Bombas Centrifugas

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CHAPTER 10

MATERIALS SCIENCE FOR
SLURRY SYSTEMS

10-0 INTRODUCTION
Slurry is essentially a mixture of solids and liquids. Abrasion, erosion, and corrosion are
so associated with pumping slurry that manufacturers of slurry pumps sometimes spend
more money dealing with wear issues than with developing new hydraulics.
Wear is very complexand too often oversimplified. It depends on many factors such as
the microstructure of the surface of the pump part, the hardness and shape of the crushed
or milled minerals, the speed of flow, the scaling of pipes, etc. Dredge and slurry pumps,
ball mill liners and shells, magnetic separators, and agitators are made from metals that
combine the ability to resist stress and impact loads,erosion, and corrosion. Excellent
books on materials science are available to the reader but, unfortunately, they too often
dedicate just few lines or one or two paragraphs to the white irons or polymers used by
the designers of slurry systems. These materials are too often classified as materials for
special applications. This chapter will therefore make an effort to expand on this topic, as
agood understanding of it can save on maintenance costs to the operator and is necessary
for the successful design of a slurry system.

10-1 THE STRESS–STRAIN RELATIONSHIP
OF METALS
The stress–strain relationship of metals under tension (Figure 10-1) is often represented in
the form of a graph of stress versus strain. Stress is essentially the load force per unit area.
It is called directstress when the load is normal to the force, and shear stress when the
load is parallel to the surface:
= FN/A

(10-1)

= FN/A

(10-2)

where
= direct stress
= tangential force
A = area

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10.2

CHAPTER TEN

Utimate Tensile Strength

u

Stress

Yield y

fracture

Elastic
Limit
e

strain

e
E

f

2% offsetFIGURE 10-1

Stress–strain relationship of metals.

FN = normal force
FT = tangential force
When a specimen bar of steel is subject to a tension load at its ends (Figure 10-2), it
stretches. Within a certain range, the elongation is elastic, and this means that if the load
is removed, the specimen will return to its original length. The maximum stress in this
elastic range is called theelastic limit and is shown in Fig 10-1 as e. The elongation L
is divided by the original length L0 of the specimen to define the strain :
= L/L0

(10-3)

It is a nondimensional measure, often expressed in percentage of original length, and mistakenly called “elongation” instead of direct strain. The direct strain under tension is correlated to the direct stress in the elastic range up to e bythe Young modulus E:
= /E

(10-4)

Steels, but not all metals, exhibit a further nonlinear elongation up to a value called the
yield stress. The elongation becomes permanent as the materials yield. Beyond the yield
point, the elongation continues to grow, until a value called the ultimate tensile strength
u is reached. This is the point at which the specimen can withstand the greatestload and
beyond which fracture is likely to occur rather quickly.
Direct stress by itself induces a degree of secondary shear stresses. For each material
there is a Poisson ratio . For steel it is 0.30 and for gray cast iron it is 0.26. Shear strain
correlates with shear strain by the modulus of rigidity G (also called shear modulus). It is
related to the Young modulus by the Poisson ratio:
G=E
2(1 + )

(10-5)

Due to fatigue considerations, steel shafts of rotating equipment are designed for maximum stresses of less than 18% to the ultimate strength or 30% of yield strength. This is
due to the combination of direct stress, torsion, and bending moments. Components of a
slurry mill or pump must be able to absorb the energy of impact without fracture. The energy due to impact...