chapter 5 metals

8/11/2019 Chapter 5 Metals

1/23

Chapter 6 and 7

Metals and heat treatment


2/23

2

Adapted from Fig. 10.28, Callister &

Rethwisch 3e.(Fig. 10.28 adapted

from Binary Alloy Phase Diagrams,

2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials

Park, OH, 1990.)

Adapted from Fig.

13.1, Callister &

Rethwisch 3e.

Classification of Metal AlloysMetal Alloys

Steels

Ferrous Nonferrous

Cast Irons


3/23

Ferrous alloy: Those which iron is the primeconstituent

Low carbon steel:

Properties: Carbon content less than 0.25 wt%, Soft

and ductile

Applications: Automobile body, Pipe line, Building,Bridge

Medium carbon steel: Properties: Carbon content 0.25 wt% to 0.60 wt%,

Low hardenabilities

Applications: Railway wheels and tracks, Gears


4/23

High carbon steel:

Properties: Carbon content 0.6 wt% to 1.4 wt%, Hardest,strongest, less ductile among the carbon steel

Applications: Knives, Razors, Hacksaw blades

Stainless steel: Properties: At least 11 wt% Cr. May be other elements (Ni,

Mo) can be added for high corrosion resistance

Applications: Automobile exhaust components, Chemical and

food processing equipment.


5/23


6/23

6

Types of Cast Iron (cont.)White iron

< 1 wt% Si

pearlite + cementite

very hard and brittle

Malleable iron

heat treat white iron at 800-900C

graphite in rosettes reasonably strong and ductile

Adapted from Fig.

13.3(c) & (d),

Callister &

Rethwisch 3e.


7/23

7

Types of Cast Iron (cont.)Compacted graphite iron

relatively high thermal conductivity

good resistance to thermal shock

lower oxidation at elevated

temperatures

Adapted from Fig. 13.3(e),

Callister & Rethwisch 3e.


8/23

8

Limitations of Ferrous Alloys

1) Relatively high densities

2) Relatively low electrical conductivities

3) Generally poor corrosion resistance


9/23

9Based on discussion and data provided in Section 13.3, Callister & Rethwisch 3e.

Nonferrous Alloys

NonFerrousAlloys

Al Alloys

-low r: 2.7 g/cm3

-Cu, Mg, Si, Mn, Zn additions-solid sol. or precip.

strengthened (struct.aircraft parts

& packaging) Mg Alloys

-very low r: 1.7g/cm3

-ignites easily-aircraft, missiles

Refractory metals-high melting Ts-Nb, Mo, W, Ta Noble metals

-Ag, Au, Pt-oxid./corr. resistant

Ti Alloys

-relativelylowr:4.5g/cm3

vs 7.9 for steel-reactiveathighTs-space applic.

Cu Alloys

Brass:

Zn is subst. impurity(costume jewelry, coins,corrosion resistant)Bronze: Sn, Al, Si, Ni aresubst. impurities(Dental equipment,

landing gear


10/23

10

Metal Fabrication

How do we fabricate metals?

Blacksmith - hammer (forged)

Cast molten metal into mold

Forming Operations

Rough stock formed to final shape

Hot working vs. Cold workingDeformation temperature

high enough for

recrystallization

Large deformations

Deformation belowrecrystallization

temperature

Strain hardening occurs

Small deformations


11/23

11

FORMING

roll

Ao

Adroll

Rolling (Hot or Cold Rolling)

(I-beams, rails, sheet & plate)

Ao Ad

force

die

blank

force

Forging (Hammering; Stamping)

(wrenches, crankshafts)

often at

elev. T

Adapted from

Fig. 14.2,

Callister &

Rethwisch 3e.

Metal Fabrication Methods (i)

ram billet

container

container

forcedie holder

die

Ao

Adextrusion

Extrusion

(rods, tubing)

ductile metals, e.g. Cu, Al (hot)

tensileforce

Ao

Addie

die

Drawing

(rods, wire, tubing)

die must be well lubricated & clean

CASTING MISCELLANEOUS


12/23

12

FORMING CASTING

Metal Fabrication Methods (ii)

Casting- mold is filled with molten metal

metal melted in furnace, perhaps alloying elements added, then

castin a mold common and inexpensive

gives good production of shapes

weaker products, internal defects

good option for brittle materials

MISCELLANEOUS


13/23

13

Sand Casting

(large parts, e.g.,

auto engine blocks)

Metal Fabrication Methods (iii)

What material will withstand T>1600Cand is inexpensive and easy to mold?

Answer: sand!!!

To create mold, pack sand around form(pattern) of desired shape

Sand Sand

molten metal

FORMING CASTING MISCELLANEOUS


14/23

14

Stage IMold formed by pouringplaster of paris around wax pattern.

Plaster allowed to harden.

Stage IIWax is melted and then

poured from moldhollow moldcavity remains.

Stage IIIMolten metal is poured

into mold and allowed to solidify.

