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During welding, one has to be extremely careful as stresses develop due to the heat produced during welding and get incorporated in the welded part, resulting in transformation of the properties of the welded part. These are mentioned as "built-in mechanical stress"

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Note added at 4 hrs (2008-08-26 12:36:38 GMT)
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The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding the weld. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.

The thermal diffusivity of the base material plays a large role—if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Alternatively, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat inputted by the welding process plays an important role as well, as processes like oxyfuel welding use high heat input and increase the size of the HAZ. Processes like laser beam welding and electron beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input. To calculate the heat input for arc welding procedures, the following formula is used:

Q = \left(\frac{V \times I \times 60}{S \times 1000} \right) \times \mathrm{Efficiency}

where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8.

Residual stresses are stresses that remain after the original cause of the stresses (external forces, heat gradient) has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment. Heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses.

While uncontrolled residual stresses are undesirable, some designs rely on them. For example, toughened glass and pre-stressed concrete depend on residual stress to prevent brittle failure. A demonstration of the effect is shown by Prince Rupert's Drop, where a molten glass globule is quenched to produce a toughened outer layer.
Prince Rupert's Drops
Prince Rupert's Drops

Bolted joints use residual stress to avoid subjecting bolts to fatigue. A gradient in martensite formation leaves residual stress in some swords with particularly hard edges (notably the katana), which can prevent the opening of edge cracks.

In certain types of gun barrels made with two tubes forced together, the inner tube is compressed while the outer tube stretches, preventing cracks from opening in the rifling when the gun is fired. Parts are often heated or dropped into liquid nitrogen to aid assembly.
these data have been collected from :
http://en.wikipedia.org/wiki/Category:Welding
I think that this will give you an idea of what I am trying to prove. I am basically a retired mechanical Engineer and I have experienced these problems.

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Note added at 4 hrs (2008-08-26 12:38:49 GMT)
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If the welding is done at site where the reactor is to be installed, the problems that are mentioned above can be taken care of.

une autre suggestion....

Jusqu'à présent, on a mis en oeuvre des contraintes relativement faibles, classiquement de l'ordre de 0,3 mm. Plus récemment, cette contrainte au montage a ...
www.freepatentsonline.com/EP1580402.html - Pages similaires

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