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Assessment of Sound Emission Phenomena at a Boundary Layer during
Steel Quenching
Franc RAVNIK and Janez GRUM
University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana
Phone: +386 1 4771765, Fax: +386 1 4771225; e-mail: franc.ravnik@fs.uni-lj.si; janez.grum@fs.uni-lj.si
Abstract The final stage in the production of machine components in the manufacturing process often represents quench-
ing and tempering. Mechanical properties, such as residual stresses and hardness layer of final part, therefore
depend on optimum parameters of a quenching process chosen and monitoring of the process itself. The paper
treats an experimental setup comprising detection of sound emission in the course of quenching process. Due to
heat transfer from a specimen’s surface to a quenching medium, film boiling and nucleate boiling occur round a
heated specimen, which strongly affects quenching. An investigation of sound emission in the quenching process
was carried out with cylindrical specimens made of chrome-molybdenum heat-treatable steel 42CrMo4
quenched in different quenching media. Sound-pressure signals demonstrated by different amplitudes depending
on time at different frequencies are shown in 3D diagrams. It has turned out that an analysis of sound-emission
signals are connected with wettings kinematics and can provide useful information that confirms differences
occurring in quenching with different quenching media, different specimen’s shapes and under different quench-
ing conditions. Analysis of sound emission signals can give useful information and confirm the differences
caused by different quenching conditions. The results lead to the applicability of the new approach to the control
of the hardening processes of steels.
Keywords: sound emission, acoustic spectrograph, cooling rate, polymeric water solution, quenching, sound
pressure level, vapor film
1. Introduction
In order to use material as much as possible, most machine parts manufacturing concluded
with quenching to obtain the desired hardness profile. A purpose of quenching is to provide a
cooling rate higher than the upper critical cooling rate in a certain depth of a machine compo-
nent in order to obtain desired mechanical properties. Critical cooling rates with carbon steels
and low-alloy steels can be achieved by cooling a machine component from the austenitic mi-
crostructure, which is obtained by immersion into quenching media such as water, oil, poly-
meric water solutions, and emulsions [7]. The rate of heat transfer during quenching depends
on the wettability of the liquid quench media. Wettability can be defined as the tendency of a
liquid to spread on a solid substrate and can be characterized by the degree and the rate of
wetting [10].
During the quenching process in fluids, the occurrence of three wetting phases of heat transfer
to quenching media with Leidenfrost temperature between 100 and 200°C and consequently
the varying of the heat transfer coefficient α, is characteristic [13, 14]:
� In the initial phase of immersion, a stable vapor film of a fluid quenching medium
forms around the workpiece, so that the heat transfer to the quenching medium is con-
siderably slowed down (Fig. 1) [13]. Heat transfer coefficient varies. aFB=100 to 250
W/m2K when quenching in water.
� In further cooling the vapor film gradually decays so that a phase of nucleate boiling
occurs at some parts of the workpiece surface having a temperature lower than the
Leidenfrost temperature. In nucleate boiling, a surface section wets with the liquid
quenching medium and the heat transfer from the workpiece increases. Vaporizing of
30th European Conference on Acoustic Emission Testing & 7th International Conference on Acoustic Emission University of Granada, 12-15 September 2012
www.ndt.net/EWGAE-ICAE2012/
media at simultaneous vertical moving of specimen enables heat transfer coefficient
anb=10 to 20 kW/m2K when quenching in water.
� At lower temperatures, the third phase of convective cooling occurs at the interface
with heat transfer coefficient aconv= approx. 700 W/m2K when quenching in water.
Spreading velocity of the wetting front depends on several physical properties of the specimen
and the quenching medium like heat transfer coefficient, surface roughness, shape of the spec-
imen, temperature of the quenching media and forced convection and other. Specimen cooling
is therefore liable to a great local variation, which influences the microstructure and mechani-
cal properties of the specimen obtained [5].
The three phases are characterized by different modes of heat transfer, which contributes to
different cooling intensities. With exacting workpiece shapes it often occurs during heat
treatment, i.e. during workpiece cooling, that all the three phases occur simultaneously on dif-
ferent workpiece surface areas, which produces considerable thermal stresses and also an ex-
plicit influence on microstructural stresses leading to distortion and residual stress, or even to
distortion and cracking, of the workpiece. Therefore, a distribution of heat transfer coefficient
is an important parameter for the distortion of the whole part [14]. Consequently, the meas-
urement of the wetting behavior of the workpiece is essential for the control of the quenching
process. Steel quenching starts at the austenitizing temperature of a given steel, the quenching
process in fluid quenching media always being accompanied by an evaporation phase of the
medium in contact with the workpiece surface, since a boiling point of most of quenching
media ranges between 100 °C and 300 °C.
