Heat treatment practices for cast iron & non-ferrous metals & alloys

Heat treatment practices for cast iron & non-ferrous metals & alloys


welcome to the lecture on heat treatment of
castings in this lecture we will discuss about heat treatment of cast iron and non ferrous
metals and alloys so we have discussed about the different kinds of cast iron and when
we cast the varieties of cast iron we need to go for the different kinds of heat treatment
so that we can get the desirable properties so coming to the variety of caste iron that
is the grey cast iron the grey cast iron if you want to have improved machinability or
improved wear resistance or improved strength or the stress removal because once we do the
casting we know that we will have the chance to go for certain heat treatment because there
will be stresses inside it because of the thermal radiance which is there during the
cooling process also although grey cast iron has good machinability because of the presence
of graphite flakes ah we still may think of having the grey cast iron having better machinability
in many cases we feel to have the improved wear resistance like when the grey cast iron
is used for the rings of the piston in that case you need to have these or cylinder liners
in those cases you need to have the wear resistance to higher degree so that its life is longer
similarly for improving the strength of the grey cast iron which has been obtained after
casting we need to do certain type of heat treatment so that its strength is increased
also the dimensional stability and stress removal so for these reasons you have different
kinds of heat treatments which is required on the cast product
now improvement in the machinability of grey cast iron so we know that for improving the
machinability normally we go for annealing process so in that case the subcritical annealing
may be done which promotes speroidization of pearlite and some degree of graphitization
so in this case the lower critical temperature below that so may be about six hundred to
seven hundred degree c of heating and holding at that temperature that is sub critical annealing
so if we do that basically holding for larger amount of time that leads to the formation
of speriods of this pearlite so that is known as speroidization of pearlite so and that
basically improves this ah machinability and also you have some degree of graphitization
so some graphites are also formed at the expense of the carbide or carbon so the subcritical
annealing is one of the methods by which the structure is speroidized so there is this
speroidization of this pearlite and the improvement in machinability of the grey cast iron
then we can also do the complete annealing so if you do the complete annealing for that
we are going above the critical temperature so we are going about around nine hundred
degree centigrade or nine to nine fifty degree centigrade and then we are basically cooling
in the furnace at a very slow rate in that case there will be so we are going into the
austenitic range and then we are very slowly cooling so certainly when we talk about grey
cast iron in presence of the graphitizes like silicon so under that slow cooling all the
complete graphitization will take place and that will make the iron very soft and in that
case it will result into a soften structure as well as it will improve the machinability
of the structure permanent mold casting are fully annealed
so that is normally the requirement because in the permanent mold castings the heat transfer
rate is normally higher than in that in the case of sand molds so here you need to go
for full annealing so that there is complete graphitization taking place and the grey cast
iron becomes softer if we do the normalizing so when we are going doing the annealing full
annealing we are going to that temperature about nine hundred degree centigrade and then
we are cooling at a very slow rate in the furnace but if we cool in the air then that
is slightly a faster cooling rate than in that case of annealing in those cases some
pearlite is retained so retention of pearlite gives the higher strength because of the fast
cooling rate the complete graphitization does not takes place and because of that pearlite
is retained and that gives more strength and hardness as compared to that in the case of
annealing so these are the methods so in this case you will have improved machinability
as well as you will have the improvement in strength as well as in hardness and in these
cases you will become completely soft and it will have a very high degree of machinability
now in the improvement in the wear strength and wear resistance as well as strength so
we have the treatment like hardening and tempering so we know that when we go for hardening in
those cases the strength is increased hardness is increased so for that we are going to about
nine hundred ten to nine twenty five degree centigrade and then we are quenching into
oil or water to produce a martensitic structure so that is a simple principle of hardening
in which we are going into the austenitic range and then we are quenching drastically
into oil that is but we can also go for water or bright solution and that results into fully
martensitic structure that increases the hardness now in those cases because hardness is quite
large and the martensite formed is very extremely little so basically you do some softening
and thats why you go for tempering so tempering will be done at various temperatures so suppose
in the case of cylinder liners you see that you first make it hard and then further you
temper it at the temperature of two hundred to two ten degree centigrade for some time
so that way you are basically giving some softness in the phases that is completely
brittle phase that is basically tempered so that martensided phase is tempered you also
do you can also go for flame or induction hardening so the so these flame or induction
hardening they are the surface hardening methods in that the core is remaining soft but only
on the surface you induce the hardness so that improves your wear resistance so wear
resistance is improved ah at the surface and for that you have flame or induction hardening
