소재열역학의 이해

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1 Chapter 2 Iron-Carbon AlloysⅡ

2 Transformation of austenite to bainite Bainite M s 약간위의적정온도까지빠르게냉각한후, transformation Cooling path for the formation of bainite

3 Transformation of austenite to bainite upper bainite (350~550 C) non-lamella 구조, bar or rod 형태의 cementite 형성 C 의 diffusion 에의해형성되므로 cementite 가먼저형성될지, ferrite 가먼저형성될지알수없음 어떤경우든, 한상이형성되고나면다른한상은먼저형성된 phase 의 boundary 에생성됨 lower bainite (250~350 C) diffusion rate 가느리기때문에 ferrite plate 내부에 iron carbide 가 precipitation supersaturated ferrite 가 austenite 로부터먼저형성된후 ferrite 내부에 cementite 가 precipitation 됨 lower bainite 는 tempered martensite 와차이가있음

4 Transformation of austenite to bainite upper bainite (350~550 C) lower bainite (250~350 C)

5 Upper bainite Upper bainite Consists of needles or laths of ferrite with precipitates between the laths carbide Lath shape Schematic growth mechanism Upper bainite in medium-carbon steel

6 Lower bainite Lower bainite at sufficiently low T (below ~ C) Microstructure change: lath plate Finer carbide dispersion Schematic growth mechanism Lower bainite in 0.69% C low-alloy steel

7 Transformation to martensite in pure Fe The displacive fcc bcc transformation in pure Fe Fraction of martensite: function of only M S : the temperature at which martensite starts to form M F : the temperature at which martensite to form M 50, M 90

8 Transformation to martensite in eutectoid steel The TTT diagram for a 0.8% carbon (eutectoid) steel Quench rate at Cs -1 miss the nose of the 1% curve Quenched into cold water not all the g will transform to martensite retained g which can only be turned into martensite below M F (-50 C)

9 Isothermal transformation of noneutectoid steels Hypoeutectoid Steel Hypereutectoid Steel nose of the diagram to shift difficult to obtain 100% martensite region exist above nose of the diagram to shift left to exist above

10 Continuous-Cooling Transformations (CCT) CCT diagram 실제산업현장에서는 IT (isothermal transformation) 이아니라 CCT 임 IT-diagram 에비해 CCTdiagram 의경우더낮은온도, 더긴시간쪽으로이동 IT 보다 CCT 가더다양한 temp range 에서 transformation 하므로똑같은 100% pearlite 라고할지라도 microstructure 차이생김

11 Continuous-Cooling Transformations (CCT) CCT diagram A: full anneal, furnace cooling pearlite B: normalizing ( 상온냉각 ) pearlite C: oil quench pearlite & (split transformation) D: water quench martensite E: cooling rate, the slowest rate of cooling without obtaining pearlite

12 TTT and CCT for 4340 steel (Ni, Cr, Mo, Mn, 0.4C)

13 TTT for 4340 steel

14 CCT for 4340 steel

15 Heat treatment for the combination of strength and ductility normalizing cold working & annealing full annealing process annealing spheroidizing annealing

16 Normalizing Austenizing + air cooling 100% austenite 로만든후 air cooling 시킨다이때, hypoeutectoid steel 은온도이상, hypereutectoid steel 은온도이상에서열처리하고 air cooling 시킴 Main purposes of normalizing I. To refine the grain structure II. To reduce segregation in castings or forgings III. To harden the steel slightly

17 Annealing (for most metals & alloys after cold work) Full annealing austenizing + slow cooling 즉, austenite 로만든후 slow cooling, 이때 hypoeutectoid steel 은 A 3 온도이상, hypereutectoid steel 은 A 1 온도이상에서 austenizing c.f. normalizing: austenizing + air cooling Process annealing 약 0.3 wt% C 이하 hypoeutectoid steel 을 A 1 온도이하에서 annealing 하는것, dislocation density 를줄여서하게만듬 frequently referred to as stress-relief or recovery

18 Microstructural change during annealing Recovery - rearrangement of into lower energy configurations Recrystallization - formation of new -free grains by the migration of angle grain boundaries Grain growth (+ Coarsening) - growth of grains at the expense of grains

