Guide - Defects Cluster Mill Rolls

8/10/2019 Guide - Defects Cluster Mill Rolls

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kers ABSE 647 83 kers StyckebrukSwedenwww.akersrolls.com

Edition 0701These data are indicative and can be

changed at any time without prior notice kers AB, All rights reserved

GUIDE Roll defects Cluster Mill Rolls 1(46)

CONTENT

1. CRACKING 2

1.1 Cracking in network 2

1.2 Longitudinal cracking 4

1.3 Helicodial Cracking 7

2. SPALLING 9

2.1 Microspalling of thermal origin 9

2.2 Microspalling of mechanical origin 12

2.3 Spalling 15

3. RUPTURE OF THE BARREL 17

3.1 Radial rupture of the barrel 17

3.2 Helicoidal rupture of the barrel 20

3.3 Longitudinal rupture of the barrel 24

4. RUPTURE OF THE NECK 27

4.1 Drive end rupture (roll dogs) 27

4.2 Drive end ruptures (splines) 29

5.1 Indentation 31

5.2 Lines 32

5.3 Grinding defects 33

5.4 Irregular surface finsh on the strip 37

6. FORMATION MECHANISMS 39

6.1 Surface fatigue 39

6.2 Damage due to overheating 42

6.3 Consequenses of a roll ing incident 45


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1. Cracking

1.1 Cracking in network

ROLL TYPEAl l

GRADEAl l

DESCRIPTION Presence of several cracks in a network, or mosaic pattern. In most cases, the struc ture and hardness are modified.

POSSIBLE CAUSE(S) Thermal shock caused by a rolling incident (sticking, slippage, blockage of the roll s, rupture of the

oil film) or a grinding mishap. The rise in temperature, above that of the heat treatment, modifies the microstructure and the

balance of internal stresses. Cracks are produced by stress relaxation.

CORRECTIVE ACTIONS Record all rolling and grinding incidents on the roll record card. Remove all cracks, work hardening or overheated areas completely before putting the roll back in

service. Carry out a non-destructive test (dye penetrant, Eddy current, magnetic particle, ultrasonic) to

make sure that the defects are eliminated.

Fig. 1 Detected with magnetic particle test

Fig. 2 Detected with Catella etching


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Fig. 3 Detected wi th dye penetrant


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1.2 Longitud inal cracking

ROLL TYPEAl l

GRADE

Al l

DESCRIPTION Longitudinal crack(s) on the barrel of a work roll (fig.1) or on a 2nd intermediate roll (fig.2).

POSSIBLE CAUSE(S) Local overheating provoked by a bearing seizure (fig.2). Local overheating provoked by an incident (fig. 3 ). Incomplete elimination of a crack that appeared during a p revious campaign. If the defect appears very early during the first campaign: delayed quench cracking (fig.1).

CORRECTIVE ACTIONS Record all rolling and grinding incidents on the roll record card.

Remove all cracks, work hardening or overheated areas completely before putting the roll back inservice.

Carry out a non-destructive test (dye penetrant, Eddy current, magnetic particle, ultrasonic) tomake sure that the defects are eliminated.

Fig. 1 Delayed quench cracking


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Fig. 2 Local overheating bearing seizure

Fig. 3 Local overheating incident, detected by Magnaflux


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ROLL TYPEAl l

GRADEAl l

DESCRIPTION

Cracks on roll face, starting from the roll centre and developing towards the outside, with fri ctionmarks (fig.1).

POSSIBLE CAUSE(S) Friction from the thrust rings.

CORRECTIVE ACTIONS Modify the thrust rings.

Fig. 1 Cracks on ro ll face


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1.3 Helicodial Cracking

ROLL TYPEIntermediate rolls

GRADE

Al l

DESCRIPTION An isolated cont inuous crack, typical of a tors ional stress that develop in a spiral pattern on a part

or on the whole barrel.

POSSIBLE CAUSE(S) Thermal chock caused by a rolling incident (sticking, slippage, blockage of the roll s, rupture of the

oil film) or a grinding mishap. The rise in temperature, above that of the heat treatment, modifiesthe microst ructure and the balance of internal stresses. Cracks are produced by stress relaxation.

