Tensile properties and fracture behavior of friction stir welded joints of Fe-32Mn-7Cr-1Mo-0.3N steel at cryogenic temperature

doi:10.1016/j.jmst.2017.11.034

Abstract

Abstract:

This study investigates the cryogenic tensile properties and fracture behavior of fiction stir welded and post-weld heat-treated joints of 32Mn-7Cr-1Mo-0.3N steel. Cryogenic brittle fracture, which occurred in the as-welded joint, is related to the residual particles that contain tungsten in the joint band structure. Post-weld water toughening resulted in the cryogenic intergranular brittleness of the joint, which is related to the non-equilibrium segregation of solute atoms during the post-weld water toughening. Annealing at 550 °C for 30 min can effectively inhibit the cryogenic intergranular brittleness of the post-weld water-toughened joint. The yield strength, ultimate tensile strength, and uniform elongation of the annealed joint are approximately 95%, 87%, and 94% of the corresponding data of the base metal.

Key words: High nitrogen steel, Friction stir welding, Post-weld heat treatment, Tensile properties, Fracture behavior, Cryogenic

Yi-jun Li, Rui-dong Fu, Yan Li, Yan Peng, Hui-jie Liu. Tensile properties and fracture behavior of friction stir welded joints of Fe-32Mn-7Cr-1Mo-0.3N steel at cryogenic temperature[J]. J. Mater. Sci. Technol., 2018, 34(1): 157-162.

Figures/Tables 10

Fig. 1. Schematic of the tensile test specimen.

Fig. 2. Microstructures of as-welded and PWWT joints: (a) cross-section of as-welded joint; (b) BM in as-welded joint; (c) HAZ in as-welded joint; (d) NZ in as-welded joint; (e) SEM image of BS in as-welded joint; (f) cross-section of PWWT joint; (g) BM in PWWT joint; (h) NZ in PWWT joint; (i) HAZ and BM in PWWT joint; (j) OIM map of HAZ in PWWT joint.

Table 1 Chemical composition of the BS (in wt%).

	Fe	Mn	Cr	Mo	W
1	41.81	20.58	7.08	0.71	26.66
2	59.74	29.5	6.62	0.69	-

Fig. 3. Cryogenic engineering stress-strain curves of FSW joint and BM under different treatment conditions.

Table 2 Tensile properties of FSW joints and BM at cryogenic temperatures.

	YS (MPa)	UTS (MPa)	UE (%)
BM-W	673(+13 -5)	864(+14 -5)	3(+1 -0)
BM-W-A	682(+8 -4)	1103(+7 -4)	36(+2 -1)
as-welded joint	884(+3 -7)	1009(+3 -7)	2(+0 -0)
PWWT joint	655(+7 -10)	834(+6 -9)	6(+1 -1)
annealed joint	650(+5 -4)	960(+5 -4)	34(+2 -1)

Fig. 4. Fe-32 Mn steel specimens after tensile fracture at cryogenic temperature: (a) as-welded joint; (b) PWWT joint; (c) BM-W; (d) BM-W-A; and (e) annealed joint.

Fig. 5. OM images showing fracture location at (a) low magnification and (b)-(c) high magnification.

Fig. 6. SEM images showing cryogenic tensile fractographs of as-welded joint.

Fig. 7. SEM images showing cryogenic tensile fractographs of BM-W and PWWT joint: (a)-(b) BM-W; and (c)-(d) PWWT joint.

Fig. 8. SEM images showing cryogenic tensile fractographs of BM-W-A and annealed joint: (a)-(c) BM-W-A; and (d)-(i) annealed joint.

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