Improving surface properties: Changing chemical composition
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Improving surface properties: Changing chemical composition

September 30, 2019

hello i welcome you all in this presentation
related with the subject fundamentals of the manufacturing processes and ah we are talking
about the methods ah used for improving the properties of the bulk materials mainly the
surface properties so in the previous presentation i have talked about the methods which are
ah used for improving the surface properties without modifying the chemical composition
and in this presentations the processes wherein the chemical composition of the surfaces modified
in order to improve the properties of the surfaces so ah there are various ah mechanisms
which are used to modify the composition of the material according to this ah ah for this
purpose means ah ah like this is the component whose surface properties are to be improved
so near surface layers of the component are ah composition of the near surface layers
is modified by ah having the suitable environment ah like say normally certain alloying elements
which ah will be helping to improve the properties of the surfaces mostly like in the ferrous
systems ah like ah introducing the carbon or nitrogen or boron or vanadium etcetera
all these helping in improving the formation of their ah phases and compounds at the surface
layers so the properties ah especially hardness wear resistance sometimes residual stresses
or also ah favourable residual stresses are produced which help in improving the properties
of the ah components being made there by the manufacturing processes so the basic approach
is to modify the near surface layer microstructure wherein basically ah the change in composition
ah helps in bringing the suitable formation of the formation of the suitable phases compounds
ah inter metallic compounds carbides borides
ah martensite etcetera so basically the compositional modification is done to have the suitable
type of the constituents near at the surface or near surface layers so that the suitable
type of the properties can be ah developed and ah for this purpose so the common elements
which are introduced in the surface ah a of the ferrous metals mostly ah are in form of
like say carburizing ah a carbon is introduced in the carburizing process nitrogen is introduced
in the nitriding process boronzing process introduces the boron vanadizing introduces
the ah vanadium so ah is is uh in these processes ah so this is one category uh all the processes
where some kind of the diffusion is involved for ah modifying the near surface layers and
ah ah this so this is one category of the process where diffusion ah is the mechanism
which is ah which helps in modifying the near surface layer composition there is another
set of the processes wherein ah we put at the surface we put the
suitable material ah like it is called pre placing [pi/put] put the suitable material
or the surface and then this material is melted with the substrate material using suitable
heat source so heat source is used ah ah maybe use the in form of a ah like a laser beam
plasma or tig arc so ah accordingly we say that the laser alloying plasma alloying or
the tig alloying so here metal is placed and it is ah melted with this wood using suitable
kind of the heat source so that it forms the integral part of the substrate and near surface
layers composition is modified so this an approach is basically the melting
so melting is another approach ah wherein the surface layer composition is modified
ah through use ah by through ah the use of these ah sources of heat may be electron beam
or laser or plasma or ah the flame also can be used for this purpose there is another
set of the process where near surface layer composition is modified through the use of
the ions so high velocity ions are directed on to the surface so what we do ah basically
in this case the metal vapours ah ah are allowed to move up so they get it deposited at the
surface of the component whose properties need to be modified and then ion beam is directed
onto the surface so a high velocity ions ah when directed on to the surface when they
impact due to the high kinetic energy near surface layers ah is affected by the two ways
one crystal structure is deformed and damaged ah and the second one whatever the metal vapours
ah are getting deposited like ah in form of the nitrogen or some another kind of the elements
whatever you want to introduce so these atoms ah we will be getting ah deposited onto the
ah surface of the component of surface of the substrate and ah the that and when the
ions impinge on the surface of the ah component to be modified it deforms the crystal structure
and as well as mixes these ions and atoms with these substrate so near surface layers
composition is also modified and in the crystal structure is also damaged and the combination
of these two will be leading to the ah modification ah of the crystal structure as well as the
surface layer composition ah through the use of the ions
so these are the three approaches melting use of the ion beam and that the diffusion
where this crystal structure as a surface layer composition is modified so that suitable
combination of the properties can be realized so if we see here this ah slide it shows the
kind of elements that we need to introduce so the kind of elements that we need to introduce
will depend upon the purpose for which ah you want to modify the surface composition
and the design of the component purpose means what is the extent of the improvement in hardness
what is the depth of a ah hardened layer is needed like in some of the cases the depth
of the hardened layer this may be means depth of the modified zone or surface hardened layer
this may be just a one or less than one micron in like ion ah beam modified methods ah uh
