Fe -Fe3C phase diagram Iron and Steel Steel Microstructures · PDF fileIron and Steel Steel...

Fe-Fe3C phase diagram

Iron and Steel

Steel Microstructures

1

Phases and Microstructure

Knowledge Incubation for TEQIP IIT Kanpur

Dr. Shashank Shekhar

2

Identify the terminal phases and its solubility

Iron-Iron carbide phase diagram

0.77

2.11

0.008 at RT

0.18

0.10

Univ. of Tennessee Handout for MSE300 Knowledge Incubation for TEQIP IIT Kanpur

Iron-Iron Carbide Phase Diagram 3

Iron-‘Iron carbide’ phase diagram

Its not a true equilibrium phase diagram because iron carbide is not a stable phase

Iron carbide decomposes into iron and carbon (graphite)

Even at elevated temperature (like 700C), it will take several years for decomposition

Hence for all practical purpose Iron-Iron carbide phase diagram represents equilibrium changes


4

Source: Wikipedia Knowledge Incubation for TEQIP IIT Kanpur

Iron-Iron Carbide Phase Diagram 5

Carbon being a very small atom gets into the interstitial of ferrite/ austenite phases to form solid solution

Ferrous metals - based on iron, comprises about 75% of metal tonnage in the world. Broadly three main alloys

Iron = C content < 0.008 wt%

Steel = Fe-C alloy (0.008 to 2.11% C)

Cast iron = Fe-C alloy (2.11% to 6.7% C)


6

Materials Science and Metallurgy by Pollack Knowledge Incubation for TEQIP IIT Kanpur

Allotropes of Iron and various phases 7

1538

1394

912 768

Te

mpe

ratu

re

-ferrite (BCC) Max solubility of C is 0.022 wt %

RT solubility of C is 0.008 wt %

-austenite (FCC) Max solubility of C is 2.14 wt %

-ferrite (BCC) Stable only at high T

Max solubility of C is 0.10 wt %

Fe3C (iron carbide/ cementite) Orthorhombic structure

Intermetallic

Brittle

Why is solubility of C higher in

FCC than in BCC?


Transformation Temperatures 8

A1 = Temperature at which austenite begins to form during heating

A2 = Temperature at which iron becomes non-magnetic

A3 = Temperature at which transformation of iron to austenite is completed during heating

A4 = Temperature at which austenite transforms to delta ferrite

Am = Temperature at which solutionizing of cementite in austenite is complete


Various Transformation Reactions and development of Microstructure

9

Peritectic Reaction:

L + ⇌

Eutectic Reaction: Eutectic of austenite and cementite is known as ledeburite

L ⇌ + Fe3C

Eutectoid Reaction: Eutectoid of ferrite and cementite is known as pearlite. The ferrite and cementite phases occur as alternate layers

⇌ + Fe3C


Definition of Microstructure


10

The microstructure of crystalline materials is defined by the type, structure, number, shape and topological arrangement of phases and/or lattice defects .

Elements of microstructure: Point defects, point-defect clusters, dislocations, stacking faults, grain boundaries, interphase interfaces are important elements of the microstructure of most materials.


Steels 11

Steels can be categorized as

Low carbon steels (C < 0.3 %)

Medium carbon steels (0.3 < C <0.6)

High carbon steels (C>0.6 %)

Steels can also be grouped as (a) plain carbon steels, (b) low alloy steels (c) stainless steels and (d) tool steels

Hypoeutectoid steels (C between 0.022 to 0.77) and Hypereutectoid steels ( C > 0.77)

Several solid state transformations take place in steel


Microstructure of Eutectoid steel


12

When Fe-alloy of 0.77% of C is cooled slowly it transforms from single phase of austenite to pearlite structure, a lamellar or layered structure of two phases: ferrite and cementite

In the micrograph, dark regions are cementite and bright regions are ferrite


Why layered structure? 13

Layered structures are formed because of redistribution of C atoms between ferrite (0.022 wt %) and cementite (6.7 wt %) by diffusion

Mechanical properties of pearlite are in between that of ferrite (soft) and cementite (brittle)

What is the fraction of ferrite for this eutectoid alloy?


