Welding metallurgy part i

WELDING METALLURGY
ME 473 WELDING TECHNOLOGY
Instructor: Assist.Prof.Dr. Oğuzhan Yılmaz

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Basic Metallurgy
 The science of joining metals by welding that relates closely to the field of
metallurgy.
 Metallurgy involves the science of producing metals from ores, of making
and compounding alloys, and the reaction of metals to many different
activities and situation.
 Heat treatment (heating and cooling of metals to obtain desired
shapes and mechanical properties)
 Steel making and processing
 Forging
 Foundry
 Welding metallurgy can be considered a special branch, since reaction
times are in the order of minutes, seconds, fraction of seconds, whereas in
the other branches reactions are in hours and minutes.
 Welding metallurgy deals with the interaction of different metals and
interaction of metals with gases and chemicals of all types.
Dr. Oğuzhan Yılmaz
Welding Technology

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 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.
Basic Metallurgy
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 The structure of metal is complex. When metal is in a liquid state, usually
hot, it has no distinct structure or orderly arrangement of atoms. So that
atoms move freely since they have high degrees of mobility due to the
heat energy involved during melting process.
 As the metal cools, atoms loose their energy and their mobility. When
temperature is further reduced, the atoms are no longer able to move and
attracted together into definite patterns.
 These patterns consist of three-dimensional lattices, which are made of
imaginary lines connecting atoms in symmetrical arrangements.
Basic Metallurgy_Crystalline structures
 Metals in a solid state possess this uniform
arrangements, which is called crystals. All metals are
crystalline solids made of atoms arranged in a
specific uniform manner.
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 There are three common types
of lattices;
(1) The face-centered cubic
(2) The body-centered cubic
(3) The hexagonal close-packed
Iron has both FCC and BCC
structures but at different temp.
This is know as ‘allotropic
change’.
The crystal lattices are only for pure
metals that are composed of
one type of atom. However,
most metals that are common
use are alloys (more than one
metal).
In alloys, the crystals will change.
According to the portion of the alloy,
there are three types of formation
occur:
(1) substitutional solid solution.
(2) interstitial solid solution and
(3) intermetallic compounds.
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 Substitutional solid solution: the atoms of the metal making up
the minor portion of the alloy will at random replace some of
the atoms of the metal making up the majority of the alloy.
 Interstitial solid solution: The atoms of the minor metal in the
alloy are much smaller than those in the major lattice, they do
not replace the atoms of the major metal in the lattice but
rather locate in points between or intervening spaces known
as interstices in the lattice.
 Intermetallic compounds: the minor metal atoms in the alloy
cannot completely dissolve either interstitially or
substitutionally. They will form the type of chemical compound
the composition of which corresponds roughly to the chemical
formula. This results in the formation of mixed kinds of atomic
groupings consisting of different and complicated crystalline
structure. [Fe3C, Cementite,Iron-Carbide]
Each group with its own crystalline structure is referred to as a
phase.
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 Different alloys, solid solutions, intermetallic compounds, and phases occur
as the molten metal solidifies.
 Solidification occurs in all direction which are normal to the nuclei crystal
that is a small crystal form. For a cubic crystal, growth progress is in six
direction simultaneously. Growth is simply the adding on of additional
crystals as tempereture decreases.
GRAIN
When the resultant structure is cut in
a flat plane, the individual dentritic
crystals, which grew until they met
adjacent dentritic crystals, form an
irregularly shaped area, known as a
‘grain’. Grains have boundaries and
are very small but much larger than
the individual crystals
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 The size of the crystals and grains depends on the rate of growth of the
crystal. The rate of crystal growth depends on the rate of cooling of the
molten solidifying metal.
 When the rate of cooling is high, the solidification process occurs more
rapidly and the crystal size and graing size tend to be smaller and vice
versa. (‘snow’ example)
 Metal structures can be characterized as having large grains (coarse
grained) or small grains (fine grained) or a mixture of large and small
grains (mixed grain).
 The arrangement of atoms is irregular in the grain boundaries, and there
are vacancies or missing atoms. The atom spacing may be larger than
normal, and individual atoms can move easily in the grain boundaries;
because of this, the diffusion of elements, which is the movement of
individual atoms through the solid structure, occurs more rapidly at grain
boundaries.
Basic Metallurgy_Grains
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Microstructure
 The overall arrangement of grains, grain boundaries, phases present in an
alloy is called its microstructure. It is largely responsible for the properties
of the metal.
 The microstructure is affected by the composition or alloy content and by
other factors such as hot or cold working, straining, heat treating etc.
 The microstructure of weld metal and adjacent metal is greatly
influenced by the welding process, which influence the properties of
