Abnormalities and Failures of Rolling Mill Rolls
Abnormalities and Failures of Rolling Mill Rolls
Rolls are changing parts of a rolling mill which are used to reduce the cross section and shape of the material being rolled. They are highly stressed parts of a rolling mill and are subject to wear. They are used both in the flat product mills as well as in the long product mills. The rolls are the most critical part of the rolling mills and the performance of the rolling mill depends very much on the quality and the performance of the rolls.
The rolls operate in severe conditions and their application demand an optimum combination of several properties such as wear resistance, and toughness etc. During rolling, rolls are under high load and the contact area between the roll and material being rolled suffers wear. Also rolls are to be capable to withstand both mechanical and thermal fluctuations to which they are generally exposed during rolling. Hence, rolls have a limited campaign life. After the campaign life is over, rolls are required to be changed for continuation of rolling. The state of the surface is one of the criteria determining the roll change.
Rolls which are removed from rolling mill are dressed in roll turning/roll grinding shop and are made ready for another rolling campaign in the mill. Rolls are discarded when their diameter reaches minimum discard diameter.
Rolling mills are increasingly demanding rolls which are capable to maintain the shape and profile much longer with the aim to extend the length of the rolling campaigns. Normally, life of rolls of any rolling mill is limited by planned roll discard. However, despite careful attention given by the roll supplier and also given during the operation of the rolling mill, abnormalities and roll failures do take place in service. A roll failure is a big catastrophe in rolling mill which not only leads to partial or total loss of the rolls, also necessitates removal of resulting cobble in the mill, causes mill stoppage and damage to rolling mill equipment. All these affect the mill performance adversely. Hence, roll failures are to be avoided.
There can be several reasons for the roll abnormalities and failures. Some of the reasons are attributable to the roll manufacturer while some other reasons are attributable to the rolling mills. Roll failures can also take place due to the reasons attributable to both the roll manufacturers and the rolling mills.
Reasons attributable to the rolls manufacturer are the internal defects of rolls which include non-uniform hardness, excessive residual stress, unreasonable micro-structure, low material strength, structure transformation to form internal stress, or loose shrinkage to decrease effective loading area etc. Reasons for roll failure can also include improper cooling system which causes thermal stress in the roll leading to barrel breakage. Rolling abnormalities can also cause roll breakage. Other reasons for roll failures include unreasonable design of rolls and the roll grooves, excessive single pass deformation, deep fire cracks, fatigue and spalling etc. Some of the important reasons for roll abnormalities and failures are described below.
Roll defects and abnormalities
Pinholes and porosities are the defects which can appear on the surface, or can be subsurface. Holes can be circular or irregular in outline, with or without a shiny interior. They are randomly dispersed on the roll barrel of chill cast rolls. A shiny interior is more often seen in subsurface defects and indicates trapped gas with no exposure to air to allow oxidation to take place. The gas can come from the mould coating or even from fire-cracks in the chill mould. Alternative causes of similar defects, normally known as porosity, are a lack of inter-dendritic feeding during solidification or possibly gas development coming from the melt. In some cases for static cast iron rolls the holes are ‘filled in’ with residual liquid and are seen as hard spots with a circular outline. This feature causes surface quality problems but rarely, if ever, leads to catastrophic failure. These defects are considered as a roll fault.
Non-metallic inclusions are a roll abnormality. Non-metallic inclusion can be of different size and appearance. Different sources are possible for non-metallic inclusions such as slag or flux entrapment or foreign particles coming from the mould or casting equipment. This is a roll abnormality and affects surface quality but does not normally lead to massive roll failures.
