FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2025 66 N anostructured bainitic steels are a class of high- carbon (C > 0.7 wt%) silicon-alloyed steels that offer an exceptional combination of wear resistance, strength, and toughness, attributed to their nanoscale dual-phase bainitic ferrite/austenite structure[1,2]. This synergy of mechanical properties makes them ideal candidates for high wear service conditions. However, their widespread adoption has been limited due to several processing challenges, including prolonged heat treatment times (spanning several days) required to achieve the desired nanostructure, and the need for precise temperature and cooling rate control to avoid cracking due to their high carbon content. To address the long isothermal heat treatment times required to produce the nanostructure, the authors have combined a Calculation of Phase Diagrams (CALPHAD), machine learning, and data mining approach to develop new alloy compositions of nanostructured bainite, with transformation kinetics that are seven times faster for the onset of bainite formation and two times faster for complete transformation[3]. Powder metallurgy can overcome the challenges associated with conventional processing routes because of its inherently lower heating and cooling rates during sintering and hot isostatic pressing (HIP). Additionally, powder metallurgy can produce near net shape parts to minimize post-fabrication machining while being scalable for production of parts up to 60-in. in diameter. Previous studies by Lonardelli et al.[4,5] have evaluated the application of powder metallurgy for nanostructured bainitic steels using mechanical alloying (MA) followed by consolidation using spark plasma sintering (SPS). Although the desired microstructure was achieved, the PRESSURELESS SINTERING OF PRE-ALLOYED NANOSTRUCTURED BAINITIC STEEL POWDERS A recent study shows that powder metallurgy and pressureless sintering is a viable pathway to realize full density bulk components of nanostructure bainitic steels. Rangasayee Kannan, Andres Marquez Rossy, Jovid Rakhmonov, Yiyu Wang, and Peeyush Nandwana Oak Ridge National Laboratory, Tennessee MA+SPS route presented drawbacks, including powder oxidation during mechanical alloying, which led to incomplete densification and reduced mechanical properties. Additionally, the scalability of the SPS approach is limited. In this study, a scalable, pressureless supersolidus liquid phase sintering (SLPS) approach to process nanostructured bainitic steel is evaluated. To address the oxidation issues associated with mechanical alloying of elemental powders, powder atomization is used to produce pre-alloyed nanostructured bainitic steel powders. POWDER ATOMIZATION Nanostructured bainitic alloy designed to have faster transformation kinetics with composition shown in Table 1 was first arc melted into a rectangular bar ingot with 1-in. cross-section and 5.5-in. length[3]. The ingot was atomized into powders using AMAZEMET rePowder Ultrasonic (40 kHz frequency) atomizer under Ar atmosphere. Figure 1 shows the obtained powder surface chemistry, powder morphology, and powder size distribution. Figures 1a and 1b show that the powders are spherical and do not contain any satellites highlighting the efficacy of the ultrasonic atomization process. The EDS elemental maps in Fig. 1c show that the main elements, i.e., Fe, Si, Co, and Al are homogeneously distributed in the powder particles. Oxygen content in the powder (estimated by inert gas fusion ASTM E 1019-18) was 0.011 wt% indi- cating minimal oxidation during the atomization process. The oxygen content is an order of magnitude lower than that obtained via the MA approach used in the literature[4,5], indicating that atomization of pre-alloyed ingots is a viable approach to fabricate nanostructure bainitic steel powders with minimum oxygen contamination. The TABLE 1 — CHEMICAL COMPOSITION OF THE DEVELOPED NANOSTRUCTURED BAINITIC STEEL, wt% C Si Mn Mo Cr V Al Co Ni 0.901 2.561 1.171 0.633 1.140 0.251 2.700 3.675 1.006 12
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