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[Music]
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steel metallurgy steel is the widest
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used metal but what constitutes a steel
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how can we affect the properties and
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what happens during the solidification
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of steel these are a few key insights
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that we will try to uncover in this
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module steel is primarily iron with up
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to 1% carbon plus other allying
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additions in the majority of Steel's
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this alloying addition generally totals
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less than 5% but in some Steel's this
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can be as great as 50% a steel
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composition can be thought of as a
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recipe different amounts of each
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ingredient make up your final product in
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steel these ingredients are known as
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alloying additions each addition affects
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the properties of the steel in a
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different way
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depending on the amount and type of
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alloying additions added we can affect
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the following properties in a different
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way strength the ability to withstand
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load in tension hardness the ability to
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resist plastic defamation usually by a
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penetration also described as resistance
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to scratching or abrasion toughness the
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ability to absorb energy ductility the
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ability to deform without fracture
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fatigue the weakening of metal caused by
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repeatedly applied load for mobility the
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ease in which metal can be molded into
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the final product
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machine ability the ease in which metal
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can be processed into the final product
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with cutting tools weld ability the ease
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in which metal can be joined corrosion
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the ability to withstand chemical
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reaction through oxidation to affect the
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steel in this manner a wide variety of
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elements can be added these include
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carbon silicon manganese phosphorus
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sulfur chromium
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molybdenum nickel aluminium niobium also
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called columbium titanium vanadium
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copper boron nitrogen tungsten cobalt
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[Music]
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carbon carbon is the most important
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element for strength and hardness as the
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level of carbon is increased the tensile
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strength and hardness also increases
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carbon is a cheap way of increasing the
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strength and it is essential for the
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formation of microstructures phosphorus
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phosphorus is usually classed as an
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impurity as it significantly reduces the
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toughness and ductility of a steel but
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it can be used as a solid solution
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strengthening this is explained later in
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this module sulphur sulphur is usually
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classed as an impurity as it reduces the
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skills ductility toughness and weld
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ability sulphur can form with iron to
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produce a low melting point in purity
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called iron sulphide this can collect
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along grain boundaries and cause the
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steel to break up during hot working
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this is called hot shortness sulphur is
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sometimes added to steel to aid
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machinability manganese manganese has a
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great effect on harden ability and can
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be found in most commercial steels
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manganese can strengthen the steel
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through solid solution strengthening
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which is explained later in this module
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it also combines with sulfur to prevent
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hot shortness
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chromium chromium increases the harden
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ability of Steel it can join together
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with carbon to form very stable carbide
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which are excellent for wear and
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abrasion resistance it is used in high
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levels in stainless steels for corrosion
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resistance it does this by creating a
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protective oxide film on the surface of
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the stainless field
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molybdenum molybdenum also increases the
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harden ability of steels when combined
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with chromium and nickel
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it has a strong multiplicative effect on
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harden ability it is both in alloy and
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stainless steels
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nickel nickel as with chromium and
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molybdenum increases the harden ability
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of steel and can increase the toughness
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of Steel's particularly at low
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temperatures again it is present in
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large amounts in stainless steels
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silicon silicon is mainly used to remove
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oxygen from steel in a process called
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the oxidation the removal of oxygen in
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Steel is important as oxygen can form
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voids in steel known as blow holds and
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porosity oxygen can also combine with
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other elements to form brittle particles
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known as oxides silicon can be used to
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increase the fluidity when casting
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Steel's Allen minium aluminium is
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primarily used to D oxidize the steel it
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can combine with nitrogen to form
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nitrites which can restrict grain growth
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niobium niobium also called columbium in
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small amounts can increase yield
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strength tensile strength and toughness
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vanadium vanadium is used to increase
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the harden ability and toughness of
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steels through its ability to restrict
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grain growth boron boron can
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significantly increase the harden
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ability of Steel's and can enhance the
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effect of other alloying elements
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nitrogen nitrogen is often added in
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combination with other elements to form
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nitrides these nitrides increase the
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hardness and tensile strength but at the
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expense of toughness and ductility
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[Music]
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when we add our elements they do not
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always work in isolation
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sometimes the elements work in
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conjunction and can cause a
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multiplication effect that would not be
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expected from the sum of the individual
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additions for example both chromium and
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molybdenum may be added individually to
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a steel in order to strengthen it but a
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small amount of molybdenum used in
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conjunction with chromium will result in
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a much greater strengthening effect than
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using one of the elements alone the
