US3055755A - Austenitic ductile iron having high notch ductility at low temperature - Google Patents

Description

United States Patent G AUSTENITIC DUCTILE IRON HAVING HIGH NOTCH DUCTILITY AT LOW TEMPERATURE Robert D. Schelleng, North Plainfield, and William K.

Abbott, Bound Brook, N..l., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed June 30, 1961, Ser. No. 120,976

Claims. (Cl. 75-123) The present invention relates to austenitic ductile cast irons and, more particularly, to austenitic ductile irons having a special composition which are characterized by high impact resistance at the temperatures employed in cryogenic service.

Cast iron is one of the oldest commercial metals and has many practical advantages, including ready castability into intricate shapes, machinability, simple melting practice, low liquid shrink-age with high percentage yields, low density as compared to non-ferrous metals such as bronzes and economic advantages over many other ferrous and non-ferrous materials for cryogenic service. The usual types of cast iron containing flake graphite have very limited ductility and impact strength even at room temperature and the use of such materials in cryogenic service is unthinkable. The recent advent of ductile cast iron has made avail-able to the art a material having substantial ductility and impact strength at room temperature. However, the ferritic types of ductile iron undergo impact transition at temperatures only slightly below room temperature with the result that such materials have severely limited application at the temperatures encountered in cryogenic service. It is accordingly necessary to employ an austenitic grade of ductile iron when provision of a material for cryogenic service is the objective. Even here, however, it is found that the commercially available grades of austenitic ductile iron suffer from the disadvantage of inadequate ductility and impact resistance when subjected to very low temperatures such at the temperature of boiling liquid hydrogen, i.e., 423 F. Although many attempts were made to overcome the foregoing difficulties, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that an austenitic ductile iron having a special composition has very good ductility and impact resistance at ordinary temperaures and retains its impact resistance and ductility down to temperatures of interest in cryogenic applications, i.e., temperatures as low as' about 423 F.

It is an object of the present invention to provide an austenitic ductile cast iron composition having improved properties.

A further object of the present invention is to provide an austenitic ductile cast iron having improved ductility and impact resistance.

Itis also an object of the present invention to provide an austenitic ductile cast iron having high ductility and impact resistance at the very low temperatures required for cryogenic service.

Another object of the present invention is to provide cast articles made of a special austenitic ductile cast iron which may be employed at very low temperatures as are encountered in cryogenic service.

Other objects and advantages will become apparent from the following description.

The present invention contemplates austenitic ductile irons containing about 20% to about 24% nickel, about 2% to about 3% carbon, about 1% to about 3% silicon, with the sum of the carbon content plus 0.06 times the nickel content plus 0.2 times the silicon content being not more than 4.4, about 3.25% to about 5% manganese, not

more than 0.25% chromium, a small amount, e.g., about 0.04% to about 0.12%, of magnesium eiiective to induce the occurrence of spheroidal graphite in the cast iron, and the balance essentially iron. These cast irons are characterized by freedom from carbides and freedom from martensite even when cooled to temperatures as low as about 423 F. and are further characterized by weldability and good founding characteristics,

The nickel and manganese contents of the alloys are very important and the contents of these elements must be maintained within the foregoing ranges in order to produce the improved results found in alloys Within the invention. Thus, the nickel content of the alloy must be at least about 20% because lower levels lead to austenite instability and martensite formation and must not exceed about 24% because no further gains in properties are evident. In addition, the manganese content must be at least about 3.25% because lower levels lead to austenite instability and martensite formation and must not exceed about 5% because of carbide formation which causes embrittlement.

The alloys produced in accordance with the invention should be substantially devoid of copper, and in any event should not contain more than about 0.25% of copper as an impurity. Since the alloy contains magnesium, which is a sulfur-avid element, sulfur in the alloy is present only in limited amounts, if at all, and usually will not be present in amount exceeding 0.02%. Phosphorus, a common impurity in cast irons, should not be present in amounts exceeding about 0.10%. The carbide-forming elements such as chromium, molybdenum, tungstem and vanadium should be substantially absent from the alloys as their eflFect in the alloys is undesirable. Thus, these elements should not be present in the alloys in amounts exceeding a total of about 0.25 Other impurities such as antimony, cerium, bismuth and lead should be kept below 0.003% total and titanium preferably should be less than 0.02% as these impurities have a deleterious effect upon the spheroidal graphite structure.

