Welded austenitic steel pipe assembly

Tapered stainless steel spool pieces are welded to the ends of stainless steel pipe lengths at the factory to provide subassemblies which are then annealed and thereafter in the field are positioned with the larger free ends of opposed spool pieces disposed to receive weld metal uniting the subassemblies while coolant liquid is maintained in contact with the inside surfaces of the spool pieces.

The present invention relates generally to corrosion prevention and is more 
particularly concerned with a novel method of constructing an austenitic 
steel pipe for use in nuclear reactor operation by welding pipe lengths 
together in the field without substantially increasing the stress 
corrosion cracking tendency of the assembly, and is also concerned with 
the resulting novel butt-welded pipe assembly. 
BACKGROUND OF THE INVENTION 
Stress corrosion cracking in weld heat affected zones of stainless steel 
piping in nuclear reactors has long been generally recognized as being a 
significant problem. One of the measures or "fixes" heretofore proposed to 
meet this problem, the overlay or backlay weld method disclosed and 
claimed in copending patent application U.S. Ser. No. 734,423, filed Oct. 
20, 1976, now U.S. Pat. No. 4,049,186 in the names of Rodney E. Hanneman, 
Richard M. Chrenko and Donald B. Kittle and assigned to the assignee 
hereof, appears to have principal merit for decreasing the stress 
corrosion cracking tendency in existing pipe installations, particularly 
in that it avoids the necessity for cutting out pipe segments or even 
opening the piping at all. There is still need for a method or means 
enabling construction or failed weld repair in the field of a welded pipe 
assembly which does not require the special inspection or precautions of 
the backlay weld or other fix for protection against stress corrosion 
cracking under nuclear reactor operating conditions, yet while allowing, 
in large part, the use of existing inventories of otherwise potentially 
susceptible stainless steel piping. 
SUMMARY OF THE INVENTION 
My present invention meets and satisfies the foregoing need in that it 
enables construction in the field of welded stainless steel piping having 
requisite resistance to stress corrosion cracking without the special 
precautions and limitations of prior fixes such as the backlay while 
permitting in large part the use of existing inventories of otherwise 
potentially susceptible stainless steel pipe. 
Moreover, this new result is comparatively easily and economically obtained 
or produced, requiring no special or additional skill or procedure in the 
field and involving as additional operations in the preferred practice 
only welding and solution heat treating at the pipe fabrication factory. 
My novel concepts enabling this new result are in essence to reduce both 
the applied stress and the residual stress in the inside diameter region 
of the weld heat affected zone, to significantly lower local sensitization 
behavior, and to greatly increase the critical stress intensity factor 
required for intergranular stress corrosion cracking (IGSCC) initiation in 
the pipe in each such heat affected zone of the pipe installation. These 
concepts are implemented in accordance with this invention by providing 
spool pieces of novel combination of form and material, butt-welding them 
to the ends of each pipe length, and joining the resulting subassemblies 
together end-to-end with welds between opposed spool pieces to provide the 
novel stress corrosion cracking-resistant stainless steel pipe assembly or 
installation. 
The spool pieces, more particularly described, are of a stainless steel 
alloy which is preferably less sensitizable than that substantially less 
expensive alloy commonly used in the fabrication of stainless steel pipe 
lengths for nuclear reactor water line use. Also, the spool piece alloy is 
one which is compatible under normal reactor operating conditions with the 
alloy of the associated pipe lengths and at the same time has yield 
strength equal to or preferably greater than that required for the base 
pipe stainless steel code. Still further, these spool pieces are each 
formed to provide a relatively short portion of enlarged wall thickness 
adjacent to one end where the weld joining the spool piece-pipe length 
subassembly to another such subassembly is to be made at the pipe 
installation location. As a practical matter, the special benefits to be 
gained through the use of spool pieces of such material are maximized by 
making the enlarged wall thickness about twice the thickness of standard 
pipe length (i.e., about 0.75 inches for a 4" schedule 80 pipe, for 
example) and by terminating the enlarged portion in a frustroconical 
surface for contact with the metal of the subassembly-joining weld to the 
base pipe. Examples of alloys having special utility in spool piece 
fabrication include 316LN (with yield strength equal to or above that 
required for 304 stainless steel code), 304LN and 347LN (grades meeting 
the said 316N criterion), 304L and 347L processed to a sufficiently small 
grain size to achieve strength equal to or above that of 304 material, and 
duplex stainless steels with ferrite levels between 8 percent and 13 
percent. 
