Microstructural Characteristics and Impact Fracture Behaviors of a Novel High-Strength Low-Carbon Bainitic Steel with Different Reheated Coarse-Grained Heat-Affected Zones

Abstract

To obtain the correlation of microstructural characteristics and toughness in a novel high-strength low-carbon bainitic structural steel with medium and heavy plate after multipass welding, a welding thermal simulation experiment was conducted to simulate different subregions in the reheated coarse-grained heat-affected zones (CGHAZ). The microstructure evolution was then analyzed and factors that influence the fracture behavior were studied. The results show that the brittle zone appeared in subcritical reheated CGHAZ, and the fractured morphology was cleavage fracture. Supercritical reheated CGHAZ had the highest impact toughness, and the fractured morphology was primarily the ductile fracture with dimples formed via the micropore polycondensation mechanism. With an increase in the secondary pass welding thermal cycle peak temperature (t p2), the average length size of martensite and austenite (M-A) decreased from 9 to 2 lm. The coarsening of M-A constituents was the main reason for decrease in the crack initiation absorbed energy. A large number of retained austenite and cementite precipitates in subcritical reheated CGHAZ clearly worsened the impact toughness, and the massive austenite and cementite precipitates more than offset the beneficial effects of high-angle boundaries. This phenomenon led to disappearance of the effect of high-angle grain boundary of prior austenite and lath bainite on arresting crack propagation. In supercritical reheated CGHAZ, crack propagation absorbed energy was increased because of grain refinement, fine precipitates, lamellar residual austenite at corners, and high-angle grain boundary.

Achieving Excellent Strength‐Ductility Balance and Lower Yield Ratio in a 690 MPa‐Grade Multiphase Construction Steel

“…[ 20 ] As mentioned earlier, large blocky M/A constituents were the source of cracks initiation. [ 21 ] Generally, stress concentration is localized at the interface between the steel substrate and coarse M/A constituents, which is difficult to relieve. Cracks were preferred to nucleate and propagate at such a weak interface, and finally deteriorated the impact toughness.…”

Influence of Vanadium Micro-Alloying on the Microstructure of Structural High Strength Steels Welded Joints

“…However, it is also reported that the loss in toughness is not just due to the presence of RA phase, but is related to the distribution and morphology of the RA constituent, and the matrix microstructure. Cui et al [ 32 ] reported that block residual austenite significantly deteriorates impact toughness of super-critical reheated coarse grain heat affected zone (SC CGHAZ).…”

Inter-Critically Reheated CGHAZ of Ultra-High-Strength Martensitic Steel with Different Cooling Rates

“…Extensive research has showed that the inter-critically reheated coarse-grained heat-affected zone (ICR CGHAZ) is the location with the poorest fracture toughness in the multi-pass welding heat-affected zone [ 5 , 6 ]. Therefore, methods to optimize mechanical properties, especially toughness, have become popular in the research on high-strength and ultra-high-strength steel [ 7 , 8 , 9 ].…”