the iron alloys

7/18/2019 The Iron Alloys

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Mech 473 Lectures

Professor Rodney Herring


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Effect of Alloying Elements on the Eutectoid

DecompositionPlain Carbon Steels contain:

0.5 .0 ! "n and

0.5 0.#0 ! Si

$o% Alloy steeels may also contain additional elements such as:

Co Cr "o & 'i ( )i and may ha*e "n contents up to 5!

&hen dissol*ed in austenite+ these alloys can affect the ,,,,. reaction by+

. Changing the eutectoid temperature )E

"n and 'i stabili-e ,,, and thus ,,.the )Eto belo% / oC.

Si+ Cr+ and "o stabili-e .....and thus ,,,,the )Eabo*e / oC.

'ote: the effect of alloying elements on the phase stability of ferrite should not be confused %ith their effect on the stability of graphite or cementite+ %hich may be uite

different.


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Effect of Alloying Elements on the Eutectoid

DecompositionAlloying additions affecting the eutectoid reaction 1cont2d3

/. $o%ering the eutectoid composition of the austenite to belo%0. !C strengthens in the order of )i4"o4&4Si4Cr4"n4'i.

#. Changing the composition of the ferrite and cementite by

partitioning in the pearlite transformation.

'ote that an element in the phase upon cooling does not 6ust gointo the phase but is ,,,,,.bet%een the ferrite and thecementite phases. &hat does this mean7

8. Changing the gro%th rate of pearlite

Co is the only element that does not retard the gro%th of pearliteAs+ Si+ Cr+ and "o raise )E%here the degree of undercooling is

increased so at temperatures abo*e the 9nee; in the )))

diagram+ the gro%th rate is increased %hereas at temperatures

belo% the 9nee;+ the gro%th rate is decreased.


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Effect of Alloying Elements on the Eutectoid Decomposition

&hat is )Efor #! Si7

&hat is !C of

eutectoid for #! Si7


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Effect of Alloying Elements on Pearlite $amellar Spacing

# =

)he addition of 0.8.? %t! Crdisplaces the plot to the right+ ie.+

to ,,,,..and up%ards to

,,,,,lamellar spacing.

)he addition of .0?.? !"ndisplaces the plot to the left+ ie.+

to lo%er )Eand do%n%ards to

larger lamellar spacing.

&hat does "n stand for7


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)emperature Dependence of the Pearlite )ransformation

At temperatures 6ust belo% the eutectoid+ it can be assumedthat:

@ )he nuclei are ,,,,..distributed throughout the austenite@ )he a*erage rate of nucleation+ '+ is ,,,,,,. .

@ )he nodules remain ,,,,..in shape

@ )he gro%th rate+ + is ,,,,,,, .

@ )he fraction of austenite transformation to pearlite as a functionof time+ f1t3 is then gi*en by the Bohnson"ehl euation:

)his euation gi*es a typical ,,,,,cur*e for the fractiontransformed as a function of time.

A considerably more comple euation is reuired at lo%er reaction

temperatures %hen the assumption of random nucleation is no

longer *alid.

( ) 8#t'Aef1t3 /3=


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)ime)emperature)ransformation Cur*es

1))) Cur*es3)ransformationtime cur*es for a gi*en temperature are deri*ed by

eamining the ,,,,,,,,..of samples remo*ed from afurnace after preset times.

)he time+ %hich is taen as the ,,,,. + of the reaction is taen as

the time to obtain ,,,..of the transformation product.

)he time+ %hich is taen as the ,,,,.+ of the reaction is taen asthe time to obtain ,,,,of the transformation.

)hese times are plotted for a series of temperatures to gi*e the )))

cur*e.


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1))) Cur*es3The TTT curve is divided into areas, which represent single constituent fields

or two component regions.

To the left of the 1% pearlite C-curve the microstructure is austenitic.

To the right of the 99% pearlite C-curve, the microstructure is pearlitic.

Between the curves, the microstructure is a miture of austenite and pearlite.

The amount of the two constituents can !e o!tained !" plotting further

C-curves for 1#%$, #%$, %$, etc.


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1))) Cur*es3)he ))) cur*es are only plotted for temperatures abo*e the 9nee; of

the temperature dependent gro%th rate cur*e for pearlite.

As the temperature is lo%ered to this nee+ the time to start the

reaction+ %hich is the ,,,,,,+ is drastically lo%ered and

is a maimum at 00 oC.


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)he ainite Feaction

ainite is formed o*er a %ide range of temperatures from /00500 oC.

@ ,,,,bainite form in the temperature region #00500o

C@ ,,,,bainite forms in the temperature region /00#00 oC

ainite lie pearlite is a miture of ferrite and carbide+ but the ,,

of the carbide depends on the its temperature of formation.

@ Gor upper bainite+ the carbide is ,,,,.Ge#C %ith ,,,,. .@ Gor lo%er bainite+ the carbide is ,,..1Hagg carbide3 Ge/./C %ith

,,,,, .


