Engineering of grain boundaries in textured ferritic-martensitic steels to increase resistance to hydrogen embrittlement

Abstract

The article examines the influence of grain boundary engineering on the resistance to hydrogen embrittlement of textured ferritic–martensitic steels used in critical structures of the energy and hydrogen infrastructure. The aim of the study is to substantiate thermomechanical processing regimes that ensure the formation of a special grain boundary state with enhanced hydrogen tolerance without changing the alloy’s chemical composition. The material investigated is a high-chromium ferritic-martensitic steel of industrial heat, subjected to a controlled sequence of hot and cold deformation, austenitizing, quenching, and tempering with variation of the reduction degree and recrystallization annealing parameters. Microstructural analysis was carried out by EBSD methods with the construction of orientation distribution functions and misorientation maps, which were used to quantitatively evaluate the fraction of special Σ-type boundaries according to the Brandon criterion, grain size, and texture intensity. Hydrogen resistance was studied by electrochemical charging, determination of diffusion parameters, and slow strain rate tensile tests combined with fractographic examination of fracture surfaces. It was established that the enhancement of crystallographic texture and grain refinement in the optimal GBE regime lead, to an increase in the fraction of special boundaries to more than 60% and a simultaneous reduction of low-angle boundaries, accompanied by a decrease in the effective hydrogen diffusion coefficient by more than four times and a significant reduction in its apparent concentration in the metal. It is shown that the formation of a branched network of special boundaries interrupts the connectivity of random grain boundaries, suppresses crack percolation along the intergranular mechanism, and provides a reduction of the plasticity loss index to about 10% while maintaining high strength characteristics, whereas the state without GBE treatment is characterized by a catastrophic loss of ductility. Threshold values of the fraction of special boundaries at which the material transforms into a new structural state with high resistance to hydrogen cracking are discussed, as well as prospects for scaling the proposed thermomechanical processing regimes to industrial production of steels for nuclear energy and the hydrogen economy.