Metal fusion bonding – Process – Critical work component – temperature – or pressure
Reexamination Certificate
1999-01-08
2001-02-27
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Critical work component, temperature, or pressure
C228S250000, C228S119000, C228S125000, C219S13700R, C219S1370WM
Reexamination Certificate
active
06193145
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a method of jointing a first part made of low alloy constructional steel and a second part made of austenitic stainless steel by heterogeneous butt-welding.
BACKGROUND OF THE INVENTION
Nuclear reactors in general and pressurised water nuclear reactors in particular include components such as the reactor containment vessel, steam generators and the pressuriser which are made of low alloy high strength constructional steel lined with a layer of stainless steel and connected by one or more connecting tubes to one or more austenitic stainless steel pipes constituting parts of the primary circuit of the reactor, for example.
The containment vessel of a nuclear reactor is generally cylindrical and has a guide tube for supporting connecting tubes in which is formed a set of connecting tubes for connecting the containment vessel to the pipes of the various loops of the primary circuit.
Steam generators of pressurised water nuclear reactors include a water box having a hemispherical wall constituting the lower part of the steam generator with two connecting tubes welded to two pipes of the primary circuit of the reactor.
The pressuriser of a pressurised water nuclear reactor includes a generally cylindrical jacket having two domed ends, the bottom end having a connecting tube for connection to the primary circuit of the nuclear reactor via an austenitic stainless steel expansion pipe, the top end being connected in the same manner to other austenitic stainless steel pipes.
In all cases the connecting tubes of the component, which are made of constructional steel lined with stainless steel, must be butt-welded to an austenitic stainless steel pipe.
SUMMARY OF THE INVENTION
It is therefore necessary to make a heterogeneous welded joint between the connecting tube and the pipe, the heterogeneous weld being effected by depositing a filler metal into a groove formed between the connecting tube and the pipe or between the connecting tube and an intermediate austenitic stainless steel connector.
Before effecting the heterogeneous weld between the connecting tube and the pipe or the intermediate connector it is necessary to deposit a thick layer of stainless steel or nickel alloy onto an end part of the connecting tube constituting one surface delimiting the groove into which the filler metal is deposited during welding.
If the welding if effected by depositing nickel alloy in the groove, a thick layer of nickel alloy is deposited on both the lateral surfaces delimiting the groove, i.e. the machined surface on the connecting tube and the machined surface on the stainless steel pipe.
Using nickel alloy as the filler metal results in welded joints of improved mechanical characteristics.
However, if the filler metal is a nickel alloy it is more difficult to weld and to inspect after welding than if the filler metal is a stainless steel, for example of the 316 L type. Nickel alloys oxidise more and are more sensitive to hot cracking than 316 L stainless steel.
BRIEF DESCRIPTION OF THE INVENTION
Furthermore, because there is a great difference between the degree to which the ferritic steel of the connecting tube and the filler metal absorb gamma rays, radiographic inspection of the weld is difficult.
The operations preceding welding, i.e. machining the groove and applying the thick layer of nickel alloy (or stainless steel), increase the time required to make the joint and lead to high costs.
Finally, some defects have been detected in the heterogeneous welded areas, although these defects do not compromise the safety of the welded joints.
The aim of the invention is therefore to propose a method of jointing by heterogeneous butt welding a first part made from a low alloy constructional steel and a second part made from austenitic stainless steel, consisting in machining an end portion of each of the parts, placing the machined end portions of the parts face-to-face to constitute a welding groove extending in a longitudinal direction between the parts and depositing a filler metal into the welding groove, this method limiting the duration and cost of the jointing operations, in particular application of the thick layer of nickel alloy (or stainless steel) to the welding groove walls before welding.
To this end, the welding groove is a narrow groove in which the angle of opening of the lateral walls parallel to the longitudinal direction and consisting of machined portions of the two parts to a median plane of the groove in the longitudinal direction is at most equal to 5° and there is deposited into the welding groove a nickel-based alloy containing by weight 18% to 32% chromium and possibly iron, manganese, silicon, molybdenum, aluminium, copper, niobium and titanium, the remainder of the alloy, with the exception of impurities resulting from its manufacture, comprising nickel constituting at least 53% by weight.
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Dunand-Roux Leon
Fournier Yves
Peigney Alain
Edmondson L.
Framatome
Pollock, Vande Sande & Amernick, R.L.L.P.
Ryan Patrick
LandOfFree
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