Characteristic carbides (MC, M2C, M3C, M7C3, M23C6) in white cast iron


Table 1: The characteristic parameters of carbides.


Table 2: Crystal type of M7C3 carbide.


Table 3: Typical composition of normal white iron (mass %).

Carbide name: MC, M2C, M3C, M7C3, M23C6
Record No.: 871
Carbide formula: MC, M2C, M3C, M7C3, M23C6
Carbide type: MC, M2C, M3C, M7C3, M23C6
Carbide composition in weight %: No data
Image type: No data
Steel name: White cast iron
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Cast irons
Steel composition in weight %: No data
Heat treatment/condition: No data
Note: Roles of alloying elements in white cast iron:
Carbon: With increasing carbon content, the hardness and wear resistance of a white cast iron are increased. However, transverse fracture toughness is decreased and brittleness is increased. The higher the carbon content, the lower the impact toughness. Increasing the carbon content increases the amount of hard and brittle eutectic carbides, and also decreases hardenability; thus when choosing the carbon content, comprehensive consideration should be taken.
Chromium: The main roles of Cr in white cast iron are: forming carbides, improving corrosion resistance and stabilizing the structure at high temperature. Increasing both the carbon and chromium contents will increase the amount of carbides, and thus improve wear resistance, but will also decrease toughness. The amount of carbides can be estimated from the following equation: Mass fraction of carbides = w(C)12.33% + w(Cr)0.55% -15.2%. When the mass fraction of Cr is lower than 7%, there exists a continuous network of M7C3 type carbides, which result in lower strength and deflection. When the mass fraction of Cr is above 9%, a discontinuous M7C3 type of carbide is formed, and the strength and the deflection are both improved. When the mass fraction of Cr is increased to 12% 19%, the properties reach their highest values. If the mass fraction of Cr exceeds 25%, hypereutectic carbide is formed; the fracture changes to a coarse needle-like appearance and the mechanical properties are decreased. In addition, a high Cr content will increase the corrosion resistance and high-temperature oxidation resistance.
Molybdenum: In white cast irons, approximately 50% of the mass fraction of Mo forms Mo2C, 25% enters other carbides and the remaining 25% dissolves in the metal matrix. The Mo which enters the metal matrix improves hardenability of the iron; with increasing Mo content, the hardenability also increases. The ability of Mo to improve the hardenability in white cast iron is related to the Cr/C ratio. When added together with any one of Cu, Ni or Cr, or with Cr+Ni together, the effect of increasing hardenability is more significant. Also, in Ni-Cr type martensitic white cast iron, Mo has the ability to replace Ni.
Nickel: Nickel is insoluble in carbides and all of it dissolves in the austenite, thus its only purpose is to improve the hardenability. The addition of 2.5% Ni to low Cr white cast iron can promote a fine and hard pearlitic structure. When w(Ni)>4.5%, the formation of pearlite can be inhibited. With further increasing in Ni content [w(Ni)>6.5%], austenite is stabilised and martensite transformation occurs at low temperature or in the as-cast state.
Copper: In low Cr and high Cr martensitic white cast irons, copper has the effect of inhibiting the formation of pearlite. Because of limited solubility in austenite, too much Cu should not be added; a suitable amount is w(Cu) < 2.5%, thus copper cannot replace Ni in Ni-hard irons. A combined addition of Cu and Mo can markedly improve the hardenability.
Vanadium: Vanadium is a strong carbide promoter and forms primary carbide or secondary carbide, and increases the degree of chilling. The strong chilling effect of vanadium can be balanced with Ni, Cu or by increasing the carbon and silicon contents. In addition, a small amount of vanadium, for example w(V) = 0.1% - 0.5% can refine coarse columnar crystals. Because it combines with carbon in the liquid iron, this reduces the carbon content in the metal matrix; vanadium increases the martensite transformation temperature and causes the microstructure to transform into martensite under casting conditions.
Silicon: Silicon is a restricted element in white cast iron since it increases carbon activity and thus easily promotes graphite formation and retards the formation of carbides. In addition, silicon reduces the hardenability and promotes pearlite formation, therefore having an adverse effect on the wear resistance. In low alloy white cast iron, w(Si) is about 1%; in high Cr white iron, silicon is often controlled w(Si) = 0.4% 0.7%. Too low a silicon content (for example, w(Si) < 0.4%) is unfavourable for deoxidation. Different from general conclusions, it was reported [4] that in medium Cr white cast iron, silicon has a tendency of increasing the amount of carbide (Fe,Cr)7C3.

