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STEEL CONSTRUCTION:
FABRICATION AND ERECTION
Lecture 3.3: Principles of Welding
OBJECTIVE/SCOPE
To present an overall view of the implications of making joints
by welding.
PREREQUISITES
Lectures 3.1: General Fabrication of Steel Structures
RELATED LECTURES
Lecture 3.4: Welding Processes
SUMMARY
This lecture describes the basic principles involved in making a
welded joint. It discusses the structure and properties of both the
weld metal and the heat affected zone. It explains the necessity
for edge preparations when butt welding, and gives examples of the
types used. It outlines how the welding procedure can be varied to
meet the needs of the particular joint being made.
ABBREVIATIONS
MAG Metal Active Gas Welding (sometimes referred to as MIG Metal
Inert Gas Welding)
MMA Manual Metal Arc Welding
SAW Submerged Arc Welding
HAZ Heat Affected Zone
1. INTRODUCTION Welding offers a means of making continuous,
load bearing, metallic joints between the components of a
structure.
In structural work, a variety of welded joints are used; these
can all be made up from the basic configurations shown in Figure 1,
which are classified as follows:
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butt joints.
tee joints. lap joints. corner joints.
As illustrated in Figure 2, a welded joint is made by fusing
(melting) the steel plates or sections (the parent metal) along the
line of the joint. The metal melted from each member at the
joint
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unites in a pool of molten metal which bridges the interface. As
the pool cools, molten metal at the fusion boundary solidifies,
forming a solid bond with the parent metal, see Figure 3. When the
solidification is complete, there is continuity of metal through
the joint.
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2. METHODS OF MAKING A WELDED JOINT Two types of weld are in
common use: butt welds and fillet welds. In the former the weld
metal is generally contained within the profiles of the welded
elements; in the latter, deposited weld metal is external to the
profile of the welded elements.
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Obviously the complete length of joint cannot be melted
simultaneously. In practice a heat source is used to melt a small
area and is then moved along the joint line, progressively fusing
the parent metal at the leading edge of the weld pool, as shown in
Figure 4. At the same time, the metal at the trailing edge of the
pool solidifies. The most commonly used heat source, in structural
work, is a low voltage (15 to 35 volt), high current (50 to 1000
amp) arc. As shown diagrammatically in Figure 5, the arc operates
between the end of a steel electrode (rod) and the work piece. It
melts both the parent metal and the electrode; molten metal from
the electrode is thereby added to the weld pool.
The molten steel in the pool will readily absorb oxygen and
nitrogen from the air, which could lead to porosity in the
solidified weld and possibly to metallurgical problems. Figure 6
shows how this is avoided by covering the pool with a molten flux,
as in Manual Metal Arc (MMA) and Submerged Arc Welding (SAW), or by
replacing the air around the arc by a non-reactive gas, as in Metal
Active Gas (MAG) Welding or cored wire welding.
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3. STRUCTURE AND PROPERTIES OF WELDS The solidified weld metal
has a cast structure and has properties characteristic of cast
steel, i.e. higher ratio of yield to ultimate strength than
structural steel. The weld metal is a mixture of parent metal and
steel melted from the electrode. In structural work the composition
of the electrode is usually chosen so that the resultant weld metal
is stronger than the connected elements. Occasionally, specific
conditions may override this chocie. For example, when joining
stainless steel to carbon-manganese steel, a highly alloyed
electrode must be used to avoid cracking in the weld metal.
When the weld pool is cooling and solidifying, the majority of
the heat flows through the parent metal alongside the joint. The
steel is thus subjected to heating and cooling cycles similar to
those experienced in heat treatment practice. As shown in Figure 7,
the structure of the steel will be changed in this region (called
the heat affected zone, HAZ). This must be taken into account in
the design in terms of notch toughness (Charpy value), etc.
The structure of the HAZ will be controlled by:
the composition of the steel (carbon equivalent). the cooling
rate in the HAZ.
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In turn the cooling rate is determined by:
arc energy, i.e. heat input to the joint. type of joint.
thickness of steel. temperature of steel plate or section prior to
welding, e.g. preheat.
A method of determining the interaction of these factors in
relation to the avoidance of cracks in the HAZ is given in the
sample chart shown in Figure 8.
In addition to its effect on the cooling rate, preheat is used
to:
Disperse hyrodgen from the weld pool and HAZ. Hydrogen in the
HAZ increases the risk of cracking if hardening has occurred. The
hydrogen comes principally from the flux. An appropriate electrode,
correctly stored, will reduce the risk of hydrogen pick-up.
Remove surface moisture in high humidity conditions or on site.
Bring the steel up to 'normal' ambient conditions (20C).
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4. EDGE PREPARATION FOR BUTT WELDS For square edge preparations
the depth of melting into the plate is called the Depth of
Penetration, see Figure 9a. As a very rough guide, the penetration
is about 1mm per 100 amp. In manual welding the current is usually
not more than 350 amp; more commonly 150-200 amp. This means that
the edges of the plate must be cut back along the joint line for
continuity through the thickness to be achieved (Figure 9b). The
groove so formed is then filled with metal melted from the
electrode (Figure 9c). Various edge profiles are used and are
illustrated in Figure 10; the edges may be planed, sawn,
guillotined or flame cut.
