Note: Descriptions are shown in the official language in which they were submitted.
This invention rela~es to a me~hod of increasing
the abrasion reslstance of a chromium-bearing heat hardenable
stainless steel while retaining good corrosion resistande and
ability to be readily converted to wrought products by hot and
cold working with conventional steel mill equipment. Steel
treated by the method of the invention is martensitic in the
heat hardened condition. The invention further relates to a
steel of critical composition which has particular utility for
fabrication into bearings,ball joints, tire ~tuds, cutlery,
materials processing equipment such as mining and ore pro-
cessing machinery, and similar products of ultimate use
wherein the above combination of properties is needed.
Currently available alloys capable of withstanding
high stress, abrasive conditions are produced as castings
onl~ and are not amena~le to production in wrought form.
Among~such prior art iron base alloys are chromium-~olybdenum
white cast iron (analyzing about 3.2% carbon, about 0.6~
silicon, a~out 15.0% chromium, about 3.0% molybdenum, and
balance iron), and high chromium white cast iron (analyzing
about 2.7% carbon, about 0.65~ silicon, about 27.0~ chromium,
and balance ironl. Other such alloys are tool steels, e.g.
AISI Type ~-2 (1.50-1.60% carbon, 0.30-0.45% silicon, 11.50-
:.
12.50% chromium, 0.75 0.85~ molybdenum, 0.70-0.90~ vanadium,
and balance iron), and AISI ~ype D-4 (2.0-2.30% carbon,
0.20-0.45~ ~ilicon, 11.50-12.50% chromium, 0.70-0.90
molybdenum, 0.30-0.50~ vanadium and balance iron).
Prior art martensitic stainless steels classified
as wrought steels, such as AISI Types 440 A, B and C, actually
can be~hot and cold worked in standard mill equipment
only with great dificulty. Moreover, these steels, which
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~ 699~ ~
contain up to about 1.2~ carbon, are deficient in abrasion
resistance under very high stress, abrasive conditions.
United States Patent ~o. 3,692,515 issued September
19, 1972 to S.G. Fletcher et al, cliscloses a steel alleged
to have improved abrasion resistance, forgeability and work-
ability consisting essentially of about 1% to about 4.25
carbon, about 1.5% maximum silicon, about 1.5~ maximum
manganese, about 10~ to about 15~ chromium, less than 2%
molybdenum, about 0.5% to about ~% titanium, less than 3%
tungsten, less than 3% nickel, less than 5% cobalt, less
than 5~ vanadium, up to 0.25% sulfur, and balance iron with
residual impurities. A preferred composition contains 2O9
carbon, 0.4% silicon,0.4% manganese, 12.5% chromium, 1.1~
molybdenum, 3% titanium, and balance substantially iron with
residual impurities. It is stated that carbon is added in
excess of that necessary to ~ive a desired hardenability and
that such excess carbon is combined with titanium in a weight
ratio of 4:1 and vanadium in a weight ratio of 4.2 (V-l):l.
The cast alloy is reduced in cross sectional area by at
2Q least 5% by working, and heat treated by austenitizing
at 1600 to 1950F and tempering at 900 to 950~F.
The maximum austenitizing temperature of 1950F
disclosed in the ~letcher patent limits the amount o~ dis-
solved carbon to about 0.7~ to 0.8% maximum. If no vanadium
is present, the excess carbon content in the preferred practice
would be Ti/4, or 3/4 (th~ preferred titanium content being
3%), i.e. 0.75~. Thus the total carbon content should be
1.45~ to 1~55~. Since the excess carbon cannot all be dis-
solved and since ~he amount~of titanium is insufficient to
combiné with all the excess carbon, that portion of the ca~bon
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not in solution and not in the form of titanium carbides
would appear as ledeburitic carbides of iron, chromium,
and such optional elements as vanadium, molybdenum and tungsten.
The limited disclosure of the Fletcher patent
regarding heat treatment gives no indication of the micro-
structure of the tempered product and would apparently
result in the presence of retained austenite.
