M7C3 and M23C6 carbides in 2.4%C-30%Cr, 2.9% C-27%Cr steels

Figure 1: SEI, Microstructures of high Cr cast iron at different conditions. Scale bars: 10, 3, 2 µm.

Figure 2: BF-TEM of 2.4%C-30%Cr primary austenite at the wear surface (a). Scale bar: 0.1 µm.

Figure 3: BF-TEM of 2.4%C-30%Cr primary austenite at the away from the wear surface (b). Scale bar: 0.2 µm.

Figure 4: Fracture appearance of 2.5%C-30%Cr iron at as-cast (a) and heat treated (b) conditions.

Figure 5: Fracture appearance of 2.5%C-30%Cr iron at as-cast (a) and heat treated (b) conditions.

Figure 6: Corrosion damage produced by potentiokinetic testing in 0.1mol/L sulphuric acid.

Figure 7: (a) Contrast from stacking faults and antiphase domain boundaries in eutectic M7C3 carbide and (b) Characteristic streaking in electron diffraction pattern of eutectic M7C3 carbide for 2.3%C-30%Cr. Scale bar: 0.1 µm.

Figure 8: BF-TEM micrographs of secondary carbides in high chroumium cast irons. Scale bars: 0.1, 0.5 µm, 100, 50 nm.

Figure 9: Duplex core-shell structure of eutectic carbides after destabilization in 2.3%C-30%Cr. Scale bar: 10 µm.

Carbide name: M7C3, M23C6
Record No.: 379
Carbide formula: M7C3, M23C6
Carbide type: M7C3, M23C6
Carbide composition in weight %: No data
Image type: SEM, TEM
Steel name: High Cr cast steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Cast iron steels
Steel composition in weight %: 2.4%C-30%Cr, 2.9% C-27%Cr.
Heat treatment/condition: No data
Note: The physical metallurgy underlying the development of cast microstructures in abrasion resistant high chromium cast irons, and their structural modification by thermal treatments is relatively complex. Structural characterisation via electron microscopy therefore has a key role to play in furthering our understanding of the phase transformations that control the microstructures and hence the service performances of these irons as wear parts. This paper shows how both scanning and especially transmission electron microscopy can provide valuable information on the nature of eutectic and secondary carbides and on the matrix structures in these irons. Particular attention is given to current characterisation research on conventionally cast 30%Cr irons that are used for applications involving corrosive wear e.g. slurry pumps and on a semi-solid cast 27%Cr iron that has a potential for applications in industry.

Nearly all the irons used for abrasion resistance are hypoeutectic, and solidifying as primary austenite dendrites followed by a eutectic of austenite and M7C3 carbides. On subsequent cooling the austenite may be largely retained or it may fully or partially transform to ferrite plus carbides, e.g. pearlite, and/or martensite. The retention or transformation of the as-cast austenite will depend on the Cr/C ratio, the presence of alloying elements and on cooling rate (cast section size). As-cast austenitic matrices are favored by high Cr/C ratios, additions of Ni, Cu, and Mo, and by faster cooling. In as-cast austenitic irons, there is always some transformation of the eutectic austenite around the eutectic carbides to martensite.

In some applications, as-cast austenitic irons can be used without heat treatment since the austenite can work harden at wear surfaces to provide a self-replacing wear resistant surface structure in a similar way to the austenitic high Mn steels. For most applications, however, some forms of heat treatment are required:
(1) To soften the casting for machining e.g. for roller tyres and pump parts. This produces pearlitic and secondary carbides in a ferrite matrix, lowering hardness levels down to 350-400 HV.
(2) To harden via destabilization treatment (at 950-1 050..), air quenching and tempering (at 450-550..). This gives a distribution of secondary carbides in a tempered martensite matrix with small amounts (ideally <5%) of residual austenite: hardness levels are 700-850 HV.
(3) To improve toughness by high temperature treatment at 1 130-1180 C. This produces a controlled amount of secondary carbide precipitation in an austenitic matrix and provides fracture toughness levels of 40-45 MPa x m 1/2 as against 20-30 MPa x m 1/2 for as-cast or normally hardened irons.

