5. ALLOY, PHASE DIAGRAM & STEELS - WordPress.com · 2017. 8. 24. · Module 2 5. Alloy, Phase...

Pruthvi Loy, Chiranth B. P. 1 SJEC, Mangaluru

5. ALLOY, PHASE DIAGRAM & STEELS

Introduction

- Solid solution: substitutional and interstitial solid solutions

- Hume Rothary rule

- Intermediate phases

- Phase diagrams

- Gibbs phase rule

Binary phase diagrams

- Types of binary phase diagram

- Construction of a binary phase diagram

- Interpretation of a phase diagram; lever rule

- Invariant reactions (Eutectic and Eutectoid)

- Iron carbon equilibrium diagram

Steels

- Classification of steels

- Specification

- Alloying additives

5.1 INTRODUCTION

Definitions – Phase, Solution, Mixture

Any material or a substance (system) is composed of a number of independent chemical species

(constituent); thus a material can be of a single constituent or a multiple constituent type. The

constituents of a system could be an element or a compound which can exist as different phases.

A phase may be defined as a physically distinct, chemically homogeneous and mechanically

separable portion of a system.

Example: Water is a single constituent material which can exist in different phases (ice, liquid

water, and water vapour)

A single phase material having one or more constituents is called a solution (liquid or solid);

when more than one phase exists, then it is called a mixture. When two phases are present in a

system, it is not necessary that there be a difference in both physical and chemical properties; a

disparity in one or the other set of properties is sufficient.

Example: Water is a solution; on adding sugar crystals to it they dissolve forming a sugar syrup

which is still a solution (single phase) but when sugar is added beyond its solubility limit in

water it will not dissolve and forms a mixture with two phases.

Module 2 5. Alloy, Phase Diagram & Steels


5.1.1 Solid Solution

The properties of materials are greatly influenced by changing its composition due to

modification of its microstructure this is known as alloying. An alloy is a substance that has

metallic properties and is composed of two or more chemical elements of which at least one is a

metal. The alloy constituents of a system can be either two metals (Cu and Ni), or a metal and a

compound (Fe and Fe3C), or two compounds (Al2O3 and Si2O3), etc. These constituents when

mixed can form a solid solution phase entirely or an intermediate phase.

a) Solid Solution

When two metals are mixed in their liquid states, they form a homogeneous solution, if this

homogeneity is maintained after solidification, then such a solid is known as a solid solution

(that is, a solution in the solid state consisting of two types of atoms in one type of space lattice).

In a solid solution the metal in major proportion is called the solvent and the one in minor

proportion is called the solute.

Types of Solid Solution:

The solid solution formed can be of two types;

Substitutional Solid Solution: In this type of solid solution the solute atoms substitute the atoms

of the solvent in the crystal structure of the solvent. Example: Cu-Ni system.

Interstitial Solid Solution: These are formed when atoms of small atomic radii fit into the

interstitial spaces of the larger solvent atoms. Example: carbon in γ-iron

Hume-Rothary rule:

These are the rules governing the formation of substitutional solid solutions;

1) Crystal structure factor: for complete solid solubility of two elements, they should have

the same type of crystal structure. For example, copper atoms may substitute for nickel

atoms without disturbing the FCC structure of nickel.

2) Relative size factor: the atoms of the solute and the solvent should be approximately of

the same size (difference in radii should be less than 15%). For example, both silver and

lead have FCC structure but the relative size factor is about 20 %. Therefore, they have

poor solubility. Whereas, silver and palladium are completely soluble in each other as

they have the same type of crystal structure (FCC) and differ in atomic radii by about 5%.

3) Chemical affinity: the two metals should have very less chemical affinity. Generally, if

the two metals are separated in the periodic table widely then they possess greater

chemical affinity and are likely to form some compound instead of solid solution.



4) Electronegativity (tendency to acquire electrons): the two metals should have less

electronegativity; higher the electronegativity of two elements greater will be the chance

of forming an intermediate phase.

5) Relative valence factor: among the metals, the one with the lower valency tends to

dissolve more of a metal of higher valency than vice versa. For example, in a Ni-Al

system; Ni has valency of 2 which dissolves 5% Al, but Al has valancy of 3 and dissolves

only 0.04% Ni.

b) Intermediate phase

An intermediate phase is a compound made up of two or more elements of which at least one of

them is metal. If the constituent elements forming an intermediate phase are exclusively metal-

metal systems, then they are called intermetallic compounds. When an intermediate phase is

formed the elements lose their individual identity and properties to a great extent and the

compound will have its own characteristic physical, chemical and mechanical properties.