As indicated above, another feature of this invention is the heat treatment 
of each pipe length-spool piece subassembly prior to use in construction 
of a pipeline installation. Actually, this operation involves a 
solution-anneal followed by a quench to eliminate sensitization resulting 
from the welding step performed at the factory. This procedure also 
relieves any prior weld and fabrication residual stresses in earlier weld 
regions. 
Still another feature of this invention in its preferred form involves 
maintaining cooling liquid in contact with the inside surface of the pipe 
assembly and particularly with the spool pieces as the subassemblies are 
being welded together once the root weld pass is completed. 
In general, then, the method of this invention includes the steps of 
providing at each end of pipe length a separately-formed spool piece of 
considerably increased wall thickness at its free end, positioning the 
resulting subassemblies with the spool pieces in abutting relation so as 
to provide an annular recess between each pair of opposed spool pieces, 
and finally welding the subassemblies together by producing molten weld 
metal in each such recess and freezing the weld metal in contact with the 
frustroconical surfaces of the spool pieces. These spool piece-pipe length 
subassemblies are then solution-annealed to desensitize the heat affected 
zones of their respective welds. 
The pipe assembly or installation of this invention similarly described 
comprises a plurality of the previously-described pipe length-spool piece 
subassemblies in which the spool pieces are each formed with an end 
portion of inside diameter and wall thickness substantially the same as 
that of the pipe lengths and another end portion of the same inside 
diameter but substantially greater outside diameter and wall thickness and 
with a tapering contour from the free open end defined by a generally 
frustroconical plane surface. Still further, the base pipe lengths of this 
assembly consist of potentially stress corrosion susceptible stainless 
steel such as 304 type, while the spool pieces and weld metal are of less 
IGSCC susceptible but at least comparable strength stainless steels, 
respectively.

DETAILED DESCRIPTION OF THE INVENTION 
As described in copending patent application U.S. Ser. No. 734,323, 
referenced above, weld shadow (i.e., due to weld geometry and local weld 
shrinkage deformation strengthening) effects appear to be operative in 
effecting the inside diameter (i.d.) strain profiles through the weld 
region of a pipe joint. Thus, as shown in FIG. 1, an as-welded, typical, 
four-inch pipe A of 304 stainless steel has a 308 stainless steel weld 
bead material B, and alpha sensitized band C of the heat affected zone of 
weld B. On the pipe i.d. surface, band C extends somewhat beyond the weld 
shadow or strengthened constraint zone of the bead of the weld and 
consequently under condition of a typical high applied axial stress or 
appropriate bending stress there can be a region of high strain coincident 
with the exposed surface of relatively highly sensitized material for 
pipes less than a certain size. When exposed to a high enough stress duty 
cycle and sufficiently aggressive boiling water reactor conditions, stress 
corrosion cracking of pipe A will occur. For schedule 80 type austenitic 
stainless steel piping sizes equal to or less than about 12" diameter can 
be susceptible to such failures. 
In the case of FIG. 2, pipe E of the same dimensions and alloy as pipe A is 
butt-welded to spool piece F of 316 or a less susceptible stainless steel 
by weld G of 308 stainless steel. Pipe E has a sensitized zone H and spool 
piece F has a similar heat affected zone J which together are like 
sensitized band C is extending beyond the constraint zone of the bead of 
weld G on the pipe and spool piece inside surfaces. In accordance with the 
preferred practice of this invention, however, this subassembly K suitably 
made at the factory rather than in the field is solution-annealed and 
quenched to eliminate the sensitized condition in this subassembly weld. 
In any event, subassemblies K and K' so-desensitized are assembled in the 
field as shown in FIG. 3 and then integrally joined by means of a weld M 
made by filling the annular recess defined by the opposed end surfaces of 
spool pieces F and F' of subassemblies K and K', respectively. 
