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/MS371/ Structure and Properties of Engineering Alloys
Chapter 1
Iron-Carbon AlloysⅠ
/MS371/ Structure and Properties of Engineering Alloys
• pure iron : to be obtained through zone refining
adding a small amount of C, Mn, P, S 증가
• pure iron 의 allotropic forms
Allotropic forms
Crystallographic form
Unit cube edge [Å] Temperature range
Alpha bcc 2.86 up to 910°C
Gamma fcc 3.65 910~1403°C
Delta bcc 2.93 1403~1535°C
Density: 7.868g/cm3, m.p.: 1535°C, b.p.: 3000°C
Iron
/MS371/ Structure and Properties of Engineering Alloys
• α-ferrite (bcc)
• γ-austenite (fcc)
• δ-ferrite (bcc)
• cementite (Fe3C)
Iron-carbon phase diagram
/MS371/ Structure and Properties of Engineering Alloys
0
200
400
600
800
1000
1200
1400
1600
TEMP
ERAT
URE_
CELS
IUS
0 1 2 3 4 5 6 7
MASS_PERCENT C
Calculated by TCCR
L + Fe3C
γL + γ
α+ Fe3C
γ + Fe3C
Liquidδ
Iron-carbon phase diagram
/MS371/ Structure and Properties of Engineering Alloys
• α-ferrite: solid solution of in α-iron
• γ-austenite: solid solution of carbon in
• δ-ferrite: solid solution of carbon in δ-iron
• cementite: Fe3C, non-equilibrium (metastable) phase
○ iron atom ● carbon atom
atomic structure of cementite
(a≠b≠c, α=β=γ=90°)12Fe and 4C per unit cell6.67wt% carbon, 93.3wt% Iron
Iron-carbon
/MS371/ Structure and Properties of Engineering Alloys
①
②
③
Invariant reaction in the Fe-Fe3C phase diagram
① peritectic reaction (L+S1→S2)L(0.53%C) + δ(0.09%C)
1495°𝐶𝐶γ(0.17%C)
② eutectic reaction (L→S1+S2)L(4.3%C)
1148°𝐶𝐶γ(2.08%C) + Fe3C(6.67%C)
③ reaction (S1→S2+S3)γ(0.8%C)
723°𝐶𝐶α(0.02%C) + Fe3C(6.67%C)
/MS371/ Structure and Properties of Engineering Alloys
hypoeutectoid hypereutectoid
①
②
① 724°C: austenizing or (homogeneous γ-austenite transformation)
② below 723°C: eutectoid
(pearlite is a mixture of α + Fe3C)
Slow cooling of plain-carbon steels
/MS371/ Structure and Properties of Engineering Alloys
a. γ-austenite
b. grain boundary 에
-eutectoid ferrite 형성
c. pro-eutectoid ferrite 성장
d. γ-austenite가
로 phase transition
Hypo-eutectoid carbon steel
/MS371/ Structure and Properties of Engineering Alloys
a. γ-austenite
b. grain boundary 에
pro-eutectoid 형성
c. pro-eutectoid Fe3C 성장
d. γ-austenite가 pearlite 로phase transition
Hyper-eutectoid carbon steel
/MS371/ Structure and Properties of Engineering Alloys
1.2
①② ③Carbon steel (near 723C)
① at 0.8 wt% (below 723C)wt% ferrite = 6.67−0.80
6.67−0.02× 100%
= 88%wt% cementite = 0.80−0.02
6.67−0.02× 100%
= 12%
② at 0.4 wt% (above 723C)wt% proeutectoid ferrite = 0.80−0.40
0.80−0.02× 100% = 50%
wt% austenite = 0.40−0.020.80−0.02
× 100% = 50%
③ at 1.2 wt% (above 723C)wt% proeutectoid cementite = 1.20−0.80
6.67−0.80× 100% = 6.8%
wt% austenite = 6.67−1.206.67−0.80
× 100% = 93.2%
/MS371/ Structure and Properties of Engineering Alloys
Experimental arrangement for determining the microscopic changes that occur during the isothermal transformation of austenite in an eutectoid plain-carbon steel
Isothermal transformation of eutectoid carbon steel
/MS371/ Structure and Properties of Engineering Alloys
Microstructural changes which occur during the isothermal transformation of an eutectoid plain carbon steel
austenite 5.8min 19.2min 24.2min 66.7min22min
Isothermal transformation of an eutectoid carbon steel
/MS371/ Structure and Properties of Engineering Alloys
Isothermal transformation diagram for an eutectoid plain-carbon steel showing its relationship to the Fe-Fe3C phase diagram
Isothermal transformation of an eutectoid carbon steel
/MS371/ Structure and Properties of Engineering Alloys
Transformation of austenite to pearlite
• γ-austenite (0.8wt%C) 𝟕𝟕𝟐𝟐𝟐𝟐℃
pearlite (α-ferrite + Fe3C)
- state phase transformation
- controlled transformation (time, T 에의존)
/MS371/ Structure and Properties of Engineering Alloys
Isothermal reaction curve
firststage
secondstage
thirdstage
• first stage:
transformation rate 가느리다
적은양의 pearlite nodule 이 nucleation & grow
• second stage:
transformation rate 가빠르다
새로운많은 nuclei가 nucleation 되고
grow 되며 nodule 은계속성장함
• third stage:
transformation rate 가느리다
nucleation rate 가감소하고, pearlite nodule
의 growth 도 impingement 에의해감소
Transformation of austenite to pearlite
/MS371/ Structure and Properties of Engineering Alloys
grain size小 大
grain size가작아지면 grain boundary area 가증가하므로transformation rate 가증가하므로 S 자형곡선이왼쪽으로이동한다
반대로 grain size 가커지면S자형곡선이오른쪽으로이동한다
Transformation of austenite to pearlite
/MS371/ Structure and Properties of Engineering Alloys
• temperature effect • effect of prior austenitegrain size
Transformation of austenite to pearlite
inter-lamellar spacing dependent ononly
/MS371/ Structure and Properties of Engineering Alloys
• martensite: metastable structure
supersaturated solid solution of C in α-ferrite
isothermal transformation diagram for an eutectoid steel
Transformation of austenite to martensite
What is Ae1?
