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Modeling of Degradation Processes of Cast Iron Carbide Phase of Mill Rolls at Operation
Abstract
In metallurgy, chromium-nickel cast iron is used in making the working layers of double-layer mill rolls. The main distinctive feature of chromium-nickel cast iron is the presence of large inclusions of carbide phase in its structure. This characteristic determines the high ruggedness of the contact surface touching hot rolled metal, and ensures wear resistance of the working tool. Operational degradation and wear of the products surfaces leads to destruction of the carbides crystalline structures; the latter contributes to reduce in hardness and quality of the working layer. The aim of our present study is to investigate the defect conditions in chromium-nickel cast iron and their influence on the development of degradation processes at high temperatures and at presence of the local deformations. We evaluate damageability and degradation of cast iron using the method of optical-mathematical analysis of microstructure images, which comprises scientific novelty of the present work. These values are estimated via modeling the diffusion processes and variability of the dislocation structure in the largest inclusions of the Me3C carbide phase. Based on the results obtained and to underline the practical value of our study, we propose a number of technological solutions that help hinder and eliminate the degradation phenomena described above, thus stabilizing the carbide phase properties and hardening the mill rolls.KeywordsChromium-nickel cast ironCarbide phase degradationDiffusionDislocation structurePhase composition variability
Crushing and grinding are widely used in mining, mineral processing, cement making, and coal power generation. The mechanical components that contact with materials being processed are working under the conditions of high abrasion and impact. How to reduce the wearing damages and prevent from the cracking failure of the mechanical parts is one of the crucial challenges facing the operations of many industries, involved crushing and grinding processes. Whit cast irons, and specifically high chromium white cast irons, have been developed and deployed worldwide for making mechanical components of crushing and grinding machines due to the combination of high wear resistance and good mechanical strength. The paper is aimed to give an overview on the white cast iron alloys, and particularly high chromium white cast iron alloys, including their classifications, their standard specifications in USA, China and Australia, their main usages and the new developments.
The Ni-hard alloys white-cast irons are generally used for high wear work. Among them, those with better impact resistance because of its low carbon content compared to the rest of the family, are studied in this paper. One experimental technique of characterizing the metallic materials is the microstructural study. Several metallographic attacks intended to reveal qualitatively each microconstituent that forms the alloy, as well as the segregation and solidification structure of casting, are studied in this article. The use of color metallography is fundamental in this case to distinguish clearly the microconstituents. The main objective of this paper is to propose a series of attacks that identify each one of the microconstituents present in the alloy that has not been reported up to date.
A new comprehensive approach is developed to characterize the structure formation of high-chromium cast iron in the working layers of mill rolls (10-12 tons weight) within the hot finishers. Application of our approach makes it possible to estimate a phase composition of the alloy. Such heterogeneous material is characterized by insufficient stability of properties and absence of reliable methods for assessing structure formation. The structural features of massive castings (up to 20 tons) are revealed in the process of crystallization and operation under the temperatures of magnetic transformation of the carbide phases (magnetostriction phenomenon). The conducted studies introduce a new understanding of phase composition of high chromium cast iron, phase composition variability at magnetic transformation of carbide phases and decomposition of retained austenite. The variability of phases and their interconnection during casting and heat treatment are estimated. It has been shown that photographs of metallographic images do not allow one to reliably estimate the alloy phase composition, since the primary image pattern after crystallization preserves, while a partial phase decomposition with the formation of their various combinations takes place. The possibility of accurate qualitative and quantitative identification of the formed phases in the cast and heat-treated state in a multiphase heterogeneous alloy (high-chromium cast iron of large masses) is shown. A new approach to characterization of structure formation in a multiphase alloy with the optical-mathematical method for the description of metallographic images is proposed. We show that the insufficient stability of the phase composition while casting under magnetostriction temperatures and with the proposed effective heat treatment method used is determined by the variability of structure formation with the buildup of certain bonds between the phases and decomposition of residual austenite.
Key words: high-chromium cast iron, massive castings, magnetic transformation of carbide phases, residual austenite, composition and interrelation of phases.
