High Chromium White Cast Iron Research Articles

Hot rolling mill roll microstructure interpretation: A computational thermodynamics study

The microstructure of the interface of a high chromium cast iron/ductile cast iron composite hot rolling mill roll has been interpreted with the help of Fe-Cr-Si-C laboratory alloys and thermodynamic calculations of their solidification paths. It has been shown that the mill roll presents two limiting microstructures: the ductile cast iron core and the white cast iron shell. While the core is heavily contaminated with chromium, originating from the partial dissolution of the shell during core pouring, the shell shows a typical high chromium white cast iron micro-structure, since it remains solid during processing. The high chromium contents in the core originate a typical “mottled” cast iron microstructure, combining nodular graphite and ledeburite. The region between the two limiting microstructures is called interface. As the distance from the core toward the shell increases in the interface, the content of chromium increases but that of silicon decreases. The resulting microstructural gradient shows ledeburitic M3C, hexagonal-section M3C, duplex M3C/M7C3 carbides, and M7C3 carbide, which could be well correlated with the calculated solidification paths of the laboratory samples.

Characterization of the microstructure of high-chromium cast irons using Mössbauer spectroscopy

The finish grinding process for cement materials uses ball mills with differently sized grinding balls. Because the grinding takes place through the impact and friction of the balls, balls with a high wear resistance are desirable and the materials of choice are the high-chromium white cast irons. This article examines the behavior of these cast irons when subjected to various heat treatments. The emphasis is on the dependence of the microstructure properties on the chemical composition of the alloys and the heat treatment parameters. The content of retained austenite and that of carbon in the martensite phase were determined by the use of Mössbauer spectroscopy. The results verify that a higher proportion of retained austenite with low carbon content and a martensite with higher hardness produce a material with good fracture toughness and resistance to abrasive wear.

Microstructure of vanadium-, niobium- and titanium-alloyed high-chromium white cast irons

In the present work, additions of 2% vanadium, 2% niobium and 2% titanium were respectively made to a high-chromium white iron (17% Cr) to analyse the effect of these elements on the as-cast structure of the alloy, and their influence on the material's hardness. This study was carried out for different solidification rates, so the alloys were cast into wedge-shaped copper moulds. The recorded solidification rates for the ingots ranged from 1.5 K sec−1 for a 2 mm thickness section, to 35 K sec−1 for a 40 mm section. It was found that the addition of these carbide forming elements considerably increased the alloy's hardness by different mechanisms. Vanadium was shown to partition completely to the eutectic carbide phase which increased hardness and the volume fraction of this phase. Niobium and titanium formed extremely hard NbC and TiC respectively, which in turn produced a refinement in the structure and contributed to the increase in hardness of the iron. There was no evidence that these elements influenced the size and morphology of the eutectic carbide M7C3 for the experienced solidification rates.

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M7C3 carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M2C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.

Effects of Mo additions on the solidification of high chromium cast iron

High chromium white cast irons present high resistance to abrasive wear, being employed fundamentally in industrial branches that manipulate hard raw materials, such as the mining industry. Eutectic and hypoeutectic alloys containing increasing contents of molybdenum (0–2 wt%) were cast and used for the investigation of the effect of Mo and of the cooling rate on the coarsening of carbides. The experimental results were obtained by DTA experiments and by casting in special moulds specially designed to obtain directionally solidified columnar grains. Molybdenum additions results in increased eutectic cell diameter and increased carbide particle spacing. Molybdenum decreases both the austenite primary solidification temperature and the starting temperature for the univariant eutectic L → γ + M7C3, but the overall effect results in increased solidification intervals both in the austenite field and in the eutectic field.

