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Simultaneous precipitation in austenitic stainless steels

Austenitic stainless steels in steam power plants

Click to enlarge

The part of a steam power plant we are interested in can be (that's simplistic) seen as a huge kettle. In the superheater tubes, high pressure 'water' circulates and is heated to temperature around 650 C before being sent to the turbines. There is interest in constantly increasing the service temperature, as this improves the efficiency of the plant.

The material used for superheater tubes must have excellent corrosion and creep resistance. Creep is a time dependent deformation under a stress which is below the yield stress of the material (that is the stress above which a material undergoes 'instantaneous' plastic deformation).

There are several creep mechanisms, which become predominant under different temperature and stresses. For example, at low stresses and relatively high temperatures, creep deformation will be essentially due to the movements of vacancies (holes in the atomic arrangement) from the region under tension to those under compression, where they have a lower energy.

At higher stresses, dislocations can move, but the stress to which they are submitted is not large enough for them to pass the obstacles. To pass the obstacle, they must climb (and this process again involves diffusion of vacancies). Click to enlarge

Creep resistance is therefore enhanced if there is a fine dispersion of particles in the matrix, that is many obstacle for the dislocations. A way to achieve this which has been exploited for many years is to add to the steel elements like Ti or Nb, which combines with the carbon or nitrogen present in the steel to form a very fine dispersion of particles in the grains.
But all precipitates are not beneficial to creep properties, either because they form coarse precipitates on the grain boundaries, or because they remove solute elements from the matrix (elements in solution also contribute to the strength)... It is therefore important to be able to predict which phases one can expect to form for a given composition of steel.

Examples of creep resistant austenitic stainless steels
Esshete 1250 (Corus / British Steel)
FeCrNiMnMoSiNb TiCNB
bal.15.59.555.631.3 0.411.0200.08500.005
NF709 (Nippon Steels)
FeCrNiMnMoSiNb TiCNB
bal.20251.01.5 0.410.060.260.060.170.005

Modelling simultaneous precipitations

Precipitations phenomena in austenitic steels are very complex, because different phases compete for solute and nucleation sites. The model which is being developed is a time step model. The basic idea is shown below:

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Experimental study

Two steels have been provided (Esshete 1250 and NF709). They are aged in furnaces for different times (up to 15000h). The experimental determination of the different precipitates forming in these steels is not straightforward: some are extremely small, some have similar structures, and some can be carbides or nitrides.

Different experimental techniques are used:

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