FIELD OF THE INVENTION
The present invention relates to a method and a system for surface and sub-
surface crack detection to determine optimal grinding condition of high speed
steel (HSS) rolls.
BACKGROUND OF THE INVENTION
During hot rolling of heavy plate and wide hot-strip material, the surface of the
employed rolls wears out because of its high loads in the roll gap. High surface
pressure combined with cyclic thermal stress leads to micro structural changes
within the material. As a result of these loads, the profile of the roll body is
deformed during use and crack propagation occurs in the roll surface. A worn
surface of a roll has a major negative impact on the surface quality of rolled
plates and strips in terms of dimensional tolerance, shape and flatness.
Therefore, the rolling process is interrupted at regular intervals to grind and to
recondition the roll surfaces so that the rolls can then be reused in the rolling
process. The material of the rolling surface should be sufficiently machined to
grind off any cracks and micro structural damage, and thus prevent propagation
of the cracks. So after each grinding, the roll surface has to be examined by non-
destructive techniques. Usually eddy current and Dye-penetrant techniques are
used for checking of surface cracks. Sometimes along with eddy current,
ultrasonic technique is also used [Takada Hazime et al. JEE Technical Report
2007]. The high speed steel (HSS) rolls have high hardness and good wear
resistance at high temperatures. This type of roll is used in finishing applications
for increased campaign times and better surface finish. But as the hardness of
HSS reaches 80/85 shore, it becomes very much crack-sensitive. Hence it is
highly recommended to eliminate all kinds of surface cracks, whenever these
rolls are reground, otherwise typical 'cats' - tongue band type spalls may occur
that lead to abnormal failure.

Accordingly, there is a need to non-destructively detect the presence or absence
of any fine cracks on the roll surface and simultaneously determine the optimal
grinding condition of the rolls to overcome the under or over grinding situations.
Using the existing ultrasonic technique with surface wave probe of frequency
2.25 MHz, assessment of the surface quality of 700mm- 1100mm diameter HSS
rolls are not possible (IN Patent IN200400104-12; IN227265B, 2004, Raj M and
Pandey J C, Development of novel ultrasonic testing technique for assessing
surface cracks depth by using ultrasonic surface wave probe)
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved
ultrasonic inspection method for non-destructive detection of surface cracks of
high hardness Rolls.
Another object of the present invention is to propose an improved system for
detecting the surface and sub-surface crack to determine optimal grinding
condition for a complete crack free surface.
A still another object of the present invention is to propose an improved
ultrasonic inspection method for non-destructive detection of surface cracks of
high hardness Rolls, which eliminates the disadvantages of prior art detection
systems such as ultrasonic, eddy current and dye-penetrant techniques for the
detection of surface cracks of Rolls with very high hardness.
Yet another object of the present invention is to propose an improved system for
detecting the surface and sub-surface crack to determine optimal grinding
condition for a complete crack free surface, which is enabled to generate a
calibration curve of ultrasonic signal amplitude from the half of the circumference

of the roll (BW) with crack-depth from a fixed distance with fixed Gain for HSS
Roll.
A further object of the present invention is to propose an improved system for
detecting the surface and sub-surface crack to determine optimal grinding
condition for a complete crack free surface, which is also enabled to calibration
curve of ultrasonic signal amplitude from the crack with crack-depth from a fixed
distance with fixed Gain for HSS Roll.
SUMMARY OF THE INVENTION
Accordingly, there is provided in one aspect of the invention, an improved
ultrasonic inspection system for non-destructive detection of HSS roll, comprising
at least one ultrasonic transducer and an ultrasonic flaw detector unit, the
transducer unit is configured to transmit surface wave of frequency about
0.5MHz to 1.0MHz into a test object, the transducer unit being excited using a
pulser of the flaw detector; a display device to exhibit the processed wave signal
which travel diametrically to scan the test object along the length in clock and
anti-clockwise directions.
According to another aspect of the invention, there is provided a method for
surface and sub-surface crack detection of high-speed steel rolls. The method
comprises: exciting a surface wave ultrasonic probe of frequency 0.5 MHz, by a
400 V peak to peak pulse and acquiring receive energy from half the
circumference (BW) of the roll of diameter ranging from 700 mm to 1200 mm;
and generating calibration curves with crack depth and crack echo amplitude
including crack depth and BW echo amplitude for the HSS Roll with fixed Gain
and fixed Distance.