Study on the structure and properties of high chromium white cast iron and its tempering standard

The structure and properties of high chromium white cast iron in different treatment states were studied. Using X-ray diffraction method, observing their metallograph and measuring their microhardness, the phases involved in the iron were determined. The notch toughness, macrohardness and the fractograph of the samples are compared and their performance was determined, which mainly depends on the matrix structure. Relatively good comprehensive properties can be obtained in the tempering state. Improvements have been made on the commonly used tempering standard. Tempering for a short period at low temperature can improve the materials’ service performance obviously.

Erosive wear and impact damage of high-chromium white cast irons

High-chromium white cast irons are used extensively in environments where small particle impact causes considerable damage. In this study, three white cast irons with a variation in carbide volume fraction were eroded by spherical and angular particles. The white cast irons eroded at a higher rate when the erodent was WC–Co spheres than when the erodent was alumina particles. When eroded by spherical WC–Co particles, the eutectic white cast iron eroded at a higher rate than the hypoeutectic white cast iron. The results of the erosion experiments were interpreted in terms of a mechanism of platelet formation involving strain localization. The greater strain hardening rate of the hypoeutectic white cast iron can account for the better erosion resistance compared to the eutectic white cast iron, based on the strain localization mechanism. The larger size and greater density of the WC–Co spheres can account for the higher erosion rates of the white cast irons when eroded with WC–Co spheres compared to the erosion rate with alumina particles.

Mechanical Properties of Chromium White Cast Irons at High Temperatures

Unidirectional solidification experiments of hypoeutectic, eutectic and hypereutectic high chromium white cast irons were conducted to clarify their deformation behavior at elevated temperatures. The relation between the microstructure and the high temperature strength was studied at temperatures reaching up to 1073K. The volume fraction of the precipitated carbide in theiron strongly depends on its chemical composition. The eutectic iron shows the highest strength within the whole temperature range. The eutectic iron shows approximately the same hardness and higher compressive strength compared with the hypereutectic iron in spite of a much a smaller carbide volume fraction than that of hypereutectic iron. The hypoeutectic iron shows a large amount of plastic deformation, while the eutectic and hypereutectic irons show very little plastic strain during deformation due to the presence of more brit tle carbides. The rule of mixtures holds for the hypoeutectic and eutectic irons but does not hold for the hypereutectic iron.

The effect of spray forming on the microstructure and properties of a high chromium white cast iron

The effect of spray forming on the structure and properties of a 17% Cr, 2.5% C white cast iron is described and compared with conventionally cast material of the same composition. Spray forming resulted in a substantial reduction in microstructural scale (eutectic (Cr,Fe)7C3 fields of up to 500 μm in conventionally cast material were replaced by discrete carbides of typically 2–8 μm diameter in the spray cast deposit). Carbide size varied as a function of position in the spray deposit, being approximately twice the size at mid section compared with either surface or interface with the collector. Carbide size was not altered by the gas to metal ratio used to atomise the spray. Spray forming increased transverse rupture stress and work of fracture by ∼50% compared with the conventionally cast material. Forging of the spray formed material was possible at ∼950°C, without inducing carbide fracture or void formation in the matrix. Quenching into iced water from 300°C induced extensive macroscopic cracking in the conventionally cast material whereas 400°C was required in the spray cast material to induce similar damage. The relationship between processing, microstructure and mechanical properties is discussed.

一方向凝固したＦｅ‐２５％Ｃｒ‐３．５％Ｃ三元共晶合金の機械的性質

Unidirectional solidification of Fe-25% Cr-3.5% C eutectic high chromium white cast irons was conducted under a solidification rate range from 1.3×10-3 mm/s to 1.3×10-1 mm/s to clarify the deformation behavior of the irons at elevated temperatures. The alloy specimens were processed in the parallel and vertical directions to the solidification direction, and a compression test was done under a strain rate range from 2.5×10-4 to 1.7×10-2 at various temperatu res up to 1073 K. The relation between the microstructure and high temperature strength was studied. The strength depended on the morphology of eutectic cells and the orientation of primary carbides. The maximum compressive strength of the parallel direction to the solidification direction was larger than that of the vertical direction within the whole temperature ranges. The strain rate sensitivity exponent showed larger value in the vertical direction than in the parallel direction. These results suggest that the strength of the parallel direction is mainly supported by the elongated primary carbide phases, while the strength of vertical direction is supported by the iron matrix and strongly depend on the deformation behavior of the iron matrix. The rule of mixtures was used for analysis of the experimental results described above.

