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Fe-Fe3C phase diagram
Iron and Steel
Steel Microstructures
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Phases and Microstructure
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Dr. Shashank Shekhar
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Identify the terminal phases and its solubility
Iron-Iron carbide phase diagram
0.77
2.11
0.008 at RT
0.18
0.10
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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
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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)
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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?
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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
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Various Transformation Reactions and development of Microstructure
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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
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Definition of Microstructure
Dr. Shashank Shekhar
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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.
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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
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Microstructure of Eutectoid steel
Dr. Shashank Shekhar
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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
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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?
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Microstructure of hypoeutectoid steel
Dr. Shashank Shekhar
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Microstructure of hypereutectoid steel 15
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Microstructures of (a) Hypoeutectoid
steel (ferrite + pearlite) (b) Eutectoid
steel (c) Hypereutectoid steel (pearlite
with network of cementite)
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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
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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
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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
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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
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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
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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
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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
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Microstructures of (a) Hypoeutectic steel
(ledeburite + pearlite +cementite) (b)
Eutectic steel (ledeburite) (c)
Hypereutectic steel (ledeburite + primary
cementite)
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Effect of alloying elements on Iron-Iron carbide phase diagram
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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
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Effect of alloying elements on Iron-Iron carbide phase diagram
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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
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Effect of Chromium (Ferrite Stabilizer) 27
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Effect of alloying elements on Iron-Iron carbide phase diagram
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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
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Effect of Nickel (austenite stabilizer) 29
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Phases and Microstructure
Dr. Shashank Shekhar
30
Microstructure of annealed SS316L Microstructure of annealed duplex steel
Microstructure
of annealed mild
steel (0.1% C)
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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
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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
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Stainless Steel
Dr. Shashank Shekhar
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Changing only the C
concentration, itself has
substantial change on the
microstructure and properties of
steel
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Phase transformations
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Dr. Shashank Shekhar
Phase diagram only tells
us about the
equilibrium phase. It
does not tell us about
the non-equilibrium
phases
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•Solid Lines are Diffusional
•Dotted is Diffusionless
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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
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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
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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
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Time-temperature path to obtain
combination of microstructures
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Dr. Shashank Shekhar
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