Microstructure of hypoeutectoid steel


14


Microstructure of hypereutectoid steel 15


16

Microstructures of (a) Hypoeutectoid

steel (ferrite + pearlite) (b) Eutectoid

steel (c) Hypereutectoid steel (pearlite

with network of cementite)

Heat Treatment by Rajan, Sharma and Sharma Knowledge Incubation for TEQIP IIT Kanpur

Microstructure of steel with C < 0.022 17

In this steel content is less than 0.022 wt % so transformation begins at temperature on intersection of NM (T3)

Transformation gets completed by T4, much earlier than for hypoeutectoid

No change until T5

Below T5, excess carbon gets rejected in the form of Fe3C

In most practical cases, since cooling is not slow enough, microstructure remains 100% ferrite

T3

T4

T5


Cast Iron 18

Cast Iron, as defined earlier, has C concentration greater than 2.11 % and less than 6.7%

Cast iron can be further subdivided into two categories

White cast iron: carbon is present in the form of cementite

Grey cast iron : carbon is present in the free form as graphite


Cast Iron 19

Grey cast iron : carbon is present in the free form as graphite Contains Si (1 to 3 %) which causes formation of graphite

flakes distributed throughout the cast product upon solidification

Presence of graphite gives a grey color to the fractured surface

Good vibration damping because of dispersion of graphite flakes

Internal lubricating qualities which make it machinable

Products made from gray cast iron include automotive engine blocks and heads, machine tool bases

When chemically treated to form spheroids rather than flakes, we end up with ductile iron. It is stronger and more ductile


Cast Iron 20

White cast iron: carbon is present in the form of cementite

Contains 0.5 to 2% Si and lower C content

Formed by rapid cooling of the molten metal

Its hard, brittle and excellent wear resistance

Applications include railway brake shoes

When cast iron is heat treated to separate carbon out of solution and form graphite, resulting metal is called malleable (upto 20%)

Malleable cast iron is utilized for pipe fitting and flanges


Microstructures of eutectic cast Iron 21

On solidification, ledeburite is formed

On further cooling, excess carbon comes out as cementite from eutectic austenite

At 727 C, eutectic austenite would contain 0.77 % C and would decompose into pearlite

0.77

2.11

0.008 at RT

0.18

0.10


Microstructures of hypoeutectic cast Iron 22

In this case, at temperature just below 1147 C, consists of proeutectic austenite and ledeburite

On further cooling, excess carbon comes out as cementite from proeutectic and eutectic austenite

At 727 C, both eutectic and proeutectic austenite would contain 0.77 % C and would decompose into pearlite

0.77

2.11

0.008 at RT

0.18

0.10


Microstructures of hypereutectic cast Iron 23

In this case, at temperature just below 1147 C, consists of proeutectic cementite and ledeburite

On further cooling, excess carbon comes out as proeutectoid cementite from eutectic austenite

At 727 C, eutectic austenite would decompose into pearlite

0.77

2.11

0.008 at RT

0.18

0.10


24

Microstructures of (a) Hypoeutectic steel

(ledeburite + pearlite +cementite) (b)

Eutectic steel (ledeburite) (c)

Hypereutectic steel (ledeburite + primary

cementite)


Effect of alloying elements on Iron-Iron carbide phase diagram

25

Steel consists of several other alloying elements Cr: improves strength, hardness, wear resistance, hot hardness,

hardenability. In significant proportions, Cr improves corrosion resistance

Mn: improves the strength and hardness of steel

Mo: increases toughness and hot hardness. Also provides hardenability and wear resistance

Ni: improves strength and toughness. In significant amounts, it improves corrosion resistance

