2-145. Low hydrogen electrodes are used in almost every case for welding material in this group, especially when low
preheat and interpass temperature are employed. However, when preheat temperatures above 600VF are used,
electrodes with cellulose type coating can be used satisfactorily. The best quality welds are made using direct current
(electrode positive) types EXX15, EXX25 but when welding is confined, alternating current electrodes may be the best
suited due to less magnetic blow. When welding high strength structural part such electrodes as E10016 and E12015 are
used. Welds made with the above electrodes should be stress relieved to prevent embrittlement, see Table 2-15.
2-146. WELDING OF CORROSION RESISTINGSTAINLESS STEELS, NICKEL CHROMIUM IRON ALLOY AND HIGH
NICKEL ALLOYS. The welding of the corrosion resisting stainless steels present some problem in fabrication that are
not encountered with the more commonly used low carbon/low alloy steels. For instance, the coefficient of thermal
expansion of the type 300 series is about 1/2-2/3 less than carbon steel and heat or thermal conductivity is about 1/2-2/3
less than carbon steel. The above will requite the use of properly designed jigs fixtures, chill plates and joints to control
expansion and contraction for successful welding. In addition tack welding and closely controlled post and preheating will
be required in many instances. General composition range of the alloys are as follows:
a. The 300 series of corrosion resisting steels are commonly referred to as the i8-8 series/chromium nickel grades
of stainless steel. The various grades normally contain 18% chromium, 8%nickeland balance iron melt small amounts of
other elements which vary with the particular alloy.
b. The 400 series of corrosion-resisting (stainless) steels are called the straight chromium grades. The
compositions of this series are divided into two groups; hardenable and non-hardenable grade analysis range from 11.50
to 27.00% chromium, 0.50-2.50% nickel, 1.0% manganese and silicon and balance iron with small amounts of other
elements varying with the particular alloy.
2-147. COMMON CHARACTERISTICS ASSOCIATED WITH WELDING.
2-148. The 300 series alloys are more susceptible to intergranular carbide precipitation than others. Actual susceptibility
is dependent on carbon content and whether the alloy contains stabilizing elements (columbium or titantium). When the
stabilized grades (321 and 347) are heated to temperatures (800°-15000F) where carbide precipitation occurs, the
stabilizing elements combine preferentially with carbon to form their own carbides tying up the carbon to avoid the
formation of chromium carbide and keeping the chromium in solution. The above is the basis for selecting certain types
as preferred welding grades.
2-149. Chromium-Nickel, Type 300 series. Of this series the best grades for welding are the annealed stabilized types
such as 321,347 and 348. These grades are stabilized with titanium (321), tantalum plus columbium (347) or columbium
(low tantalum) (348). The effect of the addition of the stabilizing element(s) is that they control carbide precipitation
which may occur at the grain boundaries in the chromium-nickel steels during heating and cooling in the range of
800°15000F. The stabilizing elements combine preferentially with carbon to form their own carbides tying up the carbon
to avoid the formation of chromium carbides keeping the chromium in solution. The degree of carbide precipitation
occurring, affects the corrosion resisting quality of the material in heat zone or area adjacent to the weld. The extent to
which carbide precipitation affects the material depends on several factors. One of the factors is the ratio of the
combined percentage of chromium and nickel to carbon percent. Another factor of importance is the length of time the
steel is held within the temperature range where carbide precipitation occurs. The shorter the time the less harmful the
carbide precipitation will be.
2-150. The other types in the 300 series which have fair to good welding characteristics and which can be welded under
controlled conditions are 304, 304L and 316L. The carbon content of these alloys is 0.08% or less and with the required
ratio of nickel and chromium to carbon percentage required for short time heating, within the carbide precipitation
formation range. In these low carbon grades the carbon content is low enough to slow the formation of carbides at grain
boundaries and, therefore, can be exposed to the 8000-1500°F temperature range, for the relatively short time required
for welding. Of these grades the one containing the lowest carbon content are the best suited for welding.
Exposure of types 304 304L and 316L to temperature of 800-1500°F for an extended period of
time shall be avoided. Carbide precipitation is not prevented, it is only delayed or reduced to
a slow rate to the extent that will permit welding and a short time stress relieving operations.
2-151. The balance of the 300 series has only fair to poor weldability. Welding of any of the corrosion steels (type 301,
302 and other types) in work hardened condition, i.e., 1/4 hard, 1/2 hard, etc., will only be accomplished as directed by
applicable technical order or other approved technical data for the specific equipment, aircraft or missile involved.