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Ipper ainiteIpper bainite formed at 80 oC is composed of a *ery fine structure

composed of laths of ferrite %ith layers of cementite bet%een the

laths.


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Ipper ainite$o%er bainite formed at /50 oC is much courser and the carbide

particles are precipitated %ith the ferrite.


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>inetics of the ainite FeactionThe isothermal decomposition of austenite to !ainite follows a t"pical '''.. ,

which can !e anal"(ed !" the )ohnson-*ehl e+uation.

This e+uation was used to generate the solid line in the transformation plots for

!ainite reaction at 1-9# oC in a 1.1% C h"pereutectoid steel.

The eperimental data conform closel" to the e+uation at high temperature !ut

deviate at the lower temperatures, which in fact lie !elow the * sof the steel,

ie., the martensitic transformation temperature, at 1# oC.

/n this !asis, the !ainite reaction can !e classified as a ''''''''''''''''''''''''''''''''''''.. .


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))) Cur*es of the ainite FeactionThe isothermal decomposition of austenite to upper and lower !ainite can !e

separated into ''''''''. TTT curves !" the addition of allo"ing

elements such as 0i, which slow down the formation of car!ides.

This has !een demonstrated in a steel containing #. C, .# *n, and .1 0i.

2n the plots !elow, the solid lines refer to &% transformation, while the dotted

lines indicate the eperimental scatter of the data.


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"echanism of the ainite Feaction*etallurgists have !een arguing for more than "ears a!out the mechanism of

the !ainite transformation mechanism.

This has arisen !ecause it does not fit precisel" under either diffusion controlled

nucleation and growth transformations li3e pearlite or shear controlled

transformations li3e martensite.

4 solution to this dilemma has !een suggested !" 5o and Cottrell6

@ 4ustenite transforms to ferrite !" a martensitic shear transformation

@ 0u!stitutional solutes remain in the same positions relative to the 7e atoms@ 2n upper !ainite, the car!on diffuses in the austenite to form cementite

@ 2n lower !ainite, the car!on diffuses in the ferrite to form -car!ide

8ence the initial phases form !" shear and then diffusion of car!on controls the

growth of !oth the car!ide phases.


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)he "artenisitic )ransformation

4thermal martensitic transformation

in #.#% C low allo" steelffect of car!on concentration on

*sand *f.

The hori(ontal lines on TTT diagrams refer to the formation of martensite.

*sis the highest temperature at which martensite forms on cooling

@ *artensite can form !" deformation at temperatures : * sand ; *d

* and *9#are the temperatures at which these percentages of martensite form.

*f is the lowest temperature at which martensite forms on cooling although the

transformation ma" not !e 1##% completed at * f

These characteristics are referred to as ''''' as opposed to isothermal.


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)he "artenisitic Phase in SteelsThe cr"stal structure of -ferrite is '''.with the car!on atoms '''

distri!uted among the interstitial sites.

4s the maimum concentration of C is #.# wt%, ver" few of the availa!lecar!on interstitial sites are actuall" occupied so the structure remains

cu!ic.


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)he "artenisitic Phase in Steels2n


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$attice Parameters of Austenite and "artensiteThe tetragonalit", ie., the c>a ratio, of the !ct cell of the martensite phase is

governed !" the car!on concentration.

7or a steel of composition wt% C, a linear dependenc" gives the followingrelationships.

The '''''''''''''..of austenite are given !"

a ? #.&&& @ #.##

The lattice parameters of martensite are given !"

a ? #.AA #.##1

c ? #.AA @ #.#1#A

F l ti hi b t th C t l St t f


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Felationship bet%een the Crystal Structures of

Austenite and "artensiteThe a lattice parameter of the tetragonal martensite unit cell is given !"

31austenite/e31martensitaa =

hile the c lattice parameter of the tetragonal martensite unit cell is given !"

31austenitee31martensit cc =


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(olume Changes Accompanying "artensite Gormation

Dsing the e+uations for the lattice parameter of austenite and martensite, the

volumes of the units cells of the two tetragonal structures can !e compared

at a common car!on concentration, e.g., 1 %C.

8ence,

volume of austenite tetragonal cell, Eat

( ) ## nm0/##.08#5JJ.0//

=== aa

aVat

Eolume of martensite tetragonal cell, Emt#nm0/8#.0/J?/.0/?5#.0/?5#.0 === caaV

mt

The increase in volume due to martensite transformation

( ) !80/##.00/##.00/8#.0 ==V2f this volume change is assumed to !e isotropic !ecause of the different orientations

of the martensitic plates, then the associated linear epansion is given !"

.#> ? 1.%

The relaation of the c>a of martensite during tempering due to the reduction of

car!on content caused !" the car!ide precipitation can thus create

''''''''''''''''''. .


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Surface Felief Effects

hen we loo3 at the surfaces of these materials using optical microscop", we see

lines which appear as scratches.

