US3396013A - Beryllium-containing maraging steel - Google Patents

Description

United States Patent 3,396,013 BERYLlLlUM-CONTAININ G MARA-GIN G STEEL John R. Mihalisin, North Caldwell, N.J., assignor to The International Nickel Company, Inc., New York, N .Y., a

corporation of Delaware Filed Mar. 21, 1966, Ser. No. 535,849 7 Claims. (Cl. 75-123) ABSTRACT OF THE DISCLOSURE A ferrous-base alloy containing at least nickel, cobalt, molybdenum and beryllium, the alloy being characterized by high strength and toughness.

The present invention relates to ferrous-base alloys and more particularly to a novel beryllium-containing, maraging steel characterized by good toughness at high levels of tensile strength.

As is generally recognized by ferrous metallurgists, the unique combination of properties characteristic of the maraging steels introduced but a few years ago added a new dimension to research and commercial activity in respect of iron-base alloys. Generally described in the US. patents to Bieber, No. 3,093,518, and to Decker, Goldman and Eash, No. 3,093,519, these steels obviated many disadvantages inherent in prior art steels while affording the highest combination of toughness and strength thereto-fore known without necessity of recourse to complicated or costly processing techniques, such as the severe plastic deformation experienced in connection with ausformin g.

Since the inception of the maraging steels, various constituents have been used or suggested for various purposes, e.g., to augment hardness and strength. Prominent among such elements have been titanium and aluminum which, indeed, contribute hardening and strengthening qualities but concomitantly lower toughness. In US. Patent No. 3,093,519 a host of other supplemental hardeners were proposed among which is beryllium. While there is no specific steel in the said patent containing beryllium, reports elsewhere reflect that when added to steels of the maraging type, beryllium confers hardness and strength but markedly adversely affects toughness. In Some Observations on the Strength and Toughness of Maraging Steels by S. Floreen and G. R. Speich, Trans. of the ASM, vol. 57, 1964, pages 714-726, it is shown that beryllium in amounts of 0.15% and 0.34% when added to nickel-cobalt, molybdenum-free maraging steels detracted from toughness. (The hardening eifect of small amounts of beryllium has also been confirmed in high nickel maraging steels, Hardening Behavoir of Ternary Alloys Based on Iron-18% Nickel, by S. Floreen, Trans. of the ASM, vol. 57, 1964, pages 38-47.)

It has now been discovered, however, that with special and quite restricted amounts of beryllium, not only can high strength and hardness levels be attained but a good level of toughness can be achieved as well.

It is an object of the invention to provide berylliumcontaining, maraging steels characterized by a satisfactory combination of strength and toughness.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGURES 1 through depict curves representing the effect of beryllium on various mechanical properties.

Generally speaking, the present invention contemplates 3,395,013 Patented Aug. 6, 1968 maraging steels of the following composition, based on weight percent: about 6% to 20% nickel, about 1% to 15% molybdenum, about 2% to 20% cobalt, from 0.01% to 0.09% beryllium and most advantageously not more than 0.05% or 0.06% beryllium, up to 0.05% carbon, up to 0.5% aluminum, up to 0.5% manganese, up to 0.5% silicon, and the balance essentially iron. Titanium can be present in an amount up to 1% but is not essential. Thus, where it might be desirable to use only very small amounts of titanium or to avoid the use thereof entirely, say, in applications requiring steels of heavy section, this can be readily accomplished in accordance herewith. In this connection, it has been found that to increase the yield strength of maraging steels otherwise contemplated herein by a predetermined amount requires a titanium content approximately six to seven times the required amount of beryllium.

As will be readily appreciated by those skilled in the art, the presence of other elements is not excluded from the subject steels, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith, in small amounts which do not adversely affect the basic characteristics of the steels. Elements such as lead, tin, arsenic, antimony, sulfur, phosphorus, bismuth, hydrogen, oxygen, nitrogen and the like should be kept as low as is consistent with good commercial steelmaking practice. Various supplemental elements can be present within the following ranges: up to 2% tantalum, up to 2% columbium, up to 2% vanadium, up to 4% copper, up to 0.1% boron and up to 0.25% zirconium, the total sum of these supplemental constituents not exceeding about 7%. For optimum toughness, boron and zirconium should be less than 0.01% and 0.1%, respectively. In addition, up to 5% chromium can be employed and the molybdenum can be replaced in whole or in part by an equal atomic percentage of tungsten; however, molybdenum is more advantageous since it contributes to improved forgeability characteristics. Calcium and/or magnesium can be used in accordance with good deoxidation practice. When the steels are produced by air melting, calcium can be used to advantage since it fixes various elements, such as sulfur.

