Microstructure and Heat Treatment of Steels

1. Microstructure and Heat Treatment of Steels
Dr. R.Narayanasamy, B.E., M.Tech., M.Engg., Ph.D., (D.Sc.)
Retired Professor (HAG),
Department of Production Engineering,
National Institute of Technology,
Tiruchirappalli-620015, Tamil Nadu, India.
Email: narayan19355@gmail.com
Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.)
Assistant Professor,
Department of Mechanical Engineering,
Mookambigai College of Engineering,
Pudukkottai-622502, Tamil Nadu, India.
Email:mv8128351@gmail.com

2. What is Hardening and Tempering?
Hardening and tempering of engineering steels is performed to
provide components with mechanical properties suitable for their
intended service.
Steels are heated to their appropriate hardening temperature
(usually between 800-900°C), held at temperature, then
"quenched" (rapidly cooled), often in oil or water. This is followed
by tempering (a soak at a lower temperature) which develops the
final mechanical properties and relieves residual stresses.

3. The actual conditions used for all three steps are determined by
steel composition, component size and the properties required.
Hardening and tempering can be carried out in "open" furnaces (in
air or combustion products), or in a protective environment
(gaseous atmosphere, molten salt or vacuum) if a surface free
from scale and decarburisation (carbon loss) is required ("neutral
hardening", also referred to as "clean hardening").
Cont.

4. What are the Benefits of Hardening and
Tempering?
 Hardening and tempering develops the optimum combination of
hardness, strength and toughness in an engineering steel.
 This offers the component designer a route to savings in weight
and material.
 Components can be machined or formed in a soft state and
then hardened and tempered to a high level of mechanical
properties.

5. Cont.
 Hardening from open furnaces is often employed for products
such as bars and forgings that are to be fully machined into
components afterwards.
 Neutrally clean hardening is applied to components that require
surface integrity to be maintained.
 Examples: Include nuts, bolts, springs, bearings and many
automotive parts.
 Neutral clean hardening is carried out under tightly-controlled
conditions to produce a precision component needing the
minimum of final finishing.

6. What Sort of Steels can be Hardened and
Tempered?
 Almost all engineering steels containing over 0.3% carbon will
respond to hardening and tempering.
 The majority of hardenable steels used for engineering
components.

7. What are the Limitations?
 The response of a steel component to hardening and tempering
depends on steel composition, component size, and method of
treatment.
 Every steel has a "limiting" section size ("ruling section") above
which full hardening cannot be achieved.
 A higher grade of steel will be required to ensure optimum
properties in a larger section.

8. Cont.
 It may be possible to harden larger components in lower-grade
steels by using non-standard treatments such as faster quench
rates or lower-temperature tempers.
 Faster quench rates always increase the risk of distortion or
cracking.
 Low-temperature tempers can seriously impair mechanical
properties such as toughness.
 Serious consideration should be given to these facts.

9. Steel Conditions
 Steels that are purchased after open treatments (e.g. 'black
bar') are liable to have lost some carbon from the surface layers
(decarburisation).
 Decarburised layers must be fully removed by machining from
all surfaces before components are hardened.
 Otherwise excessive distortion or even cracking are likely.

10. Cont.
 Steels that are purchased in cold-worked conditions, such as
'bright bar', contain residual stresses.
 These stresses can contribute to distortion during machining
and in hardening.
 It is recommended that rough-machined blanks have these
stresses removed, by normalising or soft annealing before
hardening, in order to reduce the risk of excessive distortion.

11. Temper Embrittlement
 Certain steels, particularly alloy steels containing nickel and
chromium, suffer from embrittlement.
 lf tempered in the range 250-450°C; this limits the acceptable
mechanical properties they can attain.
 Check that the selected steel is not prone to this problem, and if
in doubt consult your heat treater.

12. What Problems Could Arise?
Distortion or cracking
 Changes in size or shape can arise in hardened components
from a variety of causes.
 some inherent in the high-temperature / rapid cool process,
some attributable to component design shortcomings and
others relating to earlier manufacturing steps (e.g. thermal relief
of stresses introduced by prior forming).
 Where final dimensions are critical, finish grinding or machining
will be necessary and must be planned.

