Welding

1. *** Nice Weld Shame About The Fusion ***
Today welding is the most common method used for joining steel
fabrications largely because of the speed at which joints can be made
and the reliability of these joints in service. However because most
welding operations are now relatively simple to perform it is all too easy
to forget the complexity of the chemical and metallurgical actions that
are taking place when the weld is being deposited. Therefore not
surprisingly welds occasionally fail.
The most common causes of weld failure can be attributed to one of the
following causes:-
Overload.
Before applying the various design formulas, the problem itself must be
analysed and clearly stated. This is not always obvious, and trying to solve
the wrong problem can quickly lead to insufficient design stresses. When a
load is placed on a member, stress and strain result. Stress is the internal
resistance to the applied force. Strain is the amount of "give or deformation
caused by the stress, such as deflection in bending, elongation in tension,
contraction in compression, and angular twist in torsion.
For example of this is a lifting lug on a pressure vessel. If the vessel is
lifted by a spreader beam the loading condition on the lug consists of a
simple vertical force putting the attachment welds either in tension or
shear. However if the vessel is lifted with a rope sling the loading

2. condition becomes more complex because there is now a horizontal
component of the force to consider as well a the vertical one, which
effectively increases the loading on the welds.
Joint Design.
A welded joint should be designed such that the
welder can easily manipulate the electrode to
ensure good fusion, particularly in the root of
the joint. The profile of each run should be
roughly as wide as it is deep; wide shallow
weld beads and particularly deep narrow beads
are both ideal candidates for hot cracking.
This type of cracking occurs when the weld is starting to solidify, in the
pasty state, as it posses very little strength and therefore any residual
loading is likely to cause it to break before it has fully solidified. The
problem can be compounded by impurities that are forced out of the
solidifying weld, becoming trapped in the centre of the weld during
final solidification. Hot cracking can occur where their is a high degree
of restraint in the structure of the fabrication or where the structure
moves slightly as the weld solidifies.
A good example of this type of failure is on
the weld used to secure the small plug in
the mandrill hole of a spun dished head on
a pressure vessel, a weld that many people
do not take seriously because of its size. As
the weld cools it contracts causing the plug
to move , if the weld at the other side of the
plug is still solidifying it could easily fail.
This is because of the very high contraction
stresses generated by the plug as the weld
starts to solidify.
Bad Welding Method.
It is very important when carrying out any
welding to ensure that it is done correctly.
Consideration has to be given to all aspects
of the process and also the environment.
Often welding has to be carried out under site
conditions, the welding is often carried out in
situation so that small general purpose
electrodes are used resulting in low weld heat
input which when combined with no preheat gives very rapid heat

3. dissipation Which can create a hard micro structure particularly in the
location of the heat affected zone.
This along with high levels of
residual stress will create the ideal
condition for hydrogen induced
cracking, which although
normally associated with high
strength steels can occur in low
carbon steels if the conditions are
right. The resulting crack may not
occur immediately the weld cools
down but some time afterward,
therefore if this type of failure is
expected non destructive examination should be delayed by at least
48 hours after welding.
Metallurgical failure.
Materials that are to be welded have to tolerate severe thermal transients
created by the welding process without suffering deterioration of their
mechanical properties or adverse phase changes. The metallurgical
composition or temper conditions of certain types of metal may make
them unsuitable to weld or may require special controls to be imposed
during the welding operation. For example some steels that are easy to
machine may contain high levels of sulphur that may result in cracking
of any attaching weld. Therefore this type of material should not be used
on load bearing fabricated items such as the eye bolts that are often
found holding down manway covers on pressure vessels.
Weld Defects.
They can usually be attributed to the
welders inability to set up and manipulate
the welding equipment; although bad joint
design and faulty welding equipment can
also be responsible. The most significant
defects are cracks and those that resemble
cracks such as lack of fusion, cold overlap
etc. This is because of the risk that the crack
may become unstable and propagate when
loaded causing a dramatic failure often by
brittle fracture.
Porosity seldom causes weld failure in multi-run welds however it is a
sign that something has gone wrong with welding operation and can

