PSI - Issue 71

298 Johnny Adukwu et al. / Procedia Structural Integrity 71 (2025) 295–301 where, r avg =(r Punch +r Die )⁄2 is the average radius of the punch and die, P is the load applied. 3. Results and Discussion 3.1. Microstructure of heat treated 4340 and 300M

Figure 1(a) and (b) shows representative Secondary electron SEM images of the heat treated 4340 and 300M steels while Figure 1(c) and (d) depict EBSD inverse pole figure (IPF) maps with reconstructed prior austenite grains (PAGs) perpendicular to the loading direction (LD) of the punch.

Fig. 1. (a,b) Secondary electron SEM Images and (c,d) EBSD inverse pole figures (IPF) of 4340 and 300M steels respectively . Microstructural features of the subject steels are measured via Image J software package. PAGs of 4340 and 300M have an average size of 10.89 μ m and 10.80 μ m, respectively. The effect of the heat treatment conditions and composition difference on average PAGs grain size between the two steels is not significant. Both the SEM and EBSD maps show a tempered martensitic microstructure in both grades. Spherical precipitates, (160 to 530 nm in size) for 4340 and (120 to 240 nm in size) for 300M, are observed along the martensitic lath microstructure (insert of Figs 1(a) and (b)) in both steels. EDS Maps (Figure 2 (a) and (b)) reveal that spherical precipitates within the lath microstructure are enriched in Cr, Mn, Mo, and Si (shown by the dashed circles).

Fig. 2. Secondary electron SEM images and EDS showing distributed precipitates on the martensitic microstructure of (a) 4340 and (b) 300M