Some of the most common types of defects are defined
below.
– Hot cracking
This is the most common type of weld defect, and is
caused by, among other things, excessively large weld
pools, high impurity levels, high weldment restraint,
and too thin welds. Weld-crater cracks are a type of
hot cracking and occur if the arc is extinguished too
quickly. Ferrite in the weld metal counteracts hot
cracking. Hot cracks must be ground away.
– Strike scars
Strike scars occur if the arc strays outside the joint
briefly while the electrode is being struck. This type of
defect has high inherent stress, often in combination
with a sharp crack. It can cause stress corrosion cracking
and crevice corrosion. Strike scars in duplex steels
can give rise to 90-100% ferrite, resulting in embrittlement
and reduced corrosion resistance. Strike scars
must be ground away.
– Porosity
Porosity is caused by moisture on the work metal,
moisture in the electrodes, moisture in the gas (TIG,
MIG), contamination of the joint (oil, paint etc).
– Slag inclusions
These may result from the use of an electrode with too
large a diameter in a narrow joint, or by careless welding.
– Incomplete penetration
This results from using the wrong type of joint, or
incorrect welding parameters.
– Root defect
Incomplete penetration can cause crevice corrosion
and stress corrosion cracking.
– Incomplete fusion
This is caused by an incorrect travel speed in MIG
welding, an excessively narrow joint, excessively low
welding current, or the wrong electrode angle.
– Hydrogen cracking in 13 Cr weld metal
Preheat temperature too low, moisture content in
covering too high.
– Excessive local penetration (pipe welding)
Gap too large, heat input too high.
– Sink or concavity (pipe welding)
Incorrect joint design.
– Oxidized root side
Poor shielding can cause corrosion attacks. Remove
the oxide.
– Spatter
Grinding spatter can cause pitting and must therefore
be removed. Weld spatter can also cause pitting.
– Grinding scratches
Coarse grinding of the welded joint must be followed
by fine grinding and possibly polishing.
Practical advice
– Use standardized joint types. A single-U butt joint is
recommended for pipe welding with TIG. The single-U
butt joint is particularly advantageous in the overhead
position. A tip is to machine single-V butt joints
but grind up the single-V butt joint to a single-U butt
joint in the overhead position. Tack with a gap of
about 1.0-2.5 mm.
– Never leave grinding burr.
– Clean the joint before welding.
– When tacking with TIG, use shielding gas and grind
off or thin out the tacks.
– When welding pipe with TIG, use pure argon and
gas hoses of good quality.
Spread out the gas on the root side.
Gas flow (2)-20 I/min.
– Purge the pipe with 7-10 x the enclosed volume.
– Keep the shielding gas on until the weld has cooled
to below about 200°C.
– Using a gas lens is recommended–it provides a
better gas shield. Good in deep joint types, for
example weldolets.
– MIG welding can be carried out with pure argon or a
gas mix of argon + 30% helium + 1% oxygen.
– Heat input 0.5-1.5 kJ/mm (normal).
– If welding with covered electrodes, do not exceed
the maximum recommended current.
– Extinguish the arc carefully at the end of the weld.
– Do not exceed the recommended welding current.
– Interpass temperature <100°C (150°C).
– The joint must be completely free of low-melting
phases such as metallic copper, zinc or lead. Such
phases can otherwise cause metal penetration
during welding.
– Submerged-arc welding and resistance welding can
be used, but require special welding parameters.
Information can be obtained from our technical
customer service.
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