A1, Ac1, Ar1A3, Ac3, Ar3Acm, Accm, Arcm
/MS371/ Structure and Properties of Engineering Alloys
• Characteristics of martensitic transformation
1) martensite structures to depend on C contentlow Carbon → (dislocated) martensite
high Carbon → (twinned) martensite
effect of C content on martensitetransformation start temperature ( )
Transformation of austenite to martensite
/MS371/ Structure and Properties of Engineering Alloys
(b) mixed type(a) lath type (c) plate type
Transformation of austenite to martensite
• Characteristics of martensitic transformation
1) martensite structures to depend on C content
/MS371/ Structure and Properties of Engineering Alloys
• Characteristics of martensitic transformation2) transformation to occur, transformation 이매우빠르게일어나
no time for intermix
3) transformation 후에도 Fe atom 에대한 C atom 의상대적위치같다.
즉, no change in
Transformation of austenite to martensite
/MS371/ Structure and Properties of Engineering Alloys
• Characteristics of martensitic transformation
4) γ-austenite (fcc) → martensite ( )
martensite가 bct구조를가지는이유는 γ-austenite 는 fcc구조로bcc 구조보다 solid solubility 가큰데, bcc로 transformation 하게되면기존의 C atoms 들을다수용못하여 excess C atoms 들을수용하기위해 c축을따라 bcc 구조가 distortion 되어서 bct구조가됨
bctbccfcc
Transformation of austenite to martensite
/MS371/ Structure and Properties of Engineering Alloys
schematic representation of the martensite transformation in high-carbon iron-carbon alloys
Transformation of austenite to martensite
• Characteristics of the martensitic transformation
5) γ-austenite 𝑀𝑀𝑠𝑠 martensite (bct)
martensitic transformation to start at Ms (Ms = 220°C)
6) In the high C steels, martensite plates to be formed by displacive or -like transformation process
http://www.doitpoms.ac.uk/tlplib/superelasticity/displacive.php
/MS371/ Structure and Properties of Engineering Alloys
• Morphology of martensite in Fe-C alloys
lath martensite: dislocation martensite (slip이발생)domain 내의각각의 lath 들은일정한 orientation 을가짐lath들은 highly distortion 되어있고높은밀도의dislocation들이 tangle 된지역을이루고있음almost no retained γ-austenite
plate martensite: needle-like plate 들이 habit plane {225}에서 {259} 까지위에서 independent 하게형성됨dislocation density 가낮음plate 의크기는다양하고 {112} 위에 twin 들이존재
lath and plate (mixed) martensite: 탄소함량 0.6~1.0%, 온도범위 200~320°C
Transformation of austenite to martensite
/MS371/ Structure and Properties of Engineering Alloys
• Strength of martensite in Fe-C alloys
Hardness of fully hardened martensitic plain-carbon steel as a function of carbon content
1. refinement of the martensite cell size with increasing carbon content
2. segregation of carbon to cell walls 3. solid solution hardening 4. dispersion hardening due to
precipitation of carbide
Transformation of austenite to martensite
/MS371/ Structure and Properties of Engineering Alloys
Transformation of pure Fe (fcc) to martensite (bcc)
Displacive transformation: quench fcc iron from 914°C to room temperature at a rate of about 105°Cs-1
• to prevent the diffusive fccbcc,
• fast cooling rate: °Cs-1
• in reality, below 550°C the iron will transform to bcc by a
transformation instead
/MS371/ Structure and Properties of Engineering Alloys
Nucleation, growth & morphology of martensite
• bcc lenses– nucleation at fcc GB– growing almost instantaneously– stop growing at next GB
• martensite– a phase in any material by
displacive transformation
• martensitic transformation– displacive transformation
The mechanism of displacive transformation (martensite) in iron: nucleation and growth from grain boundary to next grain boundary
/MS371/ Structure and Properties of Engineering Alloys
Martensite lattice
• martensite lattice– with parent lattice– growing as thin
on preferred planes and in preferred direction least distortion of the lattice
The crystallographic relationships between martensite and parent lattice for pure iron
/MS371/ Structure and Properties of Engineering Alloys
Bain strain
• by atomic movementsfcc bcc
• “Bain strain” undistorted bcc cell
• This atomic “switching” involves the least shuffling of atoms. As it stands the new lattice is not coherent with the old one. But we can get coherency by rotating the bcc lattice planes as well
(a) The unit cells of fcc and bcc iron (b) Two adjacent fcc cells make a distorted bcc cell