Fig. 17 Photograph of the attrition mill, showing placement of the attrition mill wear plates In Fig. 18, a new, unused grinding plate(left)is a 230 mm(9 in diameter and 25 mm(1 in. )thick disk of gray cast iron with a hardness of 338 HV and 0. 1 mm(0.004 in graphite flakes, which are an excellent absorber of ibrations during mineral grinding. Radiating outward from the center are six cutting flutes that are used to shear and fracture the quartz particles. The 32 mm(1.25 in wear ring on the outside of the plate grinds the broken rock fragments to their final size. The procedure used to grind quartz in the disk attrition mill included (1) setting the space between the gray cast iron plates to zero clearance, (2)turning on the unit, and (3)feeding in quartz between the plates. The quartz was precrushed to 6.35 mm(0.25 in. ) diameter and then fed directly into the feed hopper at a rate of 0.5 kg(1 lb)/min. The process continued until approximately 4.5 kg(10 lb)of the 75 um product was deposited in the collection tray. The product was removed from the collection tray and the process repeated. Eventually, the machine failed due to grinding plate wear 6 Fig. 18 Macrographs of new (left)and worn(right) attrition mill wear plates Characterization of the Quartz Feed. Quartz from western Oregon was the primary feed material used in the attrition mill. This rock has a Mohs hardness of 7, corresponding to a Vickers hardness of between 700 and 800 kg/mm. The quartz mineral possesses a rupture strength of 70 MPa(10 ksi) in the transverse direction. On slow cooling, quartz forms a tetrahedral silicate structure, with silicon at the center and oxygen atoms occupying the corners. Quartz fractures conchoidally along crystallographic planes when fractured. The sharp angular shape of the quartz particles has a significant effect on the wear behavior of the grinding plates in the attrition mill, with quartz attrition leading to failure of the part in the following way When the quartz particles are pulverized by the rubbing action between the rotating plate and the stationary plate, particles become embedded in the two grinding plates and abrade through, scratching the wear ring surfaces the edge of the plate. Initially, the clearance between the plates is set to zero, but as the quartz particles flow into the space between the plates, they wedge them apart. The particles wedged between the Thefileisdownloadedfromwww.bzfxw.com
Fig. 17 Photograph of the attrition mill, showing placement of the attrition mill wear plates In Fig. 18, a new, unused grinding plate (left) is a 230 mm (9 in.) diameter and 25 mm (1 in.) thick disk of gray cast iron with a hardness of 338 HV and 0.1 mm (0.004 in.) graphite flakes, which are an excellent absorber of vibrations during mineral grinding. Radiating outward from the center are six cutting flutes that are used to shear and fracture the quartz particles. The 32 mm (1.25 in.) wear ring on the outside of the plate grinds the broken rock fragments to their final size. The procedure used to grind quartz in the disk attrition mill included: (1) setting the space between the gray cast iron plates to zero clearance, (2) turning on the unit, and (3) feeding in quartz between the plates. The quartz was precrushed to 6.35 mm (0.25 in.) diameter and then fed directly into the feed hopper at a rate of 0.5 kg (1 lb)/min. The process continued until approximately 4.5 kg (10 lb) of the 75 μm product was deposited in the collection tray. The product was removed from the collection tray and the process repeated. Eventually, the machine failed due to grinding plate wear. Fig. 18 Macrographs of new (left) and worn (right) attrition mill wear plates Characterization of the Quartz Feed. Quartz from western Oregon was the primary feed material used in the attrition mill. This rock has a Mohs hardness of 7, corresponding to a Vickers hardness of between 700 and 800 kg/mm2 . The quartz mineral possesses a rupture strength of 70 MPa (10 ksi) in the transverse direction. On slow cooling, quartz forms a tetrahedral silicate structure, with silicon at the center and oxygen atoms occupying the corners. Quartz fractures conchoidally along crystallographic planes when fractured. The sharp, angular shape of the quartz particles has a significant effect on the wear behavior of the grinding plates in the attrition mill, with quartz attrition leading to failure of the part in the following way. When the quartz particles are pulverized by the rubbing action between the rotating plate and the stationary plate, particles become embedded in the two grinding plates and abrade through, scratching the wear ring surfaces at the edge of the plate. Initially, the clearance between the plates is set to zero, but as the quartz particles flow into the space between the plates, they wedge them apart. The particles wedged between the The file is downloaded from www.bzfxw.com
