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NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT
Why Database for Cooling Capacity of Various Quenchants Should
be Developed?
NIKOLAI KOBASKO
IQ Technologies Inc, Akron, USA and Intensive Technologies Ltd, Kyiv, Ukraine
[email protected], www.intensivequench.com
Abstract: - In the paper, a necessity of the development of database for cooling capacity of quenchants is
discussed. In spite of the existence of several methods for solving an inverse heat conduction problem,
appropriate experimental techniques and funding, there is no database containing cooling characteristics
of quenchants needed for quench process computer simulations. Two reasons explain this situation. The
heat treating industry uses mostly oils for quenching alloy steels for which it is not so critical to have the
above database. A standard cylindrical Inconel 600 probe (12.5 mm dia) with the thermocouple installed
at the core cannot be used for evaluating real heat transfer coefficients (HTC). This probe can be used for
determining only effective HTC. Taking into account environmental issues, it is desirable to substitute,
when it is possible, oil quenching with intensive water quenching. Development of the proper database of
heat transfer characteristics for water based quenchants will accelerate this process.
Key - Words: - Real and effective heat transfer coefficients, Database, Standard probe, Oil, Water as a
quenchant, Environment.
1 Introduction
As known, alloy and high alloy steels are usually
quenched in oils. Often a gas quenching is
applied to decrease cooling rates within the
martensite range. When quenching in oils, no
trouble occurs because steel parts can be cooled
to the room temperature without the interruption
of the quench. There is no need to interrupt
cooling when approaching the martensite start
temperature since convective heat transfer
coefficient of oil is rather low (250 - 300
W/m2K). To control and to maintain stable the
cooling capacity of oils, a standard probe of 12.5
mm diameter, made of Inconell 600 with a
thermocouple in the core, was designed [1] .
Taking into account environmental issues, it is
desirable to substitute, when it is possible, oil
quenching with intensive water quenching.
Development of the proper database of heat
transfer characteristics for plain water and water
solutions will accelerate this process.
instead of difference ∆T = TW − Tm as it is
considered during a convection mode of heat
transfer , where: TW is wall temperature; TS is
saturation temperature; Tm is temperature of a
quenchant. As well known, the formation of
nucleating centers depends on the overheat of
the boundary layer which is determined by [2]:
Rcr ≅
(1)
where: Rcr is a critical size of a bubble which
is capable to grow and function; σ is a surface
tension (N/m); r * is a latent heat of evaporation
(J/kg); ρ ′′ is a vapor density (kg /
m3); ∆T = TW − TS is a wall overheat.
Active nucleating centers are the basic
carriers of the thermal energy that remove heat
from the part surface and transfer it to a cooler
bath. After the initiation of boiling, the bubble
continues to grow (in a saturated liquid) until
forces cause it to detach from the surface. After
the departure, a cooler liquid from the bulk of
the quench bath fills the space vacated by the
bubble and the thermal layer is reformed. When
2 Real and effective heat transfer
coefficients
A real heat transfer coefficient during nucleate
boiling relates to the difference ∆T = TW − TS ,
ISSN: 1792-4596
2σTS
,
r ρ ′′∆T
*
304
ISBN: 978-960-474-215-8
NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT
the required superheat is attained, a new bubble
starts to form at the same nucleation site.
Bubble dynamics include the processes of
growth, bubble departure, and bubble release
frequency which includes time for reformation
of the thermal layer. The bubble acts like a
pump to remove the hot liquid from the surface
and replacing it with the cooler liquid [2]. This
mechanism is the essential factor causing high
intensity of heat transfer during boiling. The
bath temperature has no essential effect on a
value of heat transfer coefficient during nucleate
boiling [2]. Therefore, when determining the
heat flux density during boiling it is necessary to
relate it to a difference of TW − TS , rather than
(63.5)
0.02
0.04
0.07
0.12
0.19
0.84
0.02
0.04
0.08
0.13
0.21
0.56
0.02
0.04
0.07
0.09
0.14
0.59
To see what is happening during nucleate
boiling, let's consider accurate experimental data
of French (see Fig. 1 and Table 1), which were
used for solving inverse heat conduction
problem (IP). For this purpose, the IQLab
software was applied.
This software was
developed by Intensive Technologies Ltd
company (Kyiv, Ukraine) [4]. To solve the IP,
thermal properties of materials are needed. Some
of them are provided in Table 2.
to TW − Tm , which can lead to large errors when
calculating the part surface temperature.
Table 2 Thermal conductivity of silver, Inconel
600, and AISI 304 steel in W/(m K) depending
on temperature (oC)
T, oC
100
300
500
700
900
Silver
392
362
366
373
Inconel 14.2 17.8 21.7 25.9
600
Steel
17.5 19.6 23
26.3 29.3
304
Fig. 1 The schematic which shows how
thermocouples were placed and accurately
flattened to the wall of spheres and polished by
French [3].
