US3816106A - Strong, corrosion resistant alloy - Google Patents

Abstract

Alloys containing special percentages of chromium, nickel, cobalt, carbon, iron, etc., are highly corrosion resistant and in the cold-worked condition, afford tensile strengths well above 150,000 psi.

Description

United States Patent 1191 Snape STRONG, CORROSION RESISTANT ALLOY [75] Inventor: Edwin Snape, Ridgewood, NJ.

[73] Assignee: The International Nickel Company,

Inc., New York, N.Y.

22 Filed: Aug. 25, 1972 [21] Appl. No.: 283,810

52 us. c1. 75/122, 75/128 B, 148/39 511 1111.01. C22c 39/00, C22C 39/20 [58] Field of Search 75/128 B, 122

[56] References Cited UNITED STATES PATENTS 2,793,948 5/1957 Wagner 75/128 June 11, 1974 6/1957 Angel 148/12 5/1970 Gibson 75/128 B Primary Examiner-Hyland Bizot Attorney, Agent, or Firm-Ewan C. MacQueen; Raymond J. Kenny ABSTRACT 4 Claims, No Drawings 1 STRONG, CORROSION RESISTANT ALLOY As is known, there are several base alloys capable of resisting, in various degrees, the ravages of corrosive attack. The conventional stainless steels, cupronickels and aluminum alloys might be cited as illustrative since they have found extensive commercial use in a variety of corrosive environments. Their acknowledged virtues notwithstanding, such materials have, nonetheless, been found wanting in respect of applications requiring high tensile strengths, e.g., upwards of 150,000 pounds per square inch (psi). Though the cupronickels and aluminum alloys are generally not hardenable to this strength plateau, many stainless steels can be sufficiently cold worked to impart such strength levels; however, this is too often accompanied by the onset of other difficulties.

On the other hand, innumerable high strength alloys on the market generally suffer from the inherent incapability of affording corrosion resistance characteristics of a magnitude comparable to those of, say, the stainless steels. And in terms of the present invention even the stainless steels are considerably less than outstanding in resisting to corrosive influence of such media as stagnant and low velocity seawater.

There are alloys commercially available, though seemingly few in number, which do afford the type of strength and corrosion resistant properties mentioned above. But insofar as 1 am aware, they are, comparatively speaking, quite costly and/or have given rise to workability difficulties.

In any-case, it has been found that by alloying such constituents as nickel, chromium, cobalt, magnesium, carbon, iron, etc., in carefully controlled percentages, alloys can be produced which are (a) markedly corrosion resistant to various media, including stagnant seawater and other chlorides, (b) hardenable to tensile strengths as high as 200,000 psi, and (c) amenable to both hot and cold working.

Generally speaking, alloys in accordance herewith contain (by weight) about 22 to 40 percent chromium, about to 25 0.04 percent nickel, about 12 to 30 percent cobalt, up to 0.2 percent magnesium, aluminum, to to 2.5 percent silicon, up to'3 manganese, and the balance essentially iron, the iron constituting at least 15 percent of the total composition.

In carrying the invention into practice and in striving for the optimum in terms of corrosion resistance, particularly against low velocity seawater, the chromium content should be at least 24 percent, e.g., 26 percent or more. It can be as low as 21 percent or possibly percent but at the sacrifice in corrosion resistance. Advantageously, it does not exceed 32 or 33 percent since hot working difl'ficulties can ensue, the higher chromium percentages tending to introduce or contribute to the formation of embrittling phases, notably sigma. A chromium range of 24 to 32 percent is quite satisfactory. I

Nickel contributes to achieving a desired microstructure. 1t greatly resists the tendency for hard martensite to form during processing, particularly during cold working. A range of 15 or 16 to 23 percent nickel is particularly beneficial.

To attain the tensile strengths contemplated herein, the alloys should be cold worked. The alloys are basically of an austenitic matrix, but by reason of cold working, relatively thin platelets, virtually parallel in arrangement, are formed and distributed throughout the austenitic matrix. These platelets appear to be much on the order of deformation twins." At least a small but efiective amount of these thin platelets should be present to enhance tensile strength, e.g., at least 2 or 3 percent by volume. In this connection, cobalt is deemed to greatly influence this platelet formation and is thus considered to play a significant role in the strenghtening process. A cobalt range of 17 to 23 or 25 percent is quite effective, although it can be as high as 30 percent as above indicated. As a practical matter, the improvements conferred at higher percentages do not appear to justify the added cost. Above 25 percent cobalt, say, 27 or 28 percent or more, it is con sidered that epsilon phase may be present.