Simulation of solidification paths in high chromium white cast irons for wear applications

Thermodynamic calculations of the solidification paths in the Fe-Si-Cr-C system using the CALPHAD approach have been used for the interpretation of the solidification microstructures of high chromium white cast irons containing high silicon contents. The results show a reasonable agreement between experiments and calculations for alloys up to 2 wt% Si. The results suggests that the stability of the cementite phase is underestimated in the thermodynamic database. The calculations predict a change in the topology of the phase diagram, with the cementite primary field disappearing in favour of a new ternary eutectic involving liquid, austenite, graphite and M 7C 3 carbide, this is partially in agreement with the observed microstructures for the alloys containing 4 wt%Si. The calculations predict the existence of a complex invariant peritectic reaction involving liquid, graphite, austenite, cementite and M 7C 3 carbide in the Fe-Si-Cr-C system.

The effect of nickel, silicon and austenite deep-freezing treatment on the microstructure and wear properties of high chromium white iron containing niobium

The influence of sub-zero treatments on erosion and abrasion resistance of a high chromium white cast iron with different amounts of nickel and silicon was investigated. It was found that nickel stabilises the residual austenite in white irons, decreasing their wear resistance. On the other hand, the corrosion resistance of these alloys is improved by nickel addition. Silicon was found to be an austenite-destabilising element increasing the γ → M transformation temperature. Up to 58% of predominantly austenitic matrix in the alloys containing 3.8% Ni is transformed to martensite by the addition of 3.6% Si and the application of sub-zero treatments. A tempered martensite matrix structure was found to offer the best resistance to both erosion and abrasion. SEM observations revealed that a martensitic matrix, because of its high hardness, can effectively support the massive carbides by preventing their fracture during abrasion and erosion processes.

Effect of high temperature and sub-ambient treatments on the matrix structure and abrasion resistance of a high-chromium white iron

The addition of NbC to high chromium white cast irons (WCIs) has potential to improve component wear lives in mining and ore beneficiation environments, due to the combined presence of Cr-rich and niobium carbides. While the effect of chromium carbide volume fraction (Cr-CVF) on abrasion performance of plain-Cr WCIs is well understood, its effect on NbC-bearing WCIs is unknown. This work studies the effect of Cr-CVF on high-stress abrasion of this newer class of dual-reinforced high-Cr WCIs, using the ball mill abrasion test with quartzite and basalt. The alloys cover a wide range of Cr-CVF, from M7C3-free NbC-reinforced steel to hyper-eutectic WCI. Bulk hardness increased linearly with Cr-CVF, at a rate of 3.9 HV per 1% Cr-CVF. In basalt, abrasion resistance increased linearly with Cr-CVF, at an average life improvement rate of 24% per 1% Cr-CVF. This confirms that performance is maximized by increasing Cr-CVF even in the presence of substantial volumes of cubic carbides. In quartzite the trend is again linear up to 30 vol% M7C3 (3.7% life increase per 1% Cr-CVF), but the hyper-eutectic alloy shows poorer resistance than slightly hypo-eutectic alloy. SEM studies of the worn surfaces demonstrate that micro-fracture of the hard carbides was the controlling damage mechanism. Primary M7C3 was observed to suffer fragmentation in high-stress abrasion in quartzite, confirming the expectation from the quantitative trend.

