TIGBook_Chpt7.pdf

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VII. Joint Design and
Types of Welds
A weld joint is the term used for the location where two or
more pieces of metal will be or have been welded together.
Figure 7.1 shows the five basic weld joint designs.
A few considerations for joint design are specific to GTAW.
Naturally the weld joint must be accessible to the GTAW torch,
making it possible for proper torch movements. Weld joints
should not be too narrow, so as to restrict access of the gas cup.
In some cases, using a narrower gas cup, or a gas lens with the
electrode extending up to an inch beyond the gas cup will help.
Edge Joints
Edge joints are often used when the members to be welded
will not be subjected to great stresses. Edge joints are not
recommended where impact or great stress may occur to one
or both of the welded members. An edge joint occurs when
the edges of parallel or nearly parallel members meet and are
joined by a weld. Figure 7.2 shows different types of edge
joints. Figure 7.2 demonstrates the various types of edges
that can be applied to the joints. If required, the joints can be
altered by grinding, cutting or machining the edges into a
groove. The groove can be a square, beveled, V, J, or U. The
main purpose of the groove is to allow proper penetration or
depth of fusion . See Figure 7.3.
To obtain a quality weld and cost-effective use of filler metal,
joint design must be considered in any type of weldment.
This will depend upon several factors including material type,
thickness, joint configuration and strength required.
It is quite possible that a welder would have little to do with how
a particular joint is designed. However, a good welder should be
familiar enough with joint design to carry out a welding job.
A proper joint design will provide the required strength and
the highest quality weld at the most economical cost. The
joint design selected will dictate what type of weld is required.
Corner
Butt
Square Groove
Bevel
Lap
T
V-Groove
U-Groove
Edge
Figure 7.1 Five basic joint designs.
J-Groove
The five basic joint designs are typically welded with the TIG
process using either a groove or a fillet weld. Groove welds
are those made into a prepared joint to get deeper penetration.
To prepare the joint, material must be removed and replaced
with weld metal. Groove welded joints are very efficient but
are more expensive than a fillet welded joint. Groove welds
generally require some form of joint preparation while fillet
welds are made on joints requiring no joint preparation.
When the edge or surface of joint members come together at
a right angle to each other, the resulting weld, which is trian-
gular in shape, is called a fillet weld. Fillet welds on lap or
T-joints are commonly used in the welding industry.
Figure 7.2 Edge Joints
Depth Of Fusion
Joint
Penetration
Root Penetration
Figure 7.3 Depth of fusion and types of penetration. Complete joint pene-
tration refers to weld metal that extends completely through the groove and
has complete fusion into the base metal. What is shown is a partial joint
penetration, which if not intended is referred to as incomplete joint penetration.
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Butt Joints
A butt joint occurs when the surfaces of the members to
be welded are in the same plane with their edges meeting.
Figure 7.4 shows butt joints with various types of grooves.
Butt joints are often used to join pressure vessels, boilers,
tanks, plate, pipe, tubing or other applications where a
smooth weld face is required. They generally require more
welding skill than other joints. Butt joints have very good
mechanical strength if properly made. They can be expensive
joints since a prepared groove is generally required to get the
proper penetration and weld size. This involves the extra
operation of joint preparation, removal of material to open up
the joint and then welding to penetrate and fill the groove.
Groove Angle
(Included Angle)
Bevel Angle
Groove
Face
Root Face
Root Opening
Figure 7.5 V-groove butt joint.
If material thickness is less than approximately 1/8" thick,
square edges butted tight together (no root opening) can be
used. (Aluminum would probably require a small root opening.)
Plate thicknesses 1/8" and greater generally require single or
double V-groove and root openings for proper penetration
and depth of fusion. Joint preparation before welding will
depend upon the joint design and the equipment available to
do the edge preparation. The oxy-fuel torch, carbon arc gouging
or plasma arc cutting/gouging is often used to cut a bevel-,
J-, U-, or square-groove edge on steel plates. Aluminum is
best prepared with mechanical cutting tools or the plasma arc
cutting/gouging process.
Square Groove
Square Groove
With Root Opening
Lap Joints
Another joint design used a great deal in the welding industry is
the lap joint. Various types of lap joints are shown in Figure 7.6.
As can be seen in the figure, lap joints occur when the surfaces
of joined members overlap one another. A lap joint has good
mechanical properties, especially when welded on both sides.
The type of weld used on a lap joint is generally a fillet weld.
