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HARDENABILITY
Dr. H. K. Khaira
Professor in MSME
MANIT, Bhopal
Introduction
• Hardenability is one of the most important
properties of a steel because it describes the
ease with which a given steel can be
quenched to form martensite or the depth to
which martensite is formed on a given
quench.
• It is an important property for welding, since it
is inversely proportional to weldability, that
is, the ease of welding a material.
Introduction
• The ability of steel to form martensite on
quenching is referred to as the hardenability.
• Hardenability is a measure of the capacity of a
steel to be hardened in depth when quenched
from its austenitizing temperature.
• Steels with high hardenability form martensite
even on slow cooling.
• High hardenability in a steel means that the steel
forms martensite not only at surface but to a
large degree throughout the interior.
Introduction
• For the optimum development of
strength, steel must be fully converted to
martensite.
• To achieve this, the steel must be quenched at
a rate sufficiently rapid to avoid the
decomposition of austenite during cooling to
such products as ferrite, pearlite and bainite.
Introduction
• Hardenability of a steel should not be
confused with the hardness of a steel.
hardness  hardenabilty
Introduction
Hardenability
• The Hardness of a steel is a
measure of a sample's
resistance to indentation or
scratching,

Hardness
• Hardenability refers to its
ability to be hardened to a
particular depth under a
particular set of conditions.
Hardenability
• 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.
• Hardenability is more related to depth of hardning 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.
• A large diameter rod quenched in a particular medium
will obviously cool more slowly than a small diameter
rod given a similar treatment. Therefore, the small rod
is more likely to become fully martensitic.
Hardenability
• The hardenability of a steel is the maximum diameter of the
rod which will have 50% martensite even in the core when
quenched in an ideal quenchant. This diameter is known as Di
or ideal diameter.

8
Relation between cooling curves for the surface and
core of an oil-quenched 95 mm diameter bar
Determination of Hardenability
• There are TWO methods to determine
hardenability of steels
– Grossman’s Method
– Jominy end quench method
Grossman’s method
• In Grossman’s method, we use round bars of different
diameters.
• These bars are quenched in a suitable quenchant.
• Further, we determine the critical diameter (dc) which
is the maximum diameter of the rod which produced
50% martensite on quenching.
• The ideal diameter (DI) is then determined from the
curve.
• This type of experiment requires multiple
austenitization and quenching treatments on
specimens of varying diameter just to quantify the
hardenability of a single material.
Radial hardness profile of cylindrical steel
samples of different diameter and composition.
Quench in water
Effect of

Composition

0.4C+1.0Cr+0.2Mo→

0.4C only →

Diameter
Hardenability Curves
Jominy End Quench Method
• Grossmans method requires multiple austenitization and quenching
treatments on specimens of varying diameter just to quantify the
hardenability of a single material.
• An alternative approach is to develop a more convenient standard test
method that can be used for relative comparison of hardenability. The
Jominy end-quench test is one such approach.
• The jominy end-quench test is specified in ASTM standard A255 and is a
widely used method for quantifying hardenability. Its wide use adds to its
value, since the utility of empirical relations and data comparison
becomes more reliable as more data are accumulated.
• Moreover, Jominy data have been collected on a large enough scale to
offer a high degree of statistical certainty for many steels.
• These data have been correlated with measurements and/or calculations
of dc.
• By using these correlations, a single Jominy test can be used to estimate
dc and DI for a given steel (and austenite grain size).
The Jominy End Quench Test
• The most commonly used method for
determining hardenability is the end quench
test developed by Jomini and Boegehold.
• The details of the test are covered in IS : 3848
– 1981 and ASTM A 255.
The Jominy End Quench Test
• The Jominy End Quench Test measures
Hardenability of steels.
• Information gained from this test is necessary
in selecting the proper combination of alloy
steel and heat treatment to minimize thermal
stresses and distortion when manufacturing
components of various sizes.
Principle
• The hardenability of a steel is measured by a Jominy
test:
• A round metal bar of standard size is transformed to
100% austenite through heat treatment, and is then
quenched on one end with room-temperature water.
• The cooling rate will be highest at the end being
quenched, and will decrease as distance from the end
increases.
• The hardenability is then found by measuring the
hardness along the bar: the farther away from the
quenched end that the hardness extends, the higher
the hardenability.
Jominy Test
The Jominy bar measures the hardenbility of a steel

