Regulating the Boron Content to Suppress Stress‐Relief Cracking in the Coarse‐Grained Heat‐Affected Zone of 7CrMoVTiB10.10 Heat‐Resistant Alloy

ABSTRACT This study demonstrates a new strategy for modifying the boron content to address stress‐relief cracking (SRC) in the 7CrMoVTiB10.10 alloy. First, coarse‐grained heat‐affected zone (CGHAZ) specimens with different boron contents were prepared via simulated welding thermal cycles. High‐temperature tensile tests were then conducted to simulate the stress‐release process, assessing the potential of boron addition at 500°C–750°C to suppress SRC. Subsequently, scanning electron microscopy, electron backscatter diffraction, electron probe microanalysis, and transmission electron microscopy were used to study fracture characteristics and microstructure, particularly boron segregation and grain boundary (GB) carbide evolution. Finally, by combining boron segregation analysis with thermodynamic and kinetic calculations of precipitation, the mechanism by which boron inhibits SRC was elucidated, and the optimal boron content was determined. An increase in boron content within the appropriate range (0.004–0.007 wt%) considerably improves SRC resistance. This is because increasing the boron content does not alter the CGHAZ microstructure but increases boron segregation at GBs, enhancing GB plasticity and inhibiting the precipitation and growth of M 23 C 6 at GBs. However, when the boron content exceeds 0.007 wt%, although boron segregation at GBs increases, excessive formation of M 23 (C, B) 6 occurs. This consumes the free boron atoms at GBs, promotes the formation of microvoids, reduces GB plasticity, and increases SRC susceptibility.