Freckles in Remelted Niobium Containing Superalloys

Abstract

Freckle defects can be found in several niobium-containing nickel base superalloys. The freckles in VAR or ESR ingots of 718. 706, 625 and other model alloys were analyzed using Scanning Electron Microscopy (SEM) 1 Energy Dispersive Spectrometry (EDS) and Differential Thermal Analysis (DTA). The compositions and the transformation temperatures of the freckled areas \\ere determined It is found that for most of the alloys the composition and melting temperature of the freckle body usually correspond to a solidification moment when about 70% liquid has transformed into solid. though in some alloys the correspondent volume fraction was found to be higher. Freckle criterion considering the compositional effect is proposed for the evaluation of the freckle potential of allojs. Quantitative calculation results of the alloys using the criterion are presented and discussed in terms of the experimental measurements.

Characteristics and Formation Tendency of Freckle Segregation in Electroslag Remelted Bearing Steel

“…According to Figure 5, if the Cr concentration is used, the lower limit of the liquid fraction for freckle formation is only 0.04 (almost complete solidification), which is, in fact, impossible. However, when the Si and Mn concentrations are used, the liquid fraction range for freckle formation can be estimated to be 0.27-0.49, which agrees, to some extent, with the reported liquid fractions of 0.3-0.5 [19], 0.3-0.6 [24,28], 0.4-0.6 [20,29,30], and 0.4-0.7 [31] for freckle formation in Ni-based superalloy.…”

“…Metals 2020, 10, 246 8 of 14 extent, with the reported liquid fractions of 0.3-0.5 [19], 0.3-0.6 [24,28], 0.4-0.6 [20,29,30], and 0.4-0.7 [31] for freckle formation in Ni-based superalloy. The modified Scheil module of the Thermo-Calc software was also used to estimate the variations of interdendritic liquid density, , and the density difference, Δ (= − , representing liquid density inversion), between the density at liquidus temperature, (calculated at the liquidus temperature and solute concentrations corresponding to the nominal compositions), and local liquid density, , at a certain temperature of T during solidification.…”

“…Therefore, the permeability of the interdendritic liquid can be viewed as a parameter of freckle formation. Many researches applied permeability parameters to evaluate the macro segregation, but most of them only adopted the mean permeability with no concern for anisotropy of permeability [12,[17][18][19]22,[33][34][35]. However, the mean permeability, with only a concern for isotropy, is appropriate for the upward directional solidification but not for the ESR solidification process due to the significant effect of the tilted solidification front.…”

“…In comparison to the criterion form of G m R n , the non-dimensional Rayleigh number receives more attention because it does not only take account of thermal conditions like cooling rate, but also incorporates thermal properties related to alloy compositions like liquid density inversion and viscosity, and also the permeability of the mushy zone in relation to solidification front angle and dendrite structure. In most of cases, this criterion form was also applied in freckle formation in unidirectionally solidified Ni-based alloys and some non-ferrous alloys rather than steel ingots [12,[17][18][19][20][21][22].…”

A Phenomenological Analysis of Freckling in Directional Solidification of Ni-Base Superalloy: The Role of Edge and Curvature in Casting Components

“…Thereafter, the eroded dendrites re-dissolve into the liquid or act as nuclei for equiaxed grains. [12][13][14][15][16][17] Apart from chemistry segregation, [18][19][20] solidification parameters and casting size also play significant roles with respect to freckle formation in Ni-base superalloy casting. [1,2,[21][22][23][24][25] During solidification, convective flow in the mushy zone can be accelerated by a lower temperature gradient (G) or solidification velocity (V), resulting in larger dendrite spacing and higher permeability.…”