Softest

Hardest
Cooling Rates at Different Jominy
Distances
Cooling rate and Jominy
distance (distance from
the quenched end) do not
change with alloying
elements as the rate of
heat transfer is nearly
independent of
composition
Steps in Jominy End Quench Test
• First, a sample specimen rod either 100mm in length and 25mm in
diameter, or alternatively, 102mm by 25.4mm is obtained.
• Second, the steel sample is normalized to eliminate differences in
microstructure due to previous forging, and
• Then it is austenitised. This is usually at a temperature of 800 to
900°C.
• Next, the specimen is rapidly transferred to the test
machine, where it is held vertically and
• Sprayed with a controlled flow of water onto one end of the
sample.
This cools the specimen from one end, simulating the effect of
quenching a larger steel component in water. Because the cooling
rate decreases as one moves further from the quenched end, you
can measure the effects of a wide range of cooling rates from vary
rapid at the quenched end to air cooled at the far end.
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

22
Jominy End Quench Test
Details of Jominy Test for Hardenability

All dimensions are in inches
Jominy End Quench Test
• After end quenching, longitudinal Flat Surfaces are ground on opposite
sides of the test piece as per dimensions.
The specimen is ground flat along its length to a depth of .38mm (15
thousandths of an inch) to remove decarburized material.
This grinding is very important for correct positioning of the sample in the
fixture and also for accurate repeatable and reliable test results.
Hardenability of Steels
• Jominy end quench test to measure hardenability.
1”
specimen
(heated to 
phase field)
24°C water

flat ground
4”
Fig. 14.5

26
Jominy End Quench Test
•The hardness is measured at intervals along its length
beginning at the quenched end. Hardness at equal intervals
(1 mm or 1/16”) to be checked and noted.
Plotting of Result
Plot the resulting
data on graph paper
with hardness value
as ordinate (Y axis)
and distance from
the quenched end as
abscissa (X axis).
Hardenability Curve
Heat Treatment of Steels: Hardenability
The

cooling rate 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.

30
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.

Figure 12.23 The
hardenability curves for
several steels.
Hardenability Curve
• Because the cooling rate decreases as one moves
further from the quenched end, we can measure
the effects of a wide range of cooling rates from
vary rapid at the quenched end to air cooled at
the far end.
• By comparing the curves resulting from end
quench tests of different grades of steels, their
relative hardenability can be established. Thus
the flatter the curve, the greater the
hardenability.
Hardenability Curve
Cooling Curves and Phases at different Jominy
Distances

• A correlation may be drawn
between position along the
Jominy specimen and
continuous cooling
transformations.
• For example, 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.
34
Cooling curves from Jominy Distances
Cooling Curves and Phases at different
Jominy Distances
Determination of Hardenability from
Jominy Test Graph
• After plotting the Jominy distance Vs Hardness curve, the Jominy
distance having hardness equal to 50 % martensite is determined.
• Then the diameter of a rod having cooling rate similar to the cooling
rate at the Jominy distance having 50 % martensite is determined
from the graph corelating the Jominy distance with the diameter of
the rod having similar cooling rate for water quenching .
• This diameter gives the hardenability of the steel in water
quenching (having H value equal to 1).
• Hardenability in any other quenchant can be determined from the
same graph.
• Di (hardenability in ideal quenching medium) can also be
determined in a similar manner.
• We can determine hardenability for any other amount of
martensite in the core in any quenchant in a similar way.
Grossman chart used to determine the hardenability of
a steel bar

For Jominy
distance 4, the
hardenability in
water quenching
is 1.1 Inch.
Hardenability Curves
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

40
Coefficient of severity of quench: H
•
•
•

Cooling capacities (Severity of quench) of quenching medium is known as H value.
H values of some of the quenchants are given below.
Cooling rates are at the center of a 2.5 cm bar.

H Value
–
–
–
–
–
–
–
–
–

Ideal Quench
Agitated brine
Brine (No agitation)
Agitated Water
Still water
Agitated Oil
Still oil
Cold gas
Still air

∞
5
2
4
1
1
0.25
0.1
0.02

Cooling Rate (0C/s)
∞
230
90
190
45
45
18
-

41
Effect of Agitation on Coefficient of
severity of quench: H
Agitation
Violent
Strong
Good
Moderate
Mild
None

Cooling Medium
Oil
Water
Brine
0.8-1.1
4.0
5.0
0.5-0.8
1.6-2.0
0.4-0.5
1.4-1.5
0.35-0.40 1.2-1.3
0.30-0.35 1.0-1.1
2.0-2.2
0.25 - 0.30 0.9-1.0
2.0
Ideal Quenchant
• Ideal quenchant is one which brings down the
surface temperature to room temperature
instantaneously and keeps it at that
temperature thereafter.