plates can see an elastic force equal to 4.05 kN, leading to compressive stresses on the particles of 0. 12 MPa (17 psi), sufficient to overcome the transverse rupture strength of 0.07 MPa(10 psi) for the quartz Additionally, the quartz particles are twice as hard as the cast iron grinding plates and cause severe wear by two distinct wear mechanisms, illustrated in Fig. 19. Because the quartz particles are sharp and angular, gouging abrasion was the primary mechanism of material removal( Fig. 20)at the wear ring on the edge of the grinding plate. The forces generated during gouging abrasion are higher than the yield strength of the cast iron, and these areas plastically deform under the compressive force of the quartz. The angle that the cutting edge of the particle makes with the wear surface is often referred to as the rake angle in cutting during machining operations. The plowing mechanism of material removal is illustrated in Fig. 21. In this case, a particle is oriented such that a"blunt edge is presented to the grinding plate, and material is pushed to the sides of the particle as it passes through the lathe, piling up material at the periphery of the wear groove. In the machining wear mechanism, also illustrated in Fig. 21, particles are oriented so a sharp" edge is presented to the grinding plate, and these particles cut into the plate and remove a chip from the surface. The chips are discontinuous, due to the flake graphite incorporated in the gray cast iron. The gouging, cutting, and plowing process continue clearance between the stationary and moving plates the point where no adjustments can be made \s ero,the ig. 19 An optical macrograph of a segment of the attrition mill wear plate showing gouging abrasion near the inner ring and grinding abrasion near the outer ring 0.5mr Fig 20 Optical micrograph of a cross section at the plate ring showing gouging abrasion. Notice the deep depressions with deformed material and cracks
plates can see an elastic force equal to 4.05 kN, leading to compressive stresses on the particles of 0.12 MPa (17 psi), sufficient to overcome the transverse rupture strength of 0.07 MPa (10 psi) for the quartz. Additionally, the quartz particles are twice as hard as the cast iron grinding plates and cause severe wear by two distinct wear mechanisms, illustrated in Fig. 19. Because the quartz particles are sharp and angular, gouging abrasion was the primary mechanism of material removal (Fig. 20) at the wear ring on the edge of the grinding plate. The forces generated during gouging abrasion are higher than the yield strength of the cast iron, and these areas plastically deform under the compressive force of the quartz. The angle that the cutting edge of the particle makes with the wear surface is often referred to as the rake angle in cutting during machining operations. The plowing mechanism of material removal is illustrated in Fig. 21. In this case, a particle is oriented such that a “blunt” edge is presented to the grinding plate, and material is pushed to the sides of the particle as it passes through the lathe, piling up material at the periphery of the wear groove. In the machining wear mechanism, also illustrated in Fig. 21, particles are oriented so a “sharp” edge is presented to the grinding plate, and these particles cut into the plate and remove a chip from the surface. The chips are discontinuous, due to the flake graphite incorporated in the gray cast iron. The gouging, cutting, and plowing process continues until the thickness of the plate has been reduced to the point where no adjustments can be made to zero, the clearance between the stationary and moving plates. Fig. 19 An optical macrograph of a segment of the attrition mill wear plate showing gouging abrasion near the inner ring and grinding abrasion near the outer ring Fig. 20 Optical micrograph of a cross section at the plate ring showing gouging abrasion. Notice the deep depressions with deformed material and cracks
公了 mm Fig. 21 Optical micrograph of a cross section of the outer section of the attrition mill wear plate showing no gouging abrasion and little plastic deformation. Along the upper wear surface are shallow wear grooves from grinding abrasion From the failure analysis study on the wear plates, suggestions for improving the wear life can be made Evidence from the wear patterns on the face of the grinding plates suggest that modifications to plate desig may reduce overall wear and improve mill efficiency. The cutting flutes on the grinding plates show uniform gouging wear on both sides of the cutting edges, as illustrated in Fig. 19. This means that the cutting edge of the grinding plate, per se, is not involved in the actual shearing of the quartz particles but only in crushing them The clearance between the flutes could, therefore, be reduced, providing more shearing action to the particles before they reach the inner wear ring Several material changes could also be made that might extend the life of the grinding