Example: H2O, NaCl, etc.

5.1.2 Phase Diagram

A phase diagram is a graphical representation of the various phases of a substance and the

conditions at which thermodynamically those distinct phases can exist at equilibrium. A phase

diagram is also called as equilibrium or constitutional diagram.

Phase diagrams are classified as:

a) Unary phase diagram or one component phase diagram

b) Binary phase diagram or two component phase diagram

c) Ternary phase diagram or three component phase diagram

a) Unary Phase Diagram

In single component systems the composition remains same for any variation of temperature and

pressure but it may undergo change of phase (solid, liquid and gaseous phase). A unary phase

diagram is a plot of pressure (P) vs. temperature (T) as shown in Figure 5.1. The lines on the

diagram represent conditions (T & P) at which a phase change occurs under equilibrium. That is,

at a point on a line, it is possible for two (or three) phases to coexist at equilibrium. At the triple-

point, three phases can coexist at equilibrium. In other regions of the plot, only one phase exists

at equilibrium.

Example: Water exists as ice, liquid water, and water vapour.



Figure 5.1: Unary phase diagram

Beyond certain value of temperature and pressure it is not possible to distinguish between the gas

and liquid phase. This is known as critical point and the corresponding temperature and pressure

are known as the critical temperature (Tc) and critical pressure (Pc) respectively.

b) Binary Phase Diagram

A binary phase is a two component system; it is the most commonly used phase diagram in alloy

designing. For most systems, pressure is constant and independently variable parameters are –

temperature and composition. Thus a binary phase diagram is a plot of temperature vs.

composition.

Example: The simplest binary system is the Cu-Ni which exhibits complete solubility in liquid

and solid state.

Figure 5.2: Binary Phase diagram for a Cu-Ni System



Figure 5.3: Unary phase diagram for water

The line above which the solid solution is liquid is called as a liquidus line.

The line below which solidification completes is called as solidus line, here only α-solid

solution exists at any temperature below the solidus line (for complete solubility).

The line between these two regions is called as an intermediate phase; where both liquid

and solid co-exist.

It can be noted that the two metals are soluble in each other in the entire range of

compositions in both liquid and solid state. This kind of system is known as

‘Isomorphous’ system.

Liquids line separates liquid from liquid + solid, solidus line separates solid from liquid +

solid

c) Ternary Phase Diagram

A ternary phase diagram has three components. The three components are usually compositions

of elements, but may include temperature or pressure also. This type of diagram is three-

dimensional but is illustrated in two-dimensions for ease of drawing and reading. Ternary phase

diagrams are needed so that three components can be compared at once.

5.1.3 Gibbs Phase rule

It states that,

P + F = C + 2

Where,

P - number of phases

F - number of degrees of freedom (the number of variables that may be changed

independently without causing the disappearance of the phase)

C - number of components in the system

2 - represent the two system variables (pressure and temperature)

Example: Water

We have, P + F = C + 2

In regions I, II & III:

P = 1, F = 2 and C = 1

1 + 2 = 1 + 2

Along the lines OA, OB and OC:

P = 2, F = 1 and C = 1

2 + 1 = 1 + 2

And at the triple point (at O):

P = 3, F = 0 and C = 1

3 + 0 = 1 + 2



Figure 5.4: Binary phase diagram

Thus, Gibbs phase rule holds good for all specified conditions.

Modified phase rule:

If the effect of pressure is ignored and assumed to be 1 atm, as in the case of binary phase

diagram, then the modified phase rule becomes,

P + F = C + 1

F = C – P + 1

Since the degrees of freedom cannot be negative, C – P + 1 ≥ 0 Or P ≤ C + 1

i.e., the number of phases present in an alloy cannot exceed the number of components plus one

In regions I (Liquid):

P = 1, F = 2 and C = 2

1 + 2 = 2 + 1

In regions II (Liquid+Solid):

P = 2, F = 1 and C = 2

2 + 1 = 2 + 1

In regions III (Solid):

P = 1, F = 2 and C = 2

1 + 2 = 2 + 1

For a binary phase diagram, the modified

phase rule is applicable.

