FIELD OF THE INVENTION
The present invention generally relates to a Rayleigh wave based ultrasonic inspection
systems and a method for residual stress evaluation of high speed steel (HSS) rolls. More
particularly, the invention relates to an ultrasonic system and a cause-detection of surface
crack generaztion during HSS rolling process in an ultrasonic system to avoid roll failure
through corrective action in roll cooling phase.
BACKGROUND OF THE INVENTION
Degradation of the work rolls while on the early stands of the finishing mill in a steel rolling
process, is a major concern for the mill operators. It reduces the length of the rolling
campaigns, increases the production costs and induces strip surface defects. As the rolls
during hot rolling are subjected to cyclic thermal variations, that may induce severe thermal
gradients in the roll for example, a hotter outside and a cooler inside. Thus, the outer part
shows more thermal expansion than the inner part of the roll and creates thermal stress
leading to compression outside and tensile inside. The thermal tensile stress adds up the
residual tensile stress and the total tensile stress reaches the yield strength of the inner
material which interalia results crack on the surface of the roll . On one hand, compressive
stresses on the roll surface prevent heat cracks and other cracks perpendicular to the roll
surface from penetrating into the roll and provide resistance toward thermal fatigue. On the
other hand, presence of compression stress in the shell may cause tensile stresses in the core
and at the bond/interface of the shell and the core in both longitudinal and radial directions.
The higher the compressive stresses in the shell, the higher the tensile stresses in the core and

in the bond zone. Rolls with higher internal stress are prone to "cat's tongue" crack
propagation and spall. Along with thermal fatigue, cracks parallel to the roll axis are
generated due to contact fatigue. The high speed steel (HSS) rolls have high hardness and
good wear resistance at high temperatures. This grade is only produced by the known
horizontal Centrifugal Cast (CC) method and has the core material in spheroidal graphite
pearlitic iron. 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 determine the cause of generation
of cracks so that preventive action can be taken in the mill to avoid any catastrophic failure.
Accordingly, there is a need to non-destructively detect the cause of generation (stress riser)
of fine cracks on the roll surface and simultaneously obtain the corrective action to overcome
the situation of "cat's tongue" crack propagation and spall. Using the well known x-ray
diffraction technique it is possible to monitor the stress condition on the surface of the roll but
it cannot predict the cause of stress. Moreover, XRD is an expensive and time consuming
technique.
It is therefore an object of the present invention to propose an improved ultrasonic inspection
method for non-destructive evaluation of residual stress of HSS Rolls with high hardness.
Another object of the present invention is to propose an improved ultrasonic inspection
method for non-destructive evaluation of residual stress of HSS Rolls with high hardness,
which allows detecting the cause of generation of surface cracks formed due to thermal or
contact fatigue and initiating corrective action.

A still another object of the present invention is to propose an improved ultrasonic inspection
method for non-destructive evaluation of residual stress of HSS Rolls with high hardness,
which is simple, cost effective, and eliminates the disadvantages of known x-ray diffraction
in the evaluation of residual stress of rolls with very high hardness.
Yet another object of the present invention is to propose an improved ultrasonic inspection
method for non-destructive evaluation of residual stress of HSS Rolls with high hardness,
which is enabled to generate calibration curve of ultrasonic velocity difference in axial and
radial direction with the stress as measured by the non-destructive evaluation technique of
residual stress of HSS rolls.
A further object of the present invention is to propose an improved ultrasonic inspection
method for non-destructive evaluation of residual stress of HSS Rolls with high hardness, in
which the cause of crack generation is detected based on the sign of velocity difference in
the axial and radial direction for HSS Roll.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method and system for determining the cause of surface
crack generation (due to thermal fatigue or contact fatigue) on the surface of high speed steel
(HSS) rolls based on Rayleigh wave ultrasound. The method comprises two surface wave
ultrasonic probes of frequency 0.5 MHz fixed at a constant distance in a designed probe
holder. One of the probes is excited by a 400 V peak to peak pulse and the other probe is used
to receive the transmitted energy. The ultrasound wave velocity along the axial and radial

