U.S. patent number 3,802,933 [Application Number 05/236,213] was granted by the patent office on 1974-04-09 for composite nitrided materials.
This patent grant is currently assigned to Surface Technology Corporation. Invention is credited to John J. Rausch, Ray J. Van Thyne.
United States Patent |
3,802,933 |
Rausch , et al. |
April 9, 1974 |
COMPOSITE NITRIDED MATERIALS
Abstract
Graded nitrided articles, surface modified in alloy composition
wherein the surface zone consists of nitrided alloys consisting
essentially of (A) one or more metals of the group columbium,
tantalum, and vanadium; (B) titanium; and (C) one or both metals of
the group molybdenum and tungsten. A minor portion of the nitrogen
may be replaced by oxygen or boron. Nitrided materials prepared
from homogeneous alloys are also included. The materials are
characterized by excellent wear and abrasion resistance.
Inventors: |
Rausch; John J. (Antioch,
IL), Van Thyne; Ray J. (Oak Lawn, IL) |
Assignee: |
Surface Technology Corporation
(Stone Park, IL)
|
Family
ID: |
26688815 |
Appl.
No.: |
05/236,213 |
Filed: |
March 20, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
16595 |
Mar 4, 1970 |
3674547 |
|
|
|
755658 |
Aug 27, 1968 |
3549427 |
|
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Current U.S.
Class: |
148/317; 428/627;
428/926; 428/610; 428/661 |
Current CPC
Class: |
C23C
8/24 (20130101); C22C 27/00 (20130101); Y10T
428/12458 (20150115); Y10T 428/12576 (20150115); Y10T
428/12812 (20150115); Y10S 428/926 (20130101) |
Current International
Class: |
C23C
8/24 (20060101); C22C 27/00 (20060101); C22c
027/00 (); C22c 029/00 (); C23c 011/14 () |
Field of
Search: |
;75/134,135,174,175.5,176,177,205,208,224
;148/13.1,16.6,20.3,31.5,34,39 ;29/182.5,182.3,195,198 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Belgian Patents Report No. 10/69, 7: Metallurgy p. 1 Nos. 720398
& 720399. .
IR 718-7(III), IIT Research, June 16, 1967-Sept. 15, 1967, pp.
51-55, 59, 60, 65 and 67..
|
Primary Examiner: Lovell; Charles N.
Attorney, Agent or Firm: Siegel; Albert
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This is a division of application Ser. No. 16,595 filed March 4,
1970, now U.S. Pat. No. 3,674,547, which is in turn a
continuation-in-part of application, Ser. No. 755,658, entitled
"WEAR RESISTANT MATERIALS" filed Aug. 27, 1968 now U.S. Pat. No.
3,549,427.
Claims
1. An article consisting essentially of: a metal or alloy substrate
member; a graded nitrided ternary or higher alloyed surface zone on
said substrate member, which surface zone consists of at least one
metal selected from each of Groups A, B, and C wherein Group A
consists of columbium, tantalum and vanadium; Group B is titanium
and Group C consists of molybdenum and tungsten and wherein:
a. the nitrogen pickup of said surface zone is at least 0.1
milligram per square centimeter of surface area and in said
zone;
b. when only columbium and molybdenum are present with titanium the
range for the columbium content is from about 20 percent to 85
percent;
c. when only columbium and tungsten are present with titanium the
range for the columbium content is from about 10 percent to 85
percent;
d. when only columbium, molybdenum and tungsten are present with
titanium the minimum amount of columbium required is determined by
the formula.
