Prevention and Control of Distortion in Arc Welding -- CC

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In many fields of modern industry,
electric arc welding is accomplishing

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miracles of production.

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Powerful diesel-electric locomotives
are being made lighter and stronger, by

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using arc welds in place of bolts, rivets
and castings.

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These penstock tubes at Shasta Dam were

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arc welded, to eliminate all joints which
might otherwise leak or corrode.

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New methods of ship construction have
resulted from the use of arc welding,

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which speeds up production, and produces
a lighter, more rigid ship.

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In the manufacture of aircraft -- arc
welding on engine mounts, fuselages and

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landing gear ensures maximum strength,
with the minimum of weight.

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But the successful application of arc

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welding depends on the use of proper
welding methods.

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Here is what happens when improper
welding methods have been used.

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Distortion has caused this job to warp
out of shape.

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The same thing has occurred to this metal tray.

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The purpose of this film is to show how

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all distortion can be controlled,
and prevented.

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But first of all, we must understand
what causes distortion.

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Let's start with this ordinary steel bar.

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If the bar is heated thoroughly and uniformly
throughout its entire volume, considerable

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expansion in all directions will take place.

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Now, if the bar is allowed to cool evenly
without restraint of any kind, we know

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that it will contract to its original
shape and size without distortion.

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However, if we place the two ends of the
bar in a vise, and then heat the bar

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uniformly, expansion in these directions
will be prevented by the vise,

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and expansion can only occur in these
other directions.

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As the bar cools, it contracts evenly
in all directions.

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The result, is a shorter bar, with a
greater thickness.

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Now let's go back to our original bar,

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and see what happens when we heat only
one side.

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Getting up close, we see that expansion
in this case is localized, and uneven.

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The surrounding cool metal acts similar

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to the vise -- and restrains expansion
in these directions.

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But there is no resistance to expansion
in this direction.

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It is obvious that this uneven expansion
causes an unnatural displacement of metal.

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When this area starts to cool and

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contract, a small amount of that
displacement becomes permanent.

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In other words, there was no control;
and the final result is distortion.

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Now several steel bars side by side,
is much the same as a steel plate;

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showing that uncontrolled contraction
always causes distortion.

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Let's see what actually happens when
we make this butt weld.

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Now keep the picture of that steel bar
in mind; because the bar's behavior when

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heated is very similar to the weld bead
we'll form to join these two plates.

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As the weld progresses, we can see that

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the molten weld metal begins to cool,
and contract immediately.

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But at the same time, the heat of the arc

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itself is causing considerable expansion
ahead of the contraction.

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Looking at the end view, we can see that
the intense heat of this molten weld

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metal, plus the heat of the arc itself,
is being transmitted to the surrounding areas.

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It is important to understand that while
the weld metal is cooling, and therefore

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contracting, the temperature of the
surrounding plates is rising and therefore

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causing considerable uneven expansion.

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As these plates cool, they will also contract.

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When we allow expansion and contraction to
occur without any control, the result is

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bound to be distortion -- caused by this
tough looking villain, Mr. Shrink himself!

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He thinks he is pretty powerful; but we

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can show that Shrink is all bronze, and
no brains.

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We know that on any welding operation,
Shrink is always right on the job.

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Look at him pull!

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And look at the distortion!

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Now that the damage has been done, let's
slice off a piece of the plate, and

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examine a typical cross-section of the weld.

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The over-welding here is a waste of time
and money, adds nothing to the strength

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and performance of the joint, and in this
case caused abnormal distortion.

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Let's find a rule we can apply in a
situation of this kind.

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Rule #1: To prevent distortion, reduce
the effective shrinkage force.

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In other words, always use as little weld

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metal as possible, and make better use of
the weld metal you need.

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We can also reduce the effective shrinkage
force through proper edge preparation.

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This amount of bevel would require more
weld metal than necessary.

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To obtain proper fusion at the root of

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the weld with a minimum of weld metal,
the bevel should be 30 degrees.

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But proper fitup is also important; so

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space the plates 1/32" to 1/16" of an
inch apart [0.794mm to 1.59mm].

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You will then need only a minimum amount
of weld metal to produce a strong joint.

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Using fewer passes is another way of
controlling distortion.

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For example, on plates that are free to

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move, distortion in this direction is
always a problem.

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In this case, if we use one or two passes

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with large electrodes, we will greatly
reduce distortion.

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In welding any structure, it is always
important to consider the neutral axis.

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With a conventional fillet, the weld is
so far off the neutral axis, that Shrink

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has plenty of leverage to pull the
plates out of alignment.

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But use of the fleet-fillet method
places the weld close to the neutral axis

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greatly reducing the leverage so that
Shrink cannot pull the plates out of line.

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Your own experience and ingenuity will
uncover other methods.

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For example, intermittent welds frequently
give all the strength required.

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In this way, you can use 2/3rds less weld

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metal, and reduce the effective shrinkage
force by that much.

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On this bulkhead, good engineering design
permitted the use of intermittent welds

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which meet all strength requirements, and
at the same time, minimize distortion.

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When a continuous weld is required, we
can control distortion if we first

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understand how expansion affects the
plates.

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Notice how expansion from the heat of
the weld along the edges causes the plates

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to spread, as shown in the magnified circle.

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As the weld progresses, the spreading
continues: and the plates become locked

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in this position by the cooling and
contraction of the weld metal.

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Welding speed will determine the amount
of this spreading action.

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But we can control this action and prevent
distortion by the use of back-stepping;

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whereby each successive bead is laid from
right to left; but the direction of your

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welding progresses from left to right.