In carrying the invention into practice, the steels should be prepared using materials of relatively high purity and/ or selected scrap as the basic melting charge. While air melting techniques can be used, vacuum melting is preferred for optimum notch strength and resistance to impact. Ingots obtained upon solidification should be rather thoroughly homogenized by soaking at temperatures of about 2100 F. to 2300" F. followed by hot working and, if desired, cold Worked to desired size. 'Prior to aging, the steels should be solution annealed at temperatures of from about 1400 F. to about 1600 F., although temperatures up to 2000 F. can be used particularly when the amount of molybdenum is at the higher end of its range. While an annealing treatment is not mandatory, it provides greater assurance of obtaining reproducible properties.

Following the solution anneal, the steels are cooled to achieve a martensitic condition, i.e., a transformation from austenite to martensite. It is important that a martensitic structure be obtained prior to aging in order to achieve the high strength levels characteristic of the steels. As used herein, the term martensite .(or martensitic) refers to steels having a structure of martensite (or substantially martensitic) in both the solution annealed condition and in the aged condition. The transformation from austenite to martensite is normally accomplished by cooling through the M M, transformation range to room temperature (from the annealing temperature). If it is desired to be assured of attaining as complete a transformation as possible, cold treatments can be utilized; for example, the steels can be refrigerated as by cooling down to temperatures of about minus 100 F. or lower. Cold working (with or without refrigeration) prior to aging can also be used to effect the completion of transformation to martensite.

The aging treatment should be conducted over the temperature range of about 800 F. to about 1000 F. for from one-half hour to about 50 hours, the longer periods being used with the lower temperatures. Aging from 850 F. to 950 F. for about one to 24 hours, e.g., three hours at 900 F., is beneficial. Aging above 1000" F. should be avoided; otherwise, retention of undesirable austenite can result.

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

Using air melting practice, various alloy compositions were prepared, the chemistry of which is set forth in Table I. The ingots formed were forged to inch bar with part of each ingot being unidirectionally rolled to /3 inch plate. This was done to obtain toughness data in respect of steel plate as distinguished from other mill forms since it is well known that the mechanical properties of plate, particularly data taken from the direction transverse to the last rolling direction (transverse orientation), is usually well below that obtained on, say, bar or rod. Put another way, notch tensile and impact properties, for example, are at their worst on transverse plate specimens since directionality effects would be expected to be at a maximum (the steels being unidirectionally rolled). Thus, smooth bar tensile properties were obtained on bar stock and notch tensile and impact properties were obtained from transverse specimens taken from the plate material. All specimens were solution treated at 1500 F. for one hour and aged for three hours at 900 F. prior to test.

it would be naturally expected that an established hardener wouldimpair toughness. And, this rule is opposite herein if the amount of beryllium is not carefully controlled. This is clearly illustrated by Alloy No. 4 which contained 0.14% beryllium and which is outside the invention. This alloy manifested, comparatively speaking, a notably inferior capability to absorb impact energy. It is worthy of mention to point out that ultimate tensile strength obtained on bar stock was continually on the upsurge whereas the notch tensile strength obtained on the plate (transverse) was starting to decline in respect of Alloy No. 3, an alloy containing but 0.057% beryllium. For an optimum combination of strength and toughness, the beryllium should not exceed 0.05% or 0.06%.

