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 37
sound effects ceased because the nucleate boiling process
ended and convection was established.
The duration of transient nucleate boiling will now be
estimated using Eq 36 and then compared with the experimental
data. The following initial conditions were used:
T 0 ¼ 850°C;
T S ¼ 120°C;
# 0 ¼ T 0 – T S ¼ 850°C – 120°C ¼ 730°C;
k ¼ 22 W/mK;
a ¼ 5.36 3 10 6 m 2 /s;
R ¼ 0.01 m; and
a conv ¼ 427 W/m 2 K.
Using these data and Eq 36, the time of transient nucleate
boiling s nb is equal to 15.5 s. Experimentally, it was
determined to be approximately 16 s, which agrees reasonably
well with the calculated result. The computational result
is less by 0.5 s because the existence of shock boiling and
possible localized vapor films were considered [36].
A portable device has been developed for recording
sound effects directly in a production quench tank so that similar
computations can be performed in the heat treating shop
[29–31]. The results of calculations of the time of the selfregulated
thermal process and time of sound effects for pins
of various diameters are shown in Table 14. Experimental and
computational results are in excellent agreement for these
parts of simple configuration. If parts possess a more complicated
configuration consisting of thin and thick sections, it is
difficult to determine the time of nonstationary nucleate boiling
precisely, since thin sections are cooled much faster, while
the quenching process (and noise associated with this portion
of the overall cooling) continues for thicker sections. However,
using Eq 36, it is still possible to estimate with sufficient precision
the average time of the self-regulated thermal process,
which coincides with nonstationary nucleate boiling.
2.7 DISCUSSION
Boiling is a phase change process in which vapor bubbles
are formed either on a heated surface or in a superheated
liquid layer adjacent to the heated surface. Dhir [2] reported
that heat flux on polished surfaces varies with wall superheat
approximately as:
q / DT m ;
ð43Þ
where m has a value between 3 and 4. This means that wall
superheat DT during quenching of steel parts does not
change significantly with a change of q. This process was
investigated by French [26] by quenching a steel ball 38.1
mm in diameter in cold water containing a small amount of
salts and making calculations of the duration of the selfregulated
thermal process and the surface temperature at
the beginning and end of the quenching process.
In this study, the steel ball was heated to 875°C and
then cooled in water at 20°C containing mineral salts. For
these calculations, the following input data were used:
T 0 ¼ 875°C;
T m (bath temperature) ¼ 20°C;
T S (saturation temperature) ¼ 100°C;
a conv ¼ 1,200 W/m 2 K;
k ¼ 22 W/mK; and
R ¼ 0.01905 m.
Overheat DT at the beginning and at the conclusion of
the self-regulated thermal process can be evaluated from Eqs
36–38, where:
K is Kondratjev form factor in m 2 (see Chapter 6);
a is the thermal diffusivity of steel (m 2 /s);
k is the thermal conductivity of steel (W/mK);
T 0 is the initial temperature (°C);
T I is the initial temperature of the surface at the beginning
of nucleate boiling (°C);
T II is the temperature of the surface at the end of nucleate
boiling (°C);
# 0 ¼ T 0 – T S ;
# I ¼ T I – T S ;
# II ¼ T II – T S ;
# uh ¼ T S – T m ;
a conv is convective heat transfer coefficient (W/m 2 K); and
b ¼ 7.36 [3].
Overheat at the beginning of the self-regulated thermal
process can be evaluated using Eq 37:
# I ¼ 1
2 3 22 3 ð775 # I Þ 0:3
¼ 10:28C;
7:36 0:01905
and overheat at the end of the process can be evaluated
using Eq 38:
# II ¼ 1 ½
7:36 1; 200 3 ð775 þ # IIÞŠ ¼ 4; 38C:
The duration of the self-regulated thermal process is
then calculated from Eq 36:
s nb ¼ 0:48 þ 3:21 ln 10:2 36:77 3 10
6
4:3 5:36 3 10 6 ¼ 22:4s:
Here K ¼ R2
p 2 ¼ 36:77 3 10 6 m 2 which is the Kondratjev
form factor for a ball of 38.1-mm diameter. The thermal diffusivity
of steel is a ¼ 5.36 3 10 6 m 2 /s. From these calculations,
the surface temperature during the self-regulated
thermal process changes from 110.2°C to104.3°C anditsduration
is 22.4 s. If an average temperature of (110.2°C þ
104.3°C)/2 ¼ 107.25°C is used, the difference between the
actual temperature and the average temperature is ±2.7 % This
difference decreases as the diameter increases, which means
that the simplified approach provided by Eq 36 can be used.
This is important, since it provides a practice approach for
heat treaters to delay the transformation of austenite into martensite
during the intensive quenching process and facilitates
the development various customized quenching processes.
2.7.1 Quench Process Investigations
Performed by French
Quenching is a very old technology. Ancient experienced
heat treaters—blacksmiths—had their own specialized knowhow
for quenching products, evaluating the duration of transient
nucleate boiling (although they probably didn’t recognize
this particular physical process in this way). The key
element of heat treating was to transfer quenched products
from one quenchant to another at the end of nucleate boiling.
The duration of nucleate boiling was evaluated simply
by the noise and small vibrations the blacksmiths felt in
their hands. Only experienced heat treaters could do this job,
and they were highly respected in ancient societies. Nowadays,
heat treating may be completely automated, and production
lines are equipped with the conveyors, robots, and
various kinds of hardware and software.