10 (Ratio E) + 20 (Ratio D)
and the maximum content of columbium is about 85 percent;
e. when only tantalum and molybdenum are present with titanium the
range for the tantalum content is from about 25 percent to 88
percent;
f. when only tantalum and tungsten are present with titanium the
range for the tantalum content is about 10 percent to 88
percent;
g. when only tantalum, molybdenum and tungsten are present with
titanium the minimum amount of tantalum required is determined by
the formula
10 (Ratio E + 25 (Ratio D)
and the maximum content of tantalum is about 88 percent;
h. when only vanadium and a metal selected from the group
consisting of molybdenum and tungsten and combinations thereof are
present with titanium the range for the vanadium content is about
15 percent to 90 percent;
i. when more than one metal of the group columbium, tantalum and
vanadium are present with only molybdenum and titanium the minimum
total content of the metals columbium, tantalum and vanadium must
be at least equal to the amount of
20 (Ratio A) + 25 (Ratio B) + 15 (Ratio C);
j. when more than one metal of the group columbium, tantalum and
vanadium are present with only tungsten and titanium, the minimum
total content of the metals columbium, tantalum and vanadium must
be at least equal to the amount of
10 (Ratio A) + 10 (Ratio B) + 15 (Ratio C);
k. when more than one metal of the group columbium, tantalum and
vanadium are present with molybdenum, tungsten and titanium, the
minimum total content of the metals columbium, tantalum and
vanadium must be at least equal to the amount of
[(Ratio A) + (Ratio B)]
[10 (Ratio E) + 25 (Ratio D)] + 15 (Ratio C);
l. when more than one metal of the group columbium, tantalum and
vanadium are present the maximum total content thereof must be
equal to or less than
85 (Ratio A) + 88 (Ratio B) + 90 (Ratio C);
m. when titanium is present with only columbium and a metal
selected from the group molybdenum and tungsten and combinations
thereof, the titanium content ranges from about 1 percent to 45
percent and the columbium to titanium ratio is greater than 1;
n. when titanium is present with only tantalum and a metal selected
from the group molybdenum and tungsten and combinations thereof,
the titanium content ranges from about 1 percent to 35 percent and
the tantalum to titanium ratio is greater than 1;
o. when titanium is present only with vanadium and a metal selected
from the group molybdenum and tungsten and combinations thereof,
the titanium content ranges from about 1 percent to 45 percent and
the vanadium to titanium ratio is greater than 0.66;
p. when titanium is present with more than one metal of the group
columbium, tantalum and vanadium and a metal selected from the
group molybdenum and tungsten and combinations thereof, the maximum
content of titanium must be equal to or less than
45 (Ratio A + Ratio C) + 35 (Ratio B)
and the ratio of the content of the metals columbium, tantalum and
vanadium to titanium must be equal to or greater than the ratio
of
(Ratio A) + (Ratio B) + 0.66 (Ratio C):1
and the minimum titanium content is 1 percent;
q. when only molybdenum, titanium and a metal selected from group
columbium and vanadium and combinations thereof are present, the
range for molybdenum content is from about 2 percent to 60
percent;
r. when only molybdenum, titanium and tantalum are present the
range of the molybdenum content is from about 2 percent to 50
percent;
s. when only tungsten, titanium and a metal selected from the group
columbium, tantalum and vanadium and combinations thereof are
present the range for tungsten content is from about 2 percent to
80 percent;
t. when molybdenum, tungsten, titanium and a metal selected from
the group columbium, tantalum, vanadium and combinations thereof
are present the maximum total content of molybdenum and tungsten
must be equal to or less than
60 (Ratio A + Ratio C) (Ratio D) +
50 (Ratio B) (Ratio D) + 80 (Ratio E)
and the minimum content of molybdenum and tungsten is 2
percent;
u. and wherein in the foregoing
Ratio A = Cb/(Cb + Ta + V)
Ratio B = Ta/(Cb + Ta + V)
Ratio C = V/(Cb + Ta + V)
Ratio D = Mo/(Mo + W)
2. The article as defined in claim 1 wherein in said surface zone
up to 3
3. The article as defined in claim 1 wherein the surface zone
thereof is
4. The article as defined in claim 1 wherein the graded, nitrided
portion of the surface zone thereof is depleted in titanium content
to the desired
5. The article as defined in claim 1 wherein said surface zone
comprises a
6. The article as defined in claim 1 wherein up to 25 percent of
the nitrogen weight pick-up is replaced by a material selected from
the group consisting of oxygen and boron and mixtures thereof.
Description
BACKGROUND OF THE INVENTION
In our parent application, Ser. No. 755,658, referenced above, we
have disclosed and claimed certain nitrided alloys consisting
essentially of
A. at least one metal of the group columbium, tantalum and
vanadium;
B. titanium; and
C. at least one metal of the group molybdenum and tungsten in
certain percentages by weight and compositional relationships as
are therein set forth. Such nitrided materials are characterized
by, among others, excellent wear and abrasion resistance and offer
substantial utility as cutting tool materials.
