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Lec 39 – Cracking of Welded Joints I: Solidification and liquation Cracks


hello i welcome you all in this presentation
this presentation is based on the topic the cracking of the welded joints since number
of types of the cracks are observed in the welded joints either during the welding or
after the welding so this topic has been divided into the two parts in one part we will be
talking about cracks observed in the weld metal and then also i will talk about the
cracks observed in the heat affected zone so we will start with the – the cracking of
welded joints and the different types of the metal systems are found sensitive for the
different types of the cracks which may be in the either heat affected zone or in the
weld metal so lets say the component like this joint using the fusion welding process
so in this case we may have cracks in the different locations which may be here like
the toe of the weld in form of toe cracks – toe crack or it may be very near to the fusion boundary
like this so this is haz cracks or crack maybe in the – in the weld metal itself so the weld
metal cracks we can say these are the three areas where the cracks can occur in addition
to them if we see the same thing in the top view and in this one this is the location
here weld joint is ending so we may find the cracks also in the crater
where the – the – the – the weld pool in the last stage of the welding where the weld pool
are solidifies this is called crater its some kind of depression of the circular family
and it is sensitive for cracks due to the high concentration of the impurities so we
can say the weld metal the toe of the cracks or the cracks in the weld metal or cracks
in the heat affected zone these are the – the that area wise the cracks which are observed so depending upon the stage when they occur
depending upon the location where they occur the different names have been given and these
cracks developed due to some of the regions which are very predominant and among these
like unfavorable – unfavorable mechanical properties of the either weld or heat affected
zone so unfavorable mechanical properties of the
weld metal or of the heat affected zone includes like extremely high hardness which makes it
brittle high yield strength which doesnt allow yielding but facilitates the crack initiation
and its growth so high yield strength and very very limited ductility not permitting
any yielding so limited ductility and low toughness in general toughness is very low so these are some of the mechanical properties
which are if unfavorable either in the weld metal or in the heat affected zone then it
will lead to the cracks in the weld in the toe region or in the heat affected zone in
addition to this we may have other points also responsible for the development of such
cracks like unfavorable constituents – unfavorable constituents sometimes presence of very unfavorable constituents
lead to the very high solidification temperature range or very low ductility so this promote
the cracking and these constituents like hydrogen in case of steels sulphur in steels lead in
steels are the common elements which promote the cracking tendency due to the different
regions like solidification temperature range is enhanced by the presence of the sulphur
and the lead while hydrogen reduces the ductility and increases
the cracking tendency due to the cold cracking possibility so in addition to these two types
of the factors we may see that the cracking is also caused by other properties related
with the weld joints and it includes say the presence of the – presence
of the tensile or shear stresses these stresses may be externally applied – externally applied
or this may develop due to the differential expansion and contraction during the welding
so leading to the development of the residual stresses so this also promote the cracking
under the favorable conditions of the weld joints and third is favorable – favorable
micro constituents micro structure these microstructural features or the constituents
like very coarse grains very large size grains promote the cracking and the nucleation of
the cracks is compared to the fine grained structure and needle shaped constituents or
the constituent having the larger aspect ratio or the constituent which are flaky in form
of the films or the layers these features are about the morphology or the shape of the
constituent so means about the grain structure in terms of the phases structure like in case
of the steels the martensite bainite and ferrite these are in the descending order of the tendency
of the cracks maximum cracking tendency is shown by the martensite thereafter somewhat
– somewhat less cracking tendency is executed by the by the bainite and the ferrite in case
of the steels so this we can say in general the four kind of the – the factors if which
promote the cracking tendency in the welded joints so but we will see that the specific types
of the cracks and what are the reasons for their the developments so as i have said the
broadly there are two locations the cracking in weld metal and cracking in the heat affected
zone so cracking in the weld metal in this category mainly we have the solidification
crack which significantly – significantly – which significantly important because it
leads to the rejection of the joints so solidification crack and as we have seen
here the solidification cracks actually run all along the length of the weld at the center
line so solidification cracks are basically the center line crack in the weld metal running
along the – the weld line and the crack crater crack is also observed in the weld metal where
the weld joint ends and the last stage of the – the fusion welding so here it actually the welding terminates
also another type of crack observed in the weld metal is the crater crack and this is
this happens primarily due to the excessive concentration of the alloying elements among
the heat affected zone the cracks which are observed in the heat affected zone very next
to the fusion boundary what we can see here commonly is – is the liquation crack and thereafter we have the heat cracking due
to embrittlement means that steels subjected to the tensile stress in heat affected zone
