Improvement in the weldability of molybdenum alloy by pre-welding solid carburising

doi:10.1016/j.jmst.2020.06.056

Abstract

Abstract:

Molybdenum (Mo), with its high chemical stability and resistance to neutron irradiation, has wide application prospects in the nuclear industry; however, the embrittlement of welded Mo joints limits its further application. In this study, the brittleness of the welded joints of Mo alloy was reduced and their strength was enhanced by adding carbon to the fusion zone (FZ) during laser welding. In the FZ of the Mo joints, carbon mainly existed as Mo₂C, and some free C atoms, and MoC and MoO_xC_y phases were also present. The distribution of Mo₂C directly influenced the bonding strength of the grain boundaries. As Mo₂C was dispersedly distributed as particles or discontinuous lines at the grain boundaries of Mo, it improved the resistance of the grain boundaries to the propagation of cracks and thereby increasing their strength. However, the Mo₂C phases distributed in a reticular pattern at the grain boundaries of Mo provided channels that enabled cracks to rapidly propagate, thereby reducing the resistance of the grain boundaries to crack propagation and weakening their strength. The emergence of the MoO_xC_y phase reduced the weakening effect of free oxygen atoms on the strength of grain boundaries of Mo.

Key words: Molybdenum, Carbon, Mo₂C, Laser beam welding, Pre-welding solid carburising

Liang-Liang Zhang, Lin-Jie Zhang, Jian Long, Xiang-Dong Ding, Jun Sun, Yuan-Jun Sun. Improvement in the weldability of molybdenum alloy by pre-welding solid carburising[J]. J. Mater. Sci. Technol., 2021, 80: 1-12.

Figures/Tables 19

Fig. 1. (a) SC and (b) laser beam welding processes.

Table 1 Joints treated by different methods.

Joints			Processing methods
Pre-welding carburising	Carb.-W	Carb.-100-W joint	First carburising for 100 h at 1200 °C, then laser beam welding
		Carb.-150-W joint	First carburising for 150 h at 1200 °C, then laser beam welding
		Carb.-200-W joint	First carburising for 200 h at 1200 °C, then laser beam welding
	AT-W	AT-100-W joint	First annealing for 100 h at 1200 °C, then laser beam welding
		AT-150-W joint	First annealing for 150 h at 1200 °C, then laser beam welding
		AT-200-W joint	First annealing for 200 h at 1200 °C, then laser beam welding
Post-welding carburising	Carb.	Carb.-100 joint	First laser beam welding, then carburising for 100 h at 1200 °C
		Carb.-150 joint	First laser beam welding, then carburising for 150 h at 1200 °C
		Carb.-200 joint	First laser beam welding, then carburising for 200 h at 1200 °C
	AT	AT-150 joint	Annealing for 150 h at 1200 °C
Without carburising	LW	LW joint	Weld joints created by laser direct melting (LDM), Mo plates without any treatments

Fig. 2. (a) Metallographic images of the cross-sections of the Mo laser weld joints: (a) Carb.-150-W joint; (b), (c), and (d) FZ, heat-affected zone (HAZ), and BM of (a), respectively; (e) AT-150-W joint; (f), (g), and (h) FZ, HAZ, and BM of (e), respectively; (i) Carb.-150 joint [32]; (j), (k), and (l) FZ, HAZ, and BM of (i), respectively [32]; (m) LW joint; (n), (o), and (p) FZ, HAZ, and BM of (m), respectively.

Fig. 3. Locations of the EPMA results of the BM, HAZ, and FZ in cross-sections of the (a) AT-150-W and (b) Carb.-150-W joints.

Table 2 Compositions of the different zones in Fig. 3(a).