plates. Lower cost is the main advantage of using gray cast iron for the wear material of the grinding plates. a negative consequence of using gray iron is that it has a high wear rate and can contaminate the product with the substantial iron debris as it wears. The ratio of iron to quartz is approximately 0.5. Because no indications of severe impact wear were noted, and because fracture and spalling were not observed, a harder plate material such as white cast iron or a work-hardening manganese steel could be used. Either material would lead to a larger value of the hardness ratio, possibly leading to lower wear. The manganese steel has a second attractive property: good high temperature strength. High-temperature strength is important, because the temperature between the grinding plates during operation can reach 900C(1650F). These high temperatures can significantly reduce the strength and wear resistance of the gray cast iron grinding plates Example 4: Impact Wear of Disk Cutters. Steel disks are used on large tunnel-boring machines to continuously cut away at rock faces. Although material is worn from the disks through indentation and scratching wear processes(Fig. 22), one of the predominant material removal mechanisms is material fracture near the edges of the disk cutting face(Fig. 23) 20 um Thefileisdownloadedfromwww.bzfxw.com
Fig. 21 Optical micrograph of a cross section of the outer section of the attrition mill wear plate showing no gouging abrasion and little plastic deformation. Along the upper wear surface are shallow wear grooves from grinding abrasion. From the failure analysis study on the wear plates, suggestions for improving the wear life can be made. Evidence from the wear patterns on the face of the grinding plates suggest that modifications to plate design may reduce overall wear and improve mill efficiency. The cutting flutes on the grinding plates show uniform gouging wear on both sides of the cutting edges, as illustrated in Fig. 19. This means that the cutting edge of the grinding plate, per se, is not involved in the actual shearing of the quartz particles but only in crushing them. The clearance between the flutes could, therefore, be reduced, providing more shearing action to the particles before they reach the inner wear ring. Several material changes could also be made that might extend the life of the grinding plates. Lower cost is the main advantage of using gray cast iron for the wear material of the grinding plates. A negative consequence of using gray iron is that it has a high wear rate and can contaminate the product with the substantial iron debris as it wears. The ratio of iron to quartz is approximately 0.5. Because no indications of severe impact wear were noted, and because fracture and spalling were not observed, a harder plate material such as white cast iron or a work-hardening manganese steel could be used. Either material would lead to a larger value of the hardness ratio, possibly leading to lower wear. The manganese steel has a second attractive property: good hightemperature strength. High-temperature strength is important, because the temperature between the grinding plates during operation can reach 900 °C (1650 °F). These high temperatures can significantly reduce the strength and wear resistance of the gray cast iron grinding plates. Example 4: Impact Wear of Disk Cutters.* Steel disks are used on large tunnel-boring machines to continuously cut away at rock faces. Although material is worn from the disks through indentation and scratching wear processes (Fig. 22), one of the predominant material removal mechanisms is material fracture near the edges of the disk cutting face (Fig. 23). The file is downloaded from www.bzfxw.com
ig. 22 Scanning electron micrograph showing the surface wear on the disk cutter, which shows elements of indentation/impact and scratching or gouging Fig 23 Disk cutter, comprised of H13 tool steel, showing material fracture at edge of contact surface ause of the dominance of the fracture process on the material removal rate, it was decided that a tougher erial should be tried to provide increased fracture resistance. The original disk material was an H13 tool steel. The proposed replacement alloy was a medium-carbon steel, which had a lower hardness but greater fracture toughness and impact toughness Material Hardness, Fracture toughness, Charpy impact energy at 24C(75 HIRC (KIc) MPa ksivin ft·Ibf H13 tool steel 55-57 14-22a 13-20 Medium-carbon 49-51 130 118 28-33 21-24 steel (a)Depending on direction of crack growth Higher-toughness disks were placed on the head of a tunnel-boring machine, along with those made of H13 Despite having significantly less toughness than the medium-carbon alloy steel, the h13 disks exhibited what could be equated to 1.7 times longer life Examination of the cutting edge of the tougher steel disk revealed even larger fracture areas, similar in appearance to spalls(Fig. 24). As