which is a variety of surface hardening that is carried out
for improving strength hardening followed by tempering at this produces at so if your
tempering temperature is higher basically that improves the tensile strength so at lower
temperature if you are doing the tempering that basically makes the phases soft that
basically so you have the extremely brittle phase formed its the brittleness is minimized
so for that you take the lower temperature but if you go for little higher temperature
then that produces the optimum tensile properties stress relief so the name indicates that you
this is a process for relieving the stresses which are generated during the casting process
or during the machining or during its use so during that many a times it also changes
its dimensions so because of the stresses which are generated and if that stress is
basically more than the strength of the material then material may change its shape there may
be deformation taking place so for that annealing or normalizing may be carried out to alleviate
the stress so for that you we are heating slowly to four eighty to five ninety degree
centigrade holding for an hour or more and then cooling slowly to two hundred to three
hundred degree centigrade so this way also you try to alleviate the stress so either
you go for normalizing or annealing or you may go in this range heating and then further
cooling so that basically relieves the stresses coming to the another variety of cast error
that we have already discussed so the variety of cast error is this white cast error which
is converted to malleable cast iron by the annealing process and we know that since white
cast iron is of no use for the engineering purpose other than wherever we need the extremely
high degree of hardness so in otherwise to get the malleability we are annealing this
white cast iron for long hours so that the carbon which is in combined form it is converted
to the free carbon or tempered carbon so this annealing heat treatment for white
cast iron is done in three steps so as we know we are heating to a temperature about
nine hundred degree centigrade or eight fifty and then we are holding the air for a very
large amount of time so during that holding so first of all when we are heating to a very
high holding temperature so during the early periods there will be a nucleation of graphite
taking place so basically this annealing is occurring in three steps or three stages first
stage is nucleation of graphite means the graphite will nucleate during the holding
period early holding period so when we are heating and going to the holding temperature
during the early period or during that period this nucleation of graphite takes place
now when we are keeping that at that temperature for very long hours forty to fifty hours during
that the graphitization will take place so that is basically dissociation of all these
combined carbon into iron plus carbon so that is f a three c will dissociate into iron plus
carbon that is free carbon and this is known as first stage graphitization so the first
stage graphitization basically is responsible for the conversion of these massive carbides
into iron plus graphite or free carbon then the second stage graphitization during
the annealing process is basically during the slow cooling through allotropic transformation
range of carbon of iron so basically when we are cooling slowly through the allotropic
transformation range during that process the formation of matrix what kind of matrix will
be formed that is governed so in that basically all the carbon further is removed and you
get the ferritic type of matrix so so that type of matrix that is ferritic matrix is
completely ferritic matrix formation that takes place during the second stage of graphitization
during the slow cooling through the allotropic transformation range so that is known as second
stage graphitization or s s g this is known as first stage graphitization that is f s
g then you can have the pearlitic malleable
iron so what we have understood that during the annealing cycle of the white cast iron
what we see is we go here and then we are cooling it slowly so so as we know this is
the first stage in in this case this is the first step so this is the first step that
is nucleation of graphite taking place then during this range this is known as f s g so
that is first stage of graphitization and then during the second so in this process
in this time all the carbides are basically ah dissolved and they are giving you iron
plus free carbon and then during that this stage your secondary stage graphitization
takes place and in that further graphitization during the slow cooling that basically gives
you completely ferritic structure now if we want to have the matrix of pearlite
that is spheroidized pearlite or temper martensite and then temper carbon modules in those cases
you have there are certain treatments and for that basically you have to see that the
secondary stage graphitization that has to be prevented so during the secondary stage
graphitization basically your result of s s g is the formation of the ferritic structure
now that is to be prevented and then if that is prevented if the cooling rate is fast in
these cases if you go and if you further heat it and then further cool fast in those cases
you are getting the pearlitic matrix so basically if the cooling rate is slow in the slow cooling
the ferritic matrix is obtained however if that is prevented this s s g is prevented
you get the pearlitic matrix so that gives you more strength also you are giving the
manganese so manganese if it is maintained at the point five to point nine percent so
in that case it retains the pearlite so complete s s g is prevented also by adding
alloying elements manganese molybdenum or chromium so if these alloying elements are
there they try to go for giving you the pearlitic matrix so one is that you arrest the anneal
during the secondary second stage graphitization another is that you are preventing the second
stage graphitization where otherwise you could have got the ferritic matrix you are getting
the pearlitic matrix because of the position because of the composition which is having
the manganese molybdenum or chromium and that gives you a pearlitic matrix