Table 1 Time required for the surface of steel
spheres to cool to different temperatures when
quenched from 875oC (1605oF) in 5% NaOH
water solution at 20 oC and moving at 3 feet per
second (0.914 m/s), according to French [3]
Size,
inch,.
(mm)
Time, s
700oC
600
500
400
300
150
1”
0.03
0.04
0.05
0.06
0.08
0.40
(25.4)
0.05
0.05
0.08
0.08
0.11
1.20
0.03
0.04
0.05
0.06
0.14
0.71
0.02
0.02
0.05
0.09
0.19
0.99
0.03
0.04
0.06
0.07
0.13
0.82
2.5”
0.03
0.04
0.06
0.07
0.08
0.65
(63.5)
0.02
0.03
0.04
0.05
0.07
0.80
0.03
0.04
0.07
0.10
0.15
0.52
ISSN: 1792-4596
Fig. 2 Real heat transfer coefficients during
quenching of spheres made of steel and
quenched in agitated ( 0.914 m/s ) 5% NaOH
water solution at 20oC: a) for sphere 38.1 mm
dia; b) for sphere 63.5 mm dia
305
ISBN: 978-960-474-215-8
NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT
The main difference is in heat transfer
evaluation is.
The real heat transfer coefficient is calculated as
α nb =
q
.
TW − TS
(2)
The effective heat transfer coefficient during
boiling is calculated as
α eff =
(3)
In practice, quenchants are tested using
standard probes, for example, silver sphere
probes or cylindrical probes made of Inconel
600. The thermocouples, as it was mentioned,
are located at the core. It is considered that
temperature field throughout the probe cross
section for silver is uniform, i.e., Bi ≤ 0 .2 .
This assumption was confirmed by the
calculation of the heat transfer coefficient from
Eq. (3), where q is a heat flux density on the
Fig. 3 Real heat transfer coefficients during
quenching of plate (14 mm) made of steel and
quenched in agitated plain water (0.914 m/s) at
20oC
As we can see from Fig. 2 and Fig. 3, the real
heat transfer coefficients during nucleate boiling
reach maximum values of 200,000 to 240,000
W/(m2K) in 2 s after the beginning of the quench
regardless of the part size and configuration.
These results were obtained for steel specimens.
As known, the thermal conductivity of steel is
16 times less as compared with silver. It means
that the calculated values of the heat transfer
coefficient during cooling of silver probes in
water and water solutions will be higher. This is
because according to the Fourier law, the heat
flux is directly proportional to the material
thermal conductivity. Assume that heat transfer
coefficients during cooling of silver spherical
probes (20 mm in dia) are the same, then we can
calculate Biot number, which corresponds to the
maximum value shown in Fig. 2 and Fig. 3, i.e.:
Bi =
probe surface; TW is the wall temperature; Tm
is the temperature of the quenchant. This is a
generally accepted approach for the film and
nucleate boiling heat transfer evaluation.
However, film and nucleate boiling heat transfer
coefficients should be determined from Eq. (2).
It means that, in reality, heat transfer
coefficients during nucleate boiling are much
higher.
Let us illustrate the above mentioned
phenomenon by the following example. Let
TW =110°С; Ts = 100°С; Tm =20°С. If α is
calculated by equation (3) then:
q
q
α1 =
= o ;
o
o
110 C − 20 C 90 C
If the calculation is performed using equation
(2) then:
q
q
α2 =
= o .
o
o
110 C − 100 C 10 C
Now consider by how many times α 2 differs
240 ,000W / m 2 K × 0.01m
= 6.6 .
366W / mK
During testing of silver probes, it is accepted by
investigators that the probe core temperature and
surface temperature are equal. It is almost true
when Bi < 0.2. However, Bi > 6.6 and it means
that the probe core temperature and the probe
surface temperature are not equal. According to
the experimental and calculated data this
difference is up to 100oC and the error can
exceed 100% because Biot number is 33 times
higher. This problem was widely discussed in
book [5].
ISSN: 1792-4596
q
.
TW − Tm
from α1 by dividing one value by the other:
α2
q
q
= o : o = 9,
α 1 10 C 90 C
i.e., α 2 is 9 times greater than α1 , which means
that, using silver probes during testing,
subsequent
miscalculations
will appear.
Therefore, it seems like Biot number Bi ≤ 0 .2 .
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ISBN: 978-960-474-215-8
NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT
established function is the same (idem).
According to the results shown in Fig. 5, it is
However, actually it is greater than
approximately 6.6, that is why the sphere core
temperature differs from the sphere surface
temperature.
