Key Words Ultrasonic testing, Eddy current testing, Shell/core disbond, Surface cracks, Rolls, Non-destructive evaluation. INTRODUCTION The integrity of rolls used in a Hot Strip Mill (HSM)/Cold Rolling Mill (CRM) affects the mills’ performance in terms of productivity and availability, inventory costs, man and machine hour costs and also, the quality of the rolled products. Immediately after commissioning of the Hot Strip Mill at Tata Steel, owing to frequent failures of the rolls, the mill suffered in terms of mills’ productivity and availability. A need was felt to check the integrity of these rolls at the Roll Shop of HSM by the standardisation of NDT techniques (ultrasonic and eddy current). TYPES OF ROLLS IN HSM/CRM Roughing Stand of HSM In the roughing stand of the 4-high Hot Strip Mill, there is normally a fire crack problem on the work roll surfaces because of the high heat impact from the hot slab. Besides the high heat impact, owing to heavy reduction of the slab, the high load and neck strength are also major concerns. Double poured chrome steel (as the shell) grades of rolls with SG iron (as the core) are used as work rolls. For back-up rolls, double poured cast steel rolls are used. The shell is made of high carbon alloyed steel (0.6-0.9 C, 1.5-2.2 Mn, 0.4-0.Si, 1.5-2.0 Cr, 0.2-0.5 Mo, 0.035% S and P max.), whereas the core consists of medium carbon unalloyed steel. In the future, some of the cast back-up rolls will be replaced by forged and hardened alloyed steel rolls. Finishing Stand of HSM The finishing stand of the Hot Strip Mill consists of six stands. Earlier, the first three stands (F1, F2 and F3) used double poured high-chrome iron rolls with high- chrome iron as the shell and SG iron as the core material, whereas the last three stands (F4, F5 and F6) used double poured indefinite chilled cast iron with indefinite chilled cast iron as the shell and SG iron as the core material. Double poured high speed steel rolls are under trial in stand F2. Cold Rolling Mill In the 6 high TCM (tandem cold mill) and the 4 high SPM (skin pass mill) of the CRM, the work as well as the intermediate rolls are made of differentially hardened forged 3% Cr steel. The back-up rolls of the SPM are made of double poured alloyed cast iron with indefinite chilled cast iron as the shell and SG iron as the core material, whereas the back-up rolls of the TCM are made of differentially hardened forged 3% Cr or 5% Cr steel. FACTORS DAMAGING THE ROLLS Shell/Core Disbond in Double Poured Rolls These rolls are centrifugally cast in which, two dissimilar metals (shell and core) are bonded with each other. The quality of this metallurgical bond depends on foundry variables like molten metal temperature, shell and core metal weights poured, spun cycle, quality of the flux applied on the hot shell, etc. In case of a poor bond or disbond area on the shell/core interface, when the roll diameter approaches the scrapping diameter, under the influence of internal stresses, the disbond size may become critical, be the source of crack propagation and lead to roll failure due to spalling. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls J. C. Pandey, Manish Raj, Umesh Singhal, N. Bandyopadhyay and O. N. Mohanty Tata Steel, Jamshedpur - 831 001, India The integrity of the rolls in the roughing and finishing stands of HSM/CRM is important for the mills’ productivity and availability, inventory costs, man and machine hour costs and also, the quality of the rolled products. Therefore, the rolls should be allowed to contain only the minimum permissible flaws. Even small surface cracks or bruises, invisible to the naked eye, may affect the product quality. Standardisation of non-destructive evaluation (NDE) techniques is, therefore, essential for roll management. The paper highlights some non-destructive evaluation techniques developed in the R&D Division of Tata Steel, such as ultrasonic and eddy current testing, to meet the above objectives. These techniques have been implemented to assess the shell thickness, shell/core bond quality, fire-cracks and pressure cracks on the double poured work and back-up rolls of the roughing as well as the finishing stands of the HSM. Double poured rolls with indefinite chilled cast iron, high-chrome iron and high speed steel as the shell material are being routinely assessed using ultrasonic and eddy current in the Roll Shop of HSM. Although on-line eddy current inspection is being carried out in the the CRM Roll Shop, ultrasonic inspection using surface waves has also been implemented to measure crack depths less than 0.70 mm. This technique has been applied for the forged steel work rolls and the intermediate rolls of the CRM. These measures have helped to bring down the roll failure rate in the Mill, with consequent savings in down time cost, reduction in roll consumption and also, improvement in the product quality. Tata Search, 2003
The integrity of rolls used in a Hot Strip Mill (HSM)/ColdRolling Mill (CRM) affects the mills’ performance interms of productivity and availability, inventory costs,man and machine hour costs and also, the quality ofthe rolled products. Immediately after commissioningof the Hot Strip Mill at Tata Steel, owing to frequentfailures of the rolls, the mill suffered in terms of mills’productivity and availability. A need was felt to checkthe integrity of these rolls at the Roll Shop of HSM bythe standardisation of NDT techniques (ultrasonic andeddy current).
