PSI - Issue 64

Maryam Mohri et al. / Procedia Structural Integrity 64 (2024) 376–383 M.Mohri et al./ Structural Integrity Procedia 00 (2019) 000–000

379

4

3. Results and discussion 3.1. Microstructural characterization

SEM analysis was employed to investigate the impact of heat treatment on the microstructure. Fig. 2 displays SEM images of the specimens, depicting the as-received, after undergoing a two-step aging process at 600°C for 20 hours followed by aging at 680°C for 8 hours and after thermomechanical training. In Fig. 2a, it is evident that the average grain size in the as-received specimen was approximately 13 μm, with some grains exhibiting large bands identified as thermal twins (Fig. 2a). These twins are indicative of boundaries with low grain boundary energy. Additionally, fine bands, assumed to be ε -martensite, were observed within some grains, likely formed during the grinding process. Following aging, elongated Cr and/or V carbides, measuring between 100 and 300 nm, precipitated inside the grains and along the grain boundaries (Fig. 2b). After training, parallel lines are present inside the grains. These may be ε martensite, twins, or slip bands (Fig. 2c), which are characteristic of cold-worked austenitic steels. (a) (b)

(c)

Fig. 2. SEM images of a) As-received, (b) Heat-treated and (c) trained samples.

3.2. Mechanical behavior The nonlinear deformation behavior observed after elastic deformation during loading is attributed to martensitic transformation followed by plastic deformation. According to Lee et al. (2013), a 0.01% yield stress is deemed more appropriate than a 0.2% yield stress for determining the onset of stress for martensite transformation. Therefore, a yield stress of 0.01% ( 0 . 01% ) is employed in this investigation. Figure 3a depicts the tensile test results up to a 4% strain for the as-received, heat-treated and trained (after 3 cycle training) samples. Table 1 lists the mechanical properties, including the SE ( ε se ), Young’s modulus, and yield stress, derived from Fig.3a. Figure 3b shows the stress strain curves of heat treated samples, which were subjected to the training cyclic. In each cycle, sample was conducted