43
Grossman chart can be used to determine the
hardenability of a steel bar for different quenchants.
If a steel is having 1.1”
hardenability, it will
have 1.6” hardenability
in a quenchant with H
value equal to 5.
Similarly, it will have
0.4” and 0.9”
hardenability in
quenchants with H
value 0.2 and 0.5
respectively.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark
used herein under license.
Effect of DI and “H” on D
Factors affecting Hardenability
• Slowing the phase transformation of austenite to ferrite
and pearlite increases the hardenability of steels.
• The most important variables which influence hardenability
are
– 1. Austenite grain size
– 2. Carbon content
– 3. Alloying elements

46
Austenitic Grain Size
• The hardenability increases with increasing
austenite grain size, because the grain
boundary area which act as nucleating site is
decreasing.
• This means that the sites for the nucleation of
ferrite and pearlite are being reduced in
number, with the result that these
transformations are slowed down, and the
hardenability is therefore increased.
Effect of austenite grain size on
Hardenability
• The more γ-grain boundary surface the easier
it is for pearlite to form rather than
martensite

• Smaller γ-grain size → lower hardenability
• Larger γ-grain size → higher hardenability
Effect of Austenitic Grain size
Carbon Content
• Carbon is primarily a hardening agent in steel.
• It also increases hardenability by slowing the
formation of pearlite and ferrite.
• But its use at higher levels is limited, because
of the lack of toughness which results in
greater difficulties in fabrication and, most
important, increased probability of distortion
and cracking during heat treatment and
welding.
Carbon and Hardenability

Hardenability of a
steel increases
with increase in C
content TTT
diagram moves to
the right.
Effect of Austenitic Grain size and
Carbon Content on Di
Effect of Alloying Elements
• most metallic alloying elements slow down the ferrite and pearlite
reactions, and so also increase hardenability. However, quantitative
assessment of these effects is needed.
• Chromium, Molybdenum, Manganese, Silicon, Nickel and Vanadium
all effect the hardenability of steels in this manner. Chromium,
Molybdenum and Manganese being used most often.
• Boron can be an effective alloy for improving hardenability at
levels as low as .0005%.
– Boron is most effective in steels of 0.25% Carbon or less.
– Boron combines readily with both Nitrogen and Oxygen and in so
doing its effect on hardenability is sacrificed.
– Therefore Boron must remain in solution in order to be affective.
– Aluminum and Titanium are commonly added as "gettering" agents to
react with the Oxygen and Nitrogen in preference to the Boron.
Effect of Alloying Elements
• The most economical way of increasing the
hardenability of plain carbon steel is to increase the
manganese content, from 0.60 wt% to 1.40 wt%, giving
a substantial improvement in hardenability.
• Chromium and molybdenum are also very
effective, and amongst the cheaper alloying additions
per unit of increased hardenabilily.
• Boron has a particularly large effect when it’s added to
fully deoxidized low carbon steel, even in
concentrations of the order of 0.001%, and would be
more widely used if its distribution in steel could be
more easily controlled.
Effect of Alloying Elements
• Hardenability of a steel increases with addition of alloying
elements such as Cr, V, Mo, Ni, W  TTT diagram moves to the
right.

temperature

Cr, Mo, W, Ni

time
Jominy hardenability curves: Hardenability improves with
increasing Mo content
Hardenability curves of 6 steels
Effect of Alloying Elements
• all steels have 0.4wt% C, but with
different alloying elements.
– At the quenched end all alloys
have the same hardness, which is
a function of carbon content
only.
– 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.
– 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.
Effect of Alloying Elements

Hardness of 42 at
center is obtained in
bars of different
diameters in
different steels
indicating different
hardenabilities.
Effect of Alloying Elements

Hardness at center
of a 3 inch bar is
different for
different steels
indicating different
amounts of
martensite at the
center
Hardenability Multiplying Factor
• The Hardenability Multiplying Factor shows the rate
at which the hardening depth is increased with the
percentage of the alloying element
• The ideal diameter (DI ) is calculated from:
DI = DIC * ƒMn *ƒSi *ƒNi*ƒCr *ƒMo
Where DIC is the basic DI factor for carbon and ƒx is the
multiplying factor for the alloying element x.
Multiplying Factors For The
Calculation Of Ideal Diameter
Base ideal diameter, DIjominy
Carbon
grain size
%
No. 6
No. 7
No.8
Mn
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00

0.0814
0.1153
0.1413
0.1623
0.1820
0.1991
0.2154
0.2300
0.2440
0.2580
0.2730
0.284
0.295
0.306
0.316
0.326
0.336
0.346
-

