The Effect of Rare Earth Metals Alloying on the Internal Quality of Industrially Produced Heavy Steel Forgings

The paper presented the findings obtained by industrial research and experimental development on the use of rare earth metals (REMs) in the production of heavy steel ingots and their impact on the internal quality of the 42CrMo4 grade steel forging. REMs alloying was carried out after vacuuming the steel. A relatively large melting loss of cerium (about 50%) and its further decrease in casting due to reoxidation were observed. Refinement of structure and better mechanical properties of forged bar containing about 0.02 wt.% of Ce compared to that of the standard production were not achieved. The wind power shaft with content of about 0.06 wt.% of Ce showed high amount of REM inclusions, which were locally chained, and in some cases, initiated cracks. Four stoichiometrically different types of REM inclusions were detected in forgings, namely (La-Ce)2O2S + (La-Ce)O2 + SiO2 (minority); oxygen, phosphorus, arsenic, and antimony bound to lanthanum and cerium probably bonded with iron oxides La + Ce, MgO, Al2O3 a SiO2; (La-Ce)2O2S, FeO, SiO2, and CaO or CaS.

THE USE OF EXTERNAL PHYSICAL INFLUENCE TO REGULATE THE FORMATION OF THE STRUCTURE OF STEEL INGOTS

В настоящей работе исследовано влияние температурных и силовых параметров внешнего воздействия на кристаллизацию и структурообразование слитков из модельных и реальных сплавов. Показано, что скоростью зарождения и роста кристаллов, а также размерами структурных зон в слитках можно управлять, изменяя температурный градиент, возникающий в жидкой фазе перед границей затвердевания. Управлять градиентом температур по сечению затвердевающего расплава можно, регулируя интенсивность теплоотвода, а также дифференцируя теплоотвод по периметру формирующегося слитка. Установлено, что от соотношения градиента температуры к скорости кристаллизации - G/R зависит формирование той или иной структурной зоны слитков. Наглядно доказана эффективность влияния вибрации на формирование структуры исследуемых слитков. Определен механизм воздействия вибрации на процессы кристаллизации и формирования структур слитков, который заключается в проявлении следующих эффектов: в разрушении и дроблении дендритов, растущих на фронте кристаллизации, под действием знакопеременных изгибающих давлений упругой волны; в зарождении кристаллов в объеме кристаллизующегося расплава вследствие кавитации; в разрушении дендритов в расплаве и на фронте кристаллизации под действием давлений, возникающих в результате схлопывания кавитационных каверн. In this paper, the influence of temperature and force parameters of external influence on the crystallization and structure formation of ingots from model and real alloys is investigated. It is shown that the rate of crystal nucleation and growth, as well as the size of the structural zones in the ingots, can be controlled by changing the temperature gradient that occurs in the liquid phase before the solidification boundary. You can control the temperature gradient along the cross-section of the solidifying melt by adjusting the intensity of the heat sink, as well as differentiating the heat sink along the perimeter of the forming ingot. It is established that the ratio of the temperature gradient to the crystallization rate - G/R depends on the formation of a particular structural zone of ingots. The effectiveness of the influence of vibration on the formation of the structure of the ingots under study is clearly proved. The mechanism of vibration influence on the processes of crystallization and formation of ingot structures is determined, which consists in the manifestation of the following effects: in the destruction and crushing of dendrites growing at the crystallization front under the action of alternating bending pressures of an elastic wave; in the nucleation of crystals in the volume of the crystallizing melt due to cavitation; in the destruction of dendrites in the melt and at the crystallization front under the action of pressures resulting from the collapse of cavitation cavities.

Devising manufacturing techniques to control the process of zonal segregation in large steel ingots

A method of physical modeling was applied to study the effect of external actions on the processes of crystallization and the formation of the structure of ingots. A brief review of existing hypotheses about the evolution of physical, structural, and chemical heterogeneities in large steel ingots is given. The parameters of the structure and the two-phase zone have been determined, as well as the nature of the distribution of segregated materials along the cross-section of ingots, depending on the conditions of their curing. The decisive importance of convective and capillary mass transfer in the interdendritic channels of hardening ingots on the formation of a zonal heterogeneity at their cross-section has been proven. Experimentally, when crystallizing a model environment (camphene), it has been visually confirmed that the flow of segregated materials in interdendritic channels occurs when a certain amount of impurities accumulates in them. A clear dependence of the speed of this flow on the rate of melt crystallization has been established. With an increase of the hardened part of the melt, the rate of segregated material movement (Vl) increases while the rate of crystallization (R) decreases due to worsening heat release conditions. At a certain distance from the ingot’s surface, these rates become equal, and impurities are carried to the curing border, which is the main cause of the formation of zonal segregation. The results reported here show that the evolution of zonal segregation in ingots can be controlled using various techniques involving external influence on the hardening melt. This study has demonstrated that the adjustable intensity of heat removal from an ingot, as well as the addition of external excess pressure on the hardening melt, could be used as such tools. In the study, to obtain ingots with a minimum level of chemical heterogeneity, it would suffice to provide the following conditions for the curing of the alloy: a value of the alloy crystallization speeds at the level of Rcr ≥ 9·10–2 mm/s, or external pressure on the free surface of ingots Рext. ≥ 135 kPa. The industrial implementation of the reported results could make it possible to improve the technology of obtaining large blacksmith ingots, provide savings in materials and energy resources, increase the yield of a suitable metal, and improve its quality

Impact of Solidification on Inclusion Morphology in ESR and PESR Remelted Martensitic Stainless Steel Ingots