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The first run to be deposited in the bottom of the groove is
called the root run. The root faces must be melted to ensure good
penetration, but at the same time the weld pool must be controlled
to avoid collapse, as seen in Figure 11. This task requires
considerable skill. The difficulties can be eased by using a
backing strip.
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The choice of edge preparation depends on:
type of process. position of welding (Figure 12). access for arc
and electrode. volume of deposited weld metal which should be kept
to a minimum. cost of preparing edges. shrinkage and distortion
(Figure 13).
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5. WELDING PROCEDURES The term welding procedure is used to
describe the complete process involved in making a weld. It covers
choice of electrode, edge preparation, preheat, welding parameters
(voltage, current and travel speed), welding position, number of
weld runs to fill the groove, and post-weld treatments, e.g.
grinding or heat treatment. Welding procedures may be devised to
meet various needs, e.g. to minimise costs, control distortion,
avoid defects or achieve good impact properties. Specific aspects
of the weld procedure are worth detailed comment.
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5.1 Current
The current controls heat input. The minimum value is fixed by
the need to fuse the plate and to keep the arc stable; the
specified minimum, however, may be higher to avoid HAZ cracks. The
maximum current depends on operating conditions. Usually, as high a
current as possible is used to achieve faster welding, and hence
lower costs. The use of maximum current may be restricted by
position; in the overhead position, for example, currents above 160
amps cannot be used. High currents usually give low impact
properties. Note that the current used is chosen to match the
electrode diameter.
5.2 Welding Position
The effect of position on current is noted above. Welding in the
overhead position requires greater skill to avoid defects, such as
poor profile, and should only be used when absolutely necessary.
Vertical welding is slower than welding in the flat position but
requires less skill than the overhead position.
5.3 Environment
If on site welding is necessary the following points must be
considered:
in cold weather the steel may need to be heated to bring it up
to 20C. overnight condensation and high humidity can lead to
porosity. care must be taken to ensure the electrodes are kept dry
in the stores. it is often difficult to achieve accurate fitting of
the joint; variable and/or large gaps
may result in defective welds, distortion and increased
costs.
6. SHRINKAGE During cooling, the hot metal in the weld zone
contracts, causing the joint to shrink. The contraction is
restrained by the cold metal surrounding the joint; stresses are
set up which, being in excess of the yield stress, produce plastic
deformation. This can lead to the distortion or buckling shown in
Figure 13. Distortion can be reduced by choice of edge preparation
and weld procedure; examples are shown in Figure 14.
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When the plastic deformation has ceased, the joint is left with
the residual stress pattern of Figure 15 with tension in the weld
metal and HAZ, and compression in the surrounding steel. The
significance of these residual stresses is discussed in other
lectures.
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7. CONCLUDING SUMMARY A welded joint is made by fusing parent
metal from both components being joined,
usually with added weld metal. The properties of both the weld
metal, which has melted and solidified, and the
surrounding heat affected zone may differ from those of the
parent metal. Welding procedures should be properly specified to
give a satisfactory welded
joint. The major parameters are: welding position, electrode
type, edge preparation, preheat, voltage, current, travel speed,
number of runs and post-weld heat treatments.
Hot metal in the weld zone contracts during cooling causing
residual stresses. Distortion will occur if appropriate control is
not exercised.
8. ADDITIONAL READING 1. Hicks, J. "Welding Design",
Granada.
details of joints and welds.
strength of welded joints.
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effects of welding on metallurgical structures, heat affected
zones, HAZ cracking.
edge preparation.
welding positions - definitions and comments.
2. Gourd, L. M. "Principles of Welding Technology", Edward
Arnold, 1980.
formation of a weld.
types of heat source.
strength of welded joints.
effects of welding on metallurgical structure, heat affected
zones, HAZ cracking.
edge preparation.
comments on residual stresses.
control of distortion.
3. Milner, D.R. and Apps, R. L. "Introduction to Welding and
Brazing", Pergamon.
effects of welding on metallurgical structure, heat affected
zones, HAZ cracking.
control of distortion.
4. Pratt, J. L. "Introduction to the Welding of Structural
Steelwork",
Steel Construction Institute - Publication No 014.
5. British Standard BS 5135: 1986, "Metal Arc Welding of Carbon
and Carbon Maganese Steels", British Standards Institution,
London.
Lecture 3.3: Principles of
WeldingOBJECTIVE/SCOPEPREREQUISITESRELATED
LECTURESSUMMARYABBREVIATIONS1. INTRODUCTION2. METHODS OF MAKING A
WELDED JOINT3. STRUCTURE AND PROPERTIES OF WELDS4. EDGE PREPARATION
FOR BUTT WELDS5. WELDING PROCEDURES5.1 Current5.2 Welding
Position5.3 Environment
6. SHRINKAGE7. CONCLUDING SUMMARY8. ADDITIONAL READING