Study of Nonmetallic Inclusion Inhomogeneity and Distribution in a Steel 38KhN3MFA Ingot Cast with a Head Section Cooler

Abstract

Results are provided for a study of a steel 38KhN3MFA metal ingot weighing 1.53 tons cast with a head section cooler intended for producing hollow forgings. The position of pores and nonmetallic inclusion distribution is studied: oxides, oxysulfides, and sulfides. Features are revealed for the position and amount of pores and nonmetallic inclusions at three ingot levels: lower, central, and head sections. The effect of ingot head section cooling on crystallization and features of oxide, oxysulfide, and sulfide is revealed.Preparation of ingots, especially large ones, is always characterized by nonuniform chemical composition throughout the volume of an ingot as a result of selective crystallization. One of the main problems is carbon liquation over an ingot height, comprising 150-200%, and forming due to a considerable temperature drop over its height. Solution of this problem is possible by using casting technology with a cooled ingot upper part and installation of a massive cooler extension in the head area ( Fig. 1), since the majority of forgings are hollow. The shrinkage sink in this case is in the axial area of an ingot in the form of a narrow region, and is removed as waste during forging. This concentrated cooling makes it possible to control the position and extent of shrinkage defects [1].It has been established from results of physical modeling of ingot crystallization with a cooling head section [2] using thermal vision monitoring [3] that the thermal center of a melt is located close to the center of an ingot height (Fig. 2). This leads to a change in crystallization conditions, and this may affect the formation and position of nonmetallic inclusions (NI) within an ingot.With the aim of confirming this assumption, the amount, size, and position of NI (oxides, oxysulfides, and sulfides) and metal pores were studied in an ingot of steel 38KhN3MFA weighing 1.53 tons, cast using a cooler extension (Fig. 1). At three different levels (bottom, center, and beneath the head area) of an ingot, sections were cut for subsequent study by method L in a MIM-8 microscope. Two hundred fields of view were analyzed in each section. Inclusions and pores were separated with respect to magnitude, in relation to dimensions in the ocular scale divisions: small up to 8.56 μm, medium 8.56-17.12 μm, and large 17.12-25.68 μm.Pore distribution was evaluated for these ingot levels. Pore distribution in the lower level is shown in Fig. 3a. It is seen that the number of fine and large pores decreases from the edge of an ingot towards the center, which is due to the strong cooling action of the massive bottom part of an ingot and cooling of crystals in the semiviscous zone of this region.With respect to pore distribution at the central level (Fig. 3b), there is a clear increase in the number of medium and large pores over the periphery towards the center. This is also connected with features of axial zone formation in the ingot level in question. Conversely, the number of small pores decreases from the periphery toward half the ing...

Phase Transformations in Novel Medium Carbon High Hardenability Steels

“…For this purpose alloyed steels with a high nickel content (up to 4.0-4.5 mass. %, for example, EN39B steel) are implemented, because nickel intensively suppresses the diffusion transformation of the austenite [3][4][5]. But high nickel content significantly increases the price of a steel products due to its expensiveness [6][7][8].…”