There is thus a real need for a method of
increasing the resistance to erosion by mechanical and/or
mechanical-chemical abrasion in a heat hardenable stainless
steel, which also exhibits ease of manuacture and abrication
into articles ~f ultimate use, and good corrosion resistance.
It is the principal object of the present invention
to provide a method of increasing the abrasion resistance of a
heat hardenable stainless steel which, by selection of heat
treatment, and observance of critical proportioning of carbon,
titanium. and silicon, will exhibit a degree of hardness and
abrasion resistance suited to a particular application, toge~her
with good hot and cold workability and good corrosion resistance.
It is a further object to provide a stainle~s
steel which in heat hardened and stre~s relieved condition
exhibits excellent abxasion resistance by reason of a sub-
stantially fully martensitic matrix and an absence of ledeburitic
carbides.
The above and other incidental objects of the
invention, which will be apparent from the discus~ion which
follows, are.obtained in a method o~ increasing the abrasion
resistance of a heat hardenable stainless ~teel while retaining
good c~rrosion re6tstance~ which comprises the steps of providing
a stainless st~el mel~ containing, in weight percent, from
: ~ . . ......... . . . . ............... . . .
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108~9~L
about 0.75% to about 10~ carbon, up to about 1,0% manganese,
about 11.5~ to about 18% chromium, up to about 1~ nickel,
up to about 1.5% molybdenum, up to about 0.1% phosphorus,
up to about 0.05% sulfur, and balance iron except ~or inci-
dental impurities, adding silicon in the range of greaterthan 1.7~ to about 4.5~, adding titanium in the range of
about 1% to about 10%, proportioning the silicon and
titanium additions relative to the carbon content in such
manner as to obtain a synergistic improvement in abxasion
resistance, casting the steel, heat treating the steel by
austenitizing within the temperature range of about 1600
to about 2250F to dissolve sufficient c~rbon to prevent
lowering the martensitic transformat~on point and to leave
a predetermined proportion of undissoIved carbon in the
form of uniformly dispersed partiales of titanium-rich
carbides of microscopic siz~, and cooling at a rate sufficient
to form a substantially fully martensitic matrix.
Within the above bxoad composition range, a
pxacticable upper limit of 5~ carbon should be observed for
wrought products formed by hot and cold working in standard
mill equipment. With carbon contents above S~, the steel can
be produced in th2 cast-to-shape condition, or in a ~m
suitable for powder metallurgy techniques, and can be hardened
and tempered.
An important aspect of the present invention is
the discovery that the increase in abxasion resistance re-
sulting ~rom addition of ti~anium alane is ;~estri~ted to a
rekatively narrow range and that an increase in th2 titanium
content above this range (which varies with the carbon content)
results in a decre~se in abrasion resistance, i.e., a reversal
1086991
of the desired effect~ However, in accoxdance with the present
invention, addition of silicon in amounts grea~er than 1.7%
results in progressive increases in abrasion resistance with
progressive increases in titanium content. The combined
silicon and titanium additions, within the limits defined
herein, must thus be reqarded as synergistic, i.e., better
abrasion resistance is achieved th~n with addi~ion of an
equal amount of either silicon or titanium alone.
In accordance with the invention, a stain-
less steel having good coxrosion resistance andexcellent abrasion resistance in a heat hardened state
consists essentially of, in weight percent, from about 1.8%
to about 10~ carbon, up to ~bout 1~0~ mangane~e, greater
than l.t~ to about 4~5~ silicon, ab~ut 11.5% to about 18%
chromium, up to about 1~ nickel, from about 1% to a~out
10% tltanium, up to about 1.5% molybde~um, up to 0.1%
phosphorus, up to 0.05~ sul~ur, and balance iron except
for incidental impurities.
Reference is made to the accompany~drawing
wherein;
Fig~ 1 is a graphic illustration of the effects
of varying titanium ~nd silicon addition~ on abrasion xesis-
tance in a chromium-iron alloy conta~ng about 1% carbon,
and
Fig. 2 i5 a graphic illustration of the effects
of varying titanium and silicon additions on abrasion resis-
tance in a chromium-iron alloy containing about 2~ carbon.
