CHAPTER 6

Iron (Fe) - Iron Carbide(Fe3C) Phase Diagram

• Dr. Talaat El-Benawy Introduction

• The study of Fe-F3C alloying system is important because it forms the basis of commercial steels and cast irons. Moreover, the basic features of this system influence the behaviour of the most complex alloy steels • Carbon is the most important alloying element in iron, which significantly affects the allotropy , structure and properties of iron • Conventionally, the complete Fe-C diagram should extend from 100% Fe to 100% carbon but it is normally studied up to around 6.67% carbon, because iron alloys of practical industrial importance contain not more than 4.5- 5.0 % carbon • Therefore, this diagram is is usually called Fe- Fe3C equilibrium phase diagram • The iron carbide Fe3C is an intermetallic compound called cementite has fixed carbon weight content can be calculated as following. • For A-B binary system, the weight percentages, wt%, can be calculated if the atomic percentages X% and atomic weights, a, are given and known for A and B alloying elements through the following equations:

• If it is considered that A refers to the carbon, C, and B to the iron, Fe. From the periodical table, it is known that the atomic weights of C and Fe are aC = 12 and aFe = 56. Then the Carbon atomic percentage of the Fe3C intermetallic compound equals: • Then iron atomic percentage equals XFe % = 75% • Accordingly, carbon weight percent of 6.67% can be calculated for the Fe3C from the previous equation. Therefore, the Fe-Fe3C equilibrium phase diagram must be plotted up to 6.67% carbon Fe–Fe3C Phase Diagram Phases in Fe–Fe3C Phase Diagram 1. α-ferrite - solid solution of C in BCC Fe • Interstitial solid solution of carbon in BCC iron • Stable form of iron at room temperature. • The maximum solubility of C is about 0.02 wt% at 727 °C which decreases to negligible amount of about < 0.00005% C at 20 °C. • α-ferrite is ferromagnetic at low temperatures and loses its magnetic properties at 768 °C and sometimes called β-ferrite instead α-ferrite. • Transforms to FCC γ-austenite at 912 °C • Soft and ductile phase. 2. γ-austenite - solid solution of C in FCC Fe • Interstitial solid solution of carbon in FCC iron. • The maximum solubility of C is 2.11 wt % at 1147 °C which decreases to 0.77% C at 727 °C. • Transforms to BCC δ-ferrite at 1394 °C • It is only stable above the temperature of 727 °C and can be obtained at room temperature by adding Ni or Mn to the اﻟﺼﻠﺐ composition (alloy steel (اﻟﺴﺒﺎﺋﻜﻲ • It is soft, ductile, tough and non- magnetic. 3. δ-ferrite solid solution of C in BCC Fe • Interstitial solid solution of carbon in BCC iron. • Same structure as α-ferrite. • The maximum solubility of C is 0.09 wt % at 1495 °C. • Stable only at temperature above 1394 °C. 4. Cementite, iron carbide Fe3C intermetallic compound • Interstitial intermetallic compound having a fixed carbon content of 6.67%, as it was calculated before. ﻟﯿﺴﺖ It is metastable (not quietly stable • phase where it (ﻣﺴﺘﻘﺮه ﺗﻤﺎﻣﺎ decomposes, very slowly (within several years), into α-Ferrite and Carbon (graphite) at 650-700 °C • It has orthorhombic crystal structure with 12 iron atoms with 4 carbon atoms. • The stable phase melts at 1227 °C. • It is slightly ferromagnetic up to 210 °C. • It is very hard and very brittle phase 5. Fe-C liquid solution

• The melting temperature of the pure iron is at 1539 °C Few comments on Fe–Fe3C system

• Maximum solubility in BCC α- ferrite is limited about 0.02 wt% at 727 °C, it has to be mentioned that BCC has relatively small interstitial positions.

• Maximum solubility in FCC γ- austenite is 2.11 wt% at 1147 °C, it has to be mentioned that FCC has larger interstitial positions.

• Cementite, Fe3C is very hard and brittle and it can strengthen steels.

