US3285738A - Ferrous-base, hardenable, corrosion-resistant, high-strength, high-ductility alloy - Google Patents

Description

United States Patent 3 285,738 FERROUS-BASE, HARDENABLE, CORROSION- RESISTANT, HIGH-STRENGTH, HIGH-DUC- TEITY ALLOY Thomas E. Johnson, Milwaukee, Wis., assignor to Stainless Foundry & Engineering, Inc., Milwaukee, Wis., a corporation of Wisconsin No Drawing. Filed May 22, 1964, Ser. No. 369,625

3 Claims. (Ci. 75-128) This invention relates to a copper-free, ferrous-base, hardenable, corrosion-resistant, high-strength, high-ductility alloy. More particularly, the invention relates to alloys of the stainless steel type composed primarily of iron, which are substantially free of copper, but which include a substantial amount of chromium and smaller amounts of other elements, including specific amounts of cobalt, which are in such relative proportions as to impart high corrosion resistance, high strength and high ductility. The terms copper-free or substantially free of copper or similar terms used herein are intended to mean less than 0.50 percent by weight of copper.

The chemical processing, food processing, and pharmaceutical industries, in handling chemicals and solutions of corrosive materials, such as acids and alkalies, is beset by many difiicult requirements as to equipment and parts in contact with the corrosive environment. Such equipment and parts must be constructed of a metallic material having a high resistance to corrosion, and further, should corrosion occur, any metallic salts transmitted to the product by the decomposition of the metal must be such a nature that they will not be deleterious to the product. Trace contamination by metallic salts of the element copper is known to be undesirable in certain processing. For example, in the manufacturing of food products from vegetable oils, copper or copper-containing alloys are not generallyused because trace contamination of the food product by copper salt products of corrosion can result in noxious discoloration which renders the product worthless.

Equipment and component parts, such as pumps, impellers, shafts, valves, pipes, bearings, pipe fittings, vessels, tanks, and the like, must be manufactured of a metal possessing adequate corrosion resistance but other desirable properties should be provided by this corrosion resistant material. Such properties of strength, hardness in conjunction with ductility, machinability, and abrasion resistance are highly desirable. Economy in the design of equipment and a reduction in the weight of equipment can be realized through utilization of high strength alloys. Abrasion resistance is provided by alloys possessing a combination of high hardness, high ductility, and high yield strength.

Austenitic chromium nickel stainless steels such as the 18% chromium, 8% nickel grade, Type 304 or the 18% chromium, 12% nickel, 2% molybdenum grade, Type 316, are extensively used in the manufacture of equipment for the chemical, food, and pharmaceutical industries. These materials possess a high degree of ductility but are relatively soft and are deficient in hardness and strength for many applications.

The element copper is frequently used in the formulation of specialty grades of stainless steels possessing both high hardness and high strength. The strengthening of these specialty grades of stainless steel is accomplished 3,285,738 Patented Nov. 15, 1966 by thermal treatment which results in the strengthening of the alloy matrix through a phenomenon known to the art as precipitation-hardening. The precipitated phase in certain of these alloys has been identified as a copper rich phase, possibly pure copper. When such copper containing alloys are exposed to corrosive solutions the possibility of product contamination by copper compounds exists, as minute particles of a copper-rich material are exposed to the corrosive environment. 7

It is the object of this invention to provide hardenable iron-base alloys in which chromium, nickel molybdenum, and cobalt are used in relative amounts to obtain optimum combination of properties including a high degree of corrosion resistance, high strength and hardness, and a high degree of ductility.

Another object of this invention is to provide hardenable iron-base corrosion resistant alloys containing definite related proportions of chromium, nickel, and molybdenum, with cobalt therein in selected and related amounts to provide a high degree of ductility together with high mechanical strength.

Still another object of this invention is to provide a hardenable iron base alloy containing little or no copper, to provide an alloy which may be utilized in corrosionresisting environments where trace contamination of copper is detrimental.

A further object of this invention is to provide an iron base corrosion resistant alloy which can be softened by thermal treatment to render it readily machinable and one which can subsequently be hardened by thermal treatment to provide a combination of desirable properties including good corrosion resistance, high strength, high ductility, and high hardness Another object of this invention is to provide a cor rosion resistant iron base alloy of high strength and high hardness which has low brittleness and high ductility as compared to prior art alloys, and can be fabricated by casting, forging, machining and welding into implements used to resist corrosion.

Various other objects will be apparent from the following description and claims.

