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4. Conosive wear

1.3 Hot roll wear modes

1.3.1 Banding

Thermal fatigue is the most important factor of wear in the hot rolling process[25,27].

Large temperature differences exist between the rolled workpiece and rolls, causing heat flow from the workpiece to rolls through the contact zone. T h e outer layer of the rolls contacting the high temperature workpiece tends to expand, but it is prevented by the body of the roll which remains cold. If the temperature difference between the surface and the body of rolls is sufficiently great, large circumferential compressive stress is induced in the surface. W h e n the outer layer is cooled by water spray, large tensile stress is also induced. These surface stresses are cyclic and have the same frequency as the rotation of the roll. They can be severe enough to exceed the yield stress of the roll material and cause the surface layer to deform plastically[33]. T h e result of these permanent surface strains is a mosaic-like pattern of surface cracks, often referred to by mill operators as "fire cracking". W . J. Williams[34] has s h o w n that these cracks predominate in the directions normal and parallel to the roll surface. Sekimoto[35] found that the depth of the fire cracks is dependent upon the roll surface temperature and the depth of the surface layer which is heated to a high temperature.

Banding of rolls was introduced to decribe the failure caused by thermal fatigue and mechanical fatigue. Banding failure is characterised by the formation of fine surface firecracks, followed by the formations and then peeling of a shiny black oxide. The peeled oxide layer often takes s o m e roll metal with it, leaving a roughnened surface unsuitable for further rolling. Banding is brought about by the tearing out of very small pieces of roll metal[29, 60]. DeBarbadillo and Trozzi studied 2 0 samples in detail including cast steel, grain iron, nodular iron and high-chromium iron during a 2-year

_ ^ Chapter 1 Literature Survey 3 0 research program, and found that fire-cracks always occurred in the roll w o r k surface

but never o n samples taken from outside of the roll w o r k zone. Furthermore, they concluded that the banding w a s essential in the roll wear mechanism and proposed a five-stage mechanism[29]. T h e five-stage mechanism is from the generation of the crack to break-down from the roll surface, which is described as following.

In the first stage, the heating of the roll surface causes stress that is sufficiently high to reach the yield stress in the roll surface, and thus deform the roll surface plastically.

W h e n the roll is cooled, the surface contraction should initiate reverse plastic yielding.

H o w e v e r , yielding does not occur in the reverse direction because the ductility of the roll material is not sufficient w h e n the roll surface is cooled, and the roll material cannot withstand the high tensile stress during cooling. H e n c e the roll surface will crack, resulting in the fire-crack pattern.

The formation of fire-cracks changes the stress pattern abruptly during the rolling cycle.

Stress arising from the roll pressure becomes m o r e important since the thermal expansion of the surface layer is no longer constrained b y the roll mass. In this second stage, transverse cracks are induced due to differential thermal expansion of the matrix and the carbides. This cracking can be considered as thermal fatigue on a microscopic scale.

In stage three, the roll surface becomes more irregular. Surface cracking pushes the surface to bulge up and causes small fragments to tilt. Transverse cracks begin to link with fire-cracks, permitting oxidation. T h e strength of the surface layer is reduced by the presence of cracks, resulting in the collapse of areas over discontinuity such as

C h a p t e r 1 Literature S u r v e y 3 1 graphite particles. T h e thermal conductivity is also reduced due to cracks, causing the

roll surface to be hotter.

In stage four, the hot roll surface rapidly becomes covered with an oxide layer that adheres tightly to the roll surface. T h e oxide is plastically deformed under conditions of high temperature and compressive stress, but has little strength under the influence of tensile stress, and the fire-crack pattern of the roll is not covered by the oxide layer. The surface oxide m a y protect the roll from wear by reducing the heat transfer, but sub-surface cracks continue to form and link up due to thermal stress and roll pressure.

The final break-down of the roll surface, during stage five, can occur in many ways. It can occur spontaneously or be stimulated by an outside force, such as shear stresses at

the roll-bar interface. Often the first step is dislodgment of small pieces of roll material at the intersection of fire-cracks. These free pieces m a y abrade and penetrate the oxide layer and m a y be rolled back into the roll surface. W h e n this process proceeds the surface layer is further weakened and later larger segments consisting of area defined by the fire-crack m e s h m a y dislodge, carrying portions of roll material with them. T h e roll surface is at this stage greatly irregular, and rolling of defect-free products is no longer possible.

Several theoretical, empirical and experimental analyses of this thermal distribution have been published[49, 50, 51] to evaluate the thermal fatigue.

C h a p t e r 1 Literature S u r v e y 32

1.3.2 A b r a s i o n

This type of wear results from the displacement of the softer of the two materials in sliding contact by an asperity of the harder material. It can also be due to a harder particle of foreign material which m a y b e c o m e partly embedded in the softer material so that the hard particle behaves as a miniature cutting tool which ploughs grooves on the harder one of the two surfaces.

In hot rolling mills the abrasive wear is caused by rubbing contact between the rolls and the workpiece. Also, in the case of four-high stands, there will be s o m e abrasion from

contact of the back-up roll and work roll.

Hot strip mills are usually installed with scale breakers before the roughing stands and the finishing stands, so that the scale which is formed on delay table can be removed.

But scale removal is rarely carried out within the finishing stands, so the scale formed between stands is rolled together with the stock. T h e scale layers which are hard and smooth, under the condition of high pressure in the roll gap, abrade the roll surface.

S o m e researchers thought that this kind of three-body wear coming from oxide scales would be a c o m m o n wear m o d e in hot rolling process[2, 11, 57]. A detailed classification of abrasion includes micro-ploughing, micro-cutting, micro-fatigue and micro-cracking.

In a multi-stand rolling mill the workpiece is accelerated as it passes through the stands, hence, the contact time for each revolution between the roll and the workpiece decreases

till the final stand. T h e workpiece's temperature also decreases from stand to stand,

C h a p t e r 1 Literature S u r v e y 33 reaching minimal at the final finishing stand. Hence, the total roll surface temperature

attained in the roll gap diminishes along the mill. As thermal fatigue is a function of temperature and contact time, it is expected to be dominant in the roughing stand but having a smaller effect in the finishing stand. However, abrasion is a function of rolling load and speed, which gets its highest in the final stands. Shuaghnessy[32] reported

from his metallurgical examination of roll surface structures, before and after rolling, that thermal fatigue is the primary cause of deterioration in the roughing and early finishing stands, and abrasion is the cause of deterioration in the finishing stand.

Other wear modes are also responsible in the rolling process. The outer layers of rolls endure large load cycles. Macroscopic mechanical fatigue must exist on the roll surfaces because of the circular compressed stresses. Fragments on the surfaces can be formed due to severe mechanical fatigue. Adhesive wear may be found in some situations.

Usually some fragments of rolled materials stick or weld to the roll surfaces. Corrosion is common in the hot rolling process because of the high temperature, large stresses in the contact zone, and oxygen in the air.

There are some special phenomena found in the hot rolling process. Some researchers found that oxide scales at high temperature sometimes benefit the resistance to abrasive wear, because oxidation of some grades would produce a hard and smooth black

magnetite scale film, which provided small friction coefficient between the rolls and rolled workpieces for some specific grades of rolls[61].

Chapter 1 Literature Survey 3 4