Damage by indentation and single impact of hard particles on a high chromium white cast iron

High chromium white cast irons are used extensively in environments where small particle impact causes considerable damage. In this study, a white cast iron containing 26 wt% Cr, and with a carbide volume fraction of 0.28, is impacted with projectiles at normal incidence using a gas gun. Both crater diameter and the size of the damage zone are measured as a function of projectile velocity, and an analytical model is developed which relates the size of the impact crater to the energy absorbed by plastic and elastic deformation of the white cast iron. Peak impact loads and strain hardening rates during impact are calculated by comparing the impact craters with craters made by quasi-static indentation. It is found that considerable strain hardening occurs in austenitic white cast irons during impact and indentation. Strain localization and dynamic softening are also observed in these materials at higher impact velocities.

Deterioration behavior of thermomechanical refiner plates

To make paper from wood raw materials, wood chips must be processed to turn them into individual fibers or fiber bundles. In mechanical pulping this is accomplished by introducing energy into the wood chip or fiber bundles as the wood chips are passed between a set of counter-rotating plates; these are up to 1.5 m in diameter and operate with a plate clearance on the order of 0.1 mm and plate peripheral velocity of 130 m s −1. The working life of refiner plates is sufficient for the paper industry; however, the deterioration of the fine surface detail of the plates reduces refining efficiency. This research was focused on the behavior of the plate materials in the refining environment, with the emphasis on understanding the deterioration behavior of the refiner plates. It combined wear analysis of normally used plates obtained from the paper mills, of plates used in designed trials in paper mills, and evaluation of plate materials in a broad range of laboratory tests. The laboratory tests were selected and suitably modified to reproduce phenomena observed on worn plates. The laboratory tests included cavitation erosion, dry-sand-rubber-wheel abrasion, and particulate erosion with high pressure water or steam as carrier fluids. Plate materials tested included 25Cr and 20Cr2Mo1Cu (both wt.%) high chromium white cast irons, Ni-hard white cast irons, and 440C and 17-4 PH stainless steels. Plate failure analysis revealed a variety of wear processes which varied with plate location and plate running time in the refiner. Cavitation erosion was clearly identified in the center of the plates where water was definitely present. Damage patterns with the same appearance as cavitation erosion were observed along plate bar edges near the periphery of the refiner plates where water is not thought to exist in a liquid form. Cavitation erosion tests in the laboratory duplicated the observed phenomena on the bar edges. Abrasion tests and erosion tests lacked such a close correlation to the observed phenomena. For all observed wear phenomena the role of the microstructure appears to be more important than the bulk hardness and for pitting-type damage such as that produced by cavitation erosion, the microstructure has a major influence on the wear process.

Verfestigung und Verschleißverhalten von weißen Gußeisen bei abrasiver Beanspruchung

AbstractWeiße Gußeisen werden häufig dort eingesetzt, wo starke abrasive Belastung auftritt. Die Mikrostruktur und die mechanischen Eigenschaften dieser Legierungen spielen eine wesentliche Rolle, um die durch Verschleiß verursachten Verluste zu kontrollieren. Hinsichtlich der mechanischen Eigenschaften wird in zahlreichen Veröffentlichungen berichtet, daß die Werkstoffhärte allein das Verschleißverhalten nicht bestimmt. Dabei gilt allgemein, daß die während des Verschleißvorganges entstehende Verfestigung den Verschleißwiderstand wesentlich beeinflußt.In der vorliegenden Arbeit werden Ni‐Hard 4 und hochchromhaltige weiße Gußeisen bezüglich ihres Verschleißverhaltens anhand eines Schleifpapierverfahrens untersucht. Die Ergebnisse dieser Untersuchungen tragen u.a. zur Klärung der Korrelations mechanismen zwischen Oberflächenverfestigung und abrasivem Verschleißverhalten bei.

