Effect of heat input on microstructure and mechanical properties of weld metal of 690 MPa grade HSLA steel

690 MPa grade high strength low alloy (HSLA) steel of 27 mm in thickness was welded by self-developed electrode of 4.0 mm in diameter without preheating. The effects of heat input on microstructure and mechanical properties of weld metal were studied and the mechanism of strength-toughness was revealed. The results show that heat input had significant effect on microstructure and mechanical properties of weld metal, which increased from 13 kJ/cm to 19 kJ/cm. With the increasing of heat input, tensile strength and hardness of butt joint decreased, −50 ℃ impact energy firstly increased and then decreased. Good strength-toughness was obtained under the heat input condition of 16 kJ/cm. The tensile strength of butt joint was 828 MPa, −50 ℃ impact energy of weld metal was 71 ~ 90 J, the average value was 80 J. The lower strength-toughness properties of weld metal under 13 kJ/cm and 19 kJ/cm heat input conditions were related with the formation of lath bainite and ferrite side plates, the formation of coarse M-A, respectively. Under the 16 kJ/cm heat input condition, good plasticity acicular ferrite was sufficiently formed and acicular ferrite and bainite were distributed in an interwoven pattern. Meanwhile, small and dispersively distributed M-A had no obviously negative effect on toughness of weld metal. Therefore, weld metal with excellent strength-toughness had been obtained.