Effects of W on Creep Behaviors of Novel Nb-Bearing Austenitic Heat-Resistant Cast Steels at 1000 ℃

doi:10.11900/0412.1961.2017.00002

Acta Metall Sin

2017, Vol. 53

Issue (9): 1025-1037 DOI: 10.11900/0412.1961.2017.00002

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Effects of W on Creep Behaviors of Novel Nb-Bearing Austenitic Heat-Resistant Cast Steels at 1000 ℃

Yinhui ZHANG, Qiang FENG

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China

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Yinhui ZHANG, Qiang FENG. Effects of W on Creep Behaviors of Novel Nb-Bearing Austenitic Heat-Resistant Cast Steels at 1000 ℃. Acta Metall Sin, 2017, 53(9): 1025-1037.

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Abstract

In order to comply with more stringent emissions and fuel economy regulations worldwide, the operation temperature of exhaust components for automotive gasoline engines is now reaching to as high as 1000 ℃, about 200 ℃ higher than the conventional standard. As a result, the incumbent materials for exhaust manifolds and turbine housings are being pushed beyond their high-temperature strength and oxidation limitations. Therefore, there is an urgent demand from automotive industries to develop novel and cost-effective alloys those durable against these increased temperatures. In this work, the effect of W additions on the creep behavior of a series of Nb-bearing austenitic heat-resistant cast steels is investigated at 1000 ℃ and 50 MPa. Microstructures before and after creep rupture tests are carefully characterized to investigate the microstructural evolution during creep deformation. The minimum creep rate of these alloys shows a trend from decline to rise as the W addition is increased. Microstructural analyses reveal that the W addition does not affect the formation of primary Nb(C, N), whereas significantly improves the precipitation of Cr-rich carbides, as well as accelerating the phase transformation from (Cr, Fe, W)₇C₃ to (Cr, Fe, W)₂₃C₆. Moreover, the excessive addition of W leads to the formation of the interme tallic χ-phase. During creep deformation, the secondary precipitation of nano-scale Nb(C, N) also aids in the strengthening of the creep resistance through pinning the dislocations. However, the cellular Cr-rich phase that contains χ-phase significantly accelerates the nucleation and propagation of creep cracks, thereby increasing the creep rate and decreasing the creep life.

Key words: automotive gasoline engine, austenitic heat-resistant cast steel creep χ;-phase solid solution strengthening

Received: 03 January 2017

ZTFLH:	TG146.1
	TG113.2

Fund: Supported by Ford China University Research Program and Fundamental Research Funds for the Central Universities (No.FRF-IC-16-005)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00002 OR https://www.ams.org.cn/EN/Y2017/V53/I9/1025

Table 1 Measured chemical compositions of the alloys with different W contents (mass fraction / %)

Fig.1 Creep strain-time curves (a) and creep strain rate-time curves (b) of the as-cast alloys under 1000 ℃ and 50 MPa

Table 2 Average creep properties of the as-cast alloys tested at 1000 ℃ and 50 MPa

Fig.2 OM images of typical microstructures in as-cast 4C3N (a), 4C3N1W (b), 4C3N3W (c), 4C3N5W columnar (d) and 4C3N5W (e) alloys

Fig.3 SEM-BSE images of typical microstructures in as-cast 4C3N (a), 4C3N1W (b), 4C3N3W (c), 4C3N5W columnar (d) and 4C3N5W (e) alloys, and high magnification SEM-SE image of the area in Fig.3e (f)

Fig.4 XRD spectrum of the extracted precipitates in as-cast 4C3N5W alloy

Fig.5 EBSD maps showing the as-cast grain size in as-cast 4C3N3W (a) and 4C3N5W columnar (b) alloys

Fig.6 SEM-SE images of typical microstructures after creep rupture tests at 1000 ℃ and 50 MPa in as-cast 4C3N (a), 4C3N1W (b), 4C3N3W (c), 4C3N5W columnar (d) and 4C3N5W (e) alloys, and high magnification SEM-BSE image of the area in Fig.6e (f)

Fig.7 XRD spectrum of the extracted precipitates in as-cast 4C3N5W alloy after creep test at 1000 ℃ and 50 MPa

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

Table 4 Average chemical compositions of γ austenite in the experimental alloys determined by EPMA(mass fraction / %)

Fig.8 Vickers hardness of the austenitic matrix in the as-cast alloys before and after creep rupture tests at 1000 ℃ and 50 MPa

Fig.9 Bright-field TEM images of the interdendritic (a) and dendritic (c) microstructures in as-cast 4C3N1W alloy after creep ruptured at 1000 ℃ and 50 MPa, and typical SAED patterns of (Cr, Fe, W)₂₃C₆ (b) and Nb(C, N) (d) acquired from the respective phases in Figs.9a and c

Fig.10 SEM-BSE images of creep cracks in as-cast 4C3N3W (a) and 4C3N5W columnar (b) alloys after creep rupture tests at 1000 ℃ and 50 MPa

Fig.11 Mass fractions of equilibrium phases as a function of W additions in Nb-bearing austenitic heat-resistant cast steels at 1000 ℃

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