The characteristic features and principal
causes of incomplete root fusion are described. General guidelines on 'best
practice' are given so welders can minimise the risk of introducing
imperfections during fabrication.
Fabrication and service defects and imperfections
As the presence of
imperfections in a welded joint may not render the component defective in the
sense of being unsuitable for the intended application, the preferred term is
imperfection rather than defect. For this reason, production quality for a
component is defined in terms of a quality level in which the limits for the
imperfections are clearly defined, for example Level B, C or D in accordance
with the requirements of EN 25817. For the American standards ASME X1 and AWS
D1.1, the acceptance levels are contained in the standards.
The application code will specify the quality levels which must be achieved
for the various joints.
Imperfections can be broadly classified into those produced on fabrication of
the component or structure and those formed as result of adverse conditions
during service. The principal types of imperfections are:
fabrication:
- lack of fusion
- cracks
- porosity
- inclusions
- incorrect weld shape and size
service:
- brittle fracture
- stress corrosion cracking
- fatigue failure
Welding procedure and welder technique will have a direct effect on
fabrication imperfections. Incorrect procedure or poor technique may produce
imperfections leading to premature failure in service.
Incomplete root fusion or penetration
Identification
Incomplete root fusion is when the weld fails to fuse one
side of the joint in the root. Incomplete root penetration occurs when both
sides of the joint are unfused. Typical imperfections can arise in the following
situations:
- an excessively thick root face in a butt weld (Fig. 1a)
- too small a root gap (Fig. 1b)
- misplaced welds (Fig. 1c)
- failure to remove sufficient metal in cutting back to sound metal in a
double sided weld (Fig. 1d)
- incomplete root fusion when using too low an arc energy (heat) input (Fig.
1e)
- too small a bevel angle,
- too large an electrode in MMA welding (Fig 2)
Fig. 1 Causes of incomplete root fusion |
 a) |
 b) |
 c) |
 d) |
 e) |
a) Excessively thick root face
b) Too small a root gap
c) Misplaced welds
d) Too low a power input
e) Too low an arc
(heat) imput |
Fig. 2 Effect of electrode size on root fusion |
 a) |
|
 b) |
a) Large diameter electrode
b) Small diameter electrode
|
Causes
These types of imperfection are more likely in consumable
electrode processes (MIG, MMA and submerged arc welding) where the weld metal is
'automatically' deposited as the arc consumes the electrode wire or rod. The
welder has limited control of weld pool penetration independent of depositing
weld metal. Thus, the non consumable electrode TIG process in which the welder
controls the amount of filler material independent of penetration is less prone
to this type of defect.
In MMA welding, the risk of incomplete root fusion can be reduced by using
the correct welding parameters and electrode size to give adequate arc energy
input and deep penetration. Electrode size is also important in that it should
be small enough to give adequate access to the root, especially when using a
small bevel angle (Fig 2). It is common practice to use a 4mm diameter electrode
for the root so the welder can manipulate the electrode for penetration and
control of the weld pool. However, for the fill passes where penetration
requirements are less critical, a 5mm diameter electrode is used to achieve
higher deposition rates.
In MIG welding, the correct welding parameters for the material thickness,
and a short arc length, should give adequate weld bead penetration. Too low a
current level for the size of root face will give inadequate weld penetration.
Too high a level, causing the welder to move too quickly, will result in the
weld pool bridging the root without achieving adequate penetration.
It is also essential that the correct root face size and bevel angles are
used and that the joint gap is set accurately. To prevent the gap from closing,
adequate tacking will be required.
Best practice in prevention
The following techniques can be used to
prevent lack of root fusion:
- In TIG welding, do not use too large a root face and ensure the welding
current is sufficient for the weld pool to penetrate fully the root
- In MMA welding, use the correct current level and not too large an
electrode size for the root
- In MIG welding, use a sufficiently high welding current level but adjust
the arc voltage to keep a short arc length
- When using a joint configuration with a joint gap, make sure it is of
adequate size and does not close up during welding
- Do not use too high a current level causing the weld pool to bridge the
gap without fully penetrating the root.
Acceptance standards
The limits for lack of penetration are specified
in BS EN 25817 (ISO 5817) for the three quality levels.
Lack of root penetration is not permitted for Quality Level B (stringent).
For Quality Levels C (intermediate) and D (moderate) long lack of penetration
imperfections are not permitted but short imperfections are permitted.
Incomplete root penetration is not permitted in the manufacture of pressure
vessels but is allowable in the manufacture of pipework depending on material
and wall thickness.
Remedial actions
If the root cannot be directly inspected, for example
using a penetrant or magnetic particle inspection technique, detection is by
radiography or ultrasonic inspection. Remedial action will normally require
removal by gouging or grinding to sound metal, followed by re-welding in
conformity with the original procedure.
Relevant standards
EN 25817:1992 (ISO 5817) Arc welded joints in steel -
Guidance on quality levels for imperfections.
EN 30042: 1994 Arc welded joints in aluminium and its weldable alloys -
Guidance on quality levels for imperfections.
This information was prepared by Bill Lucas with help from Gene Mathers.
Copies of other articles in the Job knowledge for welders series can be found
under Practical Joining Knowledge or by using the search engine.
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