Dear Prashant,
Tubing is generally seamless, but may be welded. Its stated size is the actual outside diameter. (ASTM B88 tubing, which is often used for steam tracing, is an exception in that its size designation is 1/8inch (3.2 mm) less than the actual outside diameter.) Tubing is usually made in small diameters and is mainly used for heat exchangers, instrument piping, lubricating oil services, steam tracing, and similar services.
Piping can be made from any material that can be rolled and welded, cast, or drawn through dies to form a tubular section. In all standard sizes, the outside diameter remains nearly constant regardless of the thickness. For nominal pipe sizes of 12 inches (305 mm) and smaller, the size refers to the inside diameter of standard weight pipe; for nominal pipe sizes of 14 inches (356 mm) and larger, the size denotes the actual outside diameter. The pipe diameter is expressed as nominal pipe size (NPS) which is based on these size practices
Preheating is defined as heating of the weld/surrounding base metal to a predetermined temperature prior to the start of welding. The primary purpose for preheating carbon and low-alloy steels is to reduce the tendency for hydrogen induced delayed cracking. It does this by slowing the cooling rate, which helps prevent the formation of martensite in the weld and base metal HAZ. However, preheating may be performed for many reasons, including:
a. Bring temperature up to preheat or interpass temperatures
required by the WPS.
b. Reduce shrinkage stresses in the weld and base metal,
which is especially important in weld joints with high
restraint.
c. Reduce the cooling rate to prevent hardening and a reduction
in ductility of the weld and base metal HAZ.
d. Maintain weld interpass temperatures.
e. Eliminate moisture from the weld area.
f. Meet the requirements of the applicable fabrication code,
such as the ASME Boiler and Pressure Vessel Code, depending
on the chemistry and thickness of the alloy to be welded.
Regards
Tong2je
On Tue, Jun 9, 2009 at 1:22 PM, prashant pansare <prashantpan_99@yahoo.co.in> wrote:
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