Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel

doi:10.1016/j.jmst.2017.11.049

摘要/Abstract

Abstract:

In this study, the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic (RAFM) steel friction stir welded joints were investigated. When a W-Re tool is used, the recommended welding parameters are 300 rpm rotational speed, 60 mm/min welding speed and 10 kn axial force. In stir zone (SZ), austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths. The microstructure in SZ contains lath martensite with high dislocation density, a lot of nano-sized MX and M₃C phase particles, but almost no M₂₃C₆ precipitates. In thermal mechanically affect zone (TMAZ) and heat affect zone (HAZ), refinement of prior austenite and martensitic laths and partial dissolution of M₂₃C₆ precipitates are obtained at relatively low rotational speed. However, with the increase of heat input, coarsening of martensitic laths, prior austenite grains, and complete dissolution of M₂₃C₆ precipitates are achieved. Impact toughness of SZ at -20 °C is slightly lower than that of base material (BM), and exhibits a decreasing trend with the increase of rotational speed.

Key words: Friction stir welding, Reduced activation, erritic/martensitic steel, Microstructure evolution, Impact toughness

. [J]. 材料科学与技术, 2018, 34(5): 756-766.

Chao Zhang, Lei Cui, Yongchang Liu, Chenxi Liu, Huijun Li. Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel[J]. J. Mater. Sci. Technol., 2018, 34(5): 756-766.

图/表 17

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

C	Cr	Mn	V	W	Ta	Si	Zr	N	S	P	Fe
0.1	9	0.5	0.2	1.5	0.15	0.05	0.005	0.007	<0.002	<0.002	Bal.

Fig 1. Microstructure of BM under (a) OM, (b) SEM, (c) M23C6 precipitation and (d) EDS map of the M23C6 precipitation.

Fig. 2. Dimensions of V-notch Charpy impact samples.

Fig. 3. Cross sections of the welds obtained with different rotational speeds: (a) 200 rpm, (b) 300 rpm, (c) 400 rpm.

Fig. 4. OM observations of different weld regions of the welds welded under different rotational speeds: (a)-(c) SZ welded under 200, 300, and 400 rpm respectively, (d)-(f) the TMAZ welded under 200, 300, and 400 rpm respectively, (g)-(i) the HAZ welded under 200, 300, and 400 rpm, respectively.

Fig. 5. TEM observation of SZ microstructures of the joint welded with 400 rpm rotational speed: (a) martensite laths structures, (b) and (c) retained austenite structures under bright field and dark field for BCC respectively, and (d) and (e) details of dislocations in martensite laths.

Fig. 6. TEM observation of TMAZ microstructures of the welds welded with (a) and (b) 200 rpm rotational speed, and (c) and (d) welded with 400 rpm rotational speed.

Fig. 7. TEM observation of HAZ microstructures of the joint: (a) and (b) the microstructures in the weld welded with 400 rpm rotational speed.

Fig. 8. SEM observations of different weld regions of the welds welded under different rotational speeds: (a)-(c) SZ welded under 200, 300, and 400 rpm, respectively, (d)-(f) TMAZ welded under 200, 300, and 400 rpm, respectively, (g)-(i) HAZ welded under 200, 300, and 400 rpm, respectively.

Fig. 9. TEM observations of precipitates in SZ of the weld welded under 400 rpm: (a) MX precipitates, (b) M3C precipitates.

Fig. 10. TEM observations of precipitates in TMAZ of the weld welded under 400 rpm: (a) MX precipitates, (b) M3C precipitates.

Fig. 11. TEM observations of precipitates in HAZ of the weld welded under 300 rpm: (a) MX precipitates, (b) M3C precipitates, (c) M23C6 precipitates.

Fig. 12. Hardness distribution of the welds obtained from all the welding parameters.

Fig. 13. Impact absorbed energies for SZ of the FSW joints at -20 °C under different rotational speed.

Fig. 14. SEM observations of impact fractography in the SZ: (a) 200 rpm under lower magnification, (b) 200 rpm under higher magnification, (c) 300 rpm under lower magnification, (d) 300 rpm under higher magnification, (e) 400 rpm under lower magnification, (f) 400 rpm under higher magnification.

Table 2 Heat input and dimension of HAZ during FSW process.

Rotational speed (rpm)	200	300	400
Heat input (kJ/mm)	0.681	1.021	1.362
Width of HAZ AS (mm)	1.06	1.29	2.12
Width of HAZ RS (mm)	0.82	1.12	1.65

Table 3 Precipitates in three welding regions under different parameter.

	200 rpm	300 rpm	400 rpm
SZ	M₃C, MX	M₃C, MX	M₃C, MX
TMAZ	M₃C, MX, M₂₃C₆	M₃C, MX, M₂₃C₆	M₃C, MX
HAZ	M₃C, MX, M₂₃C₆	M₃C, MX, M₂₃C₆	M₃C, MX

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