Hard and soft spots on the rolls are surface and/or subsurface defects. These defects appear as circular or semicircular, white or grey spots within the shell material and are either harder or softer than the surrounding base metal. They normally do not appear as a localized single defect but generally affect a large part of the roll body. This occurrence is limited to spun cast rolls for which different explanations can be given. Hard spots show a concentration of segregated iron carbides where as soft spots show carbide depleted or graphite enriched areas. One cause is probably the segregation effect caused by a gas bubble, which is pushed by centripetal force, through the shell metal just before final solidification. The gas originates from sudden decomposition of the water of crystallization contained in the binder of the coating material. Other reasons can be linked to excessive vibration of the mould during spin casting which influences segregation during solidification at the solid/liquid interface. This is a roll abnormality which can result in surface marking of the material being rolled but not severe failures in service.
Roll hardness variations
In case of rolls, correct hardness readings are difficult to obtain and the linear relation of hardness to other properties is always limited to a certain degree. This is because roll materials have a wide variation of composition and structures. Hence, hardness readings are more confusing than helpful in case of rolls. The views of roll manufacturers and roll users on hardness readings have always being differed from each other.
Only surface hardness can be measured in a roll non-destructively. This two dimension measurement is generally considered to be a representative for the three dimensional volume behind the surface. But in the roll there exists hardness gradient due to macroscopic and microscopic variations caused by casting (decreasing solidification speed with increasing distance from the surface) and heat treatment (decreasing cooling speed with increasing distance from the surface during quenching in relations to time-temperature- transformation curves). Further hardness depths are influenced by the compositions and the heat treatment methods.
Also, roll material being extremely hard, and hence, it is strongly influenced by work hardening (on the lathe or the grinding machine), and tempering (on the grinding machine). Hardness has impact on wear resistance. But it is also not correct that everything improves with higher hardness. Other parameters do have a higher impact on the roll performance such as composition, micro-structure, and the residual stresses.
Further there is a hardness variation on the roll surface in case of rolls since the surface area of a roll barrel is quite large. Also, after the roll has been used for some rolling in the mill, the non uniformness of hardness can increase since the centre part of the roll, which is in contact with the hot material being rolled, gets tempered causing a decrease in the hardness.
Rolls normally have residual stresses. These residual stresses are two dimensional at the surface and three dimensional in the volume. At the surface the radial stress is zero and the longitudinal stress (axial) is also zero at the barrel edge. At the main part of the barrel, axial and circumferential (tangential) are equal in sign and size. At the centre line, close to the axial area of the roll, tangential and radial stresses are equal in size and sign. Here the relation of longitudinal to tangential/radial stress is given by the relation of roll diameter to length. Which stress exceeds the material strength of the roll, causes a spontaneous breakage of the roll. The fracture can be perpendicular to the axial direction in case the longitudinal stress is too high first, or the fracture can occur in axial direction if the tangential/radial stress is too high first.
Residual stress has a high impact on the strength of the rolls. Compression strength increases the fatigue strength, reduce crack propagation, and reduce shear stress at the roll barrel surface and work hardening. Tensile residual stress may cause roll breakage. Compression and tensile residual stresses in a roll compensate each other over the cross section of the roll. The right level of residual stresses is required to be controlled in rolls.
Roll damage due to single load and thermal breakage
Roll damage can also be caused by one single load. Whenever, the roll is put into the rolling mill and the rolling starts, the roll surface heats up to a mean temperature, which stabilizes after some rolling time. During this period, a temperature gradient exists in the roll with hotter outside and cooler inside. Due to this temperature gradient, outer part of the roll has more thermal expansion than the inner part. This creates thermal stress in the roll with compression stress outside and the tensile stress inside. The thermal tensile stress adds to the residual tensile stress and if the total tensile stress reaches the strength of the material then a crack is initiated which is the starting point for the roll failure. The lower is the initial temperature of the roll when it is put into the mill; higher is the risk of the thermal breakage.
