Non-metallic inclusions in micropores of massive steel castings

Introduction

The quality of cast parts is a key economic factor not only in the production of castings, but also in their subsequent operation, for example, in the country’s oil and gas sector [1; 2]. Therefore, the study of the cast structure of such castings makes it possible to optimize the technological parameters of the casting process and improve the quality of cast billets [3; 4]. As numerous studies have shown [5], regardless of the casting method or material, defects in castings made of ferrous and non-ferrous metals – such as discontinuities and inclusions – are the main cause of production rejects. Improving the quality of cast steels is directly related to reducing the amount of non-metallic inclusions (NMIs) [6 – 10], as well as understanding their chemical composition, crystalline structure, morphology, location, size, and distribution [11 – 13]. With the advancement of scanning electron microscopy, which enables X-ray spectral analysis in micro-volumes of about 1 µm³, new opportunities have emerged for in-depth studies on the origin and formation of NMIs in steels [14 – 16].

Combining these methods enables identification of the causes of micropore formation that degrades casting quality and development of appropriate mitigation strategies. The relationship between NMIs and micropores is of particular interest as they have a significant impact on the properties of castings. The aim of this study is to investigate NMIs located near and inside micropores in a massive casting, and to develop measures for reducing their number.

Materials and methods

The chemical composition of the studied massive casting (a “housing”) weighing 10 tons without the casting head, as determined from a ladle sample, corresponds to cast microalloyed steel 20GL (GOST 977–88). However, the results of X-ray spectral analysis indicated higher levels of nickel, copper, and zinc. The steel melting process was carried out in an electric arc furnace (EAF) with basic lining and a 25-ton capacity. The molds and cores were produced using the α-set process. It should be noted that during mold filling, the resin binder in the mold and core undergoes thermal decomposition, releasing a large amount of various hydrocarbons, which subsequently, decompose into carbon and hydrogen. The latter promotes formation of gas porosity in the castings [17]. Electron microscopic studies were performed using scanning electron microscopy on the fresh fractures, microstructure, and composition of defective areas in the casting located beneath the casting head, where surface defects are most commonly found. Three billets for impact samples and two billets for preparing longitudinal and transverse microsections were cut from the central areas of the plates that showed no visible cavities. The fresh fractures (examined almost immediately after testing) of impact samples with a sharp V-shaped incision, fractured at –60 °C, were studied on a JSM-7001F scanning electron microscope using secondary electron (SEI) and backscattered electron (BSE) detectors, at magnifications ranging from 30^× to 20,000^×. The hardness of the metal plates was measured using the HV10 method, with a 10 kg load accuracy. The microstructure was examined and photographed using an Axio Observer.D1m optical microscope. In parallel with the microstructure analysis, the elemental composition of the samples was studied using an OXFORD INCA X-MAX 80 energy-dispersive X-ray spectrometer, both at discrete points and in mapping mode.

Results and discussion

The microstructure of the plates consists predominantly of ferrite and pearlite with varying degrees of dispersion. It also includes a Widmanstätten structure, consistent with the microstructure of 20GL cast steel described in the reference handbook [18]. The hardness of the metal plates varies significantly, ranging from 162 – 181 to 192 – 214 HV10 , while the impact toughness increases from 5.1 – 34.0 to 43.0 – 98.0 J/cm².

Scanning electron microscopy (SEM) of the fresh fractures revealed a mixed fracture mode in the impact samples with a sharp V-shaped incision, tested at –60 °C. Brittle fracture predominated, accompanied by a minor ductile component and the presence of opened pores (Fig. 1, a). On unetched longitudinal microsections, even at relatively low magnification with an optical microscope, areas containing numerous round or elongated pores of various sizes were observed. Some of these pores contained NMs. The diameters of the pore openings on the external surface ranged from 120 to 130 µm, while the inclusions within them measured between 1.24 and 2.7 µm. A general overview revealed grain boundaries and grain bodies free from inclusions. At higher magnification under SEM (Fig. 1, b), the inclusions were identified as multiphase, and the chemical composition of each phase was determined (Fig. 1, c). For example, the inclusion shown in Fig. 1, b consists of a central region of magnesia spinel (MgO·Al₂O₃) enclosed by a lighter calcium sulfide (CaS) shell – both phases that should have been removed with the slag.

