rizam@kukum.edu.my

6

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force

dieblank

force

• Forging (wrenches, crankshafts)

FORMING

• Drawing (rods, wire, tubing)

often atelev. T

• Rolling (I-beams, rails)

• Extrusion (rods, tubing)

Adapted from Fig. 11.7, Callister 6e.

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

roll

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roll

tensile force

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METAL FABRICATION METHODS-I

CASTING JOINING

7

• Hot working --recrystallization --less energy to deform --oxidation: poor finish --lower strength

• Cold working --recrystallization --less energy to deform --oxidation: poor finish --lower strength

• Cold worked microstructures --generally are very anisotropic!

--Forged --Fracture resistant!

(a) (b) (c)

--Swaged

FORMING TEMPERATURE

plasterdie formedaround waxprototype

8

CASTING• Sand Casting (large parts, e.g., auto engine blocks)

Sand Sand

molten metal

• Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades)

wax

• Die Casting (high volume, low T alloys)

• Continuous Casting (simple slab shapes)

molten

solidified

METAL FABRICATION METHODS-II

FORMING JOINING

9

JOINING• Powder Processing (materials w/low ductility)

pressure

heat

point contact at low T

densification by diffusion at higher T

area contact

densify

• Welding (when one large part is impractical)

• Heat affected zone: (region in which the microstructure has been changed).

Adapted from Fig. 11.8, Callister 6e.(Fig. 11.8 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

METAL FABRICATION METHODS-III

CASTINGFORMING

Annealing: Heat to Tanneal, then cool slowly.

Types of Annealing

• Process Anneal: Negate effect of cold working by (recovery/ recrystallization)

• Stress Relief: Reduce stress caused by:

-plastic deformation -nonuniform cooling -phase transform.

• Normalize (steels): Deform steel with large grains, then normalize to make grains small.

• Full Anneal (steels): Make soft steels for good forming by heating to get , then cool in furnace to get coarse P.

• Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for 15-25h.

Thermal processing of metals

Outline

• Heat Treatment of Steels– Hardenability– Influence of quenching medium, specimen

size, and geometry

• Annealing Processes– Annealing of ferrous alloys

Full annealing Normalizing

– Process annealing– Stress relief

• Precipitation Hardening

0

20

40

60

80

100

120

140

160

1 10 100

A

The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition.

A: austenite

B: bainite

M: martensite

P: pearlite

• Conventional heat treatment procedures for producing martensitic steels involves

– continuous and rapid cooling of an austenitized specimen in some type of quenching medium, such as water, oil, or air

• The optimum properties of a steel that has been quenched and then tempered can be realized only if,

– during the quenching heat treatment, the specimen has been converted to a high content of martensite

Heat Treatment of Steels

• During the quenching treatment,

– it is impossible to cool the specimen at a uniform rate throughout

– the surface will always cool more rapidly than interior regions.

• The austenite will transform over a range of temperatures, yielding a possible variation of microstructure & properties with position within a specimen.

Non-uniform Cooling Rate during Quenching

• The successful heat treating of steels to produce a predominantly martensite microstructure throughout the cross section depends mainly on three factors:

1. the composition of the alloy

2. the type and character of the quenching medium

3. the size and shape of the specimen

Heat Treatments of Steels (Cont’d)

• Hardenability: A measure of the ability a specific alloy to be hardened by forming martensite as a result of given heat treatment

• The Jominy end-quench test:

– to measure hardenability

Hardenability of Steels

• Ability to form martensite

• Jominy end quench test to measure hardenability

Adapted from Fig. 11.10, Callister 6e.

Jominy End Quench Test

24°C water

specimen (heated to phase field)

flat ground

4”

1”

• Hardness vs. distance from the quenched end

Hard

ness

, H

RC

Distance from quenched endAdapted from Fig. 11.11, Callister 6e.

Hardenability Curves

24°C water

specimen (heated to phase field)

flat ground

4”

1”

• A steel that is highly hardenable will retain large hardness values for relatively long distances; a low hardenable one will not.