The characteristics of different shapes of nonmetallic inclusions (NMIs) are determined using different methods. In this respect, various shapes of NMIs, including spherical, octahedral, elongated, bar‐like, plate‐like, polyhedral, and irregular inclusions, are observed in different steels and ferroalloys. The inclusions are investigated using three methods: 1) 2D investigations on a polished cross section (2D method); 2) 3D investigations on a film filter after electrolytic extraction and filtration (EE method); and 3) 3D investigations on a metal surface after electrolytic extraction (MS method). In addition, scanning electron microscopy (SEM) with an energy‐dispersive spectrometer (EDS) is used for the determination of the chemical composition of inclusions. The advantages and limitations of different methods for investigations of different shaped inclusions are discussed. The results show that the 2D method is less precise to detect the morphology, size, and number of inclusions; however, the EE and MS methods are used to determine a more accurate morphology. Furthermore, the MS method is found to be more advantageous in detecting large‐sized inclusions. This study also shows that the results on both the film filter and metal surface should be grouped together to obtain more comprehensive information of the inclusion characteristics.
To increase the operational durability of tools in production and operation, this paper proposes an integrated approach for processing metallographic images of tool structures at various stages of their life cycle. It is based on the use of the Thixomet Pro software and a specially developed optical-mathematical method, which supplements standard programs for searching for optimal properties and production parameters. The metallographic structural images, obtained by using both optical and electron microscopes, were evaluated with the analysis of pixels in photos. Changes in the structural components of the metal in the two zones (in the main part and at the edge of the working surface of a tool) were comparatively analyzed. During operation, the decomposition of less stable structural components occurs, and a decrease in the proportion of special carbides from 14.4% to 8.15% can be observed. This is caused by the influence of deformation localization, which leads to the fragmentation and alignment of dispersed carbides at an angle of 45° relative to the working surface of a tool deep into the tool under the action of stresses, which during operation are the centers of crack nucleation and development. At the same time, carbide decomposition as well as diffusion of carbon and chromium can be observed. Using the mathematical method for describing structural changes, it was found that under the influence of external factors at the edge of the working surface of a tool, the intensity of the resulting diffusion of chemical components is higher. In addition, zones of damage and maximum local heterogeneity associated with the presence of pores and cracks were identified. This technique made it possible to identify an increase in the anisotropy of properties, formed during operation and associated with metal degradation, and determine the degree of structural heterogeneity.
Currently, cast iron remains one of the major modern casting materials in metallurgy and machine-building industry and is sure to take the lead in the future. Chilled cast iron has high hardness and wear resistance due to a large number of carbide phases in its structure. However, low ductility and impact hardness essentially limit its applicability in terms of processing. Hot plastic working, under which the eutectic net crushing is observed, appears to be one of the most effective means of the eutectic alloy products shape and microstructure transformation. Chilled cast iron properties fundamentally improve after hot plastic working: ductility, strength and impact hardness increase by 2-3 times on retention of the high hardness factor. Chilled cast iron ductility increase can be attained when using phase transformations in eutectic cementite under lean alloying with carbide forming elements. The purpose of the paper is to study alloying effect on the chilled cast iron ductility as well as eutectic cementite behavior under hot rolling. In the paper hardening and softening of the structural components in chilled cast iron under hot working have been studied. The deformation texture forming in eutectic cementite under hot rolling has been revealed, which is connected with the dynamic softening and depends on the degree and the nature of its alloying. The mechanism and regularities of the phase transformation effect in cementite on its behavior under plastic deformation and on the alloys ductility in general have been studied. In cementite chromium alloying initiates processes, that can be characterized as the pre-precipitation stage of the new phases, and this way it contributes to the cast iron ductility reduction and embrittles cementite. Carbide transformation, that occurs in eutectic cementite under alloying with vanadium, stimulates softening of the alloy and increases its ductility level. Moreover, the multiple glide planes {130},{011},{112} in cementite have been determined. It has been found out, that in supersaturated cementite vanadium carbides precipitation stimulates the extra glide plane {111} occurrence under hot rolling. The essence of the carbide transformation phenomenon is that under hot working there occurs the lubricating effect at the transition of the metastable iron carbide condition, which is strengthened with vanadium supersaturation and mechanical hardening, to a more stable condition due to precipitation of the proeutectoid constituents on the one hand, and because of the dynamic softening processes on the other hand. At that, the autocatalyticity effect is observed: there is precipitation of carbides with hardening and softening, similar to the processes that arise as a result of the superplastic effect induced by phase transformations.
