FEATURE ADVANCED MATERIALS & PROCESSES | MAY/JUJNE 2024 48 Fig. 1 — Specific mass change vs time at 900°C in air + 10% H2O (500 hr cycles) for chromia-forming HP and alumina-forming alloys obtained in laboratory testing[2]. laboratory scale castings and industrial centrifugal castings were tested for their oxidation resistance in the laboratory at a range of temperatures between 900° and 1200°C in air + 10% water environment. Creep resistance was evaluated over the same temperature range using constant load creep tests. Subsequently, coupons were exposed to the typical furnace environment in the heat-treat furnace at the Cleveland-Cliffs facility in Coatesville, Pennsylvania. Figure 1 shows the change in the mass/unit area at 900°C in an air + 10% water vapor environment in laboratory tests conducted over 7000-9000 hr, for samples from a conventional centrifugally cast HP alloy, a laboratory cast new AFA alloy, and the centrifugal cast, scaled-up commercial version of the new AFA alloy[2]. The chromia-forming HP alloy shows a significant mass loss indicative of spallation and evaporation of the chromia-rich oxide scale under these testing conditions, followed by rapid Fe/Ni based oxide formation, spallation, and significant mass loss. In comparison, low positive mass change is observed in the cast AFA alloys (both in the laboratory scale heat and industrial scale heat) consistent with the formation of a slow-growing, protective aluminum oxide scale. Figure 2 shows differences in the oxides formed on a chromia-forming HP alloy and the oxide formed on the new developed cast alumina forming alloy after 3000 hr exposure in the industrial furnace environment at Cleveland-Cliff’s Coatesville, Pennsylvania steel plant. Note that the HP alloy shows a thicker duplex oxide layer with some evidence of internal attack. In contrast, the laboratory cast AFA alloy shows the presence of a thin oxide layer with very little internal attack as highlighted in Fig. 2b-d. O, Al, Cr maps from the near surface region from the laboratory cast ORNL AFA alloy subject to ~3000 hr exposure confirm the presence of a continuous, thin layer of the desired Al-rich oxide phase which was identified as an α-Al2O3 rich phase using x-ray diffraction. This figure shows the advantage of the cast alumina-forming alloy over the cast chromia-forming alloy in oxidation behavior. An ~8-10× increase in oxidation resistance as indicated by decrease in base material thickness loss was measured under these conditions. TESTING AND COMMERCIALIZATION Creep tests performed at Oak Ridge National Laboratory on laboratory cast AFA and at Duraloy Technologies on centrifugally cast AFA showed excellent performance of the ORNL AFA alloy. Based on the excellent combination of properties achieved in the AFA alloy, prototype rolls were fabricated by Duraloy Technologies and by MetalTek International and their performances were validated in Cleveland-Cliffs’s Coatesville plant from 2017 to 2020, leading to their larger scale commercial production and sale. Figure 3 shows an image of AFA rolls fabricated by Duraloy Technologies using ORNL’s AFA under license from ORNL[5] and sold under the trademark TMA 6350 waiting to be installed in the new furnace commissioned by Cleveland-Cliffs in 2022. 11 Fig. 2 — Optical image of a cross-section from industrially cast conventional HP alloy a er ~3000 hr exposure in Zone 6 inside the industrial austenitizing furnace at the Cleveland-Cli s Coatesville plant. (b) Backscattered SEM Image, (c) O K, (d) Al K maps of the surface region in laboratory cast AFA alloy a er ~3000 hr exposure in industrial environment showing the formation of a thin alumina scale[2]. (a) (b) (d) (c)
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