19 Deformed high purity Fe at different annealing T As cold rolled Annealed at 300 C Annealed at 370 C Annealed at 410 C Annealed at 460 C Annealed at 650 C

20 Formation of a nucleus at GB in recrystallization

21 Grain Growth (of soap cells in a flat container)

Cold working (strain hardening) 정의 : 재결정온도이하에서재료에을가하여 hardening 시키는것의미 : Cold working 을하게되면 grain 들의 elongation 이발생하고, 각 grain 내의 dislocation density 가하고그 dislocation 들이 tangle 하게되어서 dislocation cell

22 Cold working (strain hardening) 정의 : 재결정온도이하에서재료에을가하여 hardening 시키는것의미 : Cold working 을하게되면 grain 들의 elongation 이발생하고, 각 grain 내의 dislocation density 가하고그 dislocation 들이 tangle 하게되어서 dislocation cell 을형성한다. 이때 cold working 을많이할수록 dislocation density 가증가하고, cell wall 의 thickness 가증가해도 cell 의부피가감소하면서재료의는증가한다. (a) Low-Carbon steel cold-rolled 65% (b) Thin foil electron micrograph of the cold-rolled 65%

Recovery 열처리하면 dislocation density 도낮아지고 dislocation 이 lower energy state 로 dislocation climb 와 rearrangement 가일어난다. 이때 subgrain boundary 가형성되어 subgrain 을만들게된다.

23 Recovery 열처리하면 dislocation density 도낮아지고 dislocation 이 lower energy state 로 dislocation climb 와 rearrangement 가일어난다. 이때 subgrain boundary 가형성되어 subgrain 을만들게된다. 즉 subgrain 을형성하는 low-angle grain boundary 를만드는것을이라한다. recovery 동안에 mechanical change 는거의없으나는 defect 의감소로인해증가한다. recovery 의 driving force 는 stored energy 의 release 이다.

Grain growth equlibrium size 에도달할때까지 large grain 들은 smaller grain 들을 consume 함으로써 grain growth 가일어나고,

24 Recrystallization and grain growth Recrystallization cold working 시변했던 mechanical properties 들이원래상태로돌아간다. 이때의 driving force 또한 stored strain energy 의 release 이고, recrystallization 의과정은과과정이다. Grain growth equlibrium size 에도달할때까지 large grain 들은 smaller grain 들을 consume 함으로써 grain growth 가일어나고, 이때의 driving force 는 grain growth 에따른단위부피당 free energy 의감소이다. 0.06% Carbon Steel fully recrystallized partially recrystallized

Quench hardening Fully martensite 를얻기위해서는 critical cooling rate 보다빨리 cooling 시켜야한다. 그러나실제강의경우, 두께로인해 cooling 시내 외부간의온도차가생기게되어 very thin steel 을제외하고는 fully martensite 를얻기힘들다.

25 Quench hardening Fully martensite 를얻기위해서는 critical cooling rate 보다빨리 cooling 시켜야한다. 그러나실제강의경우, 두께로인해 cooling 시내 외부간의온도차가생기게되어 very thin steel 을제외하고는 fully martensite 를얻기힘들다. 이러한문제를해결하기위하여등의원소를첨가하여 nose 를더 time 쪽으로이동시켜서 slow cooling 시켜도 martensite 를쉽게만들수있게한다. (a) Water quench (b) Oil quench Residual stress - contraction due to cooling - expansion due to fcc to bcc transformation

26 Tempering (for steels after quenching) A 1 온도이하에서 process annealing 함으로써 residual stress 를시킴 martensite 는 hardness 와 strength 는크지만, toughness 와 ductility 는낮다. tempering 을통해 hardness 와 strength 는약간감소하지만 toughness 와 ductility 는크게시킬수있다.

