FIELD OF INVENTION
The present invention generally relates to. evaluation of torsional properties in
high carbon steel wires. In particular, the present invention relates to a method
to evaluate torsional properties in high carbon steel wires by eddy current
measurement.
BACKGROUND OF THE INVENTION
The torsional ductility is an important property in high carbon steel wires and it is
of great significance when these wires are subjected to undergo further plastic
deformation during manufacturing, such as the stranding tire cords, the prestresses
concrete wires or the coiling springs. The longitudinal splitting into
several layers in the wire during torsion, named as 'delamination', is one of the
qualitative indication of decrease in ductility.
The torsional ductility also is an important property in high carbon steel wires
which is usually measured by a torsion testing machine as per IS: 1717-1985.
The testing is performed on wire samples and the sample is destroyed once the
testing is complete. It is not a realistic 'approach to take decisions on the entire
coils based on results from the small test samples. Torsional property
measurement in these wires using eddy current method can assure a large
volume of coils and is non-destructive in nature.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a method to
evaluate torsional properties in high carbon steel wires by eddy current
measurement which finds a correlation between the eddy current parameter
(output voltage) in high carbon steel wires.
Another object of the present invention is to propose a method to evaluate
torsional properties in high carbon steel wires by eddy current measurement
which finds a correlation between eddy current parameter (output voltage)
torsional properties in high carbon steel wires.
A further object of the present invention is to propose a method to evaluate
torsional properties in high carbon steel wires by eddy current measurement
which develops a better method to ensure desired microstructure and torsional
properties in coils made of high carbon steel wires in a non-destructive manner.
A still further object of the present invention is to propose a method to evaluate
torsional properties in high carbon steel wires by eddy current measurement
which ensures a large volume inspection of coils for better customer satisfaction.
A still another object of the present invention is to propose a method to
evaluate torsional properties in high carbon steel wires by eddy current
measurement which helps operational people to reduce internal/external
rejection due to poor properties of the drawn wires.
An yet another object of the present invention is to propose a method to
evaluate torsional properties in high carbon steel wires by eddy current
measurement which ensures the high carbon steel wire products sent to the
customers free from undesirable properties so that they can improve the
reliability of the components/ construction made of these wires.
SUMMARY OF THE INVENTION
In the present invention, a non-destructive testing method using eddy current
principle has been developed to evaluate the high carbon steel wires with respect
to its torsional properties. A good correlation was also developed between the
torsional value and the eddy current output voltage which can be utilized to
measurethe torsional properties in such wires on line.
The measurements of important properties like torsional ductility of high carbon
steel are made on line by non-destructive method like eddy current. The entire
coils can be tested for their torsion values and decisions on the coils can be
made in a more realistic way. Before entering for field trial, it is necessary to
verify this concept in laboratory stage by developing a correlation between
the eddy current parameters like output voltage and the torsional properties in
high carbon steel wires.
In the present invention, eddy current testing was performed on high carbon
steel wire of commercial known Prestressed Concrete (PC) with different
torsional properties.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - shows correlation between eddy current frequency and penetration depth
in PCllS wire samples
Fig.2 - shows correlation between eddy current frequency and output voltage in
PCllS wire samples with torsional value 8 and 12
Fig.3 - shows correlation between eddy current frequency and output voltage in
PCllS wire samples with different torsional values
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Samples of cold drawn wires of 4.25 mm diameter were taken from a bulk
sample of commercially known PCllS wire. The chemical composition of these
wires were as mentioned in the Table 1.
Table 1: Chemical Composition of PC115grade wire
%C O/oMn O/OP 0/05 %Si %Cr
0.82 0.689 0.013 0.012 0.21 0.12
These wire samples were taken from the vicinity of these samples which were
tested for torsional properties as per 15:1717-1985. Nondestructive eddy current
output voltage in these samples was measured using an Eddy Current Tester
ZETEC Model MIZ-27-CT. The eddy current measurements were taken by
inserting these wires into an encircling coil of internal diameter (10) 5.5 mm.
Theoretical aspects of eddy current measurement
The induced output voltage V in a sensing eddy current coil carrying current I
and impedance Z is given by
V= 1.2 --------------------1
Depth of penetration 5 of induced eddy current in a test specimen is given by
11= [1/(no\Jf)1/2]---------2
The resistance offered by the test material causing increase in the output
voltage, which increases with increase in magnetic permeability, decrease in
electrical conductivity (increase in electrical resistivity) and increase in
frequency of the current. The test material variables like microstructural
constituents, type of elements, discontinuities, residual stresses etc. affect the
magnetic permeability and electrical conductivity and hence the output voltage of
the eddy current.
Experimental
A torsional testing was conducted on known as LRPCwires of diameter 4.25 mm
as per specification 15:1717-1985. The samples for eddy current measurement
were taken from the vicinity of the samples used for torsional testing. Eddy
current frequencies varied within the range 10-630 kHZ to vary penetration
depths in the tested samples. As shown in Fig.3, with decrease in eddy current
frequency penetration depth increases.As shown in Fig.4, a correlation between
eddy current frequency and output voltage for two LRPCwires with torsion value
8 and 12 from the same casting batch. A sharp peak in the output voltage were
observed at frequency 170 KHZ for both the different wires. Overall output
voltage was found to be more in the sample with higher torsional no. The output
voltage difference was found to be maximum at frequency 170 kHZ. Fig.S shows
a correlation between eddy current output voltage and torsional values in no. of
wires with different torsional no. from 3 to 14. A very good correlation was found
between output voltage and torsional no. With increase in output voltage
torsional no. increases and there is possibility to predict the torsional values
unknown samples from eddy current output voltage measurement from the
standard graph as shown in figA.
In the wire samples of commercially known grade LRPC,a good correlation was
found to exist between output voltage and torsional value. As the literature
suggest, a coarse pearlitic structure (with large interlamellar spacing) is more
prone to delamination causing low torsional values than a fine pearlitic structure
(With small interlamellar spacing).
WE CLAIM
1. A method to evaluate torsional properties in high carbon steel by eddy
current measurements comprising:
a) taking a plurality of samples of high carbon steel wire rods having known
torsional no;
b) testing torsion no. of the samples according to IS specification No:1717:
1985 by eddy current output voltage;
c) measuring the eddy current output voltage of the samples of known
torsional value;
d) preparing a standard graph of eddy current outflow voltage VS known
torsion value;
e) testing eddy current outflow voltage of a new sample (unknown torisonal
value); and
f) comparing the eddy current outflow voltage with standard graph (eddy
current outflow voltage vs torsional no.) to find out the torsional value of
unknown samples.
2. The method to evaluate properties in high carbon steel and eddy current
measurement wherein the sizes of wire sample is of 4.25 mm.