TYPES OF ROLLS IN HSM/CRM
Roughing Stand of HSM
In the roughing stand of the 4-high Hot Strip Mill, thereis normally a fire crack problem on the work roll surfacesbecause of the high heat impact from the hot slab.Besides the high heat impact, owing to heavy reductionof the slab, the high load and neck strength are alsomajor concerns. Double poured chrome steel (as theshell) grades of rolls with SG iron (as the core) are usedas work rolls. For back-up rolls, double poured caststeel rolls are used. The shell is made of high carbonalloyed steel (0.6-0.9 C, 1.5-2.2 Mn, 0.4-0.Si, 1.5-2.0Cr, 0.2-0.5 Mo, 0.035% S and P max.), whereas thecore consists of medium carbon unalloyed steel. In thefuture, some of the cast back-up rolls will be replacedby forged and hardened alloyed steel rolls.
Finishing Stand of HSM
The finishing stand of the Hot Strip Mill consists of sixstands. Earlier, the first three stands (F1, F2 and F3)used double poured high-chrome iron rolls with high-chrome iron as the shell and SG iron as the corematerial, whereas the last three stands (F4, F5 and F6)used double poured indefinite chilled cast iron withindefinite chilled cast iron as the shell and SG iron asthe core material. Double poured high speed steel rollsare under trial in stand F2.
Cold Rolling Mill
In the 6 high TCM (tandem cold mill) and the 4 high SPM(skin pass mill) of the CRM, the work as well as theintermediate rolls are made of differentially hardenedforged 3% Cr steel. The back-up rolls of the SPM are madeof double poured alloyed cast iron with indefinite chilledcast iron as the shell and SG iron as the core material,whereas the back-up rolls of the TCM are made ofdifferentially hardened forged 3% Cr or 5% Cr steel.
FACTORS DAMAGING THE ROLLS
Shell/Core Disbond in Double Poured Rolls
These rolls are centrifugally cast in which, two dissimilarmetals (shell and core) are bonded with each other. Thequality of this metallurgical bond depends on foundryvariables like molten metal temperature, shell and coremetal weights poured, spun cycle, quality of the fluxapplied on the hot shell, etc. In case of a poor bondor disbond area on the shell/core interface, when theroll diameter approaches the scrapping diameter, underthe influence of internal stresses, the disbond size maybecome critical, be the source of crack propagation andlead to roll failure due to spalling.
Non-destructive Evaluation ofthe Integrity of HSM and CRM Rolls
J. C. Pandey, Manish Raj, Umesh Singhal, N. Bandyopadhyay and O. N. MohantyTata Steel, Jamshedpur - 831 001, India
The integrity of the rolls in the roughing andfinishing stands of HSM/CRM is important for themills’ productivity and availability, inventory costs,man and machine hour costs and also, the qualityof the rolled products. Therefore, the rolls shouldbe allowed to contain only the minimum permissibleflaws. Even small surface cracks or bruises, invisibleto the naked eye, may affect the product quality.Standardisation of non-destructive evaluation (NDE)techniques is, therefore, essential for rollmanagement.
The paper highlights some non-destructive evaluationtechniques developed in the R&D Division of TataSteel, such as ultrasonic and eddy current testing,to meet the above objectives. These techniqueshave been implemented to assess the shell thickness,shell/core bond quality, fire-cracks and pressurecracks on the double poured work and back-uprolls of the roughing as well as the finishing standsof the HSM. Double poured rolls with indefinitechilled cast iron, high-chrome iron and high speedsteel as the shell material are being routinelyassessed using ultrasonic and eddy current in theRoll Shop of HSM. Although on-line eddy currentinspection is being carried out in the the CRMRoll Shop, ultrasonic inspection using surfacewaves has also been implemented to measure crackdepths less than 0.70 mm. This technique hasbeen applied for the forged steel work rolls andthe intermediate rolls of the CRM. These measureshave helped to bring down the roll failure rate inthe Mill, with consequent savings in down timecost, reduction in roll consumption and also,improvement in the product quality.
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338 J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls
EARLIER WORK
In the beginning of 1980s, when the primary Rolling Mills(1 and 2) and Sheet Mills were in operation in TataSteel, attempts were made by the R&D Division to usethe available NDT techniques in order to reduce the rollfailures, particularly cog rolls of the roughing standsof the primary rolling mills and the clear chilled rolls ofthe finishing stands of the Sheet Mill owing to deep firecracks. Thermography(5) was used to understand theeffectiveness of spray cooling in the different passes ofthe cog rolls during mills’ operation and the AC potentialdrop method was used to measure the depths of the firecracks in the cog rolls. These techniques were founduseful in finding the reason for deep fire cracks in aparticular pass where the cooling was insufficient. Thecooling system was modified and the failures owing todeep fire cracks were reduced. The development of firecracks in clear chilled rolls of the Sheet Mill was acommon phenomenon and some of the deeper cracks(particularly transverse cracks) led to the failure of therolls in two pieces. The AC potential drop method wasused to measure the depths of these fire cracks and therolls with crack depths more than 40 mm were taken outfrom the mill to reduce their failures.
