FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2025 68 conditions that resulted in excess liquid formation and distortion, solidification induced segregation regions can be observed (black arrow marks in Figs. 3c, 3e, and 3f). Figure 3g shows that for all sintering conditions the density is >99% and most of the pores are less than 50 µm in diameter. The normalized pore size distribution shows that the sample sintered at 1275°C for 5 hours has higher fraction of larger pores which reduce with increasing sintering temperature and time. Based on the results presented in Fig. 3, it can be concluded that sintering at 1275°C for 5 hours and 1300°C for 2 hours results in an optimum combination of density with minimum shape loss. Samples sintered at 1275°C for 5 hours and 1300°C for 2 hours were austenitized at 1000°C for 1 hour following which isothermal heat treatment at 200°C for 48 hours to form nanostructured bainite. Figure 4 shows the optical and secondary electron SEM image of sintered + heat treated sample showing the formation of nanostructured bainite. Micron sized pores, indicated by black arrow marks can also be seen in Figs. 4a and 4b. Secondary electron SEM images in Figs. 4c and 4d show bainitic ferrite with fine scale thin film austenite (needle-like features within bainitic ferrite) and the untransformed blocky austenite (regions marked γ). CONCLUSION This study shows that atomization of pre-alloyed nanostructured bainitic steel ingots is a viable method to produce nanostructured bainitic steel powders with minimum oxygen contamination (0.011 wt%) unlike mechanical alloying, which results in powders with significant amounts of oxygen (0.36 wt%). A CALPHAD based approach to SLPS results in near full density parts (>99%) in the sintered condition with minimal distortion. Most of the pores are <50 µm and can be potentially mitigated via HIP. Subsequent heat treatment of the sintered samples results in micro- structure and hardness comparable with nanostructured bainitic alloy processed via conventional approach. These results demonstrate that powder metallurgy is a viable pathway to realize full density sintered bulk components of nanostructure bainitic steels. ~HTPro Fig. 3 — (a) Macroscopic images of the sintered samples, (b)-(f) XCT cross-sections of samples sintered at 1275°C for 5 hours, 1300°C for 5 hours, 1300°C for 2 hours, 1325°C for 2 hours, and 1350°C for 2 hours respectively, and (g) porosity size distribution along with volume % of porosity for the sintered samples. The black arrow marks in (c), (e), and (f) indicate potential segregation regions due to liquid phase formation. Fig. 4 — (a) and (b) indicate optical micrographs of samples sintered at 1275°C for 5 hours and 1300°C for 2 hours followed by austenitization and isothermal bainite treatment at 200°C respectively. (c) and (d) indicate secondary electron SEM micrographs of samples sintered at 1275°C for 5 hours and 1300°C for 2 hours followed by austenitization and isothermal bainite treatment at 200°C respectively. The black arrow marks indicate micron-sized pores in the sample. (a) (b) (c) (d) (a) (c) (b) (d) (f) (e) (g) 14
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