Good morning, today we will talk about alpha
high strength steel. In the initial lecture, we did mention that strength of metals as
several orders of magnitude less than it is ideal strength. So, the ideal strength we
derived is of the order of g by 30; where g is the cm modules. However, in reality we
find the metals deform at a stress much lower than it is ideal strength, and this we learnt
that it is primarily, because of the presence of crystal defects like dislocation. If it
is possible to produce defects free crystals we do in fact approach these ideal strength
and which has actual event found in, which have filamentary crystals, which very small
dimension, which is virtually dislocation free. But these kinds of material, they are
not useable as it is, so is there a way of improving strength of metal, and particularly
today we will see, how strength of steel can be increase all most to the order of magnitude
of ideal strength of metals. So, under this we will talk about several
techniques, which have been a plat and we will talk about a few is; dual phase steel,
will talk about is special process called patenting, will talk about ausforming, and
again many of these cases these using these, it is possible to attain very high level of
strength, even in plain carbon steel. Now, the question then comes up, why do we then
have go for alloy steel, and unnecessarily increase this cost. Because most of the alloy
additions, which have made the quite expansive, we will know why do we go for alloy steel.
We will talk about to different class of the alloy steels, which have very high strength,
and some very interesting property, we will talk about maraging steel, we will talk about
austenitic steel, primarily we will talk about its steel laze property, and also we will
know; however understanding of the principle of physical metallurgy, it has been possible
to develop the type of steels; creep resistant steel, which have been used in several high
temperature components. Now, let us talk about this dual phase steel.
In a dual phase steel, what we do is a normal heat treatment process we heat, suppose this
is the composition of steel, this is the part of the tarrant carbon diagram; this is austenite,
this is ferrite plus cementite. So normal structure is something like this of this particular
composition, you will have politic region, and you will have ferrites; this are ferrite
frames. So it is made up of two distinct regions, this are the primary ferrite, and this is
pearlite. Now, when you heat it about this temperature, then what happens; these pearlite,
the cementite dissolves in the neighboring; a cementite reacts with ferrite, and gives
you austenite; and this reaction take place at these interfaces, and gradually the entire;
this is converted in to austenite. So what you have now, here it will be alpha
that means ferrite; plus austenite. So it is duplex structure, you can also see the
composition, this point will be giving the composition of ferrite, and this point will
give you composition of austenite. Now, what will happen, suppose you quench from here,
then what you will have is this ferrite parts they will remain unchanged. But this austenite
part will convert in to martinisite. And then what you will be left with instate of ferrite
pearilte structure; you will get ferrite martensite; and this martensite since it is carbon contains
is high; it will have very high hardness, so you have a compost material short of thing;
you have two face duplex structure and so this is advantage of this inter critical heat
treatment, and this can be implemented in mass production as well; and what you see
the depending on the temperature at which this inter critical heat treatment is given
even have different amount of martensite. You go higher; so if you go beyond this point,
this will be zero; almost time in approaching says not zero, so what will happen, beyond
this it will be totally austenite, and if you quench a part of it will convert in to
martensite, and part will form ferrite. So, if you quench directly from here; because
it has low carbon steel, it has low harden ability even, if you quench it very fast you
may not get a completely; if you quench from here you may not completely martinsitic structure.