the methods where in ion beam is used ah this so ah while in other cases the depth which
is modified ah say in case of ah in case of ah uh ah like a ah the laser cladding ah sorry
laser alloying or a tig alloying this depth maybe even a one mm or so ah while in other
cases ah like ah ah where nitrogen and the carbon are introduced at the surface in carburizing
and nitriding methods so this depth may be like say from a hundred micron to the thousand
micrometer or even more so depending upon the depth depending upon the hardness hardness
ah properties needed at the surface and ah the depth ah of the hardened zone these parameters
will be ah will be at the governing the the process that we need to select the further
modification apart from these functional requirements the component to a design
design of the component which is to be modified ah also effects because we need to accommodate
that ah into the ah the process so that the modification can be facilitated and if the
design is too complex then heating and cooling differential heating and cooling can lead
to the distortion ah and the dimensional modification so that can be a problem so under those cases
when the geometry is complex the component is vulky ah so we need to use such processes
where differential heating and cooling is minimum and ah for that purpose like ah the
plasma assisted carburizing and nitriding are ah found useful because the the rise in
temperature of the component is limited as compared to the ah conventional carburizing
and nitriding processes ah now we will see the details little bit more
ah like say the if their steal ah hardness is somewhere say ah six hundred to eight hundred
hk new hardness while the hardness of the other zones ah is um ah ah this is a ah eighteen
hundred to eighteen hundred to two thousand ah hk ah hk hardness this so this is the hardness
scale so if we see ah the low carbon steels they will be having the hardness uh ah very
low hardness this is for low carbon steels and when their properties are modified suitably
using like say carburizing ah nitriding then the properties get improved then the hardness
gets a improved and we get a much wider ah means much greater hardness and ah as compared
to this one so this may be like say carburizing this may
be for nitriding and ah so here its about the processes being used and on the other
hand we have ah the ah boronizing kind of the process which much har ah much higher
hardness ah is realises so this is what we can see in this diagram wherein as for the
hardness levels ah the low hardness for the simple steels ah but ah low carbon steels
will be having for the lower hardnesss and when they are carburized and the nitride a
nitriding is performed there is an increase in hardness takes place like say from the
eight hundred to thousand hk ah boronizing causes further significant increase in the
hardness say which may be in the ra ah range of like say sixteen hundred to thousand hk
so depending upon the kind of process um the different hardness values can be realized
and and accordingly we need to select the suitable
kind of the process so if the functionality or the property requirement as well as the
design of the component affects the selection of the suitable kind of process now we will
see the general approach in all these processes which are used whether it is carburizing or
nitriding or ah boronizing in all these processes what we do ah like the substrate component
is taken like this and then suitable rich environment is created all around component
and ah these so ah these atoms in the very nascent state at high temperature ah when
ah ah a high temperature that the high temperature conditions when ah environment ah rich in
either carbon or nitrogen or the boron is created around the component
so because of the compositional difference ah they starts getting diffused so there will
always be compositional gradient maximum will be at the surface and it will be decreasing
ah as we ah go down below the surface so accordingly the depth of the hardness means the extent
of the improvement and hardness will be ah changing so this is what i has been shown
in this ah diagram wherein the requisite ah [el/element] element rich environment either
in form of plasma ions or uh ah metal vapours ah in atomic state or in ion a state or a
plasma state is created all around the component to be modified so suitably say like chromium
or nitrogen carbon boron or silicon rich environments are created
these will be getting diffuse in to the substrate and once they are present they will be improving
the ah properties as per their action whether the are they will be forming borides or nitrites
or the carbides or it may be leading to the formation of the martensite so as ah per the
kind of elements which is being added it will be forming different types of the compounds
ah and the phases which will be having the different characteristics so accordingly ah
they will deferent ah kind of improvement in properties of the component ah up to the
a different level a carburizing is one of the most common process which is used and
as name appears in carburizing carbon ah rich environment is created all around the component
for ah introducing the carbon near the surface of the component ah
so which type of the components are carburized that is what is to be seen like the components
wherein we want that core is tough while surface is hard so this kind of the ah properties
are offered by the low carbon steels low carbon steels ah as we see this diagram carbon content
low carbon so it will be offering you higher toughness and the ah with the increase of
the carbon content so there is the reduction in the toughness value ah so if uh if the
properties are like this so at ah ah while in in terms of the hardness ah there is continuous
improvement in the hardness so when the carbon is low say point one the toughness is high
but the hardness is very low ah when the carbon is high the hardness is high but the toughness
