46 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
Heat - Affected Zone ( HAZ ) This is a region where
the base metal has been affected by the heating associ-
ated with the weld. Phase changes in the metal can
occur and produce different microstructures than the
weld bead and the base metal. The results can be dif-
ferences in mechanical and corrosion resistance
properties.
Base Metal This is a metal that has not been heated
enough to alter the metallic structure or corrosion
resistance.
The three different heat - related regions are shown in
Figure 4.20 , and some of the defects associated with
welds are shown in Figure 4.21 .
Common defects associated with welding include:
Porosity Internal porosity can be caused by trapped
gases that have inadequate time to escape from the weld
pool prior to solidifi cation. Porosity is the cause of
approximately 50% of all weld repairs.
27
Porosity can
also be a problem in cast objects, but these objects
are usually thicker than many welded structures and are
less subject to the loss of strength associated with
porosity.
Cold Cracking Hydrogen cracking is the principal
cause of this problem. Quick cooling prevents the
escape of hydrogen from the weld pool before solidifi ca-
tion. Trapped hydrogen can cause hydrogen embrittle-
ment and a variety of other problems that are discussed
in detail under the heading Environmentally Induced
Cracking in Chapter 5 , Forms of Corrosion. Cold crack-
ing is controlled by keeping fi ller metal electrodes dry
and by avoiding hydrocarbon contamination of fi ller
metal surfaces.
Hot Cracking This is also called sulfur cracking. It is
restricted to low - grade carbon steel and some forgings
with appreciable sulfur contents. Iron sulfi des have low
melting points and usually concentrate near the center
of weld beads, where the metal solidifi es last. Sulfi de
inclusions are weak and produce cracks as the weld
cools.
Slag Inclusions This is usually due to poor welding
procedures, including inadequate cleaning of surfaces
before welding.
Lack of Fusion Sharp cracks form where the weld
bead does not bond to the base metal. This may be due
to inadequate surface cleaning prior to welding or due
to insuffi cient shielding gas allowing a surface oxide to
form on the weld bead. A lack of fusion usually pro-
duces sharp, crack - like defects.
This is because there are more grain boundaries exposed
on these surfaces.
Defects in castings include inclusions, impurities
from the melted metal or from the mold, and porosity
due to entrained gases in the liquid metal. Wrought
products are formed from castings and will have the
same chemical composition and defect inclusions as the
castings from which they are formed. The inclusions are
generally ceramic materials due to material impurities
or from the mold. The forming process breaks these
inclusions, which tend to be very brittle, and spreads
them out parallel to the primary forming direction.
Metal crystals in wrought products are usually micro-
scopic, whereas many crystals in castings can be large
enough to be seen on polished surfaces with the naked
eye. Grain boundaries in metals are a primary source of
strengthening, so wrought products, with much fi ner
grain sizes, tend to be stronger and more ductile than
castings. Any porosity in the cast metal is likely to have
been removed by the compression of the forming
process. For all of these reasons, wrought metal prod-
ucts are stronger and are more reliable than castings.
Welding
Welding is the preferred joining method for most oil-
fi eld piping systems and process equipment. With most
welding processes, some of the metal is heated beyond
the melting point while some of the structure is not
heated at all. In between these two extremes is a wide
variety of temperatures and times at different tempera-
tures. This produces at least three distinct regions in the
metallic structure:
Weld Bead or Fusion Zone This is a combination of
fi ller metal and melted metal from the base metal being
joined. It solidifi es as a casting with the problems of
castings plus added stresses due to the thermal contrac-
tion caused by solidifi cation and cooling to the ambient
temperature at different rates than the surrounding
metal.
Figure 4.19 Principal directions of rolled plate.
Rolling direction
Short transverse
Long transverse
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