FIELD OF INVENTION:
The present invention generally relates to a method for measuring
interlamellar spacings of pearlite in high carbon pearlite steel wires/wire rods. In
particular, the present invention relates to a non destructive method to determine
interlamellar spacings of pearlite in high carbon steel wires by eddy current
measurement.
BACKGROUND
Interlamellar spacing of cementite lamellae in pearlite grade of drawn
steel wire is a crucial material parameter which can affect the performance of
these wires/wire rods either during further processing into thinner gauges or in
service. Interlamellar spacing in pearlite microstructure is an important
parameter to design pearlite grade steel wires of higher strengths. Reduction in
interlamellar spacings increases strength of such steels. In comparison to a fine
pearlite structure with shorter interlamellar spacings, a coarse pearlite structure
with increased interlamellar spacings, enhances the 'delamination' factor during
the torsional testing leading to low torsional values. Earlier, the only method to
measure interlamellar spacing in high carbon steel wire/wire rods was using
Scanning Electronic Microscope (SEM). Although the interlamellar spacing
measured by SEM is very accurate, it is a destructive and costly method. The
information obtained by SEM is very localized and confined to a very small spot
on a plane. It is time taking and tedious also.
The SEM requires a robust System equipped with costly devices to carry out such
measurement. (The special features of non-destructive eddy current
measurement when compared to SEM are it is simple, cheap, fast, suitable for
on-line measurement and has ability to measure large volume inspection.)
OBJECT OF THE INVENTION:
It is therefore, an object of the present invention to propose a method to
determine interlamellar spacings of pearlite in high carbon steel wires by eddy
current measurement, the interlamellar spacings being related to the eddy
current parameter (output voltage) in these wires.
A further object of the present invention is to propose a method to
determine interlamellar spacings of pearlite in high carbon steel wires by eddy
measurement in a non-destructive manner to ensure a large volume inspection
of coils.
A still another object of the present invention is to propose a method to
determine interlamellar spacings of pearlite in high carbon steel wires by eddy
current measurement with reduces rejection percentage of the drawn wires.
Yet another object of the present invention is to propose a method to
determine interlamellar spacings in high carbon steel wires by eddy current
measurement which 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 determine interlamellar spacings of
pearlite in high carbon steel wires. A correlation was also developed between
the interlamellar spacings of pearlite and the eddy current output voltage which
can be utilized to measure microstructure (interlamellar spacings of pearlite) in
such wires on line.
If the measurements of an important wire parameter like interlamellar
spacings of pearlite of high carbon steel are made on line by non-destructive
method like eddy current, the entire coils can be ensured for the desired
microstructure with respect to its interlamellar spacings of pearlite and decisions
on the coils can be made in a more realistic way.
In the present invention, eddy current testing was performed on high
carbon steel wires of commercially known Prestressed Concrete (PC) with
different interlamellar spacings of pearlite. The present method is much simpler
and non destructive in nature and is capable to measure the interlamellar
spacings in a large volume of coils. The inventive method requires that a
Calibration graph/empirical equation be generated and correlate eddy current
parameters (output voltage), including interlamellar spacing using SEM. Once
this correlation is developed, interlamellar spacing can be easily determined from
the measured eddy current parameters.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
Fig 1: Shows correlation between eddy current frequency and penetration depth
in PC 115 wire samples.
Fig 2: Shows FE-SEM micrograph showing average interlamellar spacing 0.107
urn after isothermal transformation at 550°C
Fig 3: Show FE-SEM micrograph showing average interlamellar spacing 0.121 urn
after isothermal transformation at 580°C
Fig 4: Show FE-SEM micrograph showing average interlamellar spacing 0.128 urn
after isothermal transformation at 605°C
Fig 5: Show FE-SEM micrograph showing average interlamellar spacing 0.173 urn
after isothermal transformation at 635°C
Fig 6: Show FE-SEM micrograph showing average interlamellar spacing 0.178 µM
after isothermal transformation at 660°C
Fig 7: Show FE-SEM micrograph showing average interlamellar spacing 0.290 µM
after isothermal transformation at 685°C
Fig 8: Show correlations between super cool temperature and interlamellar
spacings in a PC wire of dia 4.25 mm
Fig 9: Show correlations between eddy current out put voltage and interlamellar
spacing in PC wire
FiglO: Show correlations between interlamellar spacing predicted using empirical
equation mentioned in Fig 3 and that measured by Scanning Electron
Microscope FE-SEM in PC wire.
DETAILED DESCRIPTION QF A PREFERRED EMBODIMENT
OF THE INVENTION:
The induced output voltage V in a sensing eddy current coil carrying current I
and impedance z is given by
V=I.Z----------------------------1
Depth of penetration 5 of induced eddy current in a test specimen is given by

