Steels: Equilibrium Transformations PDF

Diagramas dediagram Fe-C phase fase Fe-C SESSION 9 STEELS: EQUILIBRIUM TRANSFORMATIONS Department of Materials Scien...

Diagramas dediagram Fe-C phase fase Fe-C SESSION 9 STEELS: EQUILIBRIUM TRANSFORMATIONS Department of Materials Science and Chemical Engineering Escuela Politécnica Superior Universidad Carlos III de Madrid Ciencia e Ingeniería Materials Science andde Materiales Engineering  Diagramas dediagram Fe-C phase fase Fe-C Content The Fe-C system Equilibrium phase transformations Development of microstructure in steels Eutectoid steels Hypoeutectoid steels Hypereutectoid steels Influence of the alloying elements in the metastable Fe-C system Ciencia e Ingeniería Materials Science andde Materiales Engineering 2  Diagramas dediagram Fe-C phase fase Fe-C Learning objectives Identify phases and microconstituents in the Fe-C phase diagram Learn the invariant reactions present in the Fe-C phase diagram Determine composition, mass fractions of the phases and the microconstituents by using the lever rule Describe the microstructure evolution regarding the phases and microconstituents upon cooling Learn how the alloying elements can affect the Fe-C phase diagram Ciencia e Ingeniería Materials Science andde Materiales Engineering 3  Diagramas dediagram Fe-C phase fase Fe-C Allotropic phases of Fe LIQUID Allotropy: existence of a chemical element in two or more forms, which may differ in the arrangement of 1535˚C atoms in crystalline solids. Fe- (BCC) 1390˚C Fe- (FCC) 910˚C Fe-α (BCC) Fe experiences two changes in Room Temperature crystal structure before it melts. Ciencia e Ingeniería Materials Science andde Materiales Engineering 4  Diagramas dediagram Fe-C phase fase Fe-C Fe-C phase diagram peritectic Metastable diagram eutectic ▪ 6.67 wt.% C ▪ Stoichiometry for Fe3C ▪ Iron carbide or cementite ▪ This phase diagram eutectoid is in fact Fe-Fe3C Hypoeutectoid Hypereutectoid Hypoeutectic Hypereutectic steels cast irons Ciencia e Ingeniería Materials Science andde Materiales Engineering 5  Diagramas dediagram Fe-C phase fase Fe-C Phases in the Fe-C System -Ferrite (BCC):  Interstitial solid solution of C in BCC iron  Low C concentration, maximum solubility 0.022 wt.% at 727ºC  Soft phase  Density 7.88 g/cm3 Ciencia e Ingeniería Materials Science andde Materiales Engineering 6  Diagramas dediagram Fe-C phase fase Fe-C Phases in the Fe-C System -Austenite (FCC):  interstitial solid solution of C in FCC iron  Maximum solubility of C 2.14 wt.% at 1147 ºC  Important for phase transitions in heat treatment of steels  Not magnetic Ciencia e Ingeniería Materials Science andde Materiales Engineering 7  Diagramas dediagram Fe-C phase fase Fe-C Phases in the Fe-C System -Ferrite (BCC):  interstitial solid solution of C in BCC iron  The same properties as -Ferrite but at higher temperatures Ciencia e Ingeniería Materials Science andde Materiales Engineering 8  Diagramas dediagram Fe-C phase fase Fe-C Phases in the Fe-C System Cementite (Fe3C):  Intermetallic compound, 6.67 %C (Fe3C)  Forms when the solubility limit of carbon in -Fe is exceeded below 727 ºC  Hard and brittle  Metastable compound Ciencia e Ingeniería Materials Science andde Materiales Engineering 9  Diagramas dediagram Fe-C phase fase Fe-C Phases in the Fe-C System Two-phase regions:  -Ferrite + Fe3C Bellow 727 ºC  -Austenite + Fe3C Between 727ºC and 1147ºC Ciencia e Ingeniería Materials Science andde Materiales Engineering 10  Diagramas dediagram Fe-C phase fase Fe-C Invariant reactions Eutectoid reaction (727ºC): γ (0.76%)   (0.022%) + Fe3C(6.67%) (pearlite) Eutectic reaction (1147ºC): L(4.3%)  γ (2.14%) + Fe3C(6.67%) (ledeburite) Peritectic reaction (1493ºC): δ (0.09%) + L(0.53%)  γ (0.16%) (austenite) Ciencia e Ingeniería Materials Science andde Materiales Engineering 11  Diagramas dediagram Fe-C phase fase Fe-C Pearlitic (Eutectoid) transformation Pearlite: Alternating layers (lamellae) of 2 phases: Fe + Fe3C: Ferrite nucleates on the austenite grain boundary, rejects C which diffuses along the interface to next layer to form cementite. - Grain boundary Carbon diffusion along the interface Coarse pearlite VE γ γ Fine Pearlite + γ Austenite (0.76%C) Ferrite (0.022%C) Decreasing the pearlitic lamella Cementite (6.67%C) spacing increases its hardness. Ciencia e Ingeniería Materials Science andde Materiales Engineering 12  Diagramas dediagram Fe-C phase fase Fe-C Eutectoid Steels 𝑪𝟎 = 𝟎. 