Page 57 - D30003

57 Parker Hannifin Corporation
Instrumentation Products Division
Huntsville, AL USA
http://www.parker.com/ipdus
General
The weld used in joining a tube to a socket weld tube fitting
is like any other type of “tee” weld. The root (i.e., the point of
intersection of the outside of the tube and annular end area
of the fitting) must be included in the weld zone.
Careful welding procedures are normally followed to assure
that this root area is included in the weld. If penetration is not
achieved, the joint will have two built-in stress risers which
may greatly reduce the strength of the weld. Upon application
of an extreme load, these stress risers could result in cracks
which could propagate out through the weld or tube depend-
ing upon the direction of the greatest load.
Often to achieve full root penetration in TIG welding of stain-
less steels, a fusion pass will be made first, followed by a final
pass utilizing a filler rod to achieve the desired fillet size.
Assembly
The codes applicable to the welding of socket weld fittings
require that the tube be inserted into the socket until bot-
tomed against the stop. The tube is then to be backed out
approximately 1/16 of an inch and then welded.
If the tube is not backed out, but welded when against a flat
bottom stop, the contraction of the weld fillet and fitting socket
can combine to produce a static stress on the weld. During
thermal transients, the fitting and the portion of the tube within
the fitting may experience a differential rate of heating or cool-
ing, again adding to the stress level in the weld.
Tacking
If the weld joint is to be “tacked” before welding, it is recom-
mended that the “Tack” weld build-up be held to a minimum.
Excessive build-up on the “tack” may cause an interrupted
final bead and a stress riser or lack of complete fusion.
Backing Gas
Backing gas is an inert gas used to flood the interior of the
fittings and tube system during welding. It serves the same
purpose internally as the shielding gas used in TIG or MIG
welding. By reducing the interior oxygen level to as low as
practicable, it also serves to control the combustion of con-
taminates that could affect weld quality.
When a backing gas is not used and nearly 100% weld pen-
etration is achieved, blisters will tend to form on the internal
tube wall. This will result in scale which may later break loose.
Therefore, in 0.050 wall or thinner tube or where the wall
thickness is such that the selected weld process may burn
through, the use of a backing gas is mandatory.
In most cases the backing gas will be argon or helium con-
nected to the system through a control regulator. Flow rates,
while small, should be high enough to purge the system.
Welds should be made in downstream sequence from the
gas connection.
Note that the entire system should be purged to insure that
there are no openings that will allow air to be drawn into the
system.
The use of backing gas, while often not mandatory, will give
a better weld joint. This is because the effects of contaminate
combustion by-products are eliminated and because the
welds are made and cooled under a shielded atmosphere,
thus eliminating internal scaling or blistering.
Welding Methods
300 Series Stainless Steels
May be welded by the TIG, MIG, or stick arc-weld process.
TIG welding is recommended as being best for welding
Weld-lok systems because it allows better operator control of
heat penetration and filler material deposition.
Stick arc welding is not recommended in many cases
because of the likelihood of excessive burn-through and
improper root penetration. In all cases where stick welding is
used, it is recommended that backing gas be used.
MIG welding gives the same characteristics as stick electrode
welding with faster deposition of the filler material. As this
process runs “hotter” than the stick process, the use of a
backing gas is mandatory. It should be noted that in welding
the relatively small fitting sizes found in the Weld-lok line, filler
deposition rate economies are not a factor and therefore the
MIG method is not commonly applied.
C1018 Steel Fittings
May be welded by the TIG, MIG, stick and oxyacetylene
methods. As scale formation remains a problem, the use of a
backing gas is still recommended.
Carbide Precipitation
When unstabilized stainless steels are heated to 800°–1500°F
during welding, the chromium in the steel combines with the
carbon to form chrome carbides which tend to form along the
grain boundaries of the metal (carbide precipitation). This low-
ers the dissolved chromium content in these areas and thus
lowers their corrosion resistance, making them vulnerable
to intergranular corrosion. Carbide precipitation is reduced
by holding the carbon content of the material to a very low
value. This limits the amount of carbon available to combine
with the chromium. The “L” series (extra low carbon) stainless
steels are often used for this purpose, but their use reduces
system design stress by approximately 15%. Parker Weld-
lok fittings are made from a select 316 series with carbon
content in the low range of 0.04 to 0.07 percent. This results
in a welded fitting with good corrosion resistance and a high
strength factor.
All Parker Weld-lok fittings in stainless steel are supplied
in the solution treated condition, capable of passing
ASTM-A-262 Tests for Detecting Susceptibility to Intergranular
Corrosion.
Arc Polarity
When welding Weld-lok fittings, best results will be obtained
by the following arc polarities:
TIG – Direct Current, straight polarity
MIG – Direct Current, reverse polarity
STICK – Polarity dependent on rod used
For further information on Parker’s Welded Fittings refer to
Parker’s Welded Fittings Catalog 4280 or contact Parker’s
Instrumentation Products Division – Product Engineering at
256-881-2040.
D30003 57 5/12/05