On Laves phase in a 9Cr3W3CoB martensitic heat resistant steel when aged at high temperatures

doi:10.1016/j.jmst.2020.12.062

Abstract

Abstract:

9Cr3W3CoB steels are developed to serve at the temperature range of 620-650 °C, and have been recognized as the most promising martensitic heat-resistant steels for supercritical power plants. Due to the high W and Co contents, the Fe₂W Laves phase in such 9Cr3 W3CoB steel possesses some specialties in thermodynamics. In the present research, it was found that even when aged at 800 °C in the 9Cr3W3CoB steel, instead of dissolving, Laves phase formed after 50 h and kept on increasing in size and number density until 1000 h, indicating that the Laves phase was marching for the thermodynamic equilibrium during aging. In this thermodynamic process, the W-rich M₃B₂ borides in as-received steel and M₂₃C₆ carbides were revealed to dissolve, supporting the growth of Laves phase. SEM investigation indicates that Laves phase tended to form clusters, and finally grow as a unit bulk Laves phase with an irregular shape. Besides, Laves phase nucleated next to M₂₃C₆ carbides and enwrapped them inside at 800 °C. In addition, the growth processes of Laves phase and M₂₃C₆ carbides were a competitive procedure, the coarsening of M₂₃C₆ carbides was prior to the growth of Laves phase at 750 °C while the growth of Laves phase was prior to the coarsening of M₂₃C₆ carbides at 800 °C.

Key words: 9Cr3W3Co steel, Aging, Laves phase, Cobalt, M₂₃C₆ carbides

Ye Liang, Wei Yan, Xianbo Shi, Yanfen Li, Quanqiang Shi, Wei Wang, Yiyin Shan, Ke Yang. On Laves phase in a 9Cr3W3CoB martensitic heat resistant steel when aged at high temperatures[J]. J. Mater. Sci. Technol., 2021, 85: 129-140.

Figures/Tables 16

Table 1 Chemical composition of experimental 9Cr3W3CoB steel (wt%).

C	Si	Mn	P	S	Cr	W	Co
0.11	0.27	0.46	0.006	0.004	9.02	2.99	3.05
V	Nb	N	B	Ti	Ni	Al	Cu
0.19	0.073	0.006	0.015	<0.005	0.028	<0.02	0.88

Fig. 1. Typical TEM micrographs of 9Cr3W3CoB steel after aging at 800 °C for different times: 0 h (a); 10 h (b); 100 h (c); 200 h (d); 800 h (e).

Fig. 2. Microstructures of as-received (a) and as-treated (b) 9Cr3W3CoB steel, microstructure of 9Cr3W3CoB steel aging at 700 °C for 1000 h (c); EDS spectra, and chemical composition of M3B2 boride marked by green circle (d), EDS spectra, SAED pattern and chemical composition of M23C6 carbides marked by blue circle (e), and Laves phase marked by orange circle (f).

Fig. 3. BSE images of Laves phase in 9Cr3 W3CoB steel after aging at different temperatures.

Fig. 4. BSE images of Laves phase in 9Cr3 W3CoB steel after aging at 800 °C for 10 h (a), 25 h (b) and 50 h (c).

Fig. 5. SIMS results of the elements distribution in Laves phase (a, b) and M23C6 carbides (c, d).

Fig. 6. Area fraction (a), Mean equivalent diameter (b) and number density (c) of Laves phase precipitates in 9Cr3W3CoB steel after aging at 750 and 800 °C.

Fig. 7. Clustered Laves phase at 750 (a) and 800 °C (c), respectively; Laves phase with irregular shape at 750 °C (b) and 800 °C (d), respectively.

Fig. 8. Variation of the volume fraction of Laves phase and M23C6 carbides in 9Cr3W3CoB steel (obtained by Thermo-Calc).

Fig. 9. Equilibrium phase diagrams of Fe-Co-W at 700 °C (a) and 800 °C (b) (obtained by Thermo-Calc).

Fig. 10. SEM image (a) and TEM image (b) of bulk M23C6 carbides with Laves phase inside; SEM image (c) and TEM image (d) of Laves phase with M23C6 carbides inside in 9Cr3W3CoB steel during aging at 750 °C (Orange circle denotes Laves phase enwrapped by M23C6 carbides, blue circle denotes M23C6 carbides enwrapped by Laves phase).

Fig. 11. SEM image (a) and TEM image (b) of bulk Laves phase with M23C6 carbides inside in 9Cr3 W3CoB steel during aging at 800 °C (Blue circle denotes M23C6 carbides enwrapped by Laves phase).

Fig. 12. Small pores in Laves phase particle after aging at 800 °C for 3000 h.

Fig. 13. Competition growth of precipitates during aging at 750 °C for 100 h (a), 600 h (c), 3000 h (e); 800 °C for 100 h (b), 600 h (d), 3000 h (f) (The upper half of each picture is the image with low magnification, the upper half of each picture is the image with high magnification).

Table 2 Variation of area fraction of M23C6 carbides during aging at 750 and 800 °C.

Aging time	0 h	100 h	600 h	3000 h
750 °C	4.43	4.69	4.44	4.42
800 °C	4.43	3.46	3.15	2.32

Fig. 14. Schematic summary of Laves phase nucleation and growth: dissolution of W-rich inclusion (a), segregation of W to the boundary (b), nulceation of Laves phase in close vicinity to M23C6 carbides (c), and growth of Laves phase by swallowing M23C6 carbides (d).

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