The initial microstructure of the steels evolves during service, due to high temperature and irradiation for prolonged times, leading to modification of defect structure and secondary phases. These changes harden the steel, leading to concomitant embrittlement, which is discussed below.
|
It is reported that carbon content in 12% chromium steel is maintained high in order to use the steel as martensitic steels. The high amount of carbon in 12% chromium steel leads to copious precipitation of carbides, that is, twice as much in 9Cr steels. Both the steels have predominantly M23C6 carbides with a small fraction of monocarbides,
|
|
|
eventually leading34 to deterioration of their resistance to brittle failure. The critical stress to propagate a crack is inversely proportional to the crack length. If it is assumed that fracture initiates at an M23C6 precipitate and the crack length at initiation equals the diameter of a carbide particle then the fracture stress will decrease with increasing precipitate size. The precipitates coarsen during irradiation in the range of 673-773 K, thus causing a decrease in fracture stress and an increase in DBTT even in the absence of further hardening.
Additionally, Cr rich, bcc a! precipitates formed35 in the higher chromium steel during thermal exposure and irradiation lead to hardening and embrittlement of the steel. The 8-ferrite, into which there is a repartitioning of chromium, is harmful, since it promotes formation of a0. The presence of very fine coherent particles of the w (Fe2Mo) phase has also been reported in the T91 and HT9 steels. The w phase was observed to form more rapidly in the 9Cr-2Mo type of steels, both in the 8-ferrite and martensite phases. This is possibly due to the higher amount of Mo in the EM12 type of steels. The w phase is enriched in Fe, Si, and Ni and contains significant amount of Mo and P. The G phase (Mn7Ni16Si17) has been found to form very occasionally in the modified 9Cr-1Mo and HT9 (12% Cr) variety of steels.
The a phase (Fe-Cr phase, enriched in Si, Ni, and P) has been observed to form around the M23C6 particles in 9-13% Cr martensitic steels after irradiation at 420-460 ° C in Dounray Fast Reactor. In addition Cr3P needles and MP (M = Fe, Cr, and Mo) particles have also been detected in the 12 and 13Cr steels in the range of 420-615 °C. The formation of these phases during irradiation may be understood in terms of the strong radiation-induced segregation (RIS) of alloying and impurity elements to point defect sinks in the steels (see Chapter 1.18, Radiation — Induced Segregation). The RIS of alloying/impurity elements could lead36 to either enrichment or depletion near the sinks, depending on the size of the atom and its binding energy with iron self-interstitials.
|
|
|
|
|
|
|
Figure 6 Effect24 of prolonged exposure (823 K per 10 000 h) of modified 9Cr-1Mo steel. Transmission electron micrograph showing (a) formation of detrimental Fe2Mo Laves intermetallic phase around the M23C6. The insets show the microdiffraction pattern and magnified view of the nucleation of Laves phase (b) EDAX spectrum confirming the enrichment of iron and molybdenum.
|
|
|
Table 5 Void swelling resistance26 29 of some commercial ferritic steels
|
Commercial
name
|
Chemistry and country of origin
|
Reactor in which irradiation was carried out
|
Burn-up achieved (dpa)
|
|
FV448
|
12Cr-MoVNb, UK
|
PFR
|
132
|
|
EM10
|
9Cr-1Mo, France
|
Phenix
|
142
|
|
1.4914
|
12CrMoVNb, Germany
|
Phenix
|
115
|
|
EP450
|
12Cr-MoVNb, Russia
|
BN-350
|
45
|
|
EP450
|
—
|
BN-600
|
144
|
|
|
Generally, a large number of alloying elements, W, Nb, Mo, Ta, V, or Ti are dissolved into the matrix of ferritic steels, some of them being larger than the iron atom. This could lead to the expansion of the unit cell of ferrite, making an element say, chromium undersized, with a positive binding energy with iron self-interstitial. Such a situation would lead to enrichment of chromium near the sink-like grain boundary. The reverse could happen if the size of the alloying elements happen to be smaller than iron.
The w, G and a phases are all enriched in Si and Ni — elements which are known to segregate to interfaces during irradiation. With the exception of G phase, all the other phases and the a0 phase are rich in Cr. In those ferritic steels, where Cr is depleted near voids and at other interfaces which act as point defect sinks, it follows that in steels containing higher than 11 or 12% Cr, the chromium enrichment within the matrix may lead to local concentrations exceeding those (>14%) at which a0 forms thermally. Further, enrichment of Cr may also result from the partial dissolution of chromium rich precipitates such as M23C6 during irradiation. In addition, RIS of phosphorus can also lead to the formation of phosphides in some of the steels. The irradiation-induced point defect clusters and loops may also facilitate and enhance nucleation of these phases. Although the relatively soft 8-ferrite improves the ductility and toughness of the 12Cr steel, the fracture could be initiated at the M23C6 precipitates on the 8-ferrite-martensite interface. The presence of 8-ferrite, extensive precipitation and radiation-induced growth of M23C6 precipitates and formation of the embrittling intermetallic phases in the 12Cr-1MoVW steel in the temperature range 573-773 K are together responsible37 for the relative change in impact behavior of 9Cr-1MoVNb and 12Cr-1MoVW between 323 and 673 K.
Irradiation-induced microstructural changes are the factors that govern the creep and embrittlement behavior, which therefore, has to be minimized using appropriate chemistry and structure.