So this means this is the critical point, so this is A 3, so this, if you say this is
A3; so if you go above A 3, then that is not match of change you have very small amount
of martensite, and it may have mostly ferrite, this is percentage phase whereas if you a
inter critical heat treatment is given here then you are likely to have some where here,
then you are likely to have here that amount of austenite amount of austenite will be proportional
to this region this line amount of ferrite it is this, but this martensite, so amount
of martensite. So, this will be that equailburium amount and as you go up that among the martensite
will changing, and also what will happen this martensite will have a much higher a carbon
contain so therefore the m s temperature is likely to be even possible little be m f certain
we will be lower than the room temperature. So even if you quench your likely get some
amount of the retained austenite, and this will goes on decreasing, so you are total
structure over here, it will consists of ferrite and this is inter critical heat treatment
temperature, it will ferrite martensite plus retained austenite an as you increase the
temperature amount of retained austenite goes on decreasing, and here off course you will
get ferrite and martensite. But advantage of having a duplex somewhere in between there
is a possibility we will get a martensite strong a martensite because we know the strength
of the martensite is a function of carbon content, so it is likely to have much higher
strength. But limitation of the dual phase is the high cooling rate is to be a adopted
to convert that austenite to martensite in plain carbon steel, and it is applicable to
thin plates and sheet, so this high limitation stays. Now, dual phase steel what is the most attractive
is, this has; this gives I mean better combination of strength, and toughness; because many application
we know the is not strength may not be enough, it must have very high degree of ductility.
Look at this; this is the region which of the dual phase steel, and how does it compare
with conventional steel; conventional steel means strengthen mechanism; is a solid solution
strengthen and some of the solid solution; phosphors has a very high degree of; little
phosphors can significant link increase the strength, but it is aggregate; is the makes
steel brittle, so these are the problem. Silicon is the common solid solution strengthens,
manganese off courses does improve, but it is not as strong as phosphors or silicon.
And grindry finding is the key strengthen mechanism in case of micro alloyed steels
so where graindry finement we look that it critically, so navobiam, and vanadium, so
carbides ,and nitrates of these steels, these are the key conteachwent which refine will
helps or which inhibits arsenic drain growth. And you get a very find structure so compute
all this, this has better combination of strength, and toughness, and it also has good formability
so that is why there is a special attraction for dual phase steel. Next, let us consider a special heat treatment
process called patenting, this is isothermal heat treatment process, what; and it gives
very ultra high strength steel wire, this is used to produce high strength steel wire
if we look at the time temperature transformation diagram of steel. So this is the A 3 temperature
and this is A 1 and here if you ferittie plus austenite; this side is austenite. And what
we thing to do is, we cool the steel from above from the austenitic region when it is
austenite. You cool it almost to the nose here, so this is the temperature of the surface
which will cool fast, this is surface, and this is core, and you maintaining in a bath
an liquid bath consisting of mostly it is let and if you have a little bath this whole
time may help in making the temperature uniform, and allow it to cool I mean allow it to transformation
to completion at the same time temperature. And what we trying to get, is fine pearlite;
so that lamellar spacing lambda; the lamellar spacing is very small, lamellar spacing is
really small. And under optical micro scope the pearlite will not resolve; so the normal
pearlite you do series kind of lamellar structure, but main idea is, if you look at in a standard;
in a optical microscope; standard optical microscope, and may be you will find that
most of this pearlite modules are not resolvable. And how it is done, so these are some this
through is guide rules and this is an idle guides, so this is the lead bath mol tent
lead and this passes through here you may straighten, this is a role. And, sometimes
you can heat it also this. You can apply some voltage between this and
this; and this is a lead bath, and these are heating coils and you can also have some;
this can setup easily in a laboratory this can setup easily in the laboratory and you
can apply some voltage and then the wire gets heated up and this temperature is about A3,
it comes in to the lead bath, and this is the just guide idle role, and this wire comes
out and it coils here, and why this treatment; it is possible to get extremely find lamellar
pearlite. And they have the excellent drawing property
that they can we cold drawn to desired strength and diameter, and you can get strength as
I look at this of the order of g p a giga paschal strength you are getting, and these
are use for very high strength applications like spring then suspension bridges were height
and side wire ropes, and strength will increase definitely as the percentage carbon grows
up. And in fact the lamellar when you give such a high cold working so maybe say 90 percentage
cold work these lamellar spacing is you know the even smaller, and in fact with transmission
electron microscope people have seen they come out be of an nano scale dimensions this
spacing as result search a large deformations, and that is responsible for such high level
of strength. Now, let us look at another space cell forming
process called ausforming. Now during a micro alloyed steel; what we found out is, if you
work austenite at low of temperature, that austenite will not re crystallize and these
austenite grains in particular one direction, and if you deform one direction, these grains
become highly long it is least so in this one directions, they become extremely as small
dimensions. And now they do not re crystallize; so they will also have high dislocation density,
so row will be high; dislocation density is high. Now, if you cool if you cool austenite
to a region we haves in most steels they have a way, and this way if you are able to cool
at this temperature and this is the core temperature, and this is the surface temperature.