𝟕𝟔%𝑪 A single microconstituent: Pearlite, formed alternating layers of 2 phases: Cementite (Fe3C) Ferrite () Ciencia e Ingeniería Materials Science andde Materiales Engineering 13  Diagramas dediagram Fe-C phase fase Fe-C Eutectoid Steels 𝑪𝟎 = 𝟎. 𝟕𝟔%𝑪 T > 727 °C (point a) Phases Present & Composition Austenite CγFe= C0= 0.76%C %γFe= 100% T < 727 °C (point b) Phases Present & Composition Ferrite (α) & Cementite (Fe3C) CFe − = 0.022%C CFe3C = 6.67%C Amount of phases (Weight fractions) CFe3C − C0 6.67 − 0.76 %Fe − = * 100 = * 100 = 89% CFe3C − C 6.67 − 0.022 %Fe3C = 100 − 89 = 11% Microconstituents Pearlite 6.67 %C Ciencia e Ingeniería Materials Science andde Materiales Engineering 14  Diagramas dediagram Fe-C phase fase Fe-C Hypoeutectoid Steels 𝑪𝟎 < 𝟎. 𝟕𝟔%𝑪 Two microconstituents: Proeutectoide ferrite Pearlite ( + Fe3C) or primary (P) Ciencia e Ingeniería Materials Science andde Materiales Engineering 15  Diagramas dediagram Fe-C phase fase Fe-C Hypoeutectoid Steels 𝑪𝟎 < 𝟎. 𝟕𝟔%𝑪 Point c C0 = 0.3%C Phases Present & Composition Austenite () C = C0 = 0.3%C Amount of phases (Weight fractions) W = 100% Ciencia e Ingeniería Materials Science andde Materiales Engineering 16  Diagramas dediagram Fe-C phase fase Fe-C Hypoeutectoid Steels Point d C0 = 0.3%C Phases Present & Composition Ferrite () + Austenite () 𝐶𝛼 = 0.02%𝐶 C = 0.4%C Amount of phases (Weight fractions) S C − C0 0.4 − 0.3 W = 100 =  100 = 100 = 26% R+S C − C  0.4 − 0.02 R C0 − C  0.3 − 0.02 W = 100 = 100 = 100 = 74% R+S C − C  0.4 − 0.02 Ciencia e Ingeniería Materials Science andde Materiales Engineering 17  Diagramas dediagram Fe-C phase fase Fe-C Hypoeutectoid Steels Point e C0 = 0.3%C Phases Present & Composition Ferrite () + Austenite () 𝐶𝛼 = 0.02%𝐶 C = 0.76%C Amount of phases (Weight fractions) S C − C0 0.76 − 0.3 W = 100 =  100 = 100 = 62% R+S C − C  0.76 − 0.02 R C0 − C  0.3 − 0.02 W = 100 = 100 = 100 = 38% R+S C − C  0.76 − 0.02 Ciencia e Ingeniería Materials Science andde Materiales Engineering 18  Diagramas dediagram Fe-C phase fase Fe-C Hypoeutectoid Steels Point f C0 = 0.3%C Phases Present & Composition Ferrite () + Cementite (Fe3C) 2 →  ( pro + E ) y Cementite 𝐶𝛼 = 0.02%𝐶 CFe3C = 6.67%C Amount of phases (Weight fractions) C Fe3C − C0 6.67 − 0.3 W (total ) = *100 = *100 = 96% C Fe3C − C 6.67 − 0.022 E 3C − C0 C Fe 0.76 − 0.3 W p = *100 = *100 = 62% CEFe3C − C 0.76 − 0.022 W E = 96 − 62 = 34% WFe3C = 100 − 96 = 4% Proeutectoid Ferrite Microconstituents Pearlite 6.67 %C Ciencia e Ingeniería Materials Science andde Materiales Engineering 19  Diagramas dediagram Fe-C phase fase Fe-C Hypereutectoid Steels 𝑪𝟎 > 𝟎. 𝟕𝟔%𝑪 Two microconstituents: Proeutectoid cementite (Fe3C)P Pearlite ( + Fe3C) Ciencia e Ingeniería Materials Science andde Materiales Engineering 20  Diagramas dediagram Fe-C phase fase Fe-C Influence of the alloying elements γ-stabilizers: Favour the γ phase → A3, A1 → displacing the eutectoid to lower T at lower %C Mn, Co, Ni, Cu, Zn, Au Ciencia e Ingeniería Materials Science andde Materiales Engineering 21  Diagramas dediagram Fe-C phase fase Fe-C Influence of the alloying elements -stabilizers: Favour the  phase → A3→ displacing the eutectoid to higher T at lower %C Si, P, Al, Be, Sn, Sb, As, Ti, Nb, V, Ta, Mo, W, Cr A3 Ciencia e Ingeniería Materials Science andde Materiales Engineering 22  Diagramas dediagram Fe-C phase fase Fe-C Influence of the alloying elements The presence of other elements in the Fe-C system can influence the temperature and composition at which the eutectoid reaction occurs. -stabilizers γ-stabilizers Ciencia e Ingeniería Materials Science andde Materiales Engineering 23  Diagramas dediagram Fe-C phase fase Fe-C Summary On the basis of composition, ferrous alloys fall into: Steels (0.008 wt% C to 2.14 wt% C) Cast irons (>2.14 wt% C) Important phases found on the Fe–C phase diagram are -ferrite (BCC), -austenite (FCC), and the intermetallic compound cementite (Fe3C). The microstructural product of an Fe-C alloy of eutectoid composition is pearlite, a microconstituent consisting of alternating layers of -ferrite and cementite. The microstructure of alloys having carbon contents less than the eutectoid (hypoeutectoid steels) are composed of proeutectoid -ferrite phase and pearlite. The microstructure of alloys having carbon contents higher than the eutectoid (hypereutectoid steels) are composed of proeutectoid cementite and pearlite. The amount of proeutectoid (ferrite or cementite) and pearlite microconstituents can be computed using the lever rule and a tie line. Ciencia e Ingeniería Materials Science andde Materiales Engineering 24