Welding solidification cracking susceptibility and behavior of a Ni-28W-6Cr alloy

doi:10.1016/j.jmst.2018.09.013

Abstract

Abstract:

Welding solidification cracking of alloys is associated with the range of solidification temperature that can be greatly affected by the amount of refractory metals and other additives. In this work, solidification cracking of Ni-28W-6Cr alloy with high W content was studied by gas tungsten arc welding, showing that the welding current, alloying elements and precipitates all affect the cracking susceptibility. The lengths of cracks increase linearly with the welding current in the range from 150 to 250?A. The relatively high cracking susceptibility is mainly attributed to the high content of Si, which tends to segregate with other elements including W, Cr, Mn as films or components with low melting point in the last solidification stage and weaken the binding force of grain boundaries. Moreover, the existence of precipitated continuous eutectic M₆C carbides in the grain boundaries also acts as nucleation sites of crack initiation, and the cracks often propagate along solidification grain boundary.

Key words: Welding solidification cracking susceptibility, Ni-W-Cr alloy, Chemical composition, Grain boundaries

Shuangjian Chen, Xiang-Xi Ye, D.K.L. Tsang, Li Jiang, Kun Yu, Chaowen Li, Zhijun Li. Welding solidification cracking susceptibility and behavior of a Ni-28W-6Cr alloy[J]. J. Mater. Sci. Technol., 2019, 35(1): 29-35.

Figures/Tables 8

Table 1 Chemical composition of Ni-28W-6Cr alloy (wt.%).

C	Si	Mn	P	Cr	W	Ni
0.04	0.31	0.34	0.32	6.85	27.91	Bal.

Fig. 1. Morphologies of welded beads at different currents. (a) 150?A, (b) 170?A, (c) 190?A, (d) 210?A, (e) 230?A, (f) 250?A, and (g) maximum crack length as a function of welding current (heat input).

Fig. 2. Microstructure of welded joint under 230?A. (a) OM image of base metal, (b) OM image of region near fusion line, red line indicates the fusion line, (c) OM image of weld metal, (d) magnified SEM images of base metal and (e) weld metal.

Fig. 3. OM images of cross-section of weld under different currents. (a) 150?A, (b) 170?A, (c) 190?A, (d) 210?A, (e) 230?A, (f) 250?A.

Fig. 4. Microstructure of solidification cracks under 230?A. (a) Main crack propagates along SGB, where region A shows the tip of main crack; (b) EBSD image of main crack tip at high magnification, and the enlarged image of region B; (c) shows some micro cracks; (d) and (e) main crack propagates along SGB, (f) and (g) show the fracture morphologies of cracks which propagated along SGB but not SSGB.

Fig. 5. Solidus-liquidus lines of several alloys. (a), (b), (c) Solidus-liquidus lines of Hastelloy N alloy, 230 alloy and Ni-28W-6Cr alloy, (d) STR of the alloys as a function of W content, all the STRs were calculated by JMatPro 7 software.

Fig. 6. Micro-segregation of elements in the weld under 230?A. (a) photomicrograph of a micro discontinuous crack along the SSGB, and the measuring line was set to cross through four dendrite arms and three SSGBs, showing microprobe line scan; (b) chemistry of the area along the measuring line.

Fig. 7. Microstructures of cracks in the weld at current of 230?A. (a) Cracks along grain boundaries; (b) crack propagates along the boundary A with carbides but not B without carbides; (c) TEM pattern of carbides in grain boundary; (d) compositions of carbides by TEM-EDX.

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