Note: Descriptions are shown in the official language in which they were submitted.
Case 5504
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ABRASION/EROSION RESISTANT WEAR ALLOY
FIELD AND BAC~GROUND OF THE INVENTION
The present invention relates, in general, to erosion
resistant wear alloys used in industry and, in particular, to an
improved abrasion/erosion resistant wear alloy suitable for use
in high or low stress applications potentially exposed to impact
loading which can be employed in coal or mineral pulverizers or
other types of crushing apparatus.
The removal of material from metal surfaces by coal and
mineral fragments is a complex process. Several types of
abrasive and/or erosive wear can occur in grinding equipment,
including gouging abrasion, high stress grinding abrasion, low
stress scratching abrasion, and erosion. Erosion is a separate
wear mode, distinct from abrasion. Erosion is a ballistic
process involving impingement of particles travelling with some
velocity while abrasion involves sliding of particles under load
over a surface.
Each of the above-described wear modes is influenced in a
different fashion by the wear component's material properties
and its microstructure. An extensive literature search revealed
that in no case has a complete correlation been made between
laboratory testing using any type of test method and a wear
alloy's performance in the field.
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Several factors are relevant to the selection of a wear
alloy for a given application. ~ The following discussion
involves selection of those properties for optimum wear alloy
performance when used in the severe service environment of a
coal pulverizer.
Wear alloy performance involves considerations of factors
that affect both wear resistance and breakage resistance.
Factors affecting wear resistance include: wear conditions,
hardness, carbide volume fraction, matrix microstructure, and
carbide morphology. Factors affecting breakage resistance
include: load conditions, fracture toughness, austenite content
of the matrix, internal stresses, carbide volume fraction (CVF),
and carbide morphology.
As discussed in Steam: its qeneration and use, 40th
Edition, Copyright ~1992 by The Babcock & Wilcox Company, at
page 6-13, cast irons and steels (containing more than 2% or
less than 2~ C, respectively) have long had wide acceptance as
wear resistant and structural components in boilers. The three
types of cast iron used in boilers are white, gray and ductile
iron. White cast iron is so known because of the silvery luster
of its fracture surface. In this alloy, the carbon is present
in combined form as the iron carbide cementite (Fe3C). This
carbide is chiefly responsible for the hardness, brittleness and
poor machinability of white cast iron. Chilled iron differs
from white cast iron only its method of manufacture and it
behaves similarly. This type of iron is cast against metal
blocks, or chills, that cause rapid cooling at the adjacent
areas, promoting the formation o~ cementite. Consequently, a
white or mottled structure, which is characterized by high
resistance to wear and abrasion, is obtained.
White cast irons can be obtained also through the use of
suitable alloy additions. The addition of chromium for example
is especially beneficial to cast iron wear properties. High
chromium irons have an austenite (or some transformation product
3S of austenite) matrix and an essentially discontinuous complex
Ca~e 5504
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network of carbides. The M7C3 carbide formed in these materials
is considerably harder than the M3C carbide found in most steels.
The replacement of the iron carbide by chromium carbide has a
dual effect on material properties. The M7C3 carbide has a
S higher hardness than that of quartz, which is one of the most
prevalent abrasives in many grinding operations. The higher
hardness carbide, therefore, is more resistant to micro-cutting
and removal by quartz particles. The morphology of the M,C3
chromium carbide network is typically discontinuous, thereby
yielding a less direct fracture path under impact loading.
A significant advance in wear alloy technology occurred in
the late 1950s and early 1960s when Ni-Hard IV was developed by
INCO. This material is essentially equivalent to B&W's Elverite
I cast wear alloy developed specifically for use in pulverizers
and other wear resistance parts. The alloy contains
approximately 4-7~ nickel and 8-10~ chromium, which produces a
discontinuous form of a complex carbide, thereby enhancing its
fracture toughness. Subsequent to the development of the Ni-
Hard IV, a range of alloys based on the ternary iron-chromium-
carbon system with additions of molybdenum and/or copper toenhance hardenability were investigated both in the U.S. and in
Europe. During this time period, detailed laboratory
investigations determined the boundaries of the ternary Fe-Cr-C
system, as shown in Figs. 1 and 2 which provided a concrete
basis for the study and development of improved wear alloy
materials.
The chemical compositions of high chromium cast irons
typically produce a hypoeutectic alloy which solidifies with a
primary austenite (~) matrix, as shown in Fig. 2. Depending
upon the alloy chemistry and cooling rates, the matrix can be
retained to room temperature or transformation can occur. The
alloy can be further modified through the use of an austenite
"destablization" heat treatment cycle. This solution treatment
precipitates secondary Cr carbides which reduces chromium and
carbon levels in the austenite. Thus, upon subsequent cooling
Case 5504
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after destablization, a martensitic transformation will occur.