with many field-return worn components, further surface examination at higher magnifications revealed no useful information. due to continued wear and oxidation. Instead. nital etched metallographic cross sections were prepared through the wear surfaces of disks made of each material Figure 25 shows a cross section taken near the edge of the contact surface of an H13 disk. This image shows that plastic flow due to the high contact pressures has led to mushrooming at the edge of the disk cutting face Cracking is subsequently initiating from these mushroomed regions. Figure 26 shows similar cracking in the lower-hardness alloy steel. The cracked zone is significantly larger and thus would lead to more extensive material removal, once crack growth is completed Closer examination of these cracked regions revealed that cracks are typically following white-etching shear localization bands(Fig. 27). These bands likely form in a single impact event and are believed to be nano-crystalline ferrite, with all other elements trapped in solution (Ref 33, 34, 35). Hardness values within the band have been found to approach 1000 HV. Due to the brittleness of this layer, subsequent load cycles lead to cracking and spallation along the shear localization band. The
Fig. 22 Scanning electron micrograph showing the surface wear on the disk cutter, which shows elements of indentation/impact and scratching or gouging Fig. 23 Disk cutter, comprised of H13 tool steel, showing material fracture at edge of contact surface Because of the dominance of the fracture process on the material removal rate, it was decided that a tougher material should be tried to provide increased fracture resistance. The original disk material was an H13 tool steel. The proposed replacement alloy was a medium-carbon steel, which had a lower hardness but greater fracture toughness and impact toughness. Material Hardness, HRC Fracture toughness, (KIc) Charpy impact energy at 24 °C (75 °F) MPa m ksi in J ft · lbf H13 tool steel 55–57 14–22(a) 13–20(a) 3–4 2–3 Medium-carbon steel 49–51 130 118 28–33 21–24 (a) Depending on direction of crack growth Higher-toughness disks were placed on the head of a tunnel-boring machine, along with those made of H13. Despite having significantly less toughness than the medium-carbon alloy steel, the H13 disks exhibited what could be equated to 1.7 times longer life. Examination of the cutting edge of the tougher steel disk revealed even larger fracture areas, similar in appearance to spalls (Fig. 24). As with many field-return worn components, further surface examination at higher magnifications revealed no useful information, due to continued wear and oxidation. Instead, nitaletched metallographic cross sections were prepared through the wear surfaces of disks made of each material. Figure 25 shows a cross section taken near the edge of the contact surface of an H13 disk. This image shows that plastic flow due to the high contact pressures has led to mushrooming at the edge of the disk cutting face. Cracking is subsequently initiating from these mushroomed regions. Figure 26 shows similar cracking in the lower-hardness alloy steel. The cracked zone is significantly larger and thus would lead to more extensive material removal, once crack growth is completed. Closer examination of these cracked regions revealed that cracks are typically following white-etching shear localization bands (Fig. 27). These bands likely form in a single impact event and are believed to be nano-crystalline ferrite, with all other elements trapped in solution (Ref 33, 34, 35). Hardness values within the band have been found to approach 1000 HV. Due to the brittleness of this layer, subsequent load cycles lead to cracking and spallation along the shear localization band. The
lower hardness and plastic flow stress of the medium-carbon alloy steel resulted in the formation of larger shear localization bands and thus, larger spalls with more rapid material removal Fig. 24 More severe material fracture/spalling on surface of disk made from tougher, medium-carbon alloy steel 1 mm Fig. 25 Micrograph showing the edge of the contact face in an H13 disk Mushrooming is occurring due to contact pressures and resulting metal flow Cracks resulting from fatigue or fracture at localized shear bands are leading to material removal Thefileisdownloadedfromwww.bzfxw.com
lower hardness and plastic flow stress of the medium-carbon alloy steel resulted in the formation of larger shear localization bands and thus, larger spalls with more rapid material removal. Fig. 24 More severe material fracture/spalling on surface of disk made from tougher, medium-carbon alloy steel Fig. 25 Micrograph showing the edge of the contact face in an H13 disk. Mushrooming is occurring due to contact pressures and resulting metal flow. Cracks resulting from fatigue or fracture at localized shear bands are leading to material removal The file is downloaded from www.bzfxw.com