In a preferred embodiment of the present invention, the test object may be a
cylindrical body of diameter ranging from 700 mm - 1200 mm with very high
surface hardness.
In an embodiment of the present invention a single transducer is used as the
receiver which acquires receive energy from half the circumference, and
determines the presence of any surface crack within half the circumference by
scanning only from two angular positions of the Roll.
In still another embodiment of the present invention, the system generates a
calibration curve for the signal amplitude from half the circumference with crack
depth from a fixed distance with fixed gain.
In a further embodiment of the present invention, the system generates a
calibration curve for the signal amplitude from half the circumference with crack
depth from a fixed distance with fixed gain will provide the decision for finding
the optimal grinding condition of the Rolls.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND TABLE
Figure 1 - A schematic diagram of the ultrasonic detection system according
to the invention.
Figure 2 - Shows the received ultrasonic signal from half-the -circumference
in a test object constituting a HSS Roll of about 700 mm dia in
finish mill.
Figure 3 - Shows the received ultrasonic signal from half-the-circumference in
a test object constituting a HSS Roll of about 1000mm dia in a
roughing mill.
Figure 4 - Shows the inventive system including the test object particularly
indicating the measurement position of the object.

Figure 5 - Shows the ultrasonic signals captured at about 1300 mm distance
from the work side.
Figure 6 - Shows the distance amplitude curve for crack depth of about 6 mm
of HSS Roll (test object).
Figure 7 - Shows graphically the back-wall amplitude vs crack depth from a
fixed distance with a fixed gain.
Table - Shows the values of back-wall echo amplitude vis-a-vis crack
depth.
DETAIL DESCRIPTION OF THE INVENTION
Accordingly, the invention system utilizes the ultrasonic surface wave to
propagate on the roll surface along the diameter. In the present system
preferably a single transmitter of about 0.5 MHz is used as a transmitter and
receiver of the surface elastic wave through the roll. The roll is scanned in both
clock and anti-clockwise direction at 0° and 90° positions along the length to
cover the entire roll surface. The received signal is analysed to determine
present or absence of crack echo. If the roll is crack-free, the echo only from half
the circumference (BW) will be observed. If any crack echo is observed in
between the BW echo, then the system compares amplitude of the BW Echo with
crack depth through the calibration curve.
The present invention thus allows to detect any crack including the crack depth
as well as the optimal grinding conditions from the developed calibration curve.
The system of the invention constitutes a faster surface crack detection
technique compared to the known eddy current technique. Moreover, this
technique can be used for rolls with high surface hardness like HSS and also for
other rolls. From the developed calibration curve, optimal grinding conditions of

the HSS roll can be estimated which reduces the rate of roll failure at the roll
shop of steel industries.
The following examples are given by way of illustration and should not be
construed to limit the scope of invention.
EXAMPLE - 1
As shown in Figure-1, Ultrasonic received signal from haft the circumference in
the test rolls of diameter of about 700 mm of finishing mill (figure 2) and about
1100 mm of roughing mill (figure 3) respectively made of HSS with no defect
using about 0.5 MHz Surface Wave Probe at 87 dB Attenuator setting.
EXAMPLE - 2
Calibration and measurements were carried out on the HSS roll having a length
of about 2000 mm and diameter about 700 mm with cracks on the surface.
Scanning was carried out along the length of the roll in both clock and anti-
clockwise directions and total scan time was 5 mins. Crack echo was observed
from a position of about 1150 mm and ends at about 1450 mm along the length
from the work side of the Roll. A schematic of the test roll is shown in figure 4.
All measurements were carried out from 350 mm in the radial direction from the
crack positions. Eddy current technique was also used to find the crack extension
on the surface of the roll and was found at 230 mm. Received signal at about
1280 mm from the work side corresponds to the position of deepest crack is
shown in figure 5.
EXAMPLE - 3
Measurements were carried out at different distances from the deepest crack to
find the change in crack echo amplitude with distance. Figure 6 shows the
variation of crack echo amplitude with distance for the deepest crack on HSS roll.

