PSI - Issue 16

Olena Berdnikova et al. / Procedia Structural Integrity 16 (2019) 89–96 Olena Berdnikova et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. The calculated values of strength (∑σ T ) and fracture toughness (К 1С ) for welded joints produced by arc (a), laser (b) and hybrid (c) welding and their fractograms: failure under the specified modes − ductile (a, upper photo, b, upper photo , c, upper photo) and brittle transcrystalline (a, lower photo), mixed (b, lower photo, c, lower photo). Under conditions of hybrid laser-arc welding at 316 J/mm (#7) and laser welding at 363 J/mm (#8) ∑σ Т = 850...1080 MPa (Fig. 2c) and ∑σ T = 900...1120 MPa (Fig. 2b), respectively, and the fracture toughness increases by 10...20%. The nature of the fracture of welded joints is ductile with a dimple relief in the absence of fragments of brittle cleavage fracture, Fig. 2b (upper photo), and Fig. 2c (upper photo). This fact is due to the predominant formation of the structure of B L with a uniform distribution of the dislocation density. Herewith, the increase of heat input to 880 J/mm (#5) in the case of laser welding does not lead to metal embrittlement. The nature of fracture is predominantly ductile with a small fraction of quasi-brittle transcrystalline fracture, Fig. 2b (lower photo). In the case of hybrid welding at 314 J/mm (#10), the mixed nature of fracture is ductile in combination with local areas of transcrystalline cleavage fracture, Fig. 2c (lower photo). The calculated estimates of local internal stresses (τ L/IN ) plotted in the diagrams of Fig. 3 show the following. Under conditions of arc welding with the modes with maximum heat input (Fig. 3a), extended zones with maximum τ L/IN (1900...3700 MPa) are formed along the intergranular boundaries of the B U in places of extended dislocation clusters ρ = 2×10 11 cm – 2 .

Fig. 3. Distribution of local internal stresses τ L/IN in the structure of welded joints with arc (a), laser (b) and hybrid laser-arc welding (c).

This leads to the incipiency of microcracks in these zones and a decrease in the crack resistance of welded joints, Fig. 2a (lower photo). The decrease of τ L/IN values is typical for welded joints obtained by the hybrid welding (τ L/IN = 1470...1867 MPa, Fig. 3b) and especially by the laser welding (τ L/IN = 1470...1663 MPa, Fig. 3c) which contributes formation of fine-grained structures B L in the welding zone in combination with a uniform distribution of the dislocation density. As a result, it was found that the optimum properties of strength, ductility and crack resistance of welded joints of high-strength steels are ensured under conditions for arc welding with a minimum heat input of 546 J/mm (#4), for laser welding − 316 J/mm (#7), and for hybrid welding − 363 J/mm (#8). This is due to the formation of the most dispersed structures − lower bainite, fine - grained martensite in the absence of extended dislocation clusters −