In such parent application, we have noted that the desired alloys
to be nitrided may be formed as free standing thin sections or clad
or by various means formed as a coating upon different substrates.
Similarly, in such parent application, we have noted that a variety
of nitriding treatments may be employed to effectuate the desired
results.
In the present application, we wish to elaborate upon the teachings
of said parent application. The compositions hereof which are
nitrided or otherwise treated are the same as the alloy
compositions which are disclosed in our parent application.
Accordingly, our parent application, Ser. No. 755,658 now U.S. Pat.
No. 3,549,427, in its entirety, is incorporated herein by
reference. We would note that a counterpart of such parent
application has issued as Belgium Pat. No. 720,398. As will be
evident, we herein provide additional features to said basic
invention and certain improvements thereof.
In our parent application, the temperatures are presented
uncorrected. In the present application, temperatures are
corrected. We used a correction factor determined by using a
tungsten-rhenium thermocouple in conjunction with the sightings of
the optical pyrometer mentioned in the parent case.
Furthermore, we would note that it is well known that titanium can
be nitrided to form a hard surface layer thereon but such material
shows a chipping propensity due to brittleness. In the practice of
our invention, such brittleness is avoided by specific alloying as
taught herein prior to nitriding. Additionally, the alloying
elements present in typical commercially available titanium alloys
do not produce the same improvement and nitrided commercial
titanium alloys show chipping similar to nitrided titanium.
The nitriding of titanium-rich alloys, i.e., containing about 90
percent titanium has been studied previously (for example, see E.
Mitchell and P. J. Brotherton, J. Institute of Metals Vol. 93
(1964) P. 381). Others have investigated the nitriding of
hafnium-base alloys (F. Holtz, et al., U.S. Air Force Report
IR-718-7 (II) (1967); molybdenum alloys (U.S. Pat. No. 3,161,949);
and tungsten alloys (D. J. Iden and L. Himmel, Acta Met, Vol. 17
(1969) P. 1483). The treatment of tantalum and certain unspecified
tantalum base alloys with air or nitrogen or oxygen is disclosed in
U.S. Pat. No. 2,170,844 and the nitriding of columbium is discussed
in the paper by R. P. Elliott and S. Komjathy, AIME Metallurgical
Society Conference, Vol. 10, 1961, P. 367.
In the present application, we wish to clearly point out the
significance of alloying surface treatments or coatings or
claddings with the present materials and surface treatments wherein
nitriding is employed as the major constituent along with
relatively minor amounts of oxygen and/or boron.
It should be noted that the alloys of the present invention may be
employed on another metal or alloy as a surface coating or cladding
and with the proper substrate selection, a highly ductile and/or
essentially unreacted substrate can be obtained. For example,
columbium or tantalum are much less reactive to nitrogen when used
in conjuction with the alloys hereof and tungsten and molybdenum do
not form stable nitrides at the nitriding temperatures employed.
Spraying and/or fusing the desired alloy onto the surface are
included in the various coating methods available. A variety of
direct deposition methods may be employed or alternate layers could
be deposited followed by a diffusion annealing treatment.
As set out in our parent application in determining whether or not
a material falls within the scope thereof, certain test criteria
were used as are set forth therein. More particularly, following
nitrided sample preparation lathe turning tests were run thereon at
surface speeds from 100 to 750 surface feet per minute (SFM) on
AISI 4340 steel having a hardness of around Rockwell C, (Rc), 43 to
45. A feed rate of 0.005 in./rev. and depth of cut of 0.050 in.
were used. A standard negative rake tool holder was employed with a
5.degree. back rake and a 15.degree. side cutting edge angle. Tool
wear was measured after removing a given amount of material.
The principal criterion in our parent application in determining
whether the nitrided materials pass or fail and thus whether or not
they are included or excluded from the scope thereof was the
ability to cut 2 cubic inch metal removal of the 4,340 steel at
speeds of both 100 and 750 SFM.
At 750 SFM our high performance, nitrided materials readily pass
the initial test of 2 cu. in. metal removal in about 1 minute. (We
would note that by "SFM" is meant the linear rate at which the
material being cut passes the cutter.)