0.0750
0.1065
0.1315
0.1509
0.1678
0.1849
0.2000
0.2130
0.2259
0.2380
0.2510
0.262
0.273
0.283
0.293
0.303
0.312
0.321
-

0.0697
0.0995
0.1212
0.1400
0.1560
0.1700
0.1842
0.1976
0.2090
0.2200
0.2310
0.2410
0.2551
0.260
0.270
0.278
0.287
0.296
-

1.167
1.333
1.500
1.667
1.833
2.000
2.167
2.333
2.500
2.667
2.833
3.000
3.167
3.333
3.500
3.667
3.833
4.000
4.167
4.333

Alloying factor, fX
Si
Ni
1.035
1.070
1.105
1.140
1.175
1.210
1.245
1.280
1.315
1.350
1.385
1.420
1.455
1.490
1.525
1.560
1.595
1.630
1.665
1.700

1.018
1.036
1.055
1.073
1.091
1.109
1.128
1.246
1.164
1.182
1.201
1.219
1.237
1.255
1.273
1.291
1.309
1.321
1.345
1.364

Cr

Mo

1.1080
1.2160
1.3240
1.4320
1.5400
1.6480
1.7560
1.8640
1.9720
2.0800
2.1880
2.2960
2.4040
2.5120
2.6200
2.7280
2.8360
2.9440
3.0520
3.1600

1.15
1.30
1.45
1.60
1.75
1.90
2.05
2.20
2.35
2.50
2.65
2.80
2.95
3.10
3.25
3.40
3.55
3.70
-
Hardenability Multiplying Factor
Exceptions
• S - reduces hardenability because of formation
of MnSand takes Mn out of solution as MnS
• Ti - reduces hardenability because it reacts
with C to form TiC and takes C out of solution;
TiC is very stable and does not easily dissolve
• Co - reduces hardenability because it
increases the rate of nucleation and growth of
pearlite
Hardenability Band
• The industrial products of steels may change
composition and average grain size from
batch to batch, therefore, 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.
Hardenability Band
• Hardenabilily data now exists for a wide range
of steels in the form of maximum and
minimum end-quench hardenability
curves, usually referred to as hardenability
bands. This data is, available for very many of
the steels listed in specifications such as those
of the American Society of Automotive
Engineers (SAE), the American Iron and Steel
Institute (AISI) and the British Standards.
Hardenability Band
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.

67
Effects of composition variation and grain size
change on the hardenability of alloy steels
Hardenability (as Range of Di Values)
of Various Steels
Example
• Calculate the approximate hardenability of an
8630 (0.3%C, 0.3%Si, 0.7%Mn, 0.5%Cr, 0.6%Ni,
0.2%Mo) alloy steel with an ASTM grain size of 7
Solution
• Find out base DI for 0.3% carbon
• Calculate multiplying factors for each element
• Ideal critical diameter found by multiplying
base diameter by the multiplying factors
Summery
• The hardenability of ferrous alloys, i.e. steels, is a function of the
carbon content and other alloying elements and the grain size of
the austenite.
• The relative importance of the various alloying elements is
calculated by finding the equivalent carbon content of the material.
• The fluid used for quenching the material influences the cooling
rate due to varying thermal conductivities and specific heats.
• Substances like brine and water cool much more quickly than oil or
air.
• Additionally, if the fluid is agitated cooling occurs even more
quickly.
• The geometry of the part also affects the cooling rate: of two
samples of equal volume, the one with higher surface area will cool
faster.
Numerical problem -1
• Predict the center hardness in a water
quenched 3” bar of 8640
Solution to Numerical Problem - 1
The cooling rate at the center of a 3” dia bar in water quenching will be
same as that at Jominy distance 17 mm.

Jominy Distance =17mm

Water Quenched

Oil Quenched
Effect of Alloying Elements
Hardness produced
at Jominy distance
17 mm in 8640 steel
will be 43 HRC.
Therefore, the
hardness at center
of a 3 inch bar will
be 43 HRC
Cooling rate and Jominy distance do not change with alloying elements
as the rate of heat transfer is nearly independent of composition
Equivalent bar diameter when
quenched
• When the end-quench hardness curve of a
steel has been found, this table enables the
user to estimate the hardnesses that would be
obtained at the centers of quenched round
bars of different diameters, when that same
steel is quenched with various severities of
quench. For each successive 1/16 in.
position, the hardness obtained in the endquench test would be found at the center of
the bar size.
Equivalent bar diameter when quenched
Cooling Rate at Each Jominy Position
for Room Temperature Water
Distance from water quenched end
1/16 in.
1
2
3
4
5
6
7
8
9
10
12
14
16
18
20

