composition and thickness of the metal being
welded.
POSTHEATING is done primarily for the
purpose of relieving stresses in the metal after it has
been welded. The temperature, the hold time, and
the cooling rate are specified for each job where
postheating is required. In general, slow cooling is
essential for stress-relieving. If the metal is cooled
rapidly, new stresses will develop and thus defeat
the purpose of the postheating.
Requirements for stress-relieving depend upon
metals to be joined, and the complexity of the
weldment. In some cases, stress-relieving may be
specified for a partly welded joint. Stress relief is
usually required for welded joints that will be
subjected to high pressure. Tables to be used as
guides for temperature control are contained in
Figure 7-23.--Example of the use of jigs and chill bars.
MIL-STD-278.
Oxyacetylene or propane torches and electric
WELDING PIPING
preheating and postheating. Portable electric
The requirements for fabricating piping by
heaters are furnished to repair ships, tenders, and
welding vary according to the class of piping
other ships and stations that normally fabricate
involved. MIL-STD-278 establishes four classes of
items made of carbon molybdenum steel and
piping: P-1, P-2, P-3, and P-LT.
chromium-molybdenum steel. The complete unit
consists of control devices mounted on a portable
CLASS P-1 includes all piping used for services
panel, electrical supply connections, heater cable
where the pressure exceeds 300 psi or the
leads, and heater coils.
temperature exceeds 650F. It also includes ALL
piping systems used for conveying deadly gases or
PREHEATING is the application of heat to the
liquids and hydrogen peroxide (regardless of the
base metal before a welding or cutting operation is
pressures and temperatures in these systems), except
performed. Preheating is not required for all welds.
where such systems are covered by classes P-2 or
When required, the preheat temperature and the
P-3. Examples of class P-1 piping include steam
length of time the temperature must be held (hold
lines, hydraulic systems, steam escape piping below
time) is specified in the welding process instruction
being used. The preheat temperature and the hold
decks, boiler generating tubes, boiler superheater
and economizer elements, and o t h e r
metal, the thickness of the metal, and to some
pressure-retaining tubes and piping. Class P-1
p i p i n g does NOT include nozzle and root
degree upon the welding method and the type of
connections to pressure vessels where such
welding rod or electrode used. Some alloys are
connections are covered by pressure vessel
more successfully welded without any preheat.
classifications.
B e c a u s e of the wide variations in preheat
requirements, you must follow the welding
specifications for each job precisely.
CLASS P-2 includes piping used for services
where the pressure does not exceed 300 psi and the
temperature does not exceed 650F. It also includes
INTERPASS TEMPERATURE is the
temperature of the deposited weld metal before the
escape piping above decks. Class P-2 piping does
next pass can be made. The interpass temperature
NOT include piping covered by class P-3.
(minimum or maximum as specified in the welding
process instruction) will vary according to the
7-18