Microanalysis of the elemental composition in regions containing NMIs, conducted in various modes, enabled the identification of their composition, nature, spatial distribution, and likely causes of their origin. For example, Fig. 2 presents a composite image combining elemental distribution maps for iron, silicon, aluminum, and carbon over an electron micrograph of the inner surface of a micropore. Against the background of the iron matrix (green), particles containing silicon (red), carbon (blue), and aluminum (orange) are observed both within the micropore and in its vicinity. These particles are typically multicomponent rather than monoelemental. A comparison of the silicon and aluminum maps clearly shows that most particles contain both elements. In addition, spectral analysis confirms the presence of oxygen and sulfur in the same region. These elements, along with carbon, are likely gaseous products of thermal decomposition of the resin in the mold, followed by pyrolysis, which releases carbon, oxygen, and sulfur [17]. These gases contribute to pore formation and subsequently condense within them.

In this micropore region, an uneven distribution of aluminum was also observed – either along grain boundaries or as discrete particles (Fig. 3, a). Aluminum frequently forms nitrides with nitrogen, as demonstrated in the corresponding elemental distribution maps (Fig. 3, b, c). In some micropores of impact samples fractured at –60 °C, non-metallic particles of MnS, CaS, MgS, and others that were not removed with slag were also identified (Fig. 4).

Scanning electron microscopy of fresh fractures from impact samples cut from other plates revealed, in addition to previously identified NMIs (MgO·Al₂O₃, CaS, SiC) located in and around pores, the presence of additional compounds (based on X-ray spectral microanalysis) containing carbon, oxygen, phosphorus (C + O + Ca + Mg + P + Si), as well as magnesium and silicon (C + O + Mg + Si). In one of the plates, near the incision area of the fracture, a region densely populated with particles вcontaining Al, S, and O was found. Some pores also contained compounds such as C + O + S and C + O + Ca while in one pore, particles composed of a wide range of light elements (C + Mg + Al + Si + S + K + Ca + Ti) were detected. Silicon carbides and iron oxides were also detected within the pores. In one of the electron micrographs (Fig. 1, a), a spherical iron oxide particle can be seen being drawn into a micropore. According to the results of X-ray spectral analysis, some small, round precipitates consist of pure zinc or its oxide, ZnO. In complex NMIs, the zinc content ranges from 4.86 to 25.57 wt. % (Table). The location, shape, and size of carbon particles on the inner surface of the micropore are clearly distinguishable (Fig. 4, c). In opened pores, calcium sulfides are sometimes observed surrounded by magnesia spinel, along with aluminum carbonitrides and copper particles.

Conclusions

Scanning electron microscopy of fresh fractures and the microstructure of plates cut from a massive casting showed that oxide films and NMIs are virtually absent at the grain boundaries and within the grain bodies. NMIs were observed primarily inside micropores or near their edges, while aluminum carbonitrides were found along grain boundaries and within the grains. The chemical composition of NMIs, determined by X-ray spectral analysis in micro-volumes of approximately 1 µm³, revealed the presence of carbon, oxygen, sulfur, nitrogen, zinc, and other elements. These findings helped to clarify the nature and likely causes of the inclusions and formed the basis for practical recommendations aimed at preventing visually detectable micropores in massive castings. To reduce the number of NMIs in castings, it is necessary not only to lower the oxygen content by a factor of 2 – 3, as proposed in [19], but also to implement the following measures.

• Use a cleaner charge that is free of zinc during steel melting to prevent its ingress into the melt. Due to its low boiling point (907 °C [20]), zinc vaporizes during mold filling and rises along with other gases toward the casting head, where it condenses during crystallization – either in pure form or as ZnO-type compounds – on the walls of forming micropores, shrinkage porosities, and cavities.

• Remove slag more thoroughly to prevent its involvement in pore formation and deposition of its particles in or near pores in the form of magnesia spinel, calcium and manganese sulfides, and oxides of iron, aluminum, calcium, etc.

• Limit the residual aluminum content in the steel after deoxidation to 0.03 %, as aluminum reacts with nitrogen to form nitrides that precipitate not only along grain boundaries, impeding grain growth and promoting an inherently fine-grained structure, but also within the grains – either as thin plates or as irregularly shaped regions in micropores.

• Avoid overheating the steel melt, as excessive arc temperatures in the furnace promote formation of atomic nitrogen, which dissolves in the molten steel and contributes to the formation of aluminum nitrides and carbonitrides.

• Minimize the resin binder content in the mold material, since excessive resin generates large volumes of gaseous thermal decomposition products (hydrocarbons), which subsequently break down into carbon and hydrogen – promoting formation of gas-related defects such as pores and cavities.