7C3 Carbides Research Articles

On microstructure and fatigue characterisation of cast iron alloyed with PTA deposits

In this paper, TiC–W–Cr powders were alloyed on grey cast iron by plasma transferred arc (PTA). The alloying samples were characterised with reference to the microstructure, microhardness, fatigue life and fatigue crack growth. From the results, it is indicated that two distinguishing region, i.e. alloying zone and heat affected zone, were formed on the surface after PTA alloying. The alloying zone mainly consists of primary austenite, martensite, a eutectic of (Fe,Cr)7C3 carbide and austenite as well as the uniformly distributed unmelted TiC particles. Plasma transferred arc alloying TiC–W–Cr eliminated the stress concentration at the edge of graphite and produced hard carbide, resulting in frequent crack deflection. As a result, the Weibull distribution of fatigue life demonstrated that PTA alloying TiC–W–Cr exhibited longer lives compared to matrix and PTA hardening without reinforcement, but more scattered. In addition, on the basis of the careful observation of fatigue crack growth, it was shown that the fatigue crack growth rate could be retarded by PTA alloying TiC–W–Cr at a low stress intensity, but speeded up at high stress intensity due to the brittleness.

Mechanical properties and chemical bonding characteristics of Cr7C3 type multicomponent carbides

The first principles calculations based on density functional theory are performed to investigate the mechanical properties and chemical bonding features of several Cr7C3 type multicomponent carbides (Fe16Cr12C12, Fe12Cr12W4C12, Fe12Cr12Mo4C12, Fe12Cr12W4C8B4, and Fe12Cr12Mo4C8B4). The full set elastic constants are calculated using stress-strain method. The Voigt–Reuss–Hill approximation is used to evaluate the mechanical moduli. The mechanical anisotropy is characterized by calculating several different anisotropic indexes and factors, such as universal anisotropic index (AU), shear anisotropic factors (A1, A2, and A3), and percent anisotropy (AB and AG). The surface constructions of bulk and Young’s moduli are illustrated to indicate the mechanical anisotropy. The obtained results indicate that the presence of B could deteriorate the mechanical moduli of (Fe,Cr)7C3 phase in Cr cast iron slightly. On the other hand, the chemical stability of (Fe,Cr)7C3 carbides is improved due to chemical bonding modifications through doping with B. These conclusions are in agreement with experimental results.

Effect of (Fe,Cr)7C3 carbide orientation on abrasion wear resistance and fracture toughness

Abstract In this study two white cast irons with different chromium contents 13.81% and 24.41% were solidified unidirectionally. Their microstructures are composed of oriented M 7 C 3 carbides and eutectic matrices. The specimens were cut parallel and transversally to the billet axe and pin-abrasion tests were carried out using fixed alumina abrasive grains. The microhardness, elastic modulus and fracture toughness were determined using an indentation technique on transversal and longitudinal sections of the M 7 C 3 carbides. M 7 C 3 carbides show higher microhardness in transverse than in longitudinal direction. Longitudinal M 7 C 3 carbides show higher fracture toughness in the direction perpendicular to the larger edge of the carbide than either parallel to the larger edge or on the transverse section. The results reveal that the white cast iron presents higher abrasion resistance in transverse section than in longitudinal section of the M 7 C 3 carbides. The wear surface was examined by scanning electron microscopy for identifying the wear micromechanisms and was correlated with the fracture toughness of the carbides. The results show that when the carbides fracture, the cracks propagate parallelly to the larger edge of the M 7 C 3 , the direction where the fracture toughness is lower.

Microstructure and wear characteristics of hypereutectic Fe–Cr–C cladding with various carbon contents

Abstract The current study used flux core arc welding to produce a series of hypereutectic Fe–Cr–C claddings with various carbon content. Depending on the carbon content, this research produced hypereutectic microstructures of γ-Fe + (Cr,Fe)7C3 carbides. As the carbon content of a cladding increased from 3.73 to 4.85 wt.%, the surface fractions of carbides increased from 33.8% to 86.1%. The morphology of primary (Cr,Fe)7C3 carbides also transited from a blade-like to a rod-like shape. With regard to wear performance, the relationship between wear resistance and hardness (H) is non-linear. However, the mean free path (λ) of primary (Cr,Fe)7C3 carbides must be considered. Wear resistance is proportional to H/λ. The primary carbides can prevent the eutectic colonies from selective abrasion. The rod-like (Cr,Fe)7C3 carbides also provide much better wear resistance because rod-like carbides have a greater hardness. After an abrasive wear process, abrasive particles cause plastic plows when the cladding has lower surface fractions of carbides. The fracture of primary carbides leads into the craters where it occurs in the worn cladding surface with higher surface fractions of carbides.

