The document discusses hardenability, which is the ability of an alloy to form martensite and harden during heat treatment. It can be tested using the Jominy end-quench test, where a bar is heated and quenched at one end in water, causing a gradient of cooling rates and hardness levels along its length. Alloying elements like chromium, molybdenum, and nickel increase hardenability by shifting the CCT diagram to allow more martensite formation at a given cooling rate. The quenching medium, sample size, and alloy composition all impact the hardness profile achieved.

1. HARDENABILITY

2. Hardenability
• The ability of an alloy to be hardened by the
formation of martensite as a results of a given
heat treatment. It is a qualitative measure of the
rate at which hardness drops off with distance
into the interior of a specimen as a result of
diminished martensite content. Steels with high
hardenabilityform martensite even on slow
cooling. It can be tested experimentally bythe
Jominy End-Quench

3. Heat Treatment of Steels: Hardenability
• Hardenability is the capability of an alloy steel to form
martensite as a result of a given heat treatment
• NOTE: hardness  hardenabilty.
• High hardenability in a steel means that the steel
forms martensite not only at surface but to a large
degree throughout the interior.
• Hardenability more related to depth of hardness of a
steel upon heat treat.
• The depth of hardening in a plain carbon steel may be
2-3 mm vs 50 mm in an alloy steel
3

4. Heat Treatment of Steels: Hardenability
• How is the hardenability of
steels assessed?
– Jominy End-Quench Test
– Test bar is heated to form
100% austenite. It is then
quenched directly at one
end with a stream of water
4

5. Jominy Test
Generally, the faster steel cools, the harder it will be.
The Jominy bar measures the hardenbility of a steel
Softest
Hardest

6. Hardenability of Steels
• Ability to form martensite
• Jominy end quench test to measure hardenability.
1”
specimen
(heated to 
phase field)
24°C water
flat ground
4”
Fig. 14.5
• Hardness versus distance from the quenched end.
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7. Jominy Test for Hardenability
• Hardenability not the same as hardness!

8. Heat Treatment of Steels: Hardenability
The
cooling rate thus varies
throughout the length of the bar, the
rate being highest at the lower end
which is in direct contact with water.
The
hardness along the length of the
bar is then measured at various
distances from the quenched end and
plotted in a graph.
The
greater the depth to which the
hardness penetrates, the greater the
hardenability of the alloy.
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9. Heat Treatment of Steels: Hardenability
• A correlation may be drawn
between position along the
Jominy specimen and
continuous cooling
transformations.
• For eg, figure shows a
continuous cooling
transformation diagram for a
eutectoid iron-carbon alloy
onto which is superimposed
the cooling curves at four
different Jominy positions, and
corresponding microstructure
that result from each.
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10. Factors that influence hardenability
• Carbon content: The
hardness at any Jominy
position increases with
the concentration of C.
10

11. Alloying and Hardenability

12. Carbon and Hardenability

13. Hardness and Hardenability
Predict the center hardness in a water quenched 3” bar of 8640
Jominy Distance =17mm
Water Quenched
Oil Quenched

14. Hardenability
• Hardenability of a steel
increases with an addition of
alloying elements such as
Cr, Mo, Ni, W,  C curve
move to the right direction in
the TTT diagram.
temperature
• Hardenability
Relative ability of a steel to be hardened in depth by quenching.
– Depends on :
1. Alloy composition : Cr, Ni, V, Mo → increase hardenability
2. Austenite grain size
Cr, Mo, W,
Ni
time

15. Alloying and Hardenability
Hardness at Center of a
3 inch bar is about 42 HRC

16. Heat Treatment of Steels: Hardenability
During the industrial
production of steel, there is
always a slight, unavoidable
variation in composition
and average grain size from
one batch to another. This
variation results in some
scatter in measured
hardenability data, which
frequently are plotted as a
band representing the max
and min values.
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20. Cooling rate and Jominy distance do not change with alloying elements
as the rate of heat transfer is nearly independent of composition

21. Jominy test and CCT diagrams

22. Effects of alloying elements on the
hardenability of alloy steels
• Example: Next slide, all alloys have 0.4wt% C, but
with different alloying elements.(1)At the quenched
end all alloys have thesame hardness, which is a
function of carbon content only.(2)The hardenability
of the 1040 is low because the hardness of the alloy
drops rapidly with Jominy distance. The drop of
hardness with Jominy distance for the other alloys is
more gradual.(3)The alloying elements delay the
austenite􀃆pearlite and/or bainite reactions, which
permits more martensite to form for a particular
cooling rate, yielding a greater hardness.

23. Effects of alloying elements on the
hardenability of alloy steels

24. Effects of carbon content on the
hardenability of alloy steels

25. Effects of composition variation and grain size change
on the hardenability of alloy steels
• The industrial products of steels may change
composition and average grain size from
batch to batch, there fore, the measured
hardenability of a given type of steel should
be presented as a band rather than a single
line, as demonstrated by the Figure at right.

26. Effects of composition variation and grain size
change on the hardenability of alloy steels

27. Quenching Media
• The fluid used for quenching the heated alloy
effects the hardenability.
– Each fluid has its own thermal properties
• Thermal conductivity
• Specific heat
• Heat of vaporization
– These cause rate of cooling differences
Spring 2001
Dr. Ken Lewis
ISAT 430
27

28. Quenching Media2
• Cooling capacities of typical quench media are
– Agitated brine
– Still water
– Still oil
– Cold gas
– Still air
Spring 2001
5.
1.
0.3
0.1
0.02
Dr. Ken Lewis
ISAT 430
28

29. Other quenching concerns
• Fluid agitation
– Renews the fluid presented to the part
• Surface area to volume ratio
• Vapor blankets
– insulation
• Environmental concerns
– Fumes
– Part corrosion
Spring 2001
Dr. Ken Lewis
ISAT 430
29

30. Influence of quench medium and sample size on the
cooling rates at different locations.
• Severity of quench: Water > Oil > Air, e.g. for a
50 mm diameter bar, the cooling rate at
center is about 27°C/s in water, but, 13.5 °C/s
in oil.
• For a particular medium, the cooling rate at
center is lower when the diameter is larger.
For example, 75mm vs. 50mm.

31. Influence of quench medium and sample size on the cooling rates at
different locations.
Quenched in water
Quenched in oil

32. Radial hardness profile of cylindrical steel
samples of different diameter and composition.
Quench in water
0.4C+1.0Cr+0.2Mo
0.4C only

33. Depth of Hardening

34. Example Problem
Determine the radial hardness profile for a 50mm (2 in.) diameter cylindrical specimen
of 1040 steel that has been quenched in moderately agitated water.
Example Problem
Determine the radial hardness
profile for a 50mm (2 in.)
diameter cylindrical specimen
of 1040 steel that has been
quenched in moderately
agitated water.

36. Thanks