While not wishing to be bound by theary, it i5
believed that the function of silicon in impro~ing abrasion
or wear resistance is the de~elopment of greater oxidation
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10~91
resistance during wear testing. This results in decrease in ~ -
the loss of matrix metal by an oxidation process and provides
extended holding o~ the small titanium-enriched carbide
particles in place within the matrix. Thus, silicon additions
lower the rate of loss of matrix me~al which, in turn, lowers
the rate of loss of carbide particles by mechanical erosion.
The stabilizing influence of silicon in retaining
or improving abrasion resistance at higher titanium levels
is believed to be due to the formation of silicon-titanium
intermetallic compounds which apparently pr~ovide continued
abrasion resistance.
The reason for the decrease in abrasion resistanc
observed for high titanium addit~ons (withou~ compensating
increases in silicon content) is unknown but may be due to
depletion of the carbon contant of the matrix metal, or
lowering of the martensitic transformation temperature thus
r sulting in retained austenite in the heat treated product.
Heat treatment of a steel of ~he broad and
preferred composition ranges set forth above produces a mar-
tensitic stainless steel matrix, containing uniformly dispersedextremely hard abrasion resistant particles of titanium carbide.
These titanium carbide particles are micrsscopic in size and
roughly sperical in shape. The creation of a martensitic
matrix of high hardness and high compressive yield strength
has been found to be necessary to provide the desired high
abrasion resistance. In this condition the hard particles of
titanium carbide are not forced into the matrix under applied
heauy service loads.
Since titanium combines with carbon in a 1 1 atomic
ratio, and since titanium carbide is o~ extreme hardness, a
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~086~a~L
highly efective resistance against abrasion can be achieved
at a relatively low alloying level. Moreover, the degree of
abrasion or wear resi~tance can be preselected for any given
application by varying the carbon and titanium contents and
by the heat treatment to which the steel is subjected, there-
by controlling the hardness of the martensitic steel matrix
and the relative volume of small titanium carbides di~persed
in`the ~atrix.
While the presence of iron and chromium make it
difficult to develop "pure" -titanium carbides as the bearing-
particle or a~rasion-resistance phase, nevertheless this condition
can be approached to the extent that only very small pro-
portions of iron and chromium exist in the carbide phase. As
is well known, the weight ratio of titanium to carbon in
titanium carbide is about 4:1. In order to harden and
strengthen the matrix a selected carbon level associated with
iron and chromium needs to be taken into solution at the
hardening temperature. Thus the titanium content will be less
than 4 times the total carbon content. The solubility of
Z carbon in iron increases with an increase in hardening temp-
erature, and this provides the mechanism for controlling
the proportion of carbon combined with titanium and hence
the relative volume o~ the titanium carbide or bearing- ~
particle phase. At a selected temperature level of soluble ;
carbon, tha undissolved or insoluble carbon is combined
with the titanium in the form of titanium carbide or titanium-
enriched carbides. It should also be understood that any
nitrogen present as an impurity will also react with titanium
to produce some titanium cyanonitrides and/or titanium nitxides
under ordinary commercial melting practice.
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More specifically, heat treatment temperatures ~or
hardening the martensitic matrix range from about 1600 to
about 2250F. A greater proportion of carbon is dissolved at
the upper limit of this range, and some chromium is dissolved
with the carbon, thereby improving the corrosion resistance
and hardness of the matrix. On the other hand, titanium car-
bides cannot dissolve in the matrix until temperatures higher
than about 2050F are attained. While not wishing to be bound
by theory, it is believed that about 0.10~ carbon is dissolved
at 1600F, about 0.8% carbon is dissolved at 1900F, and about
1.5~ carbon is dissolved at 2200F. Any undissolved carbon
remains in the form of titanium carbide. After the desired
hardening temperature is reached the steel is cooled by any
conventional system including air, a moving gas stream, oil
and the like. Thereafter, stress-relieving heat treatment
at about 550 to 700F may be applied to hardened sections,
as needed for specific applications.
