PSI - Issue 66

Johannes L. Otto et al. / Procedia Structural Integrity 66 (2024) 256–264 Johannes L. Otto et al. / Structural Integrity Procedia 00 (2025) 000–000

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Figure 1a shows the set-up with charging aid inside the large industrial vacuum furnace for diffusion brazing of the specimens from rod material. Eleven of this charging aids were placed in the furnace, so that 66 specimens were brazed at once. The filler metal foil was placed in two layers (2×20 µm) between the rod pieces which had been polished at the end faces to create butt joints with low defect density. A force of 0.5 kg on each specimen was used for fixation and to adjust a uniform brazing gap width. After the brazing process, the butt joints were turned into the geometry shown in Figure 1b. For the corrosion fatigue tests further polishing in the testing area was carried out, to reduce surface defects and the sensitivity to stress corrosion cracking, Wu et al. (2022).

Fig. 1. (a) Setup for the brazing process, (b) turned specimen geometry from the brazed butt joints for the corrosion fatigue tests.

A brazing temperature of 1160°C was selected to ensure complete melting of the filler metal. The holding time was set to one hour to enable sufficient diffusion during this time. Figure 2 shows the process and the mechanisms schematically. To improve the vacuum and temperature distribution in the furnace, further stages were used before the brazing temperature was reached and the filler metal was molten, as presented in Figure 2a). The small and light element B begins to diffuse particularly quickly from the melt into the base material, preferably along the grain boundaries. Also, other elements such as iron and nickel diffuse depending on the concentration gradient. Since B is an important melting point depressant, the solidus temperature T S of the melt rises as a result and isothermal solidification occurs, Figure 2b). The resulting final microstructure is shown in Figure 2c). In the diffusion zone (DZ), Cr-rich borides are formed on the grain boundaries, which are usually M 2 B borides, Ma et al. (2022). If no remaining melt was present at the end of the holding time, athermal solidification does not occur and only a few small precipitates form in the solidification zone (SZ). Individual small pores may be present due to gas inclusions, for example.

Fig. 2. Schematic steps of the brazing process with a) molten filler metal after reaching the brazing temperature, b) complete isothermal solidification at the end of the holding time and c) resulting microstructure after cooling with typical precipitations and few pores. By 3D reconstructing of the brittle phases from the layer-by-layer removal of the brazing seam using a focused ion beam (FIB), it was shown by Otto et al. (2023) that the Cr-rich borides in the diffusion zone form a highly cross-linked network that hardly allows any deformation. As a result, these brittle phases fracture, forming notches on the surface and pores inside, and thus influence crack initiation and propagation.