Results of a study on influence of Cr, Mo and Al on the microstructure, abrasive wear and corrosion resistance of Ni-Mn-Cu cast iron in the as-cast and heat-treated conditions are presented. Because of the chilling effect of first two elements (tendency to create hard spots), graphitising Al was added to the alloys, with the highest concentration of Cr and Mo. All castings in the as-cast condition showed an austenitic matrix, guaranteeing good machinability. Heat treatment of raw castings, consisting in annealing at 500 °C for 4 h, resulted in partial transformation of austenite. As a result the carbon-supersaturated acicular ferrite, morphologically similar to bainitic ferrite was formed. The degree of this transformation increased with increasing concentrations of Cr and Mo, which successively decreased the thermodynamic stability of austenite. A change of matrix structure made it possible to significantly increase hardness and abrasive-wear resistance of castings. The largest increment of hardness and abrasion resistance was demonstrated by the castings with the highest total concentration of Cr and Mo with an addition of 0.4% Al. Introduction of Cr and Mo also resulted in an increase of corrosion resistance. In the heat-treated specimens, increasing the concentration of Cr and Mo resulted in a successive decrease of the depth of corrosion pits, with an increase in their number at the same time. This is very favourable from the viewpoint of corrosion resistance.
The manufacturing process of the rolling rolls, as well as the quality of materials used in casting them, can have an important influence upon the quality and the safety of the exploitation. Our approaches to the issue of quality assurance of the rolling rolls, from the viewpoint of the quality of materials that are featured, can cause duration and safety in the rolling exploitation. This research is required because of the numerous flaws that cause rejection, since the phase of melting of these irons is intended to cast rolls. According to the industrial analysis in the cast iron rolls foundries, the results show that one of the main rejection categories is due to the inadequate hardness of the rolls. One of the parameters that will determine the cast iron’s structure is the chemical composition, and this factor could assure the exploitation properties of each roll in all the stands of rolling mill. In this sense, the paper presents an overview of industrial and laboratory research regarding the assurance of the chemical composition of the irons (with nodular graphite) destined for the half–hard rolls casting, and tries to draw some remarks upon the proper correlations of these irons. This study analyses iron rolls cast in combined moulds (iron chill, for the barrel and moulding sand, for the necks of rolls) and includes charges of rolls from half–hard classes, with definite structure and nodular graphite, obtained in simplex cast processes. It presents, in graphical form, the influence of the chemical composition of these irons on the hardness, measured on the barrel. The proper solution is determined through some mathematical restrictions to the input data that the mathematical modelling is initiated with. It will be determined through regression equations, which describe the mathematical dependency between the hardness and the elements of chemical composition – the basic elements (Carbon [C], Manganese [Mn] and Silicon [Si]), the particulate elements (Sulphur [S], Phosphorus [P] and Magnesium [Mg]) and the main alloying elements (Nickel [Ni], Molybdenum [Mo] and Chrome [Cr]). The main results and the graphical addenda are presented.
We analyze the influence of the size of spacing for measuring the displacements of the surface points on the corresponding macrostrains. We propose an algorithm for the determination of this size in inhomogeneous strain fields. For a specimen of 20 steel stretched by a specified force, we determine the optimal size of spacing, its relationship with the size of structural elements, and correlation with the known characteristics of the material. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
Mechanical properties of ferrite/cementite lamellar structure model in pearlite steel with the Pitsch-Petch orientation relationship are examined by a strain gradient crystal plasticity analysis. Dislocations' behavior in the lamellar structure is especially considered in the analysis and characteristic lengths of the structure used in its constitutive equations are approximately determined for each slip system. From the analysis results for different lamellar thicknesses, a scale effect on the yield stress of the pearlite model is smaller than that in case of uniform estimation of the characteristic length by the lamellar thickness because of a reduction of the Orowan stress. The yield stress shows better agreement with an experimental result. On the other hands, a scale effect on the strain hardening rate is larger than that of the uniformly estimation. This is attributed to obvious change of active slip systems in the ferrite layer associated with the thickness. It is found that while slip systems with the largest Schmid factor act in case of large thickness, slip systems which have small critical resolved shear stress act in case of small thickness due to the increase of slip anisotropy. And the change of active slip systems arises large shear incompatibility in each layer and makes the stress fields in the layer non-uniform for the model with Pitsch-Petch relationship.
p class="1Body">The formation of the structure and properties of cast irons designated for the working layer during solidification and subsequent tempering of Indefinite Chill Double Poured Centrifugal Casting Rolls (ICDP) has been studied.
