PSI - Issue 68

Swastik Soni et al. / Procedia Structural Integrity 68 (2025) 513–519 S. Soni et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 1. Microstructure of Modified P91 steel (Nital 5% etched)

2.1. Tensile testing Tensile tests were performed on cylindrical specimens of Mod 9Cr-1Mo steel with a gauge length of 25 mm and diameter of 5 mm. The tests were conducted at strain rates ranging from 3x10 -4 per seconds to 1.8x10 -2 s -1 and temperatures ranging from -70°C to -20°C to cover the DBT regime. The tensile testing was 2.2. Fractography After the tensile tests, the fracture surfaces were analyzed using scanning electron microscopy (SEM) to measure the dimple sizes. Image analysis software was employed to obtain quantitative data on the average dimple diameters for each test condition. Additionally, the fractal dimensions of the fracture surfaces were computed using the box counting method. The fractal dimension is an indicator of the complexity of the fracture surface, with lower values associated with smoother, brittle surfaces and higher values corresponding to rougher, ductile surfaces. 3. Results and Discussion 3.1. Tensile properties in DBT region It is expected and known that with increasing strain rate the yield strength in metallic material increases and it also increases with decreasing temperature, which has been described by Zener Hollomon parameter (Zener & Hollomon, 1944). However, it is also expected that the scatter which is observed in the fracture behavior in impact or fracture tests are not a part of yield strength. In tensile behavior however, post necking strain and UTS may show influence of scatter due to statistical random distribution of cleavage triggers. The recent advancement in the theory of ductile to brittle transition regime in ferritic steels have shown and discussed that the rate controlling event in DBT regime for ferritic steels is the mobility of screw dislocations which is controlled by the double kink pair nucleation and movement of dislocation in the Peierl’s valley, where the dislocation velocity follows and exponential relation with the ratio of shear stress and temperature (Barik et al. 2023, Hähner and Stamm, 1995). There are two major events which occur in DBT regime. The first one is the rise in deformation ability with increase in temperature from lower shelf to upper shelf and the second is the scatter usually seen in fracture and impact data. The first phenomena can be described by the temperature dependence of dislocation mobility. The second phenomena should not be affecting the yielding, however, the post necking fracture related parameters should be influenced by the distribution of cleavage initiators. The dependence of yield strength with change in temperature and strain rate is shown in Fig. 2. The experimental results show that the yield strength increases with increase in strain rate upto 6.7x10 -3 s -1 and after than remains constant upto 1.8x10 -2 s -1 . The yield strength shows as expected a decreasing trend with increasing temperature. The behavior

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