Incomplete elimination of a crack that appeared during a p revious campaign. Surface fatigue starting points. Problem with the dogs when the cracking starts f rom the drive end.

CORRECTIVE ACTIONS Record all rolling and grinding incidents on the roll record card. Remove all cracks, work hardening or overheated areas completely before pitting the roll back in



When the cracking s tarts from the drive end: Check and adjust the safety components settings (pins, rings) Check the sleeve (play/wear ) and the drive-end dimensions. Reduce to a minimum the setting time of the mill rapid opening device. In the case of systematic rupture, consider the possibi lity of lowering* the roll dogs hardness.

Fig. 1, After dye penetrant

Fig. 2

*NOTE: However, if the roll dogs hardness is too low, galling or deformations may occur and cause play incoupling, vibrations and possibly chatter marks on the strip.


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Fig. 3 After Magnaflux


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2. Spalling

2.1 Microspalling of thermal or igin

ROLL TYPEAl l

GRADEAl l

DESCRIPTION Spalling w ith cracks either parallel or in network and with a lower hardness.

POSSIBLE CAUSE(S) Thermal shock caused by a rolling incident (sticking, slippage, blockage of the roll s, rupture of the

oil film) or a grinding mishap. The rise in temperature, above that of the heat treatment, modifiesthe microst ructure and the balance of internal stresses. Cracks are produced by stress relaxation.

CORRECTIVE ACTIONS

Remove all cracks, work hardening or overheated areas completely before putting the roll back inservice.

Record all rolling and grinding incidents on the roll record cards. Carry out a non-destructive test (dye penetrant, Eddy current, magnetic particle, ultrasonic) to


Fig. 1 After dye penetrant


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Fig. 2 After dye penetrant

Fig. 3 After Catella etching

Fig. 4


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Fig. 5, After Magnaflux


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2.2 Microspalling of mechanical orig in

ROLL TYPEAl l (Intermediate rolls in part icular)

GRADE

Al l

DESCRIPTION Spalling not very deep, with a particular rupture face (a dull face) corresponding to the maximum

shearing s tresses zone and wi th/or without c racks and rise in the metal hardness (work hardening).

Example: cone junction on 1st intermediate rolls or back up pressure area on 2nd intermediaterolls.

POSSIBLE CAUSE(S) Mechanical origin: surface fatigue phenomenon associated with an excessive and/or accumulated

work hardening. Runs too long and insufficient grinding.

Normal run but compression st ress exceeding the steel properties.

CORRECTIVE ACTIONS Remove all cracks, work hardening or overheated areas completely before putting the roll back in

service. Record all rolling and grinding incidents on the roll record cards. Carry out a non-destructive test (dye penetrant, Eddy current, magnetic particle, ultrasonic) to

make sure that the defects are eliminated. Restore the original hardness. Turn to high speed steel grades for the work rol ls. Modify the grinding po licy in quantity and in the frequency.

Fig. 1


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Fig. 2

Fig. 3


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Fig. 4


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2.3 Spalling

ROLL TYPEAl l

GRADEAl l

DESCRIPTION Damage with small spalls having different depth due to a surface fatigue phenomenon.

The face of the rupture presents propagation fronts .- elliptical, perfectly concentric (fig. 3)- with a more confuse aspect together with very small bracelets (fig. 1-2).

POSSIBLE CAUSE(S) Residual cracking Mechanical origin: excessive and accumulated work hardening leading to a surface fatigue

phenomenon (intermediate or support rolls). Thermal origin: rolling or grinding incidents, thermal shock (cracking in network).

Combined mechanical and thermal origin (fig. 1-2).

Metallurgical orig in: inclusions (fig.3).

CORRECTIVE ACTIONS Record all rolling and grinding incidents on the roll record cards. Remove all cracks, work hardening or overheated areas completely before putting the roll back in



Fig. 1

Fig. 2


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Fig. 3


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3. Rupture of the barrel

3.1 Radial rupture of the barrel

ROLL TYPEWork rolls

GRADEAl l

DESCRIPTION Rupture of the work roll perpendicular to the axis (fig.1). Sticking of the rolled strip around the rupture area (fig.2). Dull and granulous aspect of the ruptu re zone without vis ible starting po ints (fig.3).