directions of the roll is measured. Calibration curves with stress as measured by x-ray
diffraction technique and the velocity difference in axial and radial direction is developed for
the HSS Roll with fixed Distance. The magnitude of velocity difference can be useful to
predict the stress condition on the surface of the roll whereas its sign will be useful to predict
the cause of crack formation on the surface of the roll. The present invention is useful for any
steel industries; manufacturers as well as users of HSS Rolls to take the corrective measure
on the improvement of roll cooling or lubrication based on the Rayleigh wave ultrasound
velocity difference in axial and radial direction of the roll.
The method comprises providing a system having at least two ultrasonic transducers, a probe
holder, and an ultrasonic flaw detector unit;transmitting by a first transducer a surface wave
of frequency 0.5MHz into a test object; exciting the transducer by the flaw detector;
receiving in the first transducer the transmitted signal, the probe holder holding both the
transducers at a fixed distance; monitoring in a display device of the flow detector the axial
travel of the signal by placing the probe diametrically along the length of the test object and
measuring the travel velocity of the signal in the central region of the object; monitoring in
the display the travel velocity of the signal by placing the probe perpendicularly along the
length of the test object and measuring the velocity of the signal in the central region of the
object, wherein a calibration curve for the ultrasonic wave velocity difference in the axial and
radial directions is generated with respect to the stress values measured by a x-ray
diffraction technique.

In an 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 another embodiment of the present invention, the transmitter and receiver frequency may
be in the range of 0.5 MHz-1 MHz
In yet another embodiment of the present invention one transducer is used as the transmitter
and the other as receiver fixed at a constant distance with the help of the probe holder.
In a further embodiment of the present invention, the sign of the velocity difference in the
axial and radial direction from a fixed distance allows to detect the cause of the crack
generation on the surface of the rolls including taking of corrective measure on roll cooling
or roll lubrication.
In a still further embodiment of the present invention, the sign of the velocity difference
refers the stress attributer. Stress due thermal fatigue if axial velocity (V90)» radial velocity
(Vo); whereas if V0>>V90, then stress is due to contact fatigue.
In an yet further embodiment of the present invention, if V90>>V0, roll operator should
manipulate roll cooling, whereas if V0>>V90, and the roll lubricants needs attention.
In a still another embodiment of the present invention, surface wave of frequency 0.5 MHz -
1 MHz. is produced.

The present invention system utilises the ultrasonic Rayleigh wave to propagate on the roll
surface along the axial and radial directions. In the present invention, at least two 0.5 MHz
transducers are used. One transducer is used as a transmitter and the other transducer is used
as receiver of the surface elastic wave through the roll. A probe holder is designed to hold
two probes at a fixed distance of 170 mm. The sound velocity is measured in both axial and
radial directions at the central position of the roll surface by placing the probe holder with
two transducers. The velocity difference in axial and radial directions is determined for the
presence of stress on the surface and to predict the cause of stress generation. For the new roll
or for high compressive stress on the roll surface, the velocity difference is found to be very
low (<20 m/s for HSS). In the present invention, the velocity difference in the axial and radial
directions is correlated with stress difference through the calibration curve.
The present invention allows detecting the cause of stress that leads to crack generation on
the surface of the roll based on the sign of the velocity difference.
From the developed calibration curve, optimal velocity difference value (<20m/s) for
acceptable surface compressive stress of the HSS roll is estimated to reduce the rate of roll
failure at the roll shop of steel industries.
Advantages of the present invention is that it is a faster, cost effective stress evaluation
technique compared to the prior art x-ray diffraction technique. Moreover, this technique can
be used to detect the cause of stress generation on the rolls with high surface hardness like
HSS. The developed technique indicates to the roll operator in real time to either adjust roll

lubricants or manipulate roll cooling rate based on the velocity difference in two orthogonal
directions. This in turn will reduce the rate of roll failure at the roll shop of steel industries.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- Pictorial view of the Inspection System of the invention wherein the probe is placed
(a) along the axial direction; (b) along the radial direction
Fig. 2- Stress mapping using x-ray diffraction technique
Fig. 3- Velocity mapping in axial and radial directions till 15th campaign
Fig. 4- Correlation between ∆V and ∆σ
Fig. 5- Variation of ∆V with campaign length
DETAIL DESCRIPTION OF THE INVENTION
Pictorial view of an inspection system implemented in accordance with one aspect of the
invention is shown in figure 1
For calibration, according to the invention, measurements were carried out on HSS roll of
length 2000 mm and diameter 700 mm using using an x-ray residual stress analyser and
adapting an ultrasonic surface wave technique. A stress mapping at the centre position of the
test object along the axial (0°) and radial (90°) till 15th campaign before and after grinding of
the roll using an x-ray diffraction technique, and the corresponding velocity mapping in the
same location using a Rayleigh wave ultrasonic technique have been carried-out. The results
are shown in figure 2 and 3 respectively.
Calibration curve of difference in stress (∆σ) with difference in velocity (∆v) in two
orthogonal directions as established in this invention is shown in figure 4.