If a groove weld is called for, it can be applied as shown in the
figure with a single or double bevel. The groove weld may or
may not be followed with a fillet weld. This would be indicated
by the appropriate welding symbol. The degree of overlap of the
members is generally determined by the thickness of plate. In
other words, the thicker the plate, the more overlap is required.
Beveled Butt
V-Groove
J-Groove
U-Groove
Figure 7.4 Butt joints.
Distortion and residual stresses can be problems with
butt joints.
Butt joints can be designed in various ways. They may be
welded with or without a piece of metal or ceramic backing
the joint, usually referred to as a “backing bar” or “backing
strip”. The edges can be prepared into a groove that is square,
beveled, V, J, or U grooved. Edges may be held tight togeth-
er or a small gap known as a root opening may be left
between the edges.
Figure 7.5 shows the various parts of a V-groove butt joint.
Note the groove angle, groove face, root face and root
opening. The groove angle is the total included angle of the
joint. If two 37.5˚ bevels are brought together, they form a
75˚ V-groove. The groove face is the surface of the metal in
the groove, including the root face. The root face is some-
times called the “land”. In this example, the root face is 1/8"
and the root opening 3/32". The main purpose of the various
grooves and root openings is to allow proper penetration and
depth of fusion.
Double Bevel Groove
Single Or Double Fillet
Single Bevel Groove
Figure 7.6 Lap joints.
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Corner Joints
When members to be welded come together at about 90˚ and
take the shape of an “L”, they are said to form a corner joint.
Several types of corner joints and grooves are shown in
Figure 7.7. Welds made on the inside of the “L” are considered
fillets and welds made on the outside of the “L” are considered
groove welds. Corner joints are quite easily assembled and
require little if any joint preparation. After welding, the welds
are generally finished, that is, ground smooth to present a
smooth attractive appearance. When this is the case, all effort
by the welder should be made to prevent overlap (weld material
rolling onto one of the members and not fusing), high spots,
low spots and undercut. These problems can all mean more
work since additional grinding time, rewelding and regrinding
may be required.
T-joints possess good mechanical strength, especially when
welded from both sides. They generally require little or no
joint preparation and are easily welded when the correct
parameters are used. The edges of the T-joint may be left
square if only a fillet weld is required. For groove welding they
may be altered by thermal cutting/gouging, machining
or grinding.
J-Groove
Fillet T-Joint
Figure 7.8 T-Joints
There are two main types of corner joints, open corner and
closed corner. On lighter gauge material, it may be necessary
to increase travel speed somewhat, especially on open corner
joints where excessive melt through is a possibility.
Fillet Welds
Fillet welds are approximately triangular in cross sectional
shape and are made on members whose surfaces or edges
are approximately 90˚ to each other. Fillet welds can be as
strong, or stronger than the base metal if the weld is the correct
size and the proper welding techniques are used. When
discussing the size of fillet welds, weld contour must first be
determined. Contour is the shape of the face of the weld.
Figure 7.9 shows a cross section profile of the three types of
fillet weld contours: flat, convex, and concave.
Open Corner
Flat
Closed Corner
Convex
V-Groove Corner
Figure 7.7 Corner joints.
T-Joints
A T-joint occurs when the surfaces of two members come
together at approximately right angles, or 90˚, and take the
shape of a “T”. See Figure 7.8. On this particular type of joint,
a fillet weld is used.
Concave
Figure 7.9 Fillet face contours.
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Fillet Weld Size
It is important when discussing weld size and joint design
to be familiar with the various parts of a weld. Figure 7.10
indicates the parts of a fillet weld.
Fillet welds can also be measured in a slightly more complex
way — by determining throat size. Three different throat sizes
may be referred to when discussing the size of fillet welds, as
seen in Figure 7.11 and 7.12.
Design engineers sometimes refer to the theoretical throat of
a weld. As Figure 7.11 and 7.12 show, the theoretical throat
extends from the point where the two base metal members
join (beginning of the joint root), to the top of the weld minus
any convexity on the convex fillet weld and on the concave fillet
weld, to the top of the largest triangle that can be inscribed in
the weld. The theoretical measurement looks at the weld as if
it were an actual triangle and the penetration is not figured
into the theoretical throat size.
Toe
Leg
Face
Throat
Convexity
Toe
Leg
Root
Fillet Weld Terms
Figure 7.10 Convex fillet weld.
The effective throat of a fillet weld is measured from the depth
of the joint root penetration. This is an important consideration
as the penetration is now considered part of this dimension.