In case of thermal breakage, the barrel is broken showing radial oriented fracture lines whose origin is at or near to the axis of the barrel. The fracture is perpendicular to the roll axis and usually occurs close to the centre of the barrel length. The thermal breakage is related to the maximum difference of temperature between surface and axis of the roll barrel. The temperature difference can be induced by a high heating rate of the roll surface arising from poor roll cooling or even a break-down of roll cooling or a high throughput at the beginning of the rolling campaign. This temperature difference between the outer zone and the inner part of the roll initiates thermal stresses which are superimposed on the existing residual stresses in the roll. As an example, a difference in temperature of 70 deg C between the outer surface and the axis of the roll causes additional thermal stresses in the longitudinal direction of around 1,100 kg/sq cm during the critical phase after start-up of the rolling campaign. Once the total longitudinal tensile stresses in the core exceed the ultimate strength of the core material, a sudden thermal breakage is induced. This can be either a mill fault or a roll fault.
In fact, there are three factors which are important for thermal breakage. These are (i) thermal gradient, (ii) strength and integrity of material of the core, and (iii) residual stresses. Actually, lower is the strength of core material of the roll; higher is the risk of thermal breakage.
Residual stress has a high impact on the thermal breakage. Residual stresses are always compression stress outside and tensile stress in the inner part of the roll. These stresses are reduced in all the areas where grooves are machined into the roll but remain high in the areas between the grooves. When these rolls are heated up during the rolling process thermal stress is added to residual stress and when the total tensile stress (at a maximum between the grooves) reaches the strength limits of the material, thermal breakage takes place between the grooves.
Mechanical and physical damage of rolls
Rolls can also have mechanical damage. Mechanical damage in rolls can take place because of local mechanical overload. It is quite common to find some intrusions, bruises, impressions on the rolls. These happen when any foreign material enters the rolls along with the material being rolled. The damage to roll take place when the hardness of the foreign material is high or its size is big enough to cause a deep impression on the rolls. In case of deep roll impression, it becomes necessary to machine the rolls.
Physical damage of the rolls can be of several types. These are described here.
Peeling is one of the abnormalities in the rolls. During rolling, a thin layer of oxide is formed on the roll surface within the rolling width. Partial removal of just this oxide layer is known as peeling. This peeling can be easily identified when observed as silvery circumferential streaks of parent roll material, intermingled with blue/black oxide streaks still adhering to the roll surface. The oxide layer on the roll surface grows as a function of the roll surface temperature when leaving the roll bite and time of exposure to air at elevated temperature. This oxide layer is submitted to alternating shear stresses due to the difference of surface speed of the material being rolled and the roll. Once the fatigue strength of this oxide layer is exceeded, peeling of this layer starts. Peeling is characterized as long as only the oxide layer is sheared away while the basic roll material remains intact and continues resisting the shear forces. The occurrence of this phenomenon depends on rolling conditions including surface temperature of material being rolled (which strongly determines the nature and hardness of scale), rolling reduction, roll cooling and length of the rolling campaign. Peeling of the roll is a mill feature.
Bruise or mechanical marking are local indentations on the roll barrel combined with heat-induced bruises, fire-cracks and pressure cracks within or outside the rolling width. These marks are predominantly seen on the work rolls from the rear finishing stands of the rolling mills. These are caused by local extreme overloads related to cobbles, folded and/or cold ends of material being rolled, foreign objects or thicker scale particles which have passed through the roll gap. All these abnormalities, when happening at high rolling speed, can cause high pressure as well as deformation and friction heat. These conditions can create the local surface damages on the rolls. They result from mill abnormalities if the rolling process gets out of control.
Banding is an abnormality of the rolls. It is the heavily peeled bright areas which appear on the work roll and are oriented in the circumferential direction and are very often in the form of bands with a very rough surface. Banding typically appears on ICDP (indefinite chilled double pour) work rolls in the early finishing stands of hot rolling mills, even after rather short campaign times. Banding is also possible, when high chrome work rolls are used after longer run times in the same critical stands and positions. Due to the alternating friction forces in combination with alternating thermal loads exceeding the hot fatigue shear strength of the shell material, surface parallel cracks within the depth of primary fire-cracks develop and propagate until the fire-cracked areas are sheared away from the roll. Once the roll surface is locally deteriorated, peaks of shearing forces are induced which lead to a very fast development of peeled bands around the roll barrel. The removed layer has a depth of around 0.1 mm to 0.2 mm which more or less corresponds to the depth of the primary fire-cracks. This occurrence is mainly related to the mill conditions.
