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    Mechanism analysis of fatigue spalling of roll sleeve of rol

    Time:2018-05-17 12:48  Click:

    Roller mill is a pre grinding system widely used in cement plant. It consists of two rollers with the same speed and relatively moving. Material from the upper feed roll gap between, squeezed in the double rolls (extrusion pressure generally up to 300MPa), so the roller press mainly by crushing, crushing and pre pre work wear, the wear belongs to high stress abrasive wear, its working principle as shown in figure 1. The wear resistance of roller press depends mainly on the stripe layer.

           0. Preface
    Roller press pre grinding system is widely used in cement plant, which consists of two components for the same speed, the relative motion of the roller. Material from the upper feed roll gap between, squeezed in the double rolls (extrusion pressure generally up to 300MPa), so the roller press mainly by crushing, crushing and pre pre work wear, the wear belongs to high stress abrasive wear, its working principle as shown in figure 1. The wear resistance of roller press depends mainly on the stripe layer.

           In the process of using, such as vertical mill, roller press and so on, especially roll press, the surface peeling and falling out often occur, and the depth and area are uncertain. All of these belong to the category of fatigue damage. The fatigue failure is due to the formation of microcracks in the local high stress region of the component. The weaker grains develop into macro cracks under the action of different stresses, and the cracks continue to expand and eventually lead to fatigue failure. Therefore, in addition to roll surface to ensure wear-resistant, but also to consider the ability to resist fatigue spalling, so roller press roller surface material must have good plasticity and toughness.
    Failure analysis of
           1. roller fatigue
           The fatigue limit of material known to come to a formula:
           Sigma Sigma -1=0.5 B - (1-1)
           Sigma -1, the fatigue limit of materials
           Sigma B - ultimate tensile strength
           Roller press used in normal circumstances, the extrusion pressure range by the roller surface is 50-300MPa, under normal circumstances should be around 200MPa, obviously this is far lower than the working pressure roller of 42CrMo steel matrix material commonly used (b=1080MPa, -1 = 540MPa) fatigue limit and fatigue limit of surfacing material roll the compressor is far higher than this value, the standard deduction, the roller will only wear, not fatigue spalling. The actual situation is often in the distance from the roll surface 20-30mm fatigue spalling occurs. This is the crack layer stripe extends downward, reaching the maximum shear stress and depth, fatigue, on the other hand is in the running of roller press appeared in the process of accidents caused by the roll surface pressure exceeds the strength limit of the material in somewhere or sometime in quite a long period of time over the fatigue limit of the material the micro cracks in the maximum shear depth.
           Fatigue spalling roller by cyclic stress, tensile stress and plastic strain and effect caused by the. The cyclic stress causes the crack to form, and the tensile stress expands the crack, and the plastic strain affects the whole fatigue process. If there is no one of the three, fatigue can not be formed and expanded.
           Figure 2 depicts the material three body wear fixed roller and movable roller press and between.
           The flow of material located in the two roll gap forms two systems with different wear mechanisms, such as zone I (full densification stage) in the figure and zone II (lamination crushing stage and agglomeration stage). The roll surface in the zone I is subjected to low stress abrasive wear, while the roll surface in zone II is simultaneously squeezed and worn by the material, which belongs to high stress abrasive wear. In region II, the roller surface failure due to wear or failure due to extrusion spalling, both of which one is in the leading position, mainly depends on the roll surface under pressure and roller body subjected to periodic change of material wear ability of anti fatigue.
           The extrusion pressure changes periodically, the roll surface has elastic deformation, plastic deformation and even. The roller surface produces a certain depth under the maximum shear stress, the shear stress, if the fatigue strength of roller body is lower than the maximum shear stress fatigue, fatigue is the maximum shear stress position will produce fatigue crack, the fatigue shear stress under the effect of constantly expanding, increasing, the original crack generated or produced in the process of welding roller surface, resulting in roll surface spalling, even large-scale spalling, eventually because of the extrusion, the roller surface wear, serious damage to the normal use.
            2. depth analysis of roller press shear
           On the maximum shear stress of the position is necessary. The position or depth is determined, so that the fatigue strength of the roll or surfacing layer can be increased, and the fatigue life of the extrusion roller is improved. According to the data calculated by the pure elastic theory the maximum shear stress distance and the roll surface depth should be 0.786 times of that of the roll surface and the half of the arc length of the roll surface.
           Contact roll roll surface arc length L= D pi * (Y / 3600) - - (1-2)
           D - roller diameter
           Gamma - bite angle
           According to this method can estimate fatigue depth, but the error is very large, the actual depth is far less than the value of the theory of fatigue. The integral calculation of roll to nip angle, 0.786b is about 5.2mm. If these errors into account the safety factor and the mathematical model of it, then, the maximum shear stress of the roller press depth is at 10mm ~ 15mm, is just the transition layer depth, which may be one of the theoretical basis for the design of the rolling machine of surfacing layer.
           3. conclusion:
    Through theoretical analysis, the theory of fatigue depth of roller press is about 10 ~ 15mm.
           1、 roller diameter is proportional to the fatigue and depth, the diameter is bigger, the greater depth of the fatigue may occur;
           2、 bite angle and fatigue depth is proportional to the bite angle is bigger, the greater the depth of fatigue may occur. The size of the bite angle depends on the type, size and shape of the material, and the gap between the two rollers, roll, roll surface (stripe shape), roll diameter and rotation speed, and other factors. The most obvious is the gap and roll gap, the smaller the gap, the greater the roll will make the bite angle becomes larger, so that the depth of fatigue depth;
           3、 proper control of roll gap, obviously the original roll gap is small, the material throughput is small, in order to achieve high yield must improve the silo pressure will increase by the amount of roll gap opening force, then the column increased, the bite angle increased, at the same time roll cake density in the gap increases, the pressure also increased sharply and the minimum gap, which will cause the roll surface pressure closer to the fatigue strength and shear stress and the maximum depth is proportional to the fatigue life of the adverse guarantee.
           In addition metal density and hardness is far greater than the cement clinker, therefore into the metal foreign body will suddenly increase caused by the roll, or even directly to produce micro cracks, so the feeding roller materials should be as far as possible thorough removal of iron. In addition to the iron separator on the feed belt, it is necessary to set metal detectors on the feeding belt of the separator. But in the production process, should ensure that the metal detector and pneumatic valve chain smooth, quick response, so as to eliminate the mixed metal foreign body material, avoid metal objects in the closed system of roller press and V type powder selecting machine consisting of a continuous cycle of repeated injury of roller surface.
           Anyway, to ensure the operation rules of roller press in a stable and standard, to ensure that the fatigue life of roller press, play the greatest efficiency.