POSSIBLE CAUSE(S) Excessive bending caused by mill incident. The rupture could have started from a crack, formed during a previous run and not correctly

eliminated, by concentration of stresses set up on the crack.

CORRECTIVE ACTIONS Record all rolling and grinding incidents on the roll record cards. Remove all cracks, work hardening or overheated areas completely before putting the roll back in



Fig. 1

Fig. 2


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Fig. 3


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ROLL TYPEAl l

GRADEAl l

DESCRIPTION

Rupture by fatigue perpendicular to the roll axis.

Aspects of the rupture:- Starting point at the surface (A).- Very vis ible concentric propagation f ronts (fig. 1-2) or not so visible (fig.3) but

corresponding to a gradual cracking (B).- Brutal final rupture (C).

POSSIBLE CAUSE(S) Cracks not completely eliminated by grinding, which p ropagated during previous runs under cyclic

bending stresses. The origin of the cracking may be:

- Surface fatigue.

-

Thermal fatigue (following a mill or grinding incident).




Fig. 1 Fig. 2

Fig. 3


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3.2 Helicoidal rupture of the barrel

ROLL TYPE2nd intermediate drive rol ls

GRADE

Al l

DESCRIPTION Rupture of the barrel provoked by a spiral cracking that may have been ini tiated by a mechanical or

a thermal surface defect (fig.2). Dull and granulose aspect of the rupture zone without propagation fronts (fig.3).

POSSIBLE CAUSE(S) Excessive and combined bending and torsional stresses. Retained mechanical or thermal cracks.

CORRECTIVE ACTIONS Check and adjust the safety components settings (pins, rings ).

Record all rolling and grinding incidents on the roll record card. Remove all cracks, work hardening or overheated areas completely before putting the roll back in



Fig. 1

Fig. 2


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Fig. 3


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ROLL TYPEAl l

GRADEAl l

DESCRIPTION

Rupture by fatigue in a spiral pattern (fig.1) with starting points on the surface (fig.2).

Presence of several concentric p ropagation fronts (B) tracing the development of the rupture.

POSSIBLE CAUSE(S) Cracks not correctly eliminated during the grinding operation, which propagated under excessive

bending in previous runs. The origin of the cracking may be:

- Surface fatigue.- Thermal fatigue (following a mill or grinding incident).

CORRECTIVE ACTIONS Record all rolling and grinding incidents on the roll record card.

Remove all cracks, work hardening or overheated areas completely before putting the roll back inservice. Carry out a non-destructive test (dye penetrant, Eddy current, magnetic particle, ultrasonic) to


Fig. 1

Fig. 2


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Fig. 3


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3.3 Longitudinal rupture of the barrel

ROLL TYPEAl l

GRADE

Al l

DESCRIPTION Brutal rupture of the barrel in two parts. Granulose and dull aspect of the rupture zone with the presence of oxide and calamine.

POSSIBLE CAUSE(S) Longitudinal cracking that propagated when putting the rolls in the mill under rolling stresses. Delayed quench cracking starting from the threaded centre.




Fig. 1 Face of a longitudinal rupture of a work roll


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ROLL TYPEAl l

GRADEAl l

DESCRIPTION

Rupture by fatigue.

Presence of several propagation fronts parallel to the axis (A).

POSSIBLE CAUSE(S) Cracks not correctly eliminated during the grinding operation, which propagated under cyclic

rolling stresses in previous runs. The origin of the cracking may be:

- Surface fatigue.- Thermal fatigue (following a mill or grinding incident).

Example: overheating due to the contact of the back up roller with the 2nd intermediate drive roll.