When the Sheet Mill was about to close down (1995-1996),the failure of clear chilled cast iron rolls of the finishingstands became alarming. Deeper chill depths in theserolls was suspected to be one of the reasons for the failures.An ultrasonic method was developed at R&D in order tonon-destructively measure the chill depth in these rolls(6,7).Based on these measurements, recommendations weremade to reduce the chill depth to a maximum of 7 mm.Using a 6 MHz twin crystal probe, the ability to detectback-scattered signals from the start of mottle (a globularaggregate of graphite, ferrite and cementite) was found tobe the basis of these measurements. Fig. 1 shows sometypical chilled depth profiles in two clear chilled cast ironrolls of the Sheet Mill.
Thermal Stresses
During each rotation, the work rolls of the early finishingstands or roughing stands experience differenttemperatures at different locations on the roll surface,which lead to thermal shocks and hence, generation offire cracks. Fire cracks are generally found on the rollsof the roughing and the early finishing stands where thetemperature of the rolled slab is high. During roll rotation,the maximum temperature of the roll surface which is incontact with the hot slab, has been reported as 613o Cagainst the minimum roll surface temperature of 70o C,after passing through the water cooling zone in the earlyfinishing stands (F1 - F3). It has been reported that theusual depth from the surface of the rolls at which thetemperature drops to 320o C (where there is 15 % dropin the hardness when compared with the room temperaturehardness) is 0.20 - 0.72. mm(1). This results in thermalstresses leading to fire cracks in the rolls. If such cracksjoin the poor quality shell/core bond interface, a part ofthe shell material can come out from the roll. Under theinfluence of stresses, the size of these cracks may becomecritical leading to roll failure.
Abnormal Mill Behaviour
Besides the damage mechanisms discussed above,abnormal mill behaviour during cobble or power trip, etc.can also damage the rolls. Bruise/soft spots are inducedon the roll body surface when the temperature of thatsurface area during mill service exceeds the temperingtemperature used by the roll manufacturer to obtain thedesired hardness. A residual stress within the bruise iscreated as the retempered martensite contracts from theadjacent roll material. A stress crack (fine crack) is initiatedwhich further propagates circumferentially generating afatigue path leading to brittle fracture i.e. spalling.
When there is a cobble, high loads (exceeding the normalloads), adhering metal weld on the surface, etc. localisedcontact stresses are developed, which lead to crackinitiation and propagation leading to spalling.
INSPECTION PROCEDURES ELSEWHERE ANDAT TATA STEEL
Hetchman Paul et al.(2) reported measurement of surfacecrack depths in rolls using ultrasonic surface wave probeshaving different frequencies. Takada Hazime et al.(3)
reported the use of an on-line ultrasonic inspection systemfor the measurement of surface crack depths in highspeed steel rolls in which, a broad band surface waveprobe was used. On-line automatic inspection systems(EMAT ultrasonic and eddy current) for checking soundnessof the back-up rolls at roll shops have been reported byK. Berner et al(4). Most of the advanced mills in Europe,USA and Japan use on-line NDT (ultrasonic and eddycurrent) systems and the acceptance norms are based onthe calibration blocks with artificial defects.
In the Hot Strip Mill of Tata Steel before 1996, the rolls werenot inspected non-destructively and the acceptance of therolls was based on the supplier’s inspection report. In spiteof the supplier’s acceptance of the rolls, frequent failuresof the rolls during 1996-1997 (mostly due to poor shell/corebond) led to the reinspection of the rolls at the Roll Shopat regular intervals. Hence, standardisation of non-destructive testing of rolls was taken up.
PERFORMANCE OF ROLLS IN HOT STRIP MILL
In the years 1993 - 1997 after commissioning of the HotStrip Mill, a sharp increase in the failure of HSM workrolls was a matter of great concern. The roll failure doubled
Fig. 1 : Typical chilled depth profiles in two clear chilledcast iron rolls of the finishing stand of the Sheet Mill
1 253 5 7 9 11 13 15 17 19 21 23
Positions
0.000.00
2.00
4.00
6.00
8.00
10.00
O|C - 3483
O|C - 3957
Chi
ll de
pth,
mm
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in the year 1997, compared with 1996 as is obvious fromFig. 2.
At 50 mm below the roll surface, a change in the measuredultrasonic velocity was observed, which indicated theinterface of the shell and core.
Fig. 5 shows a macro-etched sample of a broken piecefrom a failed roll. The white portion shows the shellthickness, which was also confirmed by the shell thicknessmeasured by the ultrasonic testing. Fig. 6 shows photo-micrographs of indefinite chilled cast iron shell, shell/coreinterface and the SG iron core.
From these experimental results, if the average velocitiesin the ICCI shell and the SG iron core are 5200 m/s and
axis is the time scale and the y-axis represents theamplitude of the ultrasonic signal.
