T. O. 1-1A-9
2-123. The steels that are considered the most satisfactory for welding are those that do not harden appreciably. In
selecting steels for welding it is advisable (if other requirements can be attained) to limit carbon/carbon equivalent to a
low value as the maximum hardness attainable is proportional to carbon/ carbon equivalent content. The effect of the
alloys usually tends to increase hardness, therefore, an alloy steel hardens more readily than straight carbon steel and
this factor is considered in selecting an alloy steel for welding applications. Generally the straight carbon steel with a
carbon content of 0.30% or below are the best grades for welding. These grades are usually called "welding grades" and
they can be successfully welded without preheat at temperature above 600F, unless material is relatively thick. For
welding of thick material, preheat of 100°300F is recommended.
Steel for use on aeronautical equipment or other critical applications, with a carbon
equivalent above 0.44% and hardenability greater than 4340 shall not be welded without
engineering approval by drawings, Technical Orders, etc. "Carbon Equivalent" is defined as
being the properties incurred in steel alloys by other alloying elements that result in the same
characteristics (usually to a lesser degree) as the addition of pure carbon. Carbon equivalent
is determined by the following formula: CE =C + Mn + Mo + Cr +Ni + Cu+P in accordance
No welding shall be preformed on 4340 heat treated above 200,000 PSI unless specifically
approved by the design engineer or drawing.
2-124. When metal temperature is below 60 F, preheat part or weld area as required to a 60 to 300°F range so that
metal is somewhere in this range when starting to weld. Avoid other conditions that would effect the physical/mechanical
characteristics of the weld to the extent that the part/weld will not meet specification or established
requirements/inspections. In addition to the above, medium/high carbon (straight/alloyed) steel usually requires
preheating, post-heating or other heat treatment to control stress, shrinkage, etc, necessary to produce a satisfactory
weld. For recommended general pre-heat temperatures, see Table 2-15.
2-125. When the weld joint is of such design or material thickness requires that more than one pass (commonly called
interpass) be used, the metal temperature shall be below 3000F for low carbon steel or within preheat temperature range
for medium/ high carbon (straight/alloyed) grades prior to making the next pass. When cooling is required, it shall be
allowed to occur naturally, unless otherwise specified by process specification, engineer, etc. In multiple pass welding,
the starting point for each pass should be staggered to prevent metal build-up and heat distortion. Also, each pass/layer
should be completed and cleaned before starting the next pass/layer.
2-126. Preparation for Welding. Before starting the welding operation the following factors shall be considered:
Machine/equipment capability, alignment condition, and set-up, including jigs, etc.
Safety precaution i.e., fire hazards, grounding, eye protection, etc. (See AFM 127-101 and paragraph 2-205.)
Edge preparation, joint design, and cleaning of material before and during the welding operation.
Electrode/filler metal and flux required.
Sequench for welding, including requirement for preheat, postheat, tacking, distortion, etc.
Inspection/test of finished weld/part.
Cleaning before and treatment of finished weld/ part.
2-127. Cleaning. It is important that all surfaces in or near the weld joint be free of scale, oxides, grease, oil, dirt, paint
or foreign matter which would contaminate the weld. The presence of the above in the weld can cause porposity,
inclusions, poor fusion/ penetration, oxidation, holes and difficulty in weld progression/speed.
2-128. GAS WELDING OF CARBON STEEL GENERAL
2-129. The gas welding of carbon steel (melting temperature 28000F) is normally accomplished using the oxyacetylene
process because of high heat producing capacity. By using properly designed welding torches under controlled
conditions the oxyacetylene flame can produce temperature of approximately 6000°F. Other gas heating media can be
used to a limited extent, however, they do not produce as much heat as acetylene and may materially effect weld