Iron Carbon Phase Diagram

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Information about Iron Carbon Phase Diagram

Published on March 7, 2014

Author: drsjose



Iron Carbon Phase Diagram, TTT Diagram, CCT Diagram

Metallurgy & Material Science Dr.S.Jose Professor, Dept of Mechanical Engg., TKM College of Engineering, Kollam

Module II        Diffusion in crystals Theory of Alloys Equilibrium Diagrams Iron Carbon Phase diagram TTT Diagram Heat Treatment Recovery, Recrystallisation & Grain Growth 2

Allotropes of Iron

Fe – Fe3C Phase Diagram

Five individual phases  a–ferrite (BCC) Fe-C solid solution  g-austenite (FCC) Fe-C solid solution  d-ferrite (BCC) Fe-C solid solution  Fe3C (Iron Carbide) or cementite – an inter-metallic compound  Liquid Fe-C solution

Three invariant reactions A horizontal line always indicates an invariant reaction in binary phase diagrams  Peritectic reaction at 1495˚C and 0.18%C,  d-ferrite + L↔ g-iron (austenite)  Eutectic reaction at 1147˚C and 4.3 %C,  L ↔ g-iron + Fe3C (cementite) [ledeburite]  Eutectoid reaction at 727˚C and 0.77%C,  g-iron ↔ a–ferrite+Fe3C (cementite) [pearlite]

Peritectic Reaction

Fe-C alloy classification Metals Ferrous metals Steels Non-ferrous metals Cast Irons Plain carbon steels Grey Iron Low carbon steels White Iron Medium carbon steels Malleable & Ductile Irons High carbon steels Low alloy steels High alloy steels Stainless & Tool steels

Fe-C alloy classification  Fe-C alloys are classified according to wt.% C present in the alloys  Commercial pure irons % C < 0.008  Low-carbon steels 0.008 - %C - 0.3  Medium carbon steels 0.3 - %C - 0.8  High-carbon steels 0.8- %C - 2.14  Cast irons 2.14 < %C

Cast irons  Cast irons that were slowly cooled to room temperature consists of cementite, look whitish – white cast iron.  If it contains graphite, look grayish – gray cast iron.  It is heat treated to have graphite in form of nodules – malleable cast iron.  If inoculants are used in liquid state to have graphite nodules – spheroidal graphite (SG) cast iron.

Eutectoid steel Eutectoid Reaction 727o C g ¾ cool¾® a + Fe3C ¾ 0.022 6.67 0.77 Pearlite

Hypoeutectoid steel Proeutectoid Ferrite Pearlite Microstructure of 0.38 wt% C hypoeutectoid steel

Hypereutectoid steel Pearlite Proeutectoid cementite Microstructure of 1.4 wt% C hypereutectoid steel

Eutectoid steel Hypoeutectoid steel Hypereutectoid steel a+Fe3C a+Fe3C a+Fe3C Pearlite Pearlite + proeutectoid ferrite Pearlite + proeutectoid cementite

Phase vs. Microconstituents  A phase or a mixture of phases which has a distinct identity in a microstructure is called a microconstituent  Pearlite is not a phase.  It is a microconstituent and is a mixture of two phases a- Ferrite and Fe3C.

a-Ferrite Known as a -iron Pure iron at room temperature Body-centered cubic structure Soft & ductile and imparts these properties to the steel.  Less than 0.01% carbon will dissolve in ferrite at room temperature  High temperature form is d ferrite, but the two forms are identical.  Pure ferritic steels are rare    

Austenite      Known as g -iron Face-centered cubic Much softer than ferrite Not present at room temperatures. More easily hot worked

Cementite Iron Carbide - an intermetallic compound Hard, brittle, white melts at 1837 C , density of 7.4 g/cc On the phase diagram, cementite corresponds to a vertical line at 6.7% C  Engineers care only about compounds with less carbon  Its presence in steels causes an increase in hardness and a reduction in ductility and toughness    

Pearlite  A laminated structure formed of alternate layers of ferrite and cementite with average composition 0.83% carbon  Pearly lustre in the microscope  Interference of light in its regular layers  Most common constituent of steel  It combines the hardness and strength of cementite with the ductility of ferrite and is the key to the wide range of the properties of steels.  The laminar structure also acts as a barrier to crack movement as in composites. This gives it toughness

Phase Transformations  Involve some alteration of microstructure 1. No change in number or composition of the phases present, diffusion- dependent. Solidification of pure metals, allotropic transformation. 2. Some alteration in composition and no of phases, diffusion – dependent. Eutectoid reaction 3. A metastable phase is produced, diffusionless. Martensitic transformation.

Phase Transformations  At least one new phase is formed  Do not occur instantaneously  Begin by the formation of small particles of new phase – nucleation  Homogenous – occurs uniformly throughout the parent phase.  Hetrogenous – preferentially at grain boundaries, impurities, dislocations  Size of these particles increase in size until completion - growth

Phase Transformations  Dependent on  Temperature  Time  Composition  Require some finite time for completion  Equilibrium is rarely achieved in solids  Metastable – intermediate between initial and equilibrium states.

Time-Temperature Transformation Diagram

Time-Temperature Transformation Diagram

Time-Temperature Transformation Diagram Complete

Time-Temperature Transformation Diagram

TTT Diagram

Transformation of Austenite in Eutectoid steel  Pearlite 727 - 540 C  Bainite 540 - 210 C  Martensite below 210 C

Transformations involving austenite

CCT diagram  Usually materials are cooled continuously, thus Continuous Cooling Transformation diagrams are appropriate than TTT diagrams  For continuous cooling, the time required for a reaction to begin and end is delayed, thus the isothermal curves are shifted to longer times and lower temperatures.  Main difference between TTT and CCT diagrams: no space for bainite in CCT diagram as continuous cooling always results in formation of pearlite.

CCT diagram

CCT diagram

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