Welding of material being rolled takes place on the roll barrel mainly on the work rolls of the rear finishing stands especially when rolling thin gauges. High specific rolling pressure in combination with low rolling temperature is basic conditions for welding of the material being rolled to the roll surface. In particular rolling abnormalities, such as cobbles, crimps and folded ends of material being rolled cause extreme high pressures which favour sticking of material to the roll. Thus increased plastic deformation of the roll surface in the form of indentations, or even spalling, is induced in these overloaded areas where severe heat development adds fire-cracks or bruises. Wrong choice of roll grade for the last finishing stands can lead to catastrophic sticking. This damage is caused by mill conditions.
Circumferential wavelike scratch grooves can appear on the barrel surface coinciding with the edge of the material being rolled in flat mills. The extent depends upon the variation in width of the material being rolled before a roll change. This appearance is typically observed on work rolls from the early finishing stands. This is caused by increased resistance to deformation of the edge of the material being rolled combined with high reduction rates in the early finishing stands. If the edge of the material being rolled is considerably colder than the centre and if hard scale is present on the edge then higher specific load causes increased localized wear on the roll surface. This is more prevalent when rolling stainless steels with highly alloyed roll grades. This is a mill related occurrence.
Sometimes due to the abnormalities in the rolling mill, a large torque moment is build up by the driving motors. The large moment build up takes place since the motors are always strong and powerful part of the mill. Due to the build-up of the huge torque moment, roll neck failure takes place since roll neck is usually the weakest part of the whole system. When roll neck breaks, it shows a typical structure of a fast, brittle burst with the fractured area normally inclined at 45 degree to the axial direction. However, fatigue-torsional failures of roll necks are rare during normal rolling.
The journal of the roll can suffer a cross sectional failure. It generally starts at the bottom of the radius adjacent to the barrel. The fracture face follows the radius and then continues into the side of the barrel, and shears away a portion of the barrel end face. Under shock load conditions the peak load can exceed the ultimate bending strength of the core material and fracture occurs, usually at the most highly stressed cross sectional area. In the case where a roll has been miss-handled by being dropped or by incorrect use of the porter bar during roll changes, roll necks can either crack or more often fail by fracture. The fact that a piece of the barrel is attached to the journal indicates a misuse failure. This is a mill fault.
The failure of the journal can take place due to the bending fracture. Fracture lines start from the outside and spread over the whole cross section, particularly starting in the fillet area and very often after fatigue crack propagation. This failure arises from high bending loads which exceed either the ultimate bending strength or fatigue strength of the journal. It is generally limited to 2-Hi work rolls of any grade in hot rolling mill stands. This kind of breakage can be caused by (i) high rolling loads combined with a weak roll design, (ii) rolling abnormalities with extreme bending forces, (iii) inadequate roll quality as far as journal strength is concerned, and (iv) a notch effect as a consequence of too small a fillet radius, circumferential grooves, and fatigue cracks induced by corrosion etc. This is either a mill or roll fault.
In case of journal failure from drive end torque, the fracture face is inclined to the roll axis and can show a complete shear fracture, which quite often forms a cone shaped break. This type of fracture occurs at the drive end initiating from the weakest section, which is often the split ring recess, propagating to the centre of the neck, or from the root of the spade end radius. This happens when the torque on the drive end has exceeded the torsional strength of the journal material. The strength of the journal is also get affected by the notch effect of sharp radii, i.e. in the split ring recess, or any other stress raisers such as radial bore-holes. The load can be normal for the design and operation of the mill, in which case the roll material requires upgrading or the load can be in excess of standard mill operation, which in turn is higher than the torsional resistance of the roll material. Overloads can be experienced through a variety of conditions such as (i) a mill stall due to a sticker, (ii) rolling abnormalities such as welding of material being rolled, wrong pre-set of the roll gap etc., and (iii) incorrect drive shaft fitting, either by the rolling mill, or by incorrect machining of the drive end. This is generally a mill fault.