27 Tempering Segregation of carbon atoms redistribution of carbon to lower energy sites such as dislocation, grain or lath boundaries rearrangement of carbon into clustering in high carbon martensite Carbide precipitation ε-carbide: Fe 2.4 C hcp structure, 100~200 C Hăgg (χ: chi) carbide: Fe 5 C 2 monoclinic, 200~300 C cementite: Fe 3 C orthorhombic, 250~700 C Decomposition of retained austenite austenite ferrite + cementite transformation to bainite Recovery and recrystallization of the ferrite matrix

Tempering ε-carbide: Fe 2.4 C hcp structure, 100~200 C carbon 함량이 0.2 wt% C 이상의 plain carbon steel 을 100~200 C 에서 tempering 하면 ε-carbide precipitation 됨 그러나 0.

28 Tempering ε-carbide: Fe 2.4 C hcp structure, 100~200 C carbon 함량이 0.2 wt% C 이상의 plain carbon steel 을 100~200 C 에서 tempering 하면 ε-carbide precipitation 됨 그러나 0.2 wt% C 보다낮은 plain carbon steel 의경우, carbon atoms 이 dislocation 주위보다 energy 가낮은 dislocation site 에모두수용되므로 ε- carbide 가 precipitation 되지않음 structure 이므로온도를더올리면 Hăgg carbide 나 cementite 형성 Hăgg (χ: chi) carbide: Fe 5 C 2 monoclinic metastable structure, 200~300 C sometimes used as a catalyst for chem rxn ε-carbide

precipitation 됨 낮은온도에서높은온도로올라갈경우, 형태가바뀌는이유는형성된 carbide 의 coalescence 때문이고이때 driving force 는 ferrite matrix

29 Tempering cementite: Fe 3 C orthorhombic, 250~700 C 낮은온도에서의초기 cementite 의모양은 -like 형태 낮은온도에서는 martensite lath boundary 주위에 cementite 가 needlelike 형태로생성되고, 높은온도에서는바로 ferrite grain boundary 주위에 spherical 형태로바로 precipitation 됨 낮은온도에서높은온도로올라갈경우, 형태가바뀌는이유는형성된 carbide 의 coalescence 때문이고이때 driving force 는 ferrite matrix 내의 cementite 의 surface energy 감소 Tempered martensite showing progressive agglomeration of cementite (400~600 C) Cementite

30 Tempering Recovery Recrystallization Recovered Microstructure 10 minutes at 600 C 0.18% C Steel 600 C for 96 h 0.18% C Steel 700 C for 8h partial recrystallization complete recrystallization

31 Tempering Effect of tempering on the hardness 0.026~0.39%C Steel 0.35~1.2%C Steel

32 Grain size effect - grain size 가작을수록대부분 properties 를향상시킬수있음 - 단, 의경우는예외, creep rate 은 grain size 가작을수록증가함, creep 은 grain boundary 에서함으로써 propagation 함, 따라서 grain size 가작을수록단위부피당 grain boundary 가커지므로 creep rate 증가 N: 100 배확대했을때 in 2 당 grains 의수 n: ASTM grain-size number

33 Grain size effect Effect of austenite grain size on proeutectoid ferrite distribution in hypo steel air cooled from (a) 900C (b) 1150C

34 Austempering 100% bainite 를생성하기위한 isothermal heat treatment process M s 위적정온도까지빠르게 cooling 시킨후 100% 를얻을때까지충분한시간동안 holding 시킨후상온까지 air cooling 시킴

35 Martempering (=Marquenching) 열처리된재료의을최소화시키기위해서하는 modified quenching 과정이다. 즉, 빠른 quenching 에따른 residual stress, cracking, distortion 을최소화하기위한 process steel 을 austenizing 한후 steel 을 M s 온도약간위나약간아래온도까지 quenching 시킴 steel 전체에걸쳐온도가해질때까지온도를일정시간동안유지함 steel 의 surface 와 center 사이온도차가매우커지지않을정도의 rate 로 cooling 함

Chapter 11: Phase Diagrams

Chapter 11: Phase Diagrams Chapter 11: 상태도 (Phase Diagrams) 학습목표 2 개의원소가결합이되면평형상태는어떤상태일까? 특히, 우리가다음과같은요소를정한다면.. -- 조성 (e.g., wt% Cu - wt% Ni), -- 온도 (T, temperature ) 몇개의상이형성될까? 각각의상은어떤조성이될까? 각각의상의양은얼마일까? Phase A Phase B Nickel