It is an essential feature of the invention that
the heat treatment or ~ustenitizing temperature be ~ selected
as to take enough carbon into solution that the martensite
transformation temperature (Ms) will not be lowered, thus
insuring the formation of a substantlally ~ully martensitic
matrix on cooling. The cooling rate is not a limitation
since the rate of martensite transformation is the governing
factor, and thi~ is~dependent on the alloy content of the
steel. In general, a cooling rate at least as rapid as air
cooling is preferred.
Assuming a steel having a total carbon content of
not greater than 5~, after melting and casting, it can be
hot rolled, cold rolled, heat treated to dissolve a predetermined
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percentage or proportion of carbon in the matrix and to leave
a selected proportion of the total carbon content in the form
of titanium carbides. Alternatively, at relatively low carbon
contents, all the carbon can be clissolved by hea~ treatment and
a selected proportion can be precipitated as titanium carbide
by a controlled cooling rate from the hardening temperature, or
by a selected secondary heat treatment.
Exemplary hea~ treatments which may be applied are
as follows:
A ~ heat to 1900F, hold 30 minuteS, air cool
B - heat to l9Q0F, hold 30 minutes~ air cool,
stress relieve at 600F
C - heat to 19009F, hold 30 minutest air cool
to 1300QF, hold 1 hr., and air cool or oll
~uench to room temperature
D - heat to l900~F, hold 30 minutes, air cool
to 1300F, hold 1 hx., air cool or oil quench
to room temperature, and stress-relieve at
600~F
As will be apparent from ~he above discussion, the
titanium, silicon and carbon contents, and critical propor-
tioning thereof, with conse~uent ~ormation of titanium oarbide
particles and formation o~ a hard matrix, are responsible for
the excellent abrasion re~istance of the steel of the invention.
However, in addition the titanium and carbon aontents are
further responsible ~or the ease with which the steel can be
hot and cold wor~ed. Parenthetically it should be noted at
~his point that: no prior art martensitic stainless steel con-
taining more than about 2.5% carbon can be produced in wrought
9~
form. (The previously mentioned Fletchex patent, while allegingworkability up to 4.25% carbon, actually discloses carbon
contents of only 2.35% and 2.7~ in the specific examples.)
Accordingly, a permissible increase in carbon up to and including
the S% level, while still retaining hot and cold workability,
represents a significant contribution to the art~ In the
practice of the present invention the tLtanium addition
increases the workability o the steel by raising the temper-
ature at which the alloy can be hot w~rked. By way of example,
the previously mentioned AISI D-2 and D-4 tool steels are hot
worked or forged from 1950F and ~rom l~OO~F~ respectively,
whereas the steel of the present invention is hot worked
from 2100F to 2250F. If the prior art D-2 and D-4 ~tool
steels were hot worked from 2150 to 2250F, they would over- -
heat and break up during working. Moreover, the titanium
addition significantly increases the cold workability of the
steel. For example, AISI Type 440C (containing about 1%
carbon) can accept only 15% cold reduction between anneals,
whereas a steel of the present lnvention containing about 2%
carbon and about an equal amount o~ titanium can be cold reduced
40~ between anneals.
It is believed th~t the beneficial ef~ects of
titanium on the hot and c~ld workabllity of the steel ari~e
; from the shape and size o~ the titanium carbides in the matrix.
Since these are small and spherical in shape the titanium
carbides permit easy ~low of the matrix around them during hot
and cold working~ Prior art cast alloys and so-called wrought
Types ~40 A, B or C contain ledeburitic carbide structures,
i.e., large platelets, which impede the ~low of metal around them,
~hereby causing craaking and breaking of the matrix during
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~L~86991
hot and cold working. Such ledeburitic caxbide st~uctures
are common to hypereutectoid steels generally.