This study focuses on the impact of solidification on the inclusion morphologies in different sizes of production-scale electro-slag remelting (ESR) and electro-slag remelting under a protected pressure-controlled atmosphere, (PESR), ingots, in a common martensitic stainless steel grade. The investigation has been carried out to increase the knowledge of the solidification and change in inclusion morphologies during ESR and PESR remelting. In order to optimize process routes for different steel grades, it is important to define the advantages of different processes. A comparison is made between an electrode, ESR, and PESR ingots with different production-scale ingot sizes, from 400 mm square to 1050 mm in diameter. The electrode and two of the smallest ingots are from the same electrode charge. The samples are taken from both the electrode, ingots, and rolled/forged material. The solidification structure, dendrite arm spacing, chemical analyzes, and inclusion number on ingots and/or forged/rolled material are studied. The results show that the larger the ingot and the further towards the center of the ingot, the larger inclusions are found. As long as an ingot solidifies with a columnar dendritic structure (DS), the increase in inclusion number and size with ingot diameter is approximately linear. However, at the ingot size (1050 mm in diameter in this study) when the center of the ingot converts to solidification in the equiaxial mode (EQ), the increase in number and size of the inclusions is much higher. The transition between a dendritic and an equiaxial solidification in the center of the ingots in this steel grade takes place in the region between the ingot diameters of 800 and 1050 mm.

D.K. Chernov’s role in creating and developing the doctrine of modern metallurgy and metal science. Part 2. Scientific and practical confirmation of D.K. Chernov’s ideas

The second part of the article is devoted to the practical realizing of D.K. Chernov’s scientific “predictions”. They were confirmed by him in the implementation of the concepts of points “a” and “b”, structure of the ingot and the possibility, taking into account the knowledge of these temperature boundaries, to build basic modes of deformation and subsequent heat treatment of steel ingots. These materials are supported by a modern interpretation of that provisions and additional materials of the authors.

Chemical and physical heterogeneities and gases in large steel ingot

For creation of the high-tech equipment that is used in energy, heavy engineering, chemistry and transport, the unique large-sized steel products are required. In the manufacture of such products, large forging ingots in the mass to 600 tons are used. However, an increase in the mass of the ingots leads to the formation of chemical and physical heterogeneity, enlargement and unfavorable distribution of non-metallic inclusions, of the development of segregation defects in them, which reduce the strength and exploitation characteristics of the metal. In this connection, the quality forgings and finished parts are not always meet the producing demands and the loss of metal, in the form of technological waste and rejects are reaching significant values. It is known that eccentric zonal segregation, especially it’s the most dangerous variety - cords, significantly reduce the quality and properties of products from large steel ingots. In connection with the continuous expansion of the production of large ingots, the problem of creating optimal technologies for their formation, which reduce or exclude the possibility of the formation of chemical heterogeneity and cords in steel during crystallization, it is currently important and relevant. In this paper it are presented the results of studies of the structure, gas distribution, physical and chemical heterogeneities in the cross section and height of an ingot in the mass of 140 tons, which was casted in vacuum from steel 25KHN3MFA. It is shown that depending on the temperature and time conditions of ingot hardening, among which the crystallization interval (due to the chemical composition of steels), cooling intensity in different volumes in height and cross section of ingot, temperature gradient before the crystallization front, solubility of alloying elements and gas content in the melt, etc. Based on this, when developing technology for large ingots to ensure their quality, optimal structure and properties should take into account not only their dimensions, but also the combination of these thermokinetic parameters on the crystallization process, dendritic structure formation, manifestations of liquation in different ingot volumes. Keywords: ingot, segregation strip and inclusions, dendrites, structure, oxygen, oxides, sulfides.

Feasibility of Reduced Ingot Hot-Top Height for the Cost-Effective Forging of Heavy Steel Ingots

Feasibility studies have been performed on ingots with reduced hot-top heights for the cost-effective hot forging of heavy ingots. The quality of the heavy ingots is generally affected by internal voids, which have been known to be accompanied by inclusions and segregation. To guarantee the expected mechanical performance of the forged products, these voids should be closed and eliminated during the hot open die forging process. Hence, to effectively control the internal voids, the optimum hot-top height and forging schedules need to be determined. In order to improve the utilization ratio of ingots, the ingot hot-top height needs to be minimized. To investigate the effect of the reduced hot-top height on the forged products, shaft and bar products have been manufactured via hot forging of ingots having various hot-top heights. From the operational results, the present work suggests effective forging processes to produce acceptable shaft and bar products using ingots having reduced hot tops. The mechanical properties of shop-floor products manufactured from ingots with reduced hot tops have also been measured and compared with those of conventional ingot products.

INFLUENCE OF METAL OXIDATION AND FEATURES OF VACUUMING ON THE FORMATION AND LOCATION OF NONMETALLIC INCLUSIONS IN LARGE STEEL INGOTS 38ХН3MФA

The article deals with the influence of steel oxidation and features of vacuuming on the processes of formation and distribution of non-metallic inclusions in large ingots and forgings intended for power engineering. It is shown that the distribution of sulfides and oxysulfides across the ingot cross-section is inversely related to each other, due to changes in oxygen and sulfur concentrations during the crystallization of the melt. Under the influence of intensification of degassing process at vacuum casting of ingots at the expense of purging of a jet of metal argon, the formed nonmetallic inclusions had the minimum sizes, more favorable distribution that caused increase in plastic characteristics of the received forgings on the average for 15-20 %