5.2 BINARY PHASE DIAGRAMS

A phase diagram represents various phases and the conditions for the existence of these phases at

equilibrium to understand the microstructural development of an alloy system. The principles of

microstructural control with the aid of phase diagrams can be illustrated with binary alloys even

though, in reality, most alloys contain more than two components. If more than two components

are present, phase diagrams become extremely complicated and difficult to represent and hence

binary phase diagrams are the most commonly used phase diagrams in alloy designing.

A binary phase diagram is a plot of temperature versus composition. External pressure is also a

parameter that influences the phase structure. However, in practicality, pressure remains virtually

constant in most applications; thus, the phase diagrams presented here are for a constant pressure

of one atmosphere (1 atm).



5.2.1 Types of phase diagrams

The binary phase diagrams are classified according to the various reactions or transformations

observed as follows;

a) Isomorphous system (solid solution - complete solid solubility)

b) Eutectic reaction

c) Peritectic reaction

d) Eutectoid transformation Invariant reactions

e) Peritectoid transformation

f) Monotectic and Syntectic reaction

A few alloy systems show an unlimited solid solubility for any composition, such an alloy

system is called an Isomorphous system. But for most alloy systems at some specific

temperature, there is a maximum concentration of solute atoms that may dissolve in the solvent

to form a solid solution; this is called a solubility limit. The addition of solute in excess of this

solubility limit results in the formation of another solid solution or compound that has a

distinctly different composition; these are represented as invariant reactions.

Phase diagrams are helpful in predicting these phase transformations and the resulting

microstructures.

5.2.2 Construction of a phase diagram

A phase diagram can be constructed from cooling curves; that is by plotting the variation of

temperature and the phase transformations during heating or cooling of an alloy system of

various compositions under equillibrium conditions. The equillibrium condition means that the

phases present at a particular temperature and composition will not change with time. Such an

equillibrium state can be achieved by slow heating or cooling.

Figure 5.5: Construction of a phase diagram from the cooling curves



A series of cooling curves of different compositions of A and B are plotted as shown above. The

data obtained from the cooling curve is used to construct the liquidus and solidus curves in a

phase diagram which mark the phase transformations; a liquidus curve is obtained on a phase

diagram by joining all the points that indicate the beginning of solidification in terms of

temperature from the cooling curves for the corresponding composition. Solidus curve is also

obtained similarly by joining all the points indicating the end of solidification.

5.2.3 Interpretation of a phase diagram

The interpretation of a binary phase diagram for a specified temperature and composition reveals

the following details;

a) the number of phases present

b) the composition of each phase (Tie line) and

c) the relative amount of each phase (Lever rule)

Figure 5.6: Interpretation of a binary phase diagram

In single-phase regions the above details can be obtained easily but for a point located within a

two-phase region the calculations can be done as follows;



At a point ‘x’ on the phase diagram in figure 3.19 (specified temperature and composition)

the number of phases present: two (solid and liquid)

the composition of each phase: it can obtained from a horizontal tie line (AB) connecting

the solidus and liquidus curves drawn a the specified temperature (Tx); a perpendicular

dropped from the intersection of the tie line with the liquidus and solidus curves to the

horizontal composition axis gives the composition of liquid and solid respectively.

Composition of phase 1 (liquid) = CL

Composition of phase 2 (solid) = CS

the relative amount of each phase: it can be calculated by applying lever rule.

(i.e., the amount of each phase can be calculated as the fractional length of the tie line

from the specified composition to the phase boundary of the other phase)

The amount of phase 1 (liquid) =

The amount of phase 2 (solid) =

5.3 INVARIANT REACTIONS

A phase diagram contains certain points specified by particular temperature, pressure and

composition at which multiple phases can coexist in equilibrium. Any change in these variables

causes the disappearance of phases i.e. phases are not in equilibrium anymore. These points are

known as invariant points and they represent an Invariant reaction. The alloy systems featuring

invariant reactions may show limited or no solid solubility.