The acoustic impedance (Z) of a material is given by theproduct of the material density (ρ) and velocity (v) of theultrasonic wave in that material :
Z = ρ . v .....(1)
If subscripts 1 and 2 denote the parameters for materialand ultrasonic waves for the shell and core materials ofcentrifugally cast double poured rolls respectively, then
for the shell, Z1 = ρ1 . v1 .....(2)
and for core, Z2 = ρ2 . v2 .....(3)
The reflection co-efficient, R is given by :
R = (Z2 – Z1)2 / (Z2 + Z1)
2 .....(4)
Experimental
From a broken roll of the HSM, having an indefinite chilledcast iron (ICCI) shell and a SG iron core, a sample pieceof 100 mm thickness was cut for preparing a standard.Using the through-transmission technique, the ultrasonicvelocity was measured at different locations from the rollsurface as tabulated in Table 1 and plotted in Fig. 4.
The failure analysis of these rolls revealed poor shell/corebonds, deep surface breaking cracks, and in some cases,abrupt variation in the shell thickness (eccentric shell instatic cast double poured rolls). In centrifugally castdouble poured rolls, failure beacuse of shell/core disbondwas a common phenomenon (in some cases, the wholeshell was found to be stripped out from the core). Fig. 3shows the failure of a double poured centrifugally castindefinite chilled cast iron roll because of poor shell/corebond quality at the Hot Strip Mill.
Fig. 2 : Failure of double poured rolls at the Hot Strip Millbetween 1993 to 2002
After implementation ofultrasonic and eddy currentinspection techniquesdeveloped by R & D.
Year
Fig. 3 : A double poured indefinite chilled cast iron roll ofthe finishing stand of the Hot Strip Mill showing poorshell/core bond
Hence, a need was felt to develop and implement NDTtechniques to ensure the supply of only good quality rollsto the mills which would subsequently reduce incidentsof roll failures. NDT techniques were used to detect andmeasure shell thickness, shell/core bond quality(6,7), tocheck the existence of graphite nodules in SG iron coreof these double poured rolls(8), and surface cracks on thehigh speed steel rolls of the HSM.
MEASUREMENT OF SHELL THICKNESS ANDSHELL/CORE BOND QUALITY
Theoretical Aspects
When an ultrasonic wave passes through a medium,because of the resistance offered by the medium, theultrasonic wave gets attenuated. Depending upon thecharacteristics of the medium, the extent of attenuationvaries. The resistance is characterised by “acousticimpedance” and ultrasonic waves get reflected from thepoint where there is a change in the acoustic impedance.Since the shell/core interface provides substantial changein the impedence, some parts of ultrasonic waves arereflected and displayed as a rectified signal on a calibratedCRO screen of the ultrasonic instrument, where the x-
Table 1 : Ultrasonic velocity measured at differentdepths from the roll surface
Measuring Distance from Ultrasonic velocity, Remarkspoints roll surface, mm m/s
1 10 5263 ICCI shell2 20 5172 ICCI shell3 30 5169 ICCI shell4 40 5124 ICCI shell5 50 5210 Interface6 60 5603 SG iron core7 70 5641 SG iron core8 80 5630 SG iron core9 90 5656 SG iron core
Fig. 4 : Variation of ultrasonic longitudinal velocity theindefinite chilled double poured cast iron rolls of the HotStrip Mill
5000
5100
5200
5300
5400
5500
5600
5700
0 10 20 30 40 50 60 70 80 90 100
Distance from the roll surface, mm
Ult
raso
nic
lon
git
ud
inal
vel
oci
ty, m
/s
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340 J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls
selected from a scrapped good roll which had givensatisfactory life in service. Investigations were carriedout for :
★ ultrasonic evaluation of all failed rolls, particularlydue to poor shell/core bond leading to spalling
★ ultrasonic evaluation of new in-coming rolls
★ ultrasonic evaluation of the rolls in service.
The test results are shown in Fig. 7.
5600 m/s respectively, and considering the density ofiron as 7.8 g/cm3, the reflection co-efficient (R) = 0.00137 for proper shell/core bonding. In case of shell/coredisbond (lack of metal to transmit the ultrasound), thevalue of R will increase, depending on the size of thedisbond. In case of complete disbond, the value of R willbe 0.99996, assuming air as the medium in the disbond.
Although in conventional ultrasonic flaw detectors it isnot possible to measure the absolute values of the reflectionco-efficient from the shell/core interface, it can be indicatedby the amplitude (after amplification) of the reflectedultrasonic pulsed signals.
Measurements on Rolls
Table 2 shows some typical ultrasonically measured shellthicknesses in double poured rolls of the HSM. The resultsindicated shell thickness values between 44 -�50.5 mm.This indicates that even if the rolls approach the scrap-roll diameter, there will be sufficient useful shell thicknessof 26 - 37 mm. The variation in shell thickness of about11 mm needs to be further reduced in order to economicallymanufacture and supply these rolls to the customers.
