Replacement of Ni by Mn in Commercial High-Ni Austenitic Cast Steels Used for High-Performance Turbocharger Housings
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As austenitic cast steels show excellent strengths and thermal fatigue properties above a typical exhaust-gas temperature of 1223 K (950 °C),[1–3] they have been used as turbocharger housing materials. They generally contain considerable amounts of alloying elements such as Cr, Ni, Nb, and W for the austenite stabilization, carbide formation, and improvement of high-temperature properties and corrosion resistance.[4–15] In order to use the austenitic cast steels at still further higher temperature, for example, 1323 K (1050 °C), with the increasing engine performance, high-temperature properties need to be more enhanced.
JISUNG YOO, WON-MI CHOI, BYEONG-JOO LEE, and SUNGHAK LEE are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790784, Korea. Contact e-mail: [email protected] GI-YONG KIM and HYUNGJUN KIM are with the Research and Development Center, Key Yang Precision, Gimcheon 740-180, Korea. WON-DOO CHOI and YONG-JUN OH are with the Department of Advanced Materials Engineering, Hanbat National University, Daejeon 305-719, Korea. Manuscript submitted September 12, 2018. Article published online April 10, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
A high-Ni heat-resistant austenitic steel, e.g., ASTM HK40, which is commercially used for high-temperature steam-reformers,[16,17] was suggested as a candidate material for turbocharger housings. In this steel, the Ni content reaches 20 wt pct to achieve the high austenite stability at 1323 K (1050 °C). However, a large amount of expensive Ni increases cost of the steel. Jung et al.[18] achieved an excellent high-temperature strength in the HK40 steel by a partial replacement of Ni to Mn which is another austenite former, together with the addition of Mo. Recently, a high-Ni austenitic steel with the brand name, DIN 1.4849 (0.4C-2.0Mn-1.5Si-20Cr-38Ni-1.5Nb, with a small amount of S and P (wt pct)) has been commercially used for high-performance turbocharger housings because of its excellent oxidation resistance and thermal fatigue properties.[19,20] This DIN 1.4849 steel contains a large amount of Ni (38 wt pct) for the sufficient stabilization of austenite matrix, and a small amount of Nb (strong carbide former) is present to control carbides. Here again, a partial replacement of Ni with Mn in this DIN 1.4849 steel is needed for favorably reducing alloying costs (up to 20 to 25 pct) without losing high-temperature properties. Six high-Ni austenitic cast steel ingots were fabricated by a vacuum-induction melting route (under Ar atmosphere). A master alloy (weight; 150 g) molten at 1773 K (1500 °C) was poured into a graphite module (inner size; 10093598 mm). The ‘DIN 1.4849’ steel was made as a reference cast steel, and is referred to as ‘DN38,’ based on its Ni content (38 wt pct). 6, 9, 12, 14, and 16 wt pct of Ni were replaced by 6.9, 10.4, 13.8, 16.1, and 18.4 wt pct of Mn, respectively, to produce ‘DN32,’ ‘DN29,’ ‘DN26,’ ‘DN24,’ and ‘DN22’ steel specimens (Table I). This alloy designing was based on Mn/Ni rat
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