Designing order–disorder transformation in high-entropy ferritic steels

Order–disorder transformations hold an essential place in chemically complex high-entropy ferritic steels (HEFSs) due to their critical technological application. The chemical inhomogeneity arising from mixing of multi-principal elements of varying chemistry can drive property altering changes at the atomic scale, in particular short-range order. Using density-functional theory-based linear-response theory, we predict the effect of compositional tuning on the order–disorder transformation in ferritic steels—focusing on Cr–Ni–Al–Ti–Fe HEFSs. We show that Ti content in Cr–Ni–Al–Ti–Fe solid solutions can be tuned to modify short-range order that changes the order–disorder path from BCC-B2 (Ti atomic-fraction = 0) to BCC-B2-L21 (Ti atomic-fraction > 0) consistent with existing experiments. Our study suggests that tuning degree of SRO through compositional variation can be used as an effective means to optimize phase selection in technologically useful alloys. Graphic abstract

Study of surface defects in tubes made from nondeformed continuously cast billets

Defects of outer and inner surfaces of hot-rolled tubes of various steel grades and sizes manufactured on tube-rolling unit with a continuous mill (TPA 30-102) at Interpipe Nikotube LLC from a nondeformed continuously cast billets produced by MZ Dniprostal LLC have been studied. Characteristic genetic and morphological signs of defects were revealed which makes it possible to reliably classify them, identify cause of defect formation and recommend measures to eliminate them. Defects on the outer and inner surfaces of tubes are of metallurgical origin and associated with quality of initial continuously cast billets (a consequence of violation of the smelting and continuous casting technology). Defects on the inner surface of tubes were caused on defects in the axial zone of original billets (unacceptable porosity, looseness, chemical inhomogeneity, liquation stripes and cracks, etc.) and are classified as steel-smelting films and bulges. It was found that displacement of the thermal center of crystallization (a feature of the machines for continuous steel casting of curvilinear type) had an additional negative effect on quality of the inner surface of the studied tubes. Defects on the outer surface of tubes are tears of burning in places of accumulation of low-melting inclusions and their eutectics, as well as steel-smelting scabs on rolled dirt and gas bubbles. Likelihood of formation of scabs on outer surface of the studied tubes over rolled crust introversions is not excluded. The study results will allow manufacturers to reliably classify defects, promptly reject tubes with unacceptable defects of metallurgical origin and minimize supply of low-quality products to consumers. These results will later be included in the classifier of defects in tubes manufactured on the TPA 30-102 unit from nondeformed continuously cast billets. The results of the study of natural signs of defects of metallurgical origin in the tube surface will be useful for elaboration of measures aimed at improvement of the technology of manufacturing initial tube billets. Keywords: tube surface defects, continuously cast billets, microstructure, rolled contamination, low-melting inclusions, eutectic, gas bubbles, decarburization, liquation.

On the question of using solid electrodes in the electrolysis of cryolite-alumina melts. Part 3. Electric field distribution on the electrodes

The aim of this work is to identify the theoretical limitations of molten salts electrolysis using solid electrodes to overcome these limitations in practice. We applied the theory of electric field distribution on the electrodes in aqueous solutions to predict the distribution of current density and potential on the polycrystalline surface of electrodes in molten salts. By combining the theoretical background of the current density distribution with the basic laws of potential formation on the surface of the electrodes, we determined and validated the sequence of numerical studies of electrolytic processes in the pole gap. The application of the method allowed the characteristics of the current concentration edge effect at the periphery of smooth electrodes and the distribution of current density and potential on the heterogeneous electrode surface to be determined. The functional relationship and development of the electrolysis parameters on the smooth and rough surfaces of electrodes were established by the different scenario simulations of their interaction. It was shown that it is possible to reduce the nonuniformity of the current and potential distribution on the initially rough surface of electrodes with an increase in the cathode polarisation, alumina concentration optimisation and melt circulation. It is, nonetheless, evident that with prolonged electrolysis, physical and chemical inhomogeneity can develop, nullifying all attempts to stabilise the process. We theoretically established a relationship between the edge effect and roughness and the distribution of the current density and potential on solid electrodes, which can act as a primary and generalising reason for their increased consumption, passivation and electrolytic process destabilisation in standard and low-melting electrolytes. This functional relationship can form a basis for developing the methods of flattening the electric field distribution over the anodes and cathodes area and, therefore, stabilising the electrolytic process. Literature overview, laboratory tests and theoretical calculations allowed the organising principle of a stable electrolytic process to be formulated -the combined application of elliptical electrodes and the electrochemical micro-borating of the cathodes. Practical verification of this assumption is one direction for further theoretical and laboratory research.

On the question of using solid electrodes in the electrolysis of cryolite-alumina melts. Part 2. The mechanism of passivation and conditions of stable electrolysis