5.3.1 Eutectic reaction

Most of the binary systems with limited solubility are of eutectic type. The term ‘eutectic’ is

derived from Greek word ‘eutektos’ which means ‘easily melted’. It is so called because an alloy

system having a eutectic composition melts (or solidifies) at the lowest possible temperature than

all the other compositions of the binary alloy. The point corresponding to this lowest temperature

at which the solid and liquid phases coexist under equilibrium condition is known as the eutectic

point. There are three phases present at the eutectic point; two solid phases which are obtained

upon solidification from a liquid phase.

Further, a eutectic system can be of two types based on the solid solubility; i.e. a eutectic system

with limited or no solid solubility.

𝑙𝑖𝑞𝑢𝑖𝑑 𝑐𝑜𝑜𝑙𝑖𝑛𝑔

ℎ𝑒𝑎𝑡𝑖𝑛𝑔

𝑠𝑜𝑙𝑖𝑑 + 𝑠𝑜𝑙𝑖𝑑 2



a) Eutectic with no solid solubility

+

For the above alloy system containing metals A and B, there exists unlimited liquid solubility but

upon cooling the eutectic mixture forms two solid phases without forming any compound or

intermediate phases. The two solid phases A and B has no solid solubility in each other.

Example: Bi – Cd, Al – Si alloy systems

Figure 5.7: Bi - Cd eutectic system with no solid solubility

The bismuth - cadmium alloy system forms a eutectic at 146 C and for a composition of 55%

Cd and 45% Bi which can be represented as;

𝐿 55% 𝐶𝑑 𝑐𝑜𝑜𝑙𝑖𝑛𝑔

46 𝐶 𝐵𝑖 + 𝐶𝑑



b) Eutectic with partial solid solubility

When there exists a limited solubility of one solid phase in the other;

+

i.e. for an alloy system containing metals A and B, upon solidification two solid phases are

formed which are generally represented as and ; where is a solid solution rich in metal A

and is a solid solution rich in metal B.

Example: Cu – Ag alloy system, Fe – C alloy system, etc.

Most of the eutectic alloy systems show partial solid solubility in each other. The construction

details and phase regions for a eutectic reaction with partial solubility can be explained with the

help of a copper – silver phase diagram as shown in figure 5.8.

Figure 5.8: Cu-Ag eutectic system showing partial solubility

The copper – silver alloy system forms a eutectic at 779 C and for a composition of 71.9% Ag

and 28.1% Cu which can be represented as;

𝐿 71.9% 𝐴𝑔

𝑐𝑜𝑜𝑙𝑖𝑛𝑔

779 𝐶 𝛼 8% 𝐴𝑔 + 𝛽 91.2% 𝐴𝑔



5.3.2 Eutectoid transformation

The term eutectoid means ‘eutectic-like’ i.e. the eutectoid reaction is similar to eutectic reaction

except that it features a solid to solid transformation where one solid phase is converted into two

solid phases upon cooling.

Example: Fe – C system undergoes eutectoid transformation at 723 C for a composition of 0.8

wt. % carbon in iron.

Figure 5.9: Part of Fe - C system showing eutectoid transformation

It can be represented as,

5.3.3 Peritectic reaction

A peritectic reaction is also an invariant reaction where three phases (two solids and a liquid)

coexist under equilibrium conditions. It occurs in binary alloy systems whose melting points are

significantly different where one of the solid phases precipitates out of the liquid upon cooling

and as the solidification proceeds further, the already formed solid and the remaining liquid

transforms into a new solid completely at a specified temperature and pressure.

𝑙𝑖𝑞𝑢𝑖𝑑 + 𝑠𝑜𝑙𝑖𝑑



𝑠𝑜𝑙𝑖𝑑 2

𝑠𝑜𝑙𝑖𝑑 𝑐𝑜𝑜𝑙𝑖𝑛𝑔


𝑠𝑜𝑙𝑖𝑑 2 + 𝑠𝑜𝑙𝑖𝑑 3

𝛾 0.8% 𝐶 𝑐𝑜𝑜𝑙𝑖𝑛𝑔

723 𝐶 𝛼 0.02% 𝐶 + 𝐹𝑒3𝐶 6.67% 𝐶



The term peritectic means ‘surrounded by’. It is so called because the product phase (solid 2)

will form at the boundary between the two reacting phases (liquid and solid 1) as shown in

figure5.10 (for a reaction L + ) thus separating them, and slowing down any further

reaction.

Figure 5.10: Development of a peritectic wall ( + )

Example: Fe – C system undergoes peritectic reaction at 1495 C for a composition of 0.16 wt.