Graphitization of cast irons designated for the working layer of rolls occurs under isothermal conditions at high temperatures, which vary over the cross section and resulted from casting the roll core. At subsequent slow cooling, secondary carbides precipitate and austenite partially transforms into martensite; the resulting end structure consists of martensite and austenite metallic base (primary austenite dendrites) with 11.4 per cent of retained austenite, 26 per cent of eutectic and secondary carbides, and 2.6 per cent of flake graphite.
The amount of retained austenite in the metallic base of the cast structure of cast iron designated for the working layer, heated to 430 °C, decreases from 11.4 per cent to 3.2 per cent due to the partial transformation into bainite. The martensite tetragonality decreases due to the carbide precipitation; the existing excess phases grow and the new ones are formed; the total volume fraction of the carbide phase increases to 29.8 per cent and graphite to 3.7 per cent.</p
Through in situ indentation of TiN in a high-resolution transmission electron microscope, the nucleation of full as well as partial dislocations has been observed from {001} and {111} surfaces, respectively. The critical elastic strains associated with the nucleation of the dislocations were analyzed from the recorded atomic displacements, and the nucleation stresses corresponding to the measured critical strains were computed using density functional theory. The resolved shear stress was estimated to be 13.8 GPa for the partial dislocation 1/6 <110> {111} and 6.7 GPa for the full dislocation ½ <110> {110}. Such an approach of quantifying nucleation stresses for defects via in situ high-resolution experiment coupled with density functional theory calculation may be applied to other unit processes.
The problem of self-assembly in a metal-carbon system under dynamic conditions (equilibrium and nonequilibrium) is considered
using the iron-carbon system as an example. It is proved theoretically and experimentally that the ratio of the components
of the system affects the possibility of carbon self-assembly with the formation of fractal iron structures and of metal self-assembly
with the participation of polyhapto derivatives of iron and the formation of fullerene-like carbon structures.
For numerous steel grades, detailed descriptions of different etching techniques and etching times for microstructural analysis are available. However, there are only few reference works for low-alloyed cast iron. Particularly for complex microstructures with combined fractions of bainite, ferrite, pearlite, retained austenite, carbides and martensite, there are only few detailed collections. In addition, the effects of the etchants are rarely investigated for the same image section. Therefore, this study will exclusively compare identical microstructural regions and the effect of different etchants on them.
Two specific sample areas were selected in a low-alloyed cast iron and the effect of both surface removal etching and tint etching reagents on them was examined under a reflected light optical microscope and a scanning electron microscope.
The results of the study have shown that some etchants for complex microstructures are only suitable in case potentially present phases are already known. However, the combined use of two etching solutions in particular, led to a very detailed and highcontrast image, capable of revealing and resolving microstructures with a variety of phases.
We study the causes and signs of degradation of cementite in chromium-nickel cast iron used for the production of massive forming rolls and the specific features of structural formation in cementite under the influence of operating temperatures and local strains formed in the rolls in the course of their operation. For this purpose, we use the procedure of high-temperature vacuum etching in order to detect dislocations, scanning electron microscopy, and the method of optical-mathematical description of the phases and their heterogeneity. It is shown that the degradation of cementite is connected with the formation of dislocation structures and diffusion, which promote the formation of new types of carbide phases with nonstoichiometric compositions, quasicrystalline graphite, and also ferrite and bainite. Possible combinations of newly formed phases are identified. They are also responsible for the stability of carbides. By analyzing the results of our studies of the degradation of cementite (regarded as a structural component specifying the level of hardness of the operating cast iron), we give recommendations concerning the possibility of improvement of stability of the cementite structure and decreasing the level of stresses in the rolls.
