Types of joint used in manual
Metal arc welding
The selection of the type of joint to be used for a
particular application is governed by the following
factors (Figure 11.14):
1 The load and its characteristics
2 The manner in which the load is applied
3 The cost of joint preparation and welding.
Butt joints are of several types, each having a
number of variations. However, they are generally
classified as plain, single-V, double-V, single-U
Figure 11.14Types of arc welded joint
Manual metal arc welding 305
Strength and loading conditions
The achievement of the required static strength is a
primary criterion in the selection of joint design. For
fusion welding processes which employ a filler
material of composition matching that of the base
material, it is normally only necessary to ensure
that the cross-sectional area of the loaded joint is
adequate (Figure 11.16). This is relatively easy to
achieve where simple tensile or compressive forces
are involved, but the effect of shear loading must also
be considered in the case of lap joints. In dynamic
loading situations the joint profile is more significant
and it is important to avoid discontinuities which
may act as stress raisers. This is particularly important
in joints subjected to fatigue (Figure 11.17).
Economic considerations
The cost of any weld connection will be influenced
by the joint design. In general it is desirable to
minimize the time taken to prepare and complete
the joint to the required quality. This is usually
associated with a reduction in the amount of material
added to the joint. Narrow gap butt welds, for
example, give a reduction in weld time and,
depending on the process, may reduce the complexity
of plate preparation. Overwelding, that is
the use of more weld metal than is needed to meet
the strength requirements, may involve a significant
higher cost.
(a)
(b)
(c)
Figure 11.15Basic joint types: (a) fillet (b) butt
(c) Lap (BOC Ltd)
(a)
(b)
Figure 11.16Static loading conditions: (a) butt weld
(b) fillet weld, load carrying area shaded (BOC Ltd)
Figure 11.17Acceptable and undesirable joint detail
(BOC Ltd)
306Repair of Vehicle Bodies
Practical weld constraints
Some of the practical considerations which may
affect joint design are:
1 Access
2 Welding position
3 Process/consumable/equipment availability
4 Dissimilar thicknesses.
If access is limited to one side of a butt joint
it may be necessary to employ integral backing
or complex plate preparation to ensure that
adequate quality is achieved in the weld root
(Figure 11.18).
11.7 Technique of welding
Welding mild steel plate in the flat
Position (downhand)
Prior to setting up the job it is necessary to clean the
area to be welded. Where the material is heavier than
3 mm, V-ing of the welding edges is necessary to
ensure penetration. Next make sure that the earth
lead is securely fastened to the work. Painted or
rusted surfaces provide a poor earth and should be
scraped before connecting the earth clamp. Adjust
the current setting of the transformer or generator in
accordance with the electrode maker’s specifications.
Place the electrode in the holder, making sure that
good contact is gained between the jaws of the
holder and the bare end of the electrode. Care should
be taken that the flux coating of the rod is clear of
the electrode holder jaws. Never grip this holder in a
tense manner; a firm but relaxed grip will produce a
steady hand which is essential while striking and
maintaining the arc. The angle at which the electrode
meets the plate being welded should be about 70°.
The speed of travel should be such that a continuous
penetrating bead of electrode is deposited. Welds on
heavy plates are best made by making multiple runs
rather than by excessive weaving of the electrode
from side to side to build up the weld.
Welding light-gauge steel
The manual metal arc process offers a number of
advantages for the welding of thin-gauge mild
steels. The intensely localized heat input of the arc
ensures that the minimum of disturbance occurs to
the surrounding metal plate, therefore resulting in
very little distortion. At present 1 mm is the
thinnest gauge which can be readily welded by the
MMAW process, and both butt and outside corner
welds in this thickness can be produced with complete
satisfaction. Inside corner fillet welds in this
gauge are difficult to make because of the low heat
input, which restricts the free flowing of the slag
away from the depositing metal so that fusion is
obstructed and the resulting weld is unsatisfactory.
Successful welding of light-gauge steel depends
on three important conditions:
1 The close fitting of the edges to be welded
2 The right type and gauge of electrode
3 The accurate current adjustment.
(a)
(b)
(c)
Figure 11.18Plate preparation for limited
access butt joints: (a) permanent backing strip
(b) fusible insert (c) U preparation for root access
(BOC Ltd)
In positional applications, non-symmetrical
preparations may be necessary to improve joint
profile and avoid excess weld metal. The joint
preparation may be significantly affected by the
welding process, consumables and equipment
which are available. Dissimilar thicknesses may
be joined using lap and fillet configurations, but
it may be necessary to equalize the cross-section
at the joint to perform satisfactory edge or butt
welds. In some cases it may be possible for the
joint to be self-jigging. This is particularly useful
for site assembly, saving time and improving
repeatability.
A range of approved joint designs is provided in
international standards covering welding construction
BS 5135.
Manual metal arc welding 307
Either DC or AC is suitable providing the supply is
steady and capable of fine adjustment. The opencircuit
voltage should not be less than 60 volts in
the case of DC and 70 volts in the case of AC.
Unless care is taken to see that edges or surfaces to
be joined are in close contact along the seam, the
heat conductivity of the weld area and the local
heat will cause burning through wherever a gap
occurs. Joint contact is maintained by tacking at
75–150 mm apart. The welding operation, which
takes a little time to practise, consists of striking
the arc and passing the electrode tip with the shortest
possible arc rapidly along the seam to be
welded at a uniform rate, without any sideways
movement. If this is done correctly it will produce
a satisfactory weld.
Welding stainless steels
(austenitic group only)
The technique of welding stainless steel does not
differ greatly from that of welding mild steel, but
as the material being handled is very expensive,
extra precautions and attention to detail at all
stages of welding are desirable. In principle all
stainless steel for high-class work should be
welded with a short arc using a DC supply with the
positive polarity applied to the electrode. For less
important work an AC supply can be used with an
open-circuit voltage of 100 volts. The current
ranges recommended for different thicknesses will
depend on the nature of the work and the size of
the plate. Usually the lowest convenient current
should be used; weaving should not be wider than
twice the diameter of the electrode, and electrodes
of like composition to the parent metal must be
used. The edges of the welding should be free from
scale. Clamps and jigs are advisable when welding
thin sheet. Tack welds, particularly on thin sheets,
should be placed much closer together than is the
usual practice for welding mild steel.