The following paper is condensed from chapter 4 of "Solid Particle Erosion and Erosion-Corrosion of materials" A. V. Levy, published by ASM International, 1995

In order to understand the nature of the erosivity of various kinds of particles that can occur singularly or together in particle-containing flows in fluidized-bed combustors (FBC), pulverized-coal boilers (PCB), and other types of chemical process systems, a series of experiments was carried out using several different particles. Six particle compositions were used separately to erode AISI 1020 plain carbon steel and the resulting erosion rates were compared.

Cold-rolled AISI 1020 steel was eroded at 25 ºC with particles of the six different compositions at a velocity of 80 mls. The impact angles were 30 and 90º. The 30º angle was selected as it is near the angle at which maximum erosion occurs. The 90º angle was used as it is the angle where a significantly lower erosion rate occurs on ductile metals. Hardness was used as a means of rating the overall strength and integrity of the erodents. The particles of the five different brittle material compositions were all angular in shape and were in the size range 180 to 250 m. The two different shapes of the same ductile steel particle composition used to study the effect of shape on erosion rate were spherical (shot) and angular (grit) with an average size of 100 m.

Particle Strength

The erosion rates are very low for the softest, weakest erodent materials, calcite and apatite. Once the Vickers hardness of the particle reaches approximately 700 HV, indicative of particles strong enough not to shatter when they strike the target steel, the erosion rates remain essentially constant as the hardness/strength of the particles increases further. Thus SiO₂ at 700 HV has nearly the same erosivity as silicon carbide (SiC) at 3000 HV even though there is over four times difference in their hardness. The relatively small erosivity differences among SiO₂ ADO', and SiC are primarily due to small differences in the angularity of the particles, with SiC having the sharpest angles in the as-crushed powder. The strength/integrity of the erodent particles determines whether they can impact the target surface without shattering. This, in turn, establishes their size at impact and the resultant kinetic energy they impart to the target. Their kinetic energy is a primary determinant of their erosivity.

On the surfaces of 1020 steel eroded by SiO₂ ADO', and SiC. The appearance of the craters and platelets is very similar for all three erodents. The shape of the craters caused by ADO' is a little less severe than those caused by SiC, which is thought to account for the slightly lower erosion rate caused by ADO'. The ADO' particles had a somewhat more rounded shape than the SiO₂ and SiC particles, but were generally similar. The size and shape of the three higher hardness erodents were the same after impact as they were before, indicating that they did not shatter upon impact. The striations seen in the shallow craters of eroded surfaces are imprints of the contour facets of the particles made as they translate along the crater surface.

On the AISI 1020 steel surface impacted by calcite and apatite particles significant amounts of calcite and apatite are smeared on or embedded in the steel. The production of platelets and shallow craters is not as readily defined, although the platelet-formation mechanism appears to be the active mechanism on both surfaces shown. The breakup of the weak particles and their adherence to the eroded surface both decrease the kinetic energy of incoming particle segments and cover over the eroding surface with a protective layer of particle fragments. The calcite particles showed a marked difference in size due to fracture on impact is easily seen. The basic shape of the impacted particles appears to be similar to that of the particles before impact. Thus, it is the particles' kinetic energy, which is dependent on their size and density, that has the greatest effect on their erosivity, more so than their shape.

Particle Shape

In order to determine how the shape of the particles affected their erosivity, same size steel particles in both angular and spherical shapes were used to erode the AISI 1020 steel. An impact angle of 30º at 25 ºC was used. No fracturing of the steel particles was observed, so their integrity was in the range of those of the SiO₂, ADO', and SiC particles.

For AISI 1020 steel eroded by the steel particles, the erosion rate, caused by the angular steel grit was four times greater than that caused by the spherical steel shot. The appearance of the eroded surfaces indicates the reason for the difference. The angular steel grit caused much sharper, deeper craters to form, which caused a more efficient production of extruded platelets. The spherical steel shot developed more shallow, rounded craters that did not produce platelets as efficiently. The production of platelets and the factors that influence their formation, extension, and removal are discussed below.

Crater, Platelet Formation

The erosivity of impacting particles is primarily a function of the concentration of force that the particle can cause in a microscopic area of the target metal. When particles are so weak and friable that they cannot maintain their integrity when they strike the metal surface (calcite and apatite) they shatter into many smaller pieces. These pieces do not have the mass necessary to provide the localized force that can form platelets efficiently and subsequently remove them by exceeding the local fracture stress of the target metal.

When erodent particles reach a sufficiently high level of strength and integrity where they do not fracture on impact for the velocity regime used, the erosion rate for particles of similar shape and density becomes approximately the same. Thus SiO₂, ADO', and SiC, which have similar shapes and are in the same density range, have about the same erosivity. Steel grit has a higher density and is considerably more erosive. Thus, the kinetic energy of the particles when they strike the surface plays an important role in determining the amount of force that is available to make and deform platelets and to cause subsurface work hardening. Local concentration of force is also a function of the geometry of particles. Angular particles can concentrate this force more effectively than rounded particles.

Spherical Erodent

Below 300 m, the kinetic energy of the particles appeared to be too low for them to be as effective in removing material even though there are more of the smaller beads to impact the target in a given weight of erodent. This might be explained in part by the fact that the number of particles actually striking the surface does not increase in the same way as the number of particles traveling toward the specimen due to a shielding effect provided by the rebounding particles. At particle sizes greater than 300 m, another factor became dominant in controlling the mass loss. The decrease in mass loss above this particle size was due in part to the decrease in the number of particles striking the surface. However, the major factor causing the reduction relates to the particle's ability to penetrate the target surface. While the larger diameter glass beads had more mass and, hence, more kinetic energy, the particle diameter became large enough to markedly decrease the ability of the spheres to actually penetrate the target surface and cause the severe plastic deformation that crater and platelet formation require for effective removal of material.

Angular Erodent

The steep slope of the curve (mass loss versus time) at small particle diameters can be interpreted as the steep increase in particle kinetic energy with increasing particle size, as was found in the tests using spherical particles. The more or less constant mass loss with increasing particle diameter above 200 m particle size is probably due to a combination of the relation between four characteristics of the particle stream that appear to influence mass loss:

The particle size
The number of particles striking the surface
Their kinetic energy
The interference between incoming and rebounding particles

Metallographic observation of particles of various sizes indicates that sharpness of the SiC particle edges does not change with increasing particle size and, therefore, there is no decrease in their ability to penetrate and plastically deform the metal surface as occurred with the spherical particles.

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