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Some rules and general remarks on roll repair.

September 2008 By: Karl H. Schroeder, BRC

Acknowledgement: I am very grateful to some old friends of mine who helped to improve this paper by adding pictures and good ideas and revising the text: Michael Brandner, Salzburg/Austria, Ray Schleiden, Canton-Ohio/USA and Hans-Juergen Schulz, Siegen/Germany. Introduction: I. Roll makers sometimes have problems to meet the roll requirements – and often it is evident that these requirements can be met by “roll repair”. Roll makers always take the responsibility for what they are doing and guarantee the safe performance of the rolls. There are

1. so called “cosmetic” repairs on the surface of necks or the faces of the barrel, 2. repairs which may have some impact on the strength/durability of rolls like sub-

surface cracks in necks or severe center-line segregations, and 3. repairs to meet the specified dimensions/ compensate machining errors.

II. Rolls may suffer damages in the rolling mill for various reasons, {see [1] – CAEF, Roll Failure Manual, 1st Edition 2002; [2] – Forged Hardened Steel Rolls, service problems, causes and prevention, Union Electric Steel Corporation 1999; [3] – R. F. Schleiden: Iron Roll Failures, pg. 353 ff in Rolls for the Metal working Industries, 2002} Even costs for purchase of new rolls are not crucial for rolling mills (contrary to roll makers) often it is worthwhile to think about roll repair, particularly in times of long delivery times for new rolls. I only found few articles in literature about roll repair/reclamation {see [4] – Rolls for the Metalworking Industries, 1990; [5] – Rolls for the Metalworking Industries, 2002} Often repaired rolls can continue to work without major problems.

• End spalls of back up roll: Edges of back up rolls are under high loads and prone to break of (axial stresses are always zero and the equivalent stress is high! Sigma 1 and 2 different from zero – normally compression), particularly when work- and/or back up rolls are worn out. There is much literature available how to avoid end spalls, however this is an ongoing story {see [6], W. Ji, Schumacher C., MS&T conference proceedings 2005} In case the end spall is minor, then the roll can just be machined with a chamfer (actually the barrel length is shortened by this method) – in case the end spall is huge (often when the barrel of the work roll is longer than that of the back up roll) then the back up rolls – at least for plate mills – might be repaired by applying a shrink fitted ring, see chapter shrink fit. The gap between ring and original roll needs special attention.

• Minor spalls on the barrel of back up rolls • Minor spalls in work rolls in hot flat rolling mills (particularly in plate

mills, but also in roughing stand of hot strip mills)

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• Fatigue cracks in the fillet of a back up roll (may run for some time, repair methods were not found successful)

• Cracks in any part of necks, occasionally starting in the keyways of back up rolls), sometimes in burned bearing necks [4]

• Breakage of a neck of a work roll III. In this paper I will write about repair methods but will not deal with the reasons for repair. There is much experience about successful and non successful repair, in case the reader may have some doubts, contact the roll maker or the author of this article. IV. This paper describes (what I believe is) state of the art of roll repair. It should/could be a standard for roll suppliers and may help the roll users understanding “roll repair”. As normal for e-papers it is easy to revise this paper – and in case you, the reader, have any comment/correction/improvement/addition, do not hesitate to contact the author (karlhschroeder@hotmail.com). Repair methods:

• Accepting of “not met specification or drawing tolerances”

without repair. o Very often tolerances are set very tight – sometimes without any technical

background, just: “Tighter is Nicer!” This is particularly misleading and wrong for necks for taper roller bearings, with relative movement of the inner bearing ring to the neck. The manual of the bearing suppliers show really huge allowed clearances, whatever the roll drawing may show (often less than a few percentage of the allowed clearance!). In this case it is recommended not to do any repair but to try to convince the customer that undersize is nothing bad! However, sometimes the tolerances have to be very tight and met because of special design of the bearing (or what ever) because of torque has to be transduced, and then – sorry, no excuse – the tolerances have to be met, even they are very tight!

