Cavitation Erosion Of Materials

Abstract

This investigation was undertaken to find the optimum hydraulic fluids and

materials for combating cavitation erosion in hydraulic mining machinery

operating,on 5:95 oil-in-water emulsions, Controlled cavitation was generated

in the laboratory in a vibratory cavitation apparatus employing the stationery

specimen system. The influence of separation distance between the drill tip

and specimen and of varying medium temperature on cavitation erosion were also

investigated but were of secondary importance to the studies on materials and

emulsions,

The evaluation of the influence of a number of commercial metal cutting

coolants and specific hydraulic emulsions oncavitation erosion,showed that those

emulsions with a high mineral oil content suppress erosion the most, This

phenomenon is considered to be either the result of a reduction in nucleation

of highly damaging surface cavities due to the elimination of surface nuclei

by an oil film, or to damping of cavity collapse due to the high dissolved gas

content of oil-in-water emulsions,

A large number of commercial grade alloys and polymers have been investigated,

The alloys ranged from single phase, face centered cubic nickel alloys to

complex, multi-phase, cobalt based alloys while the polymeric materials were all

linear and crystalline. The modes of erosion and mechanisms of erosion

resistance were studied in detail by optical microscopy, scanning and transmission

electron microscopy and X-ray diffractometry. The fractographic studies on the

alloys revealed, in particular, the role played by second phase particles and

other surface features such as protruding grain boundaries and surface steps

created by dislocation movement, twinning and phase transformations, Furthermore,

X-ray diffractometry coupled with theoretical considerations and the findings of

previous investigations indicated the fundamental importance of phase transitions

in contributing towards the erosion resistance of alloys,

Cobalt based Stellite alloys, nickel based cemented carbides and the

intermetallic alloy NiTi have exceptionally high erosion resistance, The

Stellites have a high yield stress and also undergo an awe phase transition which

results in a high work hardening rate and high strain to fracture, NiTi similarly

undergoes a phase transformation which allows absorption of seven per cent

reversible strain and also results in a high work hardening rate. The erosion

resistance of nickel based tungsten carbides is, in turn, attributable to the

very high yield stress of these materials,

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Ultra high molecular weight polyethylene and nylons have relatively

high erosion resistances, Their exceptional performances are due to a

high strain to fracture and high yield strength respectively. On the

other hand low tensile strength polytetrafluoroethylene has a very poor

resistance to erosion.

This work has identified various material properties which are

required for erosion resistance, These are : (a) a high yield stress,

(b) a high work hardening rate and (c) a high strain to fracture,