Erosion ductile

In the following, the effects of gliding, scattering, and lifting of the particles by the gas stream are neglected and the mechanical efficiency of impact is assumed to be 100 percent. Thus, for ductile erosions, the wear in volume loss per impact equals the work of ductile wear by the tangential force divided by the energy required to remove a unit volume of material in ductile erosion mode. Therefore, we have... 

Figure 6.6. Normalized ductile erosion energy function varying with impact angle and ductile resistance parameter (from Soo, 1977).

The above discussion clearly brings out certain features of the elevated temperature erosion of metallic materials. Almost all metallic materials exhibit ductile erosion response at room temperature, whereas at elevated temperature both brittle and ductile erosion responses are established [41,46,47]. The velocity exponents for metallic materials are around 2.5 during ambient temperature erosion. At elevated temperatures, the velocity exponents of metals and alloys vary over a wide range from 0.9 to more than 3.0 [41 3]. It is found that the erosion rate at room temperature increases with increase of particle size up to 50 pm, beyond which the particle size has no effect on the erosion rate. The reported literature indicates that the erosion rate increases with increase of particle size at high temperatures [39,40,44,49]. At ambient temperature, changing the particle shape from angular to spherical alters the erosion response from brittle to ductile [60,61], while at elevated temperatures, brittle to ductile response is noted irrespective of the particle shape [39,40, 47]. The particle feed rate has a negligible effect on the room temperature erosion rate [62-64]. A remarkable effect of the particle feed rate has been noted at elevated temperatures [39]. The mechanical properties of the erodent have a nominal influence on room temperature erosion behaviour [65-73]. However, at elevated temperature this aspect has yet to be explored. 

Erosion. The abrasive is likely to be gas borne (as in catalytic cracking units), liquid borne (as in abrasive slurries), or gravity pulled (as in catalyst transfer lines). Because of the association of velocity and kinetic energy, the severity of erosion may increase as some power (usually up to the 3d) of the velocity. The angle of impingement also influences severity. At supersonic speeds, even water droplets can be seriously erosive. There is some evidence that the response of resisting metals is influenced by whether they are ductile or brittle. Probably most abrasion involved with hydrocarbon processing is of the erosive type. 

Figure 8.12 illustrates a solid particle impinging on a surface. It has been found that the erosive wear rate depends upon the impingement angle, a, the particle velocity, vq, and the size and density of the particle, as well as the properties of the surface material. It has also been found that there is a difference in erosive wear properties of brittle and ductile materials. The maximum erosive wear of ductile materials occurs at a = 20°, whereas the maximum erosive wear for brittle materials occurs near a = 90°. Since the impingement angle is probably lower than 90° for these type of flow situations, we might consider only brittle materials, such as ceramics for this application. Let us examine brittle erosive wear in a little more detail first. 

II.1 In Section 8.2, we saw how erosive wear can be different for brittle and ductile materials. In reality, most materials exhibit behavior that is a combination of... 

Sheldon, G.L. and Finnie, I. (1966), On the ductile behaviour of nominally brittle materials during erosive cutting , Trans. ASME, 88B, 387-92. 

When a particle strikes a solid surface, the extent and nature of the damage on the surface depend on the normal compressive force Fn, tangential cutting force Fu area of contact Ac, duration of contact tc, angle of incidence a, shape of the particle, and the materials of the particle and the solid surface. The mechanisms of mechanical erosion may be explained in terms of two basic modes, i.e., the ductile mode and the brittle mode. 

In general, erosion via tangential cutting tends to be more prominent for ductile materials such as metals and plastics than for brittle materials such as ceramics and glasses. In practice, erosion is a result of the combined effect of both the ductile and brittle modes. The extent... 

As mentioned, the erosion of a solid surface depends on the collisional force, angle of incidence, and material properties of both surface and particles. Although abrasive erosion rates cannot be precisely predicted at this stage, some quantitative account of erosion modes which relates various impact parameters and properties is useful. In the following, a simple model for the ductile and brittle modes of erosion by dust or granular materials suspended in a gas medium moving at a moderate speed is discussed in light of the Hertzian contact theory [Soo, 1977]. 

Figure 6.5. Relationship between die ductile resistance parameter and the angle of impact for maximum erosion (from Soo, 1977).

Assuming that the erosion is purely ductile, derive an expression for the impact angle yielding maximum wear. 

For a general surface erosion, the wear is composed of both ductile and brittle modes. Thus, the combined directional wear can be expressed as the weighted summation of both modes. Consider a case where the weighted factor for ductile wear is ( ) = A sin a, and the weighted factor for brittle wear is r (a,) = B cos a . Both A and B are constant. What is the extent of the total erosion Derive an expression for the maximum eroding angle in terms of KD, KB, A, and B. 

Erosion Similar to abrasion cutting in ductile metal fracture (of brittle material) very small chips or particles (e.g., impellers, propellers, fans) Reduce fluid velocity to eliminate turbulence select harder alloy (high chromium) hard coatings such as cement lined pipe, rubber lining... 

Wear impact plastic deformation makes some constituents more susceptible to corrosion. Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting. Supplies kinetic energy to drive abrasion mechanism. Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material. Pressurizes mill water and gases to produce unknown temperatures, phases changes, and decomposition or reaction products from ore and water constituents. Heats ball metal, ore, fluids to increase corrosive effects. 

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