When you cool here, and then you give walking at this region; and here your hold time is
sufficiently long, but if make sure you do not go too close to M s, so in fact this working
has to be done at a temperature higher than M d temperature. This means that if you work,
then the martensitic start temperature goes up the deformation, this is the martensitic
temperature for certain percentage of deformation, so do this give cold working above M d. And
after the cool working is over, and before this is the bainite starts forming you quench
the steel, and after this; it is necessary to give some tempering may be hundred degree
centigrade you temper to relive internal stresses. But the key thing is; is avoiding bainite,
and you get martensite in cold work austenite. So austenite is also very hard within the
that also you get finer; much finer martensite. And this dislocation density will be much
higher, and here you can attain strength level is quiet high yet you will have sufficient
ductility, which is not possible in martensite of similar strength. And we will see that
although this type of heat treatment can given in plain carbon steel of an; it is not show
easy to do, so because of section size limitation. But if you go for an alloy steel possibly
you can go to a higher. In alloy steel possibly section size limitation may not be that critical
and we will look up about it. Now, let us look at why do we need alloy addition,
say when we can why go for such expensive alloy addition, when such a wide range of
strength, and toughness can be achieved in carbon and micro alloyed steel. Is it really
necessary, because alloy addition always will make steel expansive. Now, major limitation
is; in plain carbon steel they have poor harden ability, and you have section size limitations
a plain carbon steel as say may be even, in the case of infinite quenching is extremely
high rate of quenching; say ideal quenching you may be able to get through thickness hardness
that means fifty percent of martensite at the core up to latter, say may be depending
on carbon contain may be a proof certain mille meter.
So many application, this is too small; say suppose one application that comes to mind
right now, later say; this a landing gears of a craft, so this is the part of the air
craft it has to fly, so obviously your aim will be to use very high strength material.
So that strength to weight ratio is high, so it does; I mean so that structure is light
it can fly. And in this case this dimension of this may be of the order of eighty mille
meter dia. So, imagine so it will be impossible we know that best property; that you can get
in steel is trough by hardening, and tempering; that is temper martensite gives you the best
combination of strength and toughness. And this steel must have high strength as well
as toughness. So it is no way you can achieve this of such a large size.
So, here alloy addition becomes a must. And some time to get a certain very specific properties,
many applications the nuts and bolts were to make then you they are to be machine, so
this has to be a material should be machinable. So normal steels the machinability can be
improve significantly by, not only by heat treatment, say, if you have globular cementite
machinability is higher. But most cases in steel bulk of steels which goes for nuts and
bolts may not have high amount of carbite, so there machinability is improved by adding
some inclusions, particularly in any case to take care of sulfur we add manganese, and
you form this manganese sulfite inclusions. In fact for machinable steel intentionally
they are re sulfuorise, sulfur is added intensely and you little more of manganese to take care
of sulfur. And the presence of the manganese sulfite inclusions gives this steel good machinable
property. So, that means not only this hardness limitation, a harden ability limitation, but
also to gave certain very specific property you need to add some alloy elements. The another
important property the corrosion and oxidation resistance steel; it rust, to product it for
rusting, you know we use many other technique as well like coatings you galvanize the steel
to protect it for most of the steel application your galvanize steel product it for rusting,
but many cases this may not work. So, you may need to improve its corrosion
resistance, to improve corrosion resistance you had set an alloying element, and one of
the most common alloy element is chromium. If, it is present in dissolve form is greater
than around twelve percent, so twelve point seven to be specify. See, if it is around
twelve percent; more than twelve percent chromium, in that case, on the steel you have a protective;
thin protective coating of oxide, which makes steel stainless. So, all stainless steel will
have a minimum of twelve percent chromium, and this chromium must be present in solid
solution. Similarly, magnetic properties; to improve magnetic properties, say many application
steel as a known for its magnetic property it is magnetic below the curry temperature.
