chapter 1triangle.kaist.ac.kr/lectures/ms371/2019 spring/chap 1... · 2019. 2. 26. · isothermal...

/MS371/ Structure and Properties of Engineering Alloys

Chapter 1

Iron-Carbon AlloysⅠ


• 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


• α-ferrite (bcc)

• γ-austenite (fcc)

• δ-ferrite (bcc)

• cementite (Fe3C)

Iron-carbon phase diagram


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


• 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


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)


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


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


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


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%


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


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


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


Transformation of austenite to pearlite

• γ-austenite (0.8wt%C) 𝟕𝟕𝟐𝟐𝟐𝟐℃

pearlite (α-ferrite + Fe3C)

- state phase transformation

- controlled transformation (time, T 에의존)


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



grain size小大

grain size가작아지면 grain boundary area 가증가하므로transformation rate 가증가하므로 S 자형곡선이왼쪽으로이동한다

반대로 grain size 가커지면S자형곡선이오른쪽으로이동한다



• temperature effect • effect of prior austenitegrain size


inter-lamellar spacing dependent ononly


• 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


• 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 ( )



(b) mixed type(a) lath type (c) plate type



1) martensite structures to depend on C content


• 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




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



schematic representation of the martensite transformation in high-carbon iron-carbon alloys


• 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


• 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



• 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 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


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


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


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

chapter 1triangle.kaist.ac.kr/lectures/ms371/2019 spring/chap 1... · 2019. 2. 26. · isothermal...

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