Formation and liberation of vapor bubbles, their oscillation and disappearing in the liquid
generate noise, which is strongest in the transition layer between film boiling and nucleate
boiling of the quenching medium. Detection of emitted sound signals and their analysis can
therefore provide useful information on the quenching process [4]. Sound generated by bub-
bles in the water was examined by Minnaert [6]. The Minnaert model of bubble formation
frequency during immersion indicates a relation between the bubble size and the frequency of
bubble formation (Eq. 1.). In the initial phase of nucleate boiling, bubbles of smaller diame-
ters are formed and their frequency is higher, and vice versa when nucleate boiling of the
quenching medium is nearing its end.
Nucleateboiling stage
Vapourblanket stage
Convectiveheat transfer stage
Maximumcooling rate
Time t [sec] / Cooling rate v [°C/s]c
Tem
pe
ratu
re T
[°C
]
Fig. 1. Nukiyama boiling curve and dif-
ferent stages of boiling (heat
transfer).
Fig. 2. Cooling rate and temperature dur-
ing quenching.
Minnaert's frequency of oscillation bubble:
ρ
χω 031 p
RO
M
⋅= [1]
where:
p0 [N/m2] .... is hydrostatic pressure of the liquid around the bubble in conditions of stat-
ic equilibrium,
χ [ ] ............. polytrophic coefficient,
RO [m] ........ radius of the bubble in the equilibrium and
ρ [kg/m3] .... the density of the liquid
2. Experimental procedure
A measuring setup should be adapted to the expected frequency of sound phenomena. Thus it
should be known which frequencies of bubble formation and decay occur predominantly in
the range of hearing and which slightly is below this threshold [11] (Fig. 3).
The conditions at which the signals
were detected should be monitored
by temperature measurement of the
specimen during the quenching pro-
cess. The phenomena occurring at
the workpiece/medium interface
should then be logically interrelated
in film boiling, including additional
environment sound effects [4].
Narazaki [9] concluded that the
sound pressure level is a function of
the temperature of the quenching
media and cooling stage, respective-
ly. Furthermore, bubble formation
and sound generation phenomena
should be interrelated with material properties obtained.
2.1 Experimental setup
Figure 4 shows the experimental setup for detecting sound signals during wetting processes.
The setup is to be independent from the quenching medium type used and of the quenching
mode. Although the quenching process takes some seconds or even up to several minutes, the
experimental system should register individual events sensed up to 0.1 second or less.
Experimental setup used in the experiment comprises two independent lines, i.e.:
� a setup for monitoring the temperatures at the surface and in the core during quench-
ing and monitoring of the temperature of the medium itself;
� a setup for detection and processing sound pressure level (Lp).
The measuring setup for detecting and processing of sound signals includes:
� A high sensibility hydrophone in frequency ranges from 0.1 Hz to 180 kHz in a wide
range of working temperatures between 40°C and 80°C. The hydrophone is piezoelec-
tric transducer, that is, it uses piezoelectric ceramics as sensing elements permanently
bonded into sound-transparent rubber.
0 2.52.01.51.00.5Bubble s diameter ’ [mm]
Fre
qu
ency [
kH
z]
20
15
10
5
0
depth in water 100 mm
depth in water 150 mm
frequency range
Fig. 3. Expected frequency range depends on
the size of the bubble and immer-
sion depth.
� A third-octave and octave band-pass filter in combination with multi-channel measur-
ing amplifier/pre-amplifier. It enables frequency response measurements on electro-
acoustic transducers with 50 third-octave filter bands, centre frequencies from 2 Hz to
160 kHz.
� A two-channel Sound Blaster card enables an analog-to-digital conversion of the sig-
nal captured and permits 16- or 24-bit sampling of velocity up to 96 kHz.
� Specially designed program package, for the detection, recording and processing of
sound signals in their original and digitized form, permits subsequent evaluation and
monitoring of events in the specimen during quenching.
� PC with operating system MS Windows XP professional.
Simultaneously to sound emission, temperature variations at the workpiece surface (Tw) and
in the core, and quenching-medium temperatures (Tqm) prior to, during and after quenching
were monitored. Specimens were equipped with two thermocouples, the first being 2 mm be-
low the surface and the second in the core centre.
The measuring setup for monitoring temperature variations at the specimen surface and in its
core consisted of:
� Thermocouples, mounted near the surface and in the core, for measuring temperature
variations during quenching;
� USB I/O platform for signal processing of temperature signals with a 16-bit resolution
and sampling rate of up to 200 kHz;
� Software for monitoring time-dependent temperature.
