Contemporary Intensive Methods of Steel Hardening in Cold Fluids
NIKOLAI I. KOBASKO
Intensive Technologies Ltd,
Kyiv,
UKRAINE
Abstract: - In the paper, the new intensive quenching technologies are discussed which are based on controlling
the self-regulated thermal process (SRTP) which exists for a long time if any film boiling is absent. It is rather
intensive until convection starts. Despite the intense process (Kn > 0.8), when the heat transfer coefficient and
Biot number tend to infinity, there are several ways of controlling the surface temperature of steel components
by varying the boiling temperature of the fluid. To eliminate any film boiling process and provide SRTP, the
author of the paper explores the resonance effect, a thin surface insulating layer that covers the surface of
machine components and electrical negative forces to control the double electrical layer that is responsible for
destroying the film boiling mode. Based on SRTP control it is possible to delay the transformation austenite
into martensite or even accelerate these transformations. The most important are possibilities to control surface
temperature during the boiling process. All of this opens great opportunities for increasing significantly service
life of machine components and tools. In the paper also the simplified method of cooling time calculation is
proposed. It is based on the new principles concerning pure transient nucleate boiling taking place during the
hardening steel in cold fluids. Since the paper simply explains everything, results of investigations will be
widely used in the heat-treating industry.
Key-Words: - Film boiling absence, Surface temperature control, Intense quenching, New technologies, Service
life, Environment.
Received: April 23, 2023. Revised: November 14, 2023. Accepted: January 12, 2024. Published: March 28, 2024.
1 Introduction
Due to several discoveries made in the field of
material science and in the field of heat and mass
transfer, it was possible to write this paper. In 1964
in Ukraine was experimentally discovered the bell
- shaped curve, which was unexpected for material
scientists because it contradicted the existing theory
concerning the effect of cooling rate on the crack
formation and strengthening of materials. Recently
(2023), three principles of heat transfer taking place
during the transient nucleate boiling process, were
published by London Press, [1]. These discoveries
open great opportunities for intensive quenching
technologies design and for providing tools for
accurate recipe development. The paper discusses
three main directions of contemporary hardening
processes that provide great benefits and improve
essentially environmental condition. As a rule, the
strengthening of machine components is performed
by quenching them in cold fluids which include oils,
polymers for slow cooling, water and water salt
solutions, jets, or water flow for intensive
quenching. The last procedures require special
installations like powerful pumps, and rotating
propellers used for intense agitation of fluid. The
paper considers alternative intensive quenching
which is based on the fact that intensive quenching
can be performed in still water if any film boiling
during quenching is completely absent, [1].
Therefore, the main task is increasing the first
critical heat flux density responsible for the
developed film boiling, [2], [3]. Also, the deleting
of the film boiling process can be performed by
creating a thin insulating surface layer during
quenching machine components in low
concentrations of inverse solubility polymers, [4].
This thin surface layer decreases initial heat flux
which drops below the first critical value and by
this way eliminates any film boiling process. In the
last decades some new unexpected phenomena
were discovered according to which crisis of
boiling can be governed by electrical forces that are
more effective in terms of film boiling elimination,
[5]. More effectively film boiling can be eliminated
by using the resonance effect that destroys any film
boiling during quenching machine components in
cold fluids. Thus, the plan of discussion includes:
1. Explaining why during quenching steel
from high temperatures in cold fluid film
boiling in many cases is absent.
2. Providing experimental data of different
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authors to show readers that during
quenching in low agitated fluids any film
boiling can be completely absent.
3. Discussion of new less costly installations
for performing uniform and intensive
quenching to obtain super - strengthened
material, radically decrease the cost of
technological procedure, and increase
essentially service life of machine
components.
These problems are discussed below.
