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) 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.

Effect of heat input on the microstructure and impact toughness of submerged arc weld metal for high-performance weathering steel

“…In addition, for the EZ, it is the area where the upper layer of multi-layer multi-pass welding affects the next layer, the increase in E j leads to an increase in the amount of deposition, which enhances the thermal impact to the EZ, prolonging the high-temperature residence time, and also causes coarsening of the microstructures. The grain boundaries of PF and AF are typically HAGBs [26,39,40]. The coarsening of PF and AF and the decrease in AF content cause a decrease in the proportion of HAGBs and an increase in the MED MTA15°f or both CZ and EZ.…”

Insight into the Role of Mo Content on the Microstructure and Impact Toughness of X80 Thick-Walled Low-Temperature Pipeline Steel

“…Simultaneously, the formation of refined LB is promoted while suppressing PF and GB formation due to a lower Ar3 temperature. High-angle grain boundaries characterized the boundaries of PF [21], while density of dislocation substructures was higher in LB compared to GB [18]. Both factors contributed to the reduction in the high-angle grain boundary content with an increase in the Mo content (Figure 6b and Table 3).…”

“…However, in the sample with 0.25% Mo, the dimples were small and deep and exhibited a uniform size, thus indicating a higher toughness level [12,19,20]. Conversely, in the sample with 0.4% Mo, the impact toughness was indicative to be inferior based on inconsistent dimple sizes, with some regions displaying larger and shallower dimples [20,21]. Radiative region observations of the microstructure revealing the prevalence of cleavage planes are depicted in Figure 8(a2,b2).…”

“…This resulted in a decrease in the GB content with a simultaneous increase in the LB content in the microstructure. Additionally, PF formation belongs to high-temperature transformation and requires sufficient carbon atom diffusion in a higher temperature range [18,21,22,24]. However, an increase in Mo content hindered carbon atom diffusion by lowering the Ar3 temperature, which inhibited PF formation, leading to a gradual reduction in the PF content in the microstructure.…”

“…In addition, the M/A constituents and ferritic matrix have different hardness and elastic modulus. Due to uncoordinated deformation of M/A constituents during the deformation under impact load, formation of micropores or microcracks takes place at the interface of the two components [20,21].…”