FEATURE ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 47 requires periodic furnace shutdown and clean-up of the roll surface to remove the nodules and scales picked up from the steel plates. This leads to frequent shutdowns resulting in energy losses due to shutdowns and startups, increased operating costs due to higher roll maintenance costs, reduction of the product throughput capability, and higher than desirable emissions. Thus, there was an urgent need to develop new components such as furnace rolls, and other furnace hardware fabricated from alloys with excellent oxidation resistance along with good mechanical strength and creep resistance at temperature. The components highlighted in this paper have demonstrated exemplary performance and hence were selected for the Engineering Materials Achievement Award in 2023. ALUMINA-FORMING ALLOYS The key to success of the new furnace rolls is the use of a novel alloy with an ability to form an alumina scale instead of a chromia scale. Alumina typically grows at a rate 1 to 2 orders of magnitude slower than chromia and is significantly more thermodynamically stable in oxygen, resulting in superior high-temperature oxidation resistance in many environments. A further advantage of alumina over chromia is its greater stability in the presence of water vapor. Water vapor is encountered in many high- temperature energy conversion, combustion, and process industrial environments. With both oxygen and water vapor present, oxidation lifetime can decrease significantly due to the formation of volatile chromium oxy-hydroxide species, thus decreasing the allowable operating temperatures. Formation of alumina scales instead of chromia scales minimizes oxide pickup thus enabling higher product quality. Alloy developers worldwide have struggled over the past 40 years to create creep-resistant alumina-forming, iron-based austenitic stainless steels for use as high temperature structural alloys, but with limited success in balancing alloy cost, oxidation, and creep resistance. This team overcame this hurdle by identifying composition ranges that allowed the formation of a stable alumina-scale, learned from many years of studying mechanistic effects of alloy compositions on oxidation and creep resistance. The critical elemental ratios for achieving a stable austenitic alloy with good oxidation and creep resistance were co-optimized via computational thermodynamic modeling, with the alumina scale providing oxidation resistance and carbides (along with solid solution strengthening) providing the creep resistance[1-5]. With funding from ARPA-E, alloys designed using computational modeling approaches were prepared in small laboratory scale heats at Oak Ridge National Laboratory (ORNL) and their oxidation and creep properties were evaluated at temperatures in the 900° to 1150°C range. Alloy compositions were refined iteratively until composition ranges resulting in the desired combination of oxidation resistance, creep resistance, and micro- structural stability were identified using laboratory scale testing. Table 1 shows the typical composition range for HP-Nb modified, a commonly used cast alloy that forms a chromia-scale, and the new cast alumina-forming austenitic (AFA) alloy shows the range of alloy compositions that is required to achieve the required combination of oxidation resistance and creep properties[2]. Promising compositions were scaled up by industrial partners Duraloy Technologies and MetalTek International and several trial centrifugal castings were produced. The key step in technology scale-up was achieving the control of trace elements in industrial scale heats. Feedstock compositions had to be carefully specified and special manufacturing processes had to be developed to achieve tight control on trace elements and impurities shown in Table 1. Nitrogen levels had to be kept low to minimize the formation of undesirable AlN inclusions, and one or more trace elements from Y, La, Ce, Hf, and Zr were added to achieve optimal oxidation resistance from these alumina-forming alloys. Materials fabricated both in 10 TABLE 1 — NOMINAL COMPOSITION RANGES OF ALLOYS USED IN FURNACE ROLLS[2], wt% Alloy Fe Ni Cr Al Nb Si Mo W C Others HP-Nb Bal. 35 25 0 1.0 1.0 0 0 0.45 Cast AFA for furnace rolls Bal. 33-36 23-27 3.5-4.5 0.751.25 0.00.75 0-2 0-2 0.3-0.5 S, P, N, Cu, Mn must be kept as low as possible. Intentional additions of trace amounts of one or more of Y, La, Ce, Hf, and Zr are required for improved oxidation resistance.
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