The scratches are continuous !ut change direction at the interface !etween the

austenite and martensite phases

0urface relief effects in 7e-&%$t


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Surface Felief Effects

These o!servations indicate that the surface is tilted in the region of the

coherent interface so that the surface relief is of the form associated with a

mechanical ''''''as opposed to ''''''deformation.


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$ath "artensite

Fath martensite occurs in plain car!on steels with ; #.% C and 7e-Gi-*n.

/ptical micrograph showing surface relief of ''''''''.in 7-#.% Csteel.

Gote6

@ The laths are t"picall" #. ## m.

@ The ha!it plane of this t"pe of martensite is close to H111I

@ The laths are usuall" o!served in pac3ets within which each adJacentlath has the same ha!it plane variant and shape deformation incontrast to lenticular martensites.

$ i l " i


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$enticular "artensitesThese martensites occurs in high car!on steels with : #.% C and 7e-Gi-*n, 7e, and $t.

/ptical micrograph showing surface relief of 7-&% $t allo".

The plates have the shape of a conve lens. This is confirmed !" eamination of successive sections afterrepeated removal of a thin surface la"er of the sample.

0ince the interface planes are curved, the '''''''. is ta3en as the plane of the centre-line of the

plate or the mid-ri!.

*ore than one ''''''..of the ha!it plane ma" !e o!served within a single austenite grain, which

gives different contrast of the light and dar3 plates.

The martensite plates generall" etend across a prior austenite grain and narrow to a point at the grain!oundaries.

0mall martensite plates also form !etween larger prior-formed plates and etend across the availa!le

length !etween the prior plates.

Eff t f C b C t t H d f " t it


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Effect of Carbon Content on Hardness of "artensite

4 minimum of #.% C is re+uired to o!tain significant hardness.

4t car!on contents of ; #.A%, the hardness of martensite decreases

progressivel" with car!on content. 0ee plot net page.

4t car!on contents of : #.A%, the hardness plots flatten out and there is more

eperimental scatter.

This is caused !" increasing amounts of retained austenite in the steels so that

the measured hardness is not trul" indicative of 1##% martensite.

Eff t f C b C t t H d f " t it


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Effect of Carbon Content on Hardness of "artensite

Eff t f All i El t H d f " t it


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Effect of Alloying Elements on Hardness of "artensiteThe hardness of martensite is determined '''''''!" its car!on content

!ecause the interstitial car!on atoms6

@ Kistort the !cc -7e structure to !ct and there!" !uild up large elastic stresses,

@ 4re ver" efficient at loc3ing dislocations in place or ''''''''. , whichis responsi!le for the sharp "ield point in -7e

0u!stitutional allo"ing elements, such as Gi, Cr, $t, etc, do not effectthe hardness of

martensite !ecause6

@ The" do not contri!ute to the tetragonalit" of the structure

@ 4s point defects the" are not ver" efficient at loc3ing dislocations

8ence low allo" steels contain ; &% total allo" content.

The hardness of > $>* can !e predicted from the car!on content, which is

usuall" ta3en as Loc3well C&.

)h C l t ))) Di f E t t id St l


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)he Complete ))) Diagram for a Eutectoid Steel

The full diagram for a eutectoid steel shows6

@ The start and finishing times for isothermal pearlite and !ainite reactions

@ The start and finishing temperatures for athermal martensite formation

2n this diagram, the temperature of the pearlite and !ainite reactions ''''

so the transition from pearlite to !ainite is smooth and continuous.

4!ove & oC, austenite

transforms completel" to

pearliteBetween -& oC, !oth pearlite

and !ainite are formed

Between and 1# oC Hthe *sI,

onl" !ainite is formed

/n cooling !elow *s, martensite is

formed !ut in addition lower

!ainite can also form after

long holding times.

"i t t f ( i )i ) t P th


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"icrostructures from (arious )ime)emperature Paths

$ath 1 Muench to 1A# oC, hold for # min and then +uench to LT.

The pearlite transformation is suppressed !" the +uench. % martensite is

formed at 1A# oC !ut the holding time is not long enough to form !ainite

so more martensite is formed from 1A# oC to LT.

$ath Muench to oC and hold for 1## sec, then +uench to LT.

The holding time at oC is not long enough to form !ainite so the austenite

transform to martensite on cooling from oC to LT.

$ath Muench to ##o

C, hold for# s and +uench to LT.

The holding time at # oC

produces % !ainite with the

remaining austenite tranforming

to martensite on cooling to LT.$ath Muench to A## oC, hold

for 1#s and +uench to LT.

1##% pearlite forms after s at

A## oC with the additional holding

time causing no further changes.

Effect of )ransformation )emperature on the "echanical


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Effect of )ransformation )emperature on the "echanical

Properties of a Eutectoid Steel2n general,4s the transformation temperature is '''''', a finer microstructure is

'''''''., which causes the tensile strength to !e ''''''. , while

the ductilit" is '''''''' .


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the iron alloys

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