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addition of carbon to iron is probably
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the most important addition in cast
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irons and steels this makes a diagram
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called the iron carbon equilibrium
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diagram very useful equilibrium means
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that enough time has been allowed on
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heating and cooling for any reactions to
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fully complete many of the basic
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features of this diagram influence the
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behavior of the most complex steel
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alloys the diagram is used to understand
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what structures will be formed at what
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temperatures and at what carbon contents
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we can also see at what temperature
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different compositions melt and we can
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calculate how much liquid and solid will
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be present at a given temperature and
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can see when a steel will be fully solid
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we can also calculate how much of each
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structure or phase will be present at a
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given temperature on the diagram there
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are several points of interest a 1 the
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temperature at which austenite turns to
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pearlite the lowest temperature
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austenite gamma-iron
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does not exist this is 723 degrees
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Celsius
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a three the temperature when ferrite
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alpha iron transforms to austenite
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gamma-iron but pure iron this occurs at
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nine hundred and ten degrees Celsius the
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lowers in temperature along the line to
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0.8 percent carbon then it increases in
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temperature up to two percent carbon
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liquidus temperature the temperature of
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which steel of a given composition fully
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turns to a liquid below 723 degrees
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Celsius we can see fe3c which is iron
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carbide this is called cementite which
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is a ceramic compound of iron and carbon
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steels with less than 0.8 percent carbon
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consists of a structure of ferrite and
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pearlite their light is a structure that
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consists of iron carbide cementite and
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ferrite alpha iron in parallel last
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above 0.8 percent carbon cementite and
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pearlite are primary constituents
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if we take an example of a 0.3 percent
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carbon steel the steel is molten until
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we call to one thousand five hundred and
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ten degrees Celsius at this point the
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liquid iron starts to solidify into
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Delta iron from one thousand five
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hundred and ten to one thousand four
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hundred and ninety five degrees Celsius
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the amount of Delta iron increases while
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the amount of liquid decreases at one
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thousand four hundred and ninety five
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degrees Celsius the body centered cubic
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Delta iron transforms to face centered
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cubic austenite as we continue to cool
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to 1,450 four degrees Celsius the amount
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of austenite increases and the liquid
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decreases until we have a fully solid
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austenite extractor as we decrease in
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temperature further the structure
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remains austenite until we hit eight
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hundred and twenty degrees Celsius where
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it starts to form body centered cubic
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ferrite from 820 to 723 degrees Celsius
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the amount of austenite decreases and
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the amount of ferrite increases until
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the remainder of the austenite will
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transform but as austenite has a higher
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solubility for carbon than ferrite the
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ferrite that forms will not be able to
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accommodate all the carbon that was
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contained in the austenite and thus the
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remaining austenite will firm a mixture
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of ferrite alpha iron and iron carbide
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cementite this structure is known as
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pearlite here we can see some examples
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of different carbon contents and the
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structures produced low carbon structure
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consists primarily of ferrite with small
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grains of pearlite
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medium carbon structure consists
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primarily of pearlite with a small
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percentage of ferrite high carbon at 0.8
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percent carbon the structure consists
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pearlite above 0.8 percent carbon the
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structure consists of pearlite and
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cement
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in addition to the iron carbon
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equilibrium diagram there are two other
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diagram that are used extensively in
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metallurgy these are the CCT continuous
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cooling transformation diagram and the
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TTT time temperature transformation
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diagram we briefly outline these in this
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module and we'll go into these in a
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greater depth in a following module on
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heat treatment where these are primarily
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used while the iron carbon diagram
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describes the structures of steel under
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equilibrium conditions where enough time
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has been allowed on heating and cooling
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for any reactions to fully complete both
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the continuous cooling transformation
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and time temperature transformation
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diagrams allow determination of
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structures and various cooling rates
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from slow to very fast
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both these diagrams are helpful in
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selecting the optimum steel and process
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parameters to achieve a given set of
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properties continuous cooling
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transformation diagrams are generally
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more appropriate for engineering
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applications the diagrams show the
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structures that are achievable
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continuously cooling from the
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austenitization temperature at a
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constant rate these diagrams often show
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the structure that can be achieved at
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the center of different sized bars for
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cooling in water oil and air some
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diagrams also have the hardness that is
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achieved from this structure time
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temperature transformation diagram show
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how long it would take for a structure
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to be achieved by holding at a given
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temperature this diagram allows you to
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plot varying cooling rates and show the
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structure that would be achieved
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harden ability in mythology the harden
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ability of a steel is a key parameter
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and when we talk about harden ability in
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Steel's we are often describing how deep