In achieving an optimum combination of strength, ductility and toughness, it is most advantageous that the following ranges be observed: about 15 to 19%, e.g., 17% to 19%, nickel; about 4% to 8%, e.g., about 4.5% to 5.5%, molybdenum; about 5% to 15%, e.g., 7% to 8.5%, cobalt; from 0.02% to 0.05% or 0.06% beryllium; up to 0.5%, e.g., up to 0.1% or 0.2%, titanium; up to 0.2%, e.g., 0.05 to 0.2%, aluminum; up to 0.03%, e.g., 0.01% to 0.03%, carbon; up to 0.25% manganese; up to 0.25 silicon; and the balance essentially iron. A most suitable steel contains about 18% nickel, 5% molybdenum, 8% cobalt, 0.05% beryllium, 0.015% aluminum, and 0.02% carbon.

The instant invention is particularly useful in applications requiring combinations of high strength and toughness. Illustrative uses include fasteners such as bolts and the like, pressure vessels, wrenches, tools, machine parts to be subjected to stress, bearing components, etc. The steels can be produced in a variety of forms, including plate, bar, rod, sheet, castings, etc.

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.

TABLE I Chemical Composition Alloy No. Be, Ni, Co, Mo, Al, C, Si, Mn,

percent percent percent percent percent percent percent percent Calcium used as deoxidizer; balance essentially iron.

In Table II, the ultimate tensile strength (U.T.S.), I claim:

0.2% yield strength (Y.S.) and notch tensile strength (N.T.S.) are given in thousands of pounds per square inch (k.s.i.), the tensile ductility (elongation) and reduction in area values are given in percent (EL, percent and R.A., percent, respectively) and the Charpy V-notch values (C.V.N.) are given in foot-pounds (ft.-lbs.).

From the data in Table II, which is also graphically depicted in FIGURES 1 through 5, it will be noted that the addition of up to about 0.03% beryllium (Alloy No. 2) results not only in an increase in strength but also toughness. Their behavior is deemed quite unusual for 1. A maraging steel consisting essentially of about 6% to 20% nickel, about 1% to 15% molybdenum, about 2% to 20% cobalt, about 0.01% to 0.09% beryllium, up to 0.05 carbon, up to 0.5% aluminum, up to 1% titanium, up to 0.5 manganese, up to 0.5 silicon, up to 2% tantalum, up to 2% columbium, up to 2% vanadium, up to 4% copper, up to 0.1% boron, up to 0.25% zirconium, the sum of the tantalum, columbium, vanadium, copper, boron and zirconium not exceeding 7%, up to 5% chromium, and the balance essentially iron.

2. The steel as set forth in claim 1 in which the beryllium content does not exceed 0.06%.

3. The steel as set forth in claim 1 in which the molybdenum is replaced in whole or in part by an equivalent atomic percentage of tungsten.

4. The steel as set forth in claim 1 and containing about 15 to 19% nickel, about 4% to 8% molybdenum, about 5% to 15% cobalt, about 0.02% to 0.06% beryllium, up to 0.03% carbon, up to 0.5% titanium, up to 0.2% aluminum, up to 0.25 manganese, up to 0.25% silicon, and the balance essentially iron.

5 6 5. The steel as set forth in claim 1 and containing about References Cited 17% i0 19% nickel, about 4.5% to 5.5% molybdenum, NITED STATE PATENTS about 7% to 8.5% cobalt, about 0.02% to 0.05% beryl- U S liu'm, about 0.01% to 0.03% carbon, about 0.05% to 3,093,519 6/1963 Decker a1 148-31 0.2% aluminum, up to 0.2% titanium, up to 0.25% man- 5 g; g g ig t .25 '1 ,-dth bl t'll up 0 O S1 an e aance essenla y 3,243,285 3/1966 Fragetta 75-423 6. The steel as set forth in claim 4 and containing less than 0.01% boron and less than 0.1% zirconium.

7. The steel as set forth in claim 5 and containing less 10 than 0.01% boron and less than 0.1% zirconium.

HYLAND BIZOT, Primary Examiner.

(5/69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- 3.396,0l3 Dated August 6 1968 Invencol-(a) John Raymond Mihalisin It is certified that error appears in the aboveidentifiad patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 2, for "opposite" read --apposite--.

SIUNED AM FIE MED (SEAL Anon:

EdwardlLFletchu, I

. mm a. mm, .m. 0mm Commissioner or Eatents

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