Microstructural refinement mechanism and its effect on toughness in the nugget zone of high-strength pipeline steel by friction stir welding

doi:10.1016/j.jmst.2021.04.008

Abstract

Abstract:

High-strength pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 400-700 rpm, and the grain refinement mechanism of the nugget zone (NZ) was determined. The thermo-mechanical process during FSW in the NZ was simulated by multi-pass thermal compression, thereby achieving the austenitic non-recrystallization temperature (T_nr). The austenitic non-recrystallization in the NZ at the lowest rotation rate of 400 rpm caused a significant grain refinement. Furthermore, the reduced rotation rate also resulted in the formation of a high ratio of island-like martensite-austenite (M-A) constituent. The toughness of the NZs was enhanced as the rotation rate decreased, which is attributed to the fine effective grains and homogeneously distributed fine M-A constituents dramatically inhibiting crack initiation and propagation.

Key words: Pipeline steel, Friction stir welding, Grain refinement, M-A constituent, Toughness

R.H. Duan, G.M. Xie, P. Xue, Z.Y. Ma, Z.A. Luo, C. Wang, R.D.K. Misra, G.D. Wang. Microstructural refinement mechanism and its effect on toughness in the nugget zone of high-strength pipeline steel by friction stir welding[J]. J. Mater. Sci. Technol., 2021, 93: 221-231.

Figures/Tables 15

Fig. 1. K-type thermocouples insert location during FSW process.

Fig. 2. Location and geometric dimensions of Charpy v-notch specimens.

Fig. 3. Distribution maps of effective grains of BM and NZs at different rotation rates. (a) BM, (b) 700 rpm, (c) 600 rpm, (d) 500 rpm, and (e) 400 rpm.

Fig. 4. FSW thermal cycle histories of NZs at different rotation rates.

Fig. 5. The relationship between effective grain size and Z parameter.

Fig. 6. Stress-strain curves of multi-pass thermal compression at different strain rates. (a) 0.5 s-1, (b) 1 s-1, (c) 2 s-1, and (d) 5 s-1.

Fig. 7. MFS vs 1000/T plot at different strain rates. (a) 0.5 s-1, (b) 1 s-1, (c) 2 s-1, (d) 5 s-1, and (e) fitted curve.

Fig. 8. IPF maps and TEM images of NZs at different rotation rates. (a,b) 600 rpm and (c,d) 400 rpm.

Fig. 9. Schematic illustration of mechanism of recrystallization and non-recrystallization of austenite. (a) Austenite DRX, (b) Austenite non-recrystallization.

Fig. 10. SEM micrographs of BM and NZs at different rotation rates. (a) BM, (b) 600 rpm, and (c) 400 rpm.

Fig. 11. SEM and TEM micrographs of NZs at different rotation rates. (a,b) 600 rpm and (c, d) 400 rpm.

Fig. 12. Impact energy of BM and NZs at different rotation rates.

Fig. 13. SEM micrographs of crack propagation path in the NZs at different rotation rates. (a,b) 600 rpm and (c,d) 400 rpm.

Fig. 14. Distribution characteristics of M-A constituent of NZs at different rotation rates (I, II, III, and IV regions represent island-like, fine slender, coarse slender, and blocky M-A constituents, respectively.) (a) 600 rpm and (b) 400 rpm.

Fig. 15. Schematic diagram of ductile fracture and cleavage fracture. (a) Cleavage fracture, (b) Ductile fracture.

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