EXAMPLE - 4
Calibration curve has been made for the back-wall echo amplitude with the crack
depth. Table 1 depicts the crack depth and corresponding back wall echo
amplitude and figure 7 shows the variation of back wall echo amplitude with
crack depth from a fixed distance with fixed gain.
Table 1: crack depth Vs. Back wall Echo Amplitude

The advantages of the present invention are:
1. The improved inspection method of the present invention is a faster
surface and sub-surface crack detection technique compared to the prior
art eddy current technique.
2. The improved ultrasonic inspection technique can be used for rolls with
high surface hardness like HSS and also for other rolls.
3. From the generated calibration curve, optimal grinding conditions of the
HSS roll can be estimated which reduces the rate of roll failure at the roll
shop of steel industries.

WE CLAIM
1. An improved ultrasonic inspection method for the detection of fine surface
cracks of high speed steel (HSS) Rolls, said method comprising:
- providing at least one 0.5 MHz surface wave probe;
- providing a test object in the form of a HSS roll;
- acquiring receive energy from half the circumference;
- determining the presence of any surface crack in between by scanning
the test object only from two angular positions of the Roll; and
- generating calibration curves with crack-depth vs crack echo amplitude
including crack depth vs back wall echo amplitude; and
- obtaining optimum grinding conditions of the HSS roll from the
calibration curves.

2. The method as claimed in claim 1, wherein the ultrasonic surface probe is
moved along the length, at two angular positions 0° and 90° of the Roll
in clock and anti-clockwise directions and covers the entire roll surface.
3. The method as claimed in claim 1 or 2, wherein the acquiring step
comprises acquiring signal amplitude data from half the circumference
(BW) without any intermittent signal from the crack being of amplitude
higher by 50% to 80%.
4. The method as claimed in any of the preceding claims, wherein the BW
echo amplitude reduces with the presence of surface crack and the
threshold of the amplitude of the BW echo is obtained from the calibration
curve.
5. The method as claimed in claim 1, wherein the calibration curve is

generated with fixed Gain of 87 dB and fixed Distance of 350 mm for the
. HSS Roll.
6. The method as claimed in claim 1 or 3, wherein the acquiring step further
comprises adapting the BW echo amplitude to determine the surface
condition from the calibration curve.
7. The method as claimed in claim 6, wherein the BW echo amplitude
decreases linearly with the crack depth.
8. The method as claimed in any of the preceding claims, wherein the BW
echo amplitude of 50% - 70% confirms crack free surface of the Roll.
9. The method as claimed in claim 1, wherein the surface wave of frequency
0.5 MHz - 1MHz is enabled to detect sub-surface crack up to one
wavelength.
10. An improved ultrasonic inspection system for non-destructive detection of
surface and sub-surface cracks of high speed steel Rolls, comprising:

- at least one ultrasonic transducer and an ultrasonic flaw detector unit,
the transducer unit is configured to transmit surface wave of frequency
about 0.5MHz to 1.0MHz into a test object, the transducer unit being
excited using a pulser of the flaw detector; and
- a display device to exhibit the processed wave signal which travel
diametrically to scan the test object along the length in clock and anti-
clockwise directions.

11. An improved ultrasonic inspection method for the detection of fine surface
cracks of high speed steel (HSS) Rolls, as substantially herein described
and illustrated with reference to the accompanying drawings.
12. An improved ultrasonic inspection system for non-destructive detection of
surface and sub-surface cracks of high speed steel Rolls, as substantially
herein described and illustrated with reference to the accompanying
drawings.

The invention relates to an improved ultrasonic inspection method for the
detection of fine surface cracks of high speed steel (HSS) Rolls, said method
comprising providing at least one 0.5 MHz surface wave probe; providing a test
object in the form of a HSS roll; acquiring receive energy from half the
circumference; determining the presence of any surface crack in between by
scanning the test object only from two angular positions of the Roll; and
generating calibration curves with crack-depth vs crack echo amplitude including
crack depth vs back wall echo amplitude; and obtaining optimum grinding

conditions of the HSS roll from the calibration curves.