Cooling Rate
0C/s
270
170
110
70
43
31
23
18
14
11.9
9.1
6.9
5.6
4.6
3.9
Jominy test and CCT diagrams
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.
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

82
Correlation of carbon and martensite content with
Rockwell hardness
Alloy Factors For The Calculation Of
Ideal Diameter
Interconversion of ideal bar diameter
as a function of shape
Thanks

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hardenability

  • 1. HARDENABILITY Dr. H. K. Khaira Professor in MSME MANIT, Bhopal
  • 2. Introduction • Hardenability is one of the most important properties of a steel because it describes the ease with which a given steel can be quenched to form martensite or the depth to which martensite is formed on a given quench. • It is an important property for welding, since it is inversely proportional to weldability, that is, the ease of welding a material.
  • 3. Introduction • The ability of steel to form martensite on quenching is referred to as the hardenability. • Hardenability is a measure of the capacity of a steel to be hardened in depth when quenched from its austenitizing temperature. • Steels with high hardenability form martensite even on slow cooling. • High hardenability in a steel means that the steel forms martensite not only at surface but to a large degree throughout the interior.
  • 4. Introduction • For the optimum development of strength, steel must be fully converted to martensite. • To achieve this, the steel must be quenched at a rate sufficiently rapid to avoid the decomposition of austenite during cooling to such products as ferrite, pearlite and bainite.
  • 5. Introduction • Hardenability of a steel should not be confused with the hardness of a steel. hardness  hardenabilty
  • 6. Introduction Hardenability • The Hardness of a steel is a measure of a sample's resistance to indentation or scratching, Hardness • Hardenability refers to its ability to be hardened to a particular depth under a particular set of conditions.
  • 7. Hardenability • 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. • Hardenability is more related to depth of hardning 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. • A large diameter rod quenched in a particular medium will obviously cool more slowly than a small diameter rod given a similar treatment. Therefore, the small rod is more likely to become fully martensitic.
  • 8. Hardenability • The hardenability of a steel is the maximum diameter of the rod which will have 50% martensite even in the core when quenched in an ideal quenchant. This diameter is known as Di or ideal diameter. 8
  • 9. Relation between cooling curves for the surface and core of an oil-quenched 95 mm diameter bar
  • 10. Determination of Hardenability • There are TWO methods to determine hardenability of steels – Grossman’s Method – Jominy end quench method
  • 11. Grossman’s method • In Grossman’s method, we use round bars of different diameters. • These bars are quenched in a suitable quenchant. • Further, we determine the critical diameter (dc) which is the maximum diameter of the rod which produced 50% martensite on quenching. • The ideal diameter (DI) is then determined from the curve. • This type of experiment requires multiple austenitization and quenching treatments on specimens of varying diameter just to quantify the hardenability of a single material.
  • 12. Radial hardness profile of cylindrical steel samples of different diameter and composition. Quench in water Effect of Composition 0.4C+1.0Cr+0.2Mo→ 0.4C only → Diameter
  • 13.
  • 15. Jominy End Quench Method • Grossmans method requires multiple austenitization and quenching treatments on specimens of varying diameter just to quantify the hardenability of a single material. • An alternative approach is to develop a more convenient standard test method that can be used for relative comparison of hardenability. The Jominy end-quench test is one such approach. • The jominy end-quench test is specified in ASTM standard A255 and is a widely used method for quantifying hardenability. Its wide use adds to its value, since the utility of empirical relations and data comparison becomes more reliable as more data are accumulated. • Moreover, Jominy data have been collected on a large enough scale to offer a high degree of statistical certainty for many steels. • These data have been correlated with measurements and/or calculations of dc. • By using these correlations, a single Jominy test can be used to estimate dc and DI for a given steel (and austenite grain size).
  • 16. The Jominy End Quench Test • The most commonly used method for determining hardenability is the end quench test developed by Jomini and Boegehold. • The details of the test are covered in IS : 3848 – 1981 and ASTM A 255.
  • 17. The Jominy End Quench Test • The Jominy End Quench Test measures Hardenability of steels. • Information gained from this test is necessary in selecting the proper combination of alloy steel and heat treatment to minimize thermal stresses and distortion when manufacturing components of various sizes.