Assessment of Shell/Core Bond Quality
The shell/core interfacial echo in ultrasonic testing isan indicator of the bond quality. A good bond roll willshow a lower interfacial echo, whereas a poor bond rollwill show a higher interfacial echo. In case the disbondarea is more than the probe crystal diameter, multiplereflection echoes may appear on the oscilloscope screenof the ultrasonic flaw detector. Since it is difficult tocut a sample from a new roll, a reference roll was
Fig. 5 : Macro-etched sample of a broken piece from a failedroll
(a) (b) (c)
Fig. 6 : Photo-micrographs of (a) indefinite chilled castiron shell, (b) shell/core interface and (c) SG iron core(magnification X 100)
Table 2 : Ultrasonically measured shell thickness indifferent rolls of the HSM
Roll Roll Ultrasonically Scrap Useful shell thicknessNo. diameter measured shell diameter in scrapped roll (Xs)
(d), mm thickness (X), mm of roll = 1/2 [ds-(d-2X)], mm(ds), mm
Fig. 7 : Shell/core interfacial echo heights for different usedand failed, as well as new indefinite chilled cast iron rollsof the HSM.
Using a 4 MHz normal probe, ultrasonic instrument at 40dB attenuator setting, it was observed that good bond rollsshowed shell/core interfacial echo heights less than 25%.
ESTIMATION OF GRAPHITE NODULES IN SGIRON CORES
Theoretical Aspects
In solids, the ultrasonic longitudinal velocity (Vl) can beexpressed in the following manner :
.....(5)
where,E = Young’s modulus of elasticity
ρ = Densityσ = Poisson’s ratio
10 20 30 40 50 60 70
Shell thickness, mm
Ref. scrap roll
Eccentric Shell
Disbond
Good bond
100
80
60
40
20
0
Inte
rfac
ial e
cho
ht.,
%
V1 =Eρ
.E 1- σ( )
1 + σ( ) 1 − 2σ( )
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J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls 341
Theoretical Aspects
Ultrasonic surface waves are generated by wave modeconversion at the boundary of two media. When alongitudinal wave traveling at speed VL1 in medium Iarrives at the boundary medium I and II as shown inFig. 10, it is reflected(13).
It is seen from equation (5) that the ultrasonic longitudinalvelocity (Vl) is dependent on Young’s modulus of elasticity(E), Density (ρ) and Poisson’s ratio (σ).
In cast iron, the graphite phase affects the values of Eand hence, affects the measured values of the ultrasoniclongitudinal velocity (Vl) in these materials. A transformationfrom flaky to spherical form in SG iron increases the valueof the ultrasonic longitudinal velocity. Longitudinalultrasonic velocities measured by different authors E. P.Papadakis(9), R. J. Klink(10), H. E. Henderson(11) and A. G.Fuller(12) were found to be 5690, 5720, 5740 and 5560�m/srespectively.
The velocity difference between ferritic and pearlitic SGiron was found to be 10 m/s, which commercial ultrasonicequipment cannot differentiate. Since 100 m/s velocitydifference was found between cast and heat treated SGiron castings, there was no problem in using thecommercial equipment.
Measurements on Rolls
Total transit time (T) required for an ultrasonic wave totravel through the barrel diameter is :
.....(6)
where, x = shell thickness, mm
Vs = ultrasonic velocity in the shell
T = transit time required for the ultrasonic waveto travel through the barrel diameter, µs
The measurements on indefinite chilled cast iron rollswith SG iron cores are shown in Fig. 8.
Fig. 8 : Typical ultrasonic longitudinal velocity distributionin SG iron cores of indefinite chilled cast iron rolls of the HSM
The results show that the ultrasonic longitudinal velocityvalues lie within 5699 – 5782 indicating the presenceof graphite nodules in the SG iron cores.
ULTRASONIC EVALUATION OF SURFACECRACKS IN ROLLS
Surface cracks in the HSM rolls occur either due tomanufacturing defects and/or due to thermal andmechanical stresses. Fig. 9 shows a photograph of suchcracks developed during their service in the mills. Afterroll dressing, if these cracks are missed during visualinspection, they will open-up during operation in the milland leave marks on the strips causing rejection. If thedepth of these cracks approaches the scrap diameterzone, they would make the roll unsuitable for use.
Fig. 9 : A photograph of a band on the barrel surfacecontaining deep fire cracks
Fig.10 : Wave mode conversion of ultrasound at a boundaryof medium I and II
The refracted longitudinal wave in medium II deviatesfrom its original path and travels with a velocity VL2. Dueto mode conversion, another component of the wave isthe shear wave which is reflected in medium I and travelswith velocity VS1. Refracted shear wave in medium IItravels with velocity VS2. Longitudinal wave velocities arealways greater than the transverse wave velocities.Therefore, their relative angular positions are different.