The microstructures of conventionally cast 30% Cr irons in secondary carbide particles that have precipitated in the matrix and, consequently, it is much less dependent on the form of the eutectic carbides. However, if Cr iron is treated at temperatures of 1100-1180 C, an essentially austenitic matrix can be obtained containing a small number of relatively coarse secondary carbides. These carbides are sheathed in martensite and encourage the as-cast, hardened and annealed conditions are illustrated in Fig.1. The microstructure of a semi-solid cast 27% Cr iron, in which morphology of primary austenite became spheroidal and most of the eutectic carbides formed as radiating clusters are also compared.

Although SEM studies have been used extensively to investigate wear damage such as sub-surface cracking in eutectic carbides, there is very little reported use of TEM to study wear. However, as Figs. 2 and 3 shows, TEM can be used to characterize the changes that occur when austenitic irons, under certain wear conditions, work harden by formation of dislocations and stacking faults and by transformation to strain induced martensite (SIM) at the wear surface producing a self replacing hardened layer. More recently, TEM work has contributed to an understanding of SIM transformation toughening in developing improved fracture resistance. Regarding semisolid cast, preliminary results showed that dry wear rate of the heat-treated, semi-solid cast 27%Cr was lower than that of the conventionally or semi-solid as-cast, which was attributed to the presence of martensite and reduction of retained austenite. The nature of stacking faults and stacking fault energies have been examined in detail for wrought austenitic stainless steels, but there is scope to extend such research to the austenite in ascast and elevated temperature treated high Cr irons, both conventionally and semi-solid cast.

Fracture toughness of high Cr irons can be increased by reducing the volume fraction of eutectic carbides in the structure (i.e. by use of lower %C levels). Fractographic studies (see Figs. 4 and 5) show that in austenitic irons, cracks follow the eutectic carbides and do not propagate into the matrix. In martensitic irons, the fracture process is controlled by the formation of microvoids at secondary carbide particles that have precipitated in the matrix and, consequently, it is much less dependent on the form of the eutectic carbides. However, if Cr iron is treated at temperatures of 1 100-1180 C, an essentially austenitic matrix can be obtained containing a small number of relatively coarse secondary carbides. These carbides are sheathed in martensite and encourage the deviation of the crack path from the eutectic carbides into the matrix resulting in much improved fracture toughness. The form of the eutectic carbides remains unaltered during normal destabilization at 950-1 000.., but after treatment at higher temperatures, it becomes less angular and with reduced continuity. Rounding of the eutectic carbides is believed to contribute to the overall toughness improvement. Later work using TEM has suggested that the toughness improvement be mainly due to transformation induced toughening in the austenite.

SEM studies have been used to examine the behaviour of Cr irons during wet wear where corrosion damage can contribute to increase wear loss by removal of the matrix from around eutectic carbides. The unsupported carbides can then be more easily broken and removed during wear. Figure 6 shows typical corrosion damage produced during potentiokinetic corrosion testing, the SEM observations contributing to the analysis of anodic polarisation curves. As-cast irons with less than 25% Cr show very little active-passive transition, but those with 25%-35%Cr showed well defined passive regions, in the same way as stainless steel (Fig. 6 (d)). Such studies confirmed the superior behaviour of 25%-35% Cr irons experienced in service such as slurry pumps and in wet wear testing, but it appears that the corrosion resistance of these irons may be reduced by hardening heat treatments due to lowering of the matrix Cr levels. Hence, the current interest is focused on studying the effects of thermal treatments and alloying on the structure of 27%-30% Cr, and in particular on the M7C3 to M23C6 transition in the eutectic carbides.