4. often be caused by other defects that may not have been detected such
as lack of side wall fusion. Weld profile can also cause failure, if the
weld size is too small because the joint is underfilled with weld then its
load carrying capability will be reduced, if the joint contains excessive
weld metal this can create a notch effect which can lead to failure by
fatigue if the loading condition fluctuates. Bad fit up excessive root
penetration on single sided welds can create defects in the root of the
weld such as wormholes and even cracking. Distortion of welded joints
can cause failure by buckling if the welded member is subjected to
compressive loads.
Guidance on imperfection levels of welded joints is given in EN ISO
5817
To minimise these problems the following points should be
considered:-
1. Design of the weld based on the loading condition(s) the joint
will carry
2. Accessibility to enable ease of welding
3. Control of distortion
4. Careful consideration of the welding environment
5. Matching welding process with materials
6. A factor of safty applied to the design stress of the weld which
should be based on the consequance of weld failure and the
level of non destructive testing that is to be carried out.
For example a pressure vessel made to PD5500 category 3, (no
radiographic inspection), can be up to twice a thick as an equivalent
vessel made to category 2, (10% Radiography). Fillet welds and Partial
Penetration welds should be used with care as they contain lack of
fusion, they are only suitable for relatively low stressed joints that are
not subject to any form of fatigue loading and should be used with a
suitable factor of safety, which for fillet welds is at least two.
Once the weld has been designed it is then necessary to decide upon the
welding method, this is then documented in the form of a welding
procedure specification. The European Welding Standard for welding
procedures, EN ISO 15609-1 (formerly EN288 Part 2), gives guidance
on the content and format of such a specification.
However this document on its own is not sufficient because we need to

5. prove that this welding method will produce a weld of acceptable
quality possessing the right mechanical properties. Therefore it is
necessary to simulate the joint in all essential features and weld it under
normal production conditions. The completed joint can then be subject
to both non destructive and destructive examinations to determine if the
joint is going to be suitable for the application.
For most stringent applications the European Standard EN ISO 15614
Part 1 (formerly EN 288 Part 3) is preferred for welding procedure tests
in steel materials and part 2 for Aluminium and its alloys. There are
other parts of EN 288 that deal with alternative routes for qualifying
procedures, other than a procedure test, for less onerous applications.
See Welding Procedure Section for details.
Once we have established that the proposed welding method is
satisfactory we then have to ensure that the production welds will also
be of the same quality. This involves making sure the welders posses the
required skill and knowledge to deposit sound welds in accordance with
the approved procedure. Whilst we can be confident that the welder who
did the procedure will be able, any other welder used must also
demonstrate his ability by successfully completing a welder approval
test. The preferred standard for this is EN 287 Part 1 for steel and part 2
for aluminium and its alloys. This standard not only tests the
performance of the welder but also requires it to be monitored and
revalidated every 2 years to ensure that the welders skill can be relied
upon.
Finally make sure that when the welding operation is being carried out it
is supervised and coordinated by properly qualified personnel.

6. prove that this welding method will produce a weld of acceptable
quality possessing the right mechanical properties. Therefore it is
necessary to simulate the joint in all essential features and weld it under
normal production conditions. The completed joint can then be subject
to both non destructive and destructive examinations to determine if the
joint is going to be suitable for the application.
For most stringent applications the European Standard EN ISO 15614
Part 1 (formerly EN 288 Part 3) is preferred for welding procedure tests
in steel materials and part 2 for Aluminium and its alloys. There are
other parts of EN 288 that deal with alternative routes for qualifying
procedures, other than a procedure test, for less onerous applications.
See Welding Procedure Section for details.
Once we have established that the proposed welding method is
satisfactory we then have to ensure that the production welds will also
be of the same quality. This involves making sure the welders posses the
required skill and knowledge to deposit sound welds in accordance with
the approved procedure. Whilst we can be confident that the welder who
did the procedure will be able, any other welder used must also
demonstrate his ability by successfully completing a welder approval
test. The preferred standard for this is EN 287 Part 1 for steel and part 2
for aluminium and its alloys. This standard not only tests the
performance of the welder but also requires it to be monitored and
revalidated every 2 years to ensure that the welders skill can be relied
upon.
Finally make sure that when the welding operation is being carried out it
is supervised and coordinated by properly qualified personnel.