2 Absence of Film Boiling Process
during Quenching in Cold Fluids
It was widely and firmly disseminated opinion
among the leading experts that during quenching
from high temperatures of metal components in
cold fluids always three stages exist: developed
film boiling, transient nucleate boiling, and
convection. This opinion is based on well-known
law of Fourier. According to the conventional law
of Fourier, the heat flux is calculated using
equation (1):
Sd
x
T
q
(1)
Since, during immersion of heated steel
components in cold fluid at the very beginning of
cooling, the temperature gradient tends to infinity,
initial heat flux density tends to infinity too
significantly prevailing the first critical heat flux
density. That means immediate appearance of a
developed film boiling mode. Engineers from the
heat-treating industry often obtained evidences on
developed film boiling existence. However, in 1930
French performed accurate experiments with
spherical steel samples of different sizes which
were quenched from 875oC in water solution of
NaOH at 20oC where any film boiling process was
completely absent, [6]. It was shown by French that
surface temperature during quenching for all
spherical samples (6 mm and 120 mm) drops from
875oC to 200oC for 0.29 s. Within such a short time
there is no possibility for vapor bubble growth and
developing film boiling process. Later similar
results were reported by different authors who
quenched cylindrical samples in water salt
solutions of optimal concentration and didn’t
observe any film boiling process during cooling
from 850oC. It looks extraordinary because film
boiling during cooling from high temperatures in
fluids should be present. Such strange behavior can
be explained by considering modified law of
Fourier 2, [7]:
x
rx q
x
T
q
(2)
According to equation (2), the initial heat flux
density is a finite value that can drop below the first
critical heat flux density that provides the absence
of the film boiling process. For accurate evaluation
of initial heat flux density, scientists solve the
hyperbolic heat conductivity equation generated by
the modified law of Fourier which was considered
by author, [7].
As well known, the amount of thermal energy can
be calculated as:
SdxdTcdVdTcq
(3)
Using the energy conservation law, one can get
the following energy balance equation from
formulas (1) and (3):
T
xSd c SdxdT
(4)
Equation (4) can be rewritten as:
T
c
x
qx
(5)
Substituting Eq. (2) into Eq. (5) leads to:
T
c
x
q
x
Tx
r
2
2
2
(6)
The value
r
is called a relaxation time. It is a
characteristic of the free electrons movement, and it
is a constant which depends on the nature of the
material.
By differentiating Eq. (2) by
, one can get
the following:
2
2
2
T
c
x
qx
(7)
It means that Eq. (6) can be rewritten as
hyperbolic heat conductivity equation presented in
[7]. This hyperbolic heat conductivity equation
along with the boundary condition (8) representing
the transient nucleate boiling process, can be used
for accurate evaluation of initial heat flux densities
to be compared with the first critical heat flux
density.
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T
rT T
m
s
m
r R
0
(8)
The solving of the hyperbolic heat conductivity
equation (6) with the appropriate boundary and
initial conditions is a rather complicated task.
Therefore, mathematicians proceed to develop new
methods of solving them, [8].
Thus, the developed film boiling process
during the quenching of steel components from
high temperatures in cold fluids can be
completely absent if the initial heat flux density
q
in
is below the first critical value qcr1, i.e.
q q
in cr
1
.
3 Intense Quench Process in Cold
Fluids when any Film Boiling is
Absent
The above consideration of the initial process of
quenching was needed to explain absence of film
boiling when hardening metal in cold fluids. Now,
it is very important to show that transient nucleate
boiling is an intensive heat transfer mode if any
film boiling is completely absent. For this purpose,
it is enough to consider the cooling process using
regular condition theory, [9], [10] which
manipulates with Kondrat’ev number Kn
(
0 1
Kn
). According to authors, [11], cooling
is intensive if
Kn
08.
. Taking this fact into
account, real and effective numbers of Kn were
evaluated on the base of accurate experiments
(Figure 1).
As seen from Table 1 and Figure 2, the real
average Kondrat’ev numbers Kn for both sizes 12.5
and 50 mm exceed the value 0.8 therefore cooling
process is intensive despite small value of
convective HTC which was equal 500 W/m2K.