  • 18. Principle • The hardenability of a steel is measured by a Jominy test: • A round metal bar of standard size is transformed to 100% austenite through heat treatment, and is then quenched on one end with room-temperature water. • The cooling rate will be highest at the end being quenched, and will decrease as distance from the end increases. • The hardenability is then found by measuring the hardness along the bar: the farther away from the quenched end that the hardness extends, the higher the hardenability.
  • 19. Jominy Test The Jominy bar measures the hardenbility of a steel Softest Hardest
  • 20. Cooling Rates at Different Jominy Distances Cooling rate and Jominy distance (distance from the quenched end) do not change with alloying elements as the rate of heat transfer is nearly independent of composition
  • 21. Steps in Jominy End Quench Test • First, a sample specimen rod either 100mm in length and 25mm in diameter, or alternatively, 102mm by 25.4mm is obtained. • Second, the steel sample is normalized to eliminate differences in microstructure due to previous forging, and • Then it is austenitised. This is usually at a temperature of 800 to 900°C. • Next, the specimen is rapidly transferred to the test machine, where it is held vertically and • Sprayed with a controlled flow of water onto one end of the sample. This cools the specimen from one end, simulating the effect of quenching a larger steel component in water. Because the cooling rate decreases as one moves further from the quenched end, you can measure the effects of a wide range of cooling rates from vary rapid at the quenched end to air cooled at the far end.
  • 22. 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 22
  • 24. Details of Jominy Test for Hardenability All dimensions are in inches
  • 25. Jominy End Quench Test • After end quenching, longitudinal Flat Surfaces are ground on opposite sides of the test piece as per dimensions. The specimen is ground flat along its length to a depth of .38mm (15 thousandths of an inch) to remove decarburized material. This grinding is very important for correct positioning of the sample in the fixture and also for accurate repeatable and reliable test results.
  • 26. Hardenability of Steels • Jominy end quench test to measure hardenability. 1” specimen (heated to  phase field) 24°C water flat ground 4” Fig. 14.5 26
  • 27. Jominy End Quench Test •The hardness is measured at intervals along its length beginning at the quenched end. Hardness at equal intervals (1 mm or 1/16”) to be checked and noted.
  • 28. Plotting of Result Plot the resulting data on graph paper with hardness value as ordinate (Y axis) and distance from the quenched end as abscissa (X axis).
  • 30. Heat Treatment of Steels: Hardenability The cooling rate 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. 30
  • 31. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 12.23 The hardenability curves for several steels.
  • 32. Hardenability Curve • Because the cooling rate decreases as one moves further from the quenched end, we can measure the effects of a wide range of cooling rates from vary rapid at the quenched end to air cooled at the far end. • By comparing the curves resulting from end quench tests of different grades of steels, their relative hardenability can be established. Thus the flatter the curve, the greater the hardenability.
  • 34. Cooling Curves and Phases at different Jominy Distances • A correlation may be drawn between position along the Jominy specimen and continuous cooling transformations. • For example, 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. 34
  • 35. Cooling curves from Jominy Distances
  • 36. Cooling Curves and Phases at different Jominy Distances
  • 37. Determination of Hardenability from Jominy Test Graph • After plotting the Jominy distance Vs Hardness curve, the Jominy distance having hardness equal to 50 % martensite is determined. • Then the diameter of a rod having cooling rate similar to the cooling rate at the Jominy distance having 50 % martensite is determined from the graph corelating the Jominy distance with the diameter of the rod having similar cooling rate for water quenching . • This diameter gives the hardenability of the steel in water quenching (having H value equal to 1). • Hardenability in any other quenchant can be determined from the same graph. • Di (hardenability in ideal quenching medium) can also be determined in a similar manner. • We can determine hardenability for any other amount of martensite in the core in any quenchant in a similar way.
  • 38. Grossman chart used to determine the hardenability of a steel bar For Jominy distance 4, the hardenability in water quenching is 1.1 Inch.
  • 40. 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 40
  • 41. Coefficient of severity of quench: H • • • Cooling capacities (Severity of quench) of quenching medium is known as H value. H values of some of the quenchants are given below. Cooling rates are at the center of a 2.5 cm bar. H Value – – – – – – – – – Ideal Quench Agitated brine Brine (No agitation) Agitated Water Still water Agitated Oil Still oil Cold gas Still air ∞ 5 2 4 1 1 0.25 0.1 0.02 Cooling Rate (0C/s) ∞ 230 90 190 45 45 18 - 41
