δ-ferrite transformation mechanism and its effect on mechanical properties of 316H weld metal

doi:10.1016/j.jmst.2020.02.085

Abstract

Abstract:

The microstructure evolution and mechanical properties of the 316H stainless steel weld metals with different C contents were studied at the aging temperature of 550 °C for different aging holding time. The transformation behavior of δ-ferrite and precipitation mechanisms of M₂₃C₆ and σ phase in the as-aged weld metal were investigated. The results indicated that for the as-welded weld metal, with increasing C content, the yield and tensile strengths increased, while the elongation decreased owing to the increase of C solid solution strengthening effect. Moreover, both the high δ-ferrite content in low C weld metal and the precipitated M₂₃C₆ carbide in high C weld metal deteriorated the impact energy obviously. During the aging process, the rapid precipitation of M₂₃C₆ carbide occurred in δ-ferrite firstly owing to the high diffusion rate of C. Once the carbon is depleted by precipitation of M₂₃C₆, the slow formation of σ phase occurred through eutectoid transformation (δ → σ + γ) depending on the diffusion of Cr and Mo. Moreover, increasing C content promoted the formation of M₂₃C₆ carbides and inhibited the formation of σ phase. Therefore, increasing C content accelerated the transformation of δ-ferrite in weld metal during aging process. Furthermore, after a long enough aging time, a transformation from M₂₃C₆ to σ occurred. The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions. For the low C weld metal aged at 550 °C, with the increase of the aging time, fine M₂₃C₆ first precipitated, then coarsened, after that σ phase formed, which caused that the yield and tensile strengths first increased, then decreased, and finally increased slightly again. For the medium C weld metal, as the aging time increased, first the depletion of the solid solution C as a result the M₂₃C₆ precipitation deteriorated the strength, and then the formation of σ phase improved the strength. For the high C weld metal, with the increase of the aging time, the depletion of the solid solution C and the coarsening of the M₂₃C₆ precipitates deteriorated the strength. Furthermore, with increasing aging time, both the precipitation and coarsening of M₂₃C₆ and increasing σ phase content deteriorated the elongation and impact energy.

Key words: 316H weld metal, Aging treatment, δ-ferrite, M₂₃C₆, σ phase, Mechanical property

Langlang Zhao, Shitong Wei, Dong Wu, Dianbao Gao, Shanping Lu. δ-ferrite transformation mechanism and its effect on mechanical properties of 316H weld metal[J]. J. Mater. Sci. Technol., 2020, 57: 33-42.

Figures/Tables 15

Fig. 1. Schematic diagram of the welded joint.

Table 1 Chemical compositions of welding wires and corresponding weld metals (wt%).

Material	C	Cr	Ni	Mo	Mn	Si	N
C1- welding wire	0.015	19.0	12.7	2.53	1.59	0.41	0.065
C1-weld metal	0.016	19.10	12.5	2.48	1.54	0.41	0.056
C2-welding wire	0.042	18.84	12.6	2.49	1.59	0.42	0.063
C2-weld metal	0.042	19.17	12.7	2.47	1.56	0.42	0.054
C3-welding wire	0.064	18.79	12.7	2.45	1.59	0.47	0.069
C3-weld metal	0.062	18.92	12.4	2.49	1.55	0.43	0.046

Fig. 2. Configuration of the tensile sample.

Fig. 3. Configuration of the impact sample.

Fig. 4. SEM micrographs of the as-welded weld metals: (a) C1, (b) C2, (c) C3 and (d) XRD analysis of the precipitate in the as-welded C3 weld metal.

Table 2 EDS analysis result of the M23C6 carbide (wt%).

C	Cr	Fe	Mo	Mn	Ni
6.83	33.08	46.16	6.50	2.00	5.43

Fig. 5. Effects of C content on (a) yield strength, (b) tensile strength, (c) elongation and (d) impact energy.

Fig. 6. Effects of aging time on δ-ferrite transformation fraction of (a) C1 weld metal, (b) C2 weld metal and (c) C3 weld metal.

Fig. 7. TEM micrographs of C1 weld metal aged at (a) 550 °C for 500 h, (b) 550 °C for 1000 h and (c) 550 °C for 3000 h.

Fig. 8. TEM micrographs of C2 weld metal aged at (a) 550 °C for 500 h, (b) 550 °C for 1000 h and (c) 550 °C for 3000 h.

Fig. 9. TEM micrographs of C3 weld metal aged at (a) 550 °C for 500 h, (b) 550 °C for 1000 h and (c) 550 °C for 3000 h.

Fig. 10. Effects of aging time on precipitation phase type and content of electrochemical extraction residues of (a) C1 weld metal, (b) C2 weld metal and (c) C3 weld metal.

Fig. 11. Effect of aging time on (a) yield strength, (b) tensile strength, (c) elongation and (d) impact energy of C1 weld metal.

Fig. 12. Effect of aging temperature on (a) yield strength, (b) tensile strength, (c) elongation and (d) impact energy of C2 weld metal.

Fig. 13. Effect of aging temperature on (a) yield strength, (b) tensile strength, (c) elongation and (d) impact energy of C3 weld metal.

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