% carbon in iron.

Figure 5.11: Part of Fe - C system showing peritectic reaction

It can be represented as,

𝛿 . % 𝐶 + 𝐿 .5% 𝐶


1495 𝐶 𝛾

. 6% 𝐶



5.3.4 Peritectoid transformation

The peritectoid is a solid to solid transformation where the two solid phases react to form a

single product phase which is also a solid.

Example: Silver – Aluminium alloy system

5.3.5 Monotectic and Syntectic reactions

Another three phase invariant reaction that occurs in some binary system is monotectic reaction

in which a liquid transforms to another liquid and a solid.

Example: Cu-Pb system

A syntectic is formed when two liquid phases react to form a single solid product phase.

Example: Na – Zn system

𝑠𝑜𝑙𝑖𝑑 + 𝑠𝑜𝑙𝑖𝑑 2 𝑐𝑜𝑜𝑙𝑖𝑛𝑔


𝑠𝑜𝑙𝑖𝑑 3

𝑙𝑖𝑞𝑢𝑖𝑑 𝑐𝑜𝑜𝑙𝑖𝑛𝑔


𝑙𝑖𝑞𝑢𝑖𝑑 2 + 𝑠𝑜𝑙𝑖𝑑

𝑙𝑖𝑞𝑢𝑖𝑑 + 𝑙𝑖𝑞𝑢𝑖𝑑 2 𝑐𝑜𝑜𝑙𝑖𝑛𝑔


𝑠𝑜𝑙𝑖𝑑



APPENDIX

Table 5.1: Invariant reactions

Invariant reactions Symbolic representation Schematic representation Examples

Eutectic

No solid solubility

+

Bi – Cd:

55%

146 +

Partial solid solubility

+

Cu – Ag:

71.9%

779 8% + 91.2%

Peritectic +

Fe – C:

0.1% + 0.5%

495 0.16%

Eutectoid +

Fe – C:

0.8%

723 0.02% + 3 6.67%

Peritectoid +

Ag – Al

Monotectic +

Cu – Pb, Ag – Ni

Syntectic +

Na – Zn



5.4 IRON – CARBON ALLOY SYSTEM

Iron and its alloys are the most extensively used engineering material as it is abundantly

available and has fair mechanical properties. Pure iron is soft and ductile; with alloying its

mechanical properties can be enhanced. Of all the alloying elements the carbon has profound

influence on the microstructure and the properties of iron. The steel and cast iron are the alloys

of iron and carbon which are widely used in all engineering applications; and hence of all the

binary alloy systems, the Iron – Carbon systems are of at most importance.

Iron is an allotropic metal, i.e., it can crystallize in various structures, where each crystal

structure defines a separate phase which is stable in a specified temperature range. Thus, an

allotropic metal can exist in more than one solid phase at different temperatures.

There are three separate solid phases for iron, each denoted by a Greek symbol: alpha (),

gamma (γ) and delta (δ); where - iron is BCC structured being stable at room temperature and

upto 910 C, beyond 910 C iron exists as γ – iron having FCC – structure which is retained till

1395 C. At 1395 C it changes back once again to BCC structure called δ – iron and is retained

till the melting point of iron (1539 C). The allotropic changes occur in pure iron at a specific

temperature, with the addition of alloying elements the transformation temperature changes.

Figure 5.12: Cooling curve for pure iron



5.4.1 Fe – Fe3C equilibrium diagram

Pure iron exists as different phases for various range of temperatures, the addition of carbon not

only changes the transformation temperature but also alters the phase existence owing to limited

solubility of carbon in iron. Carbon when added within its solid solubility limit occupies the

interstitial position in an iron lattice forming a single phase solid solution but when exceeded

beyond its solubility limit alters the lattice structure as a result multiple phase formation occurs.

The phase diagram for an iron-carbon system with the existence of various phases at equilibrium

for different condition of temperature and carbon composition is as shown in figure 5.13.

Figure 5.13: Iron Carbon equilibrium diagram

The iron – carbon diagram is usually plotted only upto 6.67 % carbon by weight. This is because

a maximum of 6.67 % carbon is dissolved by molten iron forming a chemical compound

Cementite (Iron Carbide, Fe3C); any excess carbon will just float over the melt owing to its low

density and for this reason an Iron – Carbon diagram is actually referred to as Iron – Iron carbide

diagram (Fe – Fe3C) with pure iron and pure cementite as the extremities.



