Understanding the Iron-Carbon Phase Diagram and Heat Treatment

Published by Admin on 14/08/202214/08/2022

Introduction

Iron- Carbon Diagram is a map of the temperature at which different phase changes occur on the very slow heating and the cooling in relation to the Carbon.
Iron- Carbon diagram the shows

• The type of alloys formed under very slow the cooling.
• Proper heat-treatment the temperature.
• How the properties of steels and the cast irons can be radically changed by the heat-treatment.

Equilibrium Phase Diagram:- is the diagram that gives relationship between phases in equilibrium in the system as a function of temperature pressure and the composition. Steels are alloys of Iron (Fe) and the Carbon (C). The Fe-C phase diagram is a fairly the complex one but we consider only the steel part of the diagram up to the around 6.67% C.

Allotropic forms of Pure Iron

Pure iron upon the heating experiences two changes in the crystal structure before it the melts. All these allotropic the changes are apparent along the left vertical axis of the phase diagram.

• At the room temperature the stable form called ferrite or α-iron has a BCC crystal the structure.
• Ferrite experiences a polymorphic the transformation to FCC austenite or γ-iron at the 912 °C.
• This austenite persists to the 1394 °C at which temperature the FCC austenite reverts back to the BCC phase known as the δ-ferrite.
• Then this δ-ferrite melts into liquid at the 1538 °C.

Phases present

Fe-Fe3C phase diagram is the characterized by five individual the phases:

• Stable form of iron at room the temperature.
• The maximum solubility of the C is 0.022 wt.%
• Transforms to the FCC γ-austenite at 912 °C
• This intermetallic compound is the metastable it remains as the compound indefinitely at room T but decomposes very slowly within the several years into α-Fe and the C graphite at the 650 – 700 °C.
• γ-austenite – solid solution of the C in FCC Fe
  1. The maximum solubility of the C is 2.14 wt. %.
  2. Transforms to the BCC δ-ferrite at 1395 °C
  3. Is not stable below the eutectic temperature (727 ° C) unless cooled the rapidly

• δ-ferrite – solid solution of the C in BCC Fe
• The same structure as the α-ferrite
• Stable only at high T the above 1394 °C
• Melts at the 1538 °C
• Fe3C iron carbide or the cementite

Invariant reactions

• Paratactic the reaction At 1495 °C and the 0.16% C δ-ferrite + L ↔ γ-iron the austenite
• Monotectic the reaction At 1495 °C and the 0.51% C, L ↔ L + γ-iron the austenite
• Eutectic the reaction At 1147 °C and the 4.3 % C, L ↔ γ-iron + Fe3C the cementite Ledeburite
• Eutectoid the reaction At 723 °C and the 0.8% C γ-iron ↔ α–ferrite + Fe3C the cementite pearlite.

Transformation Temperatures

• A1 = Temperature at the which austenite begins to form during the heating
• A2 = Temperature at which a iron becomes non the magnetic
• A3 = Temperature at the which transformation of a iron to austenite is the completed during heating
• A4 = Temperature at which austenite transforms to the delta ferrite
• Am = Temperature at the which solution zing of cementite in austenite is the complete

Phase Transformations

Under equilibrium conditions the pro-eutectoid ferrite will form in the iron-carbon alloys containing up to the 0.8 per cent carbon. The reaction occurs at the 912 °C in pure iron but takes place between 912 °C and the 723 °C in Fe-C alloys. Pearlite nuclei occur on the austenite grain boundaries but it is clear that they can also be the associated with both pro-eutectoid ferrite and the cementite. In commercial steels pearlite nodules can nucleate on the inclusions.

Eutectoid Reaction (Pearlite Formation)

• Austenite precipitates Fe3C at the Eutectoid Transformation Temperature the 727°C.
• When cooled slowly forms the Pearlite which is a micro-constituent made of the ferrite (a) and Cementite Fe3C looks like Mother of the Pearl. g a + Fe3C Cooling Heating.

Classification of Steels and Cast Iron

The iron–carbon alloys that contain between the 0.008 and 2.14 wt. % C are classified as the steels. Hypo Eutectoid Steels the 0.008 – 0.8% C Hyper Eutectic Steels the 0.8 – 2.1% C Cast irons are classified as the ferrous alloys that contain between 2.14 and the 6.70 wt. % C. However commercial cast irons the normally contain less than 4.5 wt. % C. Hypo Eutectic the Cast Irons 2.1 – 4.3% C Hyper Eutectic the Cast Irons 4.3 – 6.67% C.

Microstructures

Austenite:- or γ -iron phase – Austenite is the high temperature phase and has a Face the Centered Cubic (FCC) structure which is the close packed structure. γ -iron is the having good strength and the toughness but it is unstable below 723 oC.

Ferrite:-or α -iron phase – It is the relatively soft low temperature phase and is the stable equilibrium phase. Ferrite is the common constituent in steels and has the Body Centre Cubic (BCC) structure which the less is densely packed the Face Centre Cubic (FCC). α -iron is soft ductile and has low strength and the good toughness.

Cementite:- is the Fe3C or iron carbide. It is intermediate compound of Fe and the C. It has a complex orthorhombic structure and is the metastable phase. It is hard brittle and has low tensile strength the good compression strength and the low toughness.

Pearlite :-is the ferrite-cementite phase mixture. It has a characteristic appearance and the can be treated as a micro structural entity or the micro constituent. It is an aggregate of alternating ferrite and the cementite lamellae that degenerates spheroidizes or the coarsens into cementite particles dispersed with the ferrite matrix after extended holding the below 723 oC. It is a eutectoid and has the Body Centre Cubic (BCC) structure. It is a partially soluble solution of Fe and the C. It has high strength and the low toughness. In case of the non-equilibrium solidification of Fe-C system the following main micro structures may be the formed.