And, one of the main application is, let is a transformer core. And, here when it is convert
you know increase the voltage low to high; high to low, this transformer in other core
gets heated, and this is known as hysteresis laws, to cut down the hysteresis laws of an
in steel, we use some special alloy element particularly silicon. Some steel you also
add phosphorus electrical greed steels, so you need to get certain specific property.
Similarly, to get a hard magnetic property transformer core is the soft magnetic. Similarly,
for hard magnetic property there will be other elements to be; special elements to be added.
So, that means alloy additions can give an added necessary to gives improve certain specific
properties. Another major application of steel is in power plants. And, power plant, say
stream power plant say, boilers where the steel has to be used at temperature above
say, let us say four fifty degree centigrade. So this case, creep becomes and of the important
feeliyo mechanism. So, in this particular case also you need to have some alloy addition
which gives it a better corrosion; a better creep resistance. Now, any add alloy elements to steel; where
does it go, and what is the role that they play, I think this will vary from element
to element. So, here are some common elements which are present in steel. This list is binomial,
that can we several others also possible. But these are the major alloy elements, and
these alloy elements either maybe present. So, many of these bulk of the steel which
are it is a ferritic steel, but bulk of the steel that we used they are ferritic steel.
And most many of these alloy elements they are soluble to certain even, if it is too
limited extended, they are soluble in ferrite and in high temperature free austenite also
they have some degree of solubility. So, they will be present as solute element in substitutional
solute element, and ferrite, and austenite. Some of the elements that you add, you know
they react with some of impurities which are present during the steel making stage; there
will be oxygen; it reacts with oxygen for oxide. There will be some sulfur; reacts with
that sulfur, and converts into sulfide. And some form there are silicon is also present
in steel, some form silicates. Some examples are given here manganese sulfite, manganese
silicates, so these are possibilities, you can also have oxides aluminum we have seen
is used as the deoxidizing agent, and it gives fine grain steel, so alumina is also an inclusions,
and some of these inclusions they are; they deform, and so these elements you know manganese
sulfide, if it is present they will get elongated is silicon, and presence of this elongated
inclusions makes the property directional, say suppose if there is an inclusion like
this; so in this transverse direction it will have lower strength.
So it will not a fix strength in this directions along the longitudinal directions, but transverse
properties particularly ductility they get affected, they have they also difficult to
well because of these inclusions they give dice to the defect commonly known as lamellar
theory so there should be some ways if these are that inclusion control becomes necessary
shape and size of inclusion control is key to improve properties like the weld ability
or transverse strength, where is these are hard particles they are present as globules.