• The cooling rate varies with position

Adapted from Fig. 11.12, Callister 6e.

distance from quenched end (in)Ha

rdn

ess

, HR

C

20

40

60

0 1 2 3

600

400

200A M

A P

Martensite

Martensite +

Pearlite

Fine Pearlite

Pearlite

0.1 1 10 100 1000

T(°C)

M(start)

Time (s)

0

0%100%

M(finish)

Why hardness changes with position?

Effect of Alloying Elements on Hardenability

(1.0 Cr & 0.20 Mo)

(0.55 Ni, 0.50 Cr, & 0.20 Mo)

(1.85 Ni, 0.80 Cr, & 0.25Mo)

(0.85 Cr)

(plain carbon steel)

Distance from quench end

Hardenability Curves for Five Steel Alloys (Each Containing 0.4 wt% C)

One plain carbon steel (1040) and four alloy steels

• All five alloys have identical hardness at the quenched end (57 HRC); this hardness is a function of carbon content.

• The shape of the curves relates to hardenability.

The hardenability from low to high:

1040 steel < 5140 steel < 8640 steel < 4140 steel

< 4340 steel

• A water-quenched specimen of the 1040 plain carbon steel would harden only to a shallow depth below the surface, whereas for the other four alloy steels the high quenched hardness would persist to a much greater depth.

Hardenability

FIG. 11.14 Hardenability curves for four 8600 series alloys of indicated carbon content.

The hardenability increases with the carbon content.

Effect of Carbon Content on Hardenability

• The alloying element content and carbon content change the shapes of the hardenability curve

• The principal reason for using alloying elements in the standard grades of steels is to increase hardenability.

Alloying Elements and Hardenability

• The most commonly used quenching medium

• Inexpensive and convenient to use

• Provide very rapid cooling

• Especially used for low-carbon steel, which requires a very rapid change in temperature in order to obtain good hardness and strength

• Can cause internal stresses, distortion, or cracking

Quenching Mediums (1): Water

• More gentle than water

• Used for more critical parts, such as parts that have thin sections or sharp edges

• Razor blades, springs, and knife blades

• Does not produce steel that is as hard or strong as steel quenched by water

• Less chance of producing internal stresses, distortion, or cracking

• More effective when oil is heated slightly above room temperature to 100°F or 150°F (40°C or 65°C): reduced viscosity

Quenching Mediums (2): Oil

• More gentle than oil

• Does not produce steel that is as hard or strong as steel quenched by water or oil

• Less chance of producing internal stresses, distortion, or cracking

• Generally used only on steels that have a very high alloy content

Special alloys (such as Cr and Mo) are selected because they are known to cause materials to harden even though a slower quenching method is used

Quenching Mediums (3): Air

The heated sample is placed on a screen. Cool air is blown at high speed from below it.

Medium

air

oil

water

Severity of Quench

small

moderate

large

Hardness

small

moderate

large

Effect of Quenching Medium

The severity of quench: water > oil > air

Removing Samples from a Heat-Treating Furnace

Quenching Operations in Heat Treatment

Parts Quenched in a Group

(Figs. 11-3 and 11-4 in Metallurgy Fundamentals, by D. A. Brandt and J. C. Warner)

FIG. 11.17 Radial hardness profiles for (a) 50 mm (2 in.) diameter cylindrical 1040 and 4140 steel specimens quenched in mildly agitated water, and (b) 50 and 100 mm (2 and 4 in.) diameter cylindrical specimens of 4140 steel quenched in mildly agitated water.

Effect of Part Size

• When surface-to-volume ratio increases

cooling rate increases

hardness increases

Positioncentersurface

Cooling ratesmalllarge

Hardnesssmalllarge

Effect of Part Geometry

• Annealing: a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled.

• Three stages of annealing

1. Heating to the desired temperature

2. Holding or “soaking” at that temperature

3. Cooling, usually to room temperature

Annealing Processes

1. Relieve Internal Stresses

• Internal stresses can build up in metal as a result of processing.

– Stresses may be caused by previous processing operations such as welding, cold working, casting, forging, or machining.