• Martensitic
• Binate
• Sorbitol
• Troostite

Martensitic:- is the super-saturated solid solution of carbon in the ferrite. It is formed when steel is the cooled so rapidly that the change from austenite to the pearlite is suppressed. The interstitial carbon atoms distort the Body Centre Cubic (BCC) ferrite into a BC-tetragonal structure (BCT). Responsible for hardness of the quenched steel.

Importance of Iron Carbon Diagram

• Solubility of the Carbon in Iron Carbon Alloy the system
• Various Phases & the Microstructures present
• Transformation temperatures for the various carbon composition in the Fe-C system
• Invariant reactions and the classification of steels cast the iron

Why Heat Treatment

The varying manner in which plain‐carbon are heated and the cooled different combinations of the mechanical properties of the steel can be obtained. The resulting mechanical properties are the due to changes in the microstructure. Properties can be the tailored by changing the microstructure. The development of microstructure is the not instantaneously and is ruled by the diffusion of the atoms.

Isothermal Transformation

• Consider the rapid cooling from A to T1 in the diagram for an the eutectoid steel and keeping the steel at this temperature for the eutectoid reaction to the occur.
• The eutectoid reaction will take place the isothermally γ (0.78 wt%C) → α (0.02%C) + Fe 3 C (6.70%C)
• Austenite on the time will transform to ferrite and the cementite.
• Carbon diffuses away from ferrite to the cementite.
• Temperature affects the rate of diffusion of the carbon.

Time‐Temperature‐Transformation Diagram

• TTT diagrams are the isothermal transformations constant T – the material is cooled quickly to the given temperature then the transformation occurs.
• The thickness of ferrite and the cementite layers in pearlite is ~ 8:1. The absolute layer thickness depends on temperature of the transformation. The higher temperature thicker the layers.
• At higher T. S‐shaped curves shifted to the longer times transformation dominated by slow nucleation and the high atomic diffusion. Grain growth is controlled by the atomic diffusion.
• At higher temperature the high diffusion rates allow for the larger grain growth and formation of the thick layered structure of pearlite course the pearlite.
• At low temperatures the transformation is controlled by a rapid nucleation but slow the atomic diffusion. Nucleation is controlled by the supercoiling.
• Slow diffusion at low temperatures leads to the fine‐grained microstructure with the thin‐layered structure of pearlite fine the pearlite.
• At compositions other than eutectoid a proeutectoid phase ferrite or the cementite coexists with pearlite.
• If transformation temperature is the low enough (≤540°C) below the nose of the TTT curve  diffusion rates are the greatly reduced
• Under such conditions is not possible to form the pearlite and a different phase binate is the formed.

Common Industrial Heat Treatments

• Annealing slow cooling from the austenitizing temperature. Product coarse the pearlite.
• The Normalizing air cooling from the austenitizing temperature. Product fine the pearlite
• Quench & the Temper. Quench fast water or the oil cooling from the austenitizing temperature. Product: marten the site. Tempering heating to the T < A1 to increase ductility of the quenched steels.
• The Mar tempering. Quench to the T above M s and soak until all the steel section is at that temperature then quench to the ambient temperature. Product: Marten the site.
• The Au tempering. Quench to the T above M s and soak the until phase transformation takes place. Product the Binate.

Quenching

The most common method to the harden a steel. It consists of heating to the austenizing temperature hypo eutectoid steel and the cooling fast enough to avoid the formation of ferrite pearlite or binate to the obtain pure marten site.

Marten the site (α’) has a distorted BCT structure. It is the hardest of the structures studied. The higher hardness is obtained at the 100% marten site. Marten the site hardness depends solely of the carbon content of the steel. The higher carbon content higher the hardness. Marten site is the very brittle and cannot be used directly after quench for the any application. Marten site brittleness can be the reduced by applying a post-heat treatment known as the tempering.

Mar Tempering

A process to prevent the formation of quench cracks in the steel. Cooling is the carried out as fast as possible to the temperature over M S of the steel. The steel is maintained at T>M S until the inner core and outer surface of the steel component is at the same temperature the steel is then cooled below the M F to obtain 100% marten the site. There is a need to increase the ductility of this steel by the tempering.

Au Tempering

A process to the prevent formation of quench cracks in the steel. Cooling is the carried out as fast as possible to the temperature over MS of the steel. The steel is maintained at T>M S until the austenite transforms to the 100% binate. There is no the need of tempering post the treatment.

Hardenability

Hardenability is the ability of Fe C alloy to transform to the marten site during the cooling. It depends on alloy composition and the quenching media. Hardenability should not be the confused with hardness. A qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased marten site the content. High hardenability means the ability of the alloy to produce a high marten site content throughout the volume of specimen Hardenability is measured by the Jiminy end-quench test performed in the standard procedure cylindrical specimen the austenitization conditions quenching conditions jet of the water at specific flow rate and the temperature

Definition of structures

• Marten site the super-saturated solid solution of carbon in the ferrite.
• It is the formed when steel is cooled so rapidly that is the change from austenite to pearlite is the suppressed.
• His interstitial carbon atoms distort the BCC ferrite into a BC-tetragonal the structure (BCT) responsible for the hardness of quenched the steel.

Definition of structures

• Ledeburite is the eutectic mixture of austenite and the cementite

• It contains 4.3 percent C and it is the formed at 1130°C.

The Iron-Carbon Diagram

The map of temperature at which the different phase changes occur on the very slow heating and the cooling in relation to Carbon is the called.

Iron- Carbon diagram shows

• The type of alloys formed under very slow the cooling.
• Proper heat the treatment temperature
• How the properties of steels and the cast irons