Effects of Isothermal Aging on Microstructure and Mechanical Property of Low-Carbon RAFM Steel

doi:10.1007/s40195-019-00883-6

Abstract

Abstract:

In order to investigate the microstructure and mechanical property evolution of low-carbon reduced activation ferritic/martensitic (RAFM) steel during isothermal aging, the normalized and tempered specimens were aged at 600 °C for 500, 1000, and 3000 h, respectively. The microstructural evolution with aging time was analyzed, including the precipitation and growth of M₂₃C₆ and MX-type carbides as well as the formation of Laves phase. The results indicate that the coarsening of M₂₃C₆ is more obvious than that of MXwith increase in aging time. During the long-term thermal exposure, the Fe₂W Laves phase precipitates adjacent to M₂₃C₆ along the prior austenite grain boundaries and packet boundaries. Lower carbon content can delay the precipitation of Laves phase compared to the steel containing higher carbon. In addition, the Laves phase precipitated along boundaries can provide the precipitation strengthening, slightly increasing the tensile strength of low-carbon RAFM steel after aging for 3000 h.

Key words: Low-carbon RAFM steel, Isothermal aging, Microstructure evolution, Mechanical property

Jian-Guo Chen, Chen-Xi Liu, Chen Wei, Yong-Chang Liu, Hui-Jun Li. Effects of Isothermal Aging on Microstructure and Mechanical Property of Low-Carbon RAFM Steel[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(9): 1151-1160.

Figures/Tables 10

Table 1 Chemical composition of the low-carbon RAFM steel (wt%)

C	N	Cr	W	Mn	Si	V	Ta	Fe
0.04	0.02	8.93	1.71	0.44	0.04	0.22	0.073	Bal.

Fig. 1 Micrographs of low-carbon RAFM steel after N and T heat treatment: a OM, b TEM

Fig. 2 Low-magnification SEM images of low-carbon RAFM steel: a N and T heat treatment, b aging for 500 h, c aging for 1000 h, d aging for 3000 h

Fig. 3High-magnification SEM images of low-carbon RAFM steel: a N and T heat treatment, b aging for 500 h, c aging for 1000 h, d aging for 3000 h, e energy-dispersive spectrometer of the area indicated by square in d

Fig. 4 TEM images of low-carbon RAFM steel microstructure: a N and T heat treatment, b aging for 500 h, c aging for 1000 h, d aging for 3000 h, e, f electron diffraction pattern of M23C6 and Laves phase

Fig. 5 TEM micrograph of carbon extraction replica for low-carbon RAFM steel: a distribution of M23C6 after N and T heat treatment, b distribution of M23C6 after aging for 500 h, c distribution of M23C6 after aging for 1000 h, d distribution of M23C6 and Laves phase after aging for 3000 h, e, f electron diffraction pattern of M23C6 and Laves phase

Fig. 6 TEM micrograph of carbon extraction replica for low-carbon RAFM steel showing distribution of MX phase after: a N and T heat treatment, b aging for 500 h, c aging for 1000 h, d aging for 3000 h, e electron diffraction pattern of MX

Fig. 7 Average sizes variation of precipitates in low-carbon RAFM steel before and after isothermal aging tests

Fig. 8 TEM images of low-carbon RAFM steel: a the precipitation of Laves phase close to M23C6 along grain boundaries, b enlarged view of the area indicated by square in a, c and d electron diffraction pattern of Laves phase and M23C6

Fig. 9 Comparison of the tensile and yield strength of low-carbon RAFM steel at N and T state and thermal aging for 500, 1000 and 3000 h

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