The effect of heat input on the micro-structure and impact toughness of weld metal for 690 MPa grade high strengthen steel

Abstract

Three types of welding heat inputs (20 kJ/cm, 25 kJ/cm, and 30 kJ/cm) were used to weld 690 MPa high-strength steel. OM, SEM, TEM and Lepera corrosion methods were utilized in this study. Additionally, Sisc Ias metallographic analysis software was employed to accurately determine the volume fraction and size distribution of inclusions. The reasons affecting the impact toughness of the weld were analyzed. Research has shown that as heat input increases, the impact toughness shows a trend of first increasing and then decreasing. The width of the columnar crystal in the primary structure of the weld metal increases from 70 μm (20 kJ/cm) to 149 μm (30 kJ/cm). The secondary crystal structures of the weld metal is primarily composed of lath bainite and granular bainite. The M-A constituents is mainly granular, with a slight increase in volume fraction. Additionally, the percentage of residual austenite increases from 0.46 % (20 kJ/cm) to 0.84 % (30 kJ/cm). The primary reason for the initial increase and subsequent decrease in impact toughness is due to the heat input of 25 kJ/cm. When the crack passes through the mixed structure of small bainite ferrite and lath bainite ferrite, the propagation path twists, resulting in an increase in impact absorption energy. However, when the heat input is increased to 30 kJ/cm, the micro-structure coarsens and the number of large-sized inclusions increase, leading to cleavage fracture and a decrease in impact toughness of the weld metal. The impact toughness is not significantly affected by the presence of M-A constituents and residual austenite.

Keywords

• 690 MPa high-strength steel
• Heat input
• Impact toughness
• Micro-structure
• Inclusion