• If internal stresses are allowed to remain in a metal, the part may eventually distort or crack.

• Annealing helps relieve internal stresses and reduce the chances for distortion and cracking.

Purposes for Annealing

2. Increasing Softness, Machinability, and Formability

• A softer and more ductile material is easier to machine in the machine shop.

• An annealed part will respond better to forming operations.

3. Refinement of Grain Structures

• After some types of metalworking (particularly cold working), the crystal structures are elongated.

• Annealing can change the shape of the grains back to the desired form.

Purposes for Annealing (Cont’d)

The Iron–Iron Carbide Phase Diagram

L + Fe3C

2.14 4.30

6.70

0.022

0.76

M

N

C

PE

O

G

F

H

Cementite Fe3C

• Most heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the iron

• Steel heat treating practice rarely involves the use of temperatures above 1040°C (1900°F)

Temperature Regime of Steel Heat Treatment

FIG. 11.9 The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels.

Precipitation hardening of metals(Heat treatment)

• Strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix.

• This strengthening is accomplished by phase transformations induced by heat treatment.

• Age hardness is also used to designate the process since strength develops with time, or as the alloy ages.

Precipitation hardening (Cont.)

• Requisite features of phase diagrams of alloy systems for precipitation hardening:– Appreciable maximum solubility of one

component in the other (on the order of several percent).

– Solubility limit that rapidly decreases in concentration of the major component with temperature reduction.

– Composition of precipitation-hardenable alloy must be less than the maximum solubility.

• Particles impede dislocations.• Ex: Al-Cu system• Procedure: --Pt A: solution heat treat (get solid solution) --Pt B: quench to room temp. --Pt C: reheat to nucleate small crystals within crystals.• Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al Pt A (sol’n heat treat)

Pt B

Pt C (precipitate )

Temp.

Time

Precipitation hardening

300

400

500

600

700

0 10 20 30 40 50wt%Cu(Al)

L+L

+L

T(°C)

A

B

C

composition range needed for precipitation hardening

CuAl2

Precipitation hardening (Cont.)

• Types of precipitation hardening process:– Solution heat treatment– Precipitation heat treatment.

Temperature vs. time plot showing both solution and precipitation heat treatment for precipitation hardening.

Precipitation hardening (Cont.)

Solution heat treatment: • Quenching (rapid cooling) prevent diffusion and

the formation of a new phase. Resulting phase will be supersaturated in B atoms (alloy relatively soft and weak). Phase is retained at room temperature for relatively long periods (long diffusion rate at room temperature).

Precipitation heat treatment: • The characteristics of the phase and the

strength and hardness of the alloy depends on the precipitation temperature T2 and the aging time at this temperature.

• 2014 Al Alloy:

• TS peaks with precipitation time.• Increasing T accelerates process.

• %EL reaches minimum with precipitation time.

Precipitation effect on TS, %EL

precipitation heat treat time (h)

tensi

le s

trength

(M

Pa)

300

400

500

2001min 1h 1day 1mo1yr

204°C

149°C

non-

equi

l. so

lid s

olut

ion

man

y sm

all

prec

ipita

tes

“ag

ed”

fe

wer

larg

e

pre

cipi

tate

s

“ove

rage

d”%

EL

(2in

sam

ple

)10

20

30

0 1min 1h 1day 1mo1yr

204°C 149°C

precipitation heat treat time (h)

• Peak-aged --avg. particle size = 64b --closer spaced particles efficiently stop dislocations.

Effect of aging on dislocation motion

• Over-aged --avg. particle size = 361b --more widely spaced particles not as effective.

• Steels: increase TS, Hardness (and cost) by adding --C (low alloy steels) --Cr, V, Ni, Mo, W (high alloy steels) --ductility usually decreases w/additions.• Non-ferrous: --Cu, Al, Ti, Mg, Refractory, and noble metals.• Hardenability --increases with alloy content.• Precipitation hardening --effective means to increase strength in Al, Cu, and Mg alloys.

SUMMARY