If,
α1 = Angle of incidence of a longitudinal wave in medium�Iand also the angle of reflection for the same wavein medium I
α2 = Angle of refraction for the transmitted longitudinalwave in medium II
β1 = Angle of reflection for the shear wave in medium�I
β2 = Angle of refraction for the transmitted shear wavein medium II
342 J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls
the minimum detectable beam path as 10 mm, for thecommercial flaw detectors, the minimum detectablecrack depths using different beam angles are shown inTable 3.
There is a critical angle of incidence for the longitudinalwave where α2 = 90o and the longitudinal wave doesnot penetrate in the medium II across the boundary.This is the principle of generating angular shear waveswithin a specimen where the ultrasonic transducergenerating longitudinal wave is set at an angle greaterthan the first critical angle. Plexiglass is often used ona steel specimen and the critical angle is 26.7o.
There is a second critical angle when the shear waveis also totally internally reflected and no sound wavepenetrates into the specimen which is 56.60 for steelusing a plexiglass wedge. Surface (Rayleigh) wavesare induced at solid/liquid or solid/gas interfaces.When the probe is at the second critical angle setting,the surface wave profile is approximately a shearwave at its critical angle.
Depending on the type of particle vibration andmedium used, the surface waves have been classifiedas follows :
★ Rayleigh waves in which, the particles vibrate inan elliptical orbit and the medium can be anysurface where the major axis is perpendicular tothe surface and the minor axis is parallel to thedirection of wave propagation.
★ Plate waves (Love) in which, the particles vibrateparallel to the plane layer and perpendicular tothe wave direction. The medium is the surfacelayer bonded to solid.
★ Plate waves (Lamb) in which, the components ofvibrations are perpendicular to the surface.
★ Stanely (Leaky Rayleigh waves) in which, the wavesare guided along the interface. The medium canbe the boundary of dissimilar solids.
★ Sezawa in which, the particle vibrates in anti-symmetric mode and the medium is layered solids.
Usually the depth of penetration of the surfac (Rayleigh)waves is one wavelength below the roll surface. Surfacecracks with depths less than one wave length ofultrasound can be measured if the amplitude of thereflected signals from these cracks is properly calibrated.
To calibrate the ultrasonic equipment, calibration blockswith various steps 0.55 to 3.54 mm were fabricatedfrom mild steel. Fig. 11 shows the response of theultrasonic waves towards the various steps of calibrationblocks in terms of % FSH (full scale height) on theoscilloscope screen. The graph shows that there is agradual increase in the echo amplitude with an increasein the depth of the step up to 1/2 wavelength of theultrasound (= 0.7 mm), beyond which, there is a decreasein the echo amplitude with increase in the step-depth.Further, beyond one wave length, there is no systematicresponse of ultrasonic waves, which can be used forcrack depth measurement. However, the crack depthsup to 0.7 mm can be measured using this calibrationcurve. By lowering the frequency of the surface waveprobe, the depth of the ultrasonic surface wavepenetration can be increased and higher crack depthscan be measured. For deeper cracks, more than 1.4mm, 80o, 70o, 60o, 45o probes can be tried.
As shown in Fig. 12, crack depth, AB = OB cos a,where a = probe angle and OB = beam path. Considering
Fig. 11 : Variation in echo amplitude (% FSH) in thecalibration blocks with steps of varying depths, using 2 MHz,surface wave probe at 49 dB attenuator setting
0
10
20
30
40
50
60
70
80
90
100
110
0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6
Depth of steps of calibration blocks, mm
Ech
o am
plitu
de, %
FS
H
Fig. 12 : Schematic diagram showing measurement of crackdepth using an angle beam probe
Table 3 : Maximum measurable crack depth usingsurface wave and minimum measurable crack depthusing angle probes
Surface wave probes Angle beam probe at 2 MHz frequency
Probe frequency, Max. crack depth Probe angle, o MinimumMHz measured, mm detectable crack
Figs. 13 (a) and (b) show a schematic diagram formeasurement of the surface crack depth of a typicallyoriented crack using the ultrasonic NDT method.
During one mill interruption, ultrasonic evaluationrevealed that the high chrome steel rolls of the roughingstand had developed crack depths in the range 13-35mm, whereas the indefinite chilled cast iron rolls of thefinishing stand F4 and F6 had developed fire cracks inthe range 2-14 mm.
Ultrasonic
probc
MN = Specimen surface
AB = Surface breakingcrack
OB = Ultrasonic beam pathfor angle beam probe
AM
Crack
B
O
Nα
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J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls 343
For AC :
V= iZ .....(10)
I = Io Sin ωt .....(11)
where, ω = angular frequency = 2πf (f is the frequency)
Then, V – L.di/dt = iR .....(12)
Since, the inductance opposes the applied voltage, the
induced voltage :
VL = -L.di/dt = -L.ω.io. cos ωt = - XL.io. cos ωt .....(13)
where, XL = ωL
Current i and induced voltage VL are always 90o out of
The inspection coil circuit measures the resistive componentR which is fed to the X-plate and the reactance componentXL is fed to the Y-plate of the oscilloscope.