Fig. 1


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Fig. 2


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4. Rupture of the neck

4.1 Drive end rupture (rol l dogs)

ROLL TYPE

2nd Intermediate drive roll

GRADEAl l

DESCRIPTION Rupture of one, several or all teeth (fig.1) that may lead to the rup ture of the barrel (fig.2). Brutal rupture without s tart points or propagation fronts (fig.3)

POSSIBLE CAUSE(S) Excessive stress provoked by a sudden stop of the mill. Over-dimensioned safety components (pins, rings ). Teeth hardness specification is too high.

Faulty drive-end sleeve.

CORRECTIVE ACTIONS Check and adjust the safety components settings (pins, rings ). Check the sleeve (play/wear ) and the drive end dimension. Reduce to a minimum the setting time of the mill rapid opening device. In the case of systematic rupture, consider the possibi lity of lowering* the hardness of the roll dog.

Fig. 1

Fig. 2

*NOTE: However, if the roll dogs hardness is too low, galling or deformations may occur and cause play incoupling, vibrations and possibly chatter marks on the strip.


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Fig. 3


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4.2 Drive end ruptures (splines)

ROLL TYPE2nd Intermediate drive rolls

GRADE

Al l

DESCRIPTION Rupture of a part or all of the splines.

POSSIBLE CAUSE(S) Insufficient mechanical characteristics (hardness specifi cation). Driving torque applied only on one part of the drive-end:

- Incomplete introduction of the drive sleeve (fig.1)- Wear (fig.2).- Sleeve geometry.

Surface fatigue leading to a cracking (fig. 3-4).

CORRECTIVE ACTIONS Check the drive sleeve and the splines (wear, geometry). Increase the specified hardness values. Make sure that the drive sleeve is fully introduced.

Fig. 1

Fig. 2


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Fig. 3

Fig. 4


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5. Marks on the barrel / strip

5.1 Indentation

ROLL TYPE

Work rolls

GRADEAl l

DESCRIPTION Local depressions on the surface of the barrel.

POSSIBLE CAUSE(S) Foreign body on the strip. Barrel hardness is too low or the firs t intermediate hardness too high. Soft areas fol lowing a local overheating.

CORRECTIVE ACTIONS Remove all the defects that occurred during the previous run. Improve the cleanliness / filtration of the lubricant. Increase the roll hardness specification o r decrease that of the 1st intermediate rolls.

Fig. 1

Fig. 2


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5.2 Lines

ROLL TYPE Work roll s

GRADE All

DESCRIPTION Lines on the roll parallel to the axis (from a mm to a few cm).

POSSIBLE CAUSE(S) Local overheating or overpressure (small cracks). Carbides removed from the roll surface during grinding or texturing .

CORRECTIVE ACTIONS Remove all the defects that were formed during the previous run. Re-define the metallurgical specifications (for example powder metallurgy).

Fig. 1, After Magnaflux

Fig. 2

Fig. 3, Microphotography after Nital etching


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5.3 Grinding defects


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Fig. 1

Fig. 2

Fig. 3


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Ovalization of the roll Roll centres damaged or bent. Bad alignment of the centres. Too much or too li tle play between the centres and the rolls. Angles dif ferent from the cent res. Centres of the machine in bad condition. Badly driven roll. Irregular cooling.

Concave rolls ( in case of long rolls) Steadies not correctly positioned. Cutting too deep.


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Fig. 4

Fig. 5


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5.4 Irregular surface finsh on the strip

ROLL TYPEWork rolls

GRADE

Al l

DESCRIPTION Irregular brilliance or colour on the strip.

POSSIBLE CAUSE(S) Local variation of the roll hardness, which follows a rolling incident (fig.1) or a grinding mishap.

(fig.2). Rupture of the oil film ( whitish zones).

CORRECTIVE ACTIONS Remove all the defects that were formed during the previous run. Try a steel grade with more vanadium.

Change the mill parameters and/or the oil.

Fig. 1


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Fig. 2


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6. Formation mechanisms

6.1 Surface fatigue

FORMATION MECHANISM

During rolli ng, the maximum shearing s tresses (Hertz theory) exceed the fatigue lim it of the roll materialbelow the surface.Cracks appear and develop under cyclic rolling stresses.They reach the surface and form small splinters that tear out : the phenomenon is called spalling or micro-spalling . The aspect of the rupture is unpolished as a result of the micro-movements of the metalgrains before they separate.

