Effects of Cr Reduction on High-Temperature Strength of High-Ni Austenitic Cast Steels Used for High-Performance Turbo-c
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INTRODUCTION
AUSTENITIC cast steel have been applied to heat-resistant turbo-chargers because their cast components provide an excellent opportunity for high-performance castings.[1–6] They demonstrate excellent high-temperature strengths with a stable microstructure and excellent thermal fatigue properties. These high-alloyed types of steel containing Ni, Cr, and W (or Nb) for a stabilized matrix enhanced the corrosion resistance, hardness, and carbide formation, respectively, to sustain turbo-chargers at above high temperatures of exhaust gases of 1223 K (950 °C).[7–21] To use turbo-chargers at further higher exhaust temperatures such as 1323 K (1050 °C), better high-temperature strength is required for maintaining JISUNG YOO, WON-MI CHOI, SEOK SU SOHN, BYEONGJOO 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] GI-YONG KIM is with the Research and Development Center, Key Yang Precision, Gimcheon, 740-180, Korea. YONG-JUN OH is with the Department of Advanced Materials Engineering, Hanbat National University, Daejeon, 305-719, Korea. Manuscript submitted November 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
turbo-charger structures. ASTM HK40 steel, which is a high-Ni austenitic steel commercially used for heat-resistant steam tubes or reformers in petrochemical industrial areas,[22–27] has been presented as an excellent turbo-charger candidate material.[26,28] This steel (composition; 0.4C-1.0Mn-1.2Si-25Cr-20Ni (wt pct)) contains a large amount of expensive Ni (20 wt pct) to obtain a high austenite stability even at 1323 K (1050 °C), while strong carbide-forming elements (Nb and W) are omitted to control the volume fraction of the various carbides at a minimal level.[22–27] Recently, Jung et al.[28] achieved excellent high-temperature strength compared to that of ASTM HK40 steel by adding Mo, while approximately 6 wt pct Ni was replaced by 6.9 wt pct of another inexpensive austenite former, Mn. This also led to a cost reduction of approximately 10 pct in alloying elements. In this study, three types of austenitic cast steel were fabricated by controlling Cr content in HK40 steel (25 wt pct Cr), and high-temperature strength was improved by optimizing the carbide formation and strengthening the austenite matrix. For a detailed microstructural evolution study, fractions of equilibrium phases existing at high temperatures were estimated by thermodynamic calculations, and were compared with the measured
Table I.
Nominal Compositions of the Austenitic Cast Steels (Weight Percent)
Steel Specimen
C
Si
Mn
P
S
Ni
Mo
Cr
Fe
HK40 N14 N14Mo1 N14Mo1Cr-2 N14Mo1Cr-4
0.4 0.4 0.4 0.4 0.4
1.2 1.2 1.2 1.2 1.2
1.0 7.9 7.9 7.9 7.9
0.04 0.04 0.04 0.04 0.04
0.04 0.04 0.04 0.04 0.04
20 14 14 14 14
— — 1.0 1.0 1.0
25 25 25 23 21
bal. bal. bal. bal. bal.
Fig. 1—Fractions of equilibrium phases present in the temperature range of 673 K to 1673 K (400 °C to 1400 °C) of the (a) N14M
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