In some aspects of the present invention, such test criteria of the
parent application are inoperative. This is particularly true of
the thin sections and surface zones considered herein. The nitrided
alloys are the same but in some instance in thin sections the test
criteria of the parent case are not met herein. However, the
materials still offer substantial wear and abrasion resistant
properties.
In evaluating tools and tool materials, failure is often assumed to
occur when the wearland reaches 0.030 inch. With the materials of
this invention, we selected a rather severe test -- we indicate
those which are good (i.e., pass the test), when at 750 SFM and 2
cu. in. removal, there is a uniform wearland of less than 0.025 in.
Furthermore, we would note that although chipping is seen in some
compositions upon testing at 750 SFM the chipping propensity is
aggravated at lower speeds and better assessed at 100 SFM. The
latter is one of the reasons for selecting both speeds.
Accordingly, a principal object of our invention is to provide
certain novel articles wherein the surface zone thereof is a
nitrided alloy consisting essentially of: (a) at least one metal of
the group columbium, tantalum and vanadium; (b) titanium; and (c)
at least one metal of the group molybdenum and tungsten.
Another object of our invention is to provide said novel articles
aforesaid wherein up to three percent of the titanum content is
replaced by zirconium.
A further object of our invention is to provide such nitrided
articles wherein the nitrogen pickup is at least 0.1 milligram per
square centimeter of surface area.
Still a further object of our invention is to provide such nitrided
articles wherein up to 25 percent of the nitrogen weight pickup is
replaced by oxygen and/or boron.
These and other objects, features and advantages of our invention
will become apparent to those skilled in this art from the
following detailed disclosure thereof.
DESCRIPTION OF THE INVENTION
An alloy of the composition, Cb-20V-40Ti-10Mo was readily reduced
to foil by rolling and coatings thereof were made on molybdenum by
fusing this alloy in argon at a temperature of about 3,375.degree.F
for a time of 2 minutes. The coating wet the substrate well, did
not flow excessively, and did not seriously react with the
molybdenum. A specimen with a 22 mil coating was nitrided at
2,950.degree.F for 2 hours and showed a microhardness grading and
structure similar to the nitrided material in bulk form. The
microhardness at a depth of 1/2, 1, and 2 mils was 2190, 1600, and
1365 DPN, respectively. A coating of similar thickness was produced
on tungsten by dipping tungsten stock into molten Cb-18Ti-18W
alloy.
A 3 mil coating of Cb-20V-40Ti-10 Mo was also produced on
molybdenum by fusing in argon. This was subsequently nitrided at
2,250.degree.F for one-half hour resulting in a nitrogen weight
pickup of 1.6 mg per sq. cm. The microhardness at a depth of
one-third mil from the surface was 1,680 DPN. The nitriding
temperature is sufficiently low that such alloys may be coated on a
variety of substrate materials including ferrous alloys and
successfully nitrided to produce a hard surface.
Much thinner coatings are readily produced by similar or other
procedures. As the reactive alloy coating becomes thinner, the
amount of nitrogen pickup for surface hardening is reduced since
the nitriding is concentrated near the surface. Accordingly, in
such thin sections the depth of hardening is reduced. In relatively
thin coatings, the weight pickup of nitrogen may be 0.1 to 1 mg per
sq. cm. or less and in thicker coatings the pickup will be over 1
mg per sq. cm. of surface area.
In our copending, referenced parent application, we have shown that
for noncoated homogeneous alloy stock the amount of nitriding
required for equivalent surface hardening is dependent upon sample
thickness. As the thickness is decreased, the required nitriding
temperature and weight pickup are reduced. We have observed a
pronounced effect of specimen thickness, particularly at knife
edges where the required nitrogen pickup is greatly reduced. Also,
such coated or homogeneous materials may be used for a wide variety
of applications requiring wear and abrasion resistance where the
requirement for surface hardness or depth of hardening may be less
than that required for metal cutting. Accordingly, in thin sections
of homogeneous alloy material, similar to thin coatings of the
alloys, the weight pickup of nitrogen may be 0.1 to 1 mg per sq.
cm.