• α-ferrite is magnetic below 768 °C and austenite is non- magnetic Classification of the Fe-Fe3C phase diagram Three different types of ferrous alloys can be determined in

the Fe-Fe3C phase diagram as the following: • Very soft steel of carbon percentage of C < 0.008 wt% • Steel with the following categories: o Low carbon steel as carbon percentage from 0.008 wt% and up to less than 0.25 wt%. o Medium carbon steel as carbon percentage from 0.25 wt% and up to less than 0.55 wt% o High carbon steel as carbon percentage from 0.55 wt% and up to 2.11 wt% • Cast-iron of carbon percentage more than 2.11 wt%, however, usually carbon percentage between 2.25 to 3.75 is commonly used for cast iron in practical usage Important Reactions in Fe-Fe3C equilibrium phase diagram i. Peritectic Reaction: Peritectic reaction, in general, can be represented by equation: L , S1 and S2 represent liquid and two different solids of fixed composition.

In fact, Fe-0.17% C steel is peritectic steel because only this steel undergoes above reaction completely ii. Eutectic Reaction Eutectic reaction, in general, can be represented by equation: where L represents a liquid of fixed

composition and S1 and S2 are two different solids of fixed composition

The shown Figure illustrates the eutectic region of Fe-Fe3C Phase Dia. Fe-4.3% C alloy called eutectic cast iron where the liquid of 4.3% C undergoes eutectic reaction at the eutectic temperature of 1147 °C to give a mixture of two different solids namely: γ-austenite (2.11%

C) and cementite, Fe3C (6.67% C), solidifying simultaneously. This eutectic mixture is called Ledeburite. iii. Eutectoid Reaction Eutectic reaction, in general, can be represented by equation:

where S1, S2 and S3 are three different solids each of fixed composition The shown Figure illustrates the eutectoid region of Fe-Fe3C phase diagram During cooling, γ-austenite of 0.77% C at constant eutectoid temperature of 727 °C undergoes eutectoid transformation to form a mixture of α-ferrite of 0.02% carbon and cementite, Fe3C, of 6.67% carbon in the form of alternate lamellae of both α-ferrite and cementite. This mixture is called pearlite because its pearly appearance under optical microscope Introduction to Development Microstructures in the Fe-Fe3C • The term microstructure refers to the details of a microphotograph of metal (alloy) or similar image which is taken through a microscope. • An alloy normally requires metallographic preparation before its microstructure can be seen through .

• The development microstructures of in the Fe-Fe3C alloys are mainly depending on composition (carbon content) and heat treatment (this will be explained in later lecture). • Generally, the microstructure of an alloy consists of the structure of the grains and phases, which the alloy possesses • Microstructure of the ferrous alloys are the main parameter affecting in most of the ordinary properties. Fe–Fe3C Phase Diagram Developed Microstructure due to Peritectic Reaction

• As cooling continues more δ-ferrite solidifies as shown at points 2 and 3. • At any temperature, lever rule helps to calculate the fraction of δ-ferrite and liquid. • The compositions of δ-ferrite changes with further fall of temperature. • When the peritectic temperature of 1495 °C, is just reached, the liquid has composition of 0.53% C and δ-ferrite has a composition of 0.09% carbon. Therefore, the alloy undergoes the peritectic reaction completely, i.e. the δ-ferrite reacts with the liquid to give one solid γ-austenite solution of 0.17% C as shown at point Y. Developed Microstructure due to Eutectoid Reaction Microstructure depends on the composition (carbon content) as following: i. Alloy with the eutectoid composition (0.77% C) When γ-austenite alloy of of 0.77wt% C, is cooled slowly to the eutectoid temperature of 727°C, it undergoes eutectoid transformation where a mixture of layered structure of two phases α-ferrite and cementite, Fe3C, is developed.

This mixture is called pearlite because its pearly appearance under optical microscope ii. Alloy with hypoeutectoid composition (0.02 - 0.77 % C) Compositions to the left of eutectoid (0.02-0.77wt% C) is called hypoeutectoid alloys. Where the following reactions is occurred while the temperature is decreased: The final microstructure of the hypoeutectoid alloys contain proeutectoid α-ferrite which is formed above the eutectoid temperature, plus the eutectoid pearlite mixture of α-ferrite and cementite, Fe3C as shown in the figure. In the given micrograph, the dark areas are the layers of the pearlite and the light phase is the proeutectic α-ferrite iii. Alloy with hypereutectoid composition (>0.77% C) Compositions to the right of eutectoid (0.77-2.11wt% C) is called hypereutectoid alloys. Where the following reactions is occurred while the temperature is decreased as shown in the figure: The final microstructure of the hypereutectoid alloys contain proeutectoid cementite which is formed above the eutectoid temperature, plus the eutectoid pearlite mixture of α-ferrite and cementite, Fe3C. In the shown micrograph, the dark areas are the layers of the pearlite and the light phase is the proeutectic cementite, Fe3C Calculation of the Relative Amounts of Proeutectoid Phases