In accordance with the present invention I have discovered that additions of cobalt in certain high chromium steels, without the addition of copper, results in steels which have very high strength and are ductile. The resultant alloy is capable of being strengthened by a precipitation-hardening heat treatment. One advantage of my alloy over existing commercial alloys is that it is essentially a copper-free material. Prior art commercially available alloys, such as 17-4PH, CD4MCu and V2B employ copper to gain precipitation-hardening properties. In some industries, such as the food or pharmaceutical industries, copper-bearing alloys have limited application. Furthermore, copper makes such alloys prone to cracking in heat treatment, especially when heavy sections of such alloys are hardened. The use of specific amounts of cobalt, according to this invention, in combination with related proportions of chromium, nickel and molybdenum in an iron-base alloy provides an alloy which can be readily fabricated by the usual casting, forging and welding operations, which can be softened by thermal treatment to render it readily machinable and which can be hardened subsequently by thermal treatment to provide a combination of desirable properties including good cor- In accordance with this invention, I have found. that the alloy should have the following composition:

range are more ductile than existing alloys at a given strength level.

Table II Analysis, Weight Percent Corrosion Test Data Type Heat No. Salt Spray (1) Boiling 65% Nitric Car- 8111- Manga- Chro- Nickel Molyb- Cop- Cobalt (Mg, Weight Vinegar (2) Ac bon eon nese mium denum per Increase) (Rate, (Rate.

-p-y ay) 31688 B-660 .09 1.48 .37 19 .92 9.90 2.00 0.22 Nil CD4MCu T-942- .04 .43 .61 26.00 6.04 2.15 3.40 Nil Copper Free E-069. .07 .52 1.13 25.30 5.50 2.30 0.08 0.95

Copper Free E-070- .07 .51 .61 24.96 5.50 2.30 0.10 5.08

Copper Free. 13-071-.- .08 .52 .65 25.44 5.58 2.30 0.06 8.08

Copper Free 15-072.... .07 .52 .64 25.90 5.50 2.30 0.06 12.00

In accordance with this invention, I have found that the alloy should have the following composition:

PERCENT BY WEIGHT 1 Balance, except for residual elements and impurities in amounts up to .050%.

The following table designated Table I gives examples of heats of alloys within the series covered by this inven- Corrosion tests were made on heats of alloys coming within this present invention and an alloy 31658, a substantially copper-free and cobalt-free alloy, as well as on the copper-containing alloy CD4MCu. Table 11 above is a tabulation of the results of these tests.

To obtain the data in Table II, three basic tests were involved:

(1) A 96-hour salt spray test as described in the American Society for Testing Materials Standard No. B-117. In this test all materials tested exhibited a slight gain in weight indicating the build up of a passive corrosion resistant film. The copper free alloys of this invention were equal to the Type 316 stainless steel and copper bearing CD4MCu alloy tested. There was no evident corrosion of any of the materials tested. This test series demonstrates the ability of the alloy of this invention to withstand corrosion of salt water and salt water atmospheres.

tion which are substantially copper-free. These alloys 40 (2) A 48-hour test in boiling vinegar indicates the alloy are compared in the table with a copper-free prior art of this invention to be equal to Type 316 stainless steel alloy designated HC, with a substantially copper-free and the copper bearing CD4MCu alloy. No measurable alloy that does not contain cobalt designated B868, and corrosion was encountered in any of the alloys of this also with a copper-containing alloy designated CD4MCu. invention which were tested. This test series demon- Table I B868 E069 B867 E070 D713 E493 E071 E072 Type HG OD4MCu Chemical Analysis, Percent:

Carbon 0. 11 0. 07 0. 06 0. 07 0.08 0. s 0. 0s 0. 07 1 0. 1 .04 Chr0mium 24.74 25.30 25.34 24.96 26.29 25.46 25.44 25.90 26-30 25.27 Nickel 6.14 5. 50 6.00 5. 50 5. 86 5. 40 5. 53 5. 50 1 4. 0 4. 75-6. 0 Molybdenum 2. 50 2. 30 2. 2. 2.25 2.15 2.30 2. 30 1.75 2. 25 Cobalt Nil 0. 95 4. 82 5. 03 5.12 7. 23 3. 03 12. 00 Silicon 0. 0. 52 0. 40 0. 51 0. 24 0. 52 0. 52 0. 52 1 2. 0 1 1. 0 Manganese 0. 1. 13 0. 53 0. 61 0. 41 0. 46 0. 65 0. 64 1 1. 0 Copper 0. 23 0.08 0. 32 0. 10 0.12 0.10 0.06 0. 06 2. -3. 25 Mechanical Properties:

