Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The subject invention is addressed to cast maraging steels.
As those skilled in the art are aware, more than a decade has
passed since the advent of the maraging steels. During that period, these
steels, at least those in the wrought condition, achieved a position of
prominence in various areas of application. But, all their attributes not-
withstanding, the cast versions have apparently disappeared from the com-
mercial scene. Perhaps the reason forthis was largely occasioned by the
necessity to vacuum melt and vacuum pour in an effort to achieve a satis-
factory combination of properties. Of course, this would increase cost
markedly. It would appear, therefore, that cast maraging steels which
could be produced in accordance with air melting technology but capable
of delivering a satisfactory combination of metallurgical characteristics,
notably strength and toughness, would become a more formidable competitor
in the marketplace.
It has now been found that vacuum processing can be dispensed
with and that a high level of strength and toughness can be attained using
air melting processing provided the cast steels contain special amounts
of nickel, cobalt, molybdenum, silicon, aluminum, titanium and carbon.
Generally speaking, and in accordance with the present invention,
cast maraging steels are contemplated which contain from about 15 to 19%
nickel, from about 8 or 9% to about 12.5% cobalt, about 1.5 to 2.5%
molybdenum, 0.01 to 0.2% of each of aluminum and titanium, from 0.3 to
0.6% silicon, 0.001 to 0.1%, e.g., 0.005 to 0.05%, carbon, the balance
being essentially iron. Such steels consistenLly afford a minimum yield
strength of at least 175,000 or 180,000 psi together with the capability
of absorbing high levels of impact energy. (All percentages given herein
are in weight percent.)
With regard to the chemistry above given, it is noteworthy to men-
tion that the steels contain, comparatively speaking, a rather substantial
amount of silicon. Heretofore, it has been deemed that silicon was detrimental
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particularly in respect of toughness. Indeed, in speaking of maraging steels
in general, it has been said that the silicon content thereof should be held to
a maximum of 0.1%. However, as will be shown herein, provided that cast
maraging steels contain the proper constituents, the percentages of which are
particularly correlated, not only can high strength levels be achieved, but
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more than satisfactory toughness as well. Of considerable importance, since
the instant steels accept high silicon levels, less pure materials can be used in
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production, thus offering a further economic advantage commercially. The
silicon can be extended down to 0.1% with a cobalt level from about 11.5 to 12.5%. ;~
The following illustrative data are given.
A series of steels were prepared in accordance with the invention
using conventional air melting techniques. In this regard, 30-lb. air induction
melts were made using electrolytic type charge materials. The melts were
cast into one-inch thick keel block sand molds. Tensile, Charpy V-notch
impact and, in some instances, fracture toughness tests were conducted, these
tests being performed at room temperature, The compositions of various steels
are given in Table I, Alloys 1-8 being within the invention whereas Alloys A-F
are beyond the scope thereof.
TABLE I
~ = ___ . .. _ .....
Ni Co Mo Si Al Ti C Fe
Alloy__ % % 96 % ~, % % %
18.012.2 1.7 .44 .04 .04 .022Bal.
2 17.512.4 1.9 .37 .04 .04 .010Bal.
3 17.812.6 2.0 .38 .03 .05 .011Bal.
4 18.110.2 2.0 .30 .04 .04 .019Bal.
17.910.5 1.6 .36 .03 .06 . 021 Bal .
6 18.08.1 2.1 .35 .02 .06 .031Bal.
7 17.98.4 1.7 .42 .02 .07 .047Bal.
8 17.712.5 1.9 .11 .09 .02 .006Bal.
A 17.615.3 2.6 .45 .03 .02 .015Bal .
B 17.915.4 2.5 .33 .04 .04 .039Bal.
C 12.012.4 2.1 . 03 .01 .04 .009Bal .
D 18.24.2 2.55 .23 .02 .04 .011Bal.
E 17.96.3 1.52 .36 .02 .05 .010Bal .
F 18.16.2 2. 30 .36 .02 .07 .009Bal .
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The alloys were subjected to a heat treatment consisting of
(i) solution annealing at 2100F. for 1 hour, air cooling, (ii) heating at
1100F. for 1 hour, air cooling, (iii) followed by heating at 1500F. for 1
hour and air cooling, the alloys thereafter being (iv) aged at 900F. for
3 hours. The results of these tests are reported in Table II below.
TABLE II
Yield Strength UTSElong. R.A.CVN
Alloy0.2% offset, ksi ksi % %ft.-lbs.
196 20610 4327.0
2 203 20810 5225.0
3 211 21511 5024.7
4 197 20410 4725.7
190 19414 5727.5
6 193 19811 3824.2 -
7 190 19412 5222.2
8 198 20211 5124.0
A 241 251 8 335.2
B* 260 271 2 44.0
C 141 15410 3214.7
D 169 17614 4920.5
E 149 15818 6447.0
F 163 17715 5534.5
*Aged at 800F. for 24 hours
The data reported in Tables I and II reflect the disadvantages in
appreciably departing from the cast steel compositions in accordance herewith.
For example, Alloys A and B had high cobalt levels (the molybdenum also
being at the high end of its alloying range). And while strength was high,
tensile ductility and toughness were relatively low. On the other hand,
whereas ductility and toughness were acceptable in respect of, say, Alloys E
30 and F, strength was unsatisfactory. Alloy 8 has been included in Tables I
and II to illustrate that lower percentages of silicon can be used when high
levels of cobalt are present. Such an alloy, while within the invention,
is more costly and does not provide any real benefit. Alloy 8 can be compared
with Alloy 3 in terms of silicon effect. These steels were rather similar in
composition, the latter containing 0.38% silicon versus 0.11% for the ~ormer .
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Notwithstanding the higher silicon level, Alloy 3 exhibited comparable
toughness, its yield strength being 13,000 psi higher . With cobalt per-
centages below about 9.5 or 10%, the molybdenum should be upwards of 1.7%.
The foregoing data indicate that provided a properly correlated
chemical balance is employed, "cast maraging" steels can be produced with
a quite acceptable combination of strength and toughness, this obtaining with
air melting processing.
It might also be added that it is of significance that steels in accordance
herewith combine the capability of offering high strength, e.g., 180,000 psi
and above in thick sections. As indicated in a recent National Advisory Board
Report, such a cast steel would be desirable. It was indicated that, subject to
further development work, even HY-180 probably would have to be vacuum
melted. In this regard, a 300 lb. air induction melt of the following composition
was sand cast into not only one and three inch castings, but also a six inch
thick casting with good results (193 KSI Y.S. plus 34 CVN at room temperature):
17.7% nickel, 10.1% cobalt, 1.61% molybdenum, 0.28% silicon, 0.01% aluminum,
0.04% titanium, 0. 009% carbon, balance iron and impurities .
A particularly satisfactory cast alloy contains 16. 5 to 18% nickel,
9.75 to 11% cobalt, 1 . 6 or 1. 7 to 2 . 1% molybdenum, 0 . 01 to 0 . 1% each of
aluminum and titanium, about 0.01 to 0.0596 carbon and the balance essentially
iron.
Although the invention has been described in connection with ;
preferred embodiments, modifications may be resorted to without departing
from the spirit and scope of the invention, as those skilled in the art will
readily understand. Such are considered within the purview and scope of
the invention and appended claims.