Some of the elements they may dissolve in the cementite like this, and many elements
we have seen add nayobiyan which forms carbide, nitrites like vanadium nitrate, tungsten also
forms carbide, molly also forms carbide, and then chromium also forms carbide. And there
are certain metals which are added like lead copper they are insoluble so they are present
us globules of lead or copper. Copper is added to some precipitation harden able grade of
steel, hsla steels and lead is added to steel to give it good machinability. Now, few of the common alloy elements affect
of which is listed here like chromium known for a corrosion resistance we have twelve
percent chrome; steel become stainless nickel also improves the corrosion resistance particularly
in acid: sulfuric acid environment. So, many of this stainless contains both chromium,
and nickel, and few add a very high amount nickel you can make austenitic phase stable
at room temperature. And we will talk about sub sequentially now, carbides, nitrates,
and oxides; they act us grain refiner. If you have, say tungsten carbide that will add
to abrasion and wear resistances. Some of the carbide like chromium carbides are particularly
MoC: molipdinam carbide, they add to some carbides, add to creep resistance. This precipitates
if they form along grain boundary, they actually inhibit grain boundary sliding and improve
it is strength. Similarly, if you have sulfide, silicates
they give poor through thickness properties or transverse strength, and toughness. It
also gives problem with weld ability. These inclusions we just mention they give good
machinability; silicon low magnetic hysteresis. Addition of manganese, it removes hot shortness;
we it did talk about it problem related to sulfur content steel. Sulfur aggregates to
grain boundary, and if manganese is not there it will form with addend sulfide a low melting
heated which makes steel; gives steel it is poor hard work ability, and this is known
as hot shortness. Addition of manganese is extremely important to lower ductile to brittle
temperature all cryogenic greed of steel will have sufficient some amount of manganese. Let, we look us at few common alpha high strength
steel. And many of these potential applications obviously they will have show high alloy element
their expansive and but never the less application like, aerospace application where cost may
not be major limitation. Because one major criterion is structure should be light, and
it should be able to fly. So, this is the composition a very popular grade is En24 or
AISI 4340 by this; it is know these are the composition 0.4 carbon, 1.8 chromium, 0.8
nickel, 0.25 molly. The molly takes care of many of this steel, when you add this alloy
elements they have a problem of the temp of brittleness; addition of molly overcomes that
brittleness and it is used in oil quenched and tempered condition.
So because of this alloy addition it has a good harden ability by oil quenching you will
be able to harden through and through of a particularly this landing gear of applications
this is very useful. There is another grad which is almost based on that vanadiyam addition
adds to the strength. Here also the treatment is similar oil quench, and temper. Oil quench
from that austenitic temperature range and obviously we look for fine austenite grain
we do not heat it to higher austenizing temperature. There is the great called another alloy steel
which has a much higher alloy element and so much of molly present you know makes this
benthic be very long you know this type of steel, even if you air cool possibly you will
get martensite, so you can air cool, so your quenching stresses problem are not there,
and then you give temper, and it can also given ausforming, and by this you may get
even higher strength and toughness properties. Another greatest steel we will talk about
it let us separately maraging steel here this has a very low carbon content, but high nickel,
and you have some amount of cobalt, molly and some precept; to get some precipitation
you get titanium, and aluminum. Main thing is you look at that carbon content; martensite
is hard, because of carbon, and if you reduce carbon to this level definitely the martensite
will be soft this martensite you can cold work so they may not have that high hardness,
and it is aging when you age, so these are the elements, which develop some coherent
precipitates in the matrix, and because of the formation of these fine precipitate, it
strength goes up. Now all of these steels, so in this case off course have high alloy
element, so they have high harden ability and you can always get strength of these order
of g p a order you have five percent elongation in fairly large section. And, as I mention
here this is amenable to ausforming and you can event get even higher yield strength,
and ductility. The maraging steel we mention that here, the martensite is soft, it can
be cold work, and then it can be given precipitation hardening heat treatment. And typical, and when you give these hardening
treatment, aging treatment the hardness goes up, if you do not give work; if you do not
give any cold work then possibly the hardness increases like this. Air cold plus aged; whereas,
if you cold, work and then aged then martensite; then that is the in that case, and the cold
work martensite when precipitates form it gives much higher level of strength. An on
aging this are the precipitate that form, and they have, because of low carbon content,
they have an excellent weld ability, we will see while latter that the carbon in the mean
alloy element, which is detrimental, which makes the steel difficult to weld.