5.4.2 Description of phases and structures

Following are the phases and structures obtained upon very slow cooling of iron carbon system

a) δ - ferrite: It is an interstitial solid solution of carbon in BCC iron. The maximum solid

solubility of carbon in δ – iron is 0.1% at 1495 C. It is a high temperature phase.

b) Austenite (γ): It is again an interstitial solid solution of carbon but in FCC iron. The

maximum solid solubility of carbon in γ – iron is 2.1% at 1147 C. Austenite is not stable

below 723 C and will not be present in the microstructure at room temperature upon

normal cooling.

c) - ferrite: It is also an interstitial solid solution of carbon in BCC iron. The maximum

solid solubility of carbon in – iron is 0.02% at 723 C. It is a relatively soft and low

temperature phase.

d) Cementite (Fe3C): It is a typical hard and brittle chemical compound containing 6.67%

carbon and having an orthorhombic crystal structure. It is also known as Iron carbide. It is

the hardest structure that appears on Fe – C diagram. Cementite has low tensile strength but

high compressive strength.

e) Pearlite ( + Fe3C): When austenite containing exactly 0.8% carbon is cooled very slowly

below 723 C it transforms into a lamellar structure having alternate layers of -ferrite and

cementite, this structure is known as pearlite.

f) Ledeburite (γ + Fe3C): it is the eutectic lamellar mixture of austenite and cementite. The

austenite in the eutectic mixture of ledeburite is unstable below 723 C and hence

transforms into -ferrite and cementite.

5.4.3 Invariant reactions in Fe – C system

There are three principal invariant reactions that appear on a Fe - Fe3C diagram, they are;

peritectic, eutectic and eutectoid. The peritectic reaction of a Fe - C system has already been

discussed in section 5.3.3. The eutectoid and eutectic reactions are important to understand the

microstructural development of steel and cast iron and will be discussed further in section 5.4.4.

5.4.4 Solidification of steel and cast iron

The Fe - C alloy system can be classified as steels and cast irons based on their carbon

composition.

Steels (up to 2.1% carbon)

Cast iron (beyond 2.1% carbon)



The eutectoid reaction is important to understand the solidification of steels and the eutectic

reaction for the solidification of cast iron.

5.4.4 (a) Solidification of Steels (slow cooling)

A tie line can be constructed at the eutectoid reaction ( + 3 to represent the phase

transformation and the resulting microstructure of steels as shown below;

Figure 5.14: A tie line representing eutectoid reaction

Eutectoid steel (exactly 0.8% carbon)

Iron (austenite) with a carbon composition of 0.8% upon normal cooling below 723 C forms a

complete lamellar structure comprising alternate layers of ferrite and cementite, this structure is

known as pearlite.

Figure 5.15: Solidification of eutectoid steel and the resulting microstructure



The pearlite contains -ferrite and cementite (Fe3C), the relative amount of each phases in it can

be calculated using lever rule as follows (refer tie line for carbon compositions);

-ferrite = 6.67 0.8

6.67 0.02 x 100 = 87.5 %

Cementite = 0.8 0.02

6.67 0.02 x 100 = 12.5 %

Thus, pearlite contains -ferrite and cementite in the ratio 7:1.

Hypo-eutectoid steel (less than 0.8% carbon)

The solid solubility of carbon reduces below the upper critical temperature (A3) and hence the

excess carbon comes out of austenite and forms -iron as the proeutectoid phase. The austenite

in not stable below the lower critical temperature (A1 = 723 C) and hence transforms into

pearlite. Thus at carbon percentages less than 0.8, ferrite and pearlite appear in separate patches

where ferrite is a soft and ductile phase and the cementite is hard and brittle.

Figure 5.16: Solidification of hypo-eutectoid steel and the resulting microstructure

Hyper-eutectoid steel (more than 0.8% carbon)

Beyond 0.8% carbon, the cooling of austenite below the upper critical temperature (Am) results

in rejection of carbon along the grain boundaries due to lattice reorientation, the austenite cannot

hold all the carbon below this temperature and hence the excess carbon forms proeutectoid

cementite (Fe3C). The austenite in not stable below the lower critical temperature (A1 = 723 C)



and hence transforms into pearlite. Thus, the microstructure of a hyper-eutectoid steel contains

proeutectoid cementite along with pearlite and it becomes more prevalent as the carbon

increases; hence there is an enhanced strength and hardness characteristic at the expense of

ductility.