K n = idem
possible to say that
(4).
It means that heat transfer coefficients for
boiling processes can be generalized in forms
(4). This simplifies significantly cooling time
calculations during
the transient nucleate
boiling process. Fig. 6 explains why it happens.
With increasing of the size of the cylinder the
heat flux decreases proportionally to the cylinder
size. The average values of BiV and Kn remain
the same (see Fig.5 and Fig. 6). The main
conclusion from the above calculations is that
the standard Inconel 600 probe can be used for
effective HTC calculations. However, one
should keep in mind that such data can be used
for cooling rate calculations at the core of steel
parts and cannot be used for temperature fields
calculations.
Fig. 4 Heat flux density and Kondratjev number
Kn vs. time for 1 inch (25.4 mm) cylindrical
steel probe quenched in 5% NaOH solution
Fig. 6 Temperature field distribution in
cylindrical probes 20mm and 40 mm dia. during
quenching in 5% NaOH solution at 20 oC [6]
Fig. 5 Kondratjev number (Kn) vs. Fourier
number (Fo) for cylindrical steel probes 20, 30,
and 40 mm dia. and quenched in 5% NaOH
solution [6]
3 Critical heat flux densities and
their impact on understanding the
boiling processes
It is important to keep in mind that upon the
immersion of a steel part into the quenchant, the
initial heat flux density q can be:
The software IQLab was used also for
calculations
of the effective heat transfer
coefficients, generalized Biot numbers and
Kondratjev numbers Kn. Fig. 4 shows that after
2 seconds of cooling, the Kondratjev number is a
linear function of time. In coordinates Kn vs. Fo
for different diameters of cylinders, the
ISSN: 1792-4596
q >> qcr1 ; q ≈ q cr1 or q << qcr1 .
307
ISBN: 978-960-474-215-8
NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT
4 Discussion
In the first case, q >> q cr1 , a full film
boiling is observed. A transition boiling is
observed when q ≈ q cr1 .
In the last case,
There are many institutes and universities which
investigate cooling capacity of quenchants and
use noise control systems to evaluate boiling
processes. Unfortunately, there is no database
designed for cooling capacity of quenchants
which can be used by engineers for the cooling
recipes development. We have only cooling
curves at the core of standard probes. These
data cannot be used for solving the inverse heat
conduction problem for getting real heat transfer
coefficients. Using these curves, it is possible to
calculate only average effective heat transfer
coefficients
suitable for
cooling
time
calculations at the core and not at the surface.
At present time, it is possible to design
DATABASE for cooling capacity of quenchants
since everything is prepared for this purpose:
1. Liscic probe is available, which should be
standardized since it is very suitable for solving
the inverse heat conduction problem and for
providing very accurate data [8].
2. Small silver probes and the method for
calculations of critical heat flux densities are
available, which should be standardized for
critical heat flux densities evaluation [9].
3. The noise control system and method for
processing of data obtained are available which
should be standardized for transient boiling
processes investigations [10].
Also an international team is organized
to design the above DATABASE (see
www.worldses.org/projects/Heat_and_Mass_Tra
nsfer.doc)
If such DATABASE is designed, it will be
applied for solving important problem [11-15]
and for accurate computer simulations of
technological processes [16, 17].
Switching from oils to plain water as a
quenchant will make environment cleaner. The
database, if designed, will accelerate this
process.
q << qcr1 , film boiling is absent and the main
mode of heat transfer is nucleate boiling. Each
of these three cases will produce different values
of
α = f (Tsf ) versus the part surface
temperature. Therefore, there is no unique
interrelationship of the heat transfer coefficient
α as a function of the surface temperature.
To predict what kind of heat transfer mode one
can expect, we must know critical heat flux
densities.
When evaluating critical heat flux densities,
only film boiling heat transfer coefficients are
used, which are small enough (300 - 1000
W/m2K). In this case, the condition Bi < 0.2 will
be always satisfied. It means that the surafce
temperature TW during testing will be equal to
the core temperature Tcore of the probe only at
the end of film boiling. The probe presented in
Fig. 7 can be used for the critical heat flux
evaluation [7]. Rounded ends of cylinder will
provide the second type of heat transfer mode
and accurate estimations of critical heat flux
densities.
5 Summary
1. There are two approaches in HTC
evaluation. The first approach provides real
heat transfer coefficients, which can be
used to calculate temperature fields and
residual stress distribution. The second
approach provides effective heat transfer
Fig. 7 Shape and dimensions of the silver
cylindrical probe with rounded ends that can be
recommended for evaluations of critical heat
flux densities [7].
ISSN: 1792-4596
308
ISBN: 978-960-474-215-8
NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT
[8] B.Liscic, H.M.Tensi, and W.Luty, Theory
and Technology of Quenching, SpringerVerlag, Berlin, 1992, 484 p.
