Precipitates in 8CrMoNiNb9-10 steel


Figure 83: 8CrMoNiNb9-10, 0.049% C, 0.01% N, 0.10% deltaNb, Nb / (C + N) = 8.5; tempered, extraction replica. Scale bar: 5 µm.


Figure 84: 8CrMoNiNb9-10, 0.086% C, 0.012% N, 0.23% deltaNb, Nb / (C + N) = 9.1; tempered, extraction replica. Scale bar: 2.5 µm.


Figure 85: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 1000 C 15 min/air + 750 C 30 min/air, extraction replica. Scale bar: 5 µm.


Figure 86: 8CrMoNiNb9-10, 0.057% C, 0.014% N, 0.25% deltaNb, Nb / (C + N) = 10.4 450 C 10000 h, extraction replica. Scale bar: 1 µm.


Figure 87: 8CrMoNiNb9-10, 0.086% C, 0.012% N, 0.23% deltaNb, Nb / (C + N) = 9.1 650 C 10000 h, extraction replica. Scale bar: 1 µm.


Figure 88: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 650 C 3360 h 37 N/mm2, fractured , extraction replica. Scale bar: 2 µm.


Figure 89: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 tempered, extraction replica. Scale bar: 5 µm.


Figure 90: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 heat aff. zone, extraction replica. Scale bar: 5 µm.


Figure 91: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 heat aff. zone, extraction replica. Scale bar: 5 µm.


Figure 92: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% Nb, Nb / (C + N) = 15.1 heat aff. zone, extraction replica. Scale bar: 2 µm.


Figure 93: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 heat aff. zone, extraction replica. Scale bar: 0.25 µm.


Figure 94: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 1000 C 15 min/air + 750 C 30 min/air, extraction replica. Scale bar: 0.5 µm.


Figure 95: 8CrMoNiNb9-10, 0.045% C, 0.008% N, 0.44% deltaNb, Nb / (C + N) = 15.1 1000 C 15 min/air + 750 C 30 min/air, extraction replica. Scale bar: 1 µm.


Figure 96: 8CrMoNiNb9-10, 0.057% C, 0.014% N, 0.25% deltaNb, Nb / (C + N) = 10.4, extraction replica. Scale bar: 0.5 µm.

Carbide name: See the text
Record No.: 1467
Carbide formula: See the text
Carbide type: See the text
Carbide composition in weight %: No data
Image type: TEM
Steel name: 8CrMoNiNb9-10
Mat.No. (Wr.Nr.) designation: 1.6770
DIN designation: 8CrMoNiNb9-10
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: High-temperature steels
Steel composition in weight %: 0.10% C, 0.15-0.50% Si, 0.40-0.80% Mn, 0.020% P, 0.020% S, 2.00-2.50% Cr, 0.90-1.10% Mo, 0.30-0.80% Ni, Nb/Ta: 10 x C, 0.05% N
Heat treatment/condition: See the text
Note: The steel 8CrMoNiNb9-10, which has a hot strength similar to that of 10CrMo9-10, is used in reactor construction in the secondary heat-exchanger circuit. The purpose of the niobium addition is to bind the carbon of the steel so strongly that carburization of the chromium-richer materials of the primary circuit by mass transport of the carbon through sodium or by uphill diffusion is prevented. The steel is usually tempered as follows: 970 to 1030 C at least 15 min/air + 680 to 750 C at least 60 min/air. It can be used continuously at temperatures below 600 C.

A few remarks on the precipitation behaviour: Due to the low carbon content compared with the niobium content and the extremely low solubility of niobium (and tantalum) in both austenite and ferrite, only two phases are precipitated in the steel 8CrMoNiNb9-10, namely niobium carbide NbC or niobium carbonitride Nb(C,N) and the Laves phase Fe2Nb or Fe2(Nb,Mo. Chromium-rich phases (carbides, intermetallic phases) have not been detected so far. The precipitation behaviour may be described empirically with the aid of two parameters: the stoichiometric ratio for carbide formation (the degree of stabilization) and the niobium excess (deltaNb) after carbide formation. The degree of stabilization is usually defined as: % Nb/(% C + % N).

