sept 4th alloying and heat treatment 2

8/12/2019 Sept 4th Alloying and Heat Treatment 2

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Dr.Mullany

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Topics for today

Alloying

Phase Diagrams

Heat treatment

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Al loying

The mixing of metals and semi-metals in the molten state iscalled alloying

An alloy is composed of two or more elements, the principlecomponent is a metallic element

Alloying is performed to change the physical properties of ametal

Commonly alloying is done to change

Strength

Modulus of Elasticity

Ductility Toughness

Corrosion Resistance


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Al loying

Alloys are typically prepared by melting a known mass ofmetal (solvent) in a crucible and then adding in weighedamounts of the other material (solute).

The liquid alloy is then cast and allowed to solidify. Theresulting structure depends on how the different types ofatoms behave around each other.

If the atoms are indifferent to each other they willcrystallize as a single set of crystals all the atoms willbehave as if they are similar. A single phase Solidsolution is said to form.

If the different elements crystallize separately to formdifferent crystals that meet at grain boundaries then theresulting structure is referred to as a Phase Mixture

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Al loying - Sol id Solut ion

In a solid solution the crystal structure is the sameas that of the solvent (parent metal). The Soluteatoms are distributed through in crystal. Thesolution may be formed in two different ways

Substitution Intermetallic

Interstitial

Substitutional Interstitial


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Al loying - Sol id Solut ion -substi tution

Conditions for Substitutional alloying:

-Atoms of the two metals do not differ in diameter more than15%

- The two metals must have a similar crystal structure.

An example is Brass

Solute is Zinc

Solvent is Copper(Elements are beside each other on periodic table)

Another example is that of Monel;

A mixture of Copper and Nickel

(Elements are also beside each other on periodic table)

Substitutional

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Al loying - Sol id Solut ion -intermetal lic

Intermetallic Compound: these are substituitionalsolid solutions where the solute atoms are presentin specific proportions and geometric relationships

They have sharp melting points, often higher thaneither of the two alloying elements, very goodstrength, low ductility


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Al loying Sol id Solut ion -interst itial Interstitial: Solute atoms positioned between the

atoms in the solvent

Conditions Atomic radius of the solute must be less than about 60% of

the solvent radius

An example is Steel Solute is Iron

Solvent is Carbon

Amount of carbon significantly affects material properties

Interstitial

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Solidifcation curves

The graph below shows the difference between the solidification curvesfor pure metals (one element) and alloys (several elements)

Pure metal

Al loy: the temperatureat which it solidifi es is

not sharply defined

Temperature

range for

solidification


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Phase Diagrams

Definition:A Phase has a definable structure, a uniform

and identifiable chemistry (aka composition) anddistinct boundaries or interfaces that separate it fromother different phases.

Definition:A Phase diagram (also called an equilibriumdiagram) illustrates the relationship betweentemperature, composition and the phases present in aparticular alloy.

Note: Phase diagrams are only valid under equilibrium conditions ...i.e.slow heating and cooling

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Copper-nickel phase diagram

Melting point

of pure copper

Definition of wt%: wa = wt of component a x 100

S wt of all components

Melting point

of pure nickel

After spaceflight.esa.int

80% Ni, 20% Cu


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Nickel Copper Phase diagram

Point a: 40% Cu - 60%Ni , temp >1350C, homogenous liquid form

Point b: 40% Cu - 60%Ni , temp


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Consider slow cooling of a 50%Cu liquid mixture: first solids 36% Cu(at liquidus, go left to the solidus line, then down to read composition of solids)

EQUILIBRIUM PHASE DIAGRAM!After Kalpakjian and Schmid, 5th ed

50-50% at solidification (diffusion)

Intermediate temp, go left for solid composition, right for liquid

What happens during solidification?

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Lever rule

Create a lever balanced at the

nominal composition, C0.CS represents solid composition.

CL represents the liquid

composition

Lever rule:

Wt fraction solid a distancebetween C0 and CL:

S = C0-CLS+L CS-CL

After Kalpakjian and Schmid, 5th ed


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Lever rule on Lead (Pb) Tin (Sn) phase diagram

At 250C what is the solid and liquid fraction of the alloy at 80% Lead (Pb)?

dd b

C

Solid fraction, Fs = C-db-dX 100%

= 80-64

87-64X 100%

= 69.5%

Liquid fraction, FL = b-Cb-dX 100%

= 87-8087-64

X 100%

= 30.5%

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Terminology

Definition: Eutectic: An isothermal reversiblereaction in which a liquid solution isconverted into two or more intimately mixedsolids on cooling (number of solids dependon the number of elements in the system)

Definition: Eutectoid: An isothermalreversible reaction in which a solid phase isconverted into two or more intimately mixed

solids on cooling (number of solids dependon the number of elements in the system)


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Terminology- Eutectic

There is no mixed liquid-solid at an eutectic point.On freezing at this specific composition an eutectic

mixture with the individual crystal in the form of

plates or rods or tiny particles are formed.

