Heat Treatment 1 ex

Motivation

Approx
.

15

components



-

How

do

we

select

the

best

material

for

each

component?



-

How

are

each

of

these

manufactured?

Stapler (~HK$ 5)

Car: ~ 15,000 parts;

Boeing 747 plane: ~6 million parts

Intel core 2 duo processor: 65 nm feature size, 291 million transistors

Properties of materials


Mechanical properties of materials


Strength, Toughness, Hardness, Ductility,


Elasticity, Fatigue and Creep

Chemical properties


Oxidation, Corrosion, Flammability, Toxicity, …

Physical properties


Density, Specific heat, Melting and boiling point,


Thermal expansion and conductivity,


Electrical and magnetic properties

Steels are heat treated for one of the following reasons:

Materials

Nanomaterials
, shape
-
memory alloys, superconductors
, …

Ferrous metals
: carbon
-
, alloy
-
, stainless
-
, tool
-
and
-
die steels

Non
-
ferrous metals
: aluminum, magnesium, copper, nickel,


titanium,
super alloys
, refractory metals,


beryllium, zirconium, low
-
melting alloys,


gold, silver, platinum, …

Plastics
: thermoplastics (acrylic, nylon, polyethylene, ABS,…)


thermosets (epoxies,
Polymides
,
Phenolics
, …)


elastomers (rubbers, silicones, polyurethanes, …)

Ceramics, Glasses, Graphite, Diamond, Cubic Boron Nitride

Composites
: reinforced plastics, metal
-
, ceramic matrix composites





Heat Treatment


is the controlled heating and cooling of metals to alter their
physical and mechanical properties without changing the
product shape.




Heat
Treatment


is
often associated with increasing the strength of material,
but it can also be used to alter certain manufacturability
objectives such as improve machining, improve formability,
restore
ductility.


Heat Treatment












A
: Definition


"Hardening is the process of heating a piece of steel to a
temperature within or above its critical range and than
cooling it
rapidly"


(
Begeman
, M.L.
-

Manufacturing processes
-

1977)



B
: Definition


"Hardening is that property of a material that enables it to
resist plastic deformation, penetration, indentation,
scratching"


(Lindberg, R. A.
-

Material & Manufacturing Technology
-

1968)







Hardening:


Hardening of steel is done to increase the
strenth

and wear
properties. One of the pre
-

requisites for hardening is sufficient
carbon and alloy content



Softening:


Softening is done to reduce strength or hardness, remove residual
stresses, improve tough
-
ness, restore ductility, refine grain size or
change the electromagnetic properties of the steel.



Material Modification:



Heat treatment is used to modify properties of materials in
addition to hardening and softening. These processes modify the
behavior of the steels in a beneficial manner to maximize service
life, e.g., stress relieving







Tempering


Tempering

is
a process done subsequent to quench hardening.
Quench
-
hardened parts are often too brittle. This brittleness is
removed by tempering.




Tempering
results in a desired combination of:


Hardness
, Ductility, Toughness, Strength, structural stability




QUENCHING


Cooling alloy fast enough to retain a supersaturated
solid solution of alloying constituents without
introducing adverse metallurgical or mechanical
conditions; water is most common quenching media
(immersion or spray); other media include air blasts,
soap solutions,
ind

hot oil.



HARDNESS TESTING


ROCKWELL TEST


Measures the difference in penetration between a minor and
major load


Minor load 10 Kg


Major load 60(a), 100 (B), 150 ( c) kg


A= Diamond, B= 1/16 in. ball, C= diamond



Rockwell scale runs to 130 bur only useful in range 20
-
100


HARDENING


SURFACE
HARDENING


l
. Carburizing
or Case Hardening


Steel is heated in contact with some carbonaceous material
in solid, liquid, or gas form; the steel absorbs carbon, which
is gradually diffused into the interior of the part.






Pack carburizing


(.030 to .160 thick); it employs packing parts in charcoal
or coke.






Gas carburizing



(.005 to .030 thick); it employs hydrocarbon fuels.



Liquid carburizing


(up to .250 thick); it employs a cyanide salt bath


FLAME
HARDENING:


Heating by oxyacetylene flame to above critical temperature;
heated part immediately quenched by water spray; produces
hard surface with ductile backing.