•The grain boundary engineering (GBE) is applicable to 825 tubes.•GBE is achieved through recrystallization rather than grain growth.•The low ∑ CSL grain boundaries in 825 tubes can be increased to > 75%.Grain boundary engineering (GBE) of nickel-based alloy 825 tubes was carried out with different cold drawing deformations by using a draw-bench on a factory production line and subsequent annealing at various temperatures. The microstructure evolution of alloy 825 during thermal-mechanical processing (TMP) was characterized by means of the electron backscatter diffraction (EBSD) technique to study the TMP effects on the grain boundary network and the evolution of grain boundary character distributions during high temperature annealing. The results showed that the proportion of ∑ 3n coincidence site lattice (CSL) boundaries of alloy 825 tubes could be increased to > 75% by the TMP of 5% cold drawing and subsequent annealing at 1050 °C for 10 min. The microstructures of the partially recrystallized samples and the fully recrystallized samples suggested that the proportion of low ∑ CSL grain boundaries depended on the annealing time. The frequency of low ∑ CSL grain boundaries increases rapidly with increasing annealing time associating with the formation of large-size highly-twinned grains-cluster microstructure during recrystallization. However, upon further increasing annealing time, the frequency of low ∑ CSL grain boundaries decreased markedly during grain growth. So it is concluded that grain boundary engineering is achieved through recrystallization rather than grain growth.

•Coexistent microstructure of recrystallized GBs & trace of disappeared original GBs.•GB network evolution & grain-cluster growth during GBE.•Regime of recrystallization during GBE processing.Grain boundary (GB) engineering was carried out on a Ni-based alloy with pre-precipitated carbides at GBs. Microstructure with coexistence of ∑3n boundaries formed during annealing and traces of disappeared original GBs were observed during GB-engineering. The newly formed ∑3n boundaries are in different positions with that of disappeared original GBs, which indicates the recrystallization front GBs moved into the deformed matrix and swept away original GBs. It is a typical recrystallization process rather than GB decomposition. Strain induced boundary migration initiated the recrystallization. Grain-cluster formed with the continuous occurrence of twinning-events in the wake of the migrating recrystallization front GBs during sweeping away the deformed matrix. Finally large grain-clusters with highly twinned interconnecting ∑3n boundaries were formed.

The feasibility of applying the grain boundary engineering (GBE) processing to Alloy 690 tube manufacturing for improving the intergranular corrosion resistance was studied. Through small amount of deformation by cold drawing using a draw-bench on a production line and subsequent short time annealing at high temperature, the proportion of low Σ coincidence site lattice (CSL) grain boundaries of the Alloy 690 tube can be enhanced to about 75% which mainly were of Σ3n (n = 1, 2, 3, …) type. In this case, the grain boundary network (GBN) was featured by the formation of highly twinned large size grain-clusters produced by multiple twinning during recrystallization. All of the grains inside this kind of cluster had Σ3n mutual misorientations, and hence all the boundaries inside the cluster were of Σ3n type and formed many interconnected Σ3n type triple junctions. The weight losses due to grain dropping during intergranular corrosion for the samples with the modified GBN were much less than that with conventional microstructure. Based on the characterization by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) technique, it was shown that the highly twinned large size grain-cluster microstructure played a key role in enhancing the intergranular corrosion resistance: (1) the large grain-cluster can arrest the penetration of intergranular corrosion; (2) the large grain-cluster can protect the underlying microstructure.Highlights► Grain boundary engineering (GBE) was successfully applied to Alloy 690 tube. ► Highly twinned large size grain-clusters constitute the GBE grain boundary network. ► GBE grain boundary network shows much better resistance to intergranular corrosion.

•The grain boundary engineering (GBE) is applicable to Hastelloy N alloy.•The low ∑ CSL grain boundaries in Hastelloy N alloy can be increased to more than 70%.•The primary carbide has a detrimental effect on the promotion of low Σ CSL grain boundaries.Grain boundary engineering (GBE) was carried out on Hastelloy N alloy which is an important structural material used for molten salt reactor. The proportion of low Σ coincidence site lattice (CSL) grain boundaries of the Hastelloy N alloy can be enhanced to more than 70% with the formation of large-size highly-twinned grain-cluster microstructure which was formed through extensive multiple twinning events during recrystallization. The effects of cold deformation amounts and subsequent annealing on the grain boundary character distribution (GBCD) were investigated. The effects of initial grain size and the primary carbide distribution which was affected by the content of silicon on the grain boundary network evolution were discussed. It was determined that the initial grain size and primary carbide distribution will affect the recrystallization kinetics and hence influence the formation of highly twinned grain-cluster microstructure and the GBCD. The particle stimulated nucleation (PSN) caused by the primary carbides has a detrimental effect on the promotion of low Σ CSL grain boundaries.