Automatic Process Control of Investment Casting

WITH SPECTROPYROMETER–
Automatic Process Control
of Investment Casting
by Ralph A. Felice
FAR Associates
www.pyrometry.com
It is easy to describe automatic process control of investment casting. Charge
the furnace, heat the charge uniformly to
the desired temperature, then cast. The
resulting casting cycle time would be
minimized, which would maximize productivity; temperatures would always be
the same, which would maximize quality.
Unfortunately, the second step – heat
the charge uniformly to the desired temperature – presents serious problems. The
problems are different but no less serious
depending on the technique chosen for
temperature measurement. Immersion
thermocouples can only make a very limited number of measurements per melt
cycle. Conventional pyrometers are inaccurate and have poor reproducibility.
Consequently, neither thermocouples nor
conventional pyrometers are suited for
control. Fortunately, third-generation
pyrometers – Spectropyrometers – can
make thousands of fast, accurate temperature measurements during the entire cycle, and many foundries use them
successfully to automate the investment
casting process.
Immersion Thermocouples
Historically, the most common technique for measuring the temperature of
a liquid metal melt has been the immersion thermocouple. There are two types:
the ceramic-sheathed thermocouple that
slowly equilibrates to some average temperature; and the rapid-rise thermocouple
with electronics that stop measuring and
present a temperature after a relatively
short immersion. Neither of these is
22
commonly used while the melt is being
heated at full power. This means heating must be suspended for the period of
the measurement, which adds time to the
casting cycle.
The traditional ceramic-sheathed
thermocouple can take several minutes
to reach a steady temperature. In some
foundries the operator monitors the temperature indication of the thermocouple
and decides when it has reached its maximum. While slow, this technique has
the potential for reporting accurate temperatures.1 In other foundries, the time of
immersion is set and the temperature at
the end of that time is taken as the melt
temperature. The common practice in the
latter case is to minimize the time of immersion to reduce the overall cycle time
and maximize the life of the expensive
thermocouple assembly. The result is that
the temperature is often under-reported.
Rapid-rise thermocouples have
thin thermocouple filaments and small
thermal masses. They take the operator
out of the timing of the measurement by
stopping their reading at a predetermined
rate of temperature rise to minimize the
immersion time. The result is that they
under-report the temperature of the melt.
With both ceramic-sheathed and
rapid-rise types the thermocouple is immersed when the operator believes the
melt to be near the correct temperature. If
the estimate is off the melt temperature is
adjusted by the operator’s skill. After the
adjustment period, the measurement is
repeated. A good operator can minimize
the number of immersions, but is working against a loaded deck. All there is to
go on is a charge weight and power setting. Unfortunately, settings don’t yield
the same power every time because the
efficiency of the power supply varies with
time and load, and the heating capacity of
the incoming electric power varies with
grid behavior.
Optical Pyrometers
The difficulties with immersion
thermocouples have prompted many to
attempt the non-contact, optical technique of pyrometry. Foundries around
the world are littered with the failed
hardware of these attempts. The reason is
that nature makes it very difficult to determine the temperature of a liquid metal
through non-contact optical techniques.
All pyrometry is based on a simple understanding of the physics of radiation:
every material raised to the same temperature radiates the same amount and
same colors of light. However, the simple
theory describes ideal behavior, not real
behavior. In practice, every material is
different; so a factor has been added to
make the ideal and the real agree: the
poorly-understood emissivity. Emissivity
is nothing more than the efficiency of a
radiator, and as such, varies between zero
and one.
But nature is not done with her practical joking, and she appears to have investment casters on her short list. Emissivity is wildly variable and its variations
can be huge: up to 300%. It varies between alloys; it varies with oxidation
states (a bit of slag on the surface?); it
varies with surface conditions (rough or
smooth, and a turbulent, induction-heated
liquid rivals the North Sea for changeability); it even varies with temperature
(how can temperature be measured if the
factor needed to measure the temperature
changes with the temperature?). Finally,
emissivity varies with the color of the
radiation. That is, metals typically over-
May 2013
radiate in the blue and under-radiate in
the red relative to other materials. And, of
course, each alloy is different in this behavior, also. All this variability means errors of hundreds of degrees are common.
For non-contact temperature measurement, the distinction between vacuum and air melting also becomes important. Vacuum melters must contend
with windows to the process becoming
covered with dirt and evaporated metal;
air melters must contend with smoke and
slag. It begins to be clear why so many
earlier attempts at pyrometry for investment casting have failed. Currently there
are three distinct types of pyrometer.