Hardness, toughness and cracking systems of primary (Cr,Fe)23C6 and (Cr,Fe)7C3 carbides in high-carbon Cr-based alloys by indentation

The current investigation evaluates the hardness, indentation toughness (KC) and cracking systems of primary carbides in high-carbon Cr-based alloys using the indentation technique and compares the results calculated from different model equations. The primary carbides in high-carbon Cr-based alloys are (Cr,Fe)23C6 and (Cr,Fe)7C3 carbides. The results reveal that KC evaluation of primary carbides by the Vickers indentation method strongly depends on the selection of KC equations, crack systems and crack length. The cracking systems of primary carbides shown by the polishing procedure are Palmqvist cracks. Niihara's equation with the Palmqvist crack is most appropriate to evaluate the KC of primary carbides, after experimentally assessing its applicability. The KC values of primary (Cr,Fe)23C6 and (Cr,Fe)7C3 carbides are 3.8±0.2 and 2.4±0.1MPam1/2.

Devitrification of Fe-Based Amorphous Metal SAM 1651: A Structural and Compositional Study

The effect of heat treatment on the chemistry and structure of an Fe-based bulk metallic glass (BMG) Fe48Cr15Mo14C15B6Y2 (SAM 1651) was examined experimentally. Chemical segregation was found in the as-received material, with islands 10 to 200 nm in diameter enriched in Y, Mo, and C and depleted in Fe and Cr, with respect to the surrounding matrix. Heat treatment in the range of 600 °C to 800 °C caused partial devitrification of the BMG with the formation of nanocrystalline (Fe, Cr)23C6 and (Fe, Cr)7C3 carbides in a matrix of the remaining amorphous phase. The devitrification process followed a primary crystallization route. Amorphous particle-free zones (PFZs) in the devitrified material were found, corresponding to the Y-Mo–rich islands in the fully amorphous SAM 1651. The formation of Cr-rich carbide during devitrification caused the formation of nanometer-sized Cr-depleted zones surrounding the carbide particles, which is detrimental to the corrosion performance of the alloy after thermal exposure.

Effect of molybdenum and chromium contents in sliding wear of high-chromium white cast iron: The relationship between microstructure and wear

High-chromium white cast irons are commonly used in applications requiring excellent abrasion resistance, as in the mining and mineral ore processing industry. Their excellent abrasion resistance is mainly due to their solidification microstructures. During their solidification, high levels of chromium (16–32%) lead to the formation of a high-volume fraction of eutectic M 7C 3-carbides, which may or may not be associated with primary carbides in a heterogeneous austenitic/martensitic dendritic structure. Generally, in common white high-chromium cast iron, the molybdenum content is less than 3 wt.% (by weight) so as to avoid a perlitic transformation. It is reported that by addition of molybdenum in quantities of more than 3 wt.%, new carbides (M 2C, M 6C) are formed which greatly increase the high-temperature wear resistance. In this paper, 15 high-chromium white cast alloys containing different chromium contents (16 wt.%Cr to 32 wt.%Cr) and molybdenum (Mo free to 9 wt.%Mo) are examined. For each alloy, the chemical composition and volume fraction of carbides and matrix have previously been determined. The matrix microstructure and the type of carbides depend on the relative contents of molybdenum and chromium. The wear experiments are carried out on a pin-on-disc tribometer at room temperature. The pin is made of cast iron. The wear mechanisms are observed by scanning electron microscopy (SEM). It is observed that the pin height loss, the evolution of friction versus time curves and the mean friction coefficient are largely dictated by the matrix microstructure. The carbides volume fraction and the macroscopic hardness both play only a minor role. The pin height loss is significant for a single-phased matrix and the mean friction coefficient is high. When the matrix is multiphased, the pin height loss tends towards zero and the coefficient of friction is lower. Detailed SEM observations and analysis of the evolution of the friction versus time curves indicate the substantial contribution of the large carbides in friction contact.

Microstructural Evolution of Hypoeutectic, Near-Eutectic, and Hypereutectic High-Carbon Cr-Based Hard-Facing Alloys

A series of high-carbon Cr-based hard-facing alloys were successfully fabricated on a substrate of 0.45 pct C carbon steel by gas tungsten arc welding (GTAW) process using various alloy fillers with chromium and chromium carbide, CrC (Cr:C = 4:1) powders. These claddings were designed to observe hypoeutectic, near-eutectic, and hypereutectic structures with various (Cr,Fe)23C6 and (Cr,Fe)7C3 carbides at room temperature. According to X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and optical microscopy (OM), in 3.8 pct C cladding, the microstructure consisted of the primary carbides with outer shells (Cr,Fe)23C6 surrounding (Cr,Fe)7C3 cores and [α + (Cr,Fe)23C6] eutectic structures. In 5.9 pct C cladding, the composite comprised primary (Cr,Fe)7C3 as the reinforcing phase and [α + (Cr,Fe)7C3] eutectic structures as matrix. Various morphologies of carbides were found in primary and eutectic (Cr,Fe)7C3 carbides, which included bladelike and rodlike (with a hexagonal cross section). The 5.9C cladding with great amounts of primary (Cr,Fe)7C3 carbides had the highest hardness (approximately HRC 63.9) of the all conditions.