Another useful method for utilizing our nitrided materials involves
controlled evaporation of titanium from the surface of an alloy
(detitanizing). By this procedure, an alloy, for example, with a
titanium content greater than that determined by our compositional
limitations, can be depleted in titanium content to bring the
surface alloy content within our prescribed ranges prior to
nitriding. We have heated various alloys containing the required
metals of our invention in vacuo at temperatures below the melting
point of the alloy. Titanium evaporation occurred without any
substantial change in geometry. Most importantly, this was
accomplished without the occurrence of significant amounts of
porosity. Electron microprobe analyses confirmed the significant
changes in weight that had been observed. A specimen of
Cb-45Ti-10Mo vacuum treated at a pressure of 5 .times.
10.sup.-.sup.5 Torr at 2,850.degree.F for 4 hours showed a decrease
in titanium content and a corresponding increase in columbium and
molybdenum content. The decrease in titanium content extended to a
considerable depth and in the outer 2 mils the decrease was about
10 percent. Other vacuum treatments run at 2,950.degree.F for 6
hours showed even greater titanium loss. A Cb-45Ti-20W alloy vacuum
treated at 2,850.degree.F for 4 hours lost 33 mg for a 3/8 .times.
3/8 .times. 1/8 inch specimen weighing 1.9 gram, and a similar size
sample of Cb-50Ti-20W vacuum treated at 2,950.degree.F for 6 hours
lost 60 mg.
Similar detitanizing effects were shown for Ta-Ti-Mo alloys wherein
substantial weight losses of titanium were observed without
geometry changes or the development of significant amounts of
porosity. Ta-40Ti-10Mo, initially 2.4 g., vacuum treated at
2,950.degree.F for 6 hours lost 54 mg for a 3/8 .times. 3/8 .times.
1/8 sample. Upon nitriding at 3,250.degree.F for 2 hours, this
material cut at both 750 and 100 SFM. All such vacuum treated
materials show high surface hardness. It will, of course, be
appreciated that such surface evaporation techniques can be applied
to alloys that are already within our prescribed composition ranges
to effect desirable structural and property changes.
The cutting performance of such Cb-40 Ti-10Mo vacuum treated at
2,850.degree.F for 4 hours prior to nitriding at 3,250.degree.F was
better than the same alloy when nitrided without prior
detitanizing. It should be noted that annealing per se, that is,
annealing under conditions where significant evaporation does not
occur, has an effect on the microstructural morphology. Such
morphology effects due to annealing, which result in greater
regularity of structure may produce improvements for certain uses,
but the compositional effect due to treatment in vacuo is of value
by itself.
Since our nitrided materials present as a homogeneous material or
as a coated article are in a thermodynamically metastable
condition, those skilled in the art will realize that a variety of
heat treatments, including multiple and sequential treatments, can
be used to modify the reaction structure and resulting properties
whether performed as part the over-all nitriding reaction or as
separate treatments. Improvement in cutting properties has been
noted by nitriding at lower temperatures for longer times and by
nitriding at lower temperatures followed by nitriding at higher
temperatures. However, the required weight pickup for cutting at
750 SFM is similar to the amount of nitriding necessary with a
simple 2-hour nitriding treatment. The treatments have included
typical nitriding followed by aging at lower temperatures in argon
or nitrogen. We have also nitrided at higher temperatures (and
longer times) that normally would produce some embrittlement and
then subsequently annealed in inert gas or at various partial
pressures of nitrogen as a tempering or drawing operation to
improve toughness. This duplex treatment results in a greater
reaction depth with the hardness-toughness relationship controlled
by the tempering temperature and time.
Such treatments can be employed to modify the properties of our
nitrided materials to produce various combinations of hardness and
toughness. The required annealing treatment is dependent upon the
material usage, alloy composition and degree of prior
nitriding.
The influence of annealing under various conditions for a variety
of nitrided materials may be seen from the data presented in Table
I.
TABLE I
__________________________________________________________________________
Nitriding Argon Microhardness (DPN) at Alloy Treatment Treatment
Depth (mils)
__________________________________________________________________________
Compositon .degree.F Hrs .degree.F Hrs 0.5 1 2 4 8
__________________________________________________________________________
Cb-17Ti-20W 3450 2 none 2570 2090 1890 1140 906 do do do 3450 1
1220 1017 1040 857 -- do do do 3450 1* 2190 1420 1250 835 765 do do
do 3250 2 3060 2600 2570 2160 985 Ta-20Ti-10Mo 3550 2 none 2060 --
1675 1480 1110 do do do 3250 1 1690 -- 1175 1250 946 do do do 3250
4 1790 -- 1160 996 1060
__________________________________________________________________________
*Argon -- 0.1 percent nitrogen atmosphere.