is very ah low ah so what combination we want that surfaces are hard corresponding to the
point eight level of the carbon content while the core remains is still tough so for this
purpose only the core composition is maintained while the surface composition is enriched
with the carbon so for this purpose only the carbon is introduced near the surface layers
and this can be introduced up to the depth of say ah um ah point one to one point five
mm depending about the purpose for which it is to be introduced so ah there was the carbon
gets enriched ah near the surface layers and this enrichment is done up to the pointers
like say point eight to the ah point six to point eight ah eight percentage once the carbon
is enriched at the surface so this will be ah leading to the higher carbon content in
the steel a high carbon steel but high carbon steel which is simply enriched with through
the diffusion process will not be increasing the hardness
because it will be leading to the formation of the iron carbide actually what we need
that whatever carbon has been introduced in the steel ah that is transformed into the
martensite and that is why ah what we do the kind of thermal cycle which is used for the
carburizing purpose increase in temperature up to the range of [sal/say] say eight fifty
two ah nine fifty degree centigrade and it is held for ah ah the time as per the requirement
of the depth to be modified ah wherein depth modified or depth carburized is one proportional
to the square root of that time ah so longer is the depth greater will be the ah longer
is the time for carburizing greater will be the depth which will be enriched with the
carbon and then ah it will be cooled so after cooling in again ah it is heated
to the austenitic state so that it forms the austenite for the carbon content since this
is the ah this will be leading at point eight percent it will be leading to the eutectoid
composition of the steel where in upper eutectic temperature is seven thirty degree centigrade
so heating up to say eight hundred degree centigrade we will be leading to the formation
of the gamma that is austenite so once the homogeneous austenite is formed we we it will
be quenching it hard so quenching will be leading to the formation of the martensite
so basically ah objective is with the first heating and holding is done for carburizing
purpose and once this is over carbon enrichment has taken place thereafter again it is heated
so that ah homogeneous austenite state is achieved and followed by the rapid cooling
or the quenching so that hard martensite is formed this is
the kind of heat treatment cycle which is used for ah the purpose of the carburizing
ah we know that ah the carburizing is a diffusion based mechanism wherein ah the diffusion is
found a function of the temperature as well as time so higher is the temperature greater
with the greater will be the rate of diffusion greater will be the depth of to which carbon
will be ah introduced in the modified surface or carburized surface similarly greater is
the carburizing time greater will be the depth of to which carbon will be enriched so depth
of carbo of the carburised layer will be the function of the time and temperature both
so this is what we can see here as per the another diagram wherein what we can see if
the carburizing time we mentioned here in the axis scale and
the depth of the carburizing is seen in the ah y scale carburizing ah then ah there will
be continuous increase in the depth of the carburizing with the ah increase of the time
and the rate of increase is faster for the ah like say if it is eight fifty degree centigrade
the rate of increase in the depth of carburizing is low as compared to the case when depth
of carbonizing is high so at nine hundred fifty degree centigrade this is what we can
see here in this diagram what it is showing is schematically that near surface layers
will be having the higher carbon content as compared to those which are below and
the depth up to which carbon is able to get diffuse into the substrate that is called
carburized layer and the depth of the carburized layer we can see here increasing from ah as
a function of carburizing time it is increasing at eight fifty degree centigrade increase
is limited while at nine fifty degree centigrade with the as a function of carburizing time
increase in depth of the carburizing ah layer ah or carburizing depth is increasing at much
higher rate and this is attributed to the higher rate of the diffusion of the carbon
ah into the iron at the higher temperature and we know that since the carbon directly
affects the kind of phases which will formed kind kind of the amount of the hardness ah
amount of the martensite which will be formed and the hardness of the martensite so both
ah the carbon con increase in carbon content
increases the martensite ah fraction ah and the increases the hardness of the martensite
both ah we know from this diagram that the martensite may be found but ah the hardness
is less [la/like] like this may be like say thirty or forty hrc ah hrc and with the increase
of the carbon content this may be like say sixty to sixty five hrc so what we can see
here with the carbon content point two and with the carbon content point eight so even
if the martensite is formed it is a very low carbon at ah if the carbon content is very
low in the martensite hardness will be low but ah the hardness increases with the increase
of the carbon content and ah it becomes ah maximum say around sixty to sixty five
so the carbon directly effects the amount of the carbon presented the surface directly
affects the fraction of the martensite usually formed as well as the hardness that will be
obtained and as we go down below the surface what we can see here ah there is a continuous
drop in the hardness because of the decreasing carboned content ah in the subsurface region