Significance
This work uncovers the mystery of ductile-to-brittle transition (DBT) in body-centered cubic (BCC) metals. Temperature-dependent dislocation mobility and dislocation nucleation have been proposed as the potential factors responsible for the DBT. However, there is no compelling explanation as to why the smooth/gradual temperature-dependent kink-pair formation always gives rise to an abrupt DBT. We discover that the ratio of screw dislocation velocity to edge dislocation velocity is a controlling factor responsible for the DBT. A sufficiently high relative mobility is a prerequisite for the coordinated movement of screw and edge segments to sustain dislocation multiplication. The study offers fundamental insights into the deformation mechanism of BCC metals.
Considering the dependance of materials’ properties on the microstructure, it is imperative to carry out a thorough microstructural characterization and analysis to bolster its development. This article is aimed to inform the users about the implementation of FIJI, an open source image processing software for image segmentation and quantitative microstructural analysis.
The rapid advancement of computer technology in the past years has made it possible to swiftly segment and analyze hundreds of micrographs reducing hours’ worth of analyses time to a mere matter of minutes. This has led to the availability of several commercial image processing software primarily aimed at relatively inexperienced users. Despite the advantages like ‘one-click solutions’ offered by commercial software, the high licensing cost limits its widespread use in the metallographic community.
Open-source platforms on the other hand, are free and easily available although rudimentary knowledge of the user-interface is a pre-requisite. In particular, the software FIJI has distinguished itself as a versatile tool, since it provides suitable extensions from image processing to segmentation to quantitative stereology and is continuously developed by a large user community. This article aims to introduce the FIJI program by familiarizing the user with its graphical user-interface and providing a sequential methodology to carry out image segmentation and quantitative microstructural analysis.
An analysis of mechanical properties of directionally solidified Fe-4.25%C eutectic alloy and its microstructure and fracture mechanisms is presented. Tests were conducted for macro-samples made with different growth rates. Strength tests were performed with the use of micro-samples taken from different cross-sections of macro-samples. Fe-4.25%C eutectic alloy finds application mainly in construction of structures exposed to abrasion under significant compression loads. As specified by the metastable phase diagram, Fe–Fe3C, the white eutectic or austenitic (γ) iron carbide Fe3C can be formed. The type of obtained morphology depends mostly on the temperature gradient G, the chemical composition, and growth rate, which has a substantial impact on the morphology of the γ-Fe3C eutectic. As earlier tests have proven, interphase spacing λ decreases as grow rate v increases, which improves fine-graining of the alloy. We show that directional solidification with controlled growth rate improves mechanical properties of the Fe-4.25%C eutectic alloy, enabling its use as a structural element in a significantly expanded range of applications.
According to the results of simulation of stressed states, it is shown that coarse graphite inclusions formed in the walls of case-shaped parts of transient in thickness are the sites of initiation of defects due to the changes in the dislocation structure and diffusion of carbon from graphite. The cavities formed in graphite also promote the development of oxidation.
Processes of formation of structure and properties in the functional layer from chromium-nickel cast iron of two-layer centrifugally cast indefinite forming rolls under crystallization and subsequent tempering are investigated. The temperatures of the start and finish of the phase transformations, the phase composition, and the contents of martensite and retained austenite in the material of the functional layer after the tempering are determined.
Purpose
The purpose of this paper is to develop an efficient and reliable spectral integral equation method for solving two-dimensional unsteady advection-diffusion equations.
Design/methodology/approach
In this study, the considered two-dimensional unsteady advection-diffusion equations are transformed into the equivalent partial integro-differential equations via integrating from the considered unsteady advection-diffusion equation. After this stage, by using Chebyshev polynomials of the first kind and the operational matrix of integration, the integral equation would be transformed into the system of linear algebraic equations. Robustness and efficiency of the proposed method were illustrated by six numerical simulations experimentally. The numerical results confirm that the method is efficient, highly accurate, fast and stable for solving two-dimensional unsteady advection-diffusion equations.
Findings
The proposed method can solve the equations with discontinuity near the boundaries, the advection-dominated equations and the equations in irregular domains. One of the numerical test problems designed specially to evaluate the performance of the proposed method for discontinuity near boundaries.
Originality/value
This study extends the intention of one dimensional Chebyshev approximate approaches (Yuksel and Sezer, 2013; Yuksel et al. , 2015) for two-dimensional unsteady advection-diffusion problems and the basic intention of our suggested method is quite different from the approaches for hyperbolic problems (Bulbul and Sezer, 2011).