  • 42. Effect of Agitation on Coefficient of severity of quench: H Agitation Violent Strong Good Moderate Mild None Cooling Medium Oil Water Brine 0.8-1.1 4.0 5.0 0.5-0.8 1.6-2.0 0.4-0.5 1.4-1.5 0.35-0.40 1.2-1.3 0.30-0.35 1.0-1.1 2.0-2.2 0.25 - 0.30 0.9-1.0 2.0
  • 43. Ideal Quenchant • Ideal quenchant is one which brings down the surface temperature to room temperature instantaneously and keeps it at that temperature thereafter. 43
  • 44. Grossman chart can be used to determine the hardenability of a steel bar for different quenchants. If a steel is having 1.1” hardenability, it will have 1.6” hardenability in a quenchant with H value equal to 5. Similarly, it will have 0.4” and 0.9” hardenability in quenchants with H value 0.2 and 0.5 respectively. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
  • 45. Effect of DI and “H” on D
  • 46. Factors affecting Hardenability • Slowing the phase transformation of austenite to ferrite and pearlite increases the hardenability of steels. • The most important variables which influence hardenability are – 1. Austenite grain size – 2. Carbon content – 3. Alloying elements 46
  • 47. Austenitic Grain Size • The hardenability increases with increasing austenite grain size, because the grain boundary area which act as nucleating site is decreasing. • This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased.
  • 48. Effect of austenite grain size on Hardenability • The more γ-grain boundary surface the easier it is for pearlite to form rather than martensite • Smaller γ-grain size → lower hardenability • Larger γ-grain size → higher hardenability
  • 49. Effect of Austenitic Grain size
  • 50. Carbon Content • Carbon is primarily a hardening agent in steel. • It also increases hardenability by slowing the formation of pearlite and ferrite. • But its use at higher levels is limited, because of the lack of toughness which results in greater difficulties in fabrication and, most important, increased probability of distortion and cracking during heat treatment and welding.
  • 51. Carbon and Hardenability Hardenability of a steel increases with increase in C content TTT diagram moves to the right.
  • 52. Effect of Austenitic Grain size and Carbon Content on Di
  • 53. Effect of Alloying Elements • most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability. However, quantitative assessment of these effects is needed. • Chromium, Molybdenum, Manganese, Silicon, Nickel and Vanadium all effect the hardenability of steels in this manner. Chromium, Molybdenum and Manganese being used most often. • Boron can be an effective alloy for improving hardenability at levels as low as .0005%. – Boron is most effective in steels of 0.25% Carbon or less. – Boron combines readily with both Nitrogen and Oxygen and in so doing its effect on hardenability is sacrificed. – Therefore Boron must remain in solution in order to be affective. – Aluminum and Titanium are commonly added as "gettering" agents to react with the Oxygen and Nitrogen in preference to the Boron.
  • 54. Effect of Alloying Elements • The most economical way of increasing the hardenability of plain carbon steel is to increase the manganese content, from 0.60 wt% to 1.40 wt%, giving a substantial improvement in hardenability. • Chromium and molybdenum are also very effective, and amongst the cheaper alloying additions per unit of increased hardenabilily. • Boron has a particularly large effect when it’s added to fully deoxidized low carbon steel, even in concentrations of the order of 0.001%, and would be more widely used if its distribution in steel could be more easily controlled.
  • 55. Effect of Alloying Elements • Hardenability of a steel increases with addition of alloying elements such as Cr, V, Mo, Ni, W  TTT diagram moves to the right. temperature Cr, Mo, W, Ni time
  • 56. Jominy hardenability curves: Hardenability improves with increasing Mo content
  • 58. Effect of Alloying Elements • all steels have 0.4wt% C, but with different alloying elements. – At the quenched end all alloys have the same hardness, which is a function of carbon content only. – 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. – 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.
  • 59. Effect of Alloying Elements Hardness of 42 at center is obtained in bars of different diameters in different steels indicating different hardenabilities.
  • 60. Effect of Alloying Elements Hardness at center of a 3 inch bar is different for different steels indicating different amounts of martensite at the center
  • 61. Hardenability Multiplying Factor • The Hardenability Multiplying Factor shows the rate at which the hardening depth is increased with the percentage of the alloying element • The ideal diameter (DI ) is calculated from: DI = DIC * ƒMn *ƒSi *ƒNi*ƒCr *ƒMo Where DIC is the basic DI factor for carbon and ƒx is the multiplying factor for the alloying element x.
