Method and apparatus for the differential thermal analysis of a molten metal

The invention relates to a method for measuring the cooling curve of a test sample of metal or a metal alloy, in particular cast iron or cast steel, for the differential thermal analysis in which the difference of the cooling curve of the test sample of a fixedly given comparative curve which satisfies Newton's law of cooling (U=U.sub.0 .times.e.sup.-t/RC, the U.sub.0 parameter being the maximum value at the point in time t=0 and the RC parameter being the time constant) is formed, said comparative curve being brought to coincidence with a section of the cooling curve of the sample by adjusting its parameters so that the difference can then be formed between the adjusted comparative curve and the cooling curve.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
The differential thermal analysis is used in order to be able to draw 
conclusions on the composition of materials, such as metals and metal 
alloys among others. It supplies substantially more comprehensive and 
clearer statements concerning the phase composition of the tested material 
than the normal thermal analysis (evaluation of time temperature curves). 
The method itself is carried out in laboratory conditions for test samples 
with a low microgram mass of up to 10 g. The differential thermal analysis 
cannot be carried out with conventional DTA (Differential-Thermo-Analysis) 
devices on test samples with a mass of over 10 g which must generally be 
used for a quick operational control of metal melts. Neither can the 
classical differential thermal analysis supply any statements concerning 
solidification behavior and phase composition of real castings. 
In another known, but not used, method of the differential thermal analysis 
the cooling curve of the test sample is compared with the curve of a group 
of fixedly given curves which satisfy Newton's law of cooling U=U.sub.O 
.times.e.sup.-t/RC, U.sub.O corresponding to the maximum value of the 
temperature at the point of time t=0 and RC being the time constant of the 
curve. The fixedly given curves are under some circumstances empirically 
determined taking into consideration all influencing factors (change in 
material constants and heat transfer conditions during cooling of the test 
sample to be tested) and stored by computer from which they can be called 
dependent on the expected cooling curve of the test sample. 
In such a method of differential thermal analysis, none of the curves of 
the group generally coincides fully with the cooling curve of the test 
sample in the liquid zone. For this reason, this method for metals and 
metal alloys and, in particular, cast iron and steel, is inaccurate even 
when interpolation is made between given curves to adapt them to the 
cooling curve of the test sample. 
In another known process an imaginary connecting line between the first and 
last (at the end) transformations of the test sample, which are 
characterized by thermal effects, is used as a comparative curve. All the 
results calculated on the basis of such an approximate curve are affected 
with systematic faults. 
In a known method of the aforementioned type the cooling curve is measured 
at a fixed test sample with a thermal element. The course of cooling of a 
fixed test sample, however, does not show any distinct heat effects. It 
has been attempted to increase the cooling speed in order to compensate 
this disadvantage. An increased cooling speed, however, is accompanied by 
difficulties in adjusting the comparative curve. The conclusions in the 
case of this method are also invalidated by using the thermal element as 
the measuring detector. That is, thermal elements falsely record the 
course of cooling of the test sample due to disturbing proximity 
influences. 
The object of the invention is to provide a method of the aforementioned 
type and an apparatus suited to carrying out the method, both of which are 
simpler to use and supply more exact differential values than known 
methods and apparatus. 
This object is solved according to the invention in that by using a vessel 
for molten metal, the cooling curve of the molten test sample is measured 
on the metal/vessel boundary layer and these measurements are used for 
adjusting the parameters of the comparative curve. 
Very exact difference values are obtained with the method of the invention 
since the heat effects above all in the molten zone of the curve at the 
metal/vessel boundary layer are the least inaccurate. The accuracy of the 
comparison test sample to be adjusted can be further improved by the 
determined difference values being differentiated. 
An apparatus to carry out the method comprises a temperature measuring 
device and a comparator which records the deviation of the cooling curve 
of the test sample from a fixedly given comparative curve which is 
produced by a signal transmitter working according to Newton's law 
(U=U.sub.O .times.e.sup.-t/RC, the U.sub.O parameter being the maximum 
value at the point in time t=0 and the RC parameter being the time 
constant), to which an adjustment means is arranged which, dependent on 
the parameters determining the cooling curve of the test sample in one 
curve section, adjusts the corresponding parameters U.sub.O and RC of the 
comparative curve for the corresponding curve section at the signal 
transmitter in such a manner that the two curve sections coincide, and is 
characterized in that the measuring detector of the temperature measuring 
device includes a light conductor which closes with its front side flush 
with the inside of the mold hollow space of a casting mold for the molten 
test sample. 
The comparator is preferably constructed in the form of a summation 
instrument for the deviation of the two curves. In addition, a 
differentiator can be arranged after the summation instrument. 
According to a further embodiment of the invention an elastic bonding agent 
can be arranged between the light conductor and the casting mold. This 
bonding agent should on the one hand oppose the ferro-static pressure by 
sufficient resistance to displacement of the light conductor, but on the 
other hand not hinder the movement of the light conductor in contraction 
direction of the metal on solidification and cooling. It has namely been 
shown that the front side of the light conductor adheres to the surface of 
the solidified metal block most probably caused by early formation of a 
narrow metal ridge on the light conductor/casting mold boundary surface 
which shrinks onto the light conductor and communicates hereto the 
movements of the metal wall. 