o Anywhere on roll surface micro segregations or open pores are “ugly”, but often harmless. In the areas of bearings they are particularly bad looking and in case of fixed inner bearing rings with no problem acceptable. The same is true for tapered necks – no problem.

o Whenever something (bearing rings, rubber sealing) is moving/rotating against a surface with defects it becomes different, we may experience special wear and smooth surface is required

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o Of course there are surfaces on a roll which do not have to take any loads or do not have any special function – like the neck ends, barrel faces or groves for bearing locking systems etc and “cosmetic” repair - like before or “spot welding (normally with Ni – and you will see some shiny spots after some time)” – is done for beauty, nothing else.

• Local dish grinding and nothing else

o Local shrinking defects, porosities, micro-segregations on the neck surface may create stress raisers or increase friction between neck and bearing parts. The easiest way of repair is “dish grinding” (grinding of a very smooth and shallow dish, basin), absolutely harmless, even it may look odd. This is particularly true for all areas without any relative motion of the parts relative to each other, like fixed bearing rings. But there is no risk for nothing when the areas are lubricated sufficiently.

o This procedure is not allowed in zones where a sealing moves relative to the roll, here we need some “iron cement”, or something else what guarantees a smooth surface for long terms.

o Minor spalls on the barrel of back up rolls (happens more often in cold mills) can be repaired also like this, just dished out by hand. Normally the little groove/dint will disappear after some campaigns and normal stock removal by grinding. The repaired zone never should show a sharp edge to the surface, which might create a reprint on the work roll or acts as a stress raiser for a more severe spall.

• Center-line bores

o The area along the center-line is under very low stress during bending forces acting on the roll, “neutral area”, where bending stress is zero.

o Thermal stresses due to temperature gradients in the roll are the highest where temperatures are extreme: roll surface and center-line.

o Center –line bores are state of the art for forged steel conventionally hardened work rolls for cold rolling. The center bore is important to control residual stresses. This is not to eliminate errors but to improve heat treatment and to avoid roll failures during manufacturing.

o Center- line bores are used in forged rolls – but not only – to eliminate center-line segregations and shrinking holes.

As the center-line is the neutral zone of a roll for bending it has to be discussed whether segregations really influence the strength and durability of a roll or not: more or less evenly distributed

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segregations – versus a center-line bore. When a roll has undergone (survived) a complete heat treatment cycle without any problem – then the center-line segregations are not harmful, a roll in normal roll life never again experience higher thermal stress than during heat treatment (and the centre never real bending stress anyway)!

It seems crucial that the center-bore has to eliminate the segregations completely; a bore with remaining segregations/ unsound areas is more critical than evenly distributed segregations or shrinking defects because a bore with serious surface defects represents bad “stress raisers”.

The center bore has to be done straight through; boring from two sides may create big troubles by miss-match of the bores.

The bore’s open ends must be closed – most rolls require centers for machining.

Boring not completely through but stopping somewhere in the neck area, has the advantage of only one open end, but it has the disadvantage of a special stress raiser at the end of the bore which might be critical to thermal stress – in case this happens, for instance, by a stalled bearing.

In my personal experience I did not see too many roll failures due to center-line segregations or shrinking holes – and astonishing enough for some type of roll failures “nice” shrinking holes are preventive.

Customers have to be informed about center-line boring repair and they have to agree, this repair is very evident at UT.

Shrinking defects of the type “cup and cone” (which should be identified clearly with UT) can not be repaired by center bores – rolls with this defect have to be scrapped.

• Weld repair

Welding of any kind of steel is state of the art and we find welding everywhere in machine building, often sheet or strip is welded while rolls are very massive parts with huge cross sections. Welding applications for rolls may be

1. Single spot repair of surface defects, (even successful on grey or ductile iron necks)

2. One line welding to correct - for instance - a mark of a broken tool of the lathe (To close fatigue cracks in the fillet of a back up roll was – to my knowledge – never successful in long terms)

3. Two-dimensional repair of wider areas like worn neck surface or machining errors on necks resulting in undersize,

4. Replacement of “working layers” of back up rolls or section mill rolls what requires three-dimensional welding of some (many) layers

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5. Repair of broken necks by “burn-on” /“cast-on” /”weld-on” [4, 5].