Table 1. Real and effective Kondrat’ev numbers Kn
and convective HTCs in still water versus the size
of the cylindrical probe
D,
mm
Type of
Kn
Kn
I
Kn
II
Average
Kn
HTC,
W/m2K
12.5
Real
0.95
0.81
0.88
20420
Effective
0.90
0.225
0.562
3956
50
Real
1.0
0.87
0.935
9660
Effective
0.96
0.3
0.63
1288
As seen from Figure 1, the cooling process is
very intensive.
a)
b)
Fig. 1: Surface and core cooling for cylindrical
probe of different diameters quenched from 850oC
in water salt solution at 20oC: a),50 mm; b), 80 mm
Real and effective Kondrat’ev numbers vs time
are presented in Figure 2. For example, for a
cylindrical probe 50 mm in diameter the real
Kondrat’ev number Knreal is equal to 0.995 which is
responsible for correct cooling time and cooling
rate calculations, [1].
a)
b)
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c)
Fig. 2: Real and effective Kondrat’ev numbers Kn
for cylindrical probe of different diameters
quenched from 850oC in water salt solution at 20oC:
a) 25 mm; b) 50 mm; c) 80 mm
Table 2 presents initial
Io
C,
and end
II o
C,
overheating temperatures during nucleate
boiling when convective heat transfer is 500
W/m2K..
Table 2. Initial
Io
C,
and end
II o
C,
overheating temperatures during the transient
nucleate boiling process.
Diameter,
mm
10
12.5
20
25
30
40
80
Io
C,
33.5
31
27
25
24
22
21
II o
C,
8.5
8.5
8.5
8.5
8.5
8.5
8.5
During the transient nucleate boiling process
surface temperature changes insignificantly (Table
2).
4 Conventional Batch Quenching
Process
The conventional batch quenching process is
performed in quench tanks equipped with rotating
propellers or pumps with agitated fluid (Figure 3).
Fig. 3: Illustration of a typical batch immersion
time quenching system. Agitation is provided by
two or more continuously variable impeller stirrers,
[12]
A typical load consisting of a large number of
steel parts is shown in Figure 4.
Fig. 4: A load consisting of a large number of steel
parts
When quenching in still water, the convective
heat transfer coefficient is calculated by equation
(9), [20], [21]:
conv
g T
a
0135
1 3
.
/
(9)
Some results of calculations are presented in
Table 3.
Table 3. Convective HTCs in W/m2K versus water
temperature and pressure in MPa
P, MPa
Water 10oC
Water 20oC
Water 30oC
0.1
548
640
1015
0.2
586
690
1105
0.3
609
719
1156
Convective heat transfer coefficient (HTC),
when quenching in water flow, is evaluated by
equation (10), [13]:
0.25
0.8 0.43 Pr
0.021Re Pr Prml
sf
Nu
(10)
CFD modeling shows that cooling in water
flow is not uniform (Figure 5), [14]. Not uniform
cooling can generate a local film boiling process
which results in essential distortion of steel
components during hardening in agitated fluids,
[14], [15].
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Fig. 5: Water flow velocities distribution during
intensive quenching of the forging, [14]
5 Contemporary Methods of Steel
Hardening
5.1 Hydrodynamics Wave Emitters
When quenching load consists of large number of
parts (especially relatively thin parts), a local film
boiling process takes place at the beginning of the
quenching process even in the IQ water tanks with
vigorous agitation of the water-salt solution. It has
been revealed by authors, [14], [15] that local film
boiling is the main reason for the excessive part
distortion. For gear products, a “double” distortion
occurs sometimes when the local film boiling takes
place between two teeth of the gear. That is why,
the author of the current paper suggested using
oscillatory waves produced by a hydrodynamic
emitter for destroying local large film bubbles by a
resonance effect in addition to the quenchant
agitation during the IQ – 2 process (Figure 6 and
Figure 7), [16].