then the other variety of cast iron as we have discussed is the ductile iron so ductile
iron we know that ductile iron is obtained by addition of magnesium or cerium or yttrium
in the cast iron melt and that basically ah converts this flaccid graphite into the nodular
shape of graphite so in this case you have the structure of ferrite and spheroidal graphite
so manganese phosphorus chromium nickel molybdenum should be as low as possible as they retard
the process now shorter subcritical annealing cycle may be used when very high impact properties
are not desired so basically you can go for some heat treatment of ductile iron and these
when these high impact properties are not desired you can go for shorter subcritical
annealing cycle so that gives you the desired properties
now the if you discuss about the heat treatment ah of ductile iron you can go for normalizing
of the ductile iron so again normalizing means you are going into the range of nine hundred
to nine fifty degree centigrade and further holding for some time and then you are cooling
in the normal air or or atmospheric air so that results into homogeneous structure of
fine pearlite and improvement in tensile properties so certainly if you are going for normalizing
and ah you are not cooling very slowly in those cases instead of ferrite you get the
fine pearlitic ah matrix temperature it at which the casting is removed
for air cooling affects the hardness so the thing is that if you are removing the casting
and letting it into air if it is below eight fifty degree c that will be basically affect
the hardness so you have to maintain a temperature you must not go below certain temperature
or below some somewhat like eight fifty degree centigrade so that temperature is one of the
temperature at which you have to take it from the furnace and allow it to cool in the air
heavier sections should contain alloying elements like nickel molybdenum and additional manganese
for satisfactory normalizing so that is one of the conditions that you should have this
alloying elements so that has important role in normalizing because they enhance the harden
ability of the material and normalizing is normally followed by tempering so tempering
is done so tempering will be done to a temperature may two hundred to four hundred degree centigrade
and holding for some time and then cooling so that will be essential once we go for normalizing
treatment hardening and tempering of ductile iron so
as we know that if we want to have higher hardness or higher strength we go for hardening
so for hardening it gives so if you do the hardening treatment it gives very high hardness
and oil is the preferred medium of quenching all though water or brine is also used then
after quenching casting is normally tempered to induce some softness into the very hard
physics which are formed we also go for surface hardening processes so less flame or induction
processes are used so which gives the hardness on the surfaces up to certain skin depth the
surface becomes very very hard pearlitic types of ductile iron are preferred because of short
heating cycles of these processes in the pearlitic cases you you need to give only short cycle
of heat treatments so that is preferred irons without free ferrite respond instantly to
flame or induction heating and require no holding time at austenitizing temperature
for getting fully hardened so this point tells that if it is there is no fully ferrite ah
matrix in that case you go to the austenitic range and without much of the holding you
can further cool so so that you can get sufficient hardness so that is why we have ah seen that
these pearlitic types are preferred over the ferritic type of ductile iron then we also
go for stress relieving which is done normally at five hundred to six hundred degree centigrade
so on that you retain for some time and then cool slowly so that basically stress relieves
the material one of the very important variety of the ah
ductile iron is the austempered ductile iron which is formed by a proper type of heat treatment
that is austempering type of heat treatment so that product is known as austempered ductile
iron so austempered ductile iron is having the properties as compared to steel or for
steel so many a so because its strength is as compared to that of forged steel although
you can see that its melting temperature is small so by cost in cost wise calculations
it is cheaper but its strength is as compared to steel so it is having very good toughness
with very very high strength its mixer i mean structure consists of two phase mixture of
acicular bainitic ferrite and austenite so as we have seen in the case of austempering
treatment we are heating coming below the nose of the sea curve and then we are holding
so that we are getting the benetic structure so same thing is followed here also we are
pulling fast then and but we are holding it at a temperature i mean after the point of
sea curve about three to four hundred degree centigrade then we are holding it at that
temperature so holding that gives you a acicular bainitic type of ferritic structure and austenite
so basically that gives a large amount of toughness in the material a very good strength
so it is occurring in two stages in the first stage it decomposes to bainitic ferrite and
carbon enriched austenite and then in the second stage when we are prolonging the austempering
action carbon enriched austenite decomposes to ferrite plus carbide which is in the form
of needles into the ferritic matrix and then that basically gives a large amount of strength
as well as toughness so a d i possesses very good quality
heat treatment of non ferrous metals and alloys so we have so far discussed about the heat
treatment of ferrous materials in that we discussed about different varieties of steel
carbon steel and steel and then we also discussed about different varieties of cast iron its
heat treatment then we also need to discuss about the heat treatment of non ferrous metals
and alloys now in the case of non ferrous metals and alloys normally non ferrous materials
are used in terms of alloys because whenever we talk about the pure non ferrous materials