A final tempering step is often utilized to further transform
retained austenite within the hardened structure and to stress
relieve and temper the already existing martensitic phase.
S VAM 20 , a more recent development, is a 20~ Cr white iron
with a molybdenum addition which yields an essentially
austenitic material in the as-cast condition. Subsequent
thermal processing produces a material with a carbide-in-
martensite matrix, very high hardness and good toughness
(compared to other white irons). The hardness and wear
resistance of VAM 20 are superior to those of the Elverites and
similar alloys in part due to the molybdenum addition which
forms carbides which are harder than chromium carbide. It is
always used in the heat treated condition, which accounts for
its good toughness and uniformity. VAM 20 is used in grinding
elements of coal pulverizers.
The high Cr-Mo cast wear irons as defined in A.S.T.M. A532
Type IIE have the following chemical composition ranges:
Weiqht Percent
Constituent Ranqe
Carbon 2.6 - 3.2
Manganese 0. 5 - 1.5
Silicon 1.0 max
Nickel 1.5 max
Chromium 18.0 - 23.0
Molybdenum 1.0 - 2.0
Copper 1.2 max
Phosphorous 0.10 max
Sulfur 0.06 max
The balance is essentially iron with the usual impurities.
Because the grinding of various materials such as coal
requires significant capital expense in machinery and grinding
Case 5504 2149Q 10
wear elements, improvements in the durability and wear
resistance of these components is a~constant goal. It has thus
become desirable to develop a new erosion resistant wear alloy
suitable for use in such applications.
5 SUMM~RY OF l~IE INVENTION
A review of the open literature coupled with an assessment
of current wear alloys and their in-service performance has led
the present inventors to conclude that wear alloy performance
can be improved. In particular, improved performance can be
obtained by:
(1) modifying the metal matrix by
adding elements that stabilize
certain microstructural phases
(i.e., nickel, molybdenum and
carbon are known to stabilize the
austenitic phase in ferrous
materials);
(2) using heat treatment to
homogenize the metal and to
stabilize desired microstructural
phases; and
(3) increasing carbide hardness and
volume using strong carbide
formers such as vanadium,
tungsten, niobium, tantalum or
titanium.
The concept of improving wear performance by increasing
carbide hardness has met with mixed results in the past.
~urrently there is a great deal of work was underway in Japan
exploring this concept, and initially a great deal of work was
underway in the United States and in Europe on this idea but,
due to the varied and sometimes poor results obtained, work was
abandoned by most U.S. researchers. It is also known that high
Ca~e 5504 2l49ol
volume fractions of metal carbides can cause embrittlement of
martensitic materials under some co~nditions.
It is thus an object of the present invention to balance
good wear characteristics, which can be gained from high carbide
volumes and a hard matrix, against the ductility needed_in the
metal matrix to prevent cracking. Such ductility can be
obtained by changing the matrix, i.e., from a martensitic to a
stable austenitic matrix which can provide the requisite
ductility and has the additional advantage of being work
hardenable during service. In-service work hardening of the
austenite matrix will be local to the surface and can provide
additional wear resistance without sacrificing the component
ductility that provides breakage resistance. The
abrasion/erosion resistant wear alloy of the present invention
is particularly suitable for use in high or low stress
applications potentially exposed to impact loading. In contrast
to stable austenite, it should be noted that retained,
metastable austenite can be detrimental to wear alloy
performance. Uncontrolled through-section transformation of the
metastable austenite to martensite during service is undesirable
because, a significant volume change occurs that can cause
cracking.
Accordingly, one aspect of the present invention is drawn
to an abrasion/erosion resistant wear alloy suitable for use in
high or low stress applications potentially exposed to impact
loading and having both good wear characteristics gained from a
large volume of high carbide, together with a tough, stable
austenitic matrix which provides requisite ductility and which
has the additional advantage of being work hardenable during
service, the alloy having a constituent composition consisting
range essentially of:
æll~Q~
Case 5504
Constituent wt. Percent
Carbon about 2.6 - 2.9
Manganese about 0.75 - 1.25
Silicon about 0.50 - 1.0
Nickel about 0.5 max. _
Chromium about 25 - 28
Molybdenum about 1.0 - 6.5
Copper about 0.5 max.
Phosphorous about <0.08
Sulfur about <0.05
with the balance essentially iron with the usual
impurities.
Another aspect of the present invention is drawn to a
particular composition for an abrasion/erosion resistant wear
alloy of the type described above, the alloy having a particular
constituent composition consisting essentially of:
Constituent Wt. Percent
Carbon about 2.75
Manganese about l.oo
Silicon about 0.75
Nickel about 0.2
Chromium about 27
Molybdenum about 6.25
Copper residual amounts
Phosphorous residual amounts
Sulfur residual amounts
with the balance essentially iron with the usual
impurities.