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To illustrate this method, very short
welds are used here.

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But in actual practice, each bead is
laid with one stick of electrode to allow

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time for the heat of each weld to
distribute evenly throughout the plates

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before beginning the next bead.

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When we use backstepping, notice how
heat from the first weld causes expansion

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which temporarily spreads the plates.

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But as the heat moves out across the
plates, the expansion in these outer

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areas, acting against the bead which
has cooled, forces the plates together.

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Each weld becomes a rigid section by
the time the next weld is started,

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so that the spreading action becomes
less and less which each succeeding

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bead until the weld job is completed
without further spreading or distortion.

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Rule #2 gives us a little different
slant on this fellow Shrink.

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To prevent distortion, make shrinkage
work for us.

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This is simply another way of saying
that Shrink is plain dumb, and is just

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as willing to work for us as against us,
providing we're smart enough to use him

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to our advantage.

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On a "T" weld like this, we can anticipate
Mr. Shrink's tendencies, and tip the

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perpendicular plate slightly away from
the weld side.

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Now, see how quickly Shrink goes to work

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for us, and straightens this part up to
its true position.

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Another adaptation of Rule #2 is the
spacing of parts before welding.

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In welding these searchlight trunnion arms
which be very accurately spaced when the

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welding is completed, allowance is made
for the amount of shrinkage which will occur.

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Before welding, the parts are spaced
like this.

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Then when the welding is completed,
watch how controlled shrinkage brings

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the two arms into the correct position
and perfect alignment.

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We can also make shrinkage work for us

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by prebending or springing the parts
involved.

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For example, when these two plates are
sprung away from the weld side, the

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counterforce exerted by these clamps
holding the plates firmly, overcomes

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the shrinkage tendency of the weld metal,
causing it to yield.

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But we can still use Mr. Shrink after
the clamps are removed.

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Now all he needs to do is give the

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plates a slight pull to eliminate any
signs of distortion.

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Prebending may be applied to any number
of welding operations.

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Here, it is being employed on these

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steamshovel dipper sticks to make sure
the parts will be straight after welding.

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So far, we have illustrated several

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methods by which we can control and
prevent distortion.

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First, by reducing the effective

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shrinkage force: and second, by making
shrinkage work for us.

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But on certain types of weld jobs, we

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may find that we still have a distortion
problem.

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Then we must use other methods.

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We'll call this Rule #3.

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To prevent distortion, balance shrinkage
forces with other forces.

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This rule applies automatically in welding

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this machine base, for its own structural
nature provides rigid balancing forces.

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But when these natural balancing forces
are not present, we can place Shrink in

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the position of using his own powerful
force to balance itself.

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Here's how: Use proper welding sequence.

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By welding alternately on both sides
of the neutral axis of these two plates,

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watch what happens to Shrink: first he
has to pull on this side.

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Then rush around to pull on the other side.

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And back again!

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And over again!

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Phew! It's a much harder pull each time,

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until finally we have Shrink tired out
completely. - *bong*

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The result? No distortion.

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Here's another application of the same
principle: Staggered intermittent welds

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applied in this sequence: 1... 2... 3... 4.

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Proper welding sequence permitted the
construction of this crane boom to proceed

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without delay and without distortion.

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The operation was throughly planned
beforehand, so that each cross-arm was

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tack-welded to the main members to first
make the entire crane a rigid structure.

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Following this, one set of cross-arms was
welded on one side.

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Then, one set on each opposite side; always
balancing one shrinkage force with another.

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Step by step right on up to the end,

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so that the final result was a perfectly
straight crane boom.

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The use of peening is the application of

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a balancing force to prevent distortion
in a different sense of the word.

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By peening the bead, we actually stretch

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the weld metal, counteracting its tendency
to shrink as it cools.

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When we use peening, Shrink really takes
a beating. -*hammering sounds*

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Look at him! He's groggy already.

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Peening takes the fight right out of him.

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But don't overdo it; too much peening may
damage the weld metal.

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The most important method of overcoming
distortion problems is the use of clamps,

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jigs or fixtures to hold the work in a
rigid position during welding.

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In this way, we balance the shrinkage

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forces of weld metal with sufficient
counter-forces to prevent distortion.

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For example, when we weld these two
plates, we know that when the weld metal

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cools, the plates will distort, like this.
 -However, if we hold the plates perfectly

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rigid with clamps or jigs, the restraining
forces here prevent the plates from moving.

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Consequently, the weld must
stretch as it cools.

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Now after removing the clamps, we see

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that almost all distortion has been
eliminated.

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But in most cases, the plates to be
welded are merely parts of a structure,

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and other sections will continue to hold
the plates as rigid as if they were

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clamped permanently, thereby reducing
distortion to a minimum.

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Here is a practical application of this
principal.

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These heavy fixtures clamp the aircraft

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tubing of this fuselage so rigidly that
distortion is impossible.

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The type of jig or fixture required will

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be determined of course by the nature of
the welding job.

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Here's the setup where we have every
possible shrinkage force balanced with

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other forces: the more Shrink pulls,
the more exhausted he becomes.

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*bonk*

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*bonk*

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*bonk*

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*drumroll*

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Well -- that'll take the starch out
of him for awhile!

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Remember that controlled shrinkage
prevents distortion, so be sure to

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apply one or all of these three rules
to every welding job.

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Reduce the effective shrinkage force;

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make shrinkage work for us; balance
shrinkage forces with other forces.

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Arc welding is the truly modern method
of fabrication, it is one of the great

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tools with which the leaders of modern
industry today are building a new world

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of tomorrow.