Formation of forged ingots with differential heat transfer from the upper section

Abstract

Directional solidification may be created by using heat-insulating screens to ensure differential heat transfer from the deadhead of the ingot [1]. This is an effective means of reducing or even eliminating physical inhomogeneity of the axial zone in steel ingots with constrained supply to the two-phase zone [2]. The influence of changing the heat transfer on solidification processes may be investigated on 13-t forged ingots of 40 A steel. Differential heat transfer is based on a system of screens with heat insulation consisting of a lid and a side surface. The lateral surface covers the deadhead and a quarter of the mold; the thickness of the heat insulation increases toward the deadhead. Thus, the screens cover the meniscus of the metal in the deadhead extension, its lateral surface, and the upper part of the mold [3]. Screening increases the external surface temperature of the mold wall by 120‐140 ° C, with decrease in the temperature difference over the wall cross section in the deadhead region by 80‐100 ° C; this reduces the temperature stress and increases the mold’s working life. The efficiency of screening the metal surface in the deadhead for 40 min is about 0.03; i.e., the heat transfer is reduced more than thirtyfold [2]. This eliminates the need for mixtures to heat the metal meniscus in the deadhead. To investigate the formation of forged ingots, we consider the hydrodynamics of the melt, the dynamics of the two-phase region in solidification, and the quality of the melt. The hydrodynamics is investigated by means of physical models, on the assumption that the forged ingot and the hydraulic and thermal models are identical [4, 5]. The resulting distributions of the melt fluxes and the velocity curves at different times during the solidification of the control and screened ingots are shown in Fig. 1. In the control ingot, with a considerable temperature gradient over the heat, developed thermoconcentrational convection is observed. This influences the inhomogeneity of the metal, which depends on the development of opposing fluxes, i.e., the descending fluxes along the solidification front and the ascending fluxes in the center of the ingot. As is evident from Fig. 1, the rate of convective motion in the control ingot is 2‐2.5 times that in the screened mold. At the beginning of solidification and in the cross section below the deadhead, the velocity of the descending flow is 0.45 m/s; the maximum velocity of the ascending flow is 0.12 m/s. Hydrodynamic boundary layers with near-zero velocities are formed between the descending and ascending fluxes. The boundary-layer width δ = , where Re = ∆ω l / ν is the Reynolds number; ∆ω is the velocity difference of the opposing fluxes; l is the layer height; ν is the kinematic viscosity; C = 0.48‐0.52 is a correction factor. At the beginning of solidification, Re = 0.5 × 10 6 , δ = 7.1 × 10 ‐3 m for the control ingot, and Re = 0.2 × 10 6 , δ = 10.5 × 10 ‐3 m for the screened ingot; subsequently, these values decline. At the boundary of the inverse fluxes, when Re ≥ 10 5 , secondary flows arise; they may be converted to second