T. O. 1-1A-9
2-135. WELDING OF LOW ALLOY STEEL GENERAL
2-136. "Low Alloy" steels are defined as steels whose properties are attributed to the presence of one or more elements
in addition to those commonly present in carbon steel or to more than usual amounts of elements such as manganese
and silicon. Examples of low alloy steels are: Nickel alloy, nickel-copper alloy, manganese -molybdenum, carbon-
molybdenum, chromium molybdenum, nickel-chromium-molybdenum, chromium alloy steel, manganese alloy steel and
chromium-vanadium steels.
2-137. The primary effect of the alloying elements in regard to welding of steel is on the physical properties. The
additional element/elements may form invisiable oxide firm a which interfere with fusion or cause cracking and porosity,
in addition to the principal effect of increasing hardening tendencies. The affects of the added elements occur primarily
because of metallurigical response upon the transformation (austenite) of metal during cooling from the welding
temperature.
2-138. Due to the effect of the alloy elements, the welder meat be familiar with required control factors associated with
each grade/type.
2-139. GAS WELDING (OXYACETYLENE) OF LOW ALLOY STEELS. Basically there is no difference in procedure for
oxyacetylene welding of the alloy steels than those used for welding straight carbon steel. Although the trend is to
replace oxyacetylene with the metal arc process using covered electrodes, there are applications where the oxyacetylene
method is used to an advantage. The advantage is associated with the fact that heat' before, during and after welding
can be controlled to some extent (depending on the operators ability). Using the gas process, heat is applied gradually
and the metal is not subject to extreme temperature changes (gradients) in area adjacent to weld as occurs in arc
welding. The controlled temperature that can be obtained using gas process can be important in welding the air
hardening alloy steels.
2-140. Oxyacetylene Welding of (he low alloy steels is normally accomplished with a neutral or slightly reducing flame.
In moat applications filler metal is selected to maintain weld strength, resistance to creep at elevated temperature
(characteristics of molybdenum), embrittlement at low temperature (characteristics of nickel steels) and corrosion
resistance. Special grades of filler metal, although Dually provided in standard types, are selected to maintain weld
strength, rather than using rods of the same composition as the base metal. For corrosion resistance, the filler metal is
selected to obtain a weld with similar analysis as the parent metal. For a general reference on selection of filler metal to
alloy, see Table 2-15. This table is only to be used as a guide and it is not intended to replace requirements specified by
blueprints, technical orders or other engineering data. Additional requirements for engineering design are cited in MIL-
HDBK-5.
NOTE
Postheat-treatment after gas welding the heat-treatment alloy steels is necessary to stress relieve
and to reline grain structure, unless otherwise specified.
2-141. SHIELDED METAL-ARC WELDING OF THE LOW ALLOY STEELS. The structural grades of these steels
(tensile 70,000 yield to 50,000) such as nickelcopper alloy are welded in approximately the same manner as used for the
structural grades of carbon steel. Low hydrogen carbon steel electrodes (such as flat-work and E6010-7016 for all
positions) of equal or slightly higher strength than the base metal are generally used for the applicable grades.
2-142. Low Carbon Alloy (AISI/SAE) Grades. The low carbon grades of the low alloy steels (see paragraph 2-126 are
also normally welded with low hydrogen type electrode such as E8X15 (DC electrode positive) or EXX16 (AC or DC
electrode positive). This series normally will require preheat if carbon content exceeds 18%, or when total alloy content
is high. Preheat is recommended as follows:
1.
Preheat of 200°F if alloy carbon content does not exceed 0.18% and nickel does not exceed 2.25%.
2.
200°-300°F for alloys containing 3.0-3.5% nickel and less than 0.12% carbon.
3.
300° 500°F generally and 600°F in some cases for the higher alloyed types.
NOTE
For additional information on preheat by alloys, see Table 2-15.
2-143. Edge preparation for this series will depend upon the thickness of plate and intended use of the finished part.
Typical butt weld joints of material above 3/16 and up to 1/2 inch thick are prepared with about a 14-16 degree level to
form a single U joint with an included angle of about 30-34 degrees. Material spacing at the root range from 1/32" 1/16"
for 3/16" material to approximately 1/4" for 1/2" material. A backing plate is usually used and for completion of the weld
the place is removed and a final pass (after chipping) is made to seal the root. Joints for welding heavier sections are
usually prepared with a double "U".
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