All kind of welding brings liquid metal (in case of grey iron Ni or a Ni-alloy, for steel rolls steel electrodes are used) onto more or less cold steel material. Shrinking and microstructure transformations of the weld-on material create cracks in the weld and local additional residual stresses in the roll which often have bad impact on fatigue strength. Reducing residual stresses in/around the welding is one of the serious tasks: In case 1) and 2) stress relieve is most often performed by mechanical plastic deformations (“hammering”), In case 4) and 5) heat treatment of the total roll (this would help to secure “fillet-crack-weld-repair” as well) is required (impossible with finish machined rolls) and in case 3) at least some heat treatment has to be done (minimum requirement is pre-heating of the necks before starting repair).

Welding system for 1) to 4) is electric arc welding, for 3) and 4) UP-welding is state of the art. For repair of 5) some various procedures are developed and described in literature [4, 5]. No. 1) and 2) are forbidden for any case of repair on/at the working layer; 1) to 4) are strictly forbidden to repair any crack or defect in high stressed areas like the fillet of back up rolls. Replacement/reclamation of any kind of working layer – either of back up rolls or of grooved rolls for edgers or roughers or long products – is done occasionally and seems to be a “hobby” of some specialists who look out for new application for welding wire. It seems to me even there is good performance for some while/ some campaigns real success is missing: either the welded layer spalls starting from tiny porosities in the transition zone or/and the costs (for wire, energy, operating time, additional heat treatment and difficult machining) are not justified. Despite all paper’s praise in literature {[4], [5], and others} to my knowledge it did not become a standard procedure for rolls in rolling mill stands (maybe for other applications it might be successful and state of the art: furnace rollers, rollers in continuous casters, etc).

• Spray coating

o Spray coating is one of the methods to repair an undersize of the roller

bearing seat on a neck – it seems it is much easier to perform than welding; however, definitely, it is not. To do it correctly, spray coating needs a minimum thickness, so, before it can be done the neck often has to be machined to a lower diameter. And spray coating layers are never homogeneous and free of pores.

o These restrictions do not harm when the inner bearing ring is shrink fitted – no problem.

o But in all other applications problems may arise during service: Either high pressure of lubricant will blast off some parts of the spray coating

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and ruin the bearing – or just friction may wear off the spray coating rapidly.

o For taper roller bearings always remember: A wide clearance is better than a tight one and repair of necks by spray coating is not successful – even it was tried very often. The clearance of the inner bearing ring is not related to the TIR!

• Layers of (chemical) Nickel or (galvanized) Chromium Layers of chromium or nickel reproduce the surface structure very nicely and they can achieve high wear resistance. A typical application is galvanized chromium on roughened (shot blasted or EDT [electro discharge texturing] or EBT [electron beam texturing] or what ever) cold mill work rolls to improve the rolling campaigns, the roughness stands much longer than on the original surface. The thickness of these layers can be controlled very precisely (10 to 500 micro m). Because these layers (particularly galvanic Chromium) reproduce the original surfaces so nicely, the method is limited to homogeneous materials like “steel” – the method is not applicable to graphitic materials like SG or graphitic cast steels (the layers will show little craters on the spots of graphite). o Undersize bearing areas (or others) on roll necks can be repaired by Ni or

Chromium layers. The metallurgical bond between the layer and the neck is much better than any spray coated layer. The layers are close und free of porosities – hard Chromium layers often show cracks – so the surface will withstand high pressure of lubricants nicely and the layer – when it is done correctly – will not peel of.

o However, there is a big disadvantage in this method: Equipment and

technology is needed, which is not available in roll foundries, so the help of a specialized shop is required, and these companies often do not have equipment for “bigger” parts, so this method is limited to “small rolls”. Even it is a nice technology the applications are limited and for small rolls the repair costs might be (??) too high – so, after all, it might be easier and faster to scrap a roll instead of trying to repair.