Note: Eutectic points have the lowest melting pointicrostructureof

an

eutecticmixture

Eutectic mix

Different structugrain

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Iron Carbon (steel) phase diagram

(Ferrite)

L + Fe3C

2.14 4.20

6.70

0.022

0.8

C

B D

H

Cementite

(Fe3C)

0.25

z

1.2

a + cementite (Fe3C)

g + cementite (Fe3C)

723C

Liquid

g + Liquid

(Austenite)

Eutectoid

Solid

Eutectic

Liquid

After Kalpakjian and Schmid, 5th ed


10/20

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Iron Carbon Systems

Why we need to know about Iron-carbon

Systems:

Steel is an Alloy of Iron and Carbon

Different phases of the Iron Carbon diagramhave different structures, it is important to befamiliar with them and to understand whatinfluence they have with respect to materialproperties

% Carbon content of dif ferent materials: Pure Iron (Fe) = 0.008%

Steel up to 2.11%

Cast Irons up to 6.67%

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Iron Carbon phase diagram

100% Fe 93.3% Fe

Steel Cast Iron


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Iron Carbon - Main Structures

AUSTENITE: Single phase FCCStructure

Ductile at elevated temperatures Good formability important for

manufacturing

Nonmagnetic

CEMENTITE (Fe3C): Also called Carbide

A hard and brittle intermetalliccompound that has a significantinfluence on the properties of steel

FERRITE: BCC Structure Only stable at high temperatures and

has little engineering relevance Soft, Ductile, and Magnetic

PEARLITE: lamellar aggregate ofFerrite and Cemetite

Cementite(White areas)

Ferrite(dark areas)

Austenite

Pearlite

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Cast Iron versus Steel

Cast iron has more

silicon than steel

which makes the Fe3C

decompose to Ferrite

and Graphite

4

3

2

1

0 0 1 2 3 4

% Silicon

Steels

White

Cast Iron

Grey Cast Iron

Nodular

Cast Iron

%C

arbon


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Cast Iron Main structures found

The type of cast iron found isdependant on the following: Carbon content

Alloy and impurity content

Cooling rate during and after f reezing

The Heat treatment after casting

White Cast Iron All carbon is in the combined form as

cementite

Gray Cast Iron Carbon is uncombined in the form of

graphite flakes

Nodular Cast Iron Carbon is largely uncombined in the

form of compact spheroids

Malleable Cast Iron Carbon is uncombined in the form of

irregular round particles known astemper carbon

Malleable cast iron (white cast

iron annealed to precipitate

out carbon

Gray Cast iron

White Cast iron as cast

Nodular cast iron

See also Chapter 5, Kalpakjian and Schmid, 5th ed

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Effect of Carbon on Steel Properties

Effect of carbon content on the

mechanical properties of carbon steel

Figure 3.33 from Kalpakjian and Schmid, 5th ed


13/20

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Do you remember ?

1. A stress of 10MPa is applied to

a tensile sample, if the materials

stiffness is 10GPa, how muchstrain will the sample undergo?

a) 0.001b) 0.001%

c) 1000

d) 0.01%

2. Which is false of work hardened

materials ?a) Hardness > non work hardened

b) Strength > non work hardened

c) Have equiaxed grain sizesa) They are plastically deformed

3. Which is not true about stressa) Units = Pab) Units = psi

c) = force/area

d) = area*force

4. Which of the following statementsis true

a) No part of engineering stressstrain curves should be

compared to a true stress-

strain curveb) Stiffness is the ratio of stress to

strain in the linear part of thecurve

c) Once you have gone past the

yield point you can have noelastic recovery

d) It doesnt matter under whattemperature conditions a

tensile test is preformed

Section B questions:Explain why a metal with a yield

strength of 10GPa may not be astough as a metal with a yield strength

of 5GPa

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Heat treatment

Heat treatments modify the microstructureof alloys to impart different mechanicalproperties

Effects of thermal treatment depend on The alloys composition and microstructure, The degree of cold work,

The rates of heating and cooling, The temperatures and temperature ranges,

etc.