ORIGINS Local: overpressure, indentation, slippage, excessive stresses, marks

Ex: - Back up roller in contact with the 2nd intermediate drive roll.- Junction zone of the cone with the barrel of 1st intermediate rolls.

General:runs too long, insufficient grinding

DETECTION The surface fatigue comes generally with a deep work hardening that accumulates run after run,

and can locally reach 4 HRc (40 Ld) more than the original hardness.

ACTION Grind until the roll original hardness is restored

CORRECTIVE ACTIONS Measure the Equotip hardness Ld after grinding. Measure by filiation every X campaigne (to be defined) the sensitive zones (ex. Cone junction) and

restore the hardness to the original value.


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6.2 Damage due to overheating

ASPECT Cracks on the surface parallel and aligned (fig.1) or in network (mosaic, fig.2). Disturbed hardness (lower or higher). Micro-spalling.

FORMATION MECHANISMHigh local rise in temperature, above that of the tempering (about 500C), even that of the hardening(example: when sticking, some points exceed the fusion temperature). This leads to a modif ication of themicro structure and the hardness and to unbalanced internal stresses.Cracks are then produced by stress relaxation.In case of successive insuffic ient elimination of these micro cracks, micro spallings propagate later onunder cyclic rolling stresses.

ORIGINS Strip sticking. Slippage between the rolls.

Blockage of the roller.

Grinding wheel shock. Grinding parameters inadequate. Rupture of the lubrication fi lm.

DETECTION Structure modification can be detected with an etching test. Hardness disturbance: Equotip hardness filiations of the neighbouring areas. Cracks: non destruct ive tests such as dye penetrant, magnetic, ultrasonic, Eddy current.

ACTIONS Completely eliminate the faults by machining, then carry out a non destructive test.

See also the chapter on the effect of strip sticking.

Fig. 1, After dye penetration


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Fig. 2, After magnaflux

Fig. 3, After magnaflux

Fig. 4, After Nital etching


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Fig. 5, After etching dark zones heated above tempering temperature

Fig. 6, After etching in clear hardened zones heated above hardening temperature


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6.3 Consequenses of a roll ing incident

SUBJECTWork roll (12%Cr grade) after a sticking of s tainless strip .

DESCRIPTION

The roll has small sections of folded strip s tuck to the barrel (fig.1-2).

A sample is removed for a cross-examination of the effected zone (fig.3). The macroscopic examination of the face, after polishing, shows a series of cracks, at regular

spaces, that go directly to the center of the sample (fig.4). The microscopic examination gives a closer view of these cracks and distinguish 3 layers: A, B and

C (fig.5-6).

A : On the surface, the strip which has stuck approximately 0.15 mm deep.

B : a thermally effected zone presenting a modification of the structure and a 63 HRchardness. Depth : 0.2 mm.

C : a cracked zone corresponding to the original structure affected by a gradient of thetempering temperature starting from the interface with zone B.Depth approximately 1.5 mm. Original metal hardness 61 HRc.

FORMATION MECHANISM The sticking of the strip to the barrel is accompanied with a high rise in temperature, locally higher

than that of the transformation points . The reheat treatment, checked by a rise in hardness and the changing in structure, leads

simultaneously to the zone B being placed under compressive stresses and with the zone C, whichhas been softened (tempering effect), cracking under tensile stresses

CONSEQUENCESAfter th is kind of incident, a l ight gr inding eliminates al l the stick ing marks on the surface. If not eliminatedthe underlying cracks will develop during the next runs and will mark the product, cause micro spallingsand even the rupture of the roll.The illustrated case needed a 3 mm reduction of the diameter in order to assure the complete elimination

of the consequences of an incident of an average intensity.

Fig. 1 Work roll after incident


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Fig. 4 10 X enlargement

Fig. 6 Micrography after Catella etching 100XA= str ip , B= overheated area, C= unaffected mater ial

Guide - Defects Cluster Mill Rolls

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