The output voltage of eddy current depends on magnetic permeability, 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.
Samples of cold drawn wires of 4.25 mm diameter were taken from a bulk
sample of commercially known LRPC wire. The chemical composition of theses
wires were as mentioned in the Table 1.
Table 1: Chemical Composition of LRPC wire

To vary interlamellar spacings, theses wires were given salt bath treatment in a
Salt Bath Furnace of R & D at temperatures 685, 660, 635, 605, 580 and 550°C
after holding in the salt bath for 5 minutes. After heat treatment these wires
were pickled in acid to remove scales. Non-destructive eddy current out-put
voltage in these samples was measure using suitable Scanning Electron
Microscope, at magnification more than 25000 using intercept method. Figures
2-7. show the typical micrographs with varying interlamellar spacings due to
variation in isothermal temperatures. The effect of isothermal transformations in
a salt bath at different super cool temperatures (Te-Tr), where Te is the eutectoid
equilibrium temperature and Tr reaction temperature, have been shown in Fig.8
Correlation between eddy current out-put voltage and interlamellar spacing has
been shown in Fig.9. Graph plot between interlamellar spacing predicted by
empirical equation obtained from eddy current measurements and that measured
by SEM by intercept method has been shown in Fig. 10.
WE CLAIM:
1. A non-destructive method to determine interlamellar spacing of pearlite in
high Carbon Steel wire, the method comprising the steps of:
providing a plurality of samples of high Carbon Steel wire each
subjected to isothermal transformations in a salt bath between 550°C
to 685°C and super cool temperature between Te and Tr where Te is
eutectoid equilibrium temperature & Tr is reaction temperature, and
measure interlamellar spacing of pearlite by Scanning Electron
Microscope at magnification more than 25000.
providing an eddy current tester capable of precision measurement;
measuring eddy current output voltage of said plurality of sample
pieces by using the tester;
preparing a calibration graph based on the output voltage values and
the known interlamellar spacing of said plurality of sample wires;
providing a new sample piece of high Carbon Steel wire with unknown
interlamellar spacing;
measuring eddy current output voltage of said new sample pieces; and
determining the interlamellar spacing of the new sample piece by
interpolating the output voltage values on said calibration graph.
2. The method as claimed in claim 1, whereas interlamellar spacing values of
said plurality of sample pieces are determined by scanning electron
micrograph (SEM) after subjecting the pieces to said variant temperature
leading to isothermal transformation.
3. The method as claimed in claim 2, wherein the SEM uses an intercept
method.
4. The method as claimed in claim 1, wherein the plurality of sample pieces
are subjected to super cool temperature which constitutes the differential
temperature between an eutectoid equilibrium temperature and a reaction
temperature.
5. A non-destructive method to determine interlamellar spacing of pearlite in
high Carbon Steel wire by eddy current measurement as substantially
described herein with accompanying drawing.

The present invention is a novel method to evaluate interlamellar spacings in
high Carbon Steel wires by eddy current measurement (output voltage). Sample
of wires are subjected to a isothermal transformation in a salt bath between
550°c to 685°C subsequently to a super cool temperature ranging from eutectic
equilibrium temperature to reaction temperature for isothermal transformation.
Interlamellar spacing of such wires are measured by SEM (Scanning Electron
Micrograph) and their corresponding eddy current output voltage are measured
by suitable eddy current tester. A calibration graph/empirical equation is
prepared between eddy current voltage Vs known interlamellar spacing. Eddy
current voltage of a new sample is measured and interlamellar space is predicted
by using the calibration graph/empirical equation.