Page 67 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

Basic electrodes contain low levels of H 2 , and are
therefore used where high quality welds with good
mechanical properties are required (especially high
strength welds in high restraint situations, which are
susceptible to hydrogen-induced cold cracking (HICC).
Basic electrodes require baking at temperatures above
150 8C, storing in an oven at temperatures up to 120 8C,
and being used from heated quivers at about 70 8Cto
ensure they maintain the low H 2 levels required. A modern
option is to use electrodes straight from vacuum packs,
which give low H 2 levels as long as they are used in
accordance with the manufacturer’s instructions.
. Rutile electrodes. Rutile electrodes contain titanium oxide
in the flux coating. As with basic electrodes the shielding
gas produced is mainly CO 2 . The rutile coating makes this
electrode very welder-friendly due to its ease of use, low
fume levels, low spatter levels and smooth weld beads.
They are not baked or preheated before use but may be
heated for a short period at temperatures up to 120 8Cto
ensure they are dry before use.
. Cellulosic electrodes. These are electrodes that contain
cellulose (which is an organic material) in the flux coating.
The shielding gas produced has high levels of H 2 ,
producing a hotter burning arc than CO 2 . This hotter
arc gives deeper weld penetration and faster welding
speeds and is commonly used in the stovepipe welding
technique (which entails welding pipes in the vertical down
direction).
One problem is that high levels of H 2 are introduced
into the weld from the shielding gas, leading to an
increased risk of H 2 cracking. To reduce this risk the
welding procedures specify that timed hot passes are
required; i.e. applying further passes over the root pass
while the weld is still hot. This acts as a ‘H 2 soak’
technique and allows the H 2 to remain as atomic H and
dissipate out of the weldment.

52

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200905QG Welding chap5.3d Page 52 of 66