PSI - Issue 42

Guocai Chai et al. / Procedia Structural Integrity 42 (2022) 155–162 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. EBSD images show the microstructures of 17-4PH material in different conditions, (a) grain and martensitic lath structures in the as-delivered material, (b) precipitates, NbC (red particles) at grain boundaries (black lines) and martensitic lath boundaries (light lines) in the as-delivered material, (c) grain and martensitic lath structure in the tempered material, (d) precipitates, NbC and austenitic phase (red particles) at grain boundaries (black lines) and martensitic lath boundaries (light lines) in the tempered material.

3.2. Influence of tempering and hydrogen charging on tensile behavior

Fig. 3 shows the SSRT stress versus strain curves of the 17-4PH materials in as-delivered and tempered conditions with or without hydrogen charging. Table 2 shows the tensile properties of the materials. The tempering causes a small decrease in 0.2% proof strength but an increase in elongation or failure strain used in this paper. One possible reason is that the tempering has caused a decrease in dislocation density in the material. The hydrogen charging has also led to a small decrease in 0.2% proof strength in both materials (about 2%), a softening effect. Such phenomenon has been reported in other materials, Hirth (1980) and Matsui (1979). Hydrogen induced softening can occur in a tensile testing with a low strain rate, Li et al. (2020). With hydrogen charging, the failure strain of the material decreases significantly. For the as-delivered material, the failure strain,  f, decreases about 44.3%, but about 33.8% for the tempered material. This indicates that the tempering has improved failure strain and therefore reduces susceptibility to hydrogen embrittlement. It can be attributed to the precipitation of nano Cu-rich phase and nano austenitic phase. These FCC phases itself and their phase boundaries/interfaces become hydrogen traps and increases hydrogen dissolution, which can reduce localized hydrogen induced stress concentration in the material.