EFFECTS OF N ON CREEP PROPERTIES OF AUSTENI-TIC HEAT-RESISTANT CAST STEELS DEVELOPEDFOR EXHAUST COMPONENT APPLICATIONSAT 1000 ℃

doi:10.11900/0412.1961.2015.00618

Acta Metall Sin

2016, Vol. 52

Issue (6): 661-671 DOI: 10.11900/0412.1961.2015.00618

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EFFECTS OF N ON CREEP PROPERTIES OF AUSTENI-TIC HEAT-RESISTANT CAST STEELS DEVELOPEDFOR EXHAUST COMPONENT APPLICATIONSAT 1000 ℃

Yinhui ZHANG¹,Mei LI²,Larry A GODLEWSKI²,Jacob W ZINDEL²,Qiang FENG^1,³(

)

1 State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
2 Ford Research and Advanced Engineering Laboratory, Ford Motor Company, Dearborn 48124-4356, USA
3 Beijing Key Laboratory of Special Melting and Reparation of High-End Metal Materials, University of Science and Technology Beijing, Beijing 100083, China

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Yinhui ZHANG,Mei LI,Larry A GODLEWSKI,Jacob W ZINDEL,Qiang FENG. EFFECTS OF N ON CREEP PROPERTIES OF AUSTENI-TIC HEAT-RESISTANT CAST STEELS DEVELOPEDFOR EXHAUST COMPONENT APPLICATIONSAT 1000 ℃. Acta Metall Sin, 2016, 52(6): 661-671.

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Abstract

To comply with more stringent environmental and fuel consumption regulations in recent years, automotive gasoline engines equipped with turbochargers are increasingly used to improve fuel efficiency. As a result, exhaust gas temperatures are now reaching 1050 ℃, about 200 ℃ higher than the conventional temperature. Hence, there is an urgent demand in automobile industries to develop novel and economic austenitic heat-resistant steels that are durable against these increased temperatures. In this study, the effects of N addition on creep behavior at 1000 ℃ and 50 MPa are investigated in a series of Nb-bearing austenitic heat-resistant cast steels, which are developed for exhaust component applications. Microstructures before and after creep rupture tests are carefully characterized to illustrate the microstructural evolution during creep deformation. The results of creep tests show approximately an order of magnitude increase in the minimum creep rate among the experimental alloys with variations of N addition. Microstructural analyses indicate that the morphology of NbC and Nb(C, N) is changed from “Chinese-script” to mixed flake-blocks, and then to faceted blocks as N additions increase. The best creep property occurs in an alloy with “Chinese-script” NbC, which could effectively strengthen the grain boundaries and interdendritic regions. The Cr-rich phases are adverse to creep properties, in particular those coarsened and coalesced phases along grain boundaries. They act as crack sources and accelerate the propagation of creep cracks. Moreover, the secondary precipitation of Cr-rich phase results in a significant decrease of C concentration in the matrix and thus reduces the solution strengthening ability during creep deformation. This study suggests that the strengthening of these austenitic cast steels can be achieved through the exploit of primary NbC and Nb(C, N) and the elimination of Cr-rich phases, and therefore, N additions should be strictly controlled.

Key words: gasoline engine austenitic heat-resistant cast steel creep Nb(C N) carbonitride solidification

Received: 03 December 2015

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	Yinhui ZHANG
	Mei LI
	Larry A GODLEWSKI
	Jacob W ZINDEL
	Qiang FENG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00618 OR https://www.ams.org.cn/EN/Y2016/V52/I6/661

Table 1 Measured chemical compositions of the experimental alloys

Fig.1 Creep strain curve (a) and creep strain rate curve (b) of the experimental alloys under 1000 ℃ and 50 MPa

Fig.2 OM images of typical as-cast microstructures in alloys 4C0N (a), 4C3N (b) and 4C5N (c)

Table 2 Creep properties of the experimental alloys tested at 1000 ℃ and 50 MPa

Fig.3 SEM-BSE images of typical as-cast microstructures in alloys 4C0N (a), 4C3N (b) and 4C5N (c) (Inset in Fig.3a shows the SEM-SE image of the squared area at higher magnification)

Table 3 Area fractions of precipitates and Nb(C, N) number density at grain boundaries in the as-cast microstructure of the experimental alloys

Fig.4 EPMA-BSE image (a) and compositional maps for Cr (b), C (c), N (d), Fe (e) and Nb (f) of Cr-rich phase and Nb(C, N) in alloy 4C5N

Fig.5 EBSD maps show the as-cast grain size in alloys 4C0N (a), 4C3N (b) and 4C5N (c)

Fig.6 SEM-BSE images of typical microstructures after creep rupture tests at 1000 ℃ and 50 MPa in alloys 4C0N (a), 4C3N (b) and 4C5N (c), SEM-SE image of the squared area in Fig.6c at higher magnification (d)

Fig.7 Vickers hardness of the austenitic dendrites in the experimental alloys before and after creep rupture tests at 1000 ℃ and 50 MPa

Fig.8 SEM-BSE images (unetched) after creep rupture tests at 1000 ℃ and 50 MPa in alloys 4C0N (a), 4C3N (b) and 4C5N (c)

Fig.9 Calculated phase diagrams show the temperature-dependent fractions of equilibrium phases formed in alloys 4C0N (a), 4C3N (b) and 4C5N (c)

Table 4 Average chemical compositions of γ austenite in the experimental alloys determined by EPMA

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