– tensile residual stresses in the heat affected zone and martensitic transformation takes
place then it leads to the embrittlement and such kind of embrittlement promotes the cracking
in the heat affected zone so – so this is cracking due to the embrittlement
then its cold cracking or cold crack or the hydrogen induced crack commonly known as haz
also termed as haz hydrogen assisted crack and the lamellar tearing – lamellar tearing
is also a crack commonly observed in the heat affected zone due to the decohesion of the
– decohesion of the inclusion from the metallic matrix under the tensile residual stresses so these are the common types of the cracks
and – and the locations where they are observed and based on the – based on the orientation
of the cracks this can be further classified as longitudinal crack or the transverse crack
so the cracks which are running parallel to the direction or along the direction of the
weld are termed as a longitudinal crack or the transverse crack
refer time: 13:04) longitudinal crack like the solidification
crack is a typical example of the – the longitudinal crack the cracks which are run parallel to
the welding direction or the cracks which are next to the fusion boundary running along
the welding direction parallel to the welding direction also can be termed as the longitudinal
cracks so normally crack – cracks in the haz running parallel to the welding directions
can be termed as the transverse cracks sometimes the cracks also develop perpendicular
to the welding direction in the weld as well as the heat affected zone so they are termed
as that transverse cracks so this is an example of the transverse crack and this is the center
line crack or the longitudinal crack this is an example of the longitudinal crack so
this is i can say the based on the location or orientation of the cracks
refer time: 14:35) so orientation or position wise cracks longitudinal
crack say like solidification crack or the transverse cracks
refer time: 14:42) so now i will take up the – the three types
of the cracks one by one and what can be done to overcome the reduced tendency of development
of such kind of cracks so these three cracks are the solidification cracks liquidation
cracks and the lamellar tearing which is observed in the heat affected zone so starting with
the longitudinal sorry solidification cracks refer time: 15:20) so i will be taking – talking about the solidification
crack and then liquation crack and thereafter lamellar tearing so the solidification crack
basically this type of cracks i have talked in earlier lecture also so i will be coming
directly into the remedies which can be used to avoid such kind of cracks we know that
such kind of cracks are observed in the case of the metal systems having the high solidification
temperature range so mostly the stainless steels and aluminium
alloys are found sensitive for such kind of the cracking and this kind of the cracks – cracks
are observed along weld center line so the crack will be running along the welding in
at the center line along the welding direction and it happens basically due to the segregation
of low melting point liquid metal at the center of the weld – at the center of the weld so we have to reduce the segregation of the
such kind of the low melting point liquid metal or we have to avoid the formation of
such kind of the low melting point liquid metal so there is few approaches which are
tried for reducing this solidification cracks so as far as the recognition is concerned
if we see the diagrams refer time: 17:40) this diagram the typical weld metal of the
6061 aluminium alloy showing that crack is running along the center line of the weld
and it happens and – and this kind of crack is formed like between the dendritic systems
where the low melting point liquid metal exist refer time: 18:02) so in order to control such kind of the – such
kind of the cracking the approaches which are used include the favorable developing
the suitable favorable welding conditions developing the suitable grain structure and
controlling the weld metal composition so what can be done in this case for controlling
the solidification cracking is that metal maybe very sensitive for the cracks but if
the residual stresses developed are very less than the such kind of cracking will not be
happening refer time: 18:43) so the – the these three approaches they work
on the different concepts the first one is like favorable or suitable welding condition
so that the residual tensile stresses can be reduced and for this purpose the approaches
that are used one is like using preheat so that the expansion and contraction is by enlarge
differential expansion and contraction can be reduced in order to develop – in order
to reduce the development of the residual tensile stresses or use of the low yield strength filler so
that the maximum residual stresses magnitude being developed can be reduced or reducing
the alpha or the thermal expansion coefficient of the filler metal which is being used or
– or developing the suitable weld geometry so this approach basically works on if the
residual stress magnitude tensile residual stress magnitude being developed in the weld
metal is reduced through either preheating or using the lower yield strength filler or
by developing the suitable weld geometry so instead of the tensile residual stress we
have the compressive residual stresses then that will help to reduce the solidification
crack so as far as the weld bead geometry is concerned
the whenever the weld geometry is concave in the shape like this then it leads to the
increased cracking tendency due to the – due to the setting of the tensile residual stresses
as compared to the another case when the – the weld bead geometry is of the convex type so
concave weld geometry sets in that tensile residual stresses – stresses at the top surface
of the weld bead while the convex one will be setting in the
compressive residual stresses this is what we can see this kind of weld geometry will
be reducing the solidification cracking tendency more as compared to the case when the weld
bead geometry is like this having the concave as the this top surface so now this is how the reducing the tensile