  • 62. Multiplying Factors For The Calculation Of Ideal Diameter Base ideal diameter, DIjominy Carbon grain size % No. 6 No. 7 No.8 Mn 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 0.0814 0.1153 0.1413 0.1623 0.1820 0.1991 0.2154 0.2300 0.2440 0.2580 0.2730 0.284 0.295 0.306 0.316 0.326 0.336 0.346 - 0.0750 0.1065 0.1315 0.1509 0.1678 0.1849 0.2000 0.2130 0.2259 0.2380 0.2510 0.262 0.273 0.283 0.293 0.303 0.312 0.321 - 0.0697 0.0995 0.1212 0.1400 0.1560 0.1700 0.1842 0.1976 0.2090 0.2200 0.2310 0.2410 0.2551 0.260 0.270 0.278 0.287 0.296 - 1.167 1.333 1.500 1.667 1.833 2.000 2.167 2.333 2.500 2.667 2.833 3.000 3.167 3.333 3.500 3.667 3.833 4.000 4.167 4.333 Alloying factor, fX Si Ni 1.035 1.070 1.105 1.140 1.175 1.210 1.245 1.280 1.315 1.350 1.385 1.420 1.455 1.490 1.525 1.560 1.595 1.630 1.665 1.700 1.018 1.036 1.055 1.073 1.091 1.109 1.128 1.246 1.164 1.182 1.201 1.219 1.237 1.255 1.273 1.291 1.309 1.321 1.345 1.364 Cr Mo 1.1080 1.2160 1.3240 1.4320 1.5400 1.6480 1.7560 1.8640 1.9720 2.0800 2.1880 2.2960 2.4040 2.5120 2.6200 2.7280 2.8360 2.9440 3.0520 3.1600 1.15 1.30 1.45 1.60 1.75 1.90 2.05 2.20 2.35 2.50 2.65 2.80 2.95 3.10 3.25 3.40 3.55 3.70 -
  • 64. Exceptions • S - reduces hardenability because of formation of MnSand takes Mn out of solution as MnS • Ti - reduces hardenability because it reacts with C to form TiC and takes C out of solution; TiC is very stable and does not easily dissolve • Co - reduces hardenability because it increases the rate of nucleation and growth of pearlite
  • 65. Hardenability Band • The industrial products of steels may change composition and average grain size from batch to batch, therefore, 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.
  • 66. Hardenability Band • Hardenabilily data now exists for a wide range of steels in the form of maximum and minimum end-quench hardenability curves, usually referred to as hardenability bands. This data is, available for very many of the steels listed in specifications such as those of the American Society of Automotive Engineers (SAE), the American Iron and Steel Institute (AISI) and the British Standards.
  • 67. Hardenability Band 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. 67
  • 68. Effects of composition variation and grain size change on the hardenability of alloy steels
  • 69. Hardenability (as Range of Di Values) of Various Steels
  • 70. Example • Calculate the approximate hardenability of an 8630 (0.3%C, 0.3%Si, 0.7%Mn, 0.5%Cr, 0.6%Ni, 0.2%Mo) alloy steel with an ASTM grain size of 7
  • 71. Solution • Find out base DI for 0.3% carbon • Calculate multiplying factors for each element • Ideal critical diameter found by multiplying base diameter by the multiplying factors
  • 72. Summery • The hardenability of ferrous alloys, i.e. steels, is a function of the carbon content and other alloying elements and the grain size of the austenite. • The relative importance of the various alloying elements is calculated by finding the equivalent carbon content of the material. • The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats. • Substances like brine and water cool much more quickly than oil or air. • Additionally, if the fluid is agitated cooling occurs even more quickly. • The geometry of the part also affects the cooling rate: of two samples of equal volume, the one with higher surface area will cool faster.
  • 73. Numerical problem -1 • Predict the center hardness in a water quenched 3” bar of 8640
  • 74. Solution to Numerical Problem - 1 The cooling rate at the center of a 3” dia bar in water quenching will be same as that at Jominy distance 17 mm. Jominy Distance =17mm Water Quenched Oil Quenched
  • 75. Effect of Alloying Elements Hardness produced at Jominy distance 17 mm in 8640 steel will be 43 HRC. Therefore, the hardness at center of a 3 inch bar will be 43 HRC
  • 76. Cooling rate and Jominy distance do not change with alloying elements as the rate of heat transfer is nearly independent of composition
  • 77. Equivalent bar diameter when quenched • When the end-quench hardness curve of a steel has been found, this table enables the user to estimate the hardnesses that would be obtained at the centers of quenched round bars of different diameters, when that same steel is quenched with various severities of quench. For each successive 1/16 in. position, the hardness obtained in the endquench test would be found at the center of the bar size.
  • 78. Equivalent bar diameter when quenched
  • 79. Cooling Rate at Each Jominy Position for Room Temperature Water Distance from water quenched end 1/16 in. 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 Cooling Rate 0C/s 270 170 110 70 43 31 23 18 14 11.9 9.1 6.9 5.6 4.6 3.9
  • 80. Jominy test and CCT diagrams
  • 81. 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.
  • 82. 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 82
  • 83. Correlation of carbon and martensite content with Rockwell hardness
  • 84. Alloy Factors For The Calculation Of Ideal Diameter
  • 85. Interconversion of ideal bar diameter as a function of shape