Page 38 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Analysis of a Fusion Weld

Weld toes
These are the points where the weld metal adjoins the base
metal. There will normally be four weld toes on a full
penetration butt weld (two on the face and two on the root)
and two on a fillet weld. The greater the angle at the weld toe,
the greater the risk of a fatigue failure if the component is
subjected to cyclic stress.
Fusion line
This is where fusion takes place between the melted and
unmelted material. It is commonly referred to as the fusion
boundary or weld junction.
Fusion zone
The fusion zone is the region within the weld that contains
the greatest dilution of filler metal with melted base metal.
This region of highest dilution can contain defects owing to
impurities or contaminants contained in the base metal being
drawn into the weld. The centre of the weld will be the area of
the lowest dilution and may consist of filler metal only.

HAZ
This is the region of the base metal that has not been melted
but has been affected by the heat and had a change made to
its grain structure. Cracks often occur in the HAZ after
welding has taken place owing to it being hardened by the
formation of a martensitic grain structure on cooling.
Excess penetration
Excess penetration is excess weld metal formed in the root of
the weld. Some codes may specify a limit to the penetration
due to the toe blending and/or the bore restriction caused in
piping.
Design throat thickness
The throat is where the strength is contained within the weld.
It is equal in size to the thinnest of the base materials being
joined.

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200903QG Welding chap3.3d Page 23 of 35