Micorstructure and Properties of 2Cr16Ni2MoN Stainless Steel

The effect of heat treatment on the microstructure and mechanical properties of aged hardening 2Cr16Ni2MoN stainless steel was examined. The steel contained undissolved (CrFeMo)23C6 carbides in a martensite at lower solution treated temperature. Solution treating at 1100°C resulted in coarsening of the packet and block structure of the lath martensite. The optimum solution treated temperature is 1040°C.Aging at 500°C resulted in peak strength due to the precipitation of extremely fine coherent (CrFeMo)2C carbides. The optimum combination of tensile strength and impact toughness was attained at the aging temperature of 560°C,was attributed to both the formation of fine M2C in the lath and a mixture of lath martensite with high density dislocations.

Creep behavior of INCOLOY alloy 617

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)23C6 carbides and the δ-Ni3Mo phase. M6C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni3(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)23C6 carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C.

Effect of Chromium Content on Corrosion Behaviors of Fe-9Al-30Mn-(3,5,6.5,8)Cr-1C Alloys

The corrosion behaviors of the as-quenched austenitic Fe-9%Al-30%Mn-(3,5,6.5,8)%Cr-1%C (in mass%) alloys in 3.5% NaCl solution have been examined. Passivation could be observed for all of the four alloys. The corrosion potential (Ecorr) and pitting potential (Epp) increased pronouncedly as Cr content increased from 3 to 5%, and decreased as Cr content up to 6.5 and 8%. The decrease of Ecorr and Epp of alloys containing 6.5 and 8% Cr was due to the formation of (Fe,Mn,Cr)7C3 carbides within the austenite matrix and on the grain boundaries. The present result indicates that the Fe-9%Al-30%Mn-5%Cr-1%C alloy exhibited the highest corrosion resistance in 3.5% NaCl solution. It is worthy to note that the corrosion behaviors of the austenitic FeAlMnCrC alloys with higher Cr (� 3%) content have never been reported in previous literature. [doi:10.2320/matertrans.MER2007138]

Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels

Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 µm at 600 °C to 5.85 µm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 µm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)23C6 carbides form within the grains and at the grain boundaries.

Effects of Secondary Carbide Precipitation and Transformation on Abrasion Resistance of the 16Cr-1Mo-1Cu White Iron

The relationship between secondary carbide precipitation and transformation of the 16Cr-1Mo-1Cu white iron and abrasion resistance were investigated. The results show that secondary carbide precipitation and transformation at holding stage play an important role in the hardness and abrasion resistance. After being held for a certain time at 853 K for subcritical treatment, the grainy secondary carbide, (Fe,Cr)23C6, precipitated first and then Fe2MoC or MoC carbides precipitated in the alloy, both of which improve the bulk hardness and abrasion resistance of the alloy. The reasons for these improvements are the secondary carbide precipitates from the austenite and the retained austenite transforms into the martensite, both make the matrix strengthen. So the matrix has more effective support to the harder eutectic carbide against exterior abrasion. With expanding the holding time, the in situ transformation from the granular (Fe,Cr)23C6 carbide into laminar M3C carbide causes the formation of the pearlitic matrix and an associated decrease of the alloy abrasion resistance.

Study of Mn activities in Mn–Ni–C alloys by EMF measurements

The activities of manganese in Mn–Ni–C alloys have been studied by solid-state galvanic cell technique with CaF2 as the solid electrolyte. The measurements of electromotive force (EMF) have been carried out in the temperature range 920–1240 K. The main phase compositions of the alloys have been analyzed by X-ray diffraction (XRD). It was established that the substitution of Mn by Ni in the (MnNi)23C6 carbide was limited, that the lattice parameter decreased slightly with increase in the Ni content and that a solid solution is formed between Mn and Ni. It was also found that the activity of manganese decreases with increase in the nickel content when the ratio of C/(Mn+C) is less than 8.3 wt.%, and that the negative effect of Ni on the activity of Mn in Mn–Ni–C ternary system decreases as the carbon content increases. However, when the ratio of C/(Mn+C) is equal to 8.3 wt.% or more, the activity of manganese is independent of the nickel content.