Metal Fabrication Methods (iv)


Investment Casting

(low volume, complex shapes

e.g., jewelry, turbine blades)

wax I

II

III


15/23

15

Metal Fabrication Methods (v)

Continuous Casting

-- simple shapes

(e.g., rectangular slabs,

cylinders)

molten

solidified


Die Casting

-- high volume

-- for alloys having low melting

temperatures


16/23

16

MISCELLANEOUSCASTING

Metal Fabrication Methods (vi)

Powder Metallurgy

(metals w/low ductilities)

pressure

heat

point contactat low T

densificationby diffusion athigher T

area

contact

densify

Welding

(when fabrication of one large

part is impractical)

Heat-affected zone:

(region in which the

microstructure has been

changed).

Adapted from Fig.

14.3, Callister &

Rethwisch 3e.

(Fig. 14.3 from Iron

Castings

Handbook, C.F.

Walton and T.J.

Opar (Ed.), 1981.)

piece 1 piece 2

fused base metal

filler metal (melted)base metal (melted)

unaffectedunaffectedheat-affected zone

FORMING


17/23

17

Summary of Possible TransformationsAdapted from

Fig. 11.37,

Callister &Rethwisch 3e.

Austenite (g)

Pearlite(a+ Fe3C layers + a

proeutectoid phase)

slowcool

Bainite(a+ elong. Fe3C particles)

moderatecool

Martensite(BCT phase

diffusionlesstransformation)

rapidquench

TemperedMartensite(a+ very fine

Fe3C particles)

reheat

Strength

Ductility

MartensiteT Martensite

bainitefine pearlite

coarse pearlitespheroidite

General Trends


18/23

18

Annealing: Heat to Tanneal, then cool slowly.

Based on discussion in Section 14.5, Callister & Rethwisch 3e.

Thermal Processing of Metals

Types of

Annealing

Process Anneal:

Negate effects ofcold working by(recovery/

recrystallization)

Stress Relief: Reducestresses resulting from:

- plastic deformation- nonuniform cooling- phase transform.

Normalize (steels): Deformsteel with large grains. Then heattreat to allow recrystallizationand formation of smaller grains.

Full Anneal (steels):Make soft steels forgood forming. Heatto get g, then furnace-cool

to obtain coarse pearlite.

Spheroidize (steels):Make very soft steels forgood machining. Heat just

below Teutectoid& hold for

15-25h.


19/23

19

a) Full Annealing

b) Quenching

Heat Treatment Temperature-Time Paths

c)

c) Tempering(Tempered

Martensite)

P

B

A

A

a)b)

Fig. 11.26,

Callister &

Rethwisch 3e.


20/23

20

Hardenability -- Steels Hardenability measure of the ability to form martensite

Jominy end quench test used to measure hardenability.

Plot hardness versus distance from the quenched end.

Adapted from Fig. 14.5,


(Fig. 14.5 adapted from

A.G. Guy, Essentials of

Materials Science,

McGraw-Hill Book

Company, New York,1978.)

Adapted from Fig. 14.6,


24C water

specimen(heated to g

phase field)

flat ground

Rockwell Chardness tests

Hardness,

HR

C

Distance from quenched end


21/23

21

The cooling rate decreases with distance from quenched end.


Rethwisch 3e. (Fig. 14.7 adapted from H.

Boyer (Ed.)Atlas of Isothermal

Transformation and Cooling

Transformation Diagrams, American

Society for Metals, 1977, p. 376.)

Reason Why Hardness Changes with Distance

distance from quenched end (in)Hardness,

HRC

20

40

60

0 1 2 3

600

400

200A M

0.1 1 10 100 1000

T(C)

M(start)

Time (s)

0

0%

100%

M(finish)


22/23

22

Hardenability vs Alloy Composition Hardenability curves for

five alloys each with,

C= 0.4 wt% C

"Alloy Steels"

(4140, 4340, 5140, 8640)

-- contain Ni, Cr, Mo

(0.2 to 2 wt%)-- these elements shift

the "nose" to longer times

(from A to B)

-- martensite is easier

to form


Rethwisch 3e. (Fig. 14.8 adapted from

figure furnished courtesy Republic SteelCorporation.)

Cooling rate (C/s)

Hardness,

HR

C

20

40

60

100 20 30 40 50Distance from quenched end (mm)

210100 3

4140

8640

5140

50

80

100

%M4340

T(C)

10-1

10 103

1050

200

400

600

800

Time (s)

M(start)

M(90%)

BA

TE


23/23

23

Effect of quenching medium:

Medium

air

oil

water

Severity of Quench

low

moderate

high

Hardness

low

moderate

high

Effect of specimen geometry:

When surface area-to-volume ratio increases:

-- cooling rate throughout interior increases

-- hardness throughout interior increases

Positioncenter

surface

Cooling ratelow

high

Hardnesslow

high

Influences of Quenching Medium & Specimen

Geometry

chapter 5 metals

Documents