Papers by Keyword: Non-Metallic Inclusions

Abstract: High-throughput computational screening (HTCS) based on CALPHAD (Calculation of Phase Diagram) was employed to investigate potential chemical compositions within the medium manganese steel family for achieving desired austenite stability and stacking fault energy (SFE). The primary objective was to identify optimal alloy compositions that balance the complex effects of various alloying elements on retained austenite fraction and related mechanical properties. Utilising TC-Python Thermo-Calc software coupled with a custom-developed algorithm, two optimised compositions were determined: 0.35C, 9Mn, 1Mo, 3Al, 1Si, 0.05Nb, 0.3V (alloy 353), and 0.35C, 9Mn, 1Mo, 3Al, 1Si, 0.1Nb (alloy 310) in wt.% to be the best fited composition to our selected criteria. The alloys were subsequently produced via open-air induction furnaces, and the microstructure was analysed after the hot forging condition. The initial multiphase as-cast structure, primarily composed of lath martensite, δ-ferrite (34 vol.%), and retained austenite (RA, 5–7 vol.%), experienced notable grain refinement. Forging reduced δ-ferrite grain sizes from 39 µm to 12 µm (alloy 310) and from 46 µm to 9 µm (alloy 353), accompanied by increased RA content (28 vol.% for alloy 310 and 46 vol.% for alloy 353) and reduced RA grain sizes (1.2 µm and 1.9 µm, respectively). Non-metallic inclusions (NMIs) were analysed using field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, classifying inclusions primarily as AlN, MnS, (Mo,Nb)C, or their combinations. No significant differences in inclusion types were observed, but forged samples displayed reduced inclusion sizes. In summary, the results showed that HTSC effectively identified optimal compositions with a high fraction of retained austenite.

Abstract: Characterization, classification, and size distribution of non-metallic inclusions (NMIs) in a cast high-Mn TWIP steel (Fe-20Mn-0.6C-1.5Al-0.3V, in wt.%), were studied to explore their interplay with the fracture mode during tensile deformation. NMIs were separated by electrochemical extraction, and subsequent X-ray dispersive analysis was performed to characterize their compositions. Subsequently, 70 % cold rolled TWIP sheets were processed and undergone fast heating annealing (FHA) at a heating rate of 200°C/s to anneal at temperatures 750 - 850 °C for 30 s. The grain structures achieved by FHA were evaluated by EBSD. The mechanical properties were determined by tensile testing. Distinct categories of NMIs, including Al2O3 and Mn (S,Se), and (Ti,V)N nitrides, and intricate combinations of inclusions, were identified. FHA process at low temperatures, 750-800 °C, promoted partially recrystallized microstructures. Fully recrystallized structures were obtained at 850 °C characterized by an average grain size of 2 µm at 850 °C. The structure promoted at FA 850 °C displayed noteworthy elongation of 60% with a yield strength (YS) and tensile strength (TS) of 410 and 830 MPa, respectively. A minimal effect of NMIs in TWIP steel was observed due to activating mechanical twinning mechanism, which overcomes the detrimental impact of NMIs and retard the necking induced by void formation related to NMIs.

Abstract: In this paper, the contact interface between FGH96 superalloy melts and refractory slurry with corundum powder and silica sol at 1600°C with different soaking time in 10-240 min range was investigated. The morphology and composition of the contact interface were studied by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the reaction layer formed on the contact interface between the melting superalloy and the refractory slurry, and is mainly consist of Alumina and contains a small amount of other metal elements such as Ti and Cr. The reaction layer was not smooth on the micro level, and there was a peninsula-like structure protruding into the internal part of the melting superalloy on the reaction zone, and even started to fall off at some places to form islands. With the increase of soaking time, the reaction between melt of superalloy and refractory slurry increased gradually and the reaction layer began to combine with the refractory slurry substrate and form obvious interaction layered structure, resulting in the corrosion of refractory slurry substrate. With the soaking time over 120 min, the stable contact interface was destroyed. Thermodynamic calculation shows that the substitution reaction between Al in superalloy and SiO₂ in refractory slurry meets the thermodynamic conditions, and the reaction can proceed forward.

Abstract: The paper presents the results of the study of fragments of the electric engine rotor shafts of the air-cooled units in order to establish the causes of its destruction. Chemical composition, type of the fracture, macro-and microstructure, as well as the mechanical properties of the metal are studied. Structural and technological factors contributed to the destruction are identified.