Rules of formation of the chemical composition of M23C6 multicomponent carbide in high-temperature steels

The method of phase physicochemical analysis is used to study martensite steels in the Fe−Cr−Ni−Co−Mo−W−V−C system. It is shown that the chemical compositions of two multicomponent phases, i.e., hardened martensite and (Cr, M)23C6 carbide segregated from it in high-temperature tempering are correlated where the concentrations of the elements Mo/W, V/W, and (Cr+V)/(Mo+W) are concerned. The discovered correlation is used for developing a method for calculating the chemical composition of a (Cr, Fe, Co, Ni, Mo, W, V)23C6 carbide with the help of the concentrations of the elements in the α-solid solution and the differences in the atomic radii. It is shown that the alloying elements in the carbide substitute for chromium in correspondence with the principle of dimension. It is inferred that the type of bonding in the chromium sublattice in the carbide is metallic.

The effect of Si on the relationship between orientation and carbide morphology in high chromium white irons

Electron back-scatter diffraction (EBSD) has been shown to be the most appropriate technique to study the orientation and carbide morphology of small (<0.5 μm) regions of microstructure of high chromium white irons. The carbides in a slightly hypo-eutectic Fe–Cr–C alloy show a distinct texture close to [1 0 1 1] whereas those in a 1.3 wt% Si commercial white iron have a diffuse texture, with regions near to major crystal directions, i.e., [0 0 1 1], [1 2 1 0], [0 1 1 0], unpopulated. Using EBSD, it has been shown that the interconnectivity of the eutectic (Cr, Fe)7C3 carbide is less in a 1.3 wt% Si alloy compared with a low (0.1 wt%) Si alloy. This reduced interconnectivity is consistent with the increased fracture toughness in the as-cast condition.

Transmission Electron Microscopy Examination of Hardening and Toughening Phenomena in Aermet 100

The effect of tempering on the microstructure and mechanical properties of ultrahigh strength Aermet 100 steel was examined. In the as-quenched condition, the steel contained a dispersion of relatively fine, undissolved, (CrTiFeMo)C and (CrFeMo)23C6 carbides in a martensitic matrix. Upon tempering at 427 °C, the martensite decomposed to form a high density of cementite particles concomitant with a significant drop in toughness. Tempering at 454 °C resulted in peak strength (yield strength ∼ 1756 MPa) due to the precipitation of coherent zones of fine carbides. The peak in toughness (170 MPa√m), attained at a tempering temperature of 482 °C, was attributed to both the absence of cementite and the formation of reverted, stable austenite. Tempering at higher temperatures resulted in loss of both strength and toughness, which was suggested to be the result of precipitate coarsening and formation of unstable austenite, respectively. The details of the electron microscopy studies and mechanism of strengthening and toughening are discussed in light of the current understanding of this subject.

Wear Properties of (Fe, Cr)&lt;SUB&gt;7&lt;/SUB&gt;C&lt;SUB&gt;3&lt;/SUB&gt; Carbide Bulk Alloys

(Fe, Cr)7C3 carbide bulk alloys with different chromium contents were prepared by melting pure materials. Abrasion wear resistance of the carbide alloys was examined using a pin-on-disk type wear tester in relation to microstructure and hardness. Hardness and wear resistance of the carbide alloys were raised with increasing the chromium content and the dependence of wear resistance on hardness was seen remarkably under a low applied load. For the alloys with microstructures consisting of (Cr, Fe)7C3 carbide and retained austenite, a linear relationship was seen between wear resistance and the volume fraction of carbide up to the fraction of unity, but not between wear resistance and hardness. It was also found that the carbide alloys have a relatively large coefficient of thermal expansion comparable to steels and thus a strong bonding with substrate metals can be expected for an application of carbide alloys as a hardfacing material.

Influence of M7C3 carbides orientation on the wear resistance of 300Kh20DNF white iron

1. Orientating the (Cr, Fe)7C3 carbides with respect to the working surface, has a significant influence on the wear resistance of white irons. 2. The wear resistance of 300Kh20DNF iron rings for use as end seals in the drive systems of industrial tractors, is about a factor of three greater when the (Cr, Fe)7C3 carbides were distributed along the c-axis perpendicular to the working surface, as compared to the parallel distribution. The wear resistance was increased by a factor of one and a half relative to a nonorientated carbide distribution.

Diffusion Chromising of Steel by CVD Using Chloride Compounds: 4 Influence of Steel Composition

The effects of carbon content and low titanium additions on the structure and thickness of chemical vapour deposited chromium coatings produced at 930°C have been studied. Diffusion of chromium in these steels has been examined by calculation of the average diffusion coefficients and determination of the concentration dependence of the interdiffusion coefficient by the Boltzmann- Matano method. The structure and thickness of the coating obtained depend largely on the amount of soluble carbon in the steel. The steels that contained 0.002–0.004% soluble carbon were most suitablefor chromising since only a very thin surface carbide layer was formed. Those steels which contained &gt;0·05% soluble carbon at 930°C produced coatings containing dispersions of (Cr,Fe)23C6 carbide. Such coatings remain thinner and are no longer pure diffusion coatings.