So, good weld able quality steel always attempt is make to bring down the carbon content,
and here the strength is extremely high it also have very high fracture toughness we
did talk about K one C which is the major of fracture toughness. So, such high level
of fracture toughness 120 MPa root meter square, and 1800 MPa strength is un hard of; so this
the special greed of steel, so here you have strain induced precipitation, and which is
responsible for its high strength this is no doubt expansive you have eighteen percent
nickel, the nickel is most expansive amongst the common alloy element which are added,
and but it is a unit material for like rocket casing; aerospace application. Now, let us talk a bit on stainless steel
now you have seen main alloy element which is gives strain less is chromium you must
have at least twelve percent chromium to form productive coating of chromium oxide. Along
with that we also add to improve corrosion resistance nickel, that also austenite stabilizer
we will see that even added certain amount of nickel then the steel become austenitic,
and we add molly to give it pitting resistance. Some of the common austenite stabilizer which
are listed here, even carbon is called carbon nitrogen; they are also good austenite stabilizer.
Now, a quick look at this the ferritic steel; this the chromium; herein chromium, binary
diagram, it is known as a gamma loop forming element. So, here you have ferrite, some time
call it alpha or delta does not matter. So, this side is ferritic and here, so if you
amount of chromium goes beyond the critical limit in that case if you heat the steel herein
chromium alloy, it is remain ferritic until it is melts.
So you cannot form austenite, so if you hot too much of chromium then the steel cannot
be harden given martensitic by quenching, the question of getting martensite does not
arise, because you are not able to reach this austenitic state, so this is the one important
point you must remember, this is binary alloy and this compotation is quiet important, and
this gamma loop is around thirteen percent, say if it is greater than 12.7, then it is
ferritic until it is melting point. So, this is important to remember, if it is greater
than 12.7 that you would have no chance ausforming austenite.
So, if you have stainless steel were you have carbon which is the; you have seen this is
a austenite stabilizer so if you have some carbon in the steel effective chromium concentration
goes down. So, this is the factor, seventeen times carbon contains, so if you have point
one percent carbon means, you add to this one point seven, so thirteen plus so two;
around fifteen, so if this is greater than 15 chromium, in normal steel will always have
some amount of carbon, so if it has around point one carbon, if chromium is greater than
fourteen or fifteen, then you do not expect austenite to form, and this type of steel
is known as ferritic steel. And it will be stainless because it has more
than twelve percent chromium in solution and if this is less than this then we call this
steel as a martensitic rate and common ferritic steels, and martensitic steels say compositions
are given one particular compotation is 16 chromium 0.2 carbon and this case as 0.12
carbon even find it will satisfy this relationship and this will be ferrittic it cannot be given
any hardening treatment only way you can harden is some solid solution strengthen of cold
working. Whereas, if you have in this particular case twelve chrome point one five rate so
this is the martenstic stainless steel it can be heated to austenitic region and one
quenching, so you get martensite. So this is steel commonly used for cutlery some of
the turbine blades also in engine turbine blades are made up of this type of steel. Now, to look at whether the steel; what will
be stature of the steel; whether it will be ferrittic, weather it will be martensitic;
are in the extreme case will it be austenitic that is represented very well by the diagram
called schaeffler diagram so all alloy element can be grouped in to two parts, one is rich
stabilize ferrite, and another is stabilize austenite. And, what you can find out; you
can find out nickel equiv valiant or chromium equiv valiant, so. this stand for all ferrite
stabilizer so factors which are given here, so some of the elements which are known as
very strong ferrite former that is the temperature in which ferrite is stable; look at vanadiyam,
aluminium, they have very strong ferrite former. Similarly, there are certain other alloy elements
like manganese which stabilizes austenite. Carbon is the strong austenite stabilizer,
so also nitrogen so austenitic grad you try to have high amount of nickel, and some of
this austenite stabilizer. And it is possible, if you have this chromium
equiv valiant around here, the chromium equiv valiant, and nickel equiv valiant here from
here on wards, so all these region, you know, you will have austenite stable at a room temperature.