Solution Heat Treated (Note 1):

Yield strength, p.s.i 78, 200 76,200 75, 000 72, 000 68, 500 70, 000 66, 62, 800 76, 000 50-90, 000 Tensile strength, .s.i 104,400 99,100 103,100 99, 900 98, 100 99, 300 99, 100 100,700 110,000 100-115, 000 Percent Elongation in 22. 0 36. 0 33. 0 35. 5 34. 0 37. 0 41. 0 41. 0 10 20-30 Brinell Hardness 212 212 212 217 207 217 201 197 223 250-270 Solutioned and Aged (Note 2):

Yield strength, p.s.i 90,000 90, 000 95,200 83,000 86,500 89,500 79,500 74,000 80,000 1012120, 000 Tensile strength, p.s.i 122, 000 116,000 131, 500 119, 500 123,500 125,500 120,000 120,000 115, 000 -145, 000 Percent Elongation in 2" 1e. 5 23. 5 27. 5 29. 5 31. 31. 0 36.0 23. 5 10-25 Brinell Hardness 255 255 255 255 255 241 255 241 29(2320 Max. News:

1 Solution heat treated by holding at 1,900" F. to 2,050 F. for one hour and cooling rapidly by quenching in Water.

2 Aged by thermal treatment at 850 F. for (4) hours.

It will be apparent from a study of the table that the following facts are emphasized:

(1) That the alloy of this invention is substantially copper-free and is less than the allowable maximum of 0.50 percent.

(2) That the addition of cobalt within the range previously set forth imparts age-hardenable properties and increased ductility not obtainable in alloys free of cobalt.

(3) That alloys containing cobalt within the required strates the ability of this alloy to withstand the corrosion of a typical severe environment encountered in the food industry.

(3) A 48-hour test in 65% boiling nitric acid per the American Society for Testing Materials Standard No. A262 indicates the new copper-free alloy of this invention to be substantially better in this media than Type 316 stainless and equal to CD4MCu. This is an example of an industrial environment.

(4) Corrosion ratesrnills per year.

In an experiment to demonstrate that the solution heat treatment, during age-hardening, of this corrosion resistant and mechanically strong alloy, would not produce quench-cracking, test castings having two 3" X 2%" and 2 /2" block sections connected by a A" thick Web were made. These castings from heat No. E-493, were water quenched from 2050 F. and subsequently age-hardened at 900 F. The samples were then sectioned and examined for cracking by utilizing a dye-check inspection process. No cracking was evident. Thus, the copperfree alloy of this invention is resistant to quench cracking and thermal shock. Apparently the permissible small copper content, that is, less than 0.50%, remains in solution and is not precipitated upon heat treatment.

It will be apparent that this invention provides an alloy of the stainless steel type which is corrosion resistant, of high-strength and high-ductility. By the use of specific amounts of cobalt, in combination with substantial amounts of chromium and smaller amounts of other elements, it is possible to provide a substantially copperfree alloy which can be heat treated and age-hardened to provide desirable mechanical properties.

Although some of the advantages of this alloy have been discussed above, others will be apparent.

Having thus described this invention, what is claimed 1. An alloy in percent by weight consisting essentially of:

Carbon 0.03- 0.12

Chromium 24.00-28.00 Nickel 4.00- 6.50

Cobalt 0.95-12.00 Molybdenum 1.75- 3.00

"-"'. T.-""T"T'T'TT"- T-? TC'-'7- TT Manganese Copper Iron, balance and incidental impurities.

2. An alloy in percent by weight consisting essentially Iron, balance and incidental impurities.

3. An alloy in percent by weight consisting essentially of:

Carbon 0.06 Chromium 26.00

Nickel 5.50 Cobalt 6.50 Molybdenum 2.25 Silicon 0.50 Manganese 0.50 Copper Nil Iron, balance and incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 2,306,622 12/ 1942 Krivobok 128 2,518,715 8/1950 Payson 75128 X DAVID L. RECK, Primary Examiner. P. WEINSTEIN, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,285,738 November 15, 1966 Thomas E. Johnson It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, lines 1 and 2, strike out "In accordance with this invention, I have found that the alloy should have the following composition:" and insert instead rosion resistance, high strength, high ductility and high hardness.

Signed and sealed this 19th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD]. BRENNER Commissioner of Patents

Claims (1)

1. AN ALLOY IN PERCENT BY WEIGHT CONSISTING ESSENTIALLY OF: