Transformation kinetics in controlled-power and controlled-temperature cycle testing Page: 3 of 8
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C alloy in both control modes. Both alloys were in the
normalized condition prior to testing, and therefore
consisted of fine pearlite and proeutectoid ferrite.
Samples for metallography were prepared using
standard polishing techniques and were etched using 2%
nital. Determinations of initial grain size and relative
fractions of individual microstructural constituents were
performed using quantitative image analysis software.
Controlled-Temperature Cycle Testing -
Conventional controlled-temperature cycle dilatometry
was conducted on a DSI Gleeble 1500 thermomechanical
simulator. Diametral dilatation was measured on 6.35
mm diameter samples using a high resolution
dilatometer at the location of the Cr-Al control
thermocouple percussion welded to the sample, and a free
span of 30 mm was used. The dilatometer was calibrated
for each of the heating rates by using pure nickel samples
of the same geometry. All tests were conducted in a
vacuum purged enclosed chamber under flowing high
purity argon with computer data acquisition. The tests
were conducted using programmed linear heating rates
ranging from 25 to 200'C/sec.
Gleeble Modifications - As it is currently configured,
the modification provides for open loop controlled power
inputs. The modification is compared with the
conventional Gleeble feedback temperature control
methodology in Fig. 1. The changes essentially consist of
the addition of circuitry that supplies a voltage
proportional to either the heat potentiometer setting on
the Gleeble 1531 control module or to a millivoltage
equivalent which is entered as a variable in the Gleeble
Programming Language program codes, and bypassing of
the comparator circuitry. The modification therefore
allows for any power versus time cycle, although only
constant power inputs were used in the current study.
+ Compensation Gleeble
Computer E Network Hardware
( Compensation Gleeble
Computer E Network Hardware
Figure 1. Block diagrams showing (a) normal closed loop
Gleeble temperature-control mode, and (b) open loop
In the controlled-power tests, all other conditions
such as free span, atmosphere, etc., were held constant
during the experiments. A simple power profile,
consisting of a 1 sec ramp to the desired power level,
followed by a constant power hold sufficient to allow the
sample to reach a temperature of at least 1000 C was
used. The power levels were selected to provide heating
rates in the 600 to 700 C range which were similar to
those used in the controlled-temperature cycles. Thus,
the heating rates near the lower critical temperature in
both modes are similar, and the results can be directly
Results and Discussion
Comparison of Thermal Cycles - Fig. 2 compares the
thermal cycles resulting from the constant power and
controlled-temperature cycle tests for the 0.45 wt % C
alloy at two heating rates. As shown, the thermal
25 30 35 40 45 50
1 00*C/sec (600-700*C)
. . .
Figure 2. Comparison of on-heating thermal cycles of 0.45
wt% C steel for controlled-power and controlled-energy
modes for (a) 20 C/sec and (b)100 C/sec.
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Robino, C.V.; Knorovsky, G.; Dykhuizen, R.C.; MacCallum, D.O. & Damkroger, B.K. Transformation kinetics in controlled-power and controlled-temperature cycle testing, report, June 1, 1998; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc710149/m1/3/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.