Hydrodynamics emitters generate waves in
liquid with a frequency equal to the oscillating
frequency of local film boiling. One of them is
shown in Figure 6 where 1 is a water flow rate
provided by the pump; 2 is a tube, 3 is a circulated
water stream; 4 is a generator of waves in liquid; 5
is a regulator of the wave frequency; 6 is a water
flow combined with the generated waves and
directed to the load being quenched, [16].
Fig. 6: Emitter for generating waves with a
frequency equal to the frequences of films to
provide resonance effect, [16].
The resonance effect destroys local and
developed film boiling more effectively as
compared with fluid agitation because resonance
penetraces througtout the load. Directed water flow
water faces hydrodynamic resistance if the load is
not spread enough.
Fig. 7: Emitters arrangement in quench tank, [16]:
a is the emitter, where 1 is liquid flow, 2 is a tube,
3 is circulated liquid stream, 4 is generator waves
in liquid, 5 is regulator of wave frequency, 6 is
liquid flow combined with generated waves; b is
quench tank with located in it emitters, where 1 is
quench tank; 2 is water salt solution of optimal
concentration; 3 is fixture; 4 is steel part; 5 is local
film boiling; 6 is emitter.
For calculating a frequency that provides a
resonance effect, equation (12) can be used, [16]:
C
Hz nV
fD
. (12)
Here
Hz
f
is a resonance frequency for the film
boiling process in Hz; n is a number of restrictions
or openings on the round nozzle required for
providing a resonance wave (Figure 6);
C
V
is a
circulated water flow velocity in m/s; D is a
diameter of the nozzle in m.
Note, that each restriction or opening generates
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packages of waves. The primary one is calculated
by equation (9).
5.2 Quenching Steel Covered with a Thin
Surface Insulating Layer
The absence of any film boiling process during
quenching probes in low concentration of
water polymer solutions is explained by a
decrease of initial heat flux density qo which is
calculated by Eq. (11), [4]:
qq
R
in o
coat
1 2
(11)
lR
coat
1 2
(11a)
When
coat
WmK
02. /
and
sl
WmK
20 /
then
sl
coat
100
. When the thickness of the insulating
layer is 100
m
and 2R = 0.020 m then
R
m
m
100 10
10 10 001
6
3
.
. In this case
1 2 001 100 3.
. It means that the initial
heat flux density during the quenching of a given
example (Figure 8) can be reduced 3 times that
eliminates completely any film boiling process
since
q q
in cr
1
. More data on value
are
provided in Table 4.
Table 4. Possibility of film boiling (FB)
elimination by creation a thin insulating surface
layer during quenching in low concentration of
water polymer solutions
No
R
sl
coat
l
1
0.001
100
1.2
2
0.005
100
2
3
0.01
100
3
4
0.001
200
1.4
5
0.005
200
3
6
0.01
200
5
Poly(Alkylene Glycol) polymers (PAG) of
optimal concentrations provide ideal uniform
cooling for minimizing distortion and preventing
crack formation during hardening machine
components and tools due to their inverse solubility
which is a reason for polymeric surface layer
formation, [4].
Fig. 8: Section of steel part of height L covered
with the polymeric layer of thickness
Note that an insulating layer in many cases
prevents crack formation because existing
microcracks are plugged by viscous polymer.
5.3 Use Electrical Forces to Control the
Double Electrical Layer
The discovered phenomenon can be used for
designing new quenching technology governed by
external electrical forces (Figure 9), [5]. The
negative charge is directed to load 7 which
generates electrical forces in the double electrical
layer that compresses electrolyte to the hot metal
surface destroying the film boiling process.
Fig. 9: Schematic installation to provide intensive
cooling via exploring electrical forces: 1 is a
resistor; 2 is an electrical accumulator; 3 is a relay
to interrupt electrical force in 2 seconds; 4 is
insulation; 5 is steel part; 6 is electrolyte; 7 is local
film boiling; 8 is moving system, [5].
The proposed method requires more additional
information which can be obtained by accurate
experiments.
