US3767389A

US3767389A - Maraging stainless steel particularly for use in cast condition

Info

Publication number: US3767389A
Application number: US00181234A
Authority: US
Inventors: S Floreen
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1971-02-17
Filing date: 1971-09-16
Publication date: 1973-10-23
Anticipated expiration: 1990-10-23
Also published as: FR2125519B1; US3767889A; DE2206894C3; DE2206894A1; DE2206894B2; GB1370051A; FR2125519A1

Abstract

A maraging stainless steel containing correlated amounts of nickel, chromium, silicon, and metal from the group consisting of aluminum and titanium, the steel being particularly suitable in cast form since it affords a combination of good strength, toughness and corrosion resistance and also exhibits excellent foundry characteristics.

Description

United States Patent Floreen Oct. 23, 1973 MARAGING STAINLESS STEEL 2,505,763 5 1970 GOllfll 75 124 PARTICULARLY FOR USE IN CAST 2,738,267 5/1950 Pakkala 75/124 2,820,708 1/1958 Waxweiler.... 75/128 CONDITION 3,278,298 10/1966 Perry 75/128 [75] Inventor: Stephen Floreen, Suffern, N.Y. 3,251,683 5/1966 Hammond t 75/l28 T 3,532,491 10/1970 Floreen 75/128 T 1 Asslgneel The Intematloflal Nlckel Company, 2,801,916 8/1967 Harris 75/128 T Inc., New York, N.Y.

[22] Filed: Sept 1971 Primary ExaminerHyland Bizot 2 Appl, 1 1,234 Attorney-Maurice L. Pinel Related US. Application Data [63] Continuation-in-part of Ser. No. 865,969, Oct. 13,

1969, abandoned. [57] ABSTRACT A maraging stainless steel containing correlated [52] 75/128 75/124 T amounts of nickel, chromium, silicon, and metal from [51] Int. Cl. C22c 3 /20 the group consisting of aluminum and titanium, the [58] Field of Search 75/128 T, 128 N, Steel being particularly Suitable in cast form since it 75/124 affords a combination of good strength, toughness and corrosion resistance and also exhibits excellent [56] References cued foundry characteristics.

UNITED STATES PATENTS 10/1964 Lula 75/124 17 Claims, No Drawings MARAGING STAINLESS STEEL PARTICULARLY FOR USE IN CAST CONDITION This application is a continuation-in-part of application Ser. No. 865,969, filed Oct. 13, 1969.

Nearly a decade has passed since the original discovery of the so-designated maraging steels. And while research continues on an extensive basis there is not yet on the commercial scene, at least insofar as I am aware, a low cost, cast stainless maraging steel. Conventional cast versions, however, have long been in commercial production, for example, the cast maraging steel described in US. Pat. No. 3,132,937. Indeed, significant processing improvements have been attained in respect of such cast steels as evident from the heat treating procedures set forth in US. Pat. No. 3,341,372. Nonetheless, such steels are, at least comparatively speaking, relatively costly and, more importantly, do not exhibit stainless characteristics.

Thus, there exists a hiatus in the maraging family of steels, to wit: a cast stainless maraging steel which in one package offers low cost, high corrosion resistance, good foundry characteristics and a tensile strength on the order of, say, about 150,000 psi or more, coupled with good toughness, although such steels with strengths as low as about 125,000 psi are also quite useful for many applications.

It has now been discovered that the foregoing objectives can be attained provided the steels contain properly correlated amounts of nickel, chromium, silicon, metal from the group consisting of aluminum and titanium, and other elements as set forth herein.