this is the typical method there are three ways of the carburizing one is the solid carburizing
ah there is also um this is also known as a pack carburizing then liquid carburizing
and gas carburizing so in all these cases the concept is same where carbon rich environment
is created around the components so that it can get diffused into the surface and here
in the ah so pack carburizing or solid carburizing ah the steel chamber ah filled in with the
charcoal and the limited amount of the oxygen leads to the formation of the carbon monoxide
which decomposes into the carbon and oxygen and that leads to the introduction of the
ah enrichment of the carbon into the component
being modified so this is what we can see in this typical heat treatment cycles which
ah cycle which is used for the carburizing purpose initially the heating then holding
during which carbon enrichment will be taking place then a step coo step cooling is carried
out for avoiding the ah unnecessary ah the residual stress formation or ah we can say
there a distortion tendency due to the the ah due to the thermal shock kind of conditions
so step cooling helps in avoiding the cracking as well as the distortion tendency thereafter
rapid quenching helps to have the martensitic transformation and after the martensite ah
is formed ah it is tempered so this ah many times in the solid carburizing a direct quenching
direct quenching is not practically workable so what is done first the pack carburizing
ah carburizing is done in the box then components are taken then again it is reheated ah for
a heat treatment purpose and thereafter it is quenched while in case of the liquid carburizing
it is somewhat easier because ah we can take out the sample easily from the bath and then
we can quench directly ah into the water or any other suitable medium for quenching purpose
so liquid carburizing can help directly in heating in one go then followed by quenching
to achieve the desired hardness while ah um the in case of the solid carburizing we need
to do the heating ah twice first for the carburizing purpose and second for the ah hardening purpose
another mechanism is the ah another mechanism is ah or the surface modification approaches
the nitriding ah carburizing we know ah is done in the austenitic
state ah ah so heating is done ah above the upper critical temperature so that homogeneous
austenite can be formed and a lot of carbon can be introduced in in the austenite which
is having the fcc structure and can accommodate lot of carbon and has the higher solubility
ah to the carbon while in case of the ah nitriding the approach is different nitriding is always
carried out in the ah ferritic state so the normally ah the the heating is done up to
the temperature of five hundred to five fifty degree centigrade and ah in the of course
in the nitrogen rich environment so in a box basically ah ammonia is heated so ammonia
ah breaks into the nitrogen and the hydrogen and ah
this nitrogen ah gets a diffused into the ah the surface of the component at the high
temperature in the ferritic a state of course so five hundred to five fifty degree centigrade
is the normal thing which is used so in this process basically the nitrogen will be getting
introduced the surface in the ferritic a state so basically it will be did in leading to
the formation of fe three n or fe four ah fe four and so these are the kind of compounds
which are formed at the surface will be having the more ah nitrogen concentration has compared
to the below surface regions so a typical white layer is formed at the surface where
higher concentration of the nitrogen existing and this ah white layer becomes very hard
very brittle and so it has a lot of cracking tendency and therefore normally this white
layer after the nitriding is removed so that we are ah left with the only the stable
nitrites at the surface which can offer the improved properties in the material ah simple
uh for ah the nitriding for the simple steels is not that effective [beca/because] because
it will be leading to the simple formation of the iron nitride only so ah in order to
have a more ah effective nitriding process a certain steels having the ah presence of
vanadium chromium ah aluminium these are ah added so that they can form their nitrites
effectively and thats why certain alloy steels uh are found more suitable for nitriding purpose
and these alloy steels generally have the concentration of good concentration of the
vanadium and chromium because they are nitrides are very hard and
stable so they will be leading to the more ah improved ah and effective nitriding process
and the resulting properties will also be better so this is the general principle of
the nitriding process this is what we can see here and effectiveness of the nitriding
is also different for the different alloys because of this difference so we can see as
a function of the nitriding time ah since the nitriding is also diffusion based processes
when the nitrogen rich environment is created with the help of ammonia ah it will be ah
leading to the ah diffusion of the nitrogen into the steel surface ah so a if we keep
on increasing the nitriding time the depth ah the amount and the depth up to which the
nitrogen will be introduced at the surface that will be increasing
so what we can see here ah there is increasing hardness ah increasing depth of the nitride
layer which will be produced for minimum hardness of like say fifty hrc ah so this is the condition
that ah the for minimum increase in the hardness of the fifty hrc the depth up to which this
ah this much of improvement in the hardness is achieved is like say this is this depth
is increasing ah at lower rate for for this steel for three four zero while their imp
rate of the improvement is much faster for a four one four zero and ah it is much faster
for the a nitralloy so the reason for this is ah attributed to the presence of the ah