6 Simplified Method for Cooling
Time Calculation
The generalized equation for such a statement can
be mathematically written as (Eq. (13):
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nb F
kD
a
2
(13)
Here
nb
can be considered as a width of noise
generated by vapor bubbles which is equal to its
duration measured in seconds;
is a
dimensionless parameter depending on convective
HTC;
k
F
is a dimensionless form coefficient; D is
the thickness of steel part in m; a is thermal
diffusivity of steel in m2s-1. To calculate the full
time of cooling, one should evaluate the core
temperature at the core of the steel part at the end
of nucleate boiling which now will serve as an
initial temperature for equation (14). For such
initial temperatures (Figure 10).
Fig. 10: Core temperature at the end of self-
regulated thermal process versus size in mm of
ylinder when convective heat transfer coefficient is
750 W/ m2K (A1); 1500 2W/ m2K (A2); 3000 W/
m2 K (A3); and 4000 2W/ mK (A4)/
kBi
Bi
T T
T T
K
aKn
V
V
o m
m
2095 3867. . ln
(14)
Here
is cooling time in s; k = 1, 2, 3 for
plate, cylinder accordingly; BiV is generalizes Biot
number; To is initial temperature; Tm is medium
bath temperature; K is Kondrat’ev form factor; a
is the thermal diffusivity of steel; Kn is the
dimensionless Kondrat’ev number Kn is also used
for cooling rate v evaluation (Eq. (15):
vaKn
KT T
m
(15)
The summarized time is calculated as:
full =
nb
+
conv (16)
For more information on critical heat flux
densities evaluation, [17], [18], [19]. Intensive
quenching was used by authors, [20], for direct
quenching of forging eliminating last lasting
process of hardening in oils. It improved
environment condition and saved energy. Further
investigations connected with the control of double
electrical layer, based on discovered of new
phenomenon, and exploring resonance effect will
bring to heat-treating essential benefits. Some
useful information on convective boiling one can
find in [21].
7 Conclusions
1. If any film boiling during quenching steel in
water and water salt solution of optimal
concentration is absent, the cooling process is
intensive, and the average value of
dimensionless number Kn is within 0.8 < Kn <
1.
2. To eliminate any film boiling process during
quenching, the resonance effect can be
explored which, as a rule, is generated by the
hydrodynamic emitter. Its hydrodynamic
resistance is minor compared with the directed
fluid moving throughout the load.
3. The film boiling process during quenching can
be eliminated by creating a thin insulating
polymeric layer to decrease initial heat flux
density below its critical value. For this
purpose, inverse solubility polymers are used
as the quenchants.
4. One should control the double electrical layer
that exists during quenching steel in
electrolytes to increase essentially the first
critical heat flux density. It can be done by
electrical forces to charge negatively the
quenched steel.
5. There is the possibility to control intensive self-
regulated thermal processes by controlling the
boiling point of fluid using pressure or
variation concentration of additives.
6. In this case the self-regulated thermal process
is used to delay martensite transformation to
obtain a more bainitic fine microstructure with
extraordinary mechanical properties.
7. The self-regulated thermal process is used to
obtain high compressive residual stresses and
fine or nano-microstructure at the core of
machine components.
8. The simplified method of cooling time
calculation during quenching in cold fluid
machine components of any size and form is
proposed. The main idea consists of evaluating
duration of the self-regulated thermal process
and cooling time in the convective zone to
summarize them.
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9. If the cooling process is interrupted in the
nucleate boiling zone, then is used universal
correlation for cooling time calculation within
the transient nucleate boiling zone.
10. The main attention in the future one should be
pay to critical heat flux densities to be
compared with the initial heat flux density
aiming elimination of any film boiling process
together with the powerful pumps and rotating
propellers.
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[20] Aronov, M.A., Powell, J.A., Forging Process
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[21] Collier, J.G. and Thome, J.R., 1994.
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final findings and solution.
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Conflict of Interest
The author has no conflict of interest to declare that
is relevant to the content of this article.
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