The alloy Cb-17Ti-20W, nitrided at 3,450.degree.F for 2 hours shows
substantial softening when subsequently annealed in argon for 2
hours at this same temperature. If the annealing is carried out in
an atmosphere of A-0.1%N.sub.2 it may be noted that only a moderate
decrease in hardness occurs and the material grades uniformly in a
manner similar to the nitrided condition. If annealed at
3,250.degree.F for 2 hours in argon the material hardens
significantly. The influence of annealing in argon on reducing the
uniform hardness gradient for the nitrided Ta-20Ti-10Mo alloy may
also be seen from the above data. We have found that nitrided
alloys containing higher amounts of tungsten or molybdenum soften
readily when annealed in argon. To control this softening, that is,
avoiding the formation of a surface-layer that is too soft to cut
the hardened steel at 750 SFM, we have found regulation of the
nitrogen content of the atmosphere to be a useful parameter. It
should be noted that the A-0.1%N.sub. 2 atmosphere will harden
unnitrided or moderately nitrided alloys but results in softening
when used with the highly nitrided alloys in the examples above. A
3/8 .times. 3/8 .times. 1/8 inch specimen of Cb-30Ti-20W reacted in
nitrogen at 3,250.degree.F for 2 hours, cuts well at 750 SFM. When
subsequently treated in A-0.1%N.sub.2 for 2 hours, this material
continues to nitride as evidenced by a further 8 mg. pick-up.
A number of our materials have been nitrided and subsequently
annealed. Although the nitrided alloy Cb-20V-40Ti-10Mo passed our
cutting test criteria at 750 and 100 SFM, improvement was achieved
by nitriding at 3,250.degree.F for 2 hours followed by annealing in
argon at 3,250.degree.F for 1 hour. Also, good combined performance
at 750 and 100 SFM was shown for Cb-30Ti-20W nitrided at
3,550.degree.F for 2 hours and annealed at 3,550.degree.F for 1
hour. Annealing at 3,250.degree.F for 1 hour did not produce any
significant improvement and annealing for 4 hours at 3,550.degree.F
resulted in failure in cutting at 750 SFM. Thus, one should use due
care in annealing conditions.
In most of our materials, the hardness (and nitride content) grades
and lessens as one moves from the surface inwardly. However, we
would note that in some cases such grading extends from a plateau
or from a peak hardness slightly below the surface and grades
inwardly therefrom. Such materials can be effective cutting tools
or abrasion resistant articles.
We have also nitrided materials directly in an environment
sufficiently low in nitrogen potential that the effect is noted.
Nitriding in flowing A-0.1%N.sub.2 produces reduced nitrogen
pick-up compared to 100 percent nitrogen. Another method involves
sealing the furnace with a measured amount of nitrogen and allowing
the nitrogen content to be reduced during treatment as a result of
the specimens absorbing the available nitrogen. For example,
Cb-30Ti-10Mo was reacted in an atmosphere starting with
0.45%N.sub.2 balance argon and ending with 0.03%N.sub.2. A specimen
treated in this manner cut well at both 750 and 100 SFM. The alloy
Cb-80Ti-10Mo falling outside our invention, was nitrided in
A-0.1%N.sub.2 for 2 hours at 3,050.degree.F. Similar to treating in
nitrogen, the result was a thick continuous 3 mil nitride surface
layer and such material fails immediately in testing at 750 SFM.
These various alternate nitriding treatments may be applied to the
materials of our invention whether used as a homogeneous alloy or
as a coated or surface modified material. In all of the nitriding
treatments and particularly for those involving reduced nitrogen
potential, the effect of the varying stabilities of the metal
nitrides must be considered since this can also contribute to
surface compositional effects.
Surface alloying techniques are also useful for the preparation of
the alloys to be nitrided to produce the materials of our
invention. Cb-10Mo was titanized at 2,950.degree.F for 3 hours in
vacuo by holding in a pack of fine titanium "sponge" which causes
diffusion of titanium into the surface. This treatment resulted in
a 6 mil titanized layer which upon nitriding for 2 hours at
3,250.degree.F yielded a graded reaction zone similar to Cb-Ti-Mo
materials. This contrasts with the 4 mil continuous nitride layer
formed on Cb-10Mo without the prior titanizing treatment which
exhibits cracking of the continuous nitride layer.