Chromium is also an essential element, a minimum of
about 11.5% being necessary to impart good corrosion reqistance
and haxdenability to the matrix. In this respect chromium
lowers the eutectoid carbon level (from about 0.78% carbon in
pure iron) to about 0.35% carbon at about 13~ chromium. More
than 18% chro~ium is undesirable since it would ad~ssely affect
the hot and cold working propert~es of the steel and unnec-
essarily increase the cost of the alloy with no attendantbenefit.
Silicon functions in the same manner as chromium
in lowering the eutectoid carbon level and appax~ntly is
synergistic with chromium ~n ~his function.
Manganese, nickel, phosphorus and sulfux are non-
essential elements in the steel of the invention. A maximum
of about 1~ manganese can be tolerat~d and about 0~30% is
preferred. Manganese in excess of 1% would he harmul because
of its effect of stabilizing the high temperature phase
austenite~ Up to a~out 1~ nickel may be present as an impurity
without adverse effect/ and phosphorus and sulfur samilarly can
be tolerated in amounts up to about 0.10% and 0.005~,
respectively.
zirconium may be substituted in part for titanium.
Other carbide formers auch as vanadium and molybdenum may
also be added br substituted in part for ~itanium, in amounts
up to about 1.5% each, for ~pecial purposes such as increase
in corrosion resistance. Columbium should not be added since
it adversely affects the hot workability of the steel.
The critic~l proportioning of carbon, titanium and
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silicon, and the synergistic e~ect of silicon additions
together with titanium i~ improving abrasion resis~ance, are
shown by a series of test heats of steels within and outside
the ranges of the invention, the compositions and w~ar test
results of which are set forth in Table I. For all samples,
spea~s were hot for~ed to 1~2 inch diameter, annealed
at 1450F, ma~hined, heat treated by austenitizing at 1850F,
held for 30 minutes, and then oil quenched. The surfaces of
the specimens were smoothed with 120 grit paper, and abrasion
resistance tests were conducted on the Taber Met-Abrader
Model 500.
A consideration of the data of Table I showS
that addition of increasing amounts of either silicon or
titanium improves the abrasion resistance of a nominal 1~
carbon, chromium-bearing steel (comparison of Sample 1 with
Samples 2-71, but that if the titanium addition exceeds about
1.5% and silicon is low ~less than 1.7%), abrasion resistance
decreases Samples 8 and 9). However, if silicon is added
in excess of 1.7~ when titanium exceeds 1.5%, then abraslon
resis~ance is greatly imp~oved (compare Sample 8 and 9 with
Sample 10), This efect is illustrated graphicall~ in Fig. 1
~hich is plotted from the data of Table I. It will be noted
therefrom that titanium con~ers a greater increase in
abrasion resistance (in amounts up to about 1.5%) than silicon,
but that silicon and titanium together, with silicon greater
than 1.7% and titanium greater than 1.5%, exhibit a
synergistic ef~ect (Samples 10-12).
Turning next to a consideration of a nominal 2%
carbon, chromium-bearing steel, it i~ evident that addition
of increasing amounts of either silicon or titanium increases
99~
~b~asion resistance (again w~ith titaniu~ ha~ing a gxeater
effect), but that when titanium exceeds about 4% and silicon
iS 1QW (less than 1.7%) abrasion resistance decreases (compare
Samples 13-15 with 16?. Thi~ is shown graphically in Fig. 2
which is plotted fxom Table I If silicon is added in
excess of 1,7~o when titaniu~ exceeds about 4%, abrasion
resistance is improved (compaxe Samples 15 and 16 with
Samples ~O and 23~ The synergistic effect of silicon and
- titanium at higher carbon levels is thus also evident. Figs.
- 10 1 and 2 contain curves in which titanium plus silicon are
plotted against abrasion resistance, and progressive
increases in the sum total of both cause a synergistic
increased abrasion resistance throughout the range investigated.