Generally speaking, where tensile strengths on the order of about 150,000 psi and higher are necessary together with good corrosion resistance and toughness, the present invention contemplates cast stainless maraging steels containing (in weight per cent) from percent to about 12.5 percent, e.g., 10 to 12 percent, chromium, about 7.5 to 11 percent, e.g., 8 to 10 percent, nickel, the sum of the chromium plus nickel being at least 18 percent, e.g., 19 percent, but less than 22 percent and advantageously not over 21.5 percent, about 1.5 to 3 .percent silicon, a small but effective amount, e.g., 0.01 percent, of metal from the group consisting of aluminum and titanium, up to about 1 percent manganese, up to about 0.05 percent carbon and the balance essentially iron. For tensile strengths down to about 125,000 psi, lower percentages of silicon and nickel can be utilized as explained herein. Further, the sum of chromium plus nickel can be as low as 17.5 percent. Elements such as phosphorus, sulfur, oxygen and nitrogen should be kept at low levels consistent with good commercial steelmaking practice.

In carrying the invention into practice should the chromium content fall below about 10 percent corrosion resistance is impaired. Moreover, mechanical properties can be decidedly unattractive. On the other hand, at chromium levelsmuch above 12.5 percent, problems due to delta ferrite can arise, thus adversely affecting toughness characteristics. This is particularly true with low percentages of nickel. For this reason, among others, in seeking the highest combination of strength and toughness, the nickel content should be at least 7.5 percent and is most beneficially at least 8 percent or 8.5 percent, since it both inhibits delta ferrite formation and markedly enhances toughness. However, in respect of the lower strength steels, good results can be achieved at reduced percentages of nickel,

i.e., about 6 percent or 6.5 percent, particularly with silicon contents below 1.5 percent. Silicon, as does chromium, also tends to promote delta ferrite and as the amount thereof is reduced less nickel is thus required, other factors being equal.

Although nickel counteracts delta ferrite and promotes toughness and though chromium imparts the primary resistant effect to various corrosive media, it has been found that together these constituents can be present to such an extent that mechanical properties are subverted by reason of the fact that the alloys are undesirably characterized by low M, temperatures. Therefore, the sum of the chromium plus nickel should be less than 22 percent and in striving for optimum results should not exceed 21 percent or 21.5 percent. It might be pointed out that the M, temperature should not be lower than about 325F. to 350F. to thereby as sure obtaining steels which upon transformation are characterized by an essentially martensitic structure upon cooling from say, hot working or annealing temperatures and the like.

With regard to silicon, it should not fall below about 1.5 percent where the emphasis is on the higher strength steels. Percentages much above 3 percent silicon, while imparting strength, detract from toughness, particularly the ability of the steels to absorb impact energy. A silicon range of 1.6 to 2.5 percent, e.g., 1.8 to 2.3 percent, is most satisfactory. Lower amounts of silicon can be used in connection with steels having tensile strengths between about 125,000 and 150,000 psi; however, at least about 0.5 percent silicon, e.g., at least 0.6 percent, is necessary for good foundry characteristics and to facilitate ease of castability.

In the absence of either or both aluminum and titanium mechanical properties, particularly in respect to air melted steels, are deleteriously affected. These constituents are deemed to tie up interstitials (carbon, nitrogen) and such elements as oxygen, the presence of which might otherwise bring about an undesirable loss in toughness. Only a small amount, e.g., 0.01 percent or 0.02 percent, of either or both of these constituents need be present and it is not necessary that either exceed 0.1 percent or 0.2 percent or that the total amount exceed about 0.3 percent.- Preferably, the steels contain not more than about 0.2 percent of aluminum and/or titanium. In aiming for optimum properties, from 0.02 to 0.07 percent .of each of these elements has been found to afford excellent results.

While it is above indicated that manganese can be present in the steels in accordance herewith in percentages up to about 1 percent, it is nonetheless extremelyadvantageous to maintain this constituent at much lower levels, to wit: not above about 0.4 to 0.5 percent. As will be demonstrated herein, manganese can impair the capability of the steels to absorb high levels of impact energy. As to carbon, if toughness characteristics are not to be needlessly sacrificed this element should not be present in amounts above 0.05 percent and, indeed, should be maintained at significantly lower levels, e.g., below about 0.03 percent or 0.02 percent.

For the purpose of giving those skilled in the art a better understanding of the invention the following illustrative data are given.