Carbides in white cast iron: Carbide is an important constituent phase in white cast iron and its volume fraction can reach as high as 40%; its type, chemical composition, amount, size, shape and distribution all have an important influence on the properties of the iron. The elements which can form carbides are the transition elements in the periodic table, such as Fe, Mn, Cr, W, Pt, V, Nb, Ti, etc. The atoms of all these elements have an incompletely filled d-electron shell. The tendency to form carbides is related to the degree of incompleteness of their d-electron shell; the more unfilled vacancies in the d-electron shell the element has, the stronger the ability to form carbide and the more stable the carbide. The formation ability in descending order is as follows: Ti, Nb, Zr, V, Mo, W, Cr and Mn (Fe).
Carbides have a close-packed structure or slightly distorted, close-packed structure arranged by interaction of these metal and carbon atoms, which form an interstitial structure consisting of a metal atom sub-lattice and a carbon atom sub-lattice. The sublattices of metal atoms are obviously different from the metal lattices from which they are formed, but they still belong to the typical face-centred, body-centred and close-packed hexagonal (or complex) structures. If the interstice in a metal sub-lattice is large enough to contain a carbon atom, a simple close-packed structure is formed. Therefore, the ratio of carbon atom radius rc to atom radius of transition metal rM, rc/rM, will determine the type of carbide formed.

Types of carbides: According to the structure of their crystal lattice, carbides fall into two types:
(1) Interstitial carbide with a simple, close-packed structure. When rc/rM < 0.59, carbon atoms are located at the interstices of the simple lattice, forming an interstitial phase, which is different from the original metal crystal lattice; the elements Mo, W, V, Ti, Nb and Zr belong to this type. The formed carbides include: MC type WC, VC, TiC, NbC, ZrC
M2C type W2C, Mo2C
If a variety of transition metals exist at the same time, complex carbides will form. If three conditions (lattice type, electrochemical factor and size factor) are satisfied, the metal atoms in the carbides can displace each other; for example, TiC-VC system forms (Ti,V)C; VC-NbC system forms (Nb,V)C; TiC-ZrC system forms (Ti, Zr)C, etc.
The metal atom M in MC type carbide has a simple face centred cubic structure, the octahedral interstices all are occupied by carbon atoms, so M : C = 1 : 1, and the crystal structure type is that of NaCl.

(2) Interstitial carbides with a complex hexagonal, close-packed structure. When rc/rM > 0.59, carbon cannot form a simple, closepacked interstitial phase, but forms an interstitial compound with a very complex crystal lattice. The carbides of Cr, Mn and Fe belong to this complex close-packed structure. Among them, M23C6 and M6C are complex cubic, M7C3 is complex hexagonal and M3C has an orthorhombic lattice. Commonly observed carbides with a complex close-packed structure are:
M3C type Fe3C, Mn3C or (Cr,Fe)3C, Kc for short;
M7C3 type Cr7C3, Mn7C3 or (Cr,Fe)7C3, K2 for short;
M23C6 type Cr23C6, Mn23C6, and ternary carbides Fe21W2C6, Fe21Mo2C6, (Cr, Fe)23C6, K1 for short.
M6C type Fe3W3C, Fe4W2C, FeMo3C, Fe4Mo2C ternary carbides and so on.

(a) M3C type carbide: The carbide most commonly seen is cementite in normal un-alloyed white cast iron. The crystal structure of cementite is an orthogonal lattice, with lattice constants a = 0.45144 nm, b = 0.50787 nm, c = 0.67287 nm. The crystal structure of cementite is illustrated in Fig. 5-5. Around each carbon atom there are six iron atoms which form an octahedron; all the axes of the octahedron are inclined at an angle to each other, to form a rhombohedral crystal. Because each octahedron has a carbon atom in it, and each iron atom is shared between two octahedrons, the atomic ratio of Fe and C in the molecular formula Fe3C is satisfied exactly. The projection of an octahedron of cementite is a rhombic, chain-like structure (see Fig.5-6). When observed as a whole, the rhombus planes are parallel, showing a lamellar arrangement. In each rhombohedral crystal unit, the Fe-C atoms are connected by a covalent bond, which is realized by the covalent electrons of four carbon atoms and 3d-electrons of the nearest iron atoms at the apexes of the rhombohedral unit. The other two iron atoms are situated in neighbouring rhombohedral units where the iron atoms are near to the next carbon atoms, therefore a strong connection is formed between the layers. In addition, the electronegative difference between iron and carbon strengthens the connection of Fe-C, thus the connective force of Fe-C is about twice as strong as that of Fe-Fe. Whilst the layers are connected by a metallic bond between iron atoms, the connection is weak, thus resulting in the strong anisotropy of cementite. Addition of a third element into an iron-carbon binary alloy can change the connective strength of the Fe-C bond. The elements enhancing the Fe-C bond will further stabilize cementite; whilst the elements that weaken the Fe-C bond cause Fe-C to be broken-down easily, thus reducing the stability of cementite and promoting graphitization.