In the present invention, as in the invention disclosed and claimed
in our copending parent application, when one wishes to determine
whether or not the material is useful in the nitrided state for
purposes hereof certain compositional ratios and formulae must be
employed in some cases. Such formulae represent linear
proportionate amounts based on weight percentages.
A modest mathematical statement is required. In the present
disclosure and claims, the following ratios shall have the
following meanings:
Ratio A = Cb/(Cb+Ta+V)
(that is, the concentration of columbium to total columbium,
tantalum and vanadium). Similarly,
Ratio B = Ta/(Cb+Ta+V)
Ratio C = V/(Cb+Ta+V)
Ratio D = Mo/(Mo+W)
Ratio E = W/(Mo+W)
When, in the present alloy systems, more than 1 metal of the group
columbium, tantalum and vanadium is present the maximum total
content, in terms of weight per cent of such metals must be equal
to or less than the total of
85 (Ratio A) + 88 (Ratio B) + 90 (Ratio C)
and the minimum content thereof when tungsten and/or molybdenum are
present must be equal to or greater than the total of
[(Ratio A) + (Ratio B)]
[10 (Ratio E) + 25 (Ratio D) ] + 15 (Ratio C)
Furthermore, when there is more than 1 metal of the group
columbium, tantalum and vanadium present the maximum amount of
titanium permitted in the alloy system is equal to or less than the
amount determined by the formula
45 (Ratio A + Ratio C) + 35 (Ratio B)
and the ratio of the content of such metals to the titanium must be
greater than the ratio determined by
Ratio A + Ratio B + 0.66 (Ratio C):1
Additionally, when both tungsten and molybdenum are present the
maximum amount thereof is determined by the formula
60 (Ratio A + Ratio C) (Ratio D) +
50 (Ratio B) (Ratio D) + 80 (Ratio E)
We would further note that when columbium alone is used of Group A
metals and both molybdenum and tungsten are present the minimum
amount of columbium required is determined by the formula
10 (Ratio E) + 20 (Ratio D)
The minimum titanium is 1 percent and the minimum tungsten and/or
molybdenum is 2 percent.
Up to 3 percent of the titanium content may be replaced by
zirconium.
Ta-10W was also titanized with the following procedure: vacuum pack
titanized 2,950.degree.F -- 6 hours; annealed in argon
2,950.degree.F -- 2 hours plus 3,050.degree.F -- 2 hours; nitrided
2,850.degree.F -- 2 hours. Such treatment produced a 6 mil
titanized diffusion zone. Structural grading is shown in the
microhardness traverse data presented below: Microhardness (DPN) at
depth (mils) 0.5 1. 2 4 8 ______________________________________
1520 1100 955 666 215
A strip specimen 72 mils thick was prepared using the same
titanizing and nitriding procedures and was subsequently bent
45.degree.. Cracking of the hard nitrided case occurred on the
tension (outer) side. The adherency of the hard nitrided 6 mil zone
was shown by the fact that none of it spalled from the Ta-10W
substrate which was intact.
Another surface alloying procedure involved the combined titanizing
an vanadizing of molybdenum or tungsten. This can be accomplished
by vacuum pack treatment since titanium and vanadium have similar
vapor pressures. Such treatment of molybdenum or tungsten at
2,950.degree.F for 3 hours yields a thinner diffusion zone than
that observed for the titanizing of Cb-15Mo. The depth of the
diffusion zone was about 11/2 mil with molybdenum and less with
tungsten. After nitriding at 3,250.degree.F for 2 hours the
microhardness of the molybdenum sample was 1000, 605, and 190 DPN
at 0.5, 1, and 2 mils, respectively.
Use of surface alloying or coating techniques can enhance the
utility of powder processing of the alloys prior to nitriding in a
number of ways. For example, a powder processed alloy of Cb-Mo
could be formed and then titanized or a porous molybdenum or
tungsten presintered compact could be infiltrated by coating
methods. These and other techniques can (1) lower sintering
temperatures, (2) enhance filling of pores, and (3) reduce
shrinkage as compared to making a homogeneous powder part.