13.  Components hardened and tempered to high mechanical-
property levels are often impossible to straighten later.
 In extreme circumstances, the stresses generated by
quenching can even be high enough to crack components.
 It is necessary for the manufacturer to take all reasonable steps
to minimise the risk through careful component design (e.g.
avoid stress-raising features such as sudden changes in
section, deep slots, cutouts) and steel selection.
 It is always helpful to consider potential hardening and
tempering problems at the design stage.
Cont.

14. Scaling and Decarburisation
 If open furnace treatment is selected, scaling and
decarburisation are likely.
 Large components spend longer at high temperatures and
suffer more.
 Allowance must be made for removal of affected layers after
treatment.
 The alternative is clean/neutral hardening in a protective
environment which avoids scaling and decarburisation.
Cont.

16. Time-Temperature-Transformation (TTT) Curve

17. TTT diagram showing how to make tempered steel

18. TTT diagram showing how to make Martempered steel

19. TTT diagram showing how to make Austempered steel

20.  Hardening involves controlled heating to a critical temperature
dictated by the type of steel (in the range 760-1300 C) followed
by controlled cooling.
 Dependent on the type of material, appropriate cooling rates
vary from very fast (water quench) to very slow (air cool).
Hardening of steels

21.  Tempering involves reheating the hardened tool/die to a
temperature between 150-657°C, depending on the steel type.
 A process which controls the final properties whilst relieving
stresses after hardening, tempering can be complex.
 some steels must be subjected to multiple tempering
operations.
Tempering of Steels

22.  Brittlenesss exhibited by some steels after being heated to
some temperature within the range of 300° to 650°F, and more
especially if the steel is worked at the elevated temperature.
 There can be an increase in strength and a substantial
decrease in ductility or impact strength.
 Blue brittleness temperature range must be avoided during
tempering.
Cont.
Blue Brittleness

23. Microstructure development in eutectoid steel
with increasing cooling rate in arrow direction
Source: E.C. Bain: Functions of the alloying elements in steel, American Society for
Metals, Metals Park, Ohio, USA, 1939.

25. Microstructure of Martensite Formed After
Hardening of Steel
Needle like structure indicates Martensite

26. TEM micrographs of the specimens: (a,c) for the bright and dark field image of 1.54Si-
60s; (b,d) for the bright and dark field image of 1.26Al-60s; (e,f) for the 1.54Si-1800s
and 1.26Al-1800s, respectively.
TEM micrographs with Nanoscale-Martensite
(M) and Retained Austenite (RA)

27. TEM micrographs of the specimens: (a,c) for the bright and dark field image of 1.54Si-
60s; (b,d) for the bright and dark field image of 1.26Al-60s; (e,f) for the 1.54Si-1800s
and 1.26Al-1800s, respectively.
Cont.

28. TEM micrographs of the specimens: (a,c) for the bright and dark field image of 1.54Si-
60s; (b,d) for the bright and dark field image of 1.26Al-60s; (e,f) for the 1.54Si-1800s
and 1.26Al-1800s, respectively.
Cont.
TEM micrographs with Nanoscale-Martensite
(M) and Retained Austenite (RA)

30. Figure 2.5: Martensitic microstructures (low optical micrograph) (a) lath
martensite (low-alloy steel with 0.17 wt.%C, 1200℃ hot bath) (b) plate
martensite (casehardening steel, 1100℃ hot bath). Obtained from [15].

31. Microstructure of a martensitic carbon steel
(optical micrograph)

32. Microstructure of Tempered Martensite in Carbon-Iron-Chromium Steel

33. Schematic of a TRIP steel microstructure showing a matrix of Ferrite, with
Martensite, Bainite and Retained Austenite as the additional phases.

34. Comparison between normalized and quenched and
tempered Plain Carbon Steel

35. Thank You
References:
Authors of Technical articles and Scopus Journals
are Acknowledged.