metal drastically reduces,and then brittle fracturing takes place.The ductility can be recovered by 900℃ post heat-treatment,forming a solid solution of the grain boundary precipitate. There will be then no more Widmansttätten mode distribution or acicular structure component.