Multi-variant bainitic microstructure-driven design of a 900 MPa grade high-strength weld metal with excellent strength-toughness combination

Abstract

Achieving an optimal balance between strength and toughness continues to be a fundamental challenge in the design of high-strength weld metals. In this study, a 900 MPa grade weld metal was developed through precise alloying with Ni, Cr, Mo, and V, specifically designed to promote a multi-variant bainitic microstructure while suppressing the formation of coalesced bainite (B-c). The bainitic transformation was intended to occur within the temperature range of 450-460 degrees C, facilitating the extensive autocatalytic nucleation of carbide-free bainite. Based on thermodynamic and kinetic calculations, the final weld metal composition was optimized to 0.09C-3.0Ni-0.9(Cr + Mo + V). The resulting microstructure of the multipass weld metal consisted of over 90% lath bainite, while B-c and martensite contents were less than 4%. Transmission electron diffraction analysis showed no evidence of carbide precipitation along lath boundaries or the presence of retained austenite. Instead, atom probe tomography revealed pronounced carbon segregation at the bainitic lath boundaries. Additionally, nano-scale clusters rich in Cr, Mo, and V were uniformly dispersed within the bainitic laths. Notably, the bainite laths exhibited an even distribution among the 24 crystallographic variants associated with the Kurdjumov-Sachs orientation relationship, leading to an interlocking structure with high-angle grain boundaries exceeding 40 degrees. This tailored microstructure, achieved through the proposed alloy composition, delivered an excellent balance of mechanical properties, including a yield strength of approximately 980 MPa and a Charpy impact toughness of similar to 50 J at -51 degrees C. These findings offer new insights into microstructure-guided alloy design strategies for ultra high-strength weld metals.