(Mo,Cr)2C carbides in Fe-1.7C-1.2Si-1.1Mn-0.8Ni-3.6Cr-1.7Mo-4.7V alloy


Figure 1: Scanning electron micrograph of an Fe-17C-l.2Si-l.lMn-0.8Ni-3.6Cr-1.7Mo-47V alloy slowly cooled in the form of a large casting. Enhanced contrast reveals molybdenum- enriched zones due to the dissolution of eutectic carbides. The small white acicular carbides are Mo2C. The coarse black carbides are eutectic VC, while the abundant fine dark spots are secondary VC precipitates. Courtesy INPG, Grenoble. Scale bar: 50 µm.

Carbide name: (Mo,Cr)2C
Record No.: 1184
Carbide formula: (Mo,Cr)2C
Carbide type: M2C
Carbide composition in weight %: No data
Image type: SEM
Steel name: Fe-1.7C-1.2Si-1.1Mn-0.8Ni-3.6Cr-1.7Mo-4.7V alloy
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Steels
Steel composition in weight %: Fe-1.7C-1.2Si-1.1Mn-0.8Ni-3.6Cr-1.7Mo-4.7V
Heat treatment/condition: No data
Note: Disappearance of non-equilibrium carbides in a pseudo-peritectic reaction: The above examples have shown that carbides formed during solidification remain present, in spite of the pseudo-peritectic reaction that ought to have caused them to disappear, at least partially. In multi-component alloys, according to the phase rule, it is quite normal for numerous phases to co-exist. However, the persistence of phases that are no longer in equilibrium is due to kinetic factors that make many reactions very sluggish. When carbides redissolve, the process is not the reverse of that involved in their growth, that is, gradual thinning. When epitaxial relationships exist between carbides {e.g. M7C3 and M3C, M7C3 and M23C6), the peritectic carbide nucleates on the phase that has become metastable and grows at its expense. Alloying elements are transferred from one carbide to the other. Some carbides, such as M6C, dissolve in another manner, disintegrating and becoming porous. Inside carbide particles, in regions where the carbon has left, transient phases may form, such as ferrite.
Another example, shown in Figure 1, is a white cast iron similar to that illustrated in previus images. The specimen was cut from a large, slowly cooled casting. The M6C carbides formed during solidification have completely decomposed and the molybdenum released into the matrix has re-precipitated in the form of (Mo,Cr)2C platelets. However, the conventional heat treatment has not fully homogenised the composition and the molybdenum remains concentrated in regions around the prior eutectic constituents. The lighter-coloured zones affected by the dissolution are clearly visible in Figure 1. Although only slight, the differences in local composition affect the transformation behaviour to martensite or bainite and subsequent tempering reactions. The presence of unwanted Mo2C and the general heterogeneous structure can significantly impair the mechanical properties.
Links: No data
Reference: Not shown in this demo version.

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