Brightness or one-color pyrometers have been around more than a centu-
Induction melting equipment
Heating Induction Services
ry; they are merely light meters calibrated
in temperature. Anything that changes the
amount of light – the emissivity, a dirty
window, smoke – changes the reported
temperature. Emissivity changes as the
metal melts and moves, so these fail.
This failure brought on the development of the next class of pyrometer, the
two-color pyrometer. The theory was
that the temperature could be calculated
by the ratio of two measurements: divide
the output of two one-color pyrometers
(operating at different colors) and the
emissivity cancels out. This is true for
some materials, but not metals, where
the emissivity also changes with color, so
these fail.
The most recent pyrometer type is
the many-color, or multi-wavelength
instrument. Spectropyrometers, a class
of these, use five hundred colors.2 With
this increased capability they automatically determine the emissivity for the
given alloy as the temperature and conditions change. This innovation makes
continuous accurate temperature measurement possible and allows the metal
melting process to be automated successfully.
Comparison of Results
Temperature measurement SpectroPyrometer
FAR Associates
May 2013
A vacuum investment casting foundry routinely dipped a ceramic-sheathed
thermocouple to a depth of one inch for
a fixed period. Figure 1 shows temperatures measured continuously by a Spectropyrometer in blue and thermocouple
results in red for a casting cycle. Usual
practice was a one-minute immersion,
resulting in a value 70°F below the Spectropyrometer temperature. When the operator was directed to hold the thermocouple in the melt for an extra 5 minutes,
the temperature continued to rise until it
nearly reached the pyrometer value. This
foundry used a double-sheath arrangement that resulted in a very slow thermocouple response.
Figure 1 illustrates vacuum investment casting, nickel superalloy with temperatures from a Spectropyrometer and
a double-sheathed thermocouple. For
23
colors) and the emissivity cancels out. This is true for some materials, but not metals,
where the emissivity also changes with color, so these fail.
Multi-wavelength pyrometers – Spectropyrometers
o
o
Temperature
( F)
Temperature
( F)
Temperature, F
This brings us to the most recent pyrometer type: many-color, or multi-wavelength
this
cycle andSpectropyrometers,
contrary to the usual
one-of these,Rapid-rise
thermocouples
are manIn sharp contrast, Figure 3 shows an
instruments.
a class
use five hundred
colors. 2 With
this
minute
immersion,
the
thermocouple
reually the
inserted
in air
so alloy
operaincreased capability they automatically determine
emissivity
formelts,
the given
as theinexperienced operator vainly chasing
mained
in theand
meltconditions
for the duration
shown
tor technique
critical. Inaccurate
Figure 2 the the set-point temperature; here temperatemperature
change.
This innovation
makesiscontinuous
by
the dotted measurement
line and closely
approached
temperature
possible
and allows
the metal melting
process toabe
automated
Spectropyrometer
documents
rapid
and ture differences between the immersion
the
Spectropyrometer
result.
successfully.
fairly smooth rise of temperature with measurements and the Spectropyrometer
In a different vacuum foundry the four thermocouple immersions in just are larger and much less reproducible,
sheathed
thermocouple
was routinely imComparison
of Results
over a minute with an experienced, ac- and the cycle takes longer.
mersed deeply for several minutes until
complished operator controlling.
A vacuum
foundry routinely
dipped a ceramic-sheathed thermocouple to a depth of
the
operatorICobserved
that its displayed
Figure 2measured
shows a continuously
casting cyclebyfor
one inch forhad
a fixed
period. Here
Figuremonths
1 shows temperatures
a Successful Automatic
temperature
stabilized.
air-melted
stainless
steel cycle.
with Usual
experiSpectropyrometer
in
blue
and
thermocouple
results
in
red
for
a
casting
Process Control –
of data showed exact agreement between
enced
Rapid-rise
thermocouple
practice was a one-minute immersion, resulting
in operator.
a value 70°F
below the
thermocouple and Spectropyrometer
under-reports
temperature.
Spectropyrometer
temperature. When the operator
was directed
to hold the thermocoupleSpectropyrometry
3
measurements.
in the melt for an extra 5 minutes, the temperature continued to rise until it nearly reached
Spectropyrometers have been comthe pyrometer value. This foundry used a double-sheath arrangement that resulted in a
very slow thermocouple response.
mercially available for fifteen years, and
Figure 1
thermocouple remained in the melt for the duration shown bySP
theTemperature
dotted line and closely approached have been successfully applied to IC
the Spectropyrometer
result.