Spool pieces F and F' are of substantially the same shape and dimensions, 
each having an i.d. the same as that of pipe lengths E and E' and having 
an end portion of wall thickness matching that of the latter. The other 
end portion of the spool piece in each instance is of somewhat greater 
outside diameter, i.e., approximately twice or more that of end joined to 
the pipe length. The free end surface of each of these spool pieces is 
generally frustroconical so that an adequate weld metal-receiving recess W 
is provided between the subassemblies. It will be observed also that the 
spool pieces are shaped so that the portion of enlarged cross section 
extends beyond the heat affected zone at the spool piece i.d. in each 
instance, tapering then toward the smaller cross section end portion. 
Any number of such subassemblies can be butt-welded together in the manner 
illustrated in the drawings to provide the pipeline installation required 
and this will suitably be done in the field, i.e., at the installation 
site, using subassemblies fabricated at the factory where the pipe is 
produced. 
In the case of FIG. 5, it will be understood that water may be flowing 
through the pipe assembly as in normal operation of the reactor line, or 
it may be delivered as a spray against the pipe inner surface in the 
region where the weld joining the subassemblies is being applied, but 
preferably to avoid steam pocket formation and to produce better heat 
transfer, such welding is not done when the water in that region is not 
flowing. In special cases other non-aggressive fluid coolants could be 
used on the pipe i.d. during the welding operation. As a consequence of 
this operation, the heat affected zone in the region of the interior 
surface of the pipe is avoided or substantially restricted. 
To illustrate further the special features and advantages of this invention 
and not by way of limiting the appended claims, the following detailed 
example of the practice of this invention is provided for the benefit of 
those skilled in the art: 
EXAMPLE 
For purposes of testing the concept of this invention, eight subassemblies 
were fabricated from eight four-inch diameter 304 stainless steel pipe and 
eight spool pieces of 316 stainless steel of the same inside diameter as 
the pipe and of the form and dimensions as described in detail above and 
illustrated in FIGS. 2-5. Thus, the smaller end of each spool piece was 
butt-welded to an end of its pipe length through the use of 308L Grinnel 
rings and filler metal and gas (argon) tungsten arc. The subassemblies 
were then subjected to 1100.degree. C. for one hour in argon and water 
quenched for 10 seconds, and then immersed for four minutes in a 5 percent 
hydrofluoric acid--30 percent nitric acid--65 percent water solution at 
room temperature, water rinsed, fiber brush scrubbed and finally 
reimmersed in the acid solution for four minutes and water rinsed. The 
subassemblies were thereafter arranged in four pairs with a 308L Grinnel 
ring between opposed end surfaces of each spool piece to close lateral 
access to the passage through the assembly. Two of the assembled pairs 
were joined by welds using a gas tungsten arc first to tack weld the 
Grinnel ring in place as the inside surfaces of the spool pieces were 
cooled with argon back-up gas flow, and then to fuse the Grinnel ring and 
apply the second weld layer bead with 3/32-inch 308L filler wire. The 
spool piece joint was then completed in each instance by the application 
of a series of layers of 308L weld metal through the use of a shielded 
metal arc welder. 
The other two assembled pairs were likewise joined by welds using the same 
technique and materials and equipment except that throughout the second 
and subsequent layer welding operations the interior surfaces of the 
opposed spool pieces were sprayed with water through the use of the mobile 
cooling device disclosed and claimed in copending patent application U.S. 
Ser. No. 755,670, filed Dec. 20, 1976, in the names of Pagnotta, Chrenko 
and Quinn and assigned to the assignee hereof. 
One of the first two assemblies above was given a low temperature 
sensitization heat treatment consisting of heating at 500.degree. C. in 
argon for 24 hours and furnace cooling. Similarly, one of the second pair 
of assemblies was heated at 500.degree. C. in argon for about 40 hours and 
furnace cooled. Such low temperature sensitization treatments are known to 
be effective for accelerated testing of any intergranular stress corrosion 
cracking susceptibility. 
All four pair are presently undergoing a series of characterization and 
accelerated stress corrosion cracking tests with the preliminary results 
being that the first two assemblies are proving to be superior to welded 
stainless steel pipes of installations of the prior art except for the 
complementary backlay weld type for adding margin to existing welds and 
referenced in patent application U.S. Ser. No. 734,423.