PSI - Issue 30

K.V. Stepanova et al. / Procedia Structural Integrity 30 (2020) 167–172 Stepanova K.V., , Petrov P.P., Platonov A.A. / Structural Integrity Procedia 00 (2020) 000–000

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The results of X-ray diffraction analysis in the sample of composition 7 show insignificant distortions of the iron crystal lattice parameter Δ  = 0.00112Å in the weld bead zone and Δ  = –0.00081Å in the lower boundary zone of deposited metal, whereas the value of Δ  = 0.00061 Å in the weld bead zone and Δ  = –0.00038 Å in the lower boundary in the sample, obtained by covered electrode UONI 13/55. The rare-earth metals mainly form finely dispersed refractory compounds as sulfides, silicates, oxides, carbides due to the high thermodynamic activity. A small part of rare-earth metals dissolves in solid solution, forming intermetallic compounds according to the research by Efimenko (2003). The formation in the iron fusion zone of intermetallic compounds with REE in the ferrite grains boundary contributes to its hardening. Moreover, refinement of ferrite grains increases the strength of the metal due to grain-boundary hardening of the polygonized substructure. Thus, REE reduce the diffusion mobility of the weld elements by interacting in electronic level with the elements of the matrix, contributing to a uniform distribution of dislocations with a decrease in their density. The hardening of the weld metal and HAZ is interpreted by the increase in interatomic and intergranular bonds when REE compounds are transferred into the weld pool during manual arc welding. Fig.3 shows the optical microstructure of weld metals, obtained by flux-cored wire of Composition 7 and covered electrode UONI 13/55. Fig 3a presents the non-uniform columnar formations with a width of 98 μ m in the weld zone of flux-cored wire, mixed areas of acicular ferrite are predominate. The fusion zone is characterized by grains with size of 96 μ m (Fig. 3b). The width of the HAZ is 2.96 mm. The average value of the weld metal hardness is 170 HB. Fig.3c,d present the microstructure of weld metal and fusion zone of UONI 13/55. Uniform large columnar formations occurred during cooling of the weld pool; the distance between the ferrite columns is 95 μ m (Fig. 3c). Grains with an average size of 123 μ m formed in the zone of contact of the deposited metal to the base metal (Fig. 3d), perlite areas are edged with ferrite interlayers. The width of the HAZ is 3.60 mm. The average hardness of the weld metal is 163 HB.

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Fig. 3. Micrographs of samples deposited by FCW (REE total content is 0.7 wt.%) (a,b) and UONI 13/55 (c,d)