PSI - Issue 71

G. Narasinga Rao et al. / Procedia Structural Integrity 71 (2025) 317–324

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3. Results and discussion 3.1 Microstructural & structural characterization

Optical microscopy images are shown in Fig. 2, illustrates the significant variations in the microstructure of the 17-4PH SS resulting from various material conditions. The microstructure in the initial condition (AR) was predominantly martensitic, along w ith a small quantity of δ -ferrite. After ST, the microstructure became more uniform, predominantly exhibiting martensite with reduced δ -ferrite content. This indicates that the martensite under this condition contains a lath structure with high dislocation densities (Vander Voort, Lucas et al. 2004). No evidence of precipitates was detected in the matrix under ST condition. This suggests that the martensite in this state possesses a lath morphology characterized by elevated dislocation densities. It has been suggested that the martensitic phase is supersaturated with Cu and Cr (Murayama, Hono et al. 1999). The microstructure is composed of fine spherical precipitates of the ε -Cu phase and martensite, after an hour of aging at 480 °C. These precipitates, which range in size from 40 to 100 nm and are less than 10 nm, are distributed throughout the matrix after aging at 480°C (Chung and Tzeng 2019). As the aging duration increased to 4 h (ST+Aging for 4h), the size and distribution of these precipitates became more pronounced, thereby enhancing the hardness and strength of the material. Further aging for 8 h (ST+Aging 8h) continues to coarsen the precipitates, maintaining high strength but slightly reducing toughness. Extended aging for 32 h results in over-aging, where the precipitates become too coarse and incoherent with the matrix, potentially diminishing mechanical properties, such as toughness and ductility (Yeli, Auger et al. 2017).

Fig. 2 Optical microscope images of 17-4 PH SS (a) AR, (b) ST, (c) ST + Aging 1h, (d) ST + Aging 4h, (e) ST + Aging 8h, (f) ST + Aging 32h.

The XRD data in Fig. 3 comprehensively analyzed the phase transformations and formation of precipitates in 17-4PH stainless steel under different material conditions, particularly aging at 480 °C for various durations. All samples were analyzed to obtain t he Bragg angle (2θ) and peak width (FWHM - β) values. In the samples AR, ST, ST+Aging 1h, ST+Aging 4h, ST+Aging 8h, ST+Aging 32h conditions, the XRD pattern reveals a predominantly martensitic α' phase, characterized by diffraction peaks at angles consistent with the BCC structure, specifically at 44.6° (BCC-110), 64.8° (BCC-200), 78.2° (BCC-200), 82.02° (BCC-211), and 98.6567° (BCC-220) and with the emergence of new peaks corresponding to NbCr 2 and Cr 23 C 6 (Koçak, Şimşek et al. 2023).