Carbide precipitates and mechanical properties of medium Mn steel joint with metal inert gas welding

doi:10.1016/j.jmst.2020.10.029

Abstract

Abstract:

Medium Mn steel was metal inert gas (MIG) welded with NiCrMo-3 and 307Si filler wires. The effect of filler wires on the microstructure and mechanical properties of joint was investigated, and the carbide precipitates were contrastively discussed. The results revealed that the microstructure of weld metal, heat-affected zone and base metal are austenite. Obvious grain coarsening occurred in the heat-affected zone (HAZ), and the maximum grain size grew up to 160 μm. In HAZ, C and Cr segregated at grain boundaries, the carbides was identified as Cr₇C₃. The dispersive (Nb, Mo)C phase was also found in weld metal with NiCrMo-3 filler wire. All the welded joints failed in HAZ during tensile tests. The tensile strength of welded joint with NiCrMo-3 filler wire was 675 MPa, which is much higher than that with 307Si filler wire. In comparison to base metal, higher microhardness and lower impact toughness were obtained in HAZ for these two welded joints, which was attributed to the precipitation of Cr₇C₃ phase and grain coarsening. The impact toughness around the fusion line is the worst for these two welded joints.

Key words: Medium Mn steel, MIG welding, Microstructure, Precipitate, Mechanical property

Jiang Yang, Honggang Dong, Yueqing Xia, Peng Li, Xiaohu Hao, Yaqiang Wang, Wei Wu, Baosen Wang. Carbide precipitates and mechanical properties of medium Mn steel joint with metal inert gas welding[J]. J. Mater. Sci. Technol., 2021, 75: 48-58.

Figures/Tables 22

Table 1 Chemical compositions of base and filler metals (wt%).

Materials	C	Mn	Si	S	P	Mo	Cr	Ni	Nb	Al	Fe
Mn8	1.08	8.5	0.1	0.007	0.021	0.15	2.66	0.25	-	-	Bal.
NiCrMo-3	0.01	-	0.056	-	0.014	8.816	21.97	65.14	3.59	0.233	0.16
307Si	0.077	6.57	0.86	0.004	0.014	-	19.3	8.78	-	-	Bal.

Table 2 Mechanical properties of the used medium-Mn steel plate.

Material	Yield strength, R_e (MPa)	Tensile strength, R_m (MPa)	Elongation, δ (%)	Hardness (HV)	Impact toughness (J)
Mn8	472	788	20.83	244	134.3

Fig. 1. Microstructures of the Mn8 steel: (a) OM and (b) SEM.

Fig. 2. Illustrative schematics of the clamping system.

Table 3 Welding parameters for MIG welding of the Mn8 steel.

Filler wire	Weld pass	Welding voltage (V)	Welding current (A)	Wire feed rate (m/min)
ERNiCrMo-3	1	21.5	160	10
ERNiCrMo-3	2-4	21.5-23	160-180	11.5
307Si	1	21.5	200	10
307Si	2-4	21.5-22	220-280	12

Fig. 3. Dimensions of the (a) tensile specimens, (b) Charpy impact specimens and (c) the sampling position of impact test specimens.

Fig. 4. Microstructures of welded joints: (a) HAZ with 307Si wire, (b) WM with 307Si wire, (c) HAZ with NiCrMo-3 wire, (d) WM with NiCrMo-3 wire.

Fig. 5. X-ray diffraction pattern taken from HAZ, WM and BM of different welded joints.

Fig. 6. Distribution of major alloying elements adjacent to fusion line of welded joint with 307Si wire.

Fig. 7. Distribution of major alloying elements adjacent to fusion line of welded joint with NiCrMo-3 wire.

Fig. 8. Distribution of major alloying elements marked with dotted frame in Fig. 7.

Table 4 EPMA quantitative analysis results of the corresponding locations in Fig. 8.

Location	Elements (at.%)							Possible phases
Location	C	Nb	Mo	Mn	Ni	Cr	Fe	Possible phases
1	47.9	36.4	7.0	0.1	3.4	4.2	1.0	(Nb, Mo)C
2	48.7	33.8	8.7	-	2.1	2.9	3.8	(Nb, Mo)C

Fig. 9. TEM analysis results of the precipitates in WM of welded joint with NiCrMo-3 wire, (a) FIB sampling position for TEM sample, (b) bright field image, (c) SAED1 pattern, (d) SAED2 pattern, (e) element mapping.

Fig. 10. TEM analysis results of the precipitates in HAZ: (a) precipitates distributed at grain boundaries, (b) FIB sampling position for TEM sample, (c) bright field image, (d) SAED pattern, (e) HRTEM image and FFT pattern, (f) element mapping.

Fig. 11. Microhardness profiles of these two welded joints.

Fig. 12. Tensile stress-strain curves of Mn8 steel base metal and two welded joints: (a) engineering stress-strain curves and (b) true stress-strain curves (solid lines) and strain hardening rate-strain curves (short dotted lines).

Fig. 13. Actual tensile test fracture specimens of two welded joints.

Fig. 14. Fracture morphologies of base metal and welded joints: (a, b) base metal, (c, d) welded joint with 307Si wire, (e, f) welded joint with NiCrMo-3 wire.

Table 5 Chemical composition (wt.%) of the corresponding locations in Fig. 14.

Location	Cr	Mn	Fe
1	2.74	8.21	89.05
2	3.95	11.42	84.63
3	3.16	10.32	86.52
4	2.53	7.43	90.04

Fig. 15. Impact absorbed energy of welded joints.

Fig. 16. Fractography images for the impact specimens: (a) base metal, (b) weld metal with 307Si wire, (c) fusion line with 307Si wire, (d) HAZ1 with 307Si wire, (e) HAZ2 with 307Si wire, (f) weld metal with NiCrMo-3 wire, (g) fusion line with NiCrMo-3 wire, (h) HAZ1 with NiCrMo-3 wire, (i) HAZ2 with NiCrMo-3 wire.

Fig. 17. TEM morphology and distribution of precipitates in HAZ (a) with 307Si wire and (b) with NiCrMo-3 wire.

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