Solidification structure versus hardness and impact toughness in high-chromium white cast irons

Cast macrostructure has long been considered an important metallurgical feature in determining the mechanical properties of high-chromium white irons. To explore this relationship, the US Bureau of Mines has looked at a series of high-chromium white irons cast at different superheat temperatures (75 and 250 °C) and then impact tested. All materials were cast into sand moulds, each of which produced three impact specimens (50 × 50 × 180 mm). The macrostructure of the alloys is shown to consist of columnar and equiaxed growth zones depending upon chemical composition (i.e. hypoeutectic, eutectic or hypereutectic) and superheat pouring temperature. An explanation of this solification behaviour is given by differential thermal analysis and standard solidification theories. Cast microstructure is illustrated by image analysis (carbide volume fraction and particle size) and optical metallography (carbide and matrix phase identification). Brinell hardness measurements were also taken along cut surfaces (perpendicular to the columnar and equiaxed zones) and across the top of the impact specimens (semi-chilled surface). Macrostructural and microstructural results are then related to impact toughness results. As would be expected, both macro- and micro-structure have strong effects upon the impact toughness of high-chromium white cast irons.

Surface hardening of ductile cast iron using stainless steel

A surface hardening technique for improving the wear resistance of ductile cast iron (DI) is proposed by fabricating molten DI with stainless steel. The essential idea is concerned with using the chromium source in stainless steel and the carbon source in DI to form chromium carbides. It was demonstrated that the surface of the fabricated DI consisted of firstly a layer of high chromium white cast iron ((Cr,Fe) 7C 3 eutectic carbides), which was originally stainless steel, and then a layer of medium low-chromium white cast iron ((Fe,Cr) 3C eutectic carbides), which was originally DI. Thus the hardness of the surface layer became 2–3 times that of the baseline ductile cast iron. Applying austenitic stainless steel produces a much better hardening effect than applying ferritic stainless steel. The depth of the hardened surface layer depends on the pouring temperature of the iron melt, the wall thickness of the iron casting and the cooling rate of the casting mold. The present strategy is, in principle, possibly applied to complete as well as local surface hardening of various kinds of ductile iron castings.

Effect of carbide orientation on abrasion of high Cr white cast iron

Wear resistant, high chromium white cast irons contain high volume fractions of M 7C 3 type carbides, which grow as rods and blades during solidification to produce a highly anisotropic microstructure. In this study, the effect of carbide orientation on the wear properties of these alloys is examined using both a high-stress pin-on-drum abrasion test and a single scratch test. It is found that when the carbides are oriented with their long axes perpendicular to the wear surface, abrasion resistance is considerably reduced. When the long axes of the carbides are parallel to the wear surface, the cast irons are more wear resistant, and the alloys show the least wear when the long axes of the carbides are parallel to the wear surface and oriented perpendicular to the wear direction. Scanning electron microscopy reveals that fracture of the carbides depends upon the orientation of the carbides with respect to the wear surface and the wear direction.

Wear of hardfacing white cast irons by solid particle erosion

The response of three commercial weld-hardfacing alloys to erosive wear has been studied. These were high chromium white cast irons, deposited by an open-arc welding process, widely used in the mineral processing and steelmaking industries for wear protection. Erosion tests were carried out with quartz sand, silicon carbide grit and blast furnace sinter of two different sizes, at a velocity of 40 m s −1 and at impact angles in the range 20° to 90°. A monolithic white cast iron and mild steel were also tested for comparison. Little differences were found in the wear rates when silica sand or silicon carbide grit was used as the erodent. Significant differences were found, however, in the rankings of the materials. Susceptibility to fracture of the carbide particles in the white cast irons played an important role in the behaviour of the white cast irons. Sinter particles were unable to cause gross fracture of the carbides and so those materials with a high volume fraction of carbides showed the greatest resistance to erosive wear. Silica and silicon carbide were capable of causing fracture of the primary carbides. Concentration of plastic strain in the matrix then led to a high wear rate for the matrix. At normal impact with silica or silicon carbide erodents mild steel showed a greater resistance to erosive wear than these alloys.