ASTM - Intensive Quenching Systems - Engineering and Design 2010 - N I Kobasko, M A Aronov, J A Powell, G E Totten

CHAPTER 2 n TRANSIENT NUCLEATE BOILING AND SELF-REGULATED THERMAL PROCESSES 35
Since, during the nucleate boiling, the heat transfer
coefficient is very high:
a nb >> a conv ;
ð42Þ
which is especially true for cooling in a lightly agitated or
nonagitated quenchant. From Eq 42, it follows that:
Bi Vmu >> Bi Vconv :
In Eq 41, when there is no film boiling and the nucleate
boiling is established, the generalized Biot number Bi V
approaches infinity. When the Bi V fi 1, according to Eq 41,
T Sf
T S
T V T S
! 0:
When this occurs, T Sf approximates T S , which means
that during transient nucleate boiling, the surface temperature
of a part is maintained at the boiling temperature.
If the surface temperature T Sf is less than saturation
temperature T S , then nucleate boiling stops and a singlephase
convection is established where a conv << a nb . When
the value of a conv is small, the generalized Bi V number is
also small, and according to Eq 41, at Bi V fi 0,
T Sf
T S
T V T S
! 1; or T Sf T V :
This means that equalization of the temperature field
occurs on cross-sections of parts and that the surface temperature
must increase. It follows that, during nucleate boiling,
the surface temperature is maintained at T S and cannot
be lower than T S while boiling is in progress and condition
a conv << a nb is satisfied.
Thus, the self-regulation of heat flux density—which
depends on the size and shape of parts to be hardened, the
thermal conductivity of the material, and the initial temperature
of the heated part—depends on the austenization temperature.
Self-regulation occurs during transient nucleate
boiling. The time of transient nucleate boiling is determined
by Eqs 36–38. Therefore, the time of the self-regulated thermal
process also is determined by Eq 36; however, it is less—
approximately one second, as compared with the complete
duration of the nucleate boiling process.
During full film boiling, there is no self-regulated process
because the temperature field is more uniform on crosssections
of parts to be hardened. and at the time when film
boiling starts, the temperature gradient may be insufficient for
the occurrence of the self-regulation process. This was illustrated
with experiments utilizing plate-shaped, cylindrical, and
spherical test specimens [25,33]. The time of the self-regulated
thermal process was determined by Eq 36, which was compared
with the results obtained by numerical solution and
from experimental data which are presented in Table 13.
2.6 EXPERIMENTAL DETERMINATION
OF THE TIME OF THE SELF-REGULATED
THERMAL PROCESS
The determination of the time of nonstationary nucleate
boiling or the self-regulated thermal process is a difficult
problem. Thus far, three experimental methods of determining
the time of nonstationary nucleate boiling are known
[27,30,31,33]. These methods include:
• Character of change in temperature of surface of the
part [13,27]
• Visual monitoring [13,28]
• Recording sound effects [30,31,35]
During the self-regulated thermal process, a certain
character of change in the temperature of the surface of a
part is observed. During nucleate boiling, the surface temperature
is maintained at approximately the boiling temperature
of the quenchant. When the convection cooling process
occurs, the surface temperature decreases significantly.
Using the first approach, the duration of the self-regulated
thermal process can be determined if the convective heat transfer
coefficient a conv is sufficiently large. In this case, the transition
from nucleate boiling to single-phase convection is clearly
observed. However, it should be noted that the first method is
less accurate than the other two methods. To measure the
exact temperature at the surface and the duration of nucleate
boiling using thermocouples is still a difficult problem. Most
suitable for such measurement is a Liscic-Nanmac probe [13].
The second method involves visual monitoring and
demands considerable time, material consumption, and cost
[13,28]. However, at present, with the availability of high-speed
video, the cost of such experiments has significantly decreased.
The technique includes observing the appearance and disappearance
of the vapor bubbles or moving of the wetting front [13].
The third method, recording of sound effects, is the
most promising, although a great deal of work and understanding
of the sound effects that are observed during the
nucleate boiling is required [30,31,35]. In the subsequent
TABLE 13—Time of the self-regulated thermal process when
cooling a test specimen of AISI 304 steel from 850°C in a 12 %
aqueous solution of CaCl 2
Time of self-regulated thermal process (s)
Sample
shape
Thickness or
diameter (mm)
Eq 36
Numerical
calculation
Experiment
Plate 20 25 28 —
Cylinder 10 3.54 3.7 3.8
30 24.4 22.5 23
Sphere 20 7.9 7 —
Note: Experimental studies are described in more detail in [27].