� Digital thermometer for temperature of the quenching medium measuring, prior to,
during, and after quenching.
Fig. 4. Experimental setup for temperature measurement and experimental setup for detection
of sound signals during quenching
2.2 Specimen material, form and quenching conditions
The specimens of different cylindrical forms were made of low-alloy Cr-Mo heat-treatment
steel (EN - 42CrMo4). It is characterized by good through-hardenability, i.e. even up to 57
HRC and high strength after heat treatment. Therefore it is widely used in the production of
statically and dynamically loaded machine components.
For the steel chosen an adequate quenching medium and a suitable quenching mode ensuring
a cooling rate slightly higher than the critical one should be used [2, 3, 8, 15]. In accordance
with the steel manufacturer's recommendations, austenitizing temperature between 820 and
850°C followed by water quenching and between 830 and 860°C for oil quenching was cho-
sen.
2.3 Measuring procedure
The specimen was gradually heated to the austenitizing temperature of 860°C and when the
temperature was stable, the specimen was quickly moved to a quenching bath to be quenched
in different quenching media. The initial quenching-medium temperature was about room
temperature, whereas the final temperature varied due to the different specific heats of the
quenching media used. The captured sound signals were subsequently evaluated with the pro-
gram, SpectraLAB, and the temperature signals were processed in MS EXCEL.
In the acoustic theory, sound motion in liquids and gases is always longitudinal in a form of
compression wave motion. This means that particles oscillate in longitudinal direction and
produce periodic compression and extension, which propagate through the media with the
sound velocity. Therefore material density and the sound pressure increase in the compres-
sion. The pressure changes are caused by the longitudinal wave motion and defined by the
sound pressure. The total sound pressure consists of positive and negative compression dis-
turbances measured from the equilibrium value of the sound pressure. The sum of all devia-
tions from the equilibrium position equaling zero, in practice the term effective sound pres-
sure pRMS is used and is expressed in case of sinusoids by:
( )[ ]ctxkPppRMS −⋅== 22
1
2 cos [Pa] [2]
The sound-wave motion is composed of several superimposed types of wave motions having
different frequencies, amplitudes, and phases. A change of the sound pressure can be deter-
mined at an optional point in the space in which sound propagates. Sensing sound changes
should measure the variation of timely sound pressure. Recorded acoustic signal should per-
mit the determination of sound frequency and sound intensity given in synoptic logarithmic
scale. A representation of level values with a logarithmic ratio of the acoustic quantity chosen
to its reference value can be described as a time variation. The unit of sound power level in
acoustics is expressed in decibels [dB]. Consequently, results of a given acoustic characteris-
tic shall always be given not only as a measured value but also with its reference value [1].
The sound power at a certain point is thus a difference between the instantaneous pressure and
the ambient pressure. The sound pressure (p) is expressed in [Pa]. A reference value of the
sound-pressure for liquids pref is equal to 10-6
Pa. The results of spectral analysis are graph-
ically represented in the diagrams where the complex sound pressure level Lp is a function of
frequency. The sound-pressure level Lp is defined as:
refref
pp
p
p
pL log20log10
2
2
⋅=⋅= [dB] [3]
where:
p [Pa] ........ sound pressure
pref [Pa] ..... reference value of the sound-pressure
P1 [Pa] ...... instantaneous sound pressure amplitude
K [ ] .......... is the wave number:
c [m/s] ...... is the velocity of the sound-wave motion
t [s] ........... time
3. Results
The results of spectral analysis are diagrams showing the complex sound pressure level Lp as
a time depend function of frequency. The sources of the sound pressure are consequences of
the occurrence of the phase transformation of' steel and of the boiling process at the interface
during the cooling process. The recorded diagram of the sound pressure shows a three-
dimensional variation of the sound pressure level as a function of frequency and time in dif-
ferent diagrams, such as the time variation of the pressure amplitude, a phase diagram, a pow-
er spectrum of the signal etc. The displays of the sound-pressure levels in the diagrams pro-
vide the best displays of the dependence between the acoustic signal captured and the events
at the specimen/quenching-medium interface.
Film
bo
iling
Tra
nsi
tio
n b
oilin
g
Nu
clea
te b
oili
ng
Fre
e c
on
vect
ion
Fig. 5. Typical sound pressure signal. Fig. 6. Spectrogram of sound pressure signal.