However, no credit is given for the convexity. (The convexity
by many is considered reinforcement, which would indi-
cate more strength. The exception is a fillet weld where too
much convexity is detrimental to the overall joint strength.
Excess convexity increases stresses at the weld toes and
can lead to cracking.) On convex and concave fillet welds,
effective throat is measured to the top of the largest triangle
that can be drawn in the weld. This measurement can be used
to indicate the size of the weld. The outward appearance of
the weld may look too small but if the penetration can be
assured, the weld will be of sufficient strength.
The size of a convex fillet weld is generally the length of the
leg referenced. Figure 7.11 shows a convex fillet weld and the
associated terms.
The actual throat of a fillet weld is the same as the effective
throat on a concave fillet weld. But as can be seen on Figure 7.11,
there is a difference. This throat dimension can also be used to
indicate size and strength. If anything other than the theoretical
throat is used to size a fillet weld, the welding procedure would
have to be carefully written and in-process inspection would
be required to assure that the joint is being properly pene-
trated. The overall reduction in fillet weld size, increased
speed of welding, reduced heat input and reduction of internal
stresses and distortion may make the effort worthwhile.
Figure 7.11 Convex fillet weld.
For concave fillet welding, the size and leg are two different
dimensions. The leg is the dimension from the weld toe to the
start of the joint root, however, the actual size of a convex fillet
weld as shown in Figure 7.12, is measured as the largest
triangle that can be inscribed within the weld profile. A special
fillet weld gauge is used to measure concave fillet welds. If the
weld is flat, the concave or convex fillet weld gauge can be used.
The general rule for fillet weld size is the leg should be the
same size as the thickness of the metals. If 1/4" thick plate
is being welded, a 1/4" leg fillet is needed to properly join the
members. The old saying, “If a little is good, a lot is
better,” may be true in some cases but not with fillet welds.
Consider again the 1/4" thick plate. If a lot of weld would be
better, think of 1/2" legs on the fillet. This would result in what
is termed over-welding. This weld is not just twice as large as
required, but its volume is three times that required. This
wastes weld metal, the welder’s time, causes more distortion
and may even weaken the structure because of residual
stress. Figure 7.13 shows correct and incorrect fillet welds.
Figure 7.12 Concave fillet weld.
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1/4"
A
1/4"
A
1/4"
1/4"
1/4"
1/2"
Correctly Made Fillet Weld.
Leg Equals Thickness Of Plate
1/2"
Unequal Leg Fillet (Best
Procedure, Least Possible
Chance Of Joint Failure)
B
1/2"
1/4"
1/4"
1/2"
B
1/2"
Over Welded (Base Metal Will
Break At Toes Of Weld) Legs
Of Weld Too Large For
Thickness Of Plate
1/2"
C
1/2"
Equal Leg 1/2" Fillet (Wasted Weld
Metal, Time And Extra Heat Input)
Weakest Point Will Be At The Toe
Of The Weld On The 1/4" Plate
1/8"
1/4"
1/4"
1/8"
Under Welded (Weld May
Break Through Legs Of Weld)
Need Larger Legs On Fillet
C
1/4"
Figure 7.13 Correct/incorrect fillets.
A weld or weld joint is no stronger than its weakest point.
Even though B of Figure 7.13 would appear to be much
stronger, it will not support more stress than A. It may even
support less stress due to the additional residual stresses
built up in the joint that is over-welded.
1/2"
1/4"
Equal Leg 1/4" Fillet (Less Time,
Less Weld Metal Less Heat Input
Equals Better Weld) Just As
Strong As Figure B
When metals of different thicknesses are to be joined, such
as welding a 1/4" thick plate onto a 1/2" thick plate in the form
of a T-joint, the rule for fillet weld size is size of fillet weld leg
should equal the thickness of the metal being welded .
Since there are two different thicknesses, the best weld
results will be obtained by making an unequal leg fillet weld.
Figure 7.14 shows correct and incorrect examples.
Figure 7.14 Unequal leg fillet.
The correct, unequal leg fillet weld has a 1/4" weld leg on the
1/4" plate and a 1/2" weld leg on the 1/2" plate. This would be
the best way to handle this weldment. However, consider the
results of making the weld with an equal leg fillet. There
would then be two choices: a 1/2" fillet or a 1/4" fillet. In this
instance, the 1/4" fillet would be the more practical, since a
weldment is no stronger than its weakest point. The extra
welds in the 1/2" fillet will also require more time, electrode
wire, and induce more heat into the metal, causing more
residual stress.
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