Comrison between Eddy Current and UltrasonicTesting
A detailed comparison between eddy current and ultrasonictesting is shown in Table 4.
DETECTION AND MEASUREMENT OF SURFACECRACKS IN ROLLS BY EDDY CURRENT
Theoretical Aspects
Eddy currents are the induced electric currents whichare generated in a conducting material owing tochanges in the magnetic field. An AC source is appliedto an inspection excitation coil so that the magneticlines of force penetrate into the specimen conductingsurface providing a good magnetic coupling as shownin Fig.14.
Eddy currents circulate on the specimen surface andare modified by the presence of discontinuities.(i.e.surface cracks). Eddy currents flow parallel to the planeof the windings of the coil. The detector instrumentnotes the changes with time variation of voltage andcurrent in the specimen coil(13).
The depth of penetration of the eddy current (δ) in metres,within a material is given by :
.....(9)
where, ρ = Electrical resistivity in Ohm-cm
f = Frequency of AC in Hertz
µr = Magnetic permeability of the material
The depth of penetration (δ) increases with an increasein electrical resistivity (ρ), decrease in magneticpermeability (µr) and frequency (f) of the AC current ofthe material. In rolls, the material microstructureinfluences the magnetic permeability and resistivity andhence, the depth of eddy current penetration.
An eddy current testing circuit is equivalent to a resistanceR and inductance L.
δ = 50ρ
fµr
Fig. 14 : Schematic diagram of eddy current testing
Inspectioncoil
Direction ofmagnetic
fields
Discontinuity
Detector
Eddycurrent
Conductingspecimen
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344 J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls
Experimental
For detection and measurement of surface cracks inhigh speed steel rolls, a calibration block made of specialhigh speed steel with composition similar to that of thehigh speed steel roll material was made. Fig. 15 showsthe photo-micrograph of such a block.
Figs. 16 (a) and (b) show the calibration curves usingthis calibration block with EDM notches of depths0.05, 0.10, 0.15, 0.20, 0.25, 0.30 and 0.35 mm.Fig. 16 (a) shows the calibration curves for 64 kHzfrequency whereas, Fig. 16 (b) shows it for 128 kHz.Good correlation was obtained between the depth ofthe EDM notches and eddy current response (X-axisdeflection, in units). These calibration curves were usedfor measuring the crack depth in high speed steel rolls.
Fig. 15 : Microstructure of special high speed steel calibrationblock (magnification X 400) showing complex carbide of Fe,W, V and Cr in temper martensitic matrix (Etchant : 3% Nital)
Measurements on High Speed Steel Rolls
The eddy current test results for high speed steel rollsare shown in Table 5.
Table 4 : Comparison between ultrasonic and eddycurrent testing
Ultrasonic testing
★ It is based on the acoustic propertiesof the specimen to be tested.
★ Internal soundness can be checkedi.e. suitable for inspection ofshell/core interfacial flaws and flawswithin the shell of a double pouredrolls of HSM/CRM.
★ Conventional flaw detectors are notsuitable for measuring depths ofshallow surface cracks due to theirinherent dead zone problems.
★ Conventional manual ultrasonictesting is slow in speed due toapplication of couplant on thesurface. However, EMAT systemsdo not require any couplant and arenon-contact with the specimen henceare comparable to eddy current inspeed and are being used for on-line inspection of rolls.
★ It is possible to measure depths ofthe surface cracks (normal as wellas oriented to the roll surface) inthe rolls using transverse surfacewave, transverse angle beam andlongitudinal wave normal probes.
★ In ultrasonic testing beam penetrationis not the problem like eddy currentand the crack depths which cannotbe measured by eddy current (dueto penetration problem) can bemeasured by ultrasonic testing.
★ Effective radial depth has beenreported(14) to be :
0-1.27 mm for surface wave
12.7-mm bore for straight beam
★ Surface micro-cracks <0.152 mmand macro-cracks >0.152 mm bothcan be detected using surface waveprobe(14).
★ Using pitch catch dual probe andstraight beam probe, subsurfaceflaws can be detected(14).
★ It is not possible to detect workhardened and bruise/soft spots onthe rolls(14).
Eddy current testing
It is based on the electrical andmagnetic properties of the specimento be tested.
Eddy current generation is confinedto its limited depth from the surface.Hence not suitable for checkinginternal soundness.
Eddy current is ideal non-destructivemethod for measuring the path lengthof the shallow surface cracks i.e.fire cracks and pressure cracks inthe rolls of HSM/CRM.
The probe (excitation coil) does notremain in contact with the specimenduring testing. Hence, the inspectionis fast and suitable for on-lineinspection. On-line eddy currentsystem is being used at the RollShop of CRM of Tata Steel.
Crack path lengths can bemeasured by using calibrated eddycurrent instrument, but crack depthsof the oriented cracks cannot bemeasured.
Depth of penetration of eddy currentin ferrous metals (i.e. cast iron, steel,etc.) is lower than that in non-ferrousmetals and austenitic steels. Hence,only shallow cracks can be detectedand measured in cast iron/steel rolls.