Carbon by reason of its capacity to form carbides and thus potentially detract from corrosion resistance, should advantageously not exceed 0.04 percent, particularly in the more aggressive environments. A range of from 0.001 to 0.03 or 0.04 percent is preferred, although it may less desirably be up to 0.1 or 0.15 percent.

With regard to magnesium, it is considered beneficial in respect of hot workability, especially in respect of edge cracking. Experimental data have not shown such other constituents as calcium, titanium and aluminum to be as effective in this regard. A retained magnesium levelof from about 0.002 or 0.005 percent and up to 0.1 percent or possibly 0.15 percent is deemed quite desirable.

As to other elements, aluminum and titanium can be used for added strength or other purposes but excessive amounts should be avoided to minimize hot workability problems. A range of 0.02 to 1 or 2 percent of aluminum and from 0.01 to 0.4 percent titanium can be utilized. Up to 2.5 percent silicon can be incorporated for castings, but for wrought alloys it is of benefit that the silicon content not extend beyond about 1.5 or 2 percent, this to obviate promoting edge cracking or other working difficulties. Manganese need not exceed 1 or 1.5 percent and should be held to 0.8 percent or less. A range of 0.1 or 0.2 to 0.8 percent is preferred for both silicon and manganese.

Alloys containing from 26 to 33 percent chromium, 17 to 23 percent nickel, 17 to 23 percent cobalt, carbon in an amount up to 0.03 percent, up to 0.1 percent, e.g., 0.005 to 0.05 percent magnesium, up to 1.5 percent aluminum, up to 0.3 percent titanium, up to 1.5 percent, e.g., 0.2 to 0.8 percent, each of silicon and manganese, the balance being at least 20 or 25 percent iron, are deemed particularly advantageous.

In order to give those skilled in the art a better appreciation of the invention the following data are given.

Employing vacuum induction melting, an alloy containing 31 percent chromium, 21.1 percent nickel, 19.8 percent cobalt, 0.024 percent magnesium, 0.026 percent carbon, 0.36 percent silicon, 0.48 percent manganese, balance iron and impurities was prepared using electrolytic nickel, cobalt, manganese and iron, lowcarbon ferrochromium, and spectrographic carbon. The iron, nickel, cobalt and carbon were first charged and heated to 2,850F. The carbon boil was allowed to run to completion at which point the ferrochromium, manganese and silicon were added. The melt was held for about 2 mins. and then deoxidized with magnesium (added in the form of nickel-magnesium).

values were 153,000 psi, 184,600 psi, 14.5 percent and 65 percent, respectively.

For corrosion testing purposes, a sheet specimen in the cold worked and aged condition was immersed in 10 percent ferric chloride, a most aggressive corrodent, for about 72 hrs., the solution being maintained at room temperature. The test solution is often used to simulate long term alloy behavior in relatively stagnant seawater. Duplicate samples --were used and crevices .were intentionally induced. The weight loss (average) was about 0.702 gram. This is in marked contrast to what would be expected of A181 316, a well known crevice corrosion resistant alloy. Experience has shown that the weight loss would be three times or more as high for A151 316. Moreover, the crevice corrosion of the alloy within the invention is superior in comparison with AlSl 316.

Air melting as well as vacuum processing can be used in production of the alloys. In this connection an alloy containing 32 percent chromium, 19.1 percent nickel, 18.6 percent cobalt, 0.019 percent carbon, 0.39 percent silicon, 0.51 percent manganese, and small amounts of magnesium and calcium (melt was oxidized with Ni-Mg and Ca-Si) balance iron and impurities, was produced by air melting and then processed to bar. A cold rolled 0.375 inch specimen gave a Y.S. of approximately 170,000 psi, an U.T.S. of 198,000 psi, an elongation of 9 percent and a reduction in area of 38.5 percent. The specimen underwent a loss of 0.662 gram in the above-described Fe C1 test. After aging at 900F the corresponding tensile properties were as follows: 197,900 psi, 214.4 psi, 6 and 29 percent, respectively. In the aged condition, the corrosion loss was lower, being approximately 0.344 gram.

In terms of general processing conditions, forging and hot rolling practices can be conducted over the temperature range of 2,050 to 2,350F. Temperatures below 2,000F may result in edge cracking. l-lot rolled material is preferably annealed at 2,100F or above prior to cold working.

The amount of cold working applied will, of course, depend on composition and the strength level desired. Generally speaking, sufficient cold work should be used to induce the formation of at least about percent, e.g., percent or more, by volume of the thin platelets above described. Not more than 25 to 40 percent of the platelets need be present. Aging, when used, should be carried out with the temperature span of 800 to l,000F.

