US2523838A - Metal alloy - Google Patents

Description

Patented Sept. 26, 1950 UNITED STAT TNT OFFICE METAL ALLOY Vincent T. Malcolm, Indian Orchard, Mass, as-

signor to The Chapman Valve Manufacturing Company, Indian Orchard, Mass, a corporation of Massachusetts No Drawing. Application May 13, 1949,

' Serial No. 93,190

processing equipment in the oil refinery and chemical industries are well known to design and operating engineers in these industries. Due to this fact, the oil and chemical industries are constantly searching for new materials which are tion of the product, and other damages causedby equipment failure may far exceed specific losses, all to the end that the problem of materials entering into the construction of high temperature corrosive equipment is basically an economic problem with the lowest overall cost so as to obtain the desired result.

The attack by acids on materials is one of the most important forms of corrosion especially if the acids are very hot, or boiling or are in the gaseous phase, and the prior art teaches that many ferrous alloys have been made with high nickel and high chromium contents in an attempt to meet these conditions of service.

These prior art alloys were fortified with numerous other elements most of which require certain drastic heat treatments such as heating to high temperature and quenching (as recognized by prior teachings) so that alloys may be stable in the service intended.

These particular alloys when welded into Many industries v fabricated structures, lose considerable of their corrosion resistance unless means is provided for properly heating and quenching in order to produce the necessary stability in the service intended. Many fabricated welded structures are large in dimension and/or weight, or must be welded in the field so that further heat treatment and quenching, so necessary according to the prior art, cannot be accomplished thereby resulting in reduced corrosion resistance and some loss of physical properties.

Most of the ferrous alloys of the prior art contain nickel in greater proportion than chromium, especially those alloys which were developed for resistance to boiling sulphuric acid.

My investigative and development work which was carried out with much effort and expense show, contrary to previous teachings, that the nickel content need not be greater than the chromium content. Therefore the composition of my alloy, which I am about to disclose, shows that the composition of nickel to chromium is in reverse to the generall accepted teachings, in fact being almost diametrically opposite.

My invention relates to a highly corrosion resistant ferrous alloy which has excellent strength at high temperature, is readily weldable without further heat treatment to retain corrosion resistance and is characterized by high chromium and nickel contents. The proportion of chromium to nickel being greater or at least of even content In order to obtain the properties claimed in my alloy, I use certain other element such as carbon, manganese, silicon, aluminum, copper, molybdenum, and columbium as. alloying elements, and such elements as sulphur and phosphorous are obtained in the alloy as impurities.

All of the said elements are important factors and cooperate in the production of the final stable alloy. The proportion of each of the added elements must be carefully adjusted so that they may be present in the final alloy in such a manner as to best resist the attack of corrosion, retain good physical properties at high temperature, and. be readily weldable in fabricated structures even in field welding without further heat treatment and without the loss of corrosion resistance or physical properties due to welding.

It is desired, in this disclosure, to refer to the part played by each of the elements used and to their proper proportions whereby the resulting final alloy of my invention is readily adaptable for most all services requiring excellent corrosion resistance especially in boiling acids or high temperature conditions where the corrosive media is in the gaseous or vapor phase. A description of the part played by all the Various elements, I believe, has not been previously disclosed by any prior art teachings.

The constituents and proportions thereof in one way are as follows:

Per cent Per cent Carbon .03 Copper 6.05 Manganese 1.12 Aluminum 1.04. Silicon .86 Columbium .54 Chromium 22.84 Sulphur .03 Nickel 21.64 Phosphorus .03 Molybdenum 3.30 Iron Balance The constituents of the alloy are referred to as follows:

Carbon The carbon content of the final alloy must be kept at a maximum of .0'7% in order to obtain the required corrosion resistant properties. Prior art teaching shows that even with this low carbon content there may be a certain amount of carbide precipitation to the grain boundaries causing intergranular attack unless the carbon has been dissolved by a drastic heat treatment and quench. However, in the present alloy, I use columbium in the prope proportion whereby the carbon is dissolved without drastic subsequent heat treatment. By the use of columbium I stop any intergranular attack caused by carbide precipitation to the grain boundaries. Columbium has been used before for certain austenitic alloys but never in alloys of the composition of my invention.