they are only used when we need specific properties like very high conductivity or so ah otherwise
mostly ah they are used in form of a alloys now in the cases of non ferrous materials
ah there are heat treatment processes like homogenization so what we see since mostly
we have the alloy and in case of alloys due to non equilibrium cooling there is a kind
of problem which occurs in the case of ah solidification and during the non equilibrium
cooling we have the ah coring type of structure so that is because of this range of solidification
and because of the non equilibrium cooling this coring or this is the basically variation
or gradient in the composition so because of that you have coring and dendratic segregation
that is normally observed in the case of non ferrous materials or for any material which
has which is as basically a solidification range and which ah goes while solidification
if you cool it under the non equilibrium condition then this coring and dendritic segregation
is obvious so that basically is required to be eliminated
so that is basically because of the compositional variation and also we know that there are
ah phenomena like super cooling so there are many things so that leads to this coring and
dendritic segregation so for that so normally you have isomorphous type of alloys you have
this coring and dendritic segregation which is normally observed so that is removed by
this homogenization process so in that basically the alloy will be heated at elevated temperature
for several hours so if you heat then what happens ah the composition becomes ah homogenized
and the composition becomes thoroughly same and because of the action of diffusion so
it becomes homogeneous and choice of suitable homogenizing temperature
is limiting limited by solidus line as well as retardation of the process so we cannot
go above a certain temperature because if you have a solidus range you cannot go above
this because the melting will take place above this we cannot go because melting will take
place and if you go below that too much temperature less i mean below this then at lower temperature
there may be less rate of diffusion so that is why it is written that this suitable temperature
has to be taken so that it is high enough so that there is ah quite good degree of diffusion
taking place so that composition is homogenized it is by the diffusion process the composition
is homogeneous and also the diffusion rate is somewhat higher and it should not go beyond
this line because then there will be melting of one of the phases so that should be avoided
then the treatment is solutionise quench and age treatment so this is basically in the
case of ah such alloys where on cooling one of the phase has limiting solubility in those
cases this age treatment is given is which is known as aging so this treatment is given
to alloys with solute components having decrease in solubility with decrease in temperature
so in most of these alloys like aluminum copper system aluminum silver system aluminum magnesium
system aluminum silicon system aluminum zinc system aluminum magnesium copper system aluminum
magnesium silicon system all these systems basically as the temperature comes down the
solute components has the decrease in solubility with temperature so in most of the cases what
you will see that with temp temperature decreasing there will be something like ah many cases
so as you further go this is so there will be different kinds of phases so as we see
that ones at this temperature if you go the solubilities at this temperature is this much
and once we come down then what happens as we come down the solubility goes on decreasing
so for those cases what happens you basically cool fast so that this much amount is basically
retained into the ah mixture and then that becomes a super saturated solution at this
temperature and what happens that they try to come out because a solubility is less so
that will be coming out as a coherent phase so the strength and hardness of alloys are
improved because of the formation of this coherent or semi coherent particle so what
happens when so in the matrix what happens once you are trying forcibility to return
this composition of so this solute component once it is retained in that case what happens
they try to come out as the temperature increased further so slowly what happens you have the
formation of precipitates will be coming up slowly so because your solubility was quite
low here so once we increase further the temperature little bit and hold for sometime then this
coming of this fine precipitates they try to come out so this aging treatment so solutionize
means you are going to a high temperature then you are quenching
so quenching we will do fast quenching or fast cooling so that you are taking this much
of solute that is arrested in that matrix and then since its solubility is less so it
will try to come out of it and then that basically comes in the form of coherent or semi coherent
particles so that gives an interface and basically that indeeds the dislocation movement so that
is basically the concept of strengthening in the case of non ferrous metals and alloys
so that is known as aging now if we are living it in the natural atmosphere for long time
then that is known as natural aging otherwise if we are aging to certain temperature so
we are heating to certain temperature and holding for some time in that case the sizes
of this precipitates they growth ok so that is known as precipitation hardening and then
once we age for larger time or at larger temperature then sometimes they over age
so that is normally the concept of strengthening because of the precipitation in the case of
non ferrous metals and alloys so in the case of averaging you have incoherent particles
which are formed and there will be incoherency because of that the strength decreases so
aging basically depends upon the temperature and time function and that is normally the
concept of aging in the case of non ferrous metals and alloys so that is aging is done
in the temperature hundred to two hundred degrees c range so this is normally the heat
treatment procedure normally in the case of non ferrous metals and alloys
thank you

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