2740
since 2006.
Thermocouple
In a different vacuum foundry the sheathed thermocouple was routinely immersed deeply
Figure 4 shows the setup for a Spec2720
for several
minutes until the operator observed that its displayed temperature had
thermocouple remained in the melt for the duration shown by the dotted line and closely approached tropyrometer controlling an air-melt
stabilized. Here months
of data showed exact agreement between thermocouple and
the Spectropyrometer
result.
2700
rollover furnace. A ceramic tube conSpectropyrometer measurements. 3
In a different vacuum foundry the sheathed thermocouple was routinely immersed deeply nected to the pyrometer lens is brought
2680
for
several
minutes until the
observed
its melts,
displayed
temperature
had is
Rapid-rise
thermocouples
areoperator
manually
insertedthat
in air
so operator
technique
to close proximity of the melt surface and
stabilized.
Here
months
of data showed exact
agreement
between
thermocouple
and of
critical.
In
Figure
2
the
Spectropyrometer
documents
a
rapid
and
fairly
smooth
rise
is purged with argon to remove obstruct2660
3
Spectropyrometer
measurements.
temperature with four
thermocouple immersions in just over a minute with an
ing smoke. Resulting data show a smooth
experienced,
accomplished operator controlling.
2640
temperature ramp and stable hold within
Rapid-rise
thermocouples
are manually
inserted
in16:30
air melts,
so operator
technique
is16:34
16:25
16:26
16:27
16:28
16:29
16:31
16:32
16:33
Spectropyrometerdocuments
Avg temp a rapid
Immersion
TC Temp
critical. In Figure 2 the Spectropyrometer
and fairly
smooth rise of
a few degrees.4
Tim e, hh:m m
2850 with four thermocouple immersions
temperature
in just over a minute with an
Figure 5 is a screen shot of the Specexperienced, accomplished operator controlling.
Figure 1.
Vacuum 2
IC, nickel superalloy with temperatures from a Spectropyrometer and a double- tropyrometer’s monitor during a cycle at
Figure
2800
sheathed thermocouple. For this
cycle and contrary
to the usual
one-minute
immersion, the
Spectropyrometer
Avg temp
Immersion
TC Temp
the facility shown in Figure 4. The Tol2750
2850
erance displays the quality of each temperature measurement in real time. Low
2700
2800
3
tolerances,
as shown, mean the tempera2650
2750
ture is well-known. The Signal Strength
2600
is the emissivity of the target, in this case
2700
39:07
39:17
39:27
39:37
39:47
39:57
40:07
40:17
40:27
the melt. The value shown is typical of
Time, mm:ss
2650
a liquid metal (0.15 – 0.45). Slags have
higher values, usually greater than 0.5.5
2600
Figure 2.
A casting cycle for air-melted stainless steel with experienced operator. Rapid-rise
thermocouple
under-reports
39:07
39:17 temperature.
39:27
39:37
39:47
39:57
40:07
40:17
40:27
Time, mm:ss
In sharp contrast, Figure 3 shows an inexperienced
operator vainly chasing the setpoint
temperature; here temperature differences between the immersion measurements and the
Figure 2. A casting cycle for air-melted stainless steel with experienced operator. Rapid-rise
Spectropyrometer
are larger
and much less reproducible, and the cycle takes longer.
thermocouple
under-reports
temperature.
Summary
Figure 3
Automatic process control applied
to investment casting has huge benefits.
It allows controlled, uniform heating
to the desired set point; both ramp and
hold times can be optimized. The resulting casting cycles are shorter, improving
productivity and reducing energy usage.
The improved temperature control yields
better quality due to reduced inclusions
and other temperature-related flaws. Reducing superheat can allow equipment
to last longer and reduces atmospheric
emissions. In addition, discarding im-
o
Temperature
(oF)
Temperature
( F)
Immersion
TC Temp
Spectropyrometer
Avgchasing
Temp the setpoint
In sharp contrast, Figure 3 shows
an inexperienced
operator vainly
2900 here temperature differences between the immersion measurements and the
temperature;
Spectropyrometer are larger and much less reproducible, and the cycle takes longer.