the weld.
Basic Metallurgy_Microstructures
Microstructure of a weld used in stainless steel Microstructure of base metal of the same stainless steel
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 Some metals change their crystallographic arrangement with changes in
temp. Iron has a BCC lattice structure from room temp. up to 910ºC, and
from this point to 1388 ºC it is FCC. Above this point to melting point, 1538
ºC it is again BCC. This change is called as phase transformation or
allotropic transformation. Like, titanium, zirconium and cobalt.
 Transformation occurs when metal melts or solidifies;
In melting, arrangement of atoms disappears and atoms
move randomly.
In solidifiying, crystalline arrangement reestablish itself.
 Pure metals melts or solidify at a single temperature, while alloys solidify
or melt over a range of temperature with a few exceptions.
 Phase changes can be related to alloy compositions and temp when they
are in equilibrium, and shown on a diagram (known as phase diagrams,
alloy equilibrium diagrams or constitution diagrams).
Basic Metallurgy_Phase transformation
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 Equilibrium diagrams are used to determine the phases that are present
and the percentage of each, based on the alloy composition at a temp.
And changes by increasing and decreasing temp. Most of them are
designed for alloy system containing two elements.
 In welding because of rapid changes in temperatures, equilibrium
conditions are rarely occur. In an equilibrium condition, the metal is stable
at the particular point on the diagram based on relatively slow heating and
cooling.
Basic Metallurgy_Phase transformation
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 Iron-carbon equilibrium diagram provides an insight of the behaviour of
steels in connection with welding thermal cycles and heat treatment. This
diagram represents the alloy of iron with carbon, ranging from 0% to 5%
carbon.
Basic Metallurgy_Iron-Carbon diagram
0.25
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 Pure iron is relatively weak but ductile metal. When carbon is added in
small amounts, the iron acquires a wide range of properties and uses and
becomes the most popular metal, ‘steel’.
 0% carbon, pure iron,
above 1540ºC, in liquid state, no crystalline structure
< 1540 ºC, solidification starts, BCC structure, Delta iron
< 1400 ºC, transformation occurs, FCC structure, Gamma iron
< 910 ºC, iron back to BCC, alpha iron until room temp
 Iron and carbon form a compound known as iron carbide (Fe3C) or
cementite.
 When iron carbide or cementite is heated above 1115 ºC, it decomposes
into liquid iron saturated with graphite, which is a crystalline form of
carbon.
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 Ferrite This phase has a Body Centre Cubic structure (B.C.C) which can
hold very little carbon; typically 0.0001% at room temperature. It can exist as
either: alpha or delta ferrite.
 Austenite This phase is only possible in carbon steel at high
temperature. It has a Face Centre Cubic (F.C.C) atomic structure which can
contain up to 2% carbon in solution.
 Cementite Unlike ferrite and austenite, cementite is a very hard intermetallic
compound consisting of 6.7% carbon and the remainder iron, its chemical
symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite
layers its average hardness is reduced considerably.
 Pearlite A mixture of alternate strips of ferrite and cementite in a single
grain. The name for this structure is derived from its mother of pearl
appearance under a microscope. A fully pearlitic structure occurs at 0.8%
Carbon. It is a lamellar structure, which is relatively strong and ductile.
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Ferrite
Pearlite
Austenite
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 Consider a steel with a composition of 0.25% carbon. A vertical line is
drawn up at this point;
 Above 1520ºC, the steel is molten, as the temp decreases, delta iron start to
form in the liquid.
 Just below 1500 ºC, transformation to austenite and molten metal.
 At about 1480 ºC, all the liquid metal solidifies and the form is austenite.
 Approx. 815 ºC, the austenite begins to breakdown and form a new phase,
ferrite.
 Ferrite formation continues until a temp 727 ºC
 At 727 ºC, the remaining austenite structure would disappear completely and
transforming to a structure known as pearlite+ferrite
 In welding the rise and fall of temp or the rate of change of temp is so fast
that equilibrium does not occur. Therefore, aforementioned structures will
be different.
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 At fast cooling rates, the austenite might not have sufficient time to
transform completely to ferrite and pearlite and will provide a different
microstructure. In this case, some of the untransformed austenite will be
retained and the carbon is held at supersaturated state. This new structure
is called ‘martensite’.
 If the cooling rate is sufficiently fast, the austenite might transform
completely martensite. It is harder than pearlite or ferrite-pearlite structure
and it has lower ductility.
Basic Metallurgy_Martensite formation
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 Hardness mainly depends on the carbon content but cooling rate also
influences the microstructure and causes higher hardness. This is
because the crystal lattice is changed or distorted and this hardens the
material.
 By adding different alloys to the steel, the tendency of austenite to
transform into martensite upon cooling increases, which is the basis of
hardening steels. Carbon, manganese, chromium, molybdenum etc.
 The amount of alloys and their power to create this microstructure
transformation are known as hardenbility.
