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An electromagnetic whirl theory of the genesis of planets, geological and topographical formations, weather and climatic cycles,. Skip to main Skip to similar items. HathiTrust Digital Library. Search full-text index. Available Indexes Full-text Catalog Full view only. There are two ways of doing this: Dual Property DP cemented carbides, or coatings of highly wear resistant materials such as polycrystalline diamond PCD on a cemented carbide substrate. In an ideal case, tool life and tool wear rates are inversely proportional. However, a tools service life is also determined by structural overloading, and the interval and rate of catastrophic tool failures.
Toughness tests on cemented carbides show that the critical stress intensity factor increases with Co content and WC grain size. Fracture toughness is substantially reduced at elevated temperatures. Due to the reduced fracture toughness with temperature, cemented carbides may exhibit a decrease in strength during cyclic loading at elevated temperatures. Illustration of Vickers pyramidal indentation impression and resulting corner cracks used in the Palmqvist method for determining the critical stress intensity factor KIC for cemented carbides.
However, conical drag tools are not as sensitive to catastrophic failures as radial drag tools. Catastrophic tool failure caused by impact loading is typically a result of both tool and cutterhead bouncing which occurs for an unfortunate combination of rock mass structure, cutterhead lacing design, and selected rotary speed. The impact force on the tool is caused by the striking action as it re-enters the kerf or harder portions of the rock structure.
This leads to progressive tool tip chipping and ultimately catastrophic failure of carbide inserts and disk rims. For single-rowed carbide insert studded disks, a ripple breakage effect of the studs is often experienced. Some typical examples of rock structure leading to reduced tool life are:.
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Cemented carbides are classified as brittle materials because practically no plastic deformation precedes fracture. However, cemented carbides show large variations in toughness behavior due to their microstructure. The types of fracture seen are cleavage fractures in carbide grains, grain boundary fractures between carbide grains and shear fractures in the binder. Generally, the amount of cleavage fractures increases with increased grain size and the amount shear fractures with increased binder content.
Expressed as fracture energy, the major contribution is from the latter, for example, the crack propagation through the binder. Thermal fatigue of cemented carbides is most noticeable in non-abrasive rocks since the low. One of the main properties of metallic materials required to resist abrasive wear is surface hardness. Studies of tool wear rates show that abrasive wear mechanisms is a function of the relative hardness of materials with sliding contact.
Abrasive wear can be divided into two categories: Soft and hard abrasive wear. Variable rock structure hardness or mixed face conditions which typically lead to impact loading and shattering of drag tools in particular. Plastic deformation as such is generally not regarded as a wear mechanism, but plays an important part in many wear processes. Abrasive and adhesive wear mechanisms dominate the tool wear process during of cutting rocks that contain minerals harder than the tool tip itself. Surface fatigue wear mechanisms only play a role if the wear rates are low, thereby giving necessary time for these processes to take place.
Wear rates are relatively low and do not greatly depend on the actual hardness ratio. Soft abrasive wear in cemented carbides occurs when the abrasive particles e. In the absence of thermal effect, soft abrasive wear rates are relatively low. Wear caused by sliding abrasion is divided into four basic material failure types: Micro-ploughing, micro-cutting, micro-fatigue and microcracking FIGURE 2. Wear rates increase significantly and become very sensitive to the hardness ratio. Hard abrasive wear in cemented carbides occurs when the abrasive particles harder than WC grains strike the composite and fracture WC grains on impact.
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This action causes a large degree of plastic deformation as the particles cut grooves or craters into the wearflat surface, resulting in voids and residual stresses that lead to additional fragmentation of WC grains. Silicates typically cause most of the abrasive wear on rock cutting tools. Adhesive wear contributes to the total wear when the wearflat temperature and contact stress are high enough to weaken the tool tip material so that the cutting tool becomes worn by hard abrasives.
The ability to retain hardness at high temperatures, or hot hardness, is one function of the WC-Co composite structure. WC grain hardness is not appreciably affected by temperatures reached during normal cutting operations. Critical hardness loss results when the Co binder absorbs sufficient heat to transform it into the plastic range where deformation and creep of WC-Co composites readily occurs.
Sintered cobalt within cemented carbides melts at approx. Bearing this in mind and due to the presence of asperities, localized peak contact temperatures may be as high as 2,C. Refer also to Figure 2. Typical trend of tool.
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Both rock and tool tip materials are often non-homogeneous in hardness testing and may consist of several components of varying hardness. The aggregate surface hardness of rock and wear materials is the mean value based on the hardness of their components. However, some components influence aggregate hardness more than others: - Carbides in steel, for example, have a significant effect on the wear resistance of steel cutting tools, but do not influence the overall composite material hardness since they are too small to be significant for the Vickers microindentation hardness.
