Carbides in 100Cr6 steel

Figure 1: Transmission electron micrograph of a 100Cr6 steel specimen austenitised for 20 mn at 1050 C then held for 1 h at 355 C and quenched into oil at 60 C, showing relatively coarse carbides precipitated on preferred crystal planes. These carbides are in zones that have transformed to martensite, alongside regions of upper bainite formed during holding at 355 C Courtesy Ecole des Mines de Nancy and IWT Bremen. Scale bar: 0.2 µm.

Carbide name: No data
Record No.: 1193
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: TEM
Steel name: 100Cr6
Mat.No. (Wr.Nr.) designation: 1.3505
DIN designation: 100Cr6
AISI/SAE/ASTM designation: AISI 52100
Other designation: No data
Steel group: Tool steels
Steel composition in weight %: 0.93-1.05% C, 0.15-0.35% Si, 0.25-0.45% Mn, 1.35-1.60% Cr.
Heat treatment/condition: No data
Note: Tempering is the name usually given to treatments performed between about 500 and 700 C (below A1), during which martensite converts to the thermodynamically stable phases, namely ferrite and carbides (cementite in plain carbon steels, although strictly speaking this phase is still only metastable with respect to graphite). In carbon steels, the phases are the same as in pearlite, but their distribution is different, since the carbides are formed by a continuous precipitation reaction. In the early days of metallography, the resulting structure was referred to as sorbite. The ferrite matrix is continuous and highly ductile, while the cementite particles are very fine, but not sufficiently to induce significant hardening.
In the very earliest stages of tempering, either cementite or Hagg's carbide are the first to precipitate (Fig. 1), even in alloy steels where other carbides are thermodynamically more stable. Their compositions are close to those of the matrix, with the same proportion of metallic elements. This is observed for particle sizes between about 50 and 150 nm and suggests that their formation occurs under para-equilibrium conditions, in spite of the retarding effect of the alloying additions. The system subsequently evolves slowly with time, tending towards the true equilibrium. The equilibrium carbides probably nucleate independently, on dislocations, and grow at the expense of the metastable phases. In this respect, cobalt has an indirect beneficial effect, since it retards dislocation recovery, preserving potential nucleation sites.
Links: No data
Reference: Not shown in this demo version.

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