1. Thermal processing
of metals

2. METAL FABRICATION METHODS-I
CASTING
JOINING
FORMING
• Forging
(wrenches, crankshafts)
• Rolling
(I-beams, rails)
often at
elev. T
Adapted from Fig. 11.7, Callister 6e.
• Drawing
(rods, wire, tubing)
• Extrusion
(rods, tubing) 6

3. FORMING TEMPERATURE • Hot working --recrystallization • Cold working
--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
--Swaged
--Fracture resistant!
(a)
(b)
(c) 7

4. METAL FABRICATION METHODS-II
FORMING
JOINING
CASTING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
• Die Casting
(high volume, low T alloys)
• Continuous Casting
(simple slab shapes)
• Investment Casting
(low volume, complex shapes
e.g., jewelry, turbine blades)
plaster
die formed
around wax
prototype 8

5. METAL FABRICATION METHODS-III
CASTING
FORMING
JOINING
• Powder Processing
(materials w/low ductility)
• Welding
(when one large part is
impractical)
Adapted from Fig. 11.8, Callister 6e.
(Fig from Iron Castings Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.)
• Heat affected zone:
(region in which the
microstructure has been
changed). 9

6. Thermal processing of metals
Annealing: Heat to Tanneal, then cool slowly.

7. 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

8. A: austenite B: bainite M: martensite P: pearlite
The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition.
A: austenite
B: bainite
M: martensite
P: pearlite

9. Heat Treatment of Steels
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

10. Non-uniform Cooling Rate during Quenching
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.

11. Heat Treatments of Steels (Cont’d)
The successful heat treating of steels to produce a predominantly martensite microstructure throughout the cross section depends mainly on three factors:
the composition of the alloy
the type and character of the quenching medium
the size and shape of the specimen

12. Hardenability of Steels
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

13. Jominy End Quench Test Ability to form martensite
Jominy end quench test to measure hardenability
Adapted from Fig , Callister 6e.

14. Hardenability Curves • Hardness vs. distance from the quenched end
Adapted from Fig , Callister 6e.
A steel that is highly hardenable will retain large hardness values for relatively long distances; a low hardenable one will not.

15. Why hardness changes with position?
• The cooling rate varies with position
Adapted from Fig , Callister 6e.

16. 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)

17. Hardenability 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.

18. Effect of Carbon Content on Hardenability
The hardenability increases with the carbon content.
FIG Hardenability curves for four 8600 series alloys of indicated carbon content.

19. Alloying Elements and 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.

20. Quenching Mediums (1): Water
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

21. Quenching Mediums (2): Oil
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

22. Quenching Mediums (3): Air
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
The heated sample is placed on a screen. Cool air is blown at high speed from below it.

23. Effect of Quenching Medium
air
oil
water
Severity of Quench
small
moderate
large
Hardness
The severity of quench: water > oil > air

24. Quenching Operations in Heat Treatment
Removing Samples from
a Heat-Treating Furnace
Parts Quenched in a Group
(Figs and 11-4 in Metallurgy Fundamentals, by D. A. Brandt and J. C. Warner)

25. Effect of Part Size
FIG 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.

26. Effect of Part Geometry
When surface-to-volume ratio increases
cooling rate increases
hardness increases
Position
center
surface
Cooling rate
small
large
Hardness

27. Annealing Processes
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
Heating to the desired temperature
Holding or “soaking” at that temperature
Cooling, usually to room temperature

28. Purposes for Annealing
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.

29. Purposes for Annealing (Cont’d)
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.
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.

30. The Iron–Iron Carbide Phase Diagram
L + Fe3C
2.14
4.30
6.70
0.022
0.76 M N C P E O G F H
Cementite Fe3C

31. Temperature Regime of Steel Heat Treatment
FIG The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels.
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)

32. 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.

33. 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.

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

35. Precipitation hardening (Cont.)
Types of precipitation hardening process:
Solution heat treatment
Precipitation heat treatment.

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

37. 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.

38. Precipitation effect on TS, %EL
• 2014 Al Alloy:
• TS peaks with
precipitation time.
• Increasing T accelerates
process.
• %EL reaches minimum
with precipitation time.

39. Effect of aging on dislocation motion
• Peak-aged
--avg. particle size = 64b
--closer spaced particles
efficiently stop dislocations.
• Over-aged
--avg. particle size = 361b
--more widely spaced
particles not as effective.

40. SUMMARY • 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.