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• Shrink fitting repair

In principle two ways of shrink fitting are applicable and state of the art: • Shrink fitting of rings or “caps”– where the repair ring is heated up to

expand before fitting (this is the same procedure which is used often to fix rings for universal horizontal rolls). For the repair of back up roll with this method it is important to design some axial clearance/space of a few mm between ring and barrel reaching down below scrap diameter (in radial direction), otherwise here the roll or ring will crumble and so create problems. However, in the area of the clearance there is – of course – no contact between work and back up roll (see picture 1) and this may give an imprint on the plate/strip – no good for finished hot strip! {see [7] K.H. Schroeder, MS&T conference proceedings 2004}

Picture 1: A axial clearance between a shrink fitted ring and barrel of a back up roll creates “no oxides” on the work roll, which results in a visible band on the strip [6].

• Shrink fitting of bolts – where the repair bolts are cooled down to the temperature of liquid nitrogen to achieve the necessary clearance, what limits the shrinkage, particularly for smaller parts. A typical application for this repair are (minor) spalls in plate mill work rolls – it was and is “normal” for some mills to order with new rolls plug/bolt-material of the same type for “bolt-shrink-repair”. It is possible – but happens less frequent – to repair roughing mill work rolls as well. It is

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a bit tricky to get the air out of the hole during shrink fit – otherwise it may happen that the plug moves a bit down during the first campaign(s).

Picture 1: Plugs in the barrel of a plate mill work roll after finishing a campaign.

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Picture 2: Repair of a forged plate mill back up roll (about 130 m tons) with cracked keyway

Picture 3: same roll as before after machining Shrink fitted bolts are very useful for other repair, relatively easy to apply and very safe, when it is done correctly (see pictures above and the terrible examples at the end of this paper).

In general machining of outside diameters is always easier than of inside (bore-) diameters. Therefore it is recommended to start with the inside diameter, use a reamer/broach and measure the diameter very carefully, adjust the other parts’ outside diameter adequately. It is recommended to release the end of a shrink fitting by less shrinkage at both ends.

To calculate the right oversize it is important to take the loads into consideration:

• Often the shrink fitted parts have to transfer torque • Sometimes they have to take rolling loads: repair of barrel end spalls with

shrink fitted rings. The yield strength of the material must be high enough otherwise the ring may widen under the three dimensional stress like in a rolling ring mill and the ring gets wider and falls from the roll

• And in case the shrinkage oversize is too high, then the ring (or whatever is outside) may split.

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The mathematics/formulas to calculate shrink fittings are available. In case everything is done correctly then this is the easiest and preferable method for many repairs, much better than screwed bolt joints or what ever. In most applications the oversize of the outer diameter (OD) of inner the part compared with the inner diameter (ID) of the bore of the outer part is between 0.0006 and 0.001 ID; OD = 1.0006 / 1.001 x ID • Bolted or screw joints Screws and bolted joints are used everywhere and very frequently in mechanical parts. It seems everybody understands this type of joint. And for sure there are many experts in bolted joints who design machines of all kind. Literature and internet provide knowledge about this issue. Screws and bolted joints are no easy parts, even “everybody” - like many roll makers - believe they can just design bolted joints easily. However, these self-made experts in screw joints in many foundries and roll forges ignore very basic knowledge. Luckily this does not lead automatically to catastrophic failures, because often the bolted joints are not really stressed. It always should be remembered that:

• Screws do not give a centering of any part (so, in case a part should be centered – then another device is needed) and

• The only reliable screw locking/retention are high pre-stressing of the bolt. It is evident that many designers (even sometimes designer of machines – for sure most of roll foundry-men) have not the slightest idea/knowledge about it. Otherwise they should recognize that it is virtually impossible without more technical advice beyond long armed screw wrench to fix screw bolts bigger than 32 mm (or 1 1/4 inch) diameter – it is almost impossible to apply the necessary torque / turning moment. It is easy to make a drawing with a 100 mm screw bolt – however it is impossible to apply the necessary pre-stressing (without high sophisticated measures/equipment).