Abstract: The article presents the test results of complex microcrystalline modifiers containing calcium, barium, strontium, rare earth metals. Complex modifiers were used in the processing of steel for 17G1S-U pipes in order to reduce its contamination with non-metallic inclusions, including corrosive ones. The use of modifiers allowed to reduce metal contamination by non-metallic inclusions of all kinds. The most experimental non-metallic inclusions were obtained during metal processing with INSTEEL^®5.1 and INSTEEL^®9.4 modifiers. In addition, the use of experienced modifiers ensured the production of complex oxysulfides of calcium, cerium and lanthanum with low oxygen content and thermal expansion coefficients, which increases the corrosion resistance of steel.

Abstract: X-ray structural studies have shown that with the increase of the life of the pipes, the surface layers of the metal adsorb a significant amount of gases (hydrogen, oxygen and sulfur), especially this is strongly manifested in the field of corrosion defects (ulcers, pitting). Also, it was found that in the process of long-term operation, the metal of the pipe walls is heavily flooded, which directly leads to its brittle and, as a consequence, increased hardness and reduced ductility. Such studies are unique in the subject of corrosion-mechanical resistance of metal and its degradation during long-term operation in the presence of alternating loads and at the same time, the aggressive environments of oil and gas fields.

Abstract: Fatigue cracks can start from non-metallic inclusions in the subsurface zone at low stress amplitude level and high number of cycles. This phenomenon was observed mainly at high strength steels and parts with hardened surface zone. Fatigue strength of steel can be then estimated on the basis of hardness and maximum size of defect/inclusions measurements. Many inclusions rating methods exist, some of which have been adopted as the standards. However, “standard” methods for characterization of inclusion content are not convenient for assessment of their influence on fatigue strength of materials. This contribution deals with assessment of maximum sizes of inclusions in three types of martensitic steels used for production of rotating blades of steam turbines (Böhler T552, Böhler T671, and MLX 17) using extreme value statistics methods. Murakami´s concept of √AREA_max parameter and procedure according to ASTM E2283-08 were applied. Results were compared one another and also with the result of standard methods, too. Founded parameters were used for calculation of expected limit of the fatigue strength and compared with results of fatigue test of these steels.

Abstract: Non-metallic impurities in continuous cast billets are evaluated by a growth rate of edge point impurities. The first part of our research was devoted to a relationship between the growth rate of edge point impurities and other macrostructure defects. A correlation-regression analysis of steel macrostructure quality showed the relationship between the rate of edge point impurities and segregation cracks in general, as well as corner streaks. The second part of our research indicated that impurities in billets from conventional carbon steel were crucially influenced by a method of pouring steel from a tundish into a mould. By transferring from open stream casting to shrouded casting, the quantity of non-metallic impurities in billets decreases by 7 times. In case of open stream casting, prevailing inclusions are oxides resulting from secondary oxidation, while the growth rate of edge point impurities in billets increases with an increased content of sulphur and phosphorus in steel, and decreases with an increased manganese/sulphur ratio. In case of shrouded casting, non-metallic impurities are limited to casting temperature and speed: steel overheating in the tundish above the liquidus temperature and withdrawal speed of billets. Less non-metallic impurities in case of the shrouded casting are contributed by steel overheating in the tundish above the liquidus temperature over 30oС and withdrawal speed, not exceeding 2.5 m/min.

Abstract: The analysis of the results of steel de-oxidation with manganese and silicon during tapping from an arc furnace in the Ural Steel is carried out. It is shown that the absorption of manganese and silicon during de-oxidation varies widely: from 33.0 to 88.4% (average 68.8%) for manganese and from 25.9 to 79.8% (average 63.8%) for silicon. The main causes of high varies and low absorption of deoxidizing agents were established: low carbon content and high metal overheating before de-oxidation. A statistical analysis of St 37-3 steel de-oxidation data was performed with the aim of improving the efficiency of steel de-oxidation technology in a ladle. The regression equations are obtained, allowing to calculate the assimilation of deoxidizers and the consumption of ferroalloys, depending on the temperature and carbon content in the metal during tapping from the arc furnace.

Abstract: The paper reports findings on the morphology of non-metallic inclusions in low carbon pre-peritectic and peritectic steel grades used for the fabrication of seamless pipes. It is demonstrated that the distribution of non-metallic inclusions over the cross section area of continuously cast billets is of a step-like nature conditioned by the features of billet solidification. In all the steels analyzed the non-metallic inclusions are presented by oxides, sulfides and complex oxi-sulfides not larger than 2 μm.