Replacement of Ni by Mn in High-Ni-Containing Austenitic Cast Steels used for Turbo-Charger Application
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Heat-resistant austenitic cast steels have been used for automotive turbo-chargers because they have excellent hardness, strength, thermo-mechanical fatigue life, and fracture toughness as well as thermally stable microstructure. They usually contain Cr for increasing strength and resistance to corrosion and oxidation, Ni for stabilization of austenite, and W and Nb for carbide formation to sustain turbo-chargers at high exhaust gas temperatures up to 1223 K (950 C).[1–9] As demands for more excellent sustainability are needed, a standard Fe-Cr-Ni-based cast steel [main composition;
SEUNGMUN JUNG, YONG HEE JO, CHANGWOO JEON, WON-MI CHOI, BYEONG-JOO LEE, and SUNGHAK LEE are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. Contact e-mail: [email protected] YONG-JUN OH is with the Department of Advanced Materials Engineering, Hanbat National University, Dajeon, 305-719, Korea. GI-YONG KIM and SEONGSIK JANG are with the Research and Development Center, Key yang Precision, Gimcheon, 740-180, Korea. Manuscript submitted September 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
0.4C-1.2Si-1.0Mn-20Ni-25Cr (wt pct)], viz., ASTM HK40, which are commercially used for very high-temperature applications up to 1323 K (1050 C) in petrochemical industries,[10–14] has been suggested as a promising candidate cast steel for high-performance turbo-chargers.[4,15,16] In this steel, a very high content (20 wt pct) of Ni is included to maintain the high stability of austenite at 1323 K (1050 C), and carbide formers such as W and Nb are removed to minimize the carbide formation.[1–9] Thus, a partly replacement of expensive Ni to inexpensive Mn, another austenite former, is needed, while fully taking advantages of very high austenite stability.[17] The present design of heat-resistant high-Ni-containing austenitic cast steels was focused on how a part of Ni was replaced by Mn in order to achieve cost reduction, while excellent high-temperature tensile properties were maintained. Four austenitic cast steels were fabricated by replacing a part of Ni by Mn, and room- and high-temperature tensile properties were investigated. In addition to the experimental approach, fractions of phases were verified by thermodynamic calculations, and the quantitative data were compared with the experimentally obtained data to effectively evaluate the high-temperature performance. Small buttons (weight; ~100 g) of austenitic cast steels were made by an arc-melting method.[18] The basic ‘HK40’ steel is referred to as ‘N20,’ whose composition is 0.4C-1.2Si-1.0Mn-20Ni-25Cr-0.04P-0.04S (wt pct). 4, 6, and 8 wt pct Ni was replaced by 4.6, 6.9, and 9.2 wt pct of Mn, based on a Mn/Ni ratio of 1.15 in consideration of Ni equivalent (Nieq=Ni + 30C + 0.87Mn 0.33Cu + 30(N - 0.045)) in a modified Schaeffler diagram[19] to produce ‘N16,’ ‘N14,’ and ‘N12’ steels, respectively. Steel samples were polished and etched in an etchant of 10 g K3Fe(CN)6 + 10 g KOH + 100 mL H2O, and their microstructures w
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