We have modified our nitrided material by combining nitriding with
oxidizing or boronizing. However, the amount of reaction with such
other hardening agents must be limited, a majority of the weight
pick-up is due to nitriding, and these are essentially nitrided
materials. The alloys may be preoxidized at a temperture where
little reaction would occur with nitrogen alone and then
subsequently nitrided. Also, the alloys may be reacted with a
combined oxidizing and nitriding environment although the relative
oxidizing potential must be low since for example in air the alloys
will preferentially oxidize rather than nitride. A sample of
Cb-30Ti-20W was nitrided at 3,250.degree.F for 2 hours and
subsequently boronized at 2,650.degree.F for 4 hours. The
structural features of such a material are very similar to the
alloy only nitrided; the hardness grades inwardly and of the total
weight pick-up over 90 percent due to nitriding. A smooth surface
layer about 0.4 mil thick forms due to the boronizing treatment
that is harder than the nitrided surface.
For comparison, the Cb-30Ti-20 W alloy nitrided at 3250.degree.F
for 2 hours exhibits a microhardness of 2680 DPN at a distance of
one-third mil from the surface. After the subsequent boronizing
treatment, the hardness was 4,550 DPN at the same depth. This
duplex treated material passes our test at 750 and 100 SFM but the
chipping propensity is increased. Up to 25 percent of the nitrogen
pick-up by weight may be replaced by oxygen and/or boron.
Although the alloys receptive to nitriding can be produced by
coating or surface alloying techniques, many uses involve the
forming and machining of a homogeneous alloy or a coated article.
One of the advantages in utility of these materials is our ability
to form the metallic alloys by cold or hot working and/or to
machine (or hone) to shape in the relatively soft condition prior
to final nitriding. Only minimal distortion occurs during nitriding
and replication of the starting shape and surface finish is
excellent. The final surface is reproducible and is controlled by
original surface condition, alloy composition, and nitriding
treatment. For some applications, the utility would be enhanced by
lapping, polishing, or other finishing operations after nitriding.
The nitrided surface is quite hard but only a small amount of
material removal is required to produce a highly finished
surface.
One of the nitrided effects that we have noticed is an accentuation
of sharp edges. Similar to the established technology for aluminum
oxide ceramic insert tools, we have blunted sharp cutting edges
prior to nitriding. This has been accomplished by simple tumbling
prior to nitriding or by finishing subsequent to nitriding. High
speed cutting performance will not be degraded if such edge
preparation is limited. The nitrided material can be used as a
mechanically locked insert or it can be bonded or joined by
brazing, for example, to a substrate.
We have also observed the excellent corrosion resistance of both
the alloys and the nitrided alloys in strong acids, and these
materials could effectively be employed for applications requiring
both corrosion and abrasion resistance. Both the alloys and the
nitrided alloys possess good structural strength. Thus, the
materials can be employed for applications involving wear
resistance and structural properties (hardness, strength,
stiffness, toughness) at room and elevated temperatures. Other
useful properties of the nitrided materials include good electrical
and thermal conductivity, high melting temperature, and thermal
shock resistance.
The excellent cutting properties and wear resistance of the
nitrided materials can be effectively employed with the other
useful properties of the alloys and nitrided materials to produce a
wide range of products. Some of these are: single point cutting
tools, multiple point cutting tools (including rotary burrs, files,
routers and saws), drills, taps, punches, dies for extrusion,
drawing, and other forming operations, armor, gun barrel liners,
impeller of fan blades, EDP (Electrical Discharge Machining)
electrodes, spinnerets, guides (thread, wire, and other), knives,
razor blades, scrapers, slitters, shears, forming rolls, grinding
media, pulverizing hammers and rolls, capstans, needles, gages
(thread, plug, and ring), bearings and bushings, pivots, nozzles,
cylinder liners, tire studs, pump parts, mechanical seals such as
rotary seals and valve components, engine components, brake plates,
screens, feed screws, sprockets and chains, specialized electrical
contacts, fluid protection tubes, crucibles, molds and casting
dies, and a variety of parts used in corrosion-abrasion
environments in the paper-making or petrochemical industries, for
example.
It will be understood that various modifications and variations may
be affected without departing from the spirit or scope of the novel
concepts of our invention.
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