Design and Metallurgy of Weld Joints
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Dr. Chaitanya Sharma
In this chapter we shall discuss the following:
1. Structure of fusion welds;
2. Thermal effect of welding on parent metal;
3. Effect of cooling rate;
4. Weld metal solidification and heat affected zone;
5. Heat flow - temperature distribution-cooling
6. Influence of heat input; Joint geometry; Plate
7. Significance of thermal severity number;
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What Is Metallurgy ?
• The science of joining metals by welding is closely relates to
the field of metallurgy.
• Metallurgy involves science of:
– Producing metals from ores,
– Making and compounding alloys,
– Metal reactions, Heat treatment,
– Steel making and
– Processing of metals e.g. Forging, Foundry etc.
• Welding metallurgy can be considered a special branch,
since reaction times are very small (minutes to fraction of
seconds), while in other branches reactions are large (h-m).
• Welding metallurgy deals with the interaction of different
metals and interaction of metals with gases and chemicals
of all types.
Why Welding Metallurgy?
• Welding metallurgist will examine the changes in
physical characteristics that happen in short periods.
• The solubility of gases in metals and between metals
and the effect of impurities are all of major importance
to the welding metallurgist.
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Basic Structure of Fusion Welds
• A typical fusion welded joint varies in metallurgical
structure due to melting and solidification with very high
• In general, a weld can be divided in four different zones
as shown schematically in fig. namely:
1. Fusion zone,
2. Weld Interface,
3. Heat affected Zone and,
4. Base material
Structure of Fusion Weld Joints
• The fusion zone (FZ) can be characterized as a
mixture of completely molten base metal (and filler
metal if consumable electrodes are in use) with high
degree of homogeneity where the mixing is primarily
motivated by convection in the molten weld pool.
• The main driving forces for convective heat transfer
and resulting mixing of molten metal in weld pool are:
(1) Buoyancy force,
(2) Surface tension gradient force,
(3) Electromagnetic force,
(4) Friction force.
Structure of Fusion Weld Joints
• The weld interface, (or mushy zone), is a narrow zone consisting
of partially melted base material which has not got an
opportunity for mixing. This zone separates the fusion zone and
heat affected zone.
• The heat affected zone (HAZ) is the region that experiences a
peak temperature that is well below the solidus temperature
while high enough that can change the microstructure of the
material and mechanical properties also change in HAZ.
• The amount of change in microstructure in HAZ depends on the
amount of heat input, peak temp reached, time at the elevated
temp, and the rate of cooling.
• The unaffected base metal zone surrounding HAZ does not
undergo any change in microstructure and is likely to be in a state
of high residual stress, due to the shrinkage in the fusion zone.
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Various Regions In Fusion Weld
& Corresponding Phase Diagram
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Zone 7: UBM
Zone 6: Tempered HAZ
Zone 5: Intercritical HAZ
Zone 4: Fine grain HAZ
Zone 3: Coarse grain HAZ
Zone 2: Unmixed zone + FZ
Zone 1: Solidified weld
𝛼 + 𝐹𝑒3 𝐶
γ + 𝐹𝑒3 𝐶
• The fusion zone and heat affected zone of welded
joints can exhibit very different mechanical
properties from that of the unaffected base metal as
well as between themselves. For example, the fusion
zone exhibits a typical cast structure while the heat
affected zone will exhibit a heat-treated structure
involving phase transformation, recrystallization and
grain growth. The unaffected base metal, on the
other hand, will show the original rolled structure with
a slight grain growth.
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Fig: Schematic illustration of various regions in a fusion weld zone (and the
corresponding phase diagram) for 0.30% carbon steel. Source: AWS
Weld Joint Structure
Fig: Characteristics of a typical fusion-
weld zone in oxyfuel-gas and arc
Fig: Grain structure in (a) deep weld and (b) shallow
weld. Note that the grains in the solidified weld metal
are perpendicular to their interface with the base metal
(see also Fig. 10.3). (c) Weld bead on a cold-rolled
nickel strip produced by a laser beam. (d)
Microhardness (HV) profile across a weld bead.
Fig: Intergranular corrosion of a 310-stainless-
steel welded tube after exposure to a caustic
solution. The weld line is at the center of the
photograph. SEM micrograph at 20 X.