V: grain refiner and hence improves strength and toughness

Some alloying elements affect the relative stabilities of alpha and gamma iron and as such are grouped as ferrite stabilizers or austenite stabilizers



26

Cr is also a Ferrite Stabilizer

Other elements which tend to stabilize

ferrite are W, Mo, V and Si

These elements are more soluble in α-

phase than in -phase

Most of these elements are BCC

They decrease the amount of carbon

present in the gamma-iron and thus

favor formation of larger quantity of

free carbide

Reduce the austenite region by

lowering A4 point and raising A3 point

For Cr > 12.8%, austenite phase

completely disappeare and -ferrite

and -ferrite merge


Effect of Chromium (Ferrite Stabilizer) 27



28

Ni is also a Austenite Stabilizer

Other elements which tend to

stabilize austenite are Mn and Cu

These elements are more soluble

in gamma-phase than in alpha-

phase

Most of these elements are FCC

Carbon also has stabilizing effect

(at HT)

Reduce the austenite region by

raising A4 point and lowering A3

point

These elements can make it stable

even at room temperature Heat Treatment by Rajan, Sharma and Sharma


Effect of Nickel (austenite stabilizer) 29


Phases and Microstructure


30

Microstructure of annealed SS316L Microstructure of annealed duplex steel

Microstructure

of annealed mild

steel (0.1% C)


Different grades and their applications 31

(a) plain carbon steels Low-C: automobile sheet metal

Medium-C : engine parts (crank-shaft)

High-C: springs, blades

(b) low alloy steels: Various automotive components

(c) stainless steels Austenitic: chemical & food processing equipment

Ferritic: kitchen utensils to jet engine components

Martensitic: cutlery, surgical instruments

(d) tool steels: high speed tool, shock-resistant tools, die


Different grades and their applications 32

Dual-phase steel (ferrite + martensite):

Automotive structural parts, longitudinal beams

Duplex stainless steel (austenite + ferrite):

Heat exchangers, turbocharger pumps

Precipitation hardening stainless stee:

Aerospace and nuclear application

Gray cast iron

Automotive engine blocks and heads, machine tool bases

White cast iron

pipe fitting and flanges


Stainless Steel


33

Fundamentals of Modern Manufacturing by Groover Knowledge Incubation for TEQIP IIT Kanpur

34

Changing only the C

concentration, itself has

substantial change on the

microstructure and properties of

steel

Fundamentals of Modern Manufacturing by Groover Knowledge Incubation for TEQIP IIT Kanpur

Phase transformations


36


Phase diagram only tells

us about the

equilibrium phase. It

does not tell us about

the non-equilibrium

phases


•Solid Lines are Diffusional

•Dotted is Diffusionless

37

Dr. Shashank Shekhar Knowledge Incubation for TEQIP IIT Kanpur

What is Martensite? 38

Martensite forms when austenite is rapidly cooled (quenched) to room T.

Martensite is metastable - can persist indefinitely at room temperature, but will transform to equilibrium phases on annealing at an elevated temperature.

It’s a Non Equilibrium Phase: Does not appear on Phase Diagram


What is Bainite? 39

Upper bainite consists of needles of ferrites separated by long cementite particles

It occurs in the T ~300 -540C

Lower bainite consists of thin plates of ferrite containing very fine rods or blades of cementites

It occurs in T~200-300C


Bainite 40

For T~300-450C, upper bainite consists of needles of ferrite separated by long cementite particles

For T~200-300 C, lower bainite consists of thin plates of ferrite containing very fine rods or blades of cementites


Time-temperature path to obtain

combination of microstructures



42

slide borrowed from Dr. Swati Sharma Knowledge Incubation for TEQIP IIT Kanpur

Fe -Fe3C phase diagram Iron and Steel Steel Microstructures · PDF fileIron and Steel Steel...

Documents

Transcript of Fe -Fe3C phase diagram Iron and Steel Steel Microstructures · PDF fileIron and Steel Steel...