Chapter 1. Iron-Carbon AlloysⅠ. /MS371/ Structure and Properties of Engineering Alloys

Transcription

1 Chapter 1 Iron-Carbon AlloysⅠ

2 Iron 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 ~1403 C Delta bcc ~1535 C Density: 7.868g/cm 3, m.p.: 1535 C, b.p.: 3000 C

3 Iron-carbon phase diagram α-ferrite (bcc) γ-austenite (fcc) δ-ferrite (bcc) cementite (Fe 3 C)

4 Iron-carbon phase diagram 1600 TEMPERATURE_CELSIUS δ γ L γ Liquid γ Fe 3 C L Fe 3 C α Fe 3 C MASS_PERCENT C Calculated by TCCR

5 Iron-carbon 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: Fe 3 C, non-equilibrium (metastable) phase atomic structure of cementite (a b c, α=β=γ=90 ) 12Fe and 4C per unit cell 6.67wt% carbon, 93.3wt% Iron iron atom carbon atom

6 Invariant reaction in the Fe-Fe 3 C phase diagram 1 peritectic reaction (L+S 1 S 2 ) L(0.53%C) + δ(0.09%c) γ(0.17%c) 2 eutectic reaction (L S 1 +S 2 ) L(4.3%C) γ(2.08%c) + Fe 3 C(6.67%C) 3 reaction (S 1 S 2 +S 3 ) γ(0.8%c) α(0.02%c) + Fe 3 C(6.67%C) 1 3 2

7 Slow cooling of plain-carbon steels C: austenizing or (homogeneous γ-austenite transformation) 2 below 723 C: eutectoid (pearlite is a mixture of α + Fe 3 C) 1 2 hypoeutectoid hypereutectoid

8 Hypo-eutectoid carbon steel a. γ-austenite b. grain boundary 에 -eutectoid ferrite 형성 c. pro-eutectoid ferrite 성장 d. γ-austenite 가 로 phase transition

9 Hyper-eutectoid carbon steel a. γ-austenite b. grain boundary 에 pro-eutectoid 형성 c. pro-eutectoid Fe 3 C 성장 d. γ-austenite 가 pearlite 로 phase transition

10 Carbon steel (near 723C) at 0.8 wt% (below 723C) wt% ferrite =.. 100%.. = 88% wt% cementite =.. 100%.. = 12% at 0.4 wt% (above 723C) wt% proeutectoid ferrite =.. 100% = 50%.. wt% austenite =.. 100% = 50%.. 3 at 1.2 wt% (above 723C) wt% proeutectoid cementite =.. 100% = 6.8%.. wt% austenite =.. 100% = 93.2%..

11 Isothermal transformation of eutectoid carbon steel Experimental arrangement for determining the microscopic changes that occur during the isothermal transformation of austenite in an eutectoid plain-carbon steel

12 Isothermal transformation of an eutectoid carbon steel Microstructural changes which occur during the isothermal transformation of an eutectoid plain carbon steel austenite 5.8min 19.2min 22min 24.2min 66.7min

13 Isothermal transformation of an eutectoid carbon steel Isothermal transformation diagram for an eutectoid plain-carbon steel showing its relationship to the Fe-Fe 3 C phase diagram

14 Transformation of austenite to pearlite γ-austenite (0.8wt%C) pearlite (α-ferrite + Fe 3 C) - state phase transformation - controlled transformation (time, T 에의존 )

15 Transformation of austenite to pearlite first stage: transformation rate 가느리다적은양의 pearlite nodule 이 nucleation & grow second stage: transformation rate 가빠르다 새로운많은 nuclei 가 nucleation 되고 grow 되며 nodule 은계속성장함 first stage second stage third stage third stage: transformation rate 가느리다 nucleation rate 가감소하고, pearlite nodule 의 growth 도 impingement 에의해감소 Isothermal reaction curve

16 Transformation of austenite to pearlite 小 grain size 大 grain size 가작아지면 grain boundary area 가증가하므로 transformation rate 가증가하므로 S 자형곡선이왼쪽으로이동한다 반대로 grain size 가커지면 S 자형곡선이오른쪽으로이동한다

17 Transformation of austenite to pearlite temperature effect effect of prior austenite grain size inter-lamellar spacing dependent on only

18 Transformation of austenite to martensite martensite: metastable structure supersaturated solid solution of C in α-ferrite What is Ae 1? A 1, Ac 1, Ar 1 A 3, Ac 3, Ar 3 A cm, Ac cm, Ar cm isothermal transformation diagram for an eutectoid steel

19 Transformation of austenite to martensite Characteristics of martensitic transformation 1) martensite structures to depend on C content low Carbon (dislocated) martensite high Carbon (twinned) martensite effect of C content on martensite transformation start temperature ( )

20 Transformation of austenite to martensite Characteristics of martensitic transformation 1) martensite structures to depend on C content (a) lath type (b) mixed type (c) plate type

21 Transformation of austenite to martensite Characteristics of martensitic transformation 2) transformation to occur, transformation 이매우빠르게일어나 no time for intermix 3) transformation 후에도 Fe atom 에대한C atom 의상대적위치같다. 즉, no change in

22 Transformation of austenite to martensite Characteristics of martensitic transformation 4) γ-austenite (fcc) martensite ( ) martensite 가 bct 구조를가지는이유는 γ-austenite 는 fcc 구조로 bcc 구조보다 solid solubility 가큰데, bcc 로 transformation 하게되면기존의 C atoms 들을다수용못하여 excess C atoms 들을수용하기위해 c 축을따라 bcc 구조가 distortion 되어서 bct 구조가됨 fcc bcc bct

23 Transformation of austenite to martensite Characteristics of the martensitic transformation 5) γ-austenite martensite (bct) martensitic transformation to start at M s (M s = 220 C) 6) In the high C steels, martensite plates to be formed by displacive or -like transformation process schematic representation of the martensite transformation in highcarbon iron-carbon alloys

24 Transformation of austenite to martensite Morphology of martensite in Fe-C alloys lath martensite: dislocation martensite (slip 이발생 ) domain 내의각각의 lath 들은일정한 orientation 을가짐 lath 들은 highly distortion 되어있고높은밀도의 dislocation 들이 tangle 된지역을이루고있음 almost no retained γ-austenite plate martensite: needle-like plate 들이 habit plane {225} 에서 {259} 까지위에서 independent 하게형성됨 dislocation density 가낮음 plate 의크기는다양하고 {112} 위에 twin 들이존재 lath and plate (mixed) martensite: 탄소함량 0.6~1.0%, 온도범위 200~320 C

25 Transformation of austenite to martensite Strength of martensite in Fe-C alloys 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 Hardness of fully hardened martensitic plaincarbon steel as a function of carbon content

26 Transformation of pure Fe (fcc) to martensite (bcc) to prevent the diffusive fcc bcc, fast cooling rate: Cs -1 in reality, below 550 C the iron will transform to bcc by a transformation instead Displacive transformation: quench fcc iron from 914 C to room temperature at a rate of about 10 5 Cs -1

27 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

28 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

29 Bain strain by atomic movements fcc 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