Figure 5.16: Solidification of hyper-eutectoid steel and the resulting microstructure

Although steels are marked to have a carbon percentage up to 2.1%, the commercially viable

steels contains not more than 1 to 1.5% carbon.

Note:

Critical Temperature: the temperature at which phase transformations take place is known as

critical temperature.

A1 – lower critical temperature

A3 – upper critical temperature for hypo-eutectoid steels

Am – upper critical temperature for hyper-eutectoid steels

5.4.4 (b) Solidification of Cast irons (slow cooling)

The solidification of cast iron falls in the range of eutectic reaction where the eutectic

composition of 4.3% C yields a lamellar structure of ledeburite ( + Fe3C). For any composition

within 2.1 to 4.3 % carbon other than the eutectic, the microstructure contains a proeutectic

austenite but the solubility of carbon in austenite reduces below the eutectic temperature as a

result the excess carbon comes out of austenite and forms secondary cementite. For composition

beyond 4.3% carbon, the cementite forms as the primary phase and the eutectic ledeburite forms

on these already formed cementite.



Figure 5.17: Solidification of eutectic cast iron and the resulting microstructure



The austenite within eutectic and proeutectic phase is not stable below 723 C and hence

transforms into pearlite. Thus according to Fe-C diagram, the phases that are observed in the

microstructure of cast iron at room temperature are -ferrite and cementite. But cementite being

a metastable compound under certain circumstances decomposes into -ferrite and C (graphite).

And the actual microstructure of cast irons would contain the phases -ferrite and free carbon in

the form of graphite rather than -ferrite and cementite.

Therefore, cast irons can be distinguished by the presence of carbon as an elemental carbon in

the form of graphite or in the combined carbon as Fe3C (iron carbide). The amount, size, shape

and distribution of the graphite, if present in cast irons may greatly influence their properties.

5.4.5 Effect of alloying elements

The most common alloying additives viz. Ti, Mo, Si, W, Cr, Mn, Ni, etc. when added to Fe-C

alloy system alters the phase transformation temperature; they either increase or decrease the

eutectoid transformation temperature, and are according classified as ferrite and austenite

stabilizers respectively.

Figure 5.18: Effect of alloying elements on Iron - Carbon diagram

i. Ferrite Stabilizers: these elements being more soluble in -iron than -iron raise the

eutectoid temperature.

Example: Titanium, Molybdenum, Silicon, Tungsten, Chromium, etc.

ii. Austenite Stabilizers: these elements when added lower the eutectoid temperature and

raises the peritectic point, thereby increasing the austenite stability range.

Example: Manganese, Nickel, Cobalt, Copper, etc.



5.5 STEEL

5.5.1 Types of Steel

Steel with only carbon as the alloying constituent is termed as plain carbon steel and it may be

further classified as;

1. Low-carbon steel (carbon less than 0.2%)

2. Medium-carbon steel (carbon between 0.2 and 0.5%)

3. High-carbon steel (carbon above 0.5%)

In addition to carbon, steel may also contain other alloying constituents such as manganese,

silicon, phosphorus, sulphur. Small percentages of other residual metals such as nickel,

chromium and copper may also be present. Based on the quantities of these alloying additives,

steel may also be classified as;

1. Low-alloy steel (alloy content totaling less than 8%)

2. High-alloy steel (alloy content totaling more than 8%)

Table 5.2: Typical composition of steel

Plain carbon steel Alloy steel

Apart for carbon the following alloying

elements can be expected:

Si – 0.8%

Mn – 0.5 to 1.2%

S – 0.06%

P – 0.05%

Common alloying additives are Mn, Ni, Cr, Mo,

Si, Vn, W, Cu, S, P, etc. (they either form

compounds or carbides)

Effect of alloying additives:

Mn – slightly enhance strength & hardness

Cr – corrosion resistance

Al – oxidation resistance

Mo – corrosion against sea water

Cr, Vn, Mo, W, Mn – forms carbides

Si – improve magnetic properties, promotes

graphitization

W – increases melting point

Cu – atmospheric corrosion resistance

P – increases strength & hardness but decreases

ductility & impact toughness

Alloying elements upto 50% may be added.