� Time series diagram represents the amplitude ratio during quenching process. A typi-
cal raw sound signal captured during quenching, recorded from the hydrophone in the
quenching media, is presented in Figure 5. The amplitude ratio between maximum
level at the start of the quenching process and background noise level at the very end
of the process is sufficient to identify different stages of the quenching process.
� A spectrogram is a calculated time record of sound signals, in which frequency is seen
as the ordinate value and signal intensity as color intensity: cold - blue color - low in-
tensity; hot - red color – strong intensity (Figure 6).
3.1 Analysis of the acoustic spectrum
A spectrogram is a time record of sound signals, from which a frequency is evident as a value
at the ordinate and signal intensity as color intensity (cold, blue color - low intensity, hot, red
color – strong intensity). The comparison of the duration of the vapor blanket stage calculat-
ed from the ultrasonic and the reference data shows a good agreement with respect to the
measurement uncertainty [12].
The spectrogram of the quenching process shows the presence of low frequencies at the be-
ginning of the process, i.e. in the first 10 seconds, when using water as a quenching medium.
Intense sound pulses occur due to the formation and decay of the vapor film. Their frequency
gradually reduces due to a decreasing cooling rate.
The analysis of the sound spectrum obtained during quenching comprises the detection of the
sound-pressure level and transformation into a voltage signal. It provides detailed information
on individual events in the quenching medium used. Fig. 7 shows two records of sound pres-
sure level signals detected with the hydrophone and shown in the spectrogram giving the fre-
quency of events during nucleate boiling in the water. Analyzing both records, the following
may be concluded:
12
3
5
4
7
Time (seconds)
Fre
qu
en
cy (
Hz)
SP
L
(dB
rm
s)
6
Perc
ent
Full S
ca
le
Time (100 sec/div) Fig. 7. Acoustic signal obtained in quenching in water.
� The beginning of quenching is described by the signals of different frequencies, which
in their amplitude differ from the environmental noise (Fig. 7, point 1);
� The end of quenching is described by the signal having intensity comparable to that of
the environmental noise, which means that the number and intensity of the signals
with regard to the frequency and their intensity reduces. Determination of the end of
cooling is of subjective nature and cannot be made by means of the spectrogram. (Fig.
7, point 2);
� The process duration is the difference between the beginning and end of quenching;
wherefrom an average cooling rate can be established (Fig. 7, area 3);
� From the voltage signal detected with the hydrophone and from the spectrogram, two
characteristic areas can be determined:
* an area of lower frequencies, i.e. up 1 kHz in water quenching, and up to 2 kHz in
oil quenching, where only the amplitude is changing (Fig. 7, area 4), and
* an area of higher frequencies, i.e. up 18 kHz in water quenching, and up to 20 kHz
in oil quenching, where the signal frequency is changing as well (Fig. 7, area 5).
� Individual peaks of higher sound-emission intensity given by a voltage signal of high-
er frequencies (up to 6 kHz in water quenching, and up to 14 kHz in oil quenching)
due to surface oxidation and oxide cracking (scale) at the surface (Fig. 7, points 6) are
identified. In sound emission there are also short-time peaks of high intensity with fre-
quencies of up to 12 kHz in water quenching as a result of too high a cooling rate (Fig.
7, area 7).
Generally it can be concluded that from the detected sound emission, characteristic changes at
the specimen/quenching-medium interface during quenching could be determined.
3.2 Analysis of the sound signals generated at quenching process
The time series sound pressure signals for
the specimens quenched in water (Fig.
9A) and in salt solution (Fig. 9B) are
graphically represented in the diagrams
below. Salt solution was chosen on pur-
pose to obtain too high internal stresses,
which cause the cracking of the specimen
(Fig. 8B).
10,0 20,0 Time t [s]
0,0
50,0
-50,0
Perc
en
t F
ull
scale
0,0 10,0 20,0 Time t [s]
0,0
50,0
-50,0
Perc
en
t F
ull
scale
Peak amplitude
A quenched in water B quenched in salt solution
Fig. 9. Time series sound pressure signal
Comparing sound pressure diagrams obtained indicates important characteristics shown with
differentiation in sound intensity and frequency ranges. Analysis provides detailed infor-
mation on individual events in the quenching medium used. Figures 9, 10 and 11 show sound
pressure level signals detected with the hydrophone and calculated spectrograms, giving the
frequency of events during the quenching of the specimen in pure water and salt solution. By
comparing signals of both quenching media, the following may be concluded:
Fig. 8. Specimen shape
A Before quenching B After quenching
� Significant change of the amplitude can be determined when quenching in salt solu-
tion: after approx 16 seconds of quenching process duration significant amplitude
peak shows the moment when specimen cracking occurs (Fig. 9B).
