CLASS P-3 includes all brazed piping of
Specific information pertaining to welding on
unlimited pressure and a maximum temperature of
the various classes of piping is given in the following
425°F. Fabrication and inspection of brazed piping
should be according to the requirements of
P-1 CLASS PIPING
CLASS P-LT includes all piping of design
The shielded metal-arc and gas tungsten-arc
pressure greater than 50 psi and service temperature
welding processes must be used for the shipboard
for minus 20°F and below.
welding of P-1 class piping that is 0.109 inch or
more in wall thickness and for the shop welding of
The following general considerations apply to all
P-1 piping that is more than 0.083 inch in wall
thickness. At least two layers of weld metal must be
applied around all joints in this piping.
--Whenever possible, weld pipe in the horizontal
Stress relief heat treatment is generally done
rolled position. It is much easier to weld near the
after welding on P-1 class piping. Factors that
top of the pipe than it is to work through the
determine whether or not stress relief is necessary
overhead and vertical positions. By using the
include the carbon content of the alloy, the
horizontal rolled position, you get the advantages of
chromium content, the thickness of the joint, and
increased speed and the likelihood of a sound weld.
the size of the pipe (iron pipe size [ips]). Also,
stress relief is required for some materials in P-l
--Be sure the pipe is carefully aligned.
class piping if preheat and interpass temperatures of
200° to 300°F were not maintained.
--Tack welding is used in practically all pipe
welding. The number of tack welds needed is
A l l welds in P-l piping must be tested
determined by the diameter of the pipe. The size of
hydrostatically. Most of these welds must also be
the tack welds is determined by the wall thickness of
inspected by radiographic, magnetic particle, or
the pipe. For 1/2-inch pipe, two diametrically
liquid penetrant inspection. Requirements for
opposite tack welds are used. On 12-inch pipe, you
welding and testing welds in P-l piping are normally
will need six tack welds. Four welds are usually
covered by the applicable welding procedure or
sufficient for the common pipe sizes. Tack welds
military standard. Use NSTM, chapter 074, volume
are generally incorporated into the finished weld;
1, as a starting document. It will refer you to other
therefore, the material used for tack welding MUST
applicable documents, such as MIL-STD-278 for
be the same as the filler metal. Any tack weld that
fabrication and inspection, MIL-STD-271 for
contains cracks or other defects must be removed by
nondestructive test requirements, MIL-SD-22 for
chipping, grinding, or gouging before the final weld
joint design, and numerous military specifications
for material requirements. All of these documents
are a necessary part of welding on P-l piping.
--On circumferential butt welds, provide a weld
buildup of 1/16 to 3/32 inch, depending upon wall
For more in-depth information on a particular
thickness and pipe size.
job, refer to the applicable military standards, or
procedures designated for use by NAVSEA.
--In multipass welding, remove all slag from
P-2 CLASS PIPING
each bead before depositing the next bead.
The manual shielded metal-arc process is used
--For some welds, peening is permitted as a
for the shipboard welding of P-2 class piping that is
means of correcting distortion and minimizing
0.109 inch or more in wall thickness and for the
residual stresses. However, peening MUST NOT be
shop welding of P-2 piping that is 0.083 inch in wall
done on single bead or single layer welds nor on the
thickness. Gas tungsten-arc welding may be used on
last layers of multiple layer welds.
P-2 piping that is less than 0.109 inch thick.
Thermal stress relief is required for P-2 class
p i p i n g welds in c a r b o n steel and carbon