In an alternative embodiment, measurements as described hereinabove, were carried out
using only ultrasonic technique on same pair of roll till the scrap diameter (660mm), and the
AV was monitored in the mill out after grinding on both the rolls. Variation of the AV with
campaign length for one of the test pair of roll is depicted in figure 5.
According to the present invention the signal shift is acquired in time scale due to change in
velocity in axial and radial directions. The velocity of the signal travel reduces with the
presence of surface crack and a threshold of the change in velocity can be obtained from the
calibration curve .The velocity difference is used to determine the surface stress condition
from the calibration curve. For an acceptable compressive stress on the surface of the roll, the
velocity difference should be below 20m/s for HSS roll.
The improved Rayleigh wave based ultrasonic inspection method of the invention estimates
non-destructively surface and sub-surface stresses and detects the cause of stress riser of
HSS Rolls using a surface wave probe of frequency 0.5 MHz.

We Claim:
1. An ultrasonic system and a cause-detection of surface crack generaztion during HSS
rolling process in an ultrasonic system to avoid roll failure through corrective action in roll
cooling phase, the method comprises the steps of:
-providing a system having at least two ultrasonic transducers, a probe holder, and an
ultrasonic flaw detector unit;
-transmitting by a first transducer a surface wave of frequency 0.5MHz into a test object;
- exciting the transducer by the flaw detector;
- receiving in the first transducer the transmitted signal, the probe holder holding both the
transducers at a fixed distance;
-monitoring in a display device of the flow detector the axial travel of the signal by placing
the probe diametrically along the length of the test object and measuring the travel velocity of
the signal in the central region of the object;
-monitoring in the display the travel velocity of the signal by placing the probe
perpendicularly along the length of the test object and measuring the velocity of the signal in
the central region of the object,
wherein a calibration curve for the ultrasonic wave velocity difference in the axial and radial
directions is generated with respect to the stress values measured by a x-ray diffraction
technique.

2. An ultrasonic system and a cause-detection of surface crack generaztion during HSS
rolling process in an ultrasonic system to avoid roll failure through corrective action in roll
cooling phase, wherein a signal shift is acquired in time scale due to change in velocity of
signal travel in axial and radial directions, and wherein a threshold of the velocity change is
obtained from the calibration curve.
3. An ultrasonic system and a cause-detection of surface crack generaztion during HSS
rolling process in an ultrasonic system to avoid roll failure through corrective action in roll
cooling phase, wherein the cause of surface crack generation is due to thermal fatigue when
the axial velocity (V90) is greater than radial velocity, and wherein the cause of generation of
surface crack is due to contact fatigue when the radial velocity (V0) is greater than the axial
velocity (V90).
4. An ultrasonic system and a cause-detection of surface crack generaztion during HSS
rolling process in an ultrasonic system to avoid roll failure through corrective action in roll
cooling phase, wherein the roll-cooling rate is adjusted when the axial velocity (V90) is
greater than radial velocity (V0), and wherein the roll lubricants is to be replaced when the
radial velocity (V0) is greater than axial velocity (V90).

ABSTRACT

The invention relates to an ultrasonic system and a cause-detection of surface crack
generaztion during HSS rolling process in an ultrasonic system to avoid roll failure through
corrective action in roll cooling phase, the method comprises the steps of providing a system
having at least two ultrasonic transducers, a probe holder, and an ultrasonic flaw detector
unit; transmitting by a first transducer a surface wave of frequency 0.5MHz into a test object;
exciting the transducer by the flaw detector; receiving in the first transducer the transmitted
signal, the probe holder holding both the transducers at a fixed distance; monitoring in a
display device of the flow detector the axial travel of the signal by placing the probe
diametrically along the length of the test object and measuring the travel velocity of the
signal in the central region of the object; monitoring in the display the travel velocity of the
signal by placing the probe perpendicularly along the length of the test object and measuring
the velocity of the signal in the central region of the object, wherein a calibration curve for
the ultrasonic wave velocity difference in the axial and radial directions is generated with
respect to the stress values measured by a x-ray diffraction technique.