Thin foil TEM studies of the eutectic carbides in 15%- 30% Cr irons have confirmed that these carbides are M7C3 and they are formed as rod- and/or blade-like structures. The eutectic M7C3 appears to grow as hollow pencil like crystals of hexagonal cross section, the blades being polycrystalline aggregates of rods. Regardless of whether M7C3 forms as eutectic or primary carbides during solidification or via precipitation in the solid state as secondary carbides in Cr irons, or as tempered carbides in steels, the characteristic of this carbide is that it always appears to contain a high concentration of structural faulting. Stacking sequence faults occur on {1010} and {1120} planes giving rise to characteristic elongated reflections (streaking) in electron diffraction patterns and the presence of anti-phase domain boundaries in crystals. These features are illustrated in Fig. 7. Further work is needed to study the possible influence of such structural faulting on the mechanical behavior of carbides during wear. Faulting offers a mechanism by which some plastic deformation of carbides could occur at wear surfaces delaying fracture and the subsequent removal of carbide fragments. The apparent "bending" of finer eutectic carbide rods seen at wear surfaces has yet to be explained.

The eutectic carbides in 15%-27% Cr irons do not appear to be structurally affected during conventional annealing or destabilization heat treatments. No transformation to other carbides has been observed but further TEM studies are needed to examine how heat treatment may affect the nature of the stacking faults. However, in 30%Cr iron, a transformation from M7C3 to M23C6 occurs during conventional heat treatment.
Electron microscopy has allowed studies of the type and distribution of secondary carbides formed during destabilisation and of the ferrite + carbide mixtures formed during annealing treatments. Selected area electron diffraction can be used to identify the carbides and to determine their orientation relationships with the matrix. Secondary carbides in heat treated, conventionally cast 15% and 30% Cr irons and semisolid cast 27% iron are compared in Fig. 8. In the conventionally cast 15% Cr iron the secondary carbides are M7C3 and show linear fringe contrast (and related streaking in diffraction patterns) due to the presence of stacking faults as in eutectic M7C3. In conventionally cast 30% Cr and semi-solid cast 27% Cr irons, the secondary carbides are identified as M23C6, which has a cubic lattice and does not contain faults. In conventionally cast 18- 20%Cr irons, it has been shown [25] that, as well as M7C3 secondary carbide, both M23C6 and M6C cubic carbides can also form, with the cubic carbides growing along {111} planes in the original austenite matrix. Another type of secondary carbide may be present, whose morphology is apparently non-faceted and whose electron diffraction pattern cannot match completely with either M23C6 or M6C (Fig. 8 (d)). The exact nature of this type of carbide is of interest and has yet to be identified. Various studies have revealed that the sizes of secondary carbides vary from 0.2 Ám to 0.6 Ám with both size and distribution being influenced by prior annealing and destabilisation conditions, and by the segregation patterns in the original dendritic and eutectic austenite formed on solidification. In conventionally cast irons, SEM and TEM observations have shown that secondary carbides, either M7C3 or M23C6, do not nucleate on M7C3 eutectic carbide, but form separately within the matrix and grow at subboundaries in the matrix and/or along habit directions to develop fibrous networks. However, M23C6 secondary carbide in the semi-solid cast 27%Cr iron was found to nucleate also on M7C3 eutectic carbide (Fig. 8 (e)). The eutectic carbides in as-cast 30%Cr irons are M7C3, but during destabilisation these carbides transform to M23C6 forming a duplex core-shell structure , the extent of the transformation depending on destabilisation temperature and time. This transformation can be seen by backscattering electron mode in SEM observations as in Fig. 9(a). This reveals M23C6 shells (in bright contrast) surrounding the remaining cores of eutectic M7C3. However, TEM investigation (Fig. 9(b)) is needed to reveal full details of this duplex structure and to study the mechanism by which the transformation takes place. The M23C6 is believed to nucleate at the original interface between the eutectic M7C3 and the matrix; it then grows gaining metal atoms from the matrix and consuming the M7C3 [21]. The corrosive wear resistance of hardened 30% Cr irons appears to be inferior to that of the as-cast material, while the opposite is true for 25% Cr iron and there is no evidence of a M7C3 .. M23C6 transition in the eutectic carbides during heat treatment.
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