1. ME 330 Engineering Materials
Lecture 13
Applications and Processing of Metal Alloys
Types of Metal Alloys
Fabrication of Metals
Thermal Processing of Metals
Read Chapter 11

2. Objectives for Chapter 11
Have basic understanding of types of steels, relative C content, strength, and applications.
Know fundamental differences between the types of cast iron. Describe the purpose of and procedures for process annealing, stress relief annealing, normalizing, full annealing, and spheroidizing.
Define hardenability and list the class steels that have low hardenability.
Know how to use the Jominy End Quench Test - see lab on hardenability.
Explain the procedure to precipitation harden an alloy and what characteristics on the phase diagram allow the alloy to be hardened by this mechanism.
Explain the shape of the strength vs time curve for precipitation hardened alloys in terms of the mechanism of hardening in each stage.
Distinguish between the four types of forming operations.
Distinguish between the four types of casting techniques.

3. Ferrous Alloys

4. Metal Fabrication

5. Heat Treating Steel Recall: pearlite Full Annealing
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
pearlite
Full Annealing
Heat to °C above phase transition line, hold for diffusion, and slow cool (usually in a furnace, often takes number of hours).
Returns microstructure to coarse pearlite.
Often performed on steels to be machined or formed to increase ductility

6. Heat Treating Steel Recall: Normalizing
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Normalizing
Heat to the fully austenite region and air cool.
Fine pearlite microstructure
Refine grain size and distribution (often done after a rolling operation)

7. Heat Treating Steel Recall: Spheroidizing
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Spheroidizing
Heat a medium-high carbon steel below the eutectoid line and hold for several hours
Forms small Fe3C spherical particles in  matrix.
For higher carbon steels, even pearlite is often brittle and difficult to deform
Spheroidizing minimizes hardness and is highly machinable

8. Heat Treating Steel Recall: Quenching Heat to fully austenitize steel
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Quenching
Heat to fully austenitize steel
Rapidly cool (quench in water, oil, etc)
No time for carbon to diffuse
Produces a non-equilibrium microstructure called martensite
Very hard, strong, brittle with large internal stresses

9. Heat Treating Steel Recall: Tempering
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Tempering
After quenching to form martensite
Reheat below eutectoid temperature
Hold for several hours to precipitate carbides and relieve residual stresses
Forms tempered martensite
Relatively hard, strong, much more ductile
Often ideal microstructure ...

10. Hardenability Hardness vs. Hardenability
Hardness – resistance to deformation
Hardenability – ability to be form martensite (to be hardened)
Formal definition:
Hardenability – ability of an alloy to be hardened by formation of martensite due to given heat treatment
High hardenability  martensite forms (i.e. hardens) far into specimen, not just at surface

11. Jominy End Quench Test
Standard test for measuring hardness of any material
Sample of fixed geometry suspended over water jet
Geometric factors all constant
Only effect on hardness is alloying content
Hardness measured as function of distance from quenched end
Cooling rate
Distance
Hardness, HRC
Distance from quenched end

12. Distance from quenched end, mm
Hardness Profiles
Cooling rate
Highest at quenched end
Smoothly decreases with
distance
Microstructure
Martensite
Bainite
Fine Pearlite
Course Pearlite
Often plot cooling rate and distance
Increasing carbon content
Maximum possible hardness increases
Higher overall hardness (more hardenable) 10 20 30 40 50
Distance from quenched end, mm
Cooling rate at 700 °C (°C/s ) 60
Hardness, HRC
8660
8630
8640
8620
Slower
Cooling

13. Severity of Quench
Rate of cooling!
Depends on
Medium- Fluid in which an austenetized part is plunged
Most common: air, oil, water
Agitation - Relative motion of quenching medium during cooling
Agitation rate influences cooling rate
Surface area to volume ratio -
All heat removed at surface
More surface area provides more opportunity to remove heat
Irregular shapes with edges have large quenching surfaces
For highly stressed part, want 80% martensite throughout part
Usually from a “guideline” for specific company

14. Using Cooling Curves Normal quenching takes place on radial surface
Can predict hardness across radius of a bar from Jominy tests.

15. Review: Heat Treatment of Steel
Annealing- Heat, holding at temperature, gradual cool
General term, but also used for specific processes
Tempering -Heat martensite to get diffusional transformation:
Still have ultra-fine microstructure, but more ductile
Process annealing -Reverse effects of cold work; recovery, recrystallization, but little grain growth
Stress relief - Lower temperature anneal to undo thermal stress or transformation mismatch stress
Spheroidizing - Heat pearlite just below eutectoid temp to produce spheroidal structure; makes steel easier to machine

16. Surface Hardening Techniques
Many applications, especially wear
Strong, hard, wear-resistant surface
Tough, fracture resistant inner core
Two different heat treatments - through, then surface
Methods
Chose material with steep cooling curve
Case Hardening - Change chemical composition of surface
Carburizing
Nitriding
Carbonitriding
Decremental Hardening – Very localized heat treatment
Flame Hardening
Induction Hardening

17. Alloys for Surface Hardness10 20 30 40 50
Distance from quenched end, mm
Cooling rate at 700 °C (°C/s ) 60
Hardness, HRC
100 80
Percent Martensite
4340
4140
5140
8640
1040 A B A B
Prescribe:
Minimum hardness at A
Maximum hardness at B Must know the cooling rates at each point!