In the preferred method of the present invention,
the~step of proportioning the silicon and titanium additions
relative to the carbon content thus comprises adding
silicon, in excess of 1,7~O~in direct proportion to titanium
when the titanium addition exceeds about 1.5% with a
nominal 1% caxbon content, and adding silicon, in excess
of 1.7%, in direct proportion to titanium when the titanium
addition exceeds about 4% with a nominal 2% carbon c~ntent.
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~69~1
TABLE I
Compositions - Weight Perc~nt and Abrasion Resistance
Wear Number
mg/100 cycle
Taber Met-Abrader
Sample C 9i Cr Ti Model 500
1 0.96 0.65 12.31 0.01 22,000
2 1.00 2.76 12.00 0.02 16,500
3 1.02 3.86 11.87 0.03 11,500
4 1.03 0.38 12.00 0.47 16,000
1.02 0.42 11.9~ 0.98 11,000
6 1~01 1.6~ 11.88 0.92 9,500
7 0.87 0.56 12.~0 1.43 10,800
8 1.02 ~.42 11.72 2.72 15,700
9 1.00 0.42 11.50 3~68 23,300
1.02 1.78 11.80 2.44 7-,560
11 0~95 3.11 11.99 1.72 6,500
12 0.81 4.41 11.9~ 1.54 5,500
13 2.14 0,55 11.54 1.26 4,090
14 2.34 0.50 12.03 2.14 3,400
2.35 0.51 12~00 3.84 3,000
16 2.06 0.75 12.55 5.20 4,100
17 2.30 0.46 11.87 2.28 3,150
18~ 2.05 0.67 13~]0 3.54 Mo 1.22 3,180
19* 2.28 1.75 12.01 2.31 2,~00
20* 2.12 1.83 11.97 4.18 2,550
21* 2.19 2.91 12.04 1.30 3,100
22* 2.22~ 2.8~ 12.01 2.31 2,450
23* 2.32 2.96 11.94 4.26 2,100
Residual elements in all above heats were 1% maximum Mn,
0~50~ maximum Ni, 0.030~ maximum P, 0.030~ maximum S.
*--Steels of the inve~tion.
' ~ ' .
~86g9~
The method of the invention is thus evident from
the above description and tests. It is further apparent that
articles and fabxicated products, such as materials processing
equipment, having an abrasion resistance of less than 3,S00
milligrams per 1,000 cycles by the Taber Met-Ahrader Model 500
test can be produced in heat hardened aondition by the method
of the invention fxom the steel of the invention, consisting
essentially of, in weight percent fxom about 1.8% to about 10%
carbon, up to about 1.0~ manganese, greater than 1.7% to about
4.5% silicon, about 11.5% to about 18% chromium, up to about
1% nickel, rom about 1% to about 10% titanium, up to about
1.5~ molybdenum, up to about 0~1% phosphorus, up to about 0.05
sulfur, and balance ixon except for inaidental impurities.
B~th cast and ~rought articles of ultimate use may
be involved having the above pxoperties, the steel composition
being restricted to a maximum of abou 5% carbon ~or hot worked
and cold worked articles prior to the heat treatment step.
If cold working is practiced, a stress relie treatment at
about 550 to 700F is preferably conducted a~ter the heat
hardening treatment. At carbon 1evels above 5~, cast articles
of ultimate use, and partlculate materlal suitable for powder
metallurgy processing such as compacting and sintering, may
be produced and subjected to heat hardening.
Where ext~emely high abrasion resistance and hard-
ness are desired and hot and~ar cold workability are not needed
(as in commercial tungsten carbon tooling wherein carbide
particles are bonded with nickel and/or cobalt, with the volume
proportion of carbides being about 90%), high carbon and
titanium embodiments of the steel of the invention can be sub-
stituted with resultant lower cost for total alloy additions.
16
~019699~
.
For such applications the above compositlon is utilized witha carbon range of greater than 5% to about 10%, and a titanium
range of greater than 5% to about 10%.
As will be evident from Samp~e 18 in Table I
molybdenum may be added in amounts up to about 1.5~ without
adverse effect on abrasiQn resistance, and such a modification
can be used where improved corrosion resistance i9 desired.
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