The aim was to investigate the mechanism of passivation of polycrystalline cathodes and to justify experimentally the possibility of stable electrolysis when using solid electrodes. Under laboratory conditions, the mechanism of electrode passivation and the conditions for stable electrolysis were experimentally studied. To this end, the methods of X-ray phase analysis and electron-microscopic examination of the spent electrodes were employed. A study of the electrolysis of cryolite-alumina melts showed that, in the presence of surface micro- and microdefects on a solid cathode, a precipitate consisting of impurities and electrolyte components was gradually formed. Under the selected experimental conditions, the surface of carbon cathodes was passivated with a dense double-layer precipitate of CaB6 and electrolyte components. Using the example of a carbon cathode containing both metallic titanium and titanium oxides, a method for eliminating surface microdefects is presented. This method consists in electrochemical borating of a carbon-titanium cathode. The conducted spectral electron microscopic and energy-dispersive analysis found that, during a 45-hour laboratory experiment at 980 °C and under a current density of 0.7 A/cm2, the inhomogeneous surface of the cathode was homogenized with a titanium diboride layer. At stable electrolysis parameters, an aluminum layer is electrodeposited on the cathode. A complex analysis of the electrolysis conditions, the appearance of the initial and spent carbon cathodes, and the data of analytical studies confirmed that micro- and macrodefects of the electrode cause the formation of a dense layer of deposits on the cathode. The established mechanism of passivation of a carbon cathode as a polycrystalline product can be applied to all composite electrodes, including those based on titanium diboride. A logical condition for the practical application of solid cathodes is the development of an electrolysis process with continuous surface reconditioning to decrease the chemical inhomogeneity and microdefects of the surface across the entire technological sequence.

Adhesion and friction in hard and soft contacts: theory and experiment

This paper is devoted to an analytical, numerical, and experimental analysis of adhesive contacts subjected to tangential motion. In particular, it addresses the phenomenon of instable, jerky movement of the boundary of the adhesive contact zone and its dependence on the surface roughness. We argue that the “adhesion instabilities” with instable movements of the contact boundary cause energy dissipation similarly to the elastic instabilities mechanism. This leads to different effective works of adhesion when the contact area expands and contracts. This effect is interpreted in terms of “friction” to the movement of the contact boundary. We consider two main contributions to friction: (a) boundary line contribution and (b) area contribution. In normal and rolling contacts, the only contribution is due to the boundary friction, while in sliding both contributions may be present. The boundary contribution prevails in very small, smooth, and hard contacts (as e.g., diamond-like-carbon (DLC) coatings), while the area contribution is prevailing in large soft contacts. Simulations suggest that the friction due to adhesion instabilities is governed by “Johnson parameter”. Experiments suggest that for soft bodies like rubber, the stresses in the contact area can be characterized by a constant critical value. Experiments were carried out using a setup allowing for observing the contact area with a camera placed under a soft transparent rubber layer. Soft contacts show a great variety of instabilities when sliding with low velocity — depending on the indentation depth and the shape of the contacting bodies. These instabilities can be classified as “microscopic” caused by the roughness or chemical inhomogeneity of the surfaces and “macroscopic” which appear also in smooth contacts. The latter may be related to interface waves which are observed in large contacts or at small indentation depths. Numerical simulations were performed using the Boundary Element Method (BEM).

On the question of using solid electrodes in the electrolysis of cryolite-alumina melts. Part 1.

This article is aimed at identifying issues associated with the use of solid cathodes in the electrolysis of cryolitealumina melts in order to determine conditions for their practical application. The contemporary technology of using solid anodes and cathodes is reviewed from its inception to the present time. The problems of stable electrolysis are discussed, such as effects of the electrode surface on the technological process. It is shown that all attempts undertaken over the recent 100 years to use solid electrodes, both reactive and inert, have been challenged with the emergence of electrolysis instability, formation of precipitates of varying intensity on the electrodes and impossibility of maintaining a prolonged process at current densities of above 0.4–0.5 A/cm2. Information is provided on the attempts to use purified electrolyte components with different ratios, metal-like and ceramic electrodes with a high purity and a smooth surface in order to approach real industrial conditions. However, to the best of our current knowledge, these experiments have not found commercial application. The authors believe that the most probable reason for the decreased current efficiency and passivation of solid electrodes consists in the chemical inhomogeneity and micro-defects of the bulk and surface structure of polycrystalline cathodes and anodes. It was the physical inhomogeneity of carbon electrodes that directed the development of the nascent electrolytic production of aluminium towards the use of electrolytic cells with a horizontal arrangement of electrodes and liquid aluminium as a cathode. This reason is assumed to limit the progress of electrolytic aluminium production based on the use of inert anodes and wettable cathodes in the designs of new generation electrolytic cells implying vertically arranged drained cathodes. The theoretical and experimental examination of this assumption will be presented in the following parts of the article.

Reasons for banding formation in steels of K60 strength category

Active development of pipeline transport of gas and oil with increasing working pressure to 120 atm. increases the need for pipes with large wall thickness that correspond the requirements of the DNV OS-F101 standard. Quality of continuously cast billets is decisive for improving quality of sheet metal for main pipelines. Inheritance of cast structure imperfections by a hot-rolled sheet leads to structural heterogeneity of the strip and the layered nature of the fracture surface and adversely affects the mechanical properties and corrosion resistance. Structural heterogeneity of rolled products is appeared in the form of axial ferrite-martensitic metal banding and metal banding in the main section of the sheet consisting of a mixture of ferritic and pearlitic grains — pearlitic metal banding. The flatness in the axial zone of the rolled products is due to axial chemical heterogeneity which is objectively formed during crystallization of the continuously cast billet and further phase transformations. The axial chemical inhomogeneity does not resolve despite the recrystallization of the structure and deformation and the high content of alloying elements contributes to the formation of the martensite phase and large carbonitride precipitates. The cause of pearlite bands is considered usually to be the presence of dendritic segregation. According to us the reason of this metal banding is the shift of the temperature front of γ → α transformation parallel to the sheet surface in depth as a result of which before the next volume of formed ferrite the concentration of carbon dissolved in austenite increases with the subsequent formation of pearlite. The enrichment of austenite proceeds along the boundaries preserved from the δ → γ transformation during cooling the slab and the formed pearlite structure repeats the shape of the boundaries of these grains in the section parallel to the sheet plane.