Cold Link Africa Jul/Aug 2016 Vol 1 No 6 | Página 40

TALKING TECHNICALLY
INCORPORATING COLD CHAIN

strength of the weld metal are relieved by
this straining. By the time the weld reaches
room temperature – assuming complete
restraint of the base metal so that it cannot
move – the weld will contain locked‑in
tensile stresses approximately equal to the
yield strength of the metal. If the restraints
(clamps that hold the workpiece, or an
opposing shrinkage force) are removed,
the residual stresses are partially relieved as
they cause the base metal to move, thus
distorting the weld.
Shrinkage control
To prevent or minimise weld distortion,
methods must be used both in design and
during welding to overcome the effects of
the heating and cooling cycle. Shrinkage
cannot be prevented, but it can be
controlled. Several ways can be used to
minimise distortion caused by shrinkage:
1. Do not over weld
The more metal placed in a joint, the
greater the shrinkage forces. Correctly
sizing a weld for the requirements of the
joint not only minimises distortion, but also
saves weld metal and time. The amount
of weld metal in a fillet weld can be
minimised by the use of a flat or slightly
convex bead, and in a butt joint by proper
edge preparation. The excess weld metal
in a highly convex bead does not increase
the allowable strength in code work, but it
does increase shrinkage forces.
When welding heavy plate (more than
1 inch thick) bevelling, or even double
bevelling, can save a substantial amount of
weld metal, which translates into much less
distortion automatically.
In general, if distortion is not a problem,
select the most economical joint. If
distortion is a problem, select either a joint
in which the weld stresses balance each
other or a joint requiring the least amount
of weld metal.

Poor

Weld

(c) Intermittent Welding

Poor

(d) Minimum number of
passes
Poor

Clamps
along edge
(j) Back-to-Back Clamping

Good

Good

(e) Welding near Neutral Axis

(i) Prebending

Wedge

Good

(f ) Balancing Welds around
Neutral Axis

Figure 4

(k) Sequence Welds

(l) Sequence Welds

Figure 6

of passes with small electrodes when
transverse distortion could be a problem.
Shrinkage caused by each pass tends to
be cumulative, thereby increasing total
shrinkage when many passes are used.
4. Place welds near the neutral axis
Distortion is minimised by providing a smaller
leverage for the shrinkage forces to pull the
plates out of alignment. Figure 4(e) illustrates
this. Both design of the weld and welding
sequence can be used effectively to control
distortion.
5. Balance welds around the neutral axis
This practice, shown in Figure 4(f), offsets one
shrinkage force with another to effectively
minimise distortion of the weld. Here,
too, design of the assembly and proper
sequence of welding are important factors.
6. Use back step welding
In the back step technique, the general

welds. Back-stepping may not be effective
in all applications, and it cannot be used
economically in automatic welding.
7. Anticipate the shrinkage forces
Pre-setting parts (at first glance, I thought
that this was referring to overhead or
vertical welding positions, which is not the
case) before welding can make shrinkage
perform constructive work. Several
assemblies, pre‑set in this manner, are
shown in Figure 5(h). The required amount
of pre‑set for shrinkage to pull the plates into
alignment can be determined from a few
trial welds.
Pre-bending, pre-setting or pre-springing
the parts to be welded, Figure 6(i), is a
simple example of the use of opposing
mechanical forces to counteract distortion
due to welding. The top of the weld groove
– which will contain the bulk of the weld
metal – is lengthened when the plates are
pre‑set. Thus the completed weld is slightly

clamping them tightly together. The welds
are completed on both assemblies and
allowed to cool before the clamps are
released. Pre-bending can be combined
with this method by inserting wedges
at suitable positions between the parts
before clamping.
In heavy weld, particularly, the rigidity
of the members and their arrangement
relative to each other may provide the
balancing forces needed. If these natural
balancing forces are absent, it is necessary
to use other means to counteract the
shrinkage forces in the weld metal. This
can be accomplished by balancing one
shrinkage force against another, or by
creating an opposing force. The opposing
forces may be: other shrinkage forces;
restraining forces imposed by clamps, jigs,
or fixtures; restraining forces arising from the
arrangement of members in the assembly;
or the force from the sag in a member due
to gravity.

Excessive reinforcement (greater than
dimension T) increases distortion
Direction of each
bead segment

(a) Avoid Overwelding

Reduce bevel angle
and use larger root
opening

Before Welding

300

After Welding

Direction of
Welding

1/32 to 1/160
U preparation

Double-V preperation
(g) Backstep Welding

(b) Edge preperation and Fitup
(h) Presetting Parts

Figure 3
2. Use intermittent welding
Another way to minimise weld metal is to
use intermittent rather than continuous
welds where possible, as in Figure 4(c). For
attaching stiffeners to plate, for example,
intermittent welds can reduce the weld
metal by as much as 75% to provide the
needed strength.
3. Use as few weld passes as possible
Fewer passes with large electrodes, Figure
4(d), are preferable to a greater number

40

Figure 5
progression of welding may be , say, from
left to right, but each bead segment is
deposited from right to left as in Figure
5(g). As each bead segment is placed, the
heated edges expand, which temporarily
separates the plates at B. But as the heat
moves out across the plate to C, expansion
along outer edges CD brings the plates
back together. This separation is most
pronounced as the first bead is laid. With
successive beads, the plates expand less
and less because of the restraint of prior

longer than it would be if it had been made
on the flat plate. When the clamps are
released after welding, the plates return to
the flat shape, allowing the weld to relieve its
longitudinal shrinkage stresses by shortening
to a straight line. The two actions coincide,
and the welded plates assume the desired
flatness.
Another common practice for
balancing shrinkage forces is to position
identical weld back‑to‑back, Figure 6(j),

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Figure 3 to 5: Distortion can be
prevented or minimised by techniques
that defeat – or use constructively –
the effects of the heating and cooling
cycle.

Part 2 continued in the next edition of
Cold Link Africa. CLA

COLD LINK AFRICA • July | August 2016