A series of 30 pound air-induction melts utilizing electrolytic grade metals as starting materials was prepared. The furnace was first charged with iron and nickel together with about 0.05 percent carbon (carbon boil) for the deoxidation. Thereafter, about 0.1 percent each of aluminum and titanium was added followed by the silicon addition. The heats were cast in dry said double keel block molds, the leg of each keel being 1 inch X Hinches X 7 inches in length. Standard tensile 1 inch) and Charpy V-Notch impact specimens were machined from the keel block and were thereafter solution annealed about one hour at 1900F., air cooled and maraged at about 850F. for about 3 hours. The results of these tests are reported in Tables I and II.

TABLE I Percent Ni C-r Si Al Ti Fe 6 .0 6 .2 1.86 .019 0 .04 0 .07 Bal. 8 .0 8 .2 1.82 0 .008 0 .04 0 .07 Bel. 9.9 10.1 1.77 0.011 0.01 0.03 Bal. 9.0 11.3 1 .88 0.014 0.02 0.02 Bal. 8 .0 10 .1 1.80 0 .021 0 .04 0.07 Bal. 8 .8 11 .9 1.84 0 .012 0 .02 0.03 Bal. 8 .1 12 .1 1.75 0 .013 0 .02 0.03 Ba]. 7 .9 12 .0 0.92 0 .014 0 .02 0 .02 Bal. 8.0 12.1 2.32 0.008 0.03 0.06 Ba]. 7.9 12.2 2.77 0.025 0.02 0.04 Bal.

TABLE II Y.S.,* U.T.S E1ong., R.A., CVN, Alloy k.s 1 k.s 1 percent percent ft.-lbs.

"=0.2% offset.

Concerning the above data, the effect of either low nickel and/or chromium will be observed from a pcrusal of Alloys A and B. The relatively low impact strength of these alloys is in marked contrast to those in accordance with the invention as manifested by Alloys 1 through 8. Alloys 1 and 2 offered a particularly good combination of strength plus toughness. Thus, alloys containing from about 10 to 12 percent chromium, 8.5 to 10 percent nickel and about 1.8 to 2.3 percent silicon are exceptionally good.

Tests were also conducted in which Armco iron, ferrochromium and ferrosilicon were employed rather than the ultra pure electrolytic grades of iron, chromium and silicon used in connection with Table I.

These tests were performed in much the same manner as described in connection with the alloys of Table I, the compositions (Alloys 9 and 10 are within and C and D are without the invention) and results being given in Tables III and IV, respectively.

The data set forth in Tables III and IV indicate that the use of exceptionally pure materials is not necessary. Alloy C confirms that significantly lower levels of nickel (5 percent) subvert resistance to impact. Alloy 10 reflects that as the nickel content is increased above 9 percent in these less pure materials, impact resistance is not benefitted. The low strength of Alloy D is deemed attributable to retained austenite. From overall considerations it is preferred to maintain the nickel content at not more than 9 percent regardless of silicon content.

A number of alloys given in Table l were also exposed to corrosion tests. In this regard, panels of the maraged steels (about 1 inch X 4 inches X %inch thick) were exposed to the well-known Salt Spray (Fog) Test in accordance with ASTM designation B1 17-61. Upon examination of the panels, they were re-tested in the more severe Copper-Accelerated Acetic Acid-salt Spray (CASS) test (ASTM designation B368-61T). Since both of these tests are generally well-known by those skilled in the art, a detailed description is omitted but is described in Specifications and Tests for Electro-deposited Metallic Coatings. ASTM Philadelphia 1961, 3rd ed. In any case, the results of the corrosion tests indicated that the low chromium Alloys A and B were much inferior, with variation in the silicon content seemingly having little effect.

Foundry characteristics were evaluated by pouring dry sand mold fluidity spirals at three different pouring temperatures, 2950F., 2850F. and 2775F. The steel used nominally contained 11.5 percent chromium, 8.5 percent nickel and 2 percent silicon, the balance being essentially iron. The resulting spiral lengths were 35 inches, 31 inches and 17 inches,-respectively. Each of the spirals exhibited good mold filling capability. The freezing temperature of the steel was approximately 2600F. Accordingly, in view of the pouring temperature and degree of super-heat, the fluidity measurements indicated that the castability of the subject steels would be at least as good as if perhaps not better than standard cast CF-8 stainless steel.