For values of Nb/(C+N) up to 8.5, experience has shown that carbon and nitrogen are bound only in the form of NbC or Nb(C,N). If the nitrogen is neglected, the corresponding value of the Nb/C-ratio is approximately 10. If the sum of the carbon and nitrogen contents is less than 0.06 %, primary carbonitride is formed almost exclusively. If it is greater than 0.06 %, carbide is precipitated below Ac1) (Fig. 83).
For values of Nb/(C+N) above 8.5 the Laves phase is precipitated in addition to the (primary) carbonitride. In this case also, the value of the (C+N)-content determines whether the Laves phase and carbide (Fig. 84) or the Laves phase alone (Fig. 85) are formed below Ac1.
The excess niobium deltaNb is given by:

deltaNb = % Nb = [(at, wt, Nb/at, wt, C) x % C + (at, wt. Nb/at, wt, N) x % N]

The precipitates in the microstructure
Carbides: In the microstructure three carbide forms are observed. The first is the nitrogen-rich primary phase, which precipitates already in the melt or during solidification. After rolling it is present as lines of elongated particles which show a high reflectivity in the light microscope and are usually too thick to be identified by selected-area electron diffraction. Fig. 90 shows a particle which was not extracted, as found also in the heat affected zone (HAZ). Down to Ac1 some further carbide or carbonitride may be formed. It is always in the form of globular particles, often distributed statistically (e.g. Fig. 83, only partially extracted). The carbide precipitated below Ac1 is always finely dispersed, has a square shape (plates or cubes; Fig. 93) and usually coagulates during tempering. In steels with low (C+N)-contents it dissolves slowly again above 450 C in favour of the Laves phase, which can take up some carbon. Fig. 86 shows remains of the carbide phase; the other particles are the Laves phase. In steels with higher (C+N)-contents the carbide is stable even when aging at temperatures which are high for this steel (Fig. 87).

Laves phase: In the steel considered here the Laves phase always contains molybdenum. It may contain up to 30% Mo, but no more than 2% Cr. The precipitates formed above Ac1 always contain less molybdenum than those formed below Ac1.
A small fraction of the Laves phase may be precipitated above Ac1, where the particles have the same distribution, shape and size as the corresponding carbide, so that they may be distinguished from the latter only by diffraction or X-ray microanalysis (spherical particles in Fig. 85). After short annealing times this phase shows within the grains the variety of shapes arising from its hexagonal structure (Fig. 94). On the grain boundaries it occurs now and again in the form of parallel rods (Fig. 94, top). In steels with deltaNb >0.2%, isolated grains with lamellar Laves phase are observed (Fig. 95). During tempering the intragranular precipitates are already arranged on subgrain boundaries (Fig. 85). Marked growth is observed during aging particularly in the case of the grain boundary precipitates, which coagulate only slowly, however (Fig. 87). Under the influence of stress - e.g. during creep - a considerably faster coagulation of all precipitates is observed (Fig. 88).

Precipitation in the heat affected zone (HAZ) of welds: If the steel 8CrMoNiNb9-10 is welded, the primary carbides (or carbonitrides) are dissolved in a certain region of the HAZ, even where the material does not become molten. Even on the assumption that no carbon, nitrogen or niobium diffuses into the HAZ from the weld (according to the regulations similar, stabilized weld materials are used), the total content of these alloy elements is now available for precipitation.
In the limiting case, an over-stabilized steel, in which, on account of the low (C+N)-content, only Fe2 (Nb,Mo) is present in addition to the primary Nb(C,N), behaves like a just stabilized steel with raised carbon, nitrogen and niobium contents, in which no Laves phase, but a considerable amount of finely dispersed carbide is formed.
Fig. 90 to Fig. 92 show the microstructure of the HAZ of an over-stabilized steel. Fig. 89 shows the unaffected base material. The outermost part of the HAZ is shown in Fig. 90; in the lower half of the figure a few very fine carbides may be seen besides the coarser particles of the Laves phase. Near the weld (Fig. 91) numerous carbides have been precipitated within the grains. The Laves phase is found here only on the grain boundaries. Fig. 92 shows the region which was not yet melted, containing coarse niobium carbides on the grain boundaries, and locally even a niobium carbide eutectic, besides the finely dispersed carbides.
Fig. 96 shows part of such a eutectic, which formed at the position of a primary carbide after it had been dissolved during a weld simulation treatment (maximum temperature 1350 C), although the base material had not melted at this point.
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Reference: Not shown in this demo version.

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