Cast iron, in the grade
normally used for items such as bearing housings, gearboxes or
machine beds, has a hardness of about 150 to 200 Brinell. With the
addition of alloying elements, usually chrome, nickel or copper, the
hardness can be increased to over 700 Brinell. This has opened up
the use of cast iron into areas where wear resistance was a crucial
requirement. Examples of these uses include mixer paddles, augurs
and dies in the clay industry, liner plates for ball mills and coal
chutes, wire guides for wire drawing and shotblast spares.
At the Foundry we have been producing wear resistant iron castings
for over twenty five years. As well as castings for wear resistant
applications, our customer base also covers the engineering,
decorative, architectural and artistic sectors and we can supply
castings from one off up to small to medium batch production using
Alkali Phenolic resin bonded sands. Our workforce of highly skilled moulders can work from complex loose pattern equipment, particularly
where low volumes are required, whilst our moulding line is suited
for batch production. Our long trading history, coupled with an
investment programme that has enabled us to keep up with modern
production methods and environmental legislation has meant that we
have been involved with many projects requiring abrasion resistant
castings and continue to be so.
All our wear resistant castings are produced to the current ISO
material specification, ISO 21988 2006, along with any further
certification which a customer may require. Our diverse customer
base has also given us experience in a wide range of other
applications for iron castings , including pumps and valves, forges,
foundries and rolling mills, automotive and aerospace, a wide range
of OEMs, quarries and mines, railways and rolling stock, local
authorities and artists and sculptors. We also have long term
trading relationships with local pattern makers, machine shops and
surface finishers which enable us to quote for the complete supply
of the finished Wear Resistant & Ni Hard Cast Iron Castings.
Wear resistance can be achieved in cast iron in two basic ways.
Firstly a normal grey iron can have small additions of copper,
molybdenum, vanadium or chrome added. These are usually up to 1% and
can increase the hardness from a base of about 150 Brinell up to 200
to 300 Brinell by promoting the formation of pearlite in the
casting, particularly if the carbon and silicon levels are
controlled to the lower end of the desired range. If these
hardness’s are sufficient, this method has the advantage that the
resultant alloy is still machineable.
The second method involves the use of chrome and nickel in much
higher quantities, the chrome content being as high as 28% to create
a structure that includes a high proportion of primary carbides.
Although a lot of chrome iron castings are supplied in the as cast
state, these alloys are heat treatable, which promotes the growth of
secondary carbides, and can attain hardness’s as high as 700 Brinell.
Tempering can be used to increase the toughness. This second method
is, unfortunately, only machineable using special techniques and
cutting processes which usually results in castings that are cast to
size. Where fixings are required it is possible to cast in threaded
bars or blocks of steel that can be drilled and tapped after the
casting is made to give fixing and location points.
There are a number of ways of describing this highly alloyed type of
iron – Chrome Iron, Wear Resistant Iron, Ni Hard or 28% Chrome being
the more common ones, along with a range of national and
international standards. This can cause some confusion and at Durham
Foundry we are happy to advice on a suitable grade that will give
the wear resistance required. One side effect of the levels of
chrome and nickel used in these grades is that they can have good
corrosion resistance which can lead to alternative applications.
Also, given the inherently higher melting point of chrome compared
to cast iron, they can be used for heat resistant castings,
particularly if the carbon content is controlled to the lower end of
the permissible range in some of the 28% chrome grades. These heat
resistant versions have applications in castings such as fire bars
and grate assemblies in stoves.