Source: Courtesy of B. R. Jack, Allegheny
Ludlum Steel Corp.
The Fusion Zone
• Similar to a casting process, the
microstructure in the weld zone is
expected to change significantly due to
remelting and solidification of metal at
the temperature beyond the effective
• However fusion welding is much more
complex due to physical interactions
between the heat source and the base
• Nucleation and growth of the new grains
occur at the surface of the base metal
in welding rather than at the casting mould
Weld Pool Structure
• If the weld pool is quenched,
its microstructures at
different positions can be
revealed, e.g., aluminium weld
pool structure, as shown in fig.
• Microstructure near the
fusion line consists of
– S : Solid dendrite
– L : Interdendritic liquid
• PMM – partially melted
material & partially melted
materials (PMM) and mushy
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Weld Pool Structure
• The mushy zone behind
the shaded area
consists of solid
dendrites (S) and
interdendritic liquid (L).
• Partially melted
consists of solid grains
(S) that are partially
intergranular liquid (L).
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Weld Pool Shape and Grain
• The weld pool becomes teardrop shaped at high welding
speeds and elliptical at low welding speeds.
• Since the columnar grains tend to grow perpendicular to the
weld pool boundary, therefore the trailing boundary of a
teardrop shaped weld pool is essentially straight whereas
that of elliptical weld pool is curved.
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• Axial grains can also exist in the
fusion zone, which initiate from
the fusion boundary and align along
the length of the weld, blocking
the columnar grains growing
inward from fusion lines.
Note: Axial grains has been
reported in Al alloys, Austenitic
stainless steels and indium alloys
Effect of Welding Parameters
on Weld Pool Shape
• As the heat input Q and welding
speed V both increase, the weld pool
becomes more elongated, shifting
from elliptical to teardrop shaped.
• The higher the welding speed, the
greater the length–width ratio
becomes and the more the geometric
center of the pool lags behind the
• Quenching weld pool during welding
resulted in sharp pool end of a
teardrop-shaped weld pool.
Fig: weld pools traced from photos
taken during autogenous GTAW of
304 stainless steel sheets 1.6 mm thick.
Welding parameters have more significant
effect on pool shape in stainless steel
welding than aluminum welding.
The much lower thermal conductivity of
stainless steels makes it more difficult for the
weld pool to dissipate heat and solidify
Fig: Sharp pool end observed in autogenous
GTAW of 1.6-mm 309 stainless steel
I = 85A, V=10V, WS = 4.2mm/s
Effect of Welding Speed on
GTAW of 99.96% aluminium at welding
speed of (a) 1000 and (b) 250 mm/min
Axial grains of GTAW (a) 1100 aluminium
at 12.7 mm/s welding speed, (b) 2014
aluminium at 3.6/s welding speed
WS 1100 mm/min
WS 250 mm/min
WS 12.7 mm/min
WS 3.6 mm/min
Effect of Heat Input on Weld
• A slight tendency for
the element C, Mn, Si
to decrease (in the
composition of the
weld) when the heat
• Typical macro
different heat inputs
Thermal Severity Number
• Thermal severity number defines the total thickness of
the plate through which heat could flow away from the
weld. TSN helps in detecting cracking susceptibility.
• TSN is Usually given as ‘Total Thickness’ in millimeters.
• The total thickness is the sum of thickness of all the
paths along which heat can be conducted.
• Heat flow may be along two, three and four path as
• This method can not be applied to complex shape or
made to allow effect of jigging.
• TSN can be calculated
– For bithermal welds TSN = 4 (t + b)
– For Trithermal welds, TSN = 4 (t + 2b)
where: t and b are thickness of the top and bottom plate
• A series of plate thickness which provided varying cooling rates are
tested. The crack susceptibility of the base metal – filler material
combination is determined by the minimum TSN that produces cracking.
• Controlled thermal severity testing is used to measure the cold crack
sensitivity of steels under cooling rates controlled by thickness of the
• CTST specimen consists of a square plate bolted and anchor welded to a
larger rectangular plate. After the anchor welds have cooled to room
temperature, two test welds are made on the specimen. Fillet weld
along the plate edges is controlled by the thickness of the plates and the
differences in cooling rates between bithermal and trithermal welds. This
test is primarily used to evaluate the crack sensitivity of hardenable
Thermal Severity Number