Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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I ABRASIVE RESISTANT WHITE CAST IRON
INTRODUCTION
This invention relates to cast iron and more
particularly to thy improvement in the toughness and abrasive
resistance of white cast iron along with a significant
increase in tensile strength. More specifically, the present
invention relates to a new white cast iron composition and a
process for producing such cast iron having improved
toughness, ductility and tensile strength while retaining
: desirable abrasive resistance through modification of the
carbide morphology.
Unless otherwise indicated, all references in the
disclosure and claims herein to proportions or relative
amounts in the alloy compositions by way of percent are to
percent (~) by weight.
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1 BACKGROUND OF THE INVENTION
Alloy white cast iron it well known to be a highly wear-
resistant material formed with a carbon content generally recognized to be in excess of 1 1/2% and the capability of
being alloyed with other metals, usually chromium, to combine
with the carbon to form a compound of iron-chromium carbide
such as McCoy. In many instances, the inherent abrasive
resistance of unalloyed cast iron is adequate to meet its
intended use and therefore does not pose a problem to the
user. However, when the cast iron forming an industrial
apparatus is subjected to particular kinds of wear the
inherent mechanical properties of cast iron leave much to be
desired.
It is well recognized that there are several
classifications of wear to which the cast iron material may
be subjected In the first, a gouging or grooving wear,
coarse abrasive particles penetrate the working surface of
the cast iron to induce a high rate of metal removal. In the
typical industrial experience of this type of wear, such as
in earth moving equipment, hammer mill operations and jaw
crushers, there is associated with the metal removal severe
shock loading that has been found to have a detrimental
effect upon the cast iron.
In another type of wear often referred to as high stress
abrasion, abrasive particles, such as may be encountered in a
mining operation, are crushed under grinding influence of
moving metal surfaces. Stress levels involved in this
operative wear process as occur typically in castings used
for grinding, crushing rolls or mill liners often exceed the
stress capabilities of the conventional cast iron leading to
equipment failure.
In the third category of wear, a low stress abrasion or
erosion, the abrasive operation to which the cast iron
surfaces of the equipment are subjected are not severe
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1 stressful conditions, but yet, require high abrasive
resistance.
The gouging or grooving wear that is associated with a
severe shock load requires a toughness that cast iron
typically has not characteristically possessed in the past.
A manganese steel with high plasticity and toughness has been
able to meet the severe shock resistant requirements for
material subjected to this type of wear. However, the
hardness and abrasive resistance is usually found to be
inadequate to prevent an extremely high rate of wear in the
high stress abrasion operation typical in a wide range of
pulverizing processes such as a rotary ball mill. In this
high stress operation both chrome molybdenum steel and
lo alloyed white iron may be used in various types of apparatus
depending upon the requirement of toughness and the
combination of abrasion resistance required. In the last
category of wear involving low stress operations chromium
alloyed irons with or without molybdenum or nickel additions
may be used with a desirable high martensitic matrix having a
carbide embedment.
A consideration of the categories of wear and the
knowledge of the industry concerning the types of metals
available to meet the requirements in these wear categories
has led to a dilemma to those skilled in the art. To operate
apparatus subjected to at least the first two categories of
wear there is a clear requirement or combination of optimum
wear resistance and sufficient toughness to resist the severe
impact and stress conditions characteristic to these types of
wear. Hardness and toughness are generally recognized to
typically stand at the opposite ends of the spectrum so that
those compositions possessing more of one characteristic lose
some of the other and yet both hardness and toughness are
required.
The industry that supplies abrasion resistant castings
has long sought to improve the useful life of the apparatus
utilizing the casting in the wear applications described.
