Page 39 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

Actual throat thickness
This is the actual measurement made from the weld face to
the root. The actual strength of the weld can be calculated
using this actual throat measurement (but any measurement
in excess of the design throat is ignored). This is more
relevant for partial penetration welds or where root
concavity is present.
What makes a good fusion weld?
There are four factors that must be satisfied to produce a
good fusion weld. If any of these factors is not achieved then
the result will be a weld that may not be fit for purpose. The
four factors are as follows:

. Heat input. This is the heat that melts the parent material
and filler (if required) to give the required fusion between
the parts being joined. The heating could be from an arc, a
laser, an electron beam or an oxyacetylene gas mixture.
The most common process is arc welding.
. Protection from atmosphere. If exposed, a welding arc will
pick up gases such as oxygen, nitrogen or hydrogen from
the atmosphere and pass them into the weld pool. These
gases can have a detrimental effect on the finished weld so
the arc needs to be protected from them. Common
methods of atmospheric protection used in arc welding
processes include the use of:
○ a shielding gas for processes such as TIG, MIG, MAG
and PAW;
○ a gaseous shroud, which is produced as the flux coating
on the electrode melts in MMA or FCAW;
○ a flux blanket, which covers the arc in SAW;
○ a combination of a shielding gas and a gaseous shroud
from melting flux in secondary shielded FCAW.
. Protection from external and internal contaminants
(cleaning). The weld metal can pick up contaminants
from the surface of the material so it is important that the
material surface is free from scale, rust, paint, grease,

24

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200903QG Welding chap3.3d Page 24 of 35