� Typical events are recognized: background noise, the beginning of the cooling process
and specimen cracking (Fig. 10A). Time zooming shows that the duration of cracking
signal is less than 0.05 seconds when four peaks follow one after another (Fig. 10B).
0,0 10,0 20,0 Time t [s]
0,0
50,0
-50,0
Pe
rcen
t F
ull
sca
le
Beginig of the process
Background noise
Specimen cracking
Process start
0,0
50,0
-50,0
Pe
rcent F
ull
sca
le
16,2 16,3 16,4Time t [s]16,5
1. peak
2. peak
3. peak
4. peak
Crack appearance period
Crack decay period
A typical events B “Time zooming” signal
Fig. 10. Time series sound pressure signal when quenched in salt solution
� Significant change of the amplitude shown as sudden color change can be also deter-
mined in spectrogram of sound pressure signal when specimen cracking occurs (Fig
11B). Spectrogram shows that the change occurs in all frequency ranges for a very
short time shown as more intensive red colour.
Fre
quency
[Hz]
-55
-65
-75
-85
-105
-115
-95 Rela
tive
Am
plit
ude [dB
]
0,17 24,45Time [1 sec/div]
1,0k
500
400
300
600
800
2,0k
3,0k
4,0k
5,0k6,0k
8,0k
10,0k
20,0k
Fre
quency
[Hz]
-55
-65
-75
-85
-105
-115
-95 Rela
tive
Am
plit
ude [dB
]
0,17 24,45Time [1 sec/div]
1,0k
500
400
300
600
800
2,0k
3,0k
4,0k
5,0k6,0k
8,0k
10,0k
20,0k
Pe
ak
am
pli
tud
e
A quenched in water B quenched in salt solution
Fig. 11. Spectrogram of sound pressure signal
� Figure 12 shows the calculated average spectrum for both quenching media. Figure 12B
shows in the cracking moment significant change of the intensity in frequency range be-
tween 6 and 7 kHz.
� Detailed investigation of sound pressure signal enables evaluation of frequency signals.
Figure 13 shows the course of high frequency signals 19, 20, 21.2, 22.4 and 23 kHz,
during time. In the moment of cracking of the specimen, between 16.2 and 16.25 se-
conds, a significant increase of the signal amplitude can be recognized by frequencies.
300 500 700 1,0k 2,0k 3,0k 5,0k 7,0k10.0k 20.0k
Frequency [Hz]
-50
-60
-70
-80
-90
-100
Rela
tive
Am
plit
ud
e [dB
]
300 500 700 1,0k 2,0k 3,0k 5,0k 7,0k10.0k 20.0k
Frequency [Hz]
-50
-60
-70
-80
-90
-100
Rela
tive
Am
plit
ude
[dB
]
Peak amplitude
A quenched in water B quenched in salt solution
Fig. 12. The average spectrum of samples
-140
-120
-100
-80
-60
-40
-20
0
0,1
1,3
2,5
3,6
4,8
6,0
7,2
8,3
9,5
10
,7
11
,9
13
,0
14
,2
15
,4
16
,5
17
,7
18
,9
20
,1
21
,3
22
,4
23
,6
Am
plitu
de
[dB
]
Time [sec]
23,0 kHz 22,4 kHz21,2 kHz
20,0 kHz19,0 kHz
Fig. 13. The course of frequency signals during time
3. Conclusions
This article describes the signals captured by measuring sound emission caused during
quenching. The sound emission occurring during quenching is found predominantly in the
audible frequency spectrum; therefore, detecting the sound pressure signal is efficient since
the phenomena at the interface can be monitored. The sound pressure signal, which exposes
as the amplitude and duration of the signal during quenching, was captured by the hydro-
phone. During recording of the sound pressure, a change of the signal shape appears, corre-
sponding to the formation of a vapor film phase on the specimen surface and to the nucleate
boiling phase on the specimen/quenching-medium interface. Significant change of the signal
followed when specimen cracking occurs. This change was connected with the high internal
stresses during quenching process, which caused rupture of the specimen in axial direction.
The signal was audible during the entire quenching process and the capturing of the sound
emission with hydrophone was satisfactory. Significant change of the signal was well visible
and could be recognized in the cracking of the specimen. On the basis of the experiment, the
conclusion can be drawn that the experimental setup for the capturing of the sound emission
was reliable and has given sufficient results. The analysis of the results offers an interesting
new approach to evaluation and, more importantly, to monitoring, controlling and optimizing
of the quenching process.
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