Металознавство та обробка металів (Jun 2020)
Аbout the deformation ability of overlay metal of nickel-base difficult to weld high temperature strength alloys with γ׳-phase strengthening high content
Abstract
Under conditions of multilayer microplasma powder overlay welding process of nickel-base difficult to weld high temperature strength alloys with a content of the strengthening γ׳- phase more than 50 vol. % the tendency to the reheating cracks formation in the corresponding welded joint "cast-deposited metal" from is largely determined by the structure and properties of the deposited metal. With appropriate research studies of the crack formation tendency, it was proposed not to post factum quantitatively evaluate its manifestations by the number and length of cracks, but to focus on studies of the preliminary resistance stage to their formation in the overlay metal. It is shown that an objective method for the deformation ability studying can be tensile testing at elevated temperatures (1000 ° C) of overlay metal samples. This allows us to move from the two-dimensional system of its assessment “temperature T - critical strain ε” when using a common machine test according to the international standard for hot crack testing resistance ISO 17641: 2005 to the three-dimensional system “temperature T - critical stress σ (Rmcr ) - critical deformation ε (Acr)”. For the overlay metal ZhS32 obtained by the microplasma powder overlay welding process, in the state immediately after deposition, the limited deformation capacity at 1000 °C was established: Rmcr ~ 345 МРа (tensile strength); Acr ~ 0,1 % (percentage elongation). Based on the previously established limited deformation ability of the ZhS6K overlay metal in the after deposition state, it is proposed to use the quantitative criterion ε 1000°C (А1000°C) for predictive assessments of the possibility of ensuring the technological strength of the overlay metal of nickel-base difficult to weld high temperature strength alloys. The reduced level of mechanical properties of the overlay metal ZhS32 after its crystallization in the welding pool is associated to the some its primary structure features. Using scanning electron microscopy, the following differences were established in comparison with a similar cast metal of the ZhS32 alloy: dendritic cell cross section; dispersion and uniform distribution of the carbide phase particles; the visual signs absence of the release of strengthening γ׳-phase particles on the dendrite axes. It is shown that the presence of dispersed hardening in the structure of nickel-base high temperature strength alloys of the ZhS32 and ZhS6 types depends on the cooling rate of the molten metal during crystallization, and in the case of the overlay metal it is finally formed only during subsequent heat treatments in the range of aging-homogenization modes.
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