According to a further embodiment the light conductor is composed of 
optical quartz glass for temperatures over 500.degree. C. 
The invention is explained below in further detail by means of a drawing 
representing an embodiment.

A transducer 1 transforms the temperature T.sub.p of the test sample 
established by a heat detector later to be described into a proportional 
voltage U.sub.p. This voltage U.sub.p is transmitted over a closed switch 
2 to a summation instrument 3, to which the output signal U.sub.O 
.times.e.sup.-t/RC which is delivered by a regulating circuit 4 is 
supplied to form a difference so that the regulating circuit 4 as input 
signal receives the difference value between the two aforementioned 
signals. 
The regulating circuit comprises a signal transmitter which delivers the 
output signal U=U.sub.O .times.e.sup.-t/RC and an adjustment means which 
adjusts the parameters U.sub.O and RC of the signal transmitter. In a very 
simple embodiment the signal transmitter consists of current source 30 
having an adjustable output voltage and feeding current to a series 
combination of an adjustable resistor R and a capacitor 32. The voltage 
drop of the resistor R represents the desired output signal U. If the 
output signal U of the regulating circuit 4 deviates from U.sub.p 
corresponding to the temperature T.sub.p of the test sample at t=0, which 
is established in the summation instrument 3, then the regulating circuit 
receives an input signal differing from zero which adjusts the voltage of 
the current source 30 of the signal transmitter by means of the adjustment 
means in such a way that the output signal becomes U=U.sub.p =U.sub.O. 
Directly after this adjustment the switch 2 is switched over so that the 
summing member 3 at both inputs receives the output signal of the 
regulating circuit 4. In the further course of time the output signal then 
follows the law U=U.sub.O .times.e.sup.-t/RC. 
In order now to adjust the second parameter RC (time constant) of the 
signal transmitter for the purpose of adapting it to the cooling curve of 
the test sample, the switch 2 is switched back to the point in time 
t=t.sub.1. This time t.sub.1 should preferably still lie in the liquid 
zone of the test sample. When at this time a deviation is determined 
between the output signal U of the regulating circuit 4 and the signal 
U.sub.p at the output of the transducer 1 it is noticed in the summation 
instrument 3 the time constant RC is adapted by means of the adjustment 
means while maintaining the previously adjusted value U.sub.O, so that the 
output signal of the regulating circuit 4 also coordinates with the signal 
U.sub.p of the temperature of the test sample at t.sub.1. After this 
adjustment the switch 2 is switched over again so that the summation 
instrument 3 again receives the output signal of the regulating circuit 4 
at both inputs. Due to lacking deviation the regulating circuit 4 supplies 
an output signal for the further curve which satisfies the law of cooling 
U.sub.O .times.e.sup.-t/RC. The adjustment of the time constants RC is 
achieved at resistance R with largest possible capacity of the capacitor 
32. A large adjustment area is obtained hereby. 
The output signal of the regulating circuit 4 formed in this manner for the 
comparative curve is compared in a further summation instrument 5 with the 
signal U.sub.p corresponding to the temperature T.sub.p of the test sample 
for the purpose of forming the difference. The recorded difference signal 
over the time is the DTA curve. The difference signal is preferably 
supplied to a differential member 6 to increase accuracy and said 
differential member 6 provides the desired DDTA curve (derived 
differential thermal analysis curve). 
With the apparatus of the invention it is of course not only possible to 
adapt the output signal of the regulating circuit 4 to the cooling curve 
of the test sample in the liquid zone but also in a zone of lower 
temperature. Therefore, the invention enables the comparative curve to be 
derived from each zone of the cooling curve in order to obtain the most 
exact difference values and differential value possible. 
The described apparatus is preferably used together with the light 
conductor represented in FIG. 2. 
The metal 8 situated in a casting mold 7 touches the front side 9a of the 
reflected optical quartz glass light conductor 9 and communicates its 
radiation by means of the latter and a further light conductor 10 composed 
of flexible fibre optics to an infrared radiation pyrometer 11. Further 
processing of the measured values obtained is then achieved by means of 
the transducer 1 and the DTA apparatus 2 to 5 and the differential member 
6 as described in connection with FIG. 1. The DDTA curve obtained in this 
manner is registered together with the normal cooling curve by means of a 
multi-channel recorder 12 (preferably a light beam oscillograph). 
The casting mold 7 is sealed with an elastic bonding agent 13 at the outlet 
of the light conductor 9 which is in principle movably arranged. 
The particular advantage of the method of the invention and of the use of 
the apparatus according to the invention lies in the effect that the 
advantages of the differential thermal analysis are open for test sample 
amounts of over 10 g up to the size of the real castings and thereby 
provides important information which benefits the production and use of 
castings. What is to be stressed in particular is the use of the apparatus 
of the invention in connection with optical measurement of the course of 
cooling of the metal test sample to be tested. It has been shown that 
cooling curves in zones free from transformation recorded by means of this 
optical method strictly follow Newton's law of cooling and thereby 
represent a very exact basis for adapting the comparative curve. The 
method and apparatus according to the invention on the whole increase the 
accuracy of measurement. 
While a preferred embodiment of the invention has been shown and described 
herein it will become obvious that numerous omissions, changes and 
additions may be made in such embodiment without departing from the spirit 
and scope of present invention.