Effects of N/C Ratio on Solidification Behaviors of Novel Nb-Bearing Austenitic Heat-Resistant Cast Steels for Exhaust C
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IN recent years, more stringent environmental regulations on automotive industries have been promulgated worldwide to reduce exhaust gas emissions.[1] To comply with these regulations, engine downsizing is increasingly applied to automotive gasoline engines,[2] whereas the downsizing technology requires the installation of turbochargers to improve engine power and reduce fuel consumption.[1,3] With this technology, the exhaust gas temperature is now being pushed towards 1,323 K (1,050 C), about 200 K (200 C) higher than the conventional temperature.[3–5] As a result, novel and economic alloys that are durable against this temperature are urgently demanded from automotive industries. Nb-bearing austenitic heat-resistant cast steels are competitive candidates for developing novel alloys, due to their enhanced corrosion and creep properties at YINHUI ZHANG is with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China. MEI LI, LARRY A. GODLEWSKI, and JACOB W. ZINDEL are with the Ford Research and Advanced Engineering Laboratory, Ford Motor Company, Dearborn, MI 481244356. QIANG FENG is with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing and also with the Beijing Key Laboratory of Special Melting and Reparation of High-End Metal Materials, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: qfeng@ skl.ustb.edu.cn Manuscript submitted June 26, 2016. Article published online January 5, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
temperatures around 1,073 K (800 C).[6,7] The minor modification of C and N additions to austenitic steels can both improve the NbC/Nb(C,N) precipitation strengthening and stabilize the austenitic matrix, thus beneficial to novel alloy design.[8,9] In our previous research, a series of Nb-bearing austenitic heat-resistant cast steels were designed and investigated based on variations of N/C ratio.[10] The as-cast microstructure of these alloys exhibited a significant dependence on N/C ratio, and thereby three alloy models were established based on the morphology of NbC and Nb(C,N): script, flake-blocky, and blocky. Correspondingly, these three alloy models exhibited distinct creep properties at 1,273 K (1,000 C) and 50 MPa. In particular, the alloys with ‘‘Chinese-script’’ NbC/Nb(C,N) illustrated much superior creep properties than those with blocky and flake-blocky Nb(C,N). Therefore, the precipitation of phases (NbC/Nb(C,N), d-ferrite, etc.) with different morphologies and distributions was the key to improving creep properties of these alloys at 1,273 K (1,000 C). However, the formation mechanism of different phases as a function of N/C ratio in the as-cast microstructure of these alloys remains unclear. In contrast to extensive studies on the solidification behavior of conventional austenitic and duplex stainless steels focusing on c-austenite and d-ferrite,[11–14] very limited researches have been conducted to investigate the soli
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