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| HARDFACING |
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| Hardfacing involves protecting parts exposed to different
types of wear in order to obtain a certain specifi c wear
resistance. |
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| Hardfacing is primarily used to restore worn out parts
back to usable condition in order to extend their
service life but it can be successfully employed in new
component manufacture as well. This technique will help
in manufacturing a component from cheaper material
with a wear resistant overlay providing the surface
properties. |
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| A higher hardness level does not necessarily imply
improved wear resistance or longer life. The effectiveness
of any hardfacing alloy depends of its suitability in the
operating conditions. |
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| So the appropriate hardfacing alloy has to be selected
considering the following aspects |
- The wear factors
- The base material of the component
- The surface fi nish required
- The process to be used
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| WEAR FACTORS |
| Wear factors are the action of different agents on
the metallic surface leading to degeneration and
disintegration of the metal. A number of wear factors
exist, which act individually or in combination. The weld
metals have to be selected properly to counter these
factors effectively. |
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| The major types of wear factors are as follows |
- Friction
Wear from metal parts that roll or slide
against each other. It accounts for 15% of
the total spectrum of industrial wear. This
type of wear is likely to be most severe
when parts rub together under load with
little or no lubrication. Generally, contact
between surface materials of the same
hardness will result in excessive wear. E.g.
shafts against bearing surfaces, chain links
against a roll, sprockets, steel mill rolls.
- Impact
The sudden action of a very large force
for a very short time period gives rise to
wear after some time. The mechanism
is attributed to fatigue failure that has an
incubation period prior to the appearance of
surface damage. E.g. crusher rolls, impact
hammers, railway points and crossings.
- Abrasion
About 50% of all industrial wear is abrasion
in different forms. The wear takes place
when metal is removed from a surface
by the cutting or gouging action of hard
non-metallic particles. Abrasion may be
classifi ed into three types
- Gouging abrasion : This class of
abrasion involves the removal of
sizable particles from a metallic surface
by the action of a coarse material.
The high pressure and impact cause
the particles to cut into the surface
and produce large gouge marks and
scratches. Eg. shovel diggers, chute
impact areas, pulveriser mills.
- Low stress abrasion : This form of wear
results from the sliding action of free
moving hard particles along a surface.
Material is removed by scratching or
micro-machining process. Eg. chutes,
mineral conveyors.
- High stress abrasion : It occurs where
abrasive particles are forced between
two metal parts and crushed under
heavy loads. The wear involves
surface damage accompanied by
plastic deformation. Eg. rock drills,
scraper blades, ball mills.
- Erosion
This wear is similar to abrasion but the
particles are carried by a fl uid stream like
water, steam, etc., generally at a higher
velocity compared to low-stress abrasion.
Eg. slurry transport systems, shot blasting
equipment.
- Cavitation
When a liquid is subjected to rapid changes
of pressure, vapour or gas bubbles form in
the lower pressure regions of the liquid.
Entering high pressure areas at any metal/
liquid interface, these bubbles collapse and
the immense force causes cyclic stress
and fatigue on the metal surface. E.g. ship
propellers, pump impellers.
- Heat
When metals are exposed to high
temperature for long periods, they lose
their durability mainly due to thermal fatigue
cracking. Moreover, the metals tend to
lose their strength and hardness at higher
temperatures. These factors add up to
wear. Eg. hot forging dies, extrusion dies,
stamping dies, sinter crushing equipment.
- Corrosion
It involves reaction between a metallic
surface and a corrosive environment
whence the former is dissolved away
leading to wear. In the presence of
any mechanical force, the corrosion
products may be removed leading to
virgin surface coming in contact with thecorrosive environment again causing
continued wear. E.g. valves, seating rings,
screw conveyors.
- Oxidation
In an oxidizing atmosphere, possibly
aided by high temperatures, the metal
surface builds .uP an oxide layer, which
is mostly brittle in nature. This may break
due to expansion and the entire oxidation
operation is repeated. E.g. blast furnace
parts, exhaust valves of internal combustion
engines, hot working shears.
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| BASE MATERIAL |
| The two main groups of base materials for hardfacing
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- Carbon or low-alloy steels : These steels require
preheating, post-weld heat treatment, slow cooling
for hardfacing according to the chemical composition
and the section thickness. The general guidelines
for preheating are given at the end of this section.
- Austenitic manganese steels : These steels should
be welded without any preheating or post-weld heat
treatment at all. The interpass temperature should
be kept as low as possible.
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| SURFACE FINISH |
| The surface fi nish requirements must be kept in mind
while selecting the hardfacing alloy as they cover the
entire range from easily machinable to non-machinable.
Additionally, many of these deposits will contain "relief
checks", which are formed across the bead as the bead
releases the stress generated while the hard weld metal
cools. These relief checks are not harmful to the deposit
but may propagate into the base metal if the component
is subjected to heavy impact or flexing. |
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| So, the issues of acceptability of relief checks and finishing
requirements should be addressed to when the
hardfacing deposit is decided. |
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| WELDING PROCESSES |
Shielded Metal Arc Welding
- Covers the widest range
of weld metals
- Is a versatile process for
on - site repairs and outof-
position work
- Is inexpensive
Flux - Cored Arc Welding
- The same range of alloys
available as in case of
coated electrodes
- On-site use is possible
- Self-shielded wires do
not require any additional
shielding gas
- High deposition rate
Submerged Arc Welding
- High deposition rate
no spatter loss
- Product range limited
- Positional welding not
possible
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