Improvement of impact toughness by microstructure refinement of simulated CGHAZ through enhancing welding heat input of low carbon Mo-V-Ti-N-B steel

Abstract

Welding heat input greatly influences the microstructure and impacts the affected zone’s toughness. To interpret the relationships between the welding heat input, microstructure, and low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ) of Mo-V-Ti-N-B steels, welding heat cycles with different heat inputs (25–75 kJ cm ^−1 ) were performed on a Gleeble 3500 simulator. Intragranular ferrite in the simulated samples subjected to different thermal cycles was characterized and quantified, and the impact energies of simulated samples were evaluated at −20 °C. Upon increasing the heat input, the intragranular ferrite content rose sharply from 4.3% to 76.0%. The V(C, N) enrichment on the precipitate surface increased the size of precipitates, providing favourable nucleation conditions for intragranular ferrite. The prior austenite grain (PAG) and martensite/austenite (M/A) constituents became rough, and the content of the M/A constituent increased while the impact energy of the CGHAZ increased. This behaviour occurred due to the formation of intragranular acicular ferrite (IGAF), which refined the microstructure of the CGHAZ. Grain refinement eliminated the negative influence of higher M/A content on the impact toughness of the CGHAZ.

Keywords

• low-carbon Mo-V-Ti-N-B steel
• heat input
• CGHAZ
• intragranular ferrite
• impact toughness