As indicated above herein manganese exerts a detrimental influence with regard to steels of the present invention. This is illustrated by the impact data given in .the following Table V.

TABLE V Percent CVN, Alloy Ni Cr Si 0 Al Ti Mn ft. lbs.

The cast steels of the present invention can be utilized for such applications as wearing rings, compressor wheels, corrosion resistant gears, high pressure valves, propellers, components for power plant pumps, including impellers, stage pieces, diffusers, etc., and for applications generally requiring steels which manifest a good combination of corrosion resistance, strength and toughness.

While the steels above described have been set forth solely in connection with applications as cast steels they also are useful in the wrought form. In this connection, if used in the wrought condition, the nickel content can be lowered to 5.5 percent or even 5 percent and the chromium content can be extended up to 15 percent, the sum of the chromium and nickel being less than 22 percent. This obtains over the full silicon range.

It is to be understood that the expressions balance or balance essentially used in referring to the iron content of the steels in accordance herewith, are not intended to exclude the presence of other elements, e.g., deoxidizing and cleansing constituents, and impurities normally associated therewith, in small amounts which do not adversely affect the basic characteristics of the subject steels.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A maraging stainless steel in the cast and martensitic condition consisting essentially of from about percent to about 12.5 percent chromium, about 7.5 percent to about 11 percent nickel, the sum of the chromium plus nickel being at least about 18 percent but less than 22 percent, about 1.5 percent to about 3 percent silicon, metal from the group consisting of titanium and aluminum in a small but effective amount sufficient to enhance the toughness of the steel, the titanium and aluminum not exceeding about 0.2 percent each, up to about 1 percent manganese, up to 0.05 percent carbon and the balance iron.

2. An alloy in accordance with claim 1 containing from about 11 percent to about 12 percent chromium.

3. An alloy in accordance with claim 1 containing from about 8 percent to about 10 percent nickel.

4. An alloy in accordance with claim 1 in which the sum of the nickel plus chromium does not exceed 21.5

to 2.5 percent silicon.

6. An alloy in accordance with claim 1 in which aluminum is present in an amount from about 0.01 percent to about 0.1 percent.

7. An alloy in accordance with claim 1 in which titanium is present in an amount of from about 0.01 to 0.1 percent.

8. An alloy in accordance with claim 1 containing about 0.02 percent to about 0.07 percent each of titanium and aluminum.

9. An alloy in accordance with claim 1 containing about 1 1 percent to about 12 percent chromium, about 8.5 to 9.5 percent nickel, the chromium plus nickel being from about 20 to 21.5 percent, about 1.8 percent to about 2.3 percent silicon, up to about 0.5 percent manganese, up to 0.03 percent carbon, and 0.01 to 0.1 percent of titanium and aluminum.

10. An alloy in accordance with claim 9 containing 0.02 to 0.07 percent of both titanium and aluminum.

11. An alloy in accordance wth claim 4 in which the chromium plus nickel is at least 19 percent.

12. An alloy in accordance with claim 11 containing up to 0.03 percent carbon.

13. A maraging steel in the martensitic condition and consisting essentially of about 10 percent to about 15 percent chromium, from 5 to 11 percent nickel, the sum of the chromium plus nickel being at least 18 percent but less than 22 percent, about 0.5 percent to about 3 percent silicon, metal from the groupconsisting of titanium and aluminum in a small but effective amount sufficient to enhance the toughness of the steel, the titanium and aluminum not exceeding about 0.2 percent each, up to about 1 percent manganese, up to 0.5 percent carbon and the balance iron.

14. An alloy in accordance with claim 13 containing 11 to 15 percent chromium, 7.5 to 9.5 percent nickel, 1.8 to 2.3 percent silicon, metal from the group consisting of aluminum up to 0.1 percent and titanium up to 0.1 percent, up to 0.5 percent manganese, and up to 0.03 percent carbon.