Position	Test No.	Element content (wt%)
Position	Test No.	N	O	C	Mo
Fusion zone	1	0	0.699	0.339	99.699
	2	0	0.643	0.251	99.213
	3	0	0.578	0.333	99.633
	4	0.301	0.384	0.296	99.921
	Average	0.075	0.576	0.305	99.617
Heat-affected zone	5	0.190	0.155	0.224	99.105
	6	0.183	0.281	0.184	99.538
	7	0	0.679	0.218	100.926
	8	0.165	0.681	0.377	99.824
	Average	0.135	0.449	0.251	99.848
Base metal	9	0	0.759	0.324	98.740
	10	0	0.638	0.252	99.561
	11	0.102	0.625	0.246	99.690
	12	0	0.816	0.182	99.288
	Average	0.026	0.710	0.251	99.320

Table 3 Compositions of the different zones in Fig. 3(b).

Position	Test No.	Elements (wt%)
Position	Test No.	N	O	C	Mo
Fusion zone	1	0.188	0.604	0.523	99.567
	2	0.166	0.542	0.773	99.902
	3	0.114	0.384	0.750	99.394
	4	0.102	0.299	1.042	99.825
	Average	0.143	0.457	0.772	99.672
Heat-affected zone	5	0.088	0.432	1.132	98.465
	6	0.092	0.485	1.296	99.033
	7	0.132	0.667	1.110	100.082
	8	0	0.005	0.953	98.541
	Average	0.078	0.397	1.123	99.030
Base metal	9	0.151	0.451	1.366	99.668
	10	0	0.077	0.786	99.529
	11	0.130	0.521	1.188	99.460
	12	0	0.225	0.997	99.817
	Average	0.070	0.318	1.084	99.619

Fig. 4. Micro-Vickers hardness of a cross-section of the laser beam-welded Mo joints.

Fig. 5. (a) Nanoindentation hardness of cross-sections of the FZ of the Carb.-100-W, Carb.-150-W, Carb.-200-W, AT-150-W, SC-150, AT-150, and LW joints; indentations at the (b) grain boundary and (c) grain interior.

Fig. 6. Effect of C addition on the tensile strength of the Mo alloy joints.

Fig. 7. XPS analysis results (recorded with monochromatic Al-Kα) for a cross-section of the FZ in the Carb.-150-W joint: (a) XPS survey spectrum of the FZ; (b) C 1s spectrum of the FZ.

Fig. 8. XPS C 1s spectrum (recorded with monochromatic Al-Kα) of the surface of the grain boundaries in the FZ of the Carb.-150-W joint.

Fig. 9. TEM images of the BM of the (a) non-carburised Mo alloy and (b) alloy carburised for 150 h; (c) & (d) FZ of the Carb.-150-W joints.

Fig. 10. SEM images of the surfaces of the fractures at the FZ of the Carb.-150-W joint.

Fig. 11. EBSD results of the cross-section of the Carb.-150-W joint: (a) IPF mapping; (b) enlarged image of zone A in (a); (c) mapping of the phase distribution (Phases) in (b); (d) enlarged image of zone B in (a); (e) mapping of the phase distribution (Phases) in (d).

Fig. 12. EBSD results of the cross-section of the Carb.-100-W joint: (a) IPF mapping; (b) enlarged image of zone A in (a); (c) mapping of the phase distribution (Phases) in (b).

Fig. 13. EBSD results of the cross-section of the Carb.-200-W joint: (a) IPF mapping; (b) enlarged image of zone A in (a); (c) mapping of the phase distribution (Phases) in (b); (d) enlarged image of zone B in (a); (e) mapping of the phase distribution (Phases) in (d).

Fig. 14. SEM images of the fracture surface in the FZ of the Carb.-200-W joint.

Fig. 15. Schematic diagrams of the tensile fracturing process of a carburised joint: (a) dispersed Mo2C particles at the Mo grain boundary (such as Carb., Carb.-100-W, and Carb.-150-W joints); (b) reticulated Mo2C phase at the Mo grain boundaries (such as Carb.-200-W joints).

Fig. 16. (a) Constitution diagram of the Mo-C system. The liquidus is represented by the solid line. The dotted lines correspond to its metastable regions, and the chain-dotted lines correspond to the variations in the equilibrium of the liquid phases with molybdenum and Mo2C without considering the solubility of carbon in β-Mo [ 35]; (b) interactions of carbon, molybdenum, and oxygen in the FZ of the Carb.-W joints during the thermal welding cycle.

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