 Grain size and microstructure relate directly to hardness and strength.
Fine grain size promotes both increased in strength and hardness.
 This is an advantage for heat treatment but it can be detrimental to
welding since high hardness is not desired in welds of softer materials.
Basic Metallurgy_Hardenability
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 The heat treatment of steels to increase hardness and the metallurgy of
welding have much in common.
 Most steels possess the property of hardenability, which is defined as the
property that determines the depth and distribution of hardness induced by
quenching, and this property can be measured by the ‘quench-test’, that is
used to plot hardness value from quenched end to unquenched end.
Basic Metallurgy_Hardenability
The quench-test and the
information obtained provides
usefull data for welding since it
indicates the effect of different
alloying elements on the
hardness of the quenched
steel. The microstructure of the
quenched steel can also be
studied and related to the
microstructure of welds.
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 When a weld is made, following factors occur:
 The changes of temperature
 The growth of dimensions
 The phase transformation etc.
 The rate of cooling or quench is of primary importance and this is
controlled by the process, procedure, metal and mass.
Welding Metallurgy
 Example: The electroslag has the
lowest cooling rate among welding
methods, while the gas metal arc
has a much faster cooling rate.
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 The rate of change decreases as the distance from the center of the weld
increases.
Welding Metallurgy
 It is obvious that many different
cooling rates occur and that
different microstructures will result.
Also different phases occur in the
base metal adjacent to the weld.
(a) Mixture of ferrite and pearlite
grains
(b) Pearlite transformed to Austenite
(c) Full Austenite transformation
(d) Completely liquid state
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 In addition to the complications created by the rapid cooling, there is also
the complication of composition variations.
 As weld metal is deposited on a base metal, some of the base metal melts
and mixes with the weld metal, producing a dilution of metal.
 If the compositions of the weld metal and the base metal are not identical,
variation of composition at the interface can be observed and also it
causes variation of cooling rates. This results variation of microstructures.
Welding Metallurgy
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Welding Metallurgy
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 Each microstructure has its particular characteristics and one of the
important characteristics is the hardness of the microstructure throughout
the weld area.
Welding Metallurgy
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 The area between the interface of the deposited weld metal, and
extending into the base metal far enough that any phase change occurs, is
know as the heat-affected-zone (HAZ).
 HAZ is a portion of the weld since it influences the sevice life of the weld.
 HAZ is the most critical in many welds. For instance, when welding a
hardenable steel, HAZ can increase in hardness to an undesirable level.
When welding a hardened steel, HAZ can become a softened zone since
the heat of the weld has annealed the hardended metal.
Welding Metallurgy_Heat affected zone
Heat-affected-zone (HAZ)
weld
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 It may occur in two possible ways, (1) migration of oxides along the grain
boundaries rendering them weak. (2) oxidation as in oxygen cutting.
 Protections are carefully supplied to exclude the atmosphere from the
high-temperature welding regions. Protective agents are usually in the
form of inert gases, fluxes, and electrode coatings.
Metallurgical problems in welding_Burning
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 Segregation is one of the important factor that should be considered. It
relates the solubility of elements in metals, particularly alloys.
 For instance, the composition of the first crystals that form as an alloy
freezes is different from the composition of the liquid that freezes last.
 In weld metal, because of the rapidity of freezing time, very little diffusion
occurs and there is a lack of homogeneity in the total weld.
Carbon, phosphorus, sulfur and sometimes manganese are frequently in the
segregated state in steel. This can be determined by high-magnification
study of the microstructure.
Metallurgical problems in welding_Segregation

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 Molten metal has a relatively high capacity of dissolving gases in contact
with it. As the metal cools it has less capacity for dissolved gases, and
when going from liquid to solid state the solubility of gas in metal is much
lower.
 The gas is rejected as the crystals solidify, but it may be trapped because
of almost instantaneous solidification. Entrapment of the gas causes gas
pockets and porosity in the weld.
 Carbon monoxide, which is present in many arc and fuel gas
atmospheres, is sometimes trapped. Hydrogen can also be trapped but it
may gradually disperse and escape from the weld metal over a period of
time. High temp increases the speed for hydrogen migration and removal.
 The inert gases are not soluble in molten metal and for this reason, they
are used in many gas shielded applications.
 The solubility of metals within metals is also crucial. The greater the
degree of solubility, the better the success of welding dissimilar metal
combinations.
Metallurgical problems in welding_Gas pockets

Welding metallurgy part i

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