MC, M2C and M7C3 carbides in cast high speed steel roll

Figure 1: Microstructure of the alloy Fe-1.9C-5V-2Mo-2W-4Cr solidified at 0.15K/s. Label "I" means interdendritic M2C and M7C3 eutectic carbides. Scale bar: 100 µm.

Figure 2: Microstructure of the alloy Fe-1.9C-5V-2Mo-2W-4Cr solidified at 0.15K/s. Detail of the interdendritic eutectic carbides. Scale bar: 10 µm.

Table 1: Typical composition ranges of the eutectic carbides.

Figure 3: Typical morphologies developed by the MC carbide in the HSS for rolls. Scale bar: 10 µm.

Figure 4: Detail of the microstructure of the alloy Fe-2.5C-5V-5Mo-5W-4C solidified at 0.15 K/s. Scale bar: 20 µm.

Figure 5: Thermal fatigue experiments. Nucleation of the secondary cracks at the MC eutectic carbide. Scale bar: 10 µm.

Figure 6: Thermal fatigue experiments. Propagation of the secondary cracks along coarse interdendritic M7C3 eutectic carbides. Scale bar: 40 µm.

Carbide name: MC, M2C, M7C3
Record No.: 791
Carbide formula: MC, M2C, M7C3
Carbide type: MC, M2C, M7C3
Carbide composition in weight %: No data
Image type: SEM
Steel name: High speed steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Fe-1.9C-5V-2Mo-2W-4Cr
Steel composition in weight %: 1.52.5% C; up to 6% W; up to 6% Mo; 38% Cr, 410% V.
Heat treatment/condition: No data
Note: In most cases, work rolls for the finishing stands of hot-strip mills are composite components made of an outer shell of castwear-resistantmaterial and a core of ductile iron or steel. The development of materials for the outer shell has enjoyed rapid advances beginning in the early 1980s, culminating in the application of cast alloys of the Fe-C-Cr-W-Mo-V system, which replaced high-chromium cast iron and Ni-hard cast iron. These alloys are generically termed high speed steels or, more rarely, multi-component white cast iron. The idea of using these alloys for manufacturing work rolls for hot-strip mills resulted from an insight into the requirements involved in this type of application: fundamentally, the capacity to retain a high level of hardness even when submitted to high temperatures, and also wear resistance. Both are fulfilled by the classical high speed steels. The alloy design of the high speed steels for rolls is based on the composition of the M2 steel, the main changes being the higher carbon and vanadium contents.
The degradation of the work rolls for the early finishing stands involves abrasion, oxidation, adhesion ("sticking") and thermal fatigue. This work deals with the effect of the chemical composition on the microstructure of the HSS for rolls, mainly in respect to the volume fraction and morphology of the eutectic carbides, as well as with the influence of the microstructure on their wear resistance.

Owing to the higher carbon contents of the HSS for rolls in comparison to those of the HSS for tools, austenite, instead of delta ferrite, is the primary crystallized phase and the peritectic reaction delta+L->gamma does not take place. In addition, the HSS for rolls are less hypoeutectic than the HSS for tools, that is, they have a lower volume fraction of proeutectic phase. Thus the solidification sequence of the major high speed steels for rolls is composed by just two main occurrences:
1.) primary crystallization of austenite: liquid -> austenite; 2.) eutectic decomposition of residual interdendritic liquid: liquid -> austenite + carbides.
Nevertheless, the residual interdendritic liquid decomposes through different eutectic reactions as itmoves down a eutectic trough, leading to the formation of up to three eutectics: gamma+MC, gamma+M2C and gamma+M7C3. Figures 1, 2 shows the curve related to the cooling between 1450 C and 1100 C during differential thermal analysis of the alloy Fe-1.9C-5V-2Mo-2W-4Cr and Fig. 2 shows its resultant microstructure (matrix is not etched).
The gamma+MC eutectic always precipitates first, owing to the high vanadium content of these alloys. The precipitation of the gamma+M2C and/or gamma+M7C3 eutectics in the last stages of the solidification is governed by the segregation of the alloying elements and the sequence by which they precipitate results from the competition between them, depending on the overall chemical composition and on the cooling rate. The former is favored by high W, Mo or V contents and high cooling rates while the latter is favored by high Cr or C contents and low cooling rates. Figure shows the effect of vanadium content and cooling rate on the volume fraction of the eutectic carbides in the alloy Fe-1.9C-2Mo-2W-4Cr-V (note the suppression of the M7C3 carbide for high vanadium content or cooling rate) and Table 1 shows the composition ranges of the eutectic carbides.

The as-cast microstructure of the HSS for rolls is characterized by amatrix with products of austenite decomposition (normally martensite or bainite), retained austenite and precipitated globular secondary carbides (Fig. 1(b)), MC eutectic cells (coral-like MC) and eutectic carbides distributed in the interdendritic or intercellular regions (M2C,M7C3 and idiomorphic or petallike MC). MC carbide is by far the major eutectic carbide in the microstructure. Up to three morphologies, commonly named coral-like, petallike and idiomorphic (Fig. 3), are developed, depending on the chemical composition and cooling rate. The morphology of the MC carbide is influenced both by the vanadium content and by the cooling rate, an interdependence of these variables needing be considered in determining quantitative limits of their influence. It was shown that the higher the cooling rate and the lower the vanadium content, the higher the tendency to the formation of less coupled eutectic with petal-like and/or idiomorphic MC carbide.

The volume fraction of the M2C andM7C3 carbides rarely reach 5% each one. When the formation of the M7C3 eutectic precedes that of the M2C eutectic, theM7C3 carbide develops as branched platelets thicker at the end, forming a "wall" of carbide around the eutectic cell (Fig. 1(b)), typically found in the high chromium white cast irons with low carbon and low Cr/C ratio. In this case, the M2C eutectic nucleates on theM7C3 carbide and the M2Ccarbide presents platelike and/or fine lamellarmorphologies (Fig. 1(b)), both described elsewhere [8]. When, otherwise, the formation of the M7C3 eutectic takes place after that of the M2C eutectic, the M7C3 carbide is rodlike and the M2C carbide develops only as platelets assembled as radiating clusters, playing the role of heterogeneous nucleus for the precipitation of the M7C3 eutectic (Fig. 4).

After heat treated through quenching and tempering, the microstructure has a tempered martensite or bainite matrix with remaining globular secondary carbides precipitated during solidification (size around 1 µm) and fine globular secondary carbides precipitated during tempering (size less than 1 µm), both being mainly MC, M7C3 and M23C6 carbides.

Thermal fatigue experiments show that secondary cracks nucleate at the eutectic carbide (stress concentration induced by the great difference between the thermal coefficients of carbide and matrix) and propagate along carbide/matrix interface. Since the presence of eutectic carbides, and thus crack nucleation, is unavoidable, improving thermal fatigue resistance requires their refining and homogeneous distribution, so as to avoid the formation of easy crack propagation paths, like interdendritic or intercellular coarse M7C3 or M2C carbides (Figs. 5 and 6).
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