The failure of journal can also be due to the worn and seized bearings. Score marks or deep scratches can occur on the journal in the area of the bearing, either along the axis or in the circumferential direction. There can also be indentations and inclusions of fragments of mill scale or other extraneous materials. Other damage can include oxidation and erosion of the ground surface underneath the bearing. Rotational marks and fire-cracks can be evident in the bearing area and in extreme cases thermal breakage of the neck can result. Cracks can also propagate from the oil injection holes. Inadequate, damaged or even missing seals allow intrusion of water, scale and other foreign particles into the gap between the inner bearing race and the journal. The deep scratches along the axis are caused by debris between the bearing and the journal digging into the surface when the bearing is removed for roll grinding. Grease viscosity which is too low and wrong clearance between bearing and journal together with foreign particles can cause surface damage and wear when the inner bearing ring moves around the roll journal due to micro slippage. This can even induce cold welding and cohesion between the journal and the bearing plus blockage of lubrication holes. The result can include high frictional loads, fire-cracks from the heat produced and a seized bearing. Excess wear on the journal, lack of lubrication, elliptical machining or incorrect fitting of the neck ring or any other lack of sealing can allow the mill cooling water to penetrate under the bearing and cause corrosion. This is a mill fault.
Fire cracks are thermo-shock cracks which form under a very sharp cooling rate on the roll surface. When the heated roll surface with a thermal gradient perpendicular to the roll surface during the revolution of the roll is quenched by the cooling water, surface tensile stress is build up. When the tensile stress reaches the tensile strength of the roll material, then cracks (fire cracks) are initiated. These cracks are only formed under tensile stress.
A fire crack pattern on the surface of rolls used for hot rolling with water cooling of rolls is quite normal. It helps in improving the roll bite. However, fire cracks can develop into deeper crack to cause roll failure due to spalling. The fire crack pattern is dependent on the strength of the roll material. Higher is the strength of the roll material, the wider is the fire crack network and deeper are the fire cracks. The worst type of fire cracks take place when the mill stops with the hot material between the rolls and water cooling remains on. The effect is more severe in the roughing group of stands. In a sudden mill stoppage with big cobble, it is necessary to stop the roll cooling immediately and allow the roll to cool in a normal way without quenching to avoid deep fire cracks. Also, in a sudden mill stoppage with big cobble, it is advisable to inspect the roll surface for deep cracks before restarting the mill.
There are usually three types of fire cracks. They are (i) band fire-cracks, ladder fire-cracks, and (iii) localized fire-cracks.
Band fire-cracks correspond to the width of the material being rolled and to the contact arc between work roll and the material being rolled. The appearance of these cracks is the usual mosaic type, but it is of larger mesh size than a conventional fire-crazing pattern. In the case of a mill stop, the material being rolled can remain in contact with the work rolls for a considerable time. The temperature of the roll surface increases rapidly in the contact area and heat penetrates deeper into the roll body. The thermal stresses induced exceed the hot yield strength of the roll material. When the material being rolled is removed and the rolls lifted, the roll surface cools down and due to the contraction of this localized area, the surface starts cracking. The severity of the cracks is dependent upon the contact time and the rate of cooling. This is a mill fault.
Ladder fire-cracks (Fig 1) are within a circumferential band on the barrel of the roll. These are longitudinal oriented cracks which propagate in radial planes. This type of fire-cracks can be initiated due to a lack of cooling, for example by blocked cooling nozzles. Due to pronounced heat penetration into the roll body, these fire-cracks are much deeper than usual fire-crazing. This is a mill fault.
