Caution: Wear protective clothing and leathers to
shield yourself from falling sparks and metal. Do not
stand directly under the spark stream. Wear protective
eye lenses. Never clean the slag on a weld without eye
protection. Stay aware of your environment when your
hood is down and you are welding. If flammable materi-
al is present, take measures to prevent fires. Keep a fire
extinguisher at your work area.
Design Considerations
The types of weld designs and joints should be eval-
uated to ensure the weldment does the intended job.
Areas of evaluation include:
• Material type and condition (annealed, hardened,
tempered).
• Service conditions (pressure, chemical, vibration,
shock, wear).
• Physical and mechanical properties of the com-
pleted weld and heat-affected zones.
• Preparation and welding costs.
• Assembly configuration and weld access.
• Welding equipment and tooling.
Butt Joint and Welds
Butt joints are used when strength is required. They
are reliable and can withstand stress better than any other
type of weld. To achieve full stress value, the weld must
have 100% penetration. This can be achieved by com-
pletely welding through from one side. The alternative is
to weld both sides with the weld joining in the center.
Thinner gauge materials are more difficult to fit up
for welding. They require costly tooling to maintain
proper joint configuration. Tack welding may be used as
a method of holding the components during assembly.
However, tack welds present several problems:
• Conflicting with the final weld penetration into
the weld joint.
• Adding to the final crown dimension (height).
• Cracking during the welding operation due to
heating and expansion of the joint.
• Contaminating the weld joint when cleaning the
tack weld slag, causing weld defects.
The expansion of the base material during welding
can result in mismatch, Figure 6-22. When mismatch
occurs, the weld will not completely penetrate the joint.
Many specifications limit highly stressed joints to a max-
imum mismatch of 10% of the joint thickness.
Whenever possible, butt joints should mate at the
bottom of the joint, Figure 6-23. Joints of unequal thick-
nesses should be machined in the weld area to provide
even surfaces for adequate fusion, Figure 6-24. Where
this cannot be done, the heavier piece may be machined
on the upper part of the joint.
Weld Shrinkage
Butt welds always shrink across (transverse) the
joint during welding. A shrinkage allowance must be
made if the post-welding dimensions have a small toler-
ance, or if the outer edges of the material are to be
trimmed to a final dimension. Butt welds on pipe, tubing,
and cylinders also shrink, Figure 6-25. When dimensions
Chapter 6 Weld Joints and Weld Types 55
Figure 6-22. When under stress, welds made with mismatched
joints often fail below the rated load.
Figure 6-23. Mating the joint at the bottom equalizes the load
under stress. The weld is placed on the top and penetrates
completely through the joint.
Figure 6-24. Joints of unequal thickness absorb different
amounts of heat and expand at different ratios. Equalize heat
flow by tapering the heavier material to the thickness of the
thinner material.
Figure 6-25. During welding, butt welds shrink both
transversely and longitudinally.