13. Heat Treatment of Metals

13.1 Tempering of Martensite

13.2 Annealing of Metals and Alloys

• Annealing of Non-ferrous Metals and Alloys

• Annealing of Ferrous Metals and Alloys

Tempered

Martensite

13.1 Tempering of Martensite

Martensite

Fe3C a-Ferrite

• Tempering:

a-

• Tempered

martensite consists

of extremely small

and dispersed

cementite particles

in a ferrite matrix

(much smaller than

those in

spheroidite).

EFFECTS OF TEMPERING

• Tempering at a higher temperature faster carbon diffusion

larger cementite particles grown

less ferrite – cementite phase boundary

area per unit volume

weaker and more ductile material

• Tempering for a longer tempering timemore time for carbon diffusion

larger cementite particles

less ferrite – cementite phase boundary

area per unit volume

weaker and more ductile material

• Tempered martensite has increased ductility and

toughness, while it has reduced the strength and hardness

compared to martensite.

• Tempering relieves the internal stresses that were introduced

during quenching (when martensite was formed).

Austempering and Martempering

13.2 Annealing of Metals and Alloys

• Purposes:

– Increase softness, ductility, and toughness of cold-

worked materials

– Relieve internal stresses

– Produce a specific microstructure

• Stages of annealing processes:

1. Heat the material to a desired elevated temperature (Tanneal)

2. Hold the elevated temperature (“soaking”)

3. Cool to room temperature

• Types of annealing processes:

– For non-ferrous metals and alloys:

Stress relief annealing

Process annealing

– For ferrous alloys (steels):

Spheroidizing

Full annealing

Normalizing

13.2.1 Annealing of Non-ferrous Metals

and Alloys

Stress relief annealing

Process annealing

Effects of Reheating of Non-ferrous Metals after

Cold Working

Annealing of

brass alloy

decreases TS

and increases

ductility (%EL).

The effects of

cold work are

reversed!

RECRYSTALLIZATION TEMPERATURE

• Recrystallization temperature is 1/3 -1/2 of absolute

melting temperature (K) for metals.

• The recrystallization temperature: the temperature at which recrystallization just reaches completion in 1 hour

Temperature (K) = Temperature (C) + 273

Stress Release Annealing of Non-ferrous

Metals/Alloys)• Carried out at a temperature lower than the recrystallization

temperature (Tanneal < Trecryst).

• Aims to eliminate internal stresses caused by:

– Plastic deformation processes (machining, grinding, etc.)

– Non-uniform cooling after welding or casting

– Phase transformations induced during cooling wherein parent and product phases have different densities, e.g., density decreases during Austenite (FCC crystal) Martensite (BCT crystal).

• Involves recovery ONLY.

• It can annihilate dislocations, as an

elevated temperature enhances atomic

diffusion, which hence reduces the

dislocation density in the material.

Thus, mechanical properties such as

strength and ductility are partially

recovered to their pre-cold-worked states.

Process Annealing of

Non-ferrous Metals/Alloys)

• Carried out after cold-working to:

– Allow continuation of deformation without fracture or

excessive energy consumption.

– Increase the ductility of strain-hardened metals

• Carried out at a temperature higher than the

recrystallization temperature ( ).

• Involves recovery, recrystallization, and grain

growth.

• Recrystallization occurs at temperature > Trecryst.

• In recrystallization, very fine new crystals are formed by

consuming the old cold-worked crystals. The new crystals have

much lower dislocation density than the cold-worked crystals,

thus softening the material.

33% cold

worked

brass

New crystals

nucleate after

3s. at 580C.

0.6 mm

RECRYSTALLIZATION

• All cold-worked crystals are finally consumed.

• Mechanical properties (strength, ductility) are almost restored

to the pre-cold-worked values.

After 4s

at 580C. After 8s

at 580C.

GRAIN GROWTH

• Grain growth occurs in

all polycrystalline

materials at elevated

temperature.

• Grain growth will

decrease in total grain

boundary area and

reduce total grain

surface energy.

The strength decreases

and ductility increases.

• An empirical correlation for grain size calculation:

GRAIN GROWTH (Cont’d)

K increases with temperature due to higher atomic diffusion rates.

Ktdd n

o

n elapsed time

coefficient dependent

on material and T.

grain

diam.

at time t.

exponent

typ. n = ~ 2

Initial grain

diameter

After 8 s,

580C

After 15 min,

580C

0.6 mm 0.6 mm

13.2.2 Annealing of Ferrous metals and

alloys

Spheroidizing

Full annealing

Normalizing

TYPES OF Ferrous Metals and Alloys

– Steels (<1.4 wt% C)

• Low alloy steels containing plain Fe and C, and in some

cases low levels of other alloying elements.

.

– Plain carbon steels: contain only C and some Mn as

the alloying elements.

– Other low alloy steels: contain low concentrations of

alloying elements in addition to C/Mn.

• High alloy steels containing high concentrations of

alloying elements other than C and Mn. Example:

Stainless Steel, SS 316L: 0.03% C, 17% Cr, 12% Ni, 2.5%

Mo, 2.0% Mn, <1% Si, <0.045% P, <0.03% S.

– Cast irons (> 2.5 – 4.5 wt% C)

SPHEROIDIZING

• Carried out by:

– Heating to a temperature just below the eutectoid line

(727C)

– Maintaining this temperature for more than 15-24 hours to

obtain spheroidite.

– Cooling to room temp.

• Carried out usually before machining or plastic deformation to

achieve greater ductility.

Spheroidite

FULL ANNEALING

• Carried out by:

– Heating to a temperature above the eutectoid line (727C) and for a sufficient time to convert all pearlite to austenite(austenitizing)

– Furnace cooling (slow cooling) to obtain coarse pearlite.

• Carried out usually for low- and medium-carbon steels before machining or extensive plastic deformation to:

– Produce coarse pearlite (plus a proeutectoid phase if it was present before the annealing), and

– Increase ductility.

Coarse Pearlite

NORMALIZING• Carried out by:

– Heating to a high enough temperature and for a sufficient time to convert all pearlite and any proeutectoid phases to austenite (austenitizing)

– Cooling in air (fast cooling) to obtain fine pearlite.

• Carried out usually after plastic deformation to:

– Decrease the average grain size by producing fine pearlite.

– Produce a more uniform size distribution of pearlite.

Fine Pearlite