The various features of novelty which characterize the
invention are pointed out with particularity in the claims
annexed to and forming a part of this disclosure. For a better
understanding of the present invention, its operating advantages
Case 5504 . 2i~9 0 10
and specific results attained by its uses, reference is made to
the accompanying drawings and the fo~llowing description in which
preferred embodiments of the invention are disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is a drawing showing the austenite liquidus
surface of the Fe-Cr-C system; and
Fig. 2 is a drawing showing a corner of the metastable
Fe-Cr-C liquidus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following discussion includes a description of the
range and preferred compositions of the abrasion/erosion
resistant wear alloy of the present invention. It is to be
emphasized that this particular abrasion/erosion resistant wear
alloy is suitable for use in high or low stress applications
having potential exposure to impact loading. It provides both
good wear characteristics gained from a large volume of high
hardness carbides, and a tough, stable austenitic matrix which
provide requisite ductility and which has the additional
advantage of being work hardenable during service. The alloy
has a composition consisting essentially of the following
elements as set forth in the table below, the first column being
the particular constituent, the second column being the
preferred range in weight percent of these constituents, while
the third column represents the preferred particular
composition weight percents of these constituents.
Case 5504 21~91~
g
Weiqht Percent
Preferred
Constituent Ranqe Composition
Carbon about 2.6 - 2.9 about 2.75
Manganese about 0.75 - 1.25 about 1 00
- Silicon about 0.50 - 1.0 about 0.75
Nickel about 0.5 max. about 0.2
Chromium about 25 - 28 about 27
Molybdenum about 1.0 - 6.5 about 6.25
Copper about 0.5 max. residualamounts
Phosphorous about <0.08 residualamounts
Sulfur about <0.05 residualamounts
The balance is essentially iron with the usual impurities.
In the above range and preferred compositions, the carbon is
added to increased carbide volume and alloy hardness. The
manganese is added to control cleanliness (i.e., scavenging
embrittling elements such as sulfur). The silicon promotes
castabilty while the chromium promotes carbide formation.
Finally, the molybdenum stabilizes the austenitic matrix and
forms additional high hardness carbides.
The abrasion/erosion resistant wear alloy can be used as-
cast. Alternatively, the alloy can be heat-treated or the
composition can be changed within the range and coupled with
other material processing can be made to develop an austenitic
and/or martensitic structure tailored to a variety of wear
conditions. For example, when erosive wear conditions
predominate, a martensitic matrix may be preferable. Similarly,
under more abrasive conditions, a more austenitic structure may
be preferable.
It is preferred that the present invention be heat treated
according to a two-step process, the first step during
manufacture being an austenization step carried out in a
temperature range of approximately 1750F to 1950F, the
preferred temperaturè being approximately 1850F, for a
Ca~e 5504 21~9~10
sufficient time to thoroughly homogenize the metallic structure
throughout the parts being made. 'This step is important in
obtaining a completely stable austenitic matrix.
The second step is a lower temperature tempering step
S carried out in temperature range of approximately 5~F to
1000F, the preferred temperature being approximately 950F,
again for a sufficient time to thoroughly homogenize the
tempering effects throughout the parts being made.
Another important aspect of the chemical composition of the
present invention is that the relative nominal levels of
Manganese (Mn) and Silicon (Si) are maintained such that the
weight percent of Mn is greater than the weight percent of Si,
i.e., that a ratio of Mn/Si is greater than 1Ø This is
important because the manganese addition offsets the loss of
material toughness caused by the presence of silicon in the
alloy. Whereas silicon is necessary to promote castability by
improving liquid metal fluidity; some amount of brittleness is
imparted to ferrous alloys by silicon additions.
The present invention provides large carbide volume
fractions and high carbide hardness promoting good wear
resistance. The austenitic matrix provides good resistance to
cracking and breakage resistance and the added benefit of being
able to be work hardened at the surface during service. This
promotes wear resistance without impairing ductility. The
abrasion/erosion resistance wear alloy of the present invention
can be used to produce components having substantially longer
service life, thereby reducing maintenance and replacement
costs. Particular applications where the present
abrasion/erosion resistant wear alloy can be used includes coal
pulverizers or mineral crushing equipment wear surfaces for both
abrasion and erosion-resistant applications
It will be further appreciated that although the present
abrasion/erosion resistant wear alloy is intended for use in
coal grinding and pulverizing equipment, other erosive material
handling systems and the like may benefit by employing this
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alloy. It can also be used for other applications where wear
resistance is needed. Examples would include shot blasting
equipment, mining equipment, and slurry transport. Accordingly,
while in accordance with provisions of the statutes there have
been descri~ed herein specific embodiments of the inv~ntion,
those skilled in the art will understand that nominal changes
may be made in the form of the invention covered by the appended
claims to enhance its use in various settings, and that all such
modifications and improvements have been deleted herein for the
sake of conciseness and readability but are properly within the
scope of the following claims.