The relative amounts of the proeutectoid phases α-ferrite or Fe3C as well as the pearlite can be calculated for steels of composition

C0 and C1 by applying the lever rule as shown in the figure: First, draw the tie line between α and pearlite (α + Fe3C) that extends from the eutectoid composition of 0.77% C

to α boundary of 0.02% C for hypoeutectoid alloys and the tie line between pearlite and Fe3C that extends from the eutectoid composition of 0.77% C to Fe3C boundary of 6.67% C, for hypereutectoid alloys Fraction of total α phase (proeutectoid phase and the one that existed in pearlite mixture) is determined by application of the lever rule between α and Fe3C phase. Example of the calculations made for hypereutectoid alloy of composition C1 that is shown as follows: Fraction of pearlite:

Fraction of proeutectoid cementite:

Fraction of total cementite:

Fraction of total α-ferrite:

Example 1 Determine and show the transformation experienced by slowly cooling plain carbon steel containing 0.13% C and 0.7% from the liquid stat to γ-austenite phase.

Example 2 Determine and calculate the amounts of phases in Fe-0.35% C alloy just above and just below 727 °C. For the same alloy calculate the total amount of the existed phases below 727 °C

Example 3 Calculate the amounts of α-ferrite and cementite phases in pearlite mixture of the eutectoid alloy

Example 4 Determine and calculate the amounts of phases in Fe-1.25% C alloy just above and just below the eutectoid temperature (727 °C)

Example 5 Slowly cooled plain carbon steel shows proeutectoid ferrite to be 15% by the weight of the microstructure. Estimate the carbon percent of the steel

Home Work

Problem 1:

The given above figures indicate the following: 1) Eutectoid reaction 2) (Liquid + g) zone 3) (Ledeburite + Fe3C) zone 4) (Pearlite + a-ferrite) zone 5) (g + Fe3C) zone 6) Eutectic reaction 7) (Ledeburite + g) zone 8) d phase zone 9) (Pearlite + Fe3C) zone 10)Peritectic reaction

It is required to the put corresponding correct number below each given figure Problem 2:

The given figures indicate the following: 1) Microstructure of hypoeuectoid steel. 2) Tempered Martensite. 3) Microstructure of hypereuectoid steel. 4) Martensite microstructure. 5) Microstructure of eutectoid steel. 6) γ-austenite microstructure.

It is required to the put corresponding correct number below each figure. ﻣﻠﺣوظﺔ ھﺎﻣﺔ: ﻻ ﺗﺿﻊ أي ﻛﺗﺎﺑﺔ ﻣن أي ﻧوع ﺗﺣت اﻟﺷﻛل و إﻻ ﺗﺗﻌرض ﻟﺧﺻم درﺟﺔ اﻟﺳؤال، ﻓﻘط ﺿﻊ اﻟرﻗم ﺗﺣت اﻟﺷﻛل اﻟﻣرادف

Problem 3 Slowly cooled plain carbon steel shows proeutectoid ferrite to be 10% by the weight of the microstructure. Estimate the carbon percent of the steel

Problem 4

ﺿﻊ ﻋﻼﻣﺔ (√) أﻣﺎم اﻹﺟﺎﺑﺔ اﻟﺻﺣﯾﺔ :Put sign (√) in the front of the correct answer

Steel can be existed only at carbon content: (………………) less than 0.53 wt% (………………) less than 2.11 wt% (………………) more than 2.11 wt% (………………) more than 0.09 wt%

Pearlite mixture consisting of layers from: (………………) g-ferrite and Cementite (………………) g-austenite and a-ferrite (………………) d-ferrite and Cementite (………………) a-ferrite and Cementite

The equation of Liquid phase Solid phase 1 + Solid phase 2 represents: (………………) ledburite reaction (………………) eutectoid reaction (………………) peritectic reaction (………………) eutectic reaction

For hypoeutectoid steel the existed proeutectoid phase is: (………………) g-austenite (………………) d- ferrite (………………) Fe3C (………………) a- ferrite

For hypereutectoid steel the existed proeutectoid phase is: (………………) g-austenite (………………) d- ferrite (………………) Fe3C (………………) a- ferrite Problem 5

Put the corresponding number in the circles of the given figures, which contains different transformation reaction and different important lines: