Microstructure and Mechanical Properties of RAFM-316L Dissimilar Joints by Friction Stir Welding with Different Butt Joining Modes

doi:10.1007/s40195-019-00951-x

Abstract

Abstract:

Dissimilar welded joints of reduced activation ferritic/martensitic (RAFM) steel and 316L austenitic stainless steel were prepared by friction stir welding with different butt joining modes and welding parameters. The weld quality of the joint was improved by placing the 316L steel on the advancing side and the RAFM steel on the retreating side, and using a relatively high rotational speed of 400 rpm. The microstructure of the stir zone on the 316L steel side consisted of single-phase austenite, and the microstructure of the stir zone on the RAFM steel side mainly consisted of lath martensite and equiaxed ferrite. A mechanical mixture of the two steels and diffusion of Cr and Ni could be detected near the bonding interface. Diffusion of Ni from the 316L steel to the RAFM steel resulted in the formation of a dual-phase structure consisting of austenite and ferrite. The as-welded joints showed good strength and ductility at room temperature and 550 °C, which were nearly equal to those of the 316L base material. The heat-affected zone on the RAFM side had the lowest impact toughness throughout the weld with a value of 13.2 J at - 40 °C, ~ 52% that of the RAFM base material.

Key words: Friction stir welding, Dissimilar joint, RAFM steel, 316L steel, Microstructures, Mechanical properties

He Bin, Cui Lei, Wang Dong-Po, Li Hui-Jun, Liu Chen-Xi. Microstructure and Mechanical Properties of RAFM-316L Dissimilar Joints by Friction Stir Welding with Different Butt Joining Modes[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(1): 135-146.

Figures/Tables 18

Table 1 Chemical composition of BM-RAFM and BM-316L steel (wt%)

	C	Cr	Ni	Mo	Mn	V	W	Ta	Si	N
BM-RAFM	0.10	9.2	-	-	0.47	0.21	1.0	0.087	0.12	0.0083
BM-316L	0.02	16.58	12.07	2.02	0.8	-	-	-	0.28	0.035

Fig. 1 Microstructures of the base metals: a, c OM and SEM images of RAFM; b, d OM and SEM images of 316L stainless steel

Fig. 2 Relative position of the FSW tool to the weld

Table 2 Butt joining types and the related welding parameters

Butt joining types in FSW	Rotational speed (rpm)	Travel speed (mm/min)	Tool tilt angle (°)	Axial load (kN)
RAFM(AS)-316L(RS)	300	75	2.5	10
316L(AS)-RAFM(RS)	300	75	2.5	10
RAFM(AS+1)-316L(RS)	300	75	2.5	10
RAFM(AS)-316L(RS+1)	300	75	2.5	10
RAFM(AS)-316L(RS)	400	100	2.5	10
316L(AS)-RAFM(RS)	400	100	2.5	10

Fig. 3 Dimensions of specimens for a tension, b V-notch Charpy impact, c schematic of sampling locations

Fig. 4 Cross sections of RAFM-316L dissimilar FSW joint at 300 rpm rotational speed and 75 mm/min travel speed: aRAFM(AS)-316L(RS), b 316L(AS)-RAFM(RS), c, d SEM images of rectangular regions c and d in a and b, respectively

Fig. 5 Cross sections of RAFM-316L dissimilar FSW joint at a rotational speed of 400 rpm and travel speed of 100 mm/min: aRAFM(AS)-316L(RS), b 316L(AS)-RAFM(RS), c, d SEM images of rectangular regions c and d in a and b, respectively

Fig. 6 Cross sections of RAFM-316L dissimilar FSW joint at a rotational speed of 300 rpm and a travel speed of 75 mm/min: aRAFM(AS+1)-316L(RS), b RAFM(AS)-316L(RS+1), c, d SEM images of rectangular regions c and d in a and b, respectively

Fig. 7 Microstructures in RAFM(AS)-316L(RS) weld: a HAZ-RAFM, b TMAZ-RAFM, c SZ-RAFM, d TMAZ-316L, e HAZ-316L. f TEM image of the mixed structure observed in c

Fig. 8 Microstructures in SZ at the top, middle, and bottom of RAFM(AS)-316L(RS) weld: a SZ-RAFM-TOP, b SZ-RAFM-MIDDLE, c SZ-RAFM-BOTTOM, d SZ-316L-TOP, e SZ-316L-MIDDLE, f SZ-316L-BOTTOM

Table 3 Average grain size of austenite in SZ-316L

Butt joining types in FSW	Rotational speed (rpm)	Travel speed (mm/min)	Average austenite grain size in SZ-316L (μm)
RAFM(AS)-316L(RS)	300	75	8.1 ± 0.56
316L(AS)-RAFM(RS)	300	75	6.3 ± 0.53
RAFM(AS+1)-316L(RS)	300	75	7.8 ± 0.69
RAFM(AS)-316L(RS+1)	300	75	8.4 ± 0.81
RAFM(AS)-316L(RS)	400	100	7.5 ± 0.20
316L(AS)-RAFM(RS)	400	100	6.1 ± 0.67

Fig. 9 a SEM image of the interface in the middle of SZ, b EDS results of the interface in the middle of SZ, c SEM image, d EDS results for the interface near the bottom of SZ

Fig. 10 SEM image and EDS results of the region near the bonding interface

Fig. 11 Hardness distribution of RAFM-316L dissimilar FSW joints

Fig. 12 a Room temperature engineering stress-strain curves, b tensile properties of RAFM-316L dissimilar FSW joints

Fig. 13 a Engineering stress-strain curves, b tensile properties of RAFM-316L dissimilar FSW joints at 550 °C

Fig. 14 Impact absorbed energies of the RAFM(AS)-316L(RS) joint tested at - 40 °C

Fig. 15 SEM observations of impact fractography in different regions

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