Fig 1 Ladder fire cracks and pressure cracks on the rolls
In case of localized fire-cracks the barrel shows local areas of fire-cracks, sometimes together with indentations or even local spalling. These cracks occur when the combination of mechanical and thermal stresses within these local areas pass over the yield strength of the barrel material and are exaggerated during subsequent cooling. Mill abnormalities such as a bruise through impact, welding of material being rolled, crimping (pinching) of edge or tail end of material being rolled are possible reasons for this kind of damage. The combination of fire-cracks and pressure cracks makes this damage very dangerous as it may induce ribbon fatigue (Fig 1) or even immediate spalling. This is a mill fault.
Rolls are also damaged because of fatigue. The damage due to fatigue can start at the surface or the sub-surface. The problem of fatigue in the rolls can arise due to high loads in the mills. Typical example of the fatigue failures are the barrel of the section mill rolls (Fig 2). Corrosion fatigue can also be a problem. With corrosion fatigue, there is no safe operation at all, and there is no fatigue limit. Corrosion fatigue can be reduced by reducing the nominal stresses by optimizing the roll design and high residual compression stresses. One another point connected with the corrosion fatigue is that there is no safe operation at all and there is no fatigue limit. Corrosion fatigue breakage is because of alternating stress and time.
Sometimes, after a cooling problem in the mill, roll shows some circumferential fire-cracks, which with some depth really reduce the cross-section of the roll, and due to high load the roll can break under the condition of low cycle fatigue.
Fig 2 Fatigue type roll failure and saddle shaped spall in rolls
Spalling can be another reason for roll failure. There are two different kind of spalls in the rolls. One starts at an initial surface crack while the other kind starts at the sub-surface. Surface cracks are normally caused by local overload, and all types of rolling abnormalities including abnormal rolling conditions. When the plastic deformation on the roll surface is greater than the material of the roll allows then a crack starts.
Spalls which are fatal roll damages always tend to happen with relatively low number of revolutions. This means crack initiation, crack propagation, and final spall failure can develop in one single rolling campaign.
There are five types of spalls. These are (i) saddle spalls, (ii) pressure cracks and ribbon fatigue spalls, (iii) shell/core interface-bond related spalls, (iv) spalls due to insufficient shell depth, and (v) barrel edge spalls.
Saddle shaped fatigue spalls (Fig 2) originate in the core material below the shell/core interface and break out to the barrel surface. Variable intensity of fatigue lines can be seen in the deep areas of the spall indicating the propagation direction from the core to the barrel surface. These spalls occur in work rolls with flake graphite iron core and are predominantly located in the centre of the barrel. Spalling is caused by high cyclic loads due to large reductions when rolling thin gauge and hard materials. These loads induce high alternating stresses on the core material, beyond the fatigue limit, and many micro cracks begin to form causing a progressive weakening of the core material. In the next stage these micro cracks join together and propagate to and through the shell to the barrel surface giving rise to the large and typical saddle spall. High residual tensile stresses which are thermally induced in the core during manufacture favour this type of roll damage.
In case of pressure cracks and ribbon fatigue spalls (Fig 1), initially, one or more pressure cracks is formed in an area of local overload, at or near to the barrel surface. Such a crack is usually oriented parallel to the roll axis but propagates in a non-radial direction. In the next stage, a fatigue, cat’s tongue like fracture band propagates progressively in a circumferential direction running more or less parallel to the barrel surface. The direction of propagation is opposite to the direction of roll rotation. Propagation develops within the working surface of the roll, gradually increasing in depth and width followed by a large surface spall of the overlying barrel surface. High local loads at leading edges, cobbles or doubling of the end of the material being rolled, exceed the shear strength of the shell material and initiate the crack. Subsequent rolling fatigues the material and the crack propagates until a massive spontaneous spall occurs.