18. Carburizing
Hardens steel by causing carbon to diffuse into the surface.
Furnace heat to a temperature at which carbon will diffuse
Hold until diffusion creates the proper case depth.
Must be a carbon rich environment
For steels with low carbon content (%C < 0.2)
Used extensively on gears and shafts to harden them yet maintain the core toughness.
Decarburization is opposite process for high carbon steels - softer case
From: Callister p. 92, 224

19. Flame Hardening For steels of hardenable carbon content (%C  0.4)
A high intensity oxy-acetylene flame is applied to the bring the region of interest to an austenite transformation.
The interior never reaches high temperature.
The heated region is quenched to achieve the desired hardness.
The depth of hardening can be increased by increasing the heating time.
In addition, large parts, which will not normally fit in a furnace, can be heat-treated
From: Scheer p. 43

20. Precipitation Hardening
Form extremely small, dispersed particles of a second phase
Small second phase particles called precipitates
Age hardening - strength develops as the alloy ages
Time dependent
There is a maximum attainable effect
Different than tempering martensite
Similar heat treatment procedure
Different strengthening mechanism

21. Precipitation Hardening
a Ti in b Ti matrix
Primary strengthening mechanism for
Aluminum
Nickel based superalloys
Titanium
Examples
Al/Cu
Cu/Sn
Mg/Al
Ni3Al in Ni matrix
From: Socie

22. Phase Diagrams & Treatment
Precipitation hardenable alloys have:
Appreciable maximum solubility
Decreasing solubility with temperature
Composition less than maximum solubility L
L + a
Temperature
L + b a b
Solution Treating:
Heat into the single phase region and rapidly quench to room temperature to produce a supersaturated solid solution
a + b
Aging:
Heat to a temperature below the phase transition to allow time for precipitates to form
Composition

23. Why Precipitation Harden?
Look at Aluminum rich side of Al/Cu system
- a is a substitutional solid solution of copper in aluminum
q is an intermetallic compound CuAl2
One slow cool in is NOT helpful in strengthening
Coarse q phase weakens the alloy
q + a a °C
600
700
500
400
300
200
100 10 5
time
100% a
(95.5% Al, 4.5% Cu)
Temperature
Coarse q precipitates
At a grain boundaries Al
wt % Cu

24. How to Precipitation Harden
Two reheating treatments are needed:
Solution treatment
Age hardening
Fine precipitates strengthen & harden material
q + a a °C
600
700
500
400
300
200
100 10 5
wt % Cu
100% a solid solution
Equilibrium
Microstructure
Temperature
Fine precipitates in grains
(retained after cooling) Al
time

25. Overaging If precipitates get too big, strengthening/hardening is lost
Process sped up by temperature
Some alloys age at room temperature
Why is strength lost?
100% a solid solution
Temperature
Fine precipitates in grains
Coarse precipitates
in grains
taging
time

26. Strength Development
Fine precipitates (Guinier-Preston zones  q’’ phase) are coherent with lattice
Deformation of crystal impedes dislocation motion
At optimal aging time, precipitates are dispersed, small, and coherent.
After optimal aging time, precipitates get too big, incoherent with matrix.
From: Callister

27. Age vs. Material Properties
Must age carefully to avoid overaging
Too high temperature
Too long age time
Precipitation rate maximized at intermediate temperatures
Near solvus  no driving force for nucleation
Low temperatures  diffusion is slow t T
Nucleation
Diffusion
From: Callister

28. Strengthening Mechanisms
Dislocation Looping
Dislocation cutting
From: Hertzberg

29. What is Attractive About Aluminum?
Upside
Lightweight ( )
Ductile
Heat treatable
Castable
Corrosion resistant
Good conductor
Downside
Formability is lower
Simpler shapes
For same stiffness, need larger section
Longitudinal stiffness(~EA/L)
Bending stiffness (~EI)
Rect. section, constant b b h

30. Aluminum Designations
Tempering Designations
-O: Annealed
-F: As fabricated
-H1: Strain Hardened
-H2: Strain Hardened/ Partial Annealed
-T: Thermally Processed
-T2: Annealed (cast products)
-T3: Solution treated, CW, naturally aged
-T4: Solution treated, naturally aged
-T5: Artificially aged, no solution treatment
-T6: Solution treated, artificially aged
-T7: Solution treated, over-aged
-T8: Solution treated, CW, artificially aged
-T9: Solution treated, artificially aged , CW
Alloying Composition
Aluminum 1xxx
Aluminum
Copper 2xxx
Manganese 3xxx
Silicon 4xxx
Magnesium 5xxx
Mg and Si 6xxx
Zinc 7xxx
Common alloys: T4 ,
6061-T6,
7075-T6

31. New Concepts & Terms Applications of hardness profiles (Jominy curves)
Precipitation hardening
Heat treatment process
Effect of overaging
Microstructures
Effect on strength
Applications of aluminum
Know tradeoffs, especially with steels
Which alloys are precipitation hardenable?