15. A cast maraging steel in the martensitic condition and consisting essentially of about 10 percent to about 15 percent chromium, from 5 to l 1 percent nickel, the sum of the chromium plus nickel being at least 18 percent but less than 22 percent, about 0.5 percent to about 3 percent silicon, metal from the group consisting of titanium and aluminum in a small but effective amount sufficient to enhance the toughness of the steel, the titanium and aluminum not exceeding about 0.2 percent each, up to about 1 percent manganese, up to 0.5 percent carbon, and the balance iron.

16. A cast maraging steel in accordance with claim 15 containing from 6.5 percent to about 8 percent nickel and from 0.6 to 1 percent silicon.

17. A cast maraging steel in accordance with claim 16 containing 0.01 to 0.1 percent aluminum and 0.01

to 0.1 percent titanium.

Claims

2. An alloy in accordance with claim 1 containing from about 11 percent to about 12 percent chromium.
3. An alloy in accordance with claim 1 containing from about 8 percent to about 10 percent nickel.
4. An alloy in accordance with claim 1 in which the sum of the nickel plus chromium does not exceed 21.5 percent.
5. An alloy in accordance with claim 1 containing 1.6 to 2.5 percent silicon.
6. An alloy in accordance with claim 1 in which aluminum is present in an amount from about 0.01 percent to about 0.1 percent.
7. An alloy in accordance with claim 1 in which titanium is present in an amount of from about 0.01 to 0.1 percent.
8. An alloy in accordance with claim 1 containing about 0.02 percent to about 0.07 percent each of titanium and aluminum.
9. An alloy in accordance with claim 1 containing about 11 percent to about 12 percent chromium, about 8.5 to 9.5 percent nickel, the chromium plus nickel being from about 20 to 21.5 percent, about 1.8 percent to about 2.3 percent silicon, up to about 0.5 percent manganese, up to 0.03 percent carbon, and 0.01 to 0.1 percent of titanium and aluminum.
10. An alloy in accordance with claim 9 containing 0.02 to 0.07 percent of both titanium and aluminum.
11. An alloy in accordance wth claim 4 in which the chromium plus nickel is at least 19 percent.
12. An alloy in accordance with claim 11 containing up to 0.03 percent carbon.
13. A maraging steel in the martensitic condition and consisting essentially of about 10 percent to about 15 percent chromium, from 5 to 11 percent nickel, the sum of the chromium plus nickel being at least 18 percent but less than 22 percent, about 0.5 percent to about 3 percent silicon, metal from the group consisting of titanium and aluminum in a small but effective amount sufficient to enhance the toughness of the steel, the titanium and aluminum not exceeding about 0.2 percent each, up to about 1 percent manganese, up to 0.5 percent carbon and the balance iron.
14. An alloy in accordance with claim 13 containing 11 to 15 percent chromium, 7.5 to 9.5 percent nickel, 1.8 to 2.3 percent silicon, metal from the group consisting of aluminum up to 0.1 percent and titanium up to 0.1 percent, up to 0.5 percent manganese, and up to 0.03 percent carbon.
15. A cast maraging steel in the martensitic condition and consisting essentially of about 10 percent to about 15 percent chromium, from 5 to 11 percent nickel, the sum of the chromium plus nickel being at least 18 percent but less than 22 percent, about 0.5 percent to about 3 percent silicon, metal from the group consisting of titanium and aluminum in a small but effective amount sufficient to enhance the toughness of the steel, the titanium and aluminum not exceeding about 0.2 percent each, up to about 1 percent manganese, up to 0.5 percent carbon, and the balance iron.
16. A cast maraging steel in accordance with claim 15 containing from 6.5 percent to about 8 percent nickel and from 0.6 to 1 percent silicon.
17. A cast maraging steel in accordance with claim 16 containing 0.01 to 0.1 percent aluminum and 0.01 to 0.1 percent titanium.