It is a very complex subject and we will notcover it in detail, just an overview

Definition:A combination of heating and cooling

operations, timed and applied to a metal or alloy in the

solid sate in a way that will produce desired properties


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Heat treatment - terminology

Annealing: heating to and holding at a suitable temperature above the

recrystalization temp and then cooling in the furnace at a suitable rate

(usually slow), for such purposes as reducing hardness, improving

machinability, facili tating cold working, producing a desired

microstructure or obtaining desired mechanical, physical or other

properties. Any process of annealing will usually reduce stresses.

Cold treatment: cooling to a temperature, often near -100F, for the

purpose of obtaining desired condit ions or properties such as

structural stability.

Hardening: Increasing the hardness by suitable treatment, usually

involving heating and cooling.

Under suitable cooling rates the carbon is able to diffuse out of the

austenite structure. When steel is cooled quickly the carbon becomestrapped in solution and is known as Martensite or Martenistic structure

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Heat treatment - terminology

Normalizing: Heating a ferrous alloy to a suitable temperature abovethe transformation range (as in annealing) and then cooling in air to a

temperature substantially below the transformation range. It will

produce harder and stronger steel than annealing partially due to

faster cooling rates than used in Annealing

Quenching: Rapid Cooling of a material. This increases the hardness

of the metal. Quenching mediums are (listed in order decreasing

severity):

Brine (water and 10% Sodium Chloride)

Tap water

Soluble oil

oilAir

Tempering: Reheating a quenched hardened or normalized ferrous

alloy to a temperature below the Transformation temperature and then

cooling at the desired rate. It relieves internal stresses.


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Tempering Martensite

Tempering is accomplished by heating a martensitic steel

to a temperature below the eutectoid (normally, between200-650C) for a specified time period.

By diffusion processes:

Martensite (BCT, single phase) Tempered Martensite(a + Fe3C)

Tempered martensite may be nearly as hard and strongas martensite, but with substantially enhanced ductility &toughness.

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Effect of tempering Temperature

Tensile and yield

strengths andducti lity (%RA) versus

tempering

temperature for an oi l-

quenched alloy s teel

(type 4340).

Heat treatment variablesare temperature andtime, and mosttreatments are constant-temperature p rocesses.

(Carbon diffusion isinvolved in thetransformation.)


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Jominy End Quench test

Explain why stress strain curves are so important, detailthe information that can be extracted from the curve and

from looking at the tensile sample after fracture

Hardenability curve is the dependence of hardness on distance from the

quenched end. The higher the hardness levels further away from the

quenched end the more hardenable the alloy.

Test standards:

American Society for testing and Materials (ASTM) Method A 255

Society of Automotive Engineers (SAE) standard J406

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Jominy End Quench test

Quenched end cools most rapidly, contains mostly martensite

Cooling rate decreases with distance from quenched end: greater C

diffusion, more pearlite/bainite, lower hardness

High hardenability means that the hardness curve is relatively flat.

Less Martensite

Quenched

end


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Dangers of heat treatment

Heat treatments can cause problems

such as cracking, distortions etc.

Parts incorrectly case hardened (forexample through hardened instead) canfail due to lack of toughness

Distortions must be corrected onprecision parts by finish grinding (usually) Martensitic and quench cracks

Grinding cracks

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Case hardening

Examples of such are gears, bearings,cams,

tool, dies, etc.

This technique is called case hardening

Case hardening is performed by adding other

elements to the surface or by special heat

treatments.

Many industrial applications require a hard wear

resistant surface called the case and a relatively soft

tough inside called the core


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Case Hardening

Other method involve heatTreating: Flame Hardening heating

surface with a flame andquenching

Induction hardening heatingsurface with high frequencyinduced current and quenching

This photo shows ways on 16" vise

base being hardened utilizing flame

hardening.

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Case hardening

One method involves addingsurface elements:

Carburizing adding carbon

Carbonitriding adding carbonand nitrogen

Nitriding adding nitrogen

Many recipes exist


20/20

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Homework due Wednesday the 11th Sept 13

1. A 200mm long, 10mm dia. tensile sample experienced plastic

deformation at a strain of 0.01. The applied load was 78.5 kN. What is the elastic modulus of the material?

By how many mm did sample elastically deform?

2. Looking at the Ni-Cu phase diagram answer the following

questions.

At what temperature is a Cu (20%) - NI(80%) fully solid?

What phases are present at 1300C and 50%Cu50%Ni?

3. Draw and label an engineering stress-strain diagram for a brittle

metal.

sept 4th alloying and heat treatment 2

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