Simultaneous precipitation in austenitic stainless steels
Austenitic stainless steels in steam power plants
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).
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.
| Esshete 1250 (Corus / British Steel) | ||||||||||
| Fe | Cr | Ni | Mn | Mo | Si | Nb | Ti | C | N | B |
| bal. | 15.5 | 9.55 | 5.63 | 1.3 | 0.41 | 1.02 | 0 | 0.085 | 0 | 0.005 |
| NF709 (Nippon Steels) | ||||||||||
| Fe | Cr | Ni | Mn | Mo | Si | Nb | Ti | C | N | B |
| bal. | 20 | 25 | 1.0 | 1.5 | 0.41 | 0.06 | 0.26 | 0.06 | 0.17 | 0.005 |
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:
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:
- X-ray diffraction on extration residues: in this method, the metal is dissolved, the residues filtered and then analysed in a 2-theta diffractometer. Advantages are its ease of application, informations are collected on a relatively large amount of material compared to TEM (Transmission Electron Microscope). Inconvenient: this method does not give any microstructural information (location and size of the precipitates), some precipitates are difficult to distinguish on the sole basis of their structure, some may dissolve during the extraction.
- TEM: this allows full determination of the structure (conventional diffraction, convergent beam diffccation), composition (EDX, EELS for light elements). This is done on thin foils and carbon replica.
- Optical microscopy with selective etchants.