| What
to consider in the selection of an abrasion-resisting steel
Many
considerations are involved in the selection of the proper
grade of abrasion-resisting steel. For example:
the type of service, the type of material being handled, the
type of abrasion, and the economics of operation.
Wear problems can best be solved by selecting a grade for
trial, shaping it into an experimental part and observing
the wear rate. However, the following generality can
be stated: as the hardness increases, the resistance to
abrasion increases.
Excessive hardness, however, should be avoided so as not to
cause problems in forming, or in premature failure owing to
lack of impact strength.
Sliding
Abrasion
In this type of abrasion, the surface is worn away by
friction - due to the sliding of the load on the steel.
Generally, HARDNESS is the most significant factor in
controlling the wear of the steel caused by sliding
abrasion, since the resistance to the abrading particles
penetrating the metal depends on the HARDNESS of the
metal. Higher HARDNESS provides greater wear
resistance in sliding-abrasion applications, such as frames,
chutes, hoppers and earthmoving equipment.
Impact
Abrasion
In this type of abrasion, the surface is worn away by
gouging, spalling or cutting caused by the impact from
heavy, hard materials, such as rock.
The energy of a sudden blow may crack or spall a brittle
material. TOUGHNESS must be combined with HARDNESS
for such impact-abrasion applications as mine cars, primary
chutes, wear plates, clamshell buckets, truck body liners,
and so on.
Strength
Although abrasion-resisting steels are not sold to specific
strength levels, the following table, offered for general
information purposes, shows the approximate tensile strength
for the various harness levels:
| Brinell
Hardness Number |
Approximate
Tensile Strength MPa |
321
340
360 |
1100
1170
1240 |
Fabric ability
When selecting a grade for a particular application,
consideration must be given to the fabricating
characteristics of abrasion-resisting steels, as well as
their hardness and toughness.
As outlined in the following discussion, roller quenched and
tempered abrasion-resisting plates are produced from fully
killed, fine-grained steels. The balanced chemistry
and heat treatment produce a more uniform product with
higher hardness and improved service life when compared to
as-rolled abrasion-resisting steels.
Most
Efficient Quench
The Roller-quenching facility produces the most efficient
quench possible. It exposes the entire plate surface
to a rapid high-volume, high-pressure quench, producing a
uniformity and effectively hardened plate - ideal for
abrasion-resisting applications.
Flatness
Because of the hardness of these abrasion-resisting grades,
it is not possible to produce plates with flatness
equivalent to that obtained on the 690 MPa minimum yield
strength (quenched and tempered) structural grades.
Some distortion may also be expected when plates are cut,
because the low tempering temperatures necessary do not
remove residual internal stresses.
Heat
Treatment
The quenched and tempered abrasion-resisting plate steels are:
-
Austenitized
at approximately 900oC.
-
Roller
quenched with water to produce the most effective and
uniform quench possible.
-
Tempered in
the neighborhood of 420oC to obtain the desired
hardness.
Cutting
SHEARING - if high-capacity sheers are available and provided
care is taken, abrasion-resisting quenched and tempered plates
can be sheared in thicknesses up to 25 mm. Shear
capacity will be only about 40 per cent of the rating of
conventional structural as-rolled plates. We do regard
gas cutting as an alternative.
GAS-CUTTING - Procedures used on conventional
structural grade steels are satisfactory. The burned
edges of plates are hardened by the operation to a hardness of
approximately 400 + Brinell, but this is not detrimental
unless cold forming is to be done.
Machining
Because of the high hardness, machining operations are more
difficult than on conventional structural grade steels.
However, normal machining operations can be performed by using
high speed tool steels if the cutting speeds are reduced to
about 50 per cent of those used on conventional structural
grade steels.
Punching
Punching and blanking operations are not usually practicable
on the abrasion-resisting steels.
Bending
and Forming
-
A limited
amount of cold forming or bending can be done on these
abrasion-resisting steels if proper precautions are taken,
as follows:
Except for minor forming, it is essential to condition the
burned edges of plates before forming to remove
notches and irregularities. Conditioning is most
conveniently done by grinding. Also, it may be
helpful to soften the edges by tempering with a torch,
using temperature-indicating crayons, to avoid exceeding
425oC. For severe bending it may be
necessary to completely remove the heat-affected area
resulting from burning.
-
A generous
forming radius should be used, preferably twelve times the
plate thickness or greater.
-
Major forming
should be done transverse to the rolling direction, not
parallel to it.
-
Abrasion-resisting
grades have greater spring-back than conventional
structural grade steels, and proper allowance must be made
for this characteristic.
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