Yet rock cutting tool wear in quartzitic rock occurs rapidly consistent with that produced by hard abrasives. This behavior suggests that thermal effects are important. Quartz particles may not attain the same temperature rise as the tool tip due to the limited period of time that individual quartz particles are subjected to frictional heating.
Thus, relative hardness between tool tip materials and rock mineral grains is insufficient to describe their behavior in a wear system. This is partly true due to the different nature of. For wearflat temperatures below a threshold limit, WC-Co composites in rock cutting experience in wear produced by soft abrasives; while at higher temperatures wear is accelerated and occurs by mechanisms associated with hard abrasives and adhesion.
The temperature at which tool tip materials first start to weaken is called the critical temperature, Tcritical, and the corresponding tool cutting velocity, vcritical. Critical velocity is affected by several factors such as tool tip geometry, tool tip material properties especially WC grain size since coarse WC grains improve thermal conductivity and thus enhance the transfer of heat away from the wearflat , use of waterjets for cooling and rock wear capacity FIGURE 2. Generalized summary of the behaviourand wear resistance of cemented carbides as a function of temperature.
Physical rock properties such as grain size, density and porosity. These parameters describe insintric rock properties, which are inherent only to the rock itself. Mechanical rock properties such as strength, deformability, hardness, toughness, wear capacity etc. These properties are influenced by the test method. Tool consumption depends on the following wear process parameters: Tool tip material - Carbide grade wear resistance to thermal and surface fatigue - Carbide grade resistance to catastrophic failure due to structural overload, thermal shock and shattering - Carbide insert size, shape and arrangement of attachment to tool holder Kerf profile - Fragment size and strength of kerf rock powder both dependent on mineral grain surface hardness.
Schimazek and H. Knatz in , was the result of pin-ondisk wear tests on carboniferous rock from the coal mining districts in Germany. The Wear Index F is linearly related to pin wear rates; and increases with relative mineral abrasivity, mean quartz grain size and tensile strength of the rock specimen, i.
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The equivalent quartz percentage takes both the amount and relative mineral grain abrasivity to quartz into consideration. The abrasiveness of a rock specimen is not necessarily the same as the aggregate abrasiveness of its mineral constituents; factors such as grain size and angularity, grain cementation and degree of weathering all affect wear capacity of rock.
Typical ranges for some common rock types are given in Table 2. The wear index F has been successfully used in very fine-grained and porous sedimentary rocks in Central Europe. Unfortunately, using the Wear Index F in coarse grained metamorphic and igneous rocks leads to highly misleading results. The Wear Index F was consequently modified by G.
Ewendt in It is defined as follows: A pointed steel pin with a cone angle of 90 is applied to the surface of a rock specimen, for approx. This procedure is repeated several times in various directions always using a fresh steel pin. Metamorphic Rock Phyllite 1.
The most commonly used diamond-tipped microindenters are Vickers a square based pyramid and Knoop an elongated based pyramid. Most systematic studies of ore minerals have employed Vickers microhardness determination and this technique has been widely adapted in ore microscopy. The hardness number is defined as the ratio of the applied indenter load kilogram force to the total inclined area of the permanent impression.
Microindenter hardness tests on minerals normally employ loads of For precise results, the load employed should be stated since VHN values obtained are not independent of load. The rock matrix is typically non-homogeneous on the scale of testing and may consist of several minerals of widely varying individual grain hardnesses.
The Vickers Hardness Number Rock, VHNR or the surface hardness of the rock is an aggregate value based on the weighted hardness values of its mineral constituents, i. There is an obvious trend illustrating that rock wear capacity increases with rock bulk strength and mineral surface hardness. Ultramafic rocks have relatively high bulk strength values since fractures primarily propagate through mineral grains; and not along grain boundaries. Anisotropic rocks have relatively low bulk strength values due to fracture propagation primarily along schistosity planes. This effect is especially pronounced in uniaxial compression tests of rock specimens.
These rock types have relatively low strength values due to rapid fracture propagation originating at and radiating from voids in the rock matrix under stress, thus enhancing the cuttability or drillability of intact rock. Observations show that this micro-fracturing seldom, if ever, enhances rock cuttability or drillability of intact rock.
The phenomenon is typical for Pre-Cambrian granites, granodiorites and felsic gneisses in the Fenno-Scandian Shield. Surface mapping and well-planned drilling programs as well as area mapping and geophysical preinvestigations are used to gain the necessary data and achieve a comprehensive picture complex geological structures. Importance Together with rock specimen properties, all rock mass irregularities are categorized systematically.
Depending on their frequency and relative spacing, they can either be of no importance or dominate the behavior of the rock during the excavation process. Depending on their origin, the following parting planes can be found: - Bedding planes - Schistocity planes and cleavage - Fractures and fissures.
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