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into the steel we can achieve hardening
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if the steel is described as having a
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low harden ability this will mean that
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the steel will produce a shallower depth
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of hardness when a steel has a high
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harden ability it will be the same
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hardness throughout the thickness of the
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product highly hardenable steels are
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more important in large components
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harden ability is not to be mistaken for
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hardness when describing the hardness we
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are often looking at the microstructure
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achieved during cooling for a given
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steel it can be assumed that the quicker
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the cooling rate the greater chance of
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achieving a harder structure and if that
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still has a high harden ability this
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hard structure will be present deeper
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into the thickness we can increase the
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harden ability of a steel by adding
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elements like manganese molybdenum
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chromium nickel and boron when we add
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these elements it increases the harden
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ability and this will enable us to
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achieve a harder structure deeper into
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the thickness the ability of the
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hardness to go deeper into the thickness
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is because it is easier to achieve a
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harder structure at a slower cooling
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rate the structure of a steel can be
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pure light with ferrite or cementite
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which is a softer structure martensite
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which is the harder structure and
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bainite which is in between looking at
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the continuous cooling transformation
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diagram if the steel has a low harden
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ability beright and pearlite
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transformation will be shown in the
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upper left hand side of the diagram
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meaning it will be difficult or even
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impossible to form martensite
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if a steel has high heart and ability
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transformation to martensite will be
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shown at the bottom right-hand side of
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the diagram meaning that a steel will
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fully transform to martensite over a
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large range of thicknesses strengthening
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mechanisms in the introduction to
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materials module we talked about
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dislocations being present in metals
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these dislocations reduce the strength
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of the metal the principle of
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strengthening mechanisms is to reduce
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the ability of these dislocations to
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move through the metal this can be
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achieved by the reduction in grain size
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cold working solid solution
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strengthening and dispersion or
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precipitation strengthening these
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strengthening mechanisms can be applied
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individually or in combination in metal
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it is estimated that there are 10
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million to 1 billion dislocations per
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centimeter squared and each dislocation
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has a strain field associated with it
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with grain size the grains can interact
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with the dislocations preventing further
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movement of them if we reduce the grain
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size we can increase the number of
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grains interacting with the dislocations
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preventing their movement and thus
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strengthening the metal
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[Music]
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cold work introduces a large amount of
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strain into the metal this strain
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interacts with the dislocation strain
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field impeding the movement of the
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dislocations solid solution
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strengthening is applied when we add
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other chemical elements to a metal as
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discussed in the introduction of
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materials module the element added can
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either fall between the atoms of the
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bulk material or replace the atoms
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within steel carbon atoms fall between
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iron atoms and nickel atoms replace iron
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atoms this will either be called
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interstitial or substitutional solid
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solution strengthening and will cause
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distortion in the atomic structure this
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distortion interacts with the
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dislocations preventing the dislocation
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movement and strengthening the steel
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[Music]
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dispersion or precipitation
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strengthening is highly related to the
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structure of the metal and takes place
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when a phase is finally precipitated
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through a softer matrix this hard
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precipitate acts as a barrier to
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dislocation movement the precipitate can
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also produce a strain field that
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interacts with the dislocation strain
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field so in summary
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[Music]
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we have the ability to tailor the
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properties of a steel by adding
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different Aryan elements alloying
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elements do not always work in isolation
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sometimes they cause a multiplication
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effect carbon to iron is probably the
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most important addition to cast irons
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and steel the iron carbon equilibrium
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diagram continuous cooling
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transformation diagram and time
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temperature transformation diagram are
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the most widely used diagrams in steel
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metallurgy continuous cooling
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transformation and time temperature
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transformation diagram allow
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determination of structures at a
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variation of cooling rates harden
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ability describes how deep into the
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steel we can achieve hardening hardening
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is not to be mistaken for hardness when
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describing the hardness we are often
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looking at the structure achieved from
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cooling the structure of a steel can be
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pearlite with ferrite or cementite which
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is the softest structure martensite
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which is the hardest structure and
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bainite which is in between dislocations
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reduce the strength of the metal the
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principle of strengthening mechanisms is
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to reduce the ability of these
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dislocations to move through the steel
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strengthening can be achieved by the
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reduction in grain size cold working
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solid solution strengthening and
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dispersion or precipitation
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strengthening