Effective radial depth has beenreported (14) to be :
0-0.076 mm
Surface micro-cracks can bemissed while macro-cracks can bedetected (14).
There are chances of missingsubsurface flaws(14).
Work hardened and bruise/ soft spotscan be detected(14).
Fig. 16 (b) : Calibration curve and empirical equation tomeasure crack depth in high speed steel at 128 kHz frequencyin eddy current testing
0
2
4
6
8
10
12
14
0 0.05 0.1 0.15 0.2 0.25
Notch depth, mm
X-a
xis
defle
ctio
n, u
nits
Fig. 16 (a) : Calibration curve and empirical equation tomeasure crack depth in high speed steel at 64 kHz frequencyin eddy current testing
0
1
2
3
4
5
6
7
8
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Notch depth, mm
X-a
xis
defle
ctio
n, u
nits
Tata Search, 2003
J. C. Pandey et al. Non-destructive Evaluation of the Integrity of HSM and CRM Rolls 345
For high speed steel roll numbers 1457 and 1458, themaximum crack depth measured was 0.10 mm, whichwas easily eliminated by grinding. However, the crackdepth in roll no. 1441 was found to be > 0.15 mm, whichcould not be measured by eddy current. Using a 70o angleprobe in ultrasonic testing, it could be measured and itwas found to be 3.4 mm which was removed by roll turningand grinding.
CONCLUSIONS
Non-destructive evaluation (NDE) plays an important rolein detecting and measuring the flaws in rolls to improvetheir performance. Efforts were made to develop andimplement ultrasonic and eddy current techniques at theRoll Shop of the Hot Strip Mill. The implementationresulted in reduction of roll failures.
Comparison of roll failure data for the period 1993-1997(4 years) with that for the period 1997-2001 (4 years) afterimplementation of NDT techniques in the Roll Shop ofHSM shows that roll failure was reduced from 31 rollsto 20 rolls and thus, 11 rolls were saved from failure. Thishas led to the following advantages :
★ Mills down time cost due to failure of roll in the millis approximately Rs. 1 million. Hence, there is a savingin the mill down time cost of Rs. 11 million for theperiod of 4 years.
★ Assuming 50 % life achieved by the failed rolls andcost of each roll being Rs. 0.8 million, there is a savingin the roll material of Rs. 4.4 million for the period of4 years.
★ Besides savings in mill down time cost and roll materialscost, other benefits (not quantified) were reduction instrip rejection due to roll failure, savings in man andmachine hour costs, etc.
The NDT techniques developed were used to assess :
★ Shell thickness and the shell/core bond quality incentrifugally cast double poured rolls of the finishingstand of the Hot Strip Mill by ultrasonic method usinga normal beam probe.
★ Surface cracks in high speed steel rolls up to 0.7 mmdepth using ultrasonic surface wave probes and 1.74to 5.00 mm using 80o, 70o and 60o angle probes wereseasured. For angularly oriented cracks, normal probeswere also been used.
★ Surface cracks were found using eddy current in highspeed steel rolls under trial in finishing stand F2.Calibrations have been made for crack depths up to0.25 mm.
ACKNOWLEDGEMENT
The authors are thankful to the Management of Tata Steelfor permitting to publish this paper. Thanks are also dueto the staff of the Roll Shop of the Hot Strip Mill, andM/s GIIB, Jamshedpur for their cooperation in conductingthe experiments, and implementation of different NDTtechniques.
REFERENCES
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2. Paul Hetchman, John Ballani and Roger Zorn,Surface Crack Depth Classification through ImprovedUltrasonic Techniques, 36th Mechanical Workingand Steel Processing, Iron & Steel Society, Vol. 32,1994, pp. 21-24.
3. Takada Hazime et al., Development of Roll SurfaceTechnique by Use of Broad Bandwidth SurfaceWave, 15th World Conf. on Non-destructive Testing,Rome, 2000.
4. K. Berner et. al., Automatic Inspection of Backupand Work Rolls with Ultrasonics and Eddy Currents,Proc. Int. Seminar on Roll Tech 2000, Jamshedpur,April 11-12, 1997, pp. 80-85.
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8. J. C. Pandey, Manish Raj, T. C. Janghel and B. N.Panda, Ultrasonic Velocity Technique for QualityAssurance of SG Iron Core in Centrifugal Cast D PRolls of Hot Strip Mill, Proceeding, NDT in Steel andAllied Industries ’98, Jamshedpur, April 2-3, 1998.
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Table 5 : Eddy current test results of high speed steelrolls of HSM
Roll Roll diameter, Maximum crack RemarksNo. mm depth, mm
1457 714.18 0.10 Crack was eliminated by grinding.
1458 714.13 0.10 Crack was eliminated by grinding.
1441 714.43 > 0.15 Eddy current did not inform thedepth of crack. Ultrasonic 70o angleprobe could measure the crackdepth 3.4 mm which was eliminatedby turning and grinding.