And is easy to check whether it is stainless steel austenitic or not, is to check will
this be non magnetic gamma is alpha is ferro magnetic ferro magnetic, whereas this is non
magnetic is para magnetic. so you will find it this is will be attract by magnet, but
not austenitic steel. So, many place attains you know, you are one of the main application
of austenitic steel is on this steel the when you why often you check that weather they
are attracted by magnet. Now this, we talk about ferrittic, and martenstic
grade of steel. Now, look at us austenitic grade of steels. Now, nickel is the very expansive
alloy element, so there have been attain to have which austenitic grade which are cheaper,
and they are nickel is substituted either completely or partially. And, common elements
which are used to substitute primarily is manganese, and nitrogen, and copper, these
are use to stabilize austenite to room temperature. But most common grades, these are AISI: 200
series is manganese substituted stainless; austenitic stainless steel ferrite and martinstic
stainless steel also stainless the martinstic but austenitic steel definitely this are stainless
all has high amount of chromium. One of the popular grade, the most popular
grade is AISI 304 you have 18 chromium, 8 nickel. But it does, and it has some carbon
content, and we will see they effect of carbon and latter this responsible for problem a
related to corrosion is called sensitization. So, under this condition, this does not is
settable inter angular corrosion, and to avoid that of a certain alloy elements are added
like molly it gives a better acid resistance or pitting resistance. You have little high
amount of nickel to give acid resistance. So, this is another popular grade AISI 316,
another is AISI 321, here this is called the stabilized will see later why stabilized add
certain amount of very strong carbide former, and another popular AISI 347; this contains
navobiyan is strong carbide format, this is also stabilized stainless steel.
Now, mean important property of austenitic steel; it has excellent corrosion, and oxidation
resistance. It can use for high temperature application, and it has not only high strength,
good ductility as well, the main problem is, it has mean ability heat treatment like martins
tic heat treatment is not possible only way, it can be strength and cold work, and it strength
can be increased to it should be 1000 up to around 1000 by cold work, by giving cold work,
and increase the strength significantly. Now, let us; look us what is sensitization,
we mention that oxidation or corrosion resistances of steel is derived from chromium, and this
chromium which must be present, when it is present in solid solution; then only the steel
has good corrosion resistance. No matter whether, it is austenitic or ferittic, this chromium
must be present solid solution. Now, the problem it comes up can austenite can dissolve sufficient
amount of carbon as well, and so what happen in this the austenitic stainless steel that
carbon which is dissolve in austenite, and ferrite chromium, which also dissolve in austenite,
that is possible they may be react to form carbide, and this happens heat by chance this
steel is heated to be region, and which is often very common.
And if you try to why does the steel to make a vessel or something, you know some of the
area will be heated to this region, and then type of precipitation will occur, and when
the precipitation occurs, you know it consumes significant amount of chromium; around 70
percent chromium, you have a chromium carbide, then what happen if this chromium carbide
forms at the grain boundary is a rounding region that chromium get depleted, all this
chromium get depleted, and its comes here, so you a region which is depleted a chromium,
and this users stainless corrosion resistance property. Therefore, this is where you know
it is get attack by the environment, and it is susceptible to inter granular cracking.
And, this is the chromium concentration profile that we can think of, so this is the base
chromium is twelve, and suddenly this comes down and near the precipitate is goes up to
70% and the question is and main reason for this is grain bounded area gets depleted of
chromium, and lose its stainless characteristics. Therefore, grain boundary is susceptible to
attack, and therefore, it is susceptible to inter granular cracking. And how to overcome
this; one is if you quench from above 800 degree centigrade which is not always possible
then do not get enough time for this reaction to take place, because about that most of
that, this is soluble in the matrix which is often not possible, but if a material has
been sensitize, if you can give this treatment then that problem will overcome.
