Heat Treatment Processes

Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing, and quenching.

1. HEAT TREATMENT

2. 1.INTRODUCTION OF HEAT TREATMENT:
Heat treatment is the term used to describe the
controlled heating and cooling of materials for
the purpose of altering their structures and
properties. The same material can be made weak
and ductile for ease in manufacture , and then
retreated to provide high strength and good
fracture resistance for use and application.

3. Technically , the term heat treatment applies
only to processes where the heating and cooling
are performed for the specific purpose of altering
properties , but heating and cooling often occur
as incidental phases of other manufacturing
processes , such as hot forming or welding
REFRENCE: DEGARMO’S BOOK 10
TH
EDITION

4. 2. PROCESSING HEAT TREATMENTS :
The term heat treatment is often associated with
those thermal processes that increase the
strength of a material , but the broader definition
permits inclusion of another set of processes that
we will call processing heat treatments
REF: GOOGLE

5. PROCESSING HEAT TREATMENTS FOR STEET:
Because many of the processing heat treatments are
applied to plain-carbon and low alloy steels, they
will be presented here with the simplified iron–
carbon equilibrium diagram serving as a reference
guide.
shows this diagram with the key transition lines
labeled in standard notation . The eutectoid line is
designated by the symbol and designates the
boundary between austenite and ferrite +
austenite.1 The transition from austenite to
austenite + cementite is designated as the Acm line.

7. In the process of full annealing , hypo eutectoid
steels (less than 0.77% carbon) are heated to 30°
to 60°C (50°to 100°F) above the A3 temperature
, held for sufficient time to convert the structure
to homogeneous single-phase austenite of uniform
composition and temperature, and then slowly
cooled at a controlled rate through the A1
temperature . Cooling is usually done in the
furnace by decreasing the temperature by 10°to
30°C (20°to 50°F) per hour to at least 30°C
(50°F) below the A1 temperature.
REF: DEGARMO’S BOOK 10
TH
EDITION and GOOGLE

9. Since different cooling rates do not produce a wide
variation of properties in low-carbon steels , the air
cool of a normalizing treatment often produces
acceptable uniformity. For higher carbon contents,
such as the 0.4 to 0.6% range, different cooling rates
can produce wider property variations, and the
uniform furnace cooling of a full anneal is often
preferred . Figure provides a graphical summary of
the process heat treatments.
REF: DEGARMO’S BOOK 10
TH
EDITION and GOOGLE

10. 3. HEAT TREATMENTS USED TO
INCREASE STRENGTH:
Six major mechanisms are available to increase
the strength of metals:
1. Solid-solution strengthening
2. Strain hardening
3. Grain size refinement
4. Precipitation hardening
5. Dispersion hardening
6. Phase transformations

11. Solid-solution strengthening:
In solid-solution strengthening , a base
metal dissolves other atoms , either as
substitutional solutions , where the new
atoms occupy sites in the host crystal
lattice , or as interstitial solutions , where
the new atoms squeeze into “holes”
between the atoms of the base lattice

12. Strain hardening:
Strain hardening produces an increase in
strength by means of plastic deformation
under cold-working conditions. Because
grain boundaries act as barriers to
dislocation motion , a metal with small
grains tends to be stronger than the same
metal with larger grains

13. Grain size refinement:
Grain size refinement can be used to
increase strength, except at elevated
temperatures, where grain growth can
occur and grain boundary diffusion
contributes to creep and failure.

14. Precipitation hardening:
precipitation hardening , or age hardening
, strength is obtained from a non
equilibrium structure that is produced by a
three-step heat treatment

15. Dispersion hardening:
Strength obtained by dispersing second-
phase particles throughout a base material
is known as dispersion hardening.

16. Phase transformations:
Phase transformation strengthening
involves those alloys that can be heated to
form a single phase at elevated
temperature and subsequently transform to
one or more low-temperature phases upon
cooling
REF: DEGARMO’S BOOK 10
TH
EDITION

17. 4. STRENGTHENING HEAT TREATMENTS
FOR STEEL:
Iron-based metals have been heat treated
for centuries, and today over 90% of all
heat-treatment operations are performed
on steel

18. ISOTHERMAL TRANSFORMATION DIAGRAM:

19. A useful aid to understanding non equilibrium heat
treatment processes is the isothermal-transformation
(I-T) or time-temperature-transformation (T-T-T)
diagram. The information in this diagram is obtained
by heating thin specimens of a particular steel to
produce elevated-temperature uniform-chemistry
austenite, “instantaneously” quenching to a
temperature where austenite is no longer the stable
phase
REF: DEGARMO’S BOOK 10
TH
EDITION and GOOGLE

20. TEMPERING OF MARTENSITE:
Despite its great strength , medium-or high-
carbon martensite in its as-quenched form
lacks sufficient toughness and ductility to be a
useful engineering structure . A subsequent
heating , known as tempering
Martensite is a supersaturated solid solution of
carbon in alpha-ferrite and , therefore, is a
metastable structure . When heated into the
range of 100°to 700°C (200°to 1300°F),

21. REF: GOOGLE

22. 5. SURFACE HARDENING OF STEEL:
Many products require different properties at
different locations . Quite frequently, this
variation takes the form of a hard , wear-resistant
surface coupled with a tough , fracture resistant
core . The methods developed to produce the
varied properties can be classified into three
basic groups:
(1)selective heating of the surface,
(2)altered surface chemistry, and
(3) deposition of an additional surface layer

23. SURFACE HARDENING FOR STEEL
SURFACE HARDENING FOR STEEL
REF: DEGARMO’S 10
TH
EDITION and GOOGLE

24. 6. FURNACES:
Afurnaceis a device used for high-
temperature heating. The name derives
fromGreekwordfornax, which
meansoven.

25. FURNACE TYPES:
Batch furnaces: in which the workpiece remains
stationary throughout its treatment , are preferred for
large parts or small lots of a particular part or grade of
steel.

26. Continuous furnaces: It is move the components
through the heat-treatment operation at rates
selected to be compatible with the other
manufacturing operations . Continuous furnaces
are used for large production runs where the
same or similar parts undergo the same thermal
processing

27. BELL FURNACE: In a bell furnace , the heating elements
are contained within a bottomless “bell” that is lowered
over the work.
REF: GOOGLE

28. 7. HEAT TREATMENT AND ENERGY:
Because of the elevated temperatures and the
time required at those temperatures , heat
treatments can consume considerable amounts of
energy
.However , if one considers the broader picture,
heat treatment may actually prove to be an
energy conservation measure . The manufacture
of higher-quality , more durable products can
often eliminate the need for frequent
replacements.
REFRENCE: DEGARMOS’S BOOK 10
TH
EDITION