[9] N.I.Kobasko,
Quenching
Media,
Metallovedenie i Termicheskaya Obrabotka,
Moscow, VINITI, 1989, p 127-166.
[10] A.A.Moskalenko,
N.I.Kobasko,
L.M.Protsenko,
O.V.Rasumtseva,
Acoustical System Analyzes the Cooling
Characteristics of Water and Water Salt
Solutions, Proceedings of the 7th IASME /
WSEAS International Conference on HEAT
TRANSFER, THERMAL ENGINEERING
and ENVIRONMENT (HTE '09), Moscow,
Aug. 20 -22, 2009, pp. 117 - 122.
coefficients, which can be used to calculate
the part core cooling rate. It cannot be used
to calculate correctly the temperature field
in steel parts during nucleate boiling
process.
2. The Inconel 600 cylindrical probe is used
to control stability of cooling capacity of
quenchants. It can be also used to obtain
effective heat transfer coefficients.
3. There is a need to create DATABASE
which includes critical heat flux densities,
real heat transfer coefficients, and initial
heat flux densities during immersion of
steel parts into the quench bath. If
designed, the DATABASE will accelerate
in many cases switching from oil to plain
water as a quenchant and will make
environment cleaner.
[11] N.I.Kobasko,
6,364,974 B1
Patent
No.
[12]
N.I.Kobasko, Steel superstrengthening
phenomenon,
Journal
of
ASTM
International, Vol. 2, Issue 1, 2005, paper
ID JAI12824, available online at
www.astm.org
[13]
N.I. Kobasko, W.S. Morhuniuk, B.K.
Ushakov, Design of Steel-Intensive Quench
Processes, Steel Heat Treatment: Equipment
and Process Design, (George E. Totten,
Ed.), CRC Press, New York, 2007, pp. 193
- 237, www.crcpress.com
[14] N.I.Kobasko, Intensive Steel Quenching
Methods,
Quenching
Theory
and
Technology, Second Edition, B.Liscic,
H.M.Tensi,
L.C.F.Canale,
G.E.Totten
(Eds.), CRC Press, New York, 2010
[15]
N.I. Kobasko, W.S. Morhuniuk, B.K.
Ushakov, Design of Steel-Intensive Quench
Processes, Steel Heat Treatment: Equipment
and Process Design, (George E. Totten,
Ed.), CRC Press, New York, 2007, pp. 193
- 237, www.crcpress.com
B.L.Ferguson,
[16] A.M.Freborg,
M.A.Aronov, N.I.Kobasko, J.A.Powell,
Intensive quenching theory and application
for imparting high residual surface
compressive stresses in pressure vessel
components, SAE Journal of Pressure
Vessels Technology, Vol.125, May 2003, pp.
188 - 194.
[17]
B.L. Ferguson, Z. Li and A.M. Freborg,
Modeling Heat Treatment of Steel Parts,
Computational Materials Science, Vol. 34,
Issue 3, 2005, pp. 274 - 281.
References:
[1] G.E.Totten, C.E.Bates, and N.A.Clinton,
Handbook of Quenchants and Quenching
Technology, ASM International, Materials
Park, 1993, 507 p.
[2]
V.I.Tolubinsky, Heat Transfer at Boiling,
Kyiv, Naukova Dumka, 1980, 316 p.
[3] H.J. French, The Quenching of steels,
American Society of Heat Treat., 1930.
[4] V.V.Dobryvechir, N.I.Kobasko, E.N.Zotov,
W.S.Morhuniuk, Yu.S.Sergeyev, Software
IQLab (commercially available from
Intensive Technologies Ltd., Kyiv, Ukraine,
[email protected], www.itl.kiev.ua)
[5] N.I.Kobasko, Steel Quenching in Liquid
Media Under Pressure, Naukova Dumka,
Kyiv, 1980, 206 p.
[6] N.I.Kobasko, Transient Nucleate Boiling as
a Law of Nature and a Basis for Designing
of IQ Technologies, Proceedings of the 7th
IASME / WSEAS International Conference
on
HEAT
TRANSFER,
THERMAL
ENGINEERING and ENVIRONMENT (HTE
'09), Aug. 20 - 22, Moscow, Russia, 2009,
pp. 67 - 75.
[7] M. Narazaki, S. Fuchizawa, M. Kogawara,
and M. Inaba, Effects of surface oxidation
on cooling characteristics during quenching
of heated metals in subcooled water, Tetsu to - Nagane, (J. Iron Steel Inst. Jpn.), Vol.
79, No 5, 1993, pp. 583 - 589.
ISSN: 1792-4596
US
309
ISBN: 978-960-474-215-8