Friday, September 14, 2007

WELD DEFECTS/PRACTICAL ADVICE

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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