(b) M7C3 type carbide: A typical representative of type M7C3 carbide is Cr7C3, which consists of 56 Cr atoms and 24 carbon atoms and has an even more complex crystal system than M3C. The three crystal systems of Cr7C3 are hexagonal, orthogonal and rhombohedral. The Cr in Cr7C3 can be partially replaced by Fe and Mn; if replaced above 60% by Fe, then the carbide changes to (Fe,Cr)7C3.

(c) M23C6 type carbide: This is a cubic crystal lattice cell consisting of 92 atoms; the structure is shown in Fig. 5-7. The large crystal cell is divided into 8 small cubes; on the apexes of the small cubes, there alternatively exist atom groups which become cuboctahedron or cube. Normally, the M in the carbide is mainly Cr, forming M23C6; sometimes, the M is also mainly Mn. When containing more Mo and W, Fe21Mo2C6 carbide or Fe21W2C6 carbide is formed. In the structure of Cr23C6, the centre of each small cube also has an additional atom which can only be replaced by W. When replaced by W, the crystal type (Fe, W, Cr)23C6 is formed. The carbon atoms in the Cr23C6 crystal cell are situated on the edges of the large cube, and at the same time are located in between the cuboctahedron and small cube; hence each carbon atom has 8 neighbouring metal atoms.

(d) M6C type carbide: This carbide is a complex interstitial, ternary compound consisting of W, Fe and C, which exists in high W cast iron and has a micro-hardness above 2,250 HV, and good strength and toughness properties. The carbides in as-cast, high W iron consist of M6C + M3C, or M6C + M23C6, or M6C + M7C, but the main phase is still M6C. This phase is a meta-stable structure; it will disappear after equilibrium treatment, to be replaced by WC. M6C has a face-centred cubic lattice consisting of 96 metal atoms and 16 carbon atoms, with 48 W atoms distributed at the apexes of octahedrons; the lattice structure is shown in Fig. 5-9. Among the 48 iron atoms, 32 are distributed on the apexes of 8 tetrahedrons; the centres of the tetrahedrons form a diamond lattice and the remaining16 Fe atoms are situated in free interstices. In a pure Fe- W-C system alloy, the composition of M6C is in between Fe4W2C and Fe3W3C, containing w(W) = 61% - 75%. M6C can dissolve a large amount of Si.

A high melting point and high hardness are the important features of carbides; this is because when forming carbides, there exists a strong cohesive force from the covalent bond formed by a p-electron of the carbon atom and a d-electron in the metal atom. The more unfilled vacancies the d-shell has, the stronger the covalent bond is, and the higher the melting point and hardness are. The characteristic constants of various carbides are listed in Table 5-6. Among the carbides, the MC type has the highest hardness, M7C3 has the next highest and M3C has the lowest, indicating that, as far as hardness is concerned, the connective force of the covalent bond is more important than the crystal type. In addition to high hardness (can reach as high as 2,300 2,700 HV), MC also has high oxidation resistance, hence, under high temperature and service wear conditions, this carbide is highly valued A high melting point and high hardness are the important features of carbides; this is because when forming carbides, there exists a strong cohesive force from the covalent bond formed by a p-electron of the carbon atom and a d-electron in the metal atom. The more unfilled vacancies the d-shell has, the stronger the covalent bond is, and the higher the melting point and hardness are. The characteristic constants of various carbides are listed in Table 2. Among the carbides, the MC type has the highest hardness, M7C3 has the next highest and M3C has the lowest, indicating that, as far as hardness is concerned, the connective force of the covalent bond is more important than the crystal type. In addition to high hardness (can reach as high as 2,300 2,700 HV), MC also has high oxidation resistance, hence, under high temperature and service wear conditions, this carbide is highly valued.
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