• The first pitch of screw thread takes most (80 %?) of the load, so the position of this first pitch should be chosen carefully (to avoid primarily breakages).

An actual repair sketch shows all possible mistakes and misunderstandings – however, probably it will work, because there is almost no load, no stress at this part of the roll.

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• Examples of recent roll repairs suggested by roll maker

o Repair due to center roll segregations with a center bore • Picture 4 shows an overlook of the situation. The bore is done for

good reasons only from one side – the second side would be difficult to close with an adapter, the end dimensions are smaller than the bore. The end of the bore is close to a outside diameter change, so an outside stress raiser might meet an inside stress raiser – this was changed later on. – However, where ever the bore ends there will a stress notch effect and it is to hope that no thermal gradient will be build up in this area (for what ever reason; a bearing stall may induce a neck breakage).

• Picture 5 shows the design of the adapter to close the one side bore. I do not agree to some details:

• It is impossible to pre-stress a screw of 107 mm diameter, what ever the people do (as mentioned above about 32 mm diameter is maximum!)

• If the collar of 133 mm diameter should be used for centering, then we would need very tight tolerances for the collar and the equivalent bore – but nothing!

• The idea to shrink fit a bolt of 12.7 mm diameter (some 60 mm long) is ridiculous: nobody can measure the bore (you have to believe the “reamer”) and nobody can machine the bolt to the desired precision, of course, something is done like this but just by “hammering a soft steel bolt into a hole. What actually is meant is a shrink fit with clearance ZERO, what is not a shrink fit at all! As a screw locking device this bolt is useless – as soon as there is any kind of real stress – but this adapter should not suffer from high stress – maybe the roll will be machined between centers and the roll will be transported (by accident) with a rope around the adapter. So the repair may work – even without this “shrink fitted” bolt.

• Both drawings show an incredible misunderstanding of dimensions and tolerances: it is evident that somebody just converted mm to inch and backwards. In normal technical understanding without tolerances the last digit of a figure is arguable / doubtful: So what to think about a bore length of 3927.475 mm with +/- 10mm? Or the length of an adapter is equal 291.515 mm +/- ??

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• The easy and safe way of repair would be freezing in the adapter, precise, and no additional security would be necessary.

Picture 4: Roll repair by center bore.

Picture 5: Adapter to close a center bore

o Repair of a broken very end of a work roll with an adapter.

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In principle this tiny end should not break – and it would not, if it would be handled as it should. This part is, of course, not allowed to be used for transportation of the roll with rope and crane. The repair can be done:

• Picture 6 gives an overview of the repair situation • Picture 7 shows details of the proposed repair. Again, a screw hole

of M 200 (diameter 200 mm) never can be fixed to some pre-stress of the screw bolt.

• It seems, the people in charge had some doubts themselves and they suggested some “help advice” shown in Picture 8. However, the lever should be “very long” (some meters) to be effective – and then, how to fix the roll that it will not turn??

• An easy, safe solution would be (again) freezing in the adapter. I

don’t know why they are not doing it?

Picture 6: An end of a plate mill work roll was broken of in a mill by abuse transportation

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Picture 7: Repair idea with a screwed in adapter (this was just a draft, some details should have been improved, for example: the screw head (280 mm diameter) should have chamfer on the roll side and the roll should show there a radius etc.)

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Picture 8: A funny idea to help to screw in the adapter bolt

• Glue may improve the joint Glues can be somehow helpful and are often state of the art in machine design and construction. Glues are main part of a joint or additionally used for screw joints or shrink fitting. Glues are applied to fit/fix horizontal universal rings on to arbors; but whenever glues are joining parts then very special know how is around for surface finish, clearance determination, fixing temperatures, waiting times etc. In my experience I never found successful repair of rolls with “just glues” – maybe I did not see the right repair or …. ; I have my doubt for many reasons whether it may work or not.