When apiece of metal is
heated, the metal expands. Upon cooling, the metal contracts and tries to resume its
original shape. The effects of this expansion and contraction are shown in figure 3-31.
View A shows a bar that is not restricted in any way. When the bar is heated, it is free
to expand in all directions. If the bar is allowed to cool without restraint, it contracts
to its original dimensions.
When the bar is clamped in a
vise (view B) and heated, expansion is limited to the unrestricted sides of the bar. As
the bar begins to cool, it still contracts uniformly in all directions. As a result, the
bar is now deformed. It has become narrower and thicker, as shown in view C.
These same
expansion and contraction forces act on the weld metal and base metal of a welded joint;
how-ever, when two pieces of metal are welded together, expansion and contraction may not
be uniform throughout all parts of the metal. This is due to the difference in the
temperature from the actual weld joint out to the edges of the joint. This difference in
temperature leads to internal stresses, distortion, and warpage. Figure 3-32 shows some of
the most common difficulties that you are likely to encounter.
When you are welding a
single-V butt joint (fig.3-32, view A), the highest temperature is at the surface of the
molten puddle. The temperature decreases as you move toward the root of the weld and away
from the weld. Because of the high temperature of the molten metal, this is where
expansion and contraction are greatest. When the weld begins to cool, the surface of the
weld joint contracts (or shrinks) the most, thus causing warpage or distortion. View B
shows how the same principles apply to a tee joint. Views C and D show the distortions
caused by welding a bead on one side of a plate and welding two plates together without
proper tack welds.
All metals, when exposed to
heat buildup during welding, expand in the direction of least resistance. Conversely, when
the metal cools, it contracts by the same amount; therefore, if you want to prevent or
reduce the distortion of the weldment, you have to use some method to overcome the effects
of heating and cooling.
CONTROLLING DISTORTION
You can control the
distortion caused by expansion and contraction during welding by following the simple
procedures listed here.
Proper Edge Preparation
and Fit-up
As discussed earlier in this chapter, proper edge preparation and fit-up are essential to
good quality welds. By making certain the edges are properly beveled and spacing is
adequate, you can restrict the effects of distortion. Additionally, you should use tack
welds, especially on long joints. Tack welds should be spaced at least 12 inches apart and
run approximately twice as long as the thickness of the weld.
Control the Heat Input
You should understand that the faster a weld is made, the less heat is absorbed by the
base metal. As you gain welding experience, it will become easier for you to weld a seam
with the minimum amount of heat by simply speeding up the welding process.
Regardless of your
experience, it is often necessary to use a welding technique designed to control heat
input. An intermittent weld (sometimes called a skip weld) is often used instead of one
continuous weld. When you are using an intermittent weld, a short weld is made at the
beginning of the joint. Next, you skip to the center of the seam and weld a few inches.
Then, you weld at the other end of the joint. Finally, you return to the end of the first
weld and repeat the cycle until the weld is finished. Figure 3-33 shows the intermittent
weld.
Another technique to control
the heat input is the back-step method (fig. 3-34). When using this tech-nique, you
deposit short weld beads from right to left along the seam.
Preheat the Metal
As discussed earlier, expansion and contraction rates are not uniform in a structure
during welding due to the differences in temperature throughout the metal. To control the
forces of expansion and contraction, you preheat the entire structure before welding.
After the welding is complete, you allow the structure to cool slowly. More about
preheating and postheating is discussed later in this training manual.
Limit the Number of Weld
Passes
You can keep distortion to a minimum by using as few weld passes as possible. You should
limit the num-ber of weld passes to the number necessary to meet the requirements of the
job. (See fig. 3-35.)
Use Jigs and Fixtures
Since holding the metal in a fixed position prevents excessive movements, the use of jigs
and fixtures can help prevent distortion. A jig or fixture is simply a device used to hold
the metal rigidly in position during the welding operation.
Allow for Distortion
A simple remedy for the distortion caused by expan-sion and contraction is to allow for it
during fit-up. To reduce distortion, you angle the parts to be welded slightly in the
opposite direction in which the contrac-tion takes place. When the metal cools,
contraction forces pull the pieces back into position. Figure 3-36 shows how distortion
can be overcome in both the butt and tee joints.
There is more to being a good
welder than just being able to lay a good bead. There are many factors that must be
considered. Later, we discuss additional techniques that you can apply to specific welding
situations.
Published
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy