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1 Various iron carbon compositions alloyed and non-alloyed do
not have a high toughness in the martensitic state with the
carbon starting as low as .04~. Hypereutectoid steels and
white irons exhibit insufficient toughness because of the
morphology of the cementite (Fake). Alloying the
iron-carbon composition produces carbides (McCoy) with
increased hardness thus meeting some requirements for greater
abrasion resistance. However, while abrasion resistance
increases the toughness or resistance to fracture decreases
as the carbide volume increases, unless at any given carbide
volume the carbide size is decreased. Metallurgists have
long recognized the complexity of white cast iron because the
two main micro-constituents, the carbide and the matrix act
essentially independent of each other. Nevertheless, the
ultimate characteristics of the material result from the
interdependence between the two components if the white iron
is subjected to abrasive and shock conditions. When impact
takes place upon such material, the carbides shatter and if
the carbides are continuous and of relatively large size the
cracks will propagate throughout the structure often leading
to failure or at least accelerated wear of the material.
There is thus to date no recognized iron-carbon alloy
whose carbon content exceeds 1.7~ by weight that meets the
requirements of high abrasive resistance and good shock
stress absorption
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SUMMARY OX THE PRESENT INVENTION
Accordingly the present invention seeks to provide a white
cast iron having characteristics of high hardness or wear
resistance and improved toughness.
Further the present invention seeks to provide a white cast
iron possessing not only desirable wear resistance and toughness
characteristics but also having improved tensile strength.
The invention in one broad aspect pertains to an alloy cast
iron composition comprising a base of the element iron, and one
or more of the following alloying elements: 0.001% to 30% ,1
vanadium, titanium, niobium, tantalum, molybdenum, nickel, copper ,
our chromium, or mixtures thereof, and 2.0~ to 4.5~ carbon, the
composition of the foregoing having a solidification point within
15F of the eutectic temperature of the cast iron formed with the
selected alloying elements, and further containing 0.001~ to 4.0
boron whereby the cast iron possesses desirable wear resistance,
toughness and tensile strength properties.
Another aspect of the invention pertains to the process of
forming globular shaped carbides in cast iron comprising adding
0~001~ to 4.0% boron to an alloy cast iron comprising 0.001% to
30~ vanadium, titanium, niobium, molybdenum, nickel, copper,
tantalum or chromium ox mixtures thereof and 2.0% to 4.5% carbon
to form a molten cast iron composition, and cooling the molten
alloy cast iron composition below equilibrium solidification
temperature to a super cooled temperature, solidifying the molten
cast iron composition to produce globular shaped carbides having
an average size less than the average conventional cast iron
carbide particle.
still further aspect of the invention pertains to the
process of super cooling molten cast iron to improve the
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toughness and abrasion resistance and tensile strength of cast
iron comprising increasing the entropy of a molten cast iron
mixture of carbon, iron and vanadium, titanium, molybdenum,
nickel, copper, tantalum or chromium or mixtures thereof, to
form a molten cast iron composition, super cooling the molten
cast iron composition to a temperature below the equilibrium
solidification temperature of the molten cast iron composition,
and solidifying the molten cast iron composition while producing
globular shaped carbides having an average size less than the
average size of the conventional cast iron carbide.
The invention also pertains to a super cooled alloy cast
iron composition consisting of a balance of iron, and one or
more of the following alloying elements: .001% to 30% vanadium,
titanium, niobium, tantalum, molybdenum, nickel, copper or
mixtures thereof, 18% to 28% chromium, 1.5% to 3.0~ carbon,
and further containing .001~ to 1.0~ boron, the composition of
the foregoing having a solidification point within 15F of the
eutectic temperature of the cast iron formed with the selected
alloying elements. The composition when molten is capable of
being super cooled below the equilibrium solidification temperature,
a substantial portion of the carbon being present in the form
of globular carbides, whereby the cast iron possesses desirable
wear resistance, toughness and tensile strength properties
and when solidified having a matrix essentially free of graphite.
Another broad aspect also comprehends the process ox
forming globular shaped carbides in cast iron comprising
adding 0.001% to 4.0% boron to an alloy cast iron comprising
0.001~ to 30% vanadium, titanium, niobium, molybdenum, nickel,
copper, tantalum or mixtures thereof, 18% to 28% chromium,
and 2.0% to 4.5% carbon to form a molten cast iron composition,
and cooling the molten alloy cast iron composition below
equilibrium solidification temperature to a super cooled
temperature, solidifying the molten cast iron composition
to produce globular shaped carbides having an average size less
than the average conventional cast iron carbide particle.
The present invention also seeks to provide a cast iron
composition having high abrasive resistance and toughness wherein
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the carbides are in the form of globules that approach spherical
form and particularly a cast iron that is tough and wear
resistant in which the carbides are of smaller than conventional
average size and substantially evenly distributed throughout the
matrix.
More particularly the present invention is a unique
discovery of an alloy cast iron composition comprising as a base
the element iron, with or without 0.001% to 30% by weight singly
or cumulatively vanadium, titanium, niobium, molybdenum, nickel,
copper, tantalum or chromium or mixtures thereof, 2.0 to 4.5% by
weight carbon forming an alloy composition and introducing 0.001%
to 4.0% by weight boron to improve wear-resistance, toughness and
tensile strength properties. The alloy has a solidification
point between 2200F and 2400F and generally is in a range
between 2260F to 2300F. This solidification point is within
15F of the eutectic temperature of the cast iron with the
selected alloying elements. The carbides present in the form of
globules that are approaching spherical form and are of a size
that average less than 4 microns which is considerably less than
I the average particle size of carbides in conventional cast iron.
In the process of the present invention an alloy white cast
iron containing 0.001% to 30~ vanadium, titanium, niobium,
molybdenum, nickel, copper, tantalum or chromium or mixtures
thereof and 2.0% to 4.5% carbon forming a molten cast iron
composition is provided with an entropy increasing additive such
as 0.001~ to 4.0% boron then cooling the molten cast iron
composition at least 5F below the equilibrium solidification
temperature of between 2200F and 2400F to a super cooled
temperature and thereafter solidifying the molten cast iron
composition to produce globular shaped carbides having an average
size less than the average conventional cast iron or carbide
particle and, on the average, less than 4 microns.
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1 DESCRIPTION OF THE PREFERRED EMBODIMENT
It has long been recognized that white cast iron
inherently possesses the wear-resistant characteristics
desirable to meet the various wear conditions to which the
apparatus composed of cast iron is subjected. It now has
been discovered that the carbide morphology of the alloyed
cast iron can be altered to retain the characteristic wear-
resistance and not only increases the tensile strength but
more importantly provides measurable plastic deformation and
significant toughness improvement. It has been well known
that in the prior cast irons either the free (in excess of
that found in the matrix of austenite, puerility or
marten site) carbon was in the form of graphite that takes a
three-dimensional form somewhat similar to a corn flake or in
the form of a carbide in a plate or rod-like shape. In
either form the particles are microscopic in size but usually
would be larger than lo microns or an average particle size
assuming normal heat abstraction from a sand mold and a metal
section size in excess ox loom.
It is known that these graphite flakes are the origin of
the fractures along the plane of the flakes. Typically a
good grade of cast iron would have a tensile strength of
about 50,000 psi with 0% elongation producing a very brittle
or non-tough material with no capability of deformation
whatsoever. When properly alloyed, the free carbon
partitions to an inter metallic metal carbide usually chromium
carbide shaped generally in the form of the plates or rods
and may be continuous or discontinuous within the matrix but
again are of an average size greater than 10 microns The
carbide particles may also take the form of needles but
whatever appearance they may have microscopically, their long
dimension on the average is still at least lo microns which
increases the propensity for crack initiation under stress
which often leads to an ultimate apparatus failure.
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1 In the present invention it has been found that this
normal rod or plate geometry of the carbides can be changed
into a globular form that approximates a spherical shape
producing not only the desired toughness but a significant
tensile strength increase. This change in the morphology of
the carbides of cast iron has altered the non-ductile,
brittle, non deformable cast iron of the past to one that has
the capability of plastic deformation, higher tensile
strength with retention of the superior wear-resistant
characteristics.
It has been found for instance, that the cast iron of
the present invention will bend prior to breaking and the
stress level to which it is subjected is significantly higher
without fracture as compared to prior known cast irons. The
cast iron of the present invention is preferably alloyed with
chromium but depending upon various additions of vanadium,
titanium, niobium, tantalum, nickel, molybdenum or copper
from .001% to 30% to substitute for the chromium, the
properties of the resultant cast iron vary.
In general, the cast iron of the present invention has
been found to have a tensile strength as high as 120,000 psi
compared to the traditional 50,000 to 60,000 psi tensile
strength of prior known cast irons. Typical cast irons have
had a I elongation characteristic while the present cast
iron has a I elongation capability. Those skilled in the
art would immediately recognize the significant advantages of
an increase in elongation or plastic deformation as providing
a toughness capability so important in those apparatuses
subjected to great wear and shock loading such as, for
instance, crushers and pulverizers for the mining industry
and also in pumps for the transpiration of fluids containing
abrasive solids. To achieve only the change in the shape of
the carbides in the cast iron would be desirable but not
nearly as effective as if the shape of the carbides would
change to globules and the particle size was reduced
substantially below the typical average 10 to 14 micron size
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1 of the particles of prior cast irons down to a size less than
4 microns. sty a reduction of this magnitude in the size of
the particle of the carbide, it is possible to minimize the
mean-free path between the smaller discrete globular shaped
particles in order to contribute to higher strength levels,
better wear-resistance and greater deformation, capability.
Thus, in accordance with the present invention not only are
the carbides changed in shape to spherical or near
spherically shaped globules, but the globular particles have
been reduced in average size to below 4 microns.
Cast iron is well recognized to be an iron-carbon coup-
position that may be alloyed. It is generally recognized in
the art that the dividing line between cast iron and steel is
the volubility of carbon in iron in the solid state. At
higher levels of carbon, the carbon would be in the form of
free graphite unless it was alloyed. Typically, the alloying
element used to form carbides in cast iron and to improve
various properties is chromium. However, molybdenum, vane-
drum, titanium, copper, nickel, niobium and tantalum in any combination may optionally be added to the chromium or sub-
statute for the chromium. when used in conjunction with
chromium these metal elements are usually present in an
amount up to about I though preferably vanadium and niobium
may range from .001~ to 5%, molybdenum and copper from .001%
to I nickel from oily to 7% and titanium and tantalum
range from .001'~ to I with the total in combination with
chromium or with chromium alone should be in the range of
.001% to 30~. Preferably the chromium is in the range of 7%
to 29% and more preferably in the ranges of 25~ to 28~ or 14
to 226 or 7% to 12~ which ranges of chromium represent the
three major groups of commercial alloy white irons The
carbon content is preferably not less than 2.0% and no more
than about 4.5~ and preferably in the range of 2.06 to 3% for
cast iron with a content of 25~ to 28% chromium and 14~ to
22~ chromium or 2% to 3.5~ for 7% to 12~ chromium.
The typical cast iron compositions outlined above can
achieve a changed carbide morphology by the addition of boron
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1 generally in the range of .001% to I and preferably .01~ to
lo and most preferably between .01~ to 0.4~. This addition
of boron is found to produce globular carbide particles but
is more pronounced when the alloyed iron-carbon composition
selected is related to the eutectic temperature.
The solidification point of pure iron is about 2800F
and as carbon is added, the solidification point decreases.
As alloyed with or without the addition of boron, the
solidification temperature varies between 2200F and nephew
varying primarily in accordance with the amount of chromium
present but also varying due to the selection of the
particular alloying elements. More desirably it is found
that the solidification temperature of the alloyed
iron-carbon system should be in the range of 2260F to 2300F
or approximately 2280F plus or minus 10 to 20F. Any
specific cast iron composition with the selected alloying
elements present in amounts in accordance with this invention
will solidify within 15F of the eutectic temperature for
that system of cast irons formed with those particular
alloying elements.