different elements in the different amounts which are will will have which will be having
more affinity to the nitrogen ah and daily forming more hard and stable nitrides
so this is what we can see in the typical nitride surface will be having a higher concentration
of the nitrogen so eta kind of the [ni/nitride] nitride is formed at the surface then gamma
nitride is found below that and then diffuse nitrogen is present below that so since the
because of the difference in the nitrogen concentration maximum at the surface nitrogen
concentration and ah then below that the it is somewhat lesser but this is the zone where
na ah then nitrides will be formed that is the compound formation will be taking place
well further below the nitrogen concentration is lower and the nitrogen is there in at the
diffused state in the steel so these are the zones which will be subjected to the greater
hardness ah levels due to the ah nitrate formation plasma nitriding and the plasma assisted carburizing
these are the another set of the processes where ah instead of the decomposition of the
gases into the atomic state ah here directly these ah constituents like active
nitrogen and ah carbon are obtained directly through the plasma formation and when the
plasma is formed ah for using the suitable kind of the gases so that we have desired
carbon and nitrogen ah in the environment and this is achieved the using quite low temperature
like ah three hundred to the eight hundred degree centigrade and under the pressure of
three hundred two thousand pa and when this kind of ah the nitriding is carried out when
the nitriding is carried out in the plasma state of the carbon and nitrogen ah it can
ah it if the process is a much faster it takes lesser time and the maximum temperature generated
to the component is also lower so one ah the modified zone near the surface layer is formed
ah after the modification ah the laser alloying ah is the another method
where the surface composition is made through the surface completion is modified through
the alloying ah purpose and ah the general approach for this is the basically pre placement
method and the component or the constraints that ah we want to add is identified as per
the requirement of the purpose like say carbon steel if we take a an example of the carbon
steel if its composition if its corrosion resistance is to be improved then we will
be putting a layer of the chromium over it so because chromium is going to ah improve
the corrosion resistance and then chromium is melted using the beam of the laser
so basically laser beam is directed on to the pre placed ah chromium and then uh when
laser beam directed it melts the chromium as well as it melts little bit the surface
of the base metal so ah this is how ah ah the kind of the mixing of the element placed
at the surface and then ah is taking place with the ah substrate so ah the delusion levels
in this case are much higher may be the say twenty five to fifty percent where lot of
melting of the base metal takes place ah for mixing of the elements being placed at the
surface so this is how the composition of the surface ah layers can be modified in order
to achieve the desired properties at the surface so this is what we can see here or in the
laser alloying ah the this is the substrate and the element to be introduced is ah applied
in form of the paste over the substrate and then laser beam is passed
so wherever there is a laser beam there will be the molten state mixing and the mixing
of the substrate ah mat substrate material with the module placed over the substrate
and after the solidification will be leading after the solidification we we will be getting
the modified zone so this is how we introduced the ah the elements at the surface and ah
that is how ah thats why it is called laser allowing so if we see this component ah which
has been laser alloyed what we can see here the this is the substrate and near the surface
layers are modified using the nickel and [phosphor/phosphorus] phosphorus coating and nick ah and ah this
modified zone is extremely spoiled maybe like say five to ten ah micrometer and this is
realized like by before pli pre placing ah by before we pre place the material to be
alloyed and then laser beam is passed over the surface so that the mixing of the ah the
material pre placed with the base material after the melting ah
leads to the formation of the ah leads to the formation of the ah alloyed layer this
is the another example of the laser alloying where the material placed at the surface it
properly mixed with the base metal and forms the very ah coherent ah modified surface layer
which is ah very solid this is the ion implantation method as i have explained this is the substrate
material and ah and atomic arrangement in the substrate material and the material to
be ah incorporated at the surface is ah evaporated and these metal vapours will be getting deposited
over the surface and ah when the ion beam is directed at high ah velocity onto the surface
so this will be leading to the ah increased mixing of the ah these at atoms with the substrate
ah material and thus the surface ah layers ah get modified and in this process um ah
the surface layer crystal structure is also modified so we will see that ah the most of
the the ions and the metal vapour atoms ah all these are present ah at the surface and
their concentration will keep on reducing with the increase of the depth from the surface
so ah here now i will summarize this ah presentation in this presentation basically i have talked
about ah few approaches like carburizing nitriding ah ion implantation ah ah basically it it
is the nitrogen ion implantation which is used very commonly for improving the properties
and then laser annoying these are the methods which can be used to modify the surface layer
composition ah for improved properties of the components
thank you for your attention

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