Then other alternative way is the reduce carbon. So, there are certain grade like a three zero
four a very low carbon; three zero four with a very low carbon. In that case, it is possible,
that will not be susceptible to the sensitization problem. Another more common way of avoiding
it; add strong carbide former like titanium or nayobiyam then what happens this reacts
with carbon and then this formes titanium carbide or nayobiyam carbide and they stronger
affinity for carbon then chromium, so therefore chromium does not enough, I mean carbon to
react with it, and chromium is forced to remain in solid solution. So these are the very common
way of avoiding that sensitization problem in steel. Now, quick look at creep resistance steel;
now, we know, we talk about creep, which is time dependent deformation, any component
which is used at a high temperature. In that case, there will be time dependent deformation,
and this is a thermally activated process. Now, if you are therefore looking for is steel
or any alloy which can withstand high temperature or high stress at high temperature. In that
case, what you will look for; you look for the material must be able to withstand and
that environment, that temperature it must a good oxidation resistance or it must have
good creep resistance. Oxidation resistance common alloy element is chromium; creep resistance
common alloy element is molly. Another is structural stability, which should have very
stable structure, and it should have preferably some stable precipitate; and stable precipitates
are coherent precipitate coherent or semi coherent precipitate is more stable.
So, look for such future in the alloy, and also it often, you know this creep resistance
steel many cases, if it is if you are making the boiler, the super heater, you have to
see that not only creep resistance, it must have good weld ability. Because main fabrication
technique will be welding, and these tubes are welded, so often you know there is a limit
that it will have lower carbon. Now, the earlier creep resistance steel, older power plant
which does not operate at very high temperature may be the temperature is round 450 degree
centigrade. So, there half molly steel is good for creep resistance, but this is known
for a problem called graphitization prolonged use at high temperature, carbides gets convert
it to graphite. So, that is the major problem, so how to over
this; this is over come by adding chromium and add a little bit of molly. So, two quarter
chrome, one molly is a very common creep resistance steel. And this is used a 565 degree centigrade
so mean is a question that comes up how to improve creep resistance, and it is a time
dependent deformation, and deformation we know is a main reason is dislocation glide,
if you make this is difficult, how do you make it difficult, you have precipitate large,
and many precipitate you need to have to make this dislocation glide difficult. Sometimes
you can make dislocation movement difficult by loading stacking fault energy like austenitic
steel they have low staking fault energy. So therefore, two dislocations will be difficult
movement will cannot crossly. Then another important factor is coarse grain and obviously,
when you looking for creep resistance steel anything walking at high; you will definitely
you looking for steel which has or material which has high melting point. And these are the common creep resistant alloys,
which are listed we talk about, and this is new grade steel currently used super critical
power plant; it can go up to 600 degree centigrade, it has higher amount of chromium, and some
amount of strong carbide formers, which gives stable carbides, and it forces molly to remain
in solid solution to give it better creep resistance. Austenitic steels are good creep
resistance, and higher temperature you go for nickel base super alloy. There is often
a debate going on whether ferritic steel or austenitic steel a better for creep resistance
alloy. So, no doubt at a higher temperature a space ability considered austenitic steel,
you do not have alternative this has higher temperature capability. But ferritic steel,
it is possible to improve its temperature capability to 650 degree centigrade. And because
it has certain advantage like it has low thermal co efficient of expansion, then has higher
thermal conductivity which are also quiet important. So, some of these properties which
are comparison is given for ferritic and austenitic steel. And this diagram is actually time to represent,
and will quickly go to that. So, pictorial shown as how do you improve the creep resistance,
so you have precipitates the lambda is the distance between the two, and the key reason
is, so this is the creep rate equation this is dislocation density velocity. And, key
thing you can assume that always time to claim is this total time; this time has to component,
these are the layer arrange a dislocation pile which has formed which against a piratical.
Now dislocation has to climb to overcome this, and this to make this closes very difficult,
that means if you make this practical bega, it will take longer time to climb. And always
t climb is much larger than the t glide. So, you can approximate the expression like this.
So, what it means for the higher creep resistance, you have shorter inter particle spacing and
larger piratical. So, that is the key. So, with this we stop here, and to some up
whatever we have cover today is a dual phase steel, a special heat treatment call patenting,
another heat treatment called ausforming, we talked about alloy steel; why it is necessary
to add alloy element to steel, we talked about special grade of steel called maraging steel,
and creep resistance steel. Thank you.