With this alloyed cast iron composition and the addition
of boron, it has been found possible to modify the carbide
morphology to produce globular carbide particles that are
approximating spherical shape.
To achieve this important particle size modification and
to attain a substantially uniform distribution of the
globular carbide particles, it has been found that if the
cast iron composition were cooled below the equilibrium
solidification temperature by at least 5F, and preferably it
is believed 8 to 10F or more, prior to solidification that
the particle size of the carbide particles would be
dramatically reduced from their usual average size of 10
microns or more to an average size of less than 4 microns.
This super cooling was found to be difficult to achieve and
only upon a thermodynamic approach to the problem was it
discovered that by increasing the entropy of the cast iron
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melt, the disol-deL- of the system its increased to allow the
melt to be under cooled. A higher entropy value decreases
the Gibbs free energy value of a liquid-solid system, and the
phase with the lowest free energy will be the most stable.
The relationship is Go = _ S where G is Gibbs free energy, T
is the absolute temperature and S the entropy. Additionally,
the thermodynamic relationship OH = TO + UP reduces to
OH = TO because UP = O for solids indicates that US = OH
where S is the entropy and the heat of fusion and T the
absolute solidification point. An increase in entropy
produces a decrease in the solidification point with a
constant heat of fusion for the system.
It was discovered that boron will, when added to the
cast iron composition, increase the entropy that produced the
higher randomness within the system and allow the requisite
under cooling. rho exact changes occurring are not
completely understood and the explanation as set forth above
should be considered to be theoretical.
As the alloy cast iron composition of this invention is
cooled below the equilibrium solidification temperature into
the super cooling range of at least 5F below the equilibrium
solidification temperature, when the solidification does
occur it is more instantaneous than when super cooling does
not take place. Thus, the super cooling avoids the usual
lengthy period of crystal or particle growth that
conventionally occurs. Rather, the solidification is more
rapid before the growth of the particles can be achieved.
Thus, the minute carbide particles instead of agglomerating
into rods or plates as occurs in the conventional cast iron
do not have the opportunity to agglomerate with the rapid
solidification in the alloy cast iron composition of the
present invention nor is there a migration of these particles
to agglomerate to form a plate or rod so as to produce
non-uniformity in the distribution of the carbides. Rather,
the uniformity in the carbide distribution is inherent in the
; melt phase even during the super cooling phase of the alloy. .
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1 cast iron composition so that the uniformity of the carbide
distribution is retained during solidification. The result
of solidification of the super cooled melt below the
equilibrium solidification temperature is a substantial
reduction in the size of the particle and a more uniform
distribution of the carbides throughout the matrix of the
cast iron which is the basis for the strength, toughness and
abrasion resistance of the cast iron composition of the
present invention.
Specific_Exam~le
A typical cast iron composition containing 27.2
chromium, 2.04% carbon is an alloy composition with
solidification in the range of 2280F which is above the
eutectic temperature of about 2263F. With the addition ox
0.17~ boron the alloy can be super cooled to a temperature of
5 degrees below that equilibrium solidification temperature
and to about slightly below 2275F. Between this
temperature point and below the equilibrium solidification
temperature the melt is super cooled and remains liquid.
Further cooling produces carbides having a globular shape
that is nearly spherical and of an average particle size of
less than 4 microns. The tensile strength of the resulting
cast iron is in the range of 120,000 psi with approximately
I elongation permitted. Such a white cast iron is quite
wear-resistant and additionally has improved tensile strength
and toughness characteristics that make it particularly
useful in high wear and stress operations.
Similar results are obtained with a composition of 3.32%
carbon, 9.12% chromium, 5.18~ nickel and 0.17~ boron having
an equilibrium solidification temperature at about the
eutectic temperature of 2287F. Supercooling then takes
place down to 2280F before solidification occurs.
It is believed that the objects of the present invention
have been met by the invention as described above and it is
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1 believed that the invention should only be restricted in
accordance with the following claims in which