Alloys contemplated within the subject invention are useful as fasteners particularly in connection with oxidizing chloric solutions and marine environments. Other marinehardware would include parts for pumps,

.valves, shafting, shocks, cleats, wrought fittings, etc.

The alloys can also be used in chemical plant equipmerit and for such diverse utility as aerospace, desalinization and undersea mining applications. Also, they can be produced and used in conventional mill forms, including sheet, strip, bar and rod.

In referring to the iron content of the alloys as constituting the balance or balance essentially, it is to be understood, as will be appreciated by those skilled in the art, that the presence of other elements is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing constituents, and impurities normally associated therewith in small amounts that do not adversely affect the basic characteristics of the alloys. Non essential elements that can be present include up to 2 percent each of copper and zirconium, up to 0.05 percent boron and up to 0.05 percent selenium.

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 un derstand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

1 claim: I

l. A corrosion resistant alloy characterized by a microstructure comprised of an austenitic matrix throughout which thin platelets are dispersed, the platelets being substantially parallel and present in an effective amount of at least about 5 percent by volume sufficient to' impart enhanced tensile strength to the matrix, said alloy consisting essentially of from 22 to 33 percent chromium, about 10 percent to 23 percent nickel, about 12 percent to 23 percent cobalt, up to about 0.04 percent carbon, up to about 0.2 percent magnesium, up to less than 0.5 percent titanium, up to about 3 percent aluminum, up to 2.5 percent silicon, up to 3 percent manganese, and the balance essentially iron, the iron constituting at least 15 percent of the alloy.

2. A corrosion resistant alloy characterized by a microstructure comprised of an austenitic matrix throughout which thin platelets are dispersed, the platelets being present at least in a small but effective amount sufficient to impart enhanced tensile strength to the matrix, said alloy consisting essentially of about 26 to 33 percent chromium, about 17 to 23 percent nickel, about 17 to 23 percent cobalt, about 0.001 percent to about 0.03 percent carbon, up to 0.1 percent magnesium, up to 0.4 percent titanium, up to 1.5 percent aluminum, up to 0.8 percent silicon, up to'about 0.8 percent manganese and the balance being essentially iron, the iron constituting at least 15 percent of the alloy.

3. A corrosion resistant alloy characterized by a microstructure comprised of an austenitic matrix throughout which thin platelets are dispersed, the platelets being present at least in a small but effective amount sufiicientto impart enhanced tensile strength to the matrix, said alloy consisting essentially of from 22 to 40 percent chromium, about '10 to 25 percent nickel, about 12 to 30 percent cobalt, up to about 0.04 percent carbon, about 0.002 percent to about 0.15 percent magnesium, up to less than 0.5 percent titanium, up to about 3 percent aluminum, up to 2.5 percent silicon, up to 3 percent manganese,'and the balance essentially iron, the iron constituting at least 15 percent of the alloy.

6 to about 0.8 manganese, and the balance essentially iron, the iron constituting at least 15 percent of the alloy.

Claims (3)

1. 2. A corrosion resistant alloy characterized by a microstructure comprised of an austenitic matrix throughout which thin platelets are dispersed, the platelets being present at least in a small but effective amount sufficient to impart enhanced tensile strength to the matrix, said alloy consisting essentially of about 26 to 33 percent chromium, about 17 to 23 percent nickel, about 17 to 23 percent cobalt, about 0.001 percent to about 0.03 percent carbon, up to 0.1 percent magnesium, up to 0.4 percent titanium, up to 1.5 percent aluminum, up to 0.8 percent silicon, up to about 0.8 percent manganese and the balance being essentially iron, the iron constituting at least 15 percent of the alloy.
2. 3. A corrosion resistant alloy characterized by a microstructure comprised of an austenitic matrix throughout which thin platelets are dispersed, the platelets being present at least in a small but effective amount sufficient to impart enhanced tensile strength to the matrix, said alloy consisting essentially of from 22 to 40 percent chromium, about 10 to 25 percent nickel, about 12 to 30 percent cobalt, up to about 0.04 percent carbon, about 0.002 percent to about 0.15 percent magnesium, up to less than 0.5 percent titanium, up to about 3 percent aluminum, up to 2.5 percent silicon, up to 3 percent manganese, and the balance essentially iron, the iron constituting at least 15 percent of the alloy.
3. 4. A corrosion resistant alloy consisting essentially of about 24 to 33 percent chromium, about 16 to 23 percent nickel, about 17 to 23 percent cobalt, about 0.001 to 0.03 percent carbon, up to 0.4 percent titanium, up to 1.5 percent aluminum, up to 0.8 percent silicon, up to about 0.8 manganese, and the balance essentially iron, the iron constituting at least 15 percent of the alloy.