M23C6 carbides in 1.4532 steel

Figure 18: X7CrNiMoAl15-7, change of hardness due to aging.

Figure 19: X7CrNiMoAl15-7, 1040 C 1 h/W + 760 C 1.5 h/air, thin foil. Scale bar: 0.5 µm.

Figure 20: X7CrNiMoAl15-7, 1040 C 1 h/W + 760 C 1.5 h/air, thin foil. Scale bar: 0.5 µm.

Figure 21: X7CrNiMoAl15-7, 1040 C 1 h/W + 760 C 1.5 h/air + 560 C 2 h/air, thin foil, dark field. Scale bar: 0.1 µm.

Carbide name: M23C6
Record No.: 1495
Carbide formula: M23C6
Carbide type: M23C6
Carbide composition in weight %: No data
Image type: TEM
Steel name: X7CrNiMoAl15-7
Mat.No. (Wr.Nr.) designation: 1.4532
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Austenitic-martensitic steels
Steel composition in weight %: max. 0.09% C, max. 1.00% Si, max. 1.00% Mn, max. 0.030% P, max. 0.030% S, 14.00-16.00% Cr, 2.00-2.50% Mo, 6.50-7.75% Ni, 0.75-1.50% Al
Heat treatment/condition: See the text
Note: The chemical composition and the heat treatment are so adjusted to each other that the matrix consists of austenite, apart from high-temperature ferrite, after the solution treatment, and of martensite after hardening. The transformation from austenite to martensite is brought about by an intermediate heat treatment.

For the usual series of heat treatments, Fig. 18 shows the hardness variations associated with the changes of microstructure. Starting from the solution treatment at 1040 C, the first increase in hardness from 160 to 300 HV 30 occurs after the intermediate heat treatment at 760 C due to the formation of martensite. During the subsequent aging at 560 C, precipitation causes a further increase in hardness, which reaches a maximum of about 450 HV 30 already after an aging time of 1.5 to 2 h. Overaging occurs very slowly in comparison.

The microstructure after the intermediate heat treatment is shown in thin foil micrographs in Fig. 19 and Fig. 20. The martensite formation is brought about by the M23C6 precipitates, which lie preferentially on the austenite grain boundaries Fig. 19. On the ferrite-austenite boundaries, on the other hand, they grow into the high-temperature ferrite in the form of thin lamellae (Fig. 20), the ferrite transforming to austenite in the form of an eutectoid reaction. The amount of austenite therefore grows at the expense of the ferrite. On cooling, this newly formed austenite transforms together with the original austenite to martensite.

The microstructure after hardening is illustrated in Fig. 21 by a thin foil micrograph from the region of the hardness maximum. It is seen that the precipitates causing the hardness increase have a diameter of only about 6 nm. They are coherent B2-ordered NiAl particles with a very small missmatch, which are precipitated not only in the martensite (Fig. 21) but also in the ferrite.
Links: No data
Reference: Not shown in this demo version.

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