Current applications.  Shape welding has been used for many years to add weld metal on forgings,  as shown for instance in Fig. 15, where arrows indicate the location of material completions of this kind at the priviously  mentioned "Atucha 2" shell flange. The major benefits of this technology are:           - simplified and less expensive forging,           - short production time,           - low additional expenditures,           - except for stress-relieving there is no need for             other heat-treatments (such as quenching and             tempering).

Fig. 15

The two main production methods of shape welding                                                                                   

Starting from this positive experience, development works had been performed in nuclear apparatus fabrication aiming at qualifying of shape welding as a process eligible for sub-components manufacture.

As a result of these studies, two different methods of production were used for applications in nuclear engineering, i.e.

- shape welding around a vertical axis, and

- shape welding around a horizontal axis.

Fig. 16

Fig. 17

The first approach of welding around a vertical axis is the simplest one, and is to be preferred especially for rotationally symmetric products of large size. It is also preferred whenever the product contours are of a complicated configuration.

A typical example of small size application is shown in Fig. 16 illustrating the design of a SG feed-water nozzle with complicated geometrical contours.

For nozzles with more favorable geometrical contours the shape welding around a horizontal axis is more suitable, especially when several identical products are required as shown in Fig. 17. Pursuing this approach, a total of 4 identical nozzles were shape welded in one operation.

Fig. 18 shows the semi-finished product after stress-relieving, while Fig. 19 illustrates a machined blank taken from the semi-product for the manufacture of 2 nozzles.

Fig. 18

Fig. 19

Procedure qualification 

The submerged-arc technology is a fully qualified welding process that has been in use for many years in heavy apparatus construction. When adopting suitable wire/flux combinations, physical properties are achieved in the material which safely fulfills the high quality requirements to be satisfied by steel in nuclear engineering. 

For manufacturing shape welded parts same wire/flux combination have been used as for conventional welding of the forged MnMoNi reactor steel quality (20 MnMoNi 5 5 according to KTA 3201.1, Part 1). 

To qualify this production method, the nozzles in stress-relieved condition were  tested comprehensively. All physical properties to be achieved according to the applicable specifications for primary-loop components of light-water reactors were fully accomplished (i.e. German nuclear standards KTA 3201.1 relating to materials and KTA 3201.3 relating to manufacture, as well as VdTÜV series 401 material standards). 

The most important test results can be seen in Figs 20 to 25.

The strength properties, which were tested at 20 °C (Figs 20 and 21) and 350 °C (Figs 22 and 23), are within very close tolerance limits without any dependence on the sample location.

Fig. 20	Fig. 21
Fig. 22	Fig. 23

The most prominent toughness values characterizing the isotropic toughness response have been represented by the A_v-t (energy absorbed / temperature) curve in Fig. 27, and by the energy absorbed during impact testing at 0 °C       (Fig. 28).

Fig. 24

Fig. 25

Further requirements such at the fracture appearance transition temperature T_NDT ≤ -5 °C are fulfilled with a wide safety tolerance.

Major advantages in terms of quality include the following:

- High homogeneity with regard to both chemical analysis and grain structure,

- isotropic physical properties,

- high ductility, and

- high toughness at low temperatures.

According to the German Safety Standard  KTA 3201.1, Part 1, Chapter 29, Product forms and components from ferritic steels fabricated by shape-welding, the following two types 

                                                            8 MnMoNi 5 5, and 

                                                          10 MnMoNi 5 5 

are already acceptable for the production of nuclear primary components. The first one - 8 MnMoNi 5 5 - was developed by MAN GHH.

Future outlook

Shape-welding technique opens up a new perspective for designing heavy nuclear components mainly with a view to realizing the idea of “seamless components” of any size. This point is illustrated in Fig. 26, based on the example of a 1300 MW reactor pressure vessel.  

Fig. 26

In the shape welded version (B in Fig. 26) the number of circular welded joints are not only reduced from 5 to 2, but at the same time the remaining 2 welds have lost the classical meaning of “welds” because no difference exist between the three characteristic zones of  “weld metal”, “HAZ” and “base metal”; all three zones consist of the same isotropic        MnMoN all weld metal and have identical chemical composition, microstructure and mechanical properties.

For this reason, in case of seamless components the question of economy cannot be decided simply by comparing the prices of shape-welded parts and forged parts. Savings realized with the new steel in wall thicknesses, in circular welds, in complete corrosion resistant cladding layers (single layer instead of multiple layer), in the extent of examinations, and in repeat tests at the user, as well as in reduced delivery periods and grater availability of plants, can all tip the scales in the end in favour of shape-welded heavy components. 

In conclusion, the German experience of manufacturing primary nuclear components of very large dimensions and wall thicknesses is a solid basis for the development in the 21^st of reactor pressure vessels and steam generators made from isotropic shape welded steel 8 MnMoNi 5 5.