5.5.2 Steel Designations

There are various standard organizations who have classified the steel grades by their

composition, physical properties, etc.

AISI – American Iron & Steel Institute

SAE – Society of Automotive Engineers

ASTM – American Society for Testing & Materials

BIS – Bureau of Indian Standards

AISI/SAE Steel Designation System:

The AISI and SAE were both involved in efforts to standardize such a numbering system for

steels which was united into a joint system designated the AISI/SAE steel grades.

Steels are designated by a four digit number, where the first digit indicates the main alloying

element, the second digit indicates the secondary alloying element, and the last two digits

indicate the amount of carbon in weight percentage.

The following table specifies the types of steels based on the main alloying constituent.

Table 5.3: AISI/SAE Steel Designation System

Designation Type

1xxx Carbon steels

2xxx Nickel steels

3xxx Nickel-chromium steels

4xxx Molybdenum steels

5xxx Chromium steels

6xxx Chromium-vanadium steels

7xxx Tungsten steels

8xxx Nickel-chromium-molybdenum steels

9xxx Silicon-manganese steels

XX

Type of

material

selected

XX

Amount of

carbon present

in steel



Table 5.4: AISI/SAE Steel Designation System

Class of Steels SAE-AISI number Composition

Carbon steel

10XX Plain carbon steel (Mn < 1%)

11XX High sulphur carbon steel

12XX High sulphur & phosphorous carbon steel

15XX Plain carbon steel (Mn, 1 – 1.65%)

Manganese Steel 13XX Manganese – 1.75%

Nickel Steel 23XX Ni – 3.50%

25XX Ni – 5.00%

Nickel-Chromium steel

31XX Ni 1.25, Cr 0.65

32XX Ni 1.75, Cr 1.07

33XX Ni 3.50, Cr 1.50

34XX Ni 3.00, Cr 0.77

Molybdenum Steel 40XX Mo 0.20 – 0.50

44XX Mo 0.40 – 0.52

Chromium-Molybdenum Steel 41XX Cr – 0.5, 0.8 & 0.95, Mo 0.1 – 0.3

Ni-Cr-Mo Steel 43XX Ni – 1.82, Cr – 0.5 & Mo – 0.25

Nickel-Molybdenum Steel 46XX Ni – 0.85 & 1.82, Mo – 0.20 & 0.25

Chromium Steel 50XX Cr 0.27 – 0.65

Chromium-Vanadium Steel 61XX Cr – 0.6, 0.8 & 0.95, V – 0.1 & 0.15

Tungsten-Chromium Steel 72XX W – 1.75, Cr – 0.75

Silicon-Manganese Steel 92XX Si – 1.4 & 2.0, Mn 0.65 – 0.85

References:

1. Fundamentals of Materials Science & Engineering – William D. Callister

2. Material Science and Metallurgy – K.R.Phaneesh

3. Material Science and Metallurgy – Kesthoor Praveen

4. Materials Science for Engineers – James F. Shackleford

5. Physical Metallurgy Principles – Robert E. Reed-Hill

5. ALLOY, PHASE DIAGRAM & STEELS5.1 INTRODUCTION5.1.1 Solid Solution5.1.2 Phase Diagram5.1.3 Gibbs Phase rule

5.2 BINARY PHASE DIAGRAMS5.2.1 Types of phase diagrams5.2.2 Construction of a phase diagram5.2.3 Interpretation of a phase diagram

5.3 INVARIANT REACTIONS5.3.1 Eutectic reaction5.3.2 Eutectoid transformation5.3.3 Peritectic reaction5.3.4 Peritectoid transformation5.3.5 Monotectic and Syntectic reactions

APPENDIX5.4 IRON – CARBON ALLOY SYSTEM5.4.1 Fe – Fe3C equilibrium diagram5.4.2 Description of phases and structures5.4.3 Invariant reactions in Fe – C system5.4.4 Solidification of steel and cast iron5.4.4 (a) Solidification of Steels (slow cooling)5.4.4 (b) Solidification of Cast irons (slow cooling)

5.4.5 Effect of alloying elements

5.5 STEEL5.5.1 Types of Steel5.5.2 Steel Designations

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