Should higher carbon be used in alloys of the type of my invention the carbides will accelerate corrosion because they are cathodic and the film that forms may be easily broken down by these cathodic areas. Therefore, in order to produce the best stability in my alloy, the carbon must be low and no carbide precipitation to the grain boundaries allowed to take place.

Manganese Manganese is added to the alloy in proper proportion because it improves the workability of the alloy for casting and fabrication. While found to be unnecessary as a corrosion resistant addition to the alloy, the proportions of 5% to 2.5% do not detract from the corrosion resistant properties of the final alloy. Manganese also has that property of helping to secure the carbon in the final alloy in the combined form.

Silicon Despite previous teaching, my investigative work shows that silicon is unnecessary as an added element for best corrosion resistance. It is added to my alloy, in certain proportions of .5% to 1.50%, in order to promote fluidity in the pouring of castings where it has considerable effect. When used in the proper proportions, as outlined, it controls the proper casting of different thicknesses of sections. It is believed that high silicon tends to promote the appearance of sigma at elevated temperatures.

If the alloy is cast into ingots to be used in a wrought product, the silicon content may be necessarily lower than as set forth here without any detrimental results to the corrosion rQsistai ce Ql the final alloy. My investigations, therefore, show no necessity for the use of high silicon regardless of any prior art teaching on the corrosion resistance of the final alloy.

Chromium Chromium is an essential constituent of my alloy because it is an element by which resistance to oxidizing conditions is secured, particularly unde the oxidizing operation of service where oxidizing conditions are present as, for instance, where hot or boiling nitric acid may be used. When chromium is combined with nickel in the correct proportion, as 20-25%, it provides an excellent base for the alloy of my invention. Oxidation resistance and corrosion resistance are not necessarily coexistent but they are closely allied so that they may be grouped broadly as manifestations of the same group phenomenon. Although the common cause may be basic in that it is a direct function of chemical composition, differences in degree must be limited by keeping the alloy within the limits as herein set forth.

Nickel Nickel, as previously stated, when combined with chromium provides an excellent base for the alloy of my invention. Nickel in the proper proportion of 20- 5% is an excellent alloying addition, not only because it resists the action of sulphuric acid but also because it adds toughness and ductility to the final alloy. In addition, nickel adds to the alloy very excellent creep resistant properties when used under very high temperature service conditions.

Since the alloy of my invention contains a certain amount of copper, the nickel provides a suitable matrix so that it forms a solid solution with the copper in all proportions. When chromium is also alloyed, the correct proportion of the nickel copper additions must be carefully adjusted within the proportions shown so that there is no free copper. Under no circumstances should free copper exist in the alloy because it is not only detrimental to the casting and rolling properties but also may be the cause of certain contamination when used in the service intended.

Nickel has little effect on oxidation resistance but does play an important part in improving the properties of the alloy at temperatures which usually promote oxidation. On the other hand, nickel has a very important influence on corrosion resistance, particularly in the media of relatively low oxidizing power.

Nickel in the alloy renders the alloy austenitic; I have therefore been very careful, after much investigative work, not to obtain higher nickel than specified heretofore because if higher nickel than herein specified is used, while the alloy may be resistant to acid attack, it will not serve as a high temperature alloy because, even at moderate temperature, the austenite will revert to ferrite with loss of properties; especially is this true if molybdenum is present. Prior teachings have not shown this but present day teachings do.

Furthermore, high nickel (if the carbon has not been properly dissolved by heat treatment or the addition of columbium) will reject the carbon from the solid solution with the formation of grain boundary precipitation which will result in localized susceptibility to corrosive attack.

It is at this time necessary to decisively state that nickel as the alloying element must be definitely controlled within the limits herein shown, and with the columbium added in lieu of heat treatment in order for the final corrosion resistant alloy to be passive and stable at moderate or high temperature regardless of its. corrosionresistance at temperatures of not greater than200 F..

' 7 Copper Copper is an alloying element essential to the further control of the corrosion resistant properties of the alloy of my invention, when used in the proper proportions of 5 to 6.5% as set forth in combination with other constituents, because copper produces in the final alloy that property of resistance to hot or boiling sulphuric acid of any concentration. Copper must be adjusted to the limits herein specified because below 5% there is insufficient copper present for best resistance and above 6.5%, with the amount of chromium specified, it will not all be in solid solution with the nickel and will remain as free copper, and be highly detrimental to the final alloy.