Description
Get 14 peer-reviewed papers on metallography and related ASTM standards. You’ll learn about the use of metallographic techniques to better understand and control problems and quality and get an in-depth review of current ASTM metals standards.
Topics include
The papers in this book were presented at a symposium celebrating the 100th anniversary of ASTM International Committee E04 on Metallography held in November 2017.
The paper presents an analysis of the influence of inoculating feedings Superseed® 75 (Sp) and Reseed® (Rd) Inoculant on the structure formation in the centrifugal cast rolls made from the nickel-chromium cast iron. It is shown that when increasing the share of the inoculant from 1 to 4 kg the top and bottom pinion rings have a similar structure of the graphite form as well as in the grain size and the ratio of the phases. Formation of a uniform structure along the length of roll pinion ring will enhance their service life and reduce the tendency to spalling.
The influence of heat treatment on the structural uniformity, degree of residual-austenite decomposition, working-layer hardness, and residual-stress distribution in two-layer high-chromium rollers is studied in production conditions. A new method is proposed for the cyclic heat treatment of two-layer rollers.
A metallurgical investigation of failed samples of hot-strip mill work-rolls used in an integrated steel plant was made to
determine the influence of microstructural characteristics on failure susceptibility and roll life. The samples investigated
pertained to prematurely failed indefinite chill double-poured (ICDP) iron work-rolls, which exhibited varying roll lives
under similar mill operating environments. Although microstructures of all the investigated rolls showed similar graphite
morphologies irrespective of their mill performance, discernible differences in carbide characteristics could be observed
between high and low life rolls. Microstructural observation of nital-etched roll specimens revealed that lower life rolls
were characterized by carbide microcracking. The propensity for cracking was particularly high in carbides exhibiting microhardness
greater than 1020 VPN. Electron-probe microanalysis (EPMA) indicated that carbides in the spalled rolls were mostly of M3C type, where M was Fe and Cr. Quantitative image analysis of phases in the investigated rolls revealed that while graphite
volume fraction in the range of 4.0 to 6.4% did not significantly affect roll life, carbide content higher than 28.5 vol%
was found detrimental. In fact, a carbide content in the range of 24.0 to 28.50 vol% was found to be desirable for higher
roll life. The study thus revealed that although carbides are indispensable for high hardness, resistance to wear, and thermal
cracking, an excessive volume fraction (>30 vol%) of high hardness (microhardness > 1020 VPN) carbides accentuated microcracking,
which ultimately induced premature spalling of hot-strip mill work-rolls.
Heat treatment of high-alloy white cast irons
- J R Keough
- K L Hayrynen
Keough, J.R., Hayrynen, K.L.: Heat treatment of high-alloy white cast irons. In: Stefanescu,
Doru M. (eds.) Cast iron science and technology, vol. 1A. ASM International (2017).
https://doi.org/10.31399/asm.hb.v01a.a0006305
Some results from the investigation of effects of heat treatment on properties of ni-hard cast irons
- Blagoj Rizov
- Lj
Rizov, Blagoj Lj.: Some results from the investigation of effects of heat treatment on properties of ni-hard cast irons. International Journal of Engineering Research and Development
13(2), 30-35 (2017).
Abrasion-Resistant Cast Iron Handbook
- G Laird
- R Gundlach
- K Röhrig
Laird, G., Gundlach, R., Röhrig, K.: Abrasion-Resistant Cast Iron Handbook. In: Frink, Karen E. (eds.) American Foundry Society, Schaumburg, IL 60173-4555 USA (2015).
Methods of identification etching figures in cementite iron
- N I Sandler
Sandler, N.I.: Methods of identification etching figures in cementite iron. Industrial laboratory 2, 202-203 (1967).
Development of new approaches to estimation of the structure formation in high-chromium cast iron. Industrial laboratory
- T S Skoblo
- O Klochko
- Yu
- E L Belkin
Skoblo, T.S., Klochko, O.Yu., Belkin, E.L., et al.: Development of new approaches to estimation of the structure formation in high-chromium cast iron. Industrial laboratory. Diagnostics of materials 83 (5), 27-38 (2017).