70
Abrasive Erosion and Corrosion of Hydraulic Machinery
25° as previously but with somewhat increased frequency, distorted
formations of symmetrical shape.
As the angle between the abrasive particles and the surface being worn
out increases up to 45°, the effect of impingement contacts, in combination
with slipping, is observed more distinctly. At the time of an impingement-
slipping contact there simultaneously arise both compressive strain and
shearing tension in the surface layer, which are evidenced by a considerable
number of hollows and banks of material displaced, per unit surface area;
these hollows and banks of material are extended along the action of forces
applied, as a result of the action effected by abrasive particles. The surface
failure effected by shearing continues to proceed, however, the penetration of
particles, at the initial stage of contacts made, develops more effectively due
to a considerable increase of the interactive force component, i.e. the depth
and width of grooves being developed exceed significantly, by their sizes, the
corresponding sizes of micro erosions observed at a = 10°, whereas the
respective sizes of longitudinal orientation are preserved (or insufficiently
altered). The strip area size being removed at singular impingements ensures a
considerable rise in the amount of the material removed as a whole, when
compared with the erosion amount observed at a = 10°.
When the angle of interaction between the abrasive particles and the
surface being worn is increased up to 65°, the surface of metal is basically
subjected to direct erosion, with the traces of plastic deformation appearing
more frequently. Within the elasticity distorted micro areas, when the surface
is subjected to failure caused by low-concentration streams of hydro-mixtures,
with the latter affecting this surface at high angles, the different stages in
formation of erosion products and the adequate discreteness of the process
developed are traced more distinctly.
When the hydro-mixture stream is oriented vertically (a = 90°) the
abrasive particles effect a direct impingement thus causing the deformation of
adequate nature. The main cause of erosion in this case refers to plastic
deformation. There arise here main sites of erosion presented on the surface in
the form of hollows, dints and scaly metal surface layer.
The results obtained by experimental studies reveal that the main cause of
the erosion, being developed under conditions of contacts made between the
surface and fine-grain particles (d
<
0.25 mm), attributes evidently to fatigue
failure and shearing of the surface (at a = 10°).