2850
Immersion TC Temp
2800
2900
Spectropyrometer Avg Temp
2750
2850
2700
2800
2650
2750
31:30
2700
31:50
32:10
32:30
32:50
33:10
33:30
33:50
Time, mm:ss
2650
Figure 3. Another casting cycle from the same foundry as Figure 2 but with an inexperienced
31:30 is erratic
31:50 and32:10
32:50
33:10
33:30
33:50
operator. Heating
the cycle 32:30
is much longer.
Time, mm:ss
24
Figure 3. Another casting cycle from the same foundry as Figure 2 but with an inexperienced
operator. Heating is erratic and the cycle is much longer.
May 2013
4
Figure 5.
Figure 5. View of monitor during melt cycle at facility of Figure 4. Tolerance is the on-line measure
of the quality of the temperature measurement;
strength
is emissivity
of target.
View of signal
monitor
during
melt
cycle at
Figure 5 is a screen shot of the facility
Spectropyrometer’s
monitor
during a cycle is
at the
of Figure
4. Tolerance
thefacility
shown in Figure 4. The Tolerance displays the quality of each temperature measurement
on-line
measure
of
the
quality
of
the
in real time. Low tolerances, as shown, mean the temperature is well-known. The Signal
temperature
measurement;
signal
Figure 4. Spectropyrometer
optical probe
withprobe
ceramicwith
tubeceramic
for smoketube
suppression
in of the target, in this case the melt. The value shown is typical of
Strength
theshown
emissivity
Spectropyrometer
optical
forissmoke
a liquid metal (0.15 – 0.45). Slags
have higher
values, usuallyofgreater
than 0.5. 5
measuring
position.
strength
is emissivity
target.
suppression in measuring position.
Figure 4.
Figure 4 shows the setup for a Spectropyrometer controlling an air-melt
rollover furnace.
Summary
A ceramic tube connected to the pyrometer lens is brought to close proximity of the melt
Automatic process
controla applied to investment casting has huge benefits. It allows
surface
is purged
with argonexpensive
to remove obstructing smoke. Resulting
data show
mersionandsystems
eliminates
Acknowledgment
desired
setpoint;
bothProceedings
ramp and holdof
times
Gray,
R. A.
Felice,
thecan
In-be
smooth
temperature
ramp and stable hold within a few degrees. 4controlled, uniform heating to the
thermocouple
consumables.
optimized. The resulting casting
cycles areCasting
shorter, improving
productivity
and reducing
vestment
Institute
55th
Technical
To achieve automatic process conenergyto
usage.
The Kermit
improved temperature control yields better quality due to reduced
The author wishes
thank
Conference,
14 –superheat
17, 2007
inclusions and other temperature-related
flaws.Oct.
Reducing
can allow equipment
trol, a fast, accurate, reproducible tem- Buntrock for his encouragement
and
suplast longer and reduces atmospheric
addition, discarding
immersion
4 emissions.
Video Inavailable
at http://pyromperature measurement system capable port for this articleto and
technology
systems the
eliminates
expensive thermocouple consumables.
etry.com/pt009.wmv
of continuous measurement is necessary. described within
To achieve automatic process control,
accurate, reproducible
temperature
Immersion systems are not fast enough
5a fast,
“Pyrometry
of Materials
With
Endnotesmeasurement system capable ofChanging,
continuous measurement is necessary. Immersion
and conventional pyrometers are incapaSpectrally-Dependent Emissystems are not fast enough and conventional pyrometers are incapable of dealing with
ble of dealing with the optical properties
sivity
—
Solid
&IC
Liquid
Metals,”
the
optical
properties
of
the
melt.
Since
2006,
many
foundries
have usedR. A.
a type of
1 Video showing good ceramicof the melt. Since 2006, many investment
multi-wavelength pyrometry, Spectropyrometry,
to automate
process control
Felice,
D.
A.
Nash,
Proceedings
of
the
sheathed thermocouple
technique is
successfully.
casting foundries have used a type of
9th
International
Temperature
Sympomulti-wavelength pyrometry, Spectropy- available here: http://pyrometry.com/apAcknowledgment
sium, March 19 – 23, 2012 Anaheim,
rometry, to automate process control suc- plications/investment-casting-melt-movThe author wishes to thank Kermit
for his encouragement
and support
CA;Buntrock
“Temperature:
Its Measurement
andfor this
ie/. Note depth and article
periodandofthe
insertion.
cessfully
technology described within.
Control in Science and Industry, Vol. 8,”
2 US Patents 5772323, 6379038
3 “Successful Pyrometry in In- to be published by the American Institute
6
vestment Casting” D. M. Olinger, J. V. 5of Physics.
May 2013
25