Evolution of second phase particles during thermal cycling in high heat input welding.

This study examines the evolution of Nb, Ti, and V-containing second-phase particles at varying stages of a 100 kJ/cm welding line energy. It utilizes extraction replication techniques to observe the impact of quenching at temperature of 1000°C, 1200°C and 1350°C during heating, while the cooling temperature at 1300°C, 1100°C, and 900°C, respectively. A high-temperature confocal microscope is used to monitor the dynamic changes and effects on the austenite grain boundaries in particles during the entire welding thermal cycle. It is found that in the below 1000°C during the heating process, the predominant phenomenon is the dissolution and vanishing of smaller particles. As the temperature rises to the between 1000°C and 1350°C of the heating process, larger particles undergo partial dissolution, resulting in a reduction in their size, while the smaller ones completely dissolve and vanish. During the heating stage of the thermal cycle, the number of particles per unit decrease from 17.17 to 4.37 N/um². During the above 1100°C of the thermal cycle cooling process, the particles undergo precipitation and growth. Upon the cooling temperature to 900°C, this stage is mainly characterized by particle precipitation. During the below 900°C of the thermal cycle cooling process, the particle precipitation and growth occur simultaneously. During the cooling stage of the thermal cycle, the number of particles per unit increase from 4.37 to 16.56 N/um².The influence of second phase particles on pinning austenite grain boundaries and hindering grain growth tends to diminish initially and then rise again, reaching its minimum impact near the temperature peak. [ABSTRACT FROM AUTHOR]