In case of double poured rolls, a large area of shell material separates from the core following the weakly bonded interface until an area of full metallurgical bond is reached. At this point the fracture propagates rapidly towards the barrel surface resulting in a large spall. During the casting of a roll, the aim is to achieve the full metallurgical bond between the shell and the core metal. Disbonding of the shell from the core during operation is favoured by any reason which reduces the strength of the bond such a (i) residuals of oxide layer between shell and core, (ii) presence of flux or slag at the interface, and (iii) excess of carbides, micro-porosities, graphite flakes or non-metallic inclusions such as sulphides etc. Other reasons for separation of the shell and core can be the excessive local overload during mill abnormalities initiating a local disbonding which continues to grow by fatigue crack propagation following the shell/core interface until a critical size is attained. This leads to a spontaneous secondary big spalling. This kind of damage can happen even if there is no metallurgical defect in the bonding zone. Excessive radial tensile stresses in the bonding zone due to abnormal heating conditions (failure of the roll cooling system, sticker etc.) can also be a reason for separation of the shell and core. This is normally a roll fault if bonding defects are present.
Spalls due to insufficient shell depth takes place when the interface between the shell metal and the core is completely welded but the depth of shell is insufficient to reach scrap diameter. The core material which contains more graphite and lesser alloy is much softer than the shell material and shows as grey in colour. As the interface follows the solidification front of the shell metal, the areas of soft core metal showing at the barrel surface, are patchy and not continuous. The depth of shell depends upon a number of factors which controls the centrifugal casting process such as metal weight, casting temperatures, and time base etc. It is when one of these critical parameters has not been met then insufficient shell depth is obtained. This is a roll fault.
Surface and/or subsurface cracks and associated spalls form on the work roll barrel in case of flat mills around 100 mm to 300 mm from the end of the barrel in a circumferential direction. These cracks are extended towards the freeboard of the barrel surface. In extreme cases, these cracks can enter into the neck radius. This cracked edge either can stick to the roll body or break out as a large spall. Excessive pressure on the end of the work roll barrel, reinforced by positive work roll bending, lack of barrel end relief of the back up roll, poor shape of the strip, thick edges (dog bone shape) or wrong set-up process induces a local overload which exceeds the shear strength of the work roll material. Excessive wear profiles due to long campaigns can cause localized overloads at the barrel ends which favour the initial crack. Continued rolling propagates the crack, exiting at the freeboard and failure then occurs. This is a mill fault.
Damage of steel roll due to hydrogen
Hydrogen (H2) can cause two types of problems in steel rolls. One is special fatigue shown by starting of one or more round of cracks perpendicular to the longitudinal direction of rolls and growing conically into both the directions. It takes a long time until this fatigue becomes evident. The cracks do not really work as stress raisers through bending. The cracks are situated more or less in the stress free area and it is only the thermal stress which really alternates from campaign to campaign. These rolls are in service for many years until the problem become evident. It has been observed that these phenomena always start in the upper barrel end, upper neck, where hydrogen concentrates during solidification and where during primary cooling in that volume and where the ferrite-austenite transition takes place last. It is really a progressive fatigue situation, however the only stresses in this area are related to residual and thermal stress and the number of alterations of loads is very small. But ultimately H2 is found to be active.
The second H2 related issue is that delayed brittle fraction happens unexpectedly, without any rolling load on the roll. Sometimes it happens when rolls are still on stock, even years after delivery. This phenomenon is well known. The material is made brittle by H2 and when subjected to a load, sooner or later (depending on the content of H2 and the stress) the roll begins to disintegrate without any sign of deformation, not even anywhere in the area of fracture topography which shows only cleavage face.
H2 is critical for steel only as long as the atoms of H2 are dissolved in the microstructure and can move by free diffusion. As soon as two atoms combines into H2 gas and is present in cavities or porosity, the gas is no more harmful. Shrinkage cavities are the traps for catching H2. Hence, H2 related failure is not normally found in rolls with large shrinkage cavities. Since the cast rolls always have at least micro cavities, the tolerable content of H2 is much higher than the forged rolls.
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