Molybdenum Molybdenum has been used for many years as an alloying agent for austenitic steels of the high chromium-nickel type in quantities of from 2 to 4% because it imparts good inherent passive corrosion resistance to the final alloy, particularly preventing the so-called pitting type of corrosion frequently found under oxidizing conditions of service. Therefore, in the alloy of my invention, I use a molybdenum addition of 2 to 4% because previous test results on austenitic steels of 18 chromium-8 nickel type over a period of many years have shown molybdenum in this proportion to be eifective as an alloying agent in the prevention of pitting.

Aluminum As most of the alloys of the high chromium nickel type owe a great part of their corrosion resistance to a thin film forming characteristic and. the speed in which another film is formed in the event of the original protecting film being broken, my invention teaches the use of aluminum as an alloying element because I find from my investigations that aluminum has that property of quickly replacing a broken film. As the result of my experiments, I find there exists a certain relation between the minimum amount of aluminum that must be employed to produce an effective film protection and the chromium content which must be employed with the said aluminum addition. To produce this film forming characteristic in my alloy, I employ not less than 0.5% aluminum and not more than 1.5% because aluminum above this proportion is difficult to alloy and is of no value in the alloy.

Columbium In most of the high chromium-nickel alloys as shown by previous teaching, the alloys must be properly heat treated and quenched in water from at least 2000 F. in order to insure the best corrosion resistance and freedom from intergranular attack. In certain austenitic alloys containing 18% chromium and 8% nickel, columbium has been added so as to eliminate this drastic quench, and it is only necessary to heat treat at 1650 F. and cool in air to insure dimensional stability.

The alloy of my invention is the first to recognize the value of columbium in such a complicated alloy. By the use of columbium in a ratio of 8 to times the carbon content of the alloy and not more than 1%, we obtain a very desira- Phosphorus and, sulphur Both phosphorus and sulphur are contaminating elements which are inherent in all alloys but definitely each is held to a limit of .03%.

The physical properties of the alloy of this invention are as follows:

Yield strength, lbs. per sq. in. 45,000 Tensile strength, lbs. per sq. in 75,000 Elongation in 2?, percent 16 Brinell hardness number (approx.) 150 Modulus of elasticity 22,500,000 Density 8.02 Coefficient of expansion between and 1200 F. .0000097 Limiting creep strength .0001% per hour rate: Lbs. per sq. in.

1100 F 8,000 1200 F 7,200 1300 F. 6,000 1400 F. 4,800

Stress rupture 100 hrs. to fracture:

Lbs. per sq. in.

1100 F. .l 22,000 1200 F. 21,000 1300 F 16,000 1400 F. 13,000

The creep tests were carried out in accordance with A. S. T. M. specification E 22-41; the expansion tests were carried out in accordance with A. S. T. M. specification B -39 and the fracture test under an A. S. T. M. contemplated standard.

While there is a complete series of corrosion resistant tests on the alloy of this invention in various corrosive media, only two of the most severe are set forth as follows:

Boiling 65% nitric acid-temperature 254 F. five 48 hour periods for a total of 240 hours. The inches penetration per month as calculated in accordance with A. S. T. M. A 262-44T was found to be .0014.

The same test procedure was used for 78% sulphuric acid at a temperature of 176 F. and the inches penetration per month was calculated to be .00095.

Having thus described my invention, and the best manner of practicing the new process of forming this novel composition, but without limiting myself to the order of steps, of such process recited, or the proportions of parts employed. therein, or to the precise ingredients named, as itis evident that each of these ingredients has a considerable range of equivalents and that the order and proportions of the process may be varied without departing from its scope and purposes.

What it is desired to claim and secure by Letters Patent of the United States is:

A non-heat treated corrosive resistant and temperature-resistant ferrous alloy of substantially the following analysis: carbon between F 0.03% and 0.07%, manganese between 0.5% and.

2.5%, silicon between 0.5% to 1.5%, chromium between 20.0% and 25.0%, nickel between 20.0% and 25.0%, molybdenum between 2.0% and 4.0%, copper between 5.0% and 6.5%, aluminum between 0.5% and- 1.5%, columbium between 0.5% and 1.0%, sulphur no more than 0.03%, phosphorus no more than 0.03%, and the balance being substantially all iron.

VINCENT T. MALCOLM.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number Great Britain Oct. '14, 1947