residual stresses through preheat or use of the lower fill yield strength filler or using
suitable weld geometry can help to reduce that tensile stresses and thereby it can help
to reduce the solidification cracking tendency in addition to this developing suitable favorable
conditions if we reduce the degree of restraint during the welding that also will help to
reduce the tensile residual stresses or residual stresses is being developed
refer time: 22:00) another one is the refining the grain structure
if the grain structure of the weld metal is very coarse like this then – then it will
be leading to the segregation of the alloying element at the center more segregation at
the center as compared to the case when the grain structure is fine so basically the coarse
structure increases the concentration of the low melting
point impurities at the boundaries or the grain boundaries because the grain boundary area is limited
in a given volume if they are just three or four number of the grains then the grain boundary
area is limited and this will be resulting in the higher concentration of the impurities
in the grain boundary area and leading to the easier solidification cracking as compared
to the case when the same area is having the large number of the fine grains so fine grains will be resulting in the large
grain boundary area which will be reducing the concentration of the low melting point
things so it is so refinement of the grain – grain structure helps in reducing the concentration
of the low melting point constituents at the grain boundaries and that in turn helps to
reduce the solidification cracking tendency and this what can be achieved through the
inoculation approach or through the arc oscillation so grain boundary refined grain structural
refinement is possible through the inoculation which wherein the alloying elements like titanium
vanadium or zirconium are added in the metal system so they will refine like vanadium and
zirconium or aluminium are added in the steels or the titanium is added in the aluminium
alloys so the weld metal structure is refined magnetic arc oscillation also helps to refine
the grain structure or modify the grain structure so controlling the weld metal composition
in this case basically we tried to manipulate the weld metal composition in such a way that
the solidification temperature range of the weld metal is reduced so if the base metal is actually sensitive
for the cracking like say the base metal composition is sensitive for the cracking and leading
to the solidification temperature range of 100 degree centigrade then will modify the
weld -weld metal composition by adding suitable filler so that the filler – filler and the
base metal when both of the mix up together will be modifying the composition of the weld
metal and which will be changing the microstructure
of which will be changing the solidification temperature range this what we can understand
from this diagram where in like say this is the typical diagram for most of the aluminium
– aluminium alloys where in like say this is the eutectic system and for – for this
concentration of the alloying element of a particular alloying elements say b the solidification
temperature range is high so when filler with the b is higher concentration
of the b is added so weld metal composition is shifted to this location that will have
the somewhat lower solidification temperature range i mean to say the selection of the filler
metal in such a way is made in such a way that the weld metal composition modified composition
of the weld metal will be leading to the reduction in solidification temperature range and that
in turn will be reducing the cracking tendency so use of the suitable filler or the controlled
dilution all helps to modify the composition of the weld metal in such a way that the solidification
temperature range of the weld metal is reduced and so the cracking tendency of the weld metal
– so the solidification cracking tendency of the weld metal is reduced now we will see that these are the typical
methods which are used here like when there is no arc acceleration the grain size the
crack length we will see the solidification cracking tendency the crack length is more
and when the arc oscillation is implemented it refines the grain structure and modifies
the grain structure and which in turn reduces the – the – the crack length which is observed
of – after the welding like say this kind of structure having the
columnar structure from both the sides is found to more crack sensitive as compared
to this case when the arc is manipulated in such a way that the grain – grains do not
grow from both the sides and meet at center but the grain – grains are completely twisted
in such a way that even if the crack is trying to – trying to grow it will not find the common
path along the common week area for its growth at the center line so it will be resisting
the growth of the cracks this is what i have explained the weld joint
with the higher with the low depth to – low width to the depth ratio like this will have
the more cracking tendency as compared to the case when the width to the depth ratio
is high so the weld width to the depth ratio high weld width to the depth ratio reduces
the cracking tendency so this is what the same i have explained that the concave bead
promotes the cracking tendency as compared to the concave bead – so convex bead so this is the convex bead example this is
the concave this is the same thing when the in fillet weld when we have concave weld bead
geometry this will be leading to the development of the tensile residual stresses while when
they set the tensile residual stresses basically it try to – tries to compress the metal at
the center and thereby it reduces the cracking tendency this is the another one the liquation cracking
as i have said the liquidation cracking tendency is observed also in the high temperature solidification
range metal systems and this primary occurs due to the – the formation of the two phase
zone next to the fusion boundary and the such formation of liquid and the presence of the
liquid and the solid mixture next in the very area very close to the fusion boundary in
presence of the residual tensile stresses promotes the cracking tendency so – so the liquation cracking tendency is
basically observed in aluminium alloys or the other alloys also which are showing the
higher solidification temperature range and it is observed next to the fusion boundary
and especially in the area where two phase zone exist like part of the liquid metal and
part of the solid so presence of the tensile residual stresses lead to the development
of the liquidation crack about this also i have talked earlier so basically
i will be talking about the methods used to control the liquation cracks the first one
is the filler metal filler metal is selected in such a way that the weld metal solidifies
after the solidification of the heat affected zone so what we do basically the filler is selected
in such a way that the liquidus of the filler is – is lower than the base metal so the liquidus
of the weld is lower than the base metal so this is what we can say liquidus of the base
is sorry here the solidus of the solidus of the weld is lower than the solidus of the
base so in this case the when the filler is selected in such a way that if the weld is
still in the liquidus state because solidus is slower so the weld maybe in the liquidus state while
the since the solidus of the base is high so whatever the partial melting is taking
place next to the fusion boundary that will be in this zone pmz zone basically it will
be solidifying first before the solidification of the weld metal will be starting so if the solidification of the heat affected
zone is taking first then it will reduce the crack it will reduce the cracking tendency
because – and it will be able to resist the tensile residual stresses if they are being
set in heat source – heat source is to be used in such a way heat source the purpose
of the using heat source is to reduce the partial melting zone region of the partial
melting zone so like this if the partial melting zone is very limited
then the cracking tendency will also be less and if the partial melting zone is wide then
cracking tendency will be more so this difference in the zone of the partial melting zone can
be achieved by – can also be achieved through the two approaches by reducing the heat input
– reducing the net heat input and for this one approach is use of the multi-pass welding so limited heat input will help to reduce
the heat affect will – will help to reduce the – the partial melting zone size or use
of the high energy density process – use of high energy density process so high energy
density process also help to reduce the – reduce the net amount of the heat required for achieving
the fusion so that the weld can be made so the processes
like the laser beam or the electron beam helps in achieving the same thing with the limited
heat input and that in turn helps to reduce the size of the heat affected zone or the
partial melting zone and reduction in partial melting zone will help to reduce the cracking
tendency then degree of restraints we know that if
there is no residual stresses in the weld metal then there will not be any crack even
if the metal is sensitive for the cracking so the that the target of – the target of
means the target should also be to reduce the residual tensile strength stresses in
the weld joints and the one of the approach is to reduce the degree of restraint or use
the – reduce the volume of the weld metal or reduce or use the filler metal of the low
yield strength so all these approaches can be used in order to reduce the residual stress
magnitude as for as so the one of the points means one of the
ways of reducing the cracking in the liquation – liquation cracking in the pmz region or
partial melting zone is to reduce the residual tensile stresses for which we can use the
low yield strength filler or we can use preheat or we can reduce the degree of restraint – degree
of restraint so all these approaches will help to reduce the magnitude of the residual
stresses being developed and the base metal so base metal composition
should be such that the grain structure – grain structure is fine this is one and the base
metal is – is not sensitive to the composition of – not sensitive to the – the liquidation
crack due to the wide solidification temperature range so this is what we will be seeing in terms
of the base metal its the composition that decides the solidification temperature range
the composition need to be adjusted of the weld metal of the way need to be adjusted
of the base metal in such a way that the solidification temperature range can be reduced and the grain
structure the fine grain structure are reduces the liquation cracking as compared to the
coarse grain structure this is what we can see here when average
grain structure average grain size is less than the crack length is also less while the
when the grain size is increased it increases the cracking tendency at the same time the
presence of the impurities in the base metal like sulphur and phosphorus also need to be
avoided and we need to refine the grain structure so that the segregation tendency of these
impurities in the – in the base metal can be reduced so here it shows the typical way by which
the – the grain structure affects the cracking tendency so when the grain size is fine the
crack length is limited and the grain size is large the crack size is more this is what
we can see here this is weld zone this is a base metals side so this is the area where
the two phase means the mushy zone will be forming means liquid and the solid both will
be formed and under the tensile residual stresses will be leaded to the development of cracks so such wide cracks can be observed in the
heat affected zone so this is where i will summarize this lecture in this lecture in
this presentation i have talked about the – the development of the cracks in the weld
joints primarily in the weld metal and next to the fusion boundary so the cracks observed in the weld metal is
the solidification crack and the crack which is observed next to the fusion boundary is
the liquation crack and in this presentation i have talked about the remedial methods which
can be used in order to reduce the these two types of the cracks thank you for your attention

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