It is important that the graphite structure in the alloys contemplated in accordance with this invention consist of spheroids. The presence of chunky and flake graphite in the alloys in any quantity should be avoided as otherwise the strength, ductility and impact resistance of the alloys are all detrimentally affected. In order to insure the presence of spheroidal graphite structures in the alloys of the invention, the composition must be maintained within the foregoing limits to insure that the total graphitizing power of a melt from which castings are to be made is controlled such that the alloy freezes in the iron graphite system, e.g., free of carbide.

The alloy may be prepared in the usual melting equipment used for producing cast iron, e. g., the cupola furnace, the electric arc furnace, the induction furnace, the air furnace. The ingredients required for producing the alloy are melted together, brought to proper temperature, e.g., about 2750 F. to about 2850 F., at which point the magnesium is added, e.g., as a nickel-magnesium alloy containing about 12% to about 30% magnesium with the balance essentially nickel, the melt is then inoculated with a graphitizing inoculant and metal from the melt is then cast. The final graphitizing inoculation with about 0.25% to about 1% of silicon, e.g., about 0.5% silicon, is a very important step and is required in order to produce good spheroidal graphite structures. Usually, ferrosilicon, a commercial graphitizing alloy containing about 70% to about silicon, about 0.5 to about 1.0% calcium and the balance essentially iron, is employed for the final graphitizing addition. However, other silicon-containing agents or alloys such as nickel-silicon alloys, or nickel silicide, calcium-silicon alloys or calcium silicide, silicon metal, and various proprietary inoculating alloys commonly used for reducing dendriticism and reducing chill in foundry gray cast iron may be employed for this purpose. While it has been pointed out hereinbefore that the magnesium may advantageously be introduced into the melt by adding magnesium to the melt as a nickel-magnesium alloy, other well known magnesium containing addition alloys may be employed for this purpose.

Alloys produced in accordance with the present invention have very good properties at atmospheric temperatures and at very low temperatures, even in the ascast condition. However, it is advantageous to subject castings made of the alloy to a normalizing treatment comprising heating to the temperature range of about 1600 F. to about 1900 F., e.g., 1700 F. to 1800 F., for at least about one hour per inch of section followed by air cooling. The normalizing treatment retains carbon in solution in the austenite and increases the austenite stability preventing martensite formation at low temperatures.

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

EXAMPLE I Two melts of the alloy contemplated in accordance with the invention were made by melting charges of pig iron, ingot iron, electrolytic nickel and ferro alloys in an induction furnace. Each charge was brought to a temperature of about 2900 F., cooled to 2800 F. and was then treated with an addition of about 1% of an addition alloy containing about magnesium, about 2% carbon and the balance essentially nickel after which each charge was inoculated with about 0.5% silicon as a calcium-bearing ferrosilicon containing about 85% silicon. Metal from each of the thus-treated charges was cast into castings, including l-inch, 2-inch and 3-inch thick plates. It was found that the microstructures of the two alloys were devoid of carbide and of martensite and contained good spheroidal graphite. Metal from each of the melts was analyzed with the results set forth in the following Table I:

Table I Alloy TC, S Mn, Ni, Cr, Mg, P,

No. Percent Percent Percent Percent Percent Percent Percent Metal from these alloys was subjected to tensile tests with the results set forth in the following Table II:

In addition, metal from these alloys was subjected to Charpy V-notch impact tests with the following results:

The specimens employed for the foregoing impact tests measured 0.394 inch by 0.394 inch on each side, with a 45 notch 0.079 inch deep and had a radius at the bottom thereof 0.010 inch in length. The foregoing data reported is an average of three tests with the exception of the first value reported for Alloy No. 1 which represents an average of six tests and of the last value reported for Alloy No. 1 which represents an average of five tests.

EXAMPLE II In another example, a 300 pound commercial heat of the alloy was made in an induction furnace. The alloy (Alloy No. 3) contained about 21.2% nickel, about 3.75% manganese, about 2.48% carbon, about 1.75% silicon, about 0.019% phosphorus and about 0.032% magnesium. A number of castings including bars about 3 inches thick were made. The castings contained good spheroidal graphite. The bars were normalized from about 1700 F. Metal from these bars was subjected to a tensile test at room temperature and to Charpy V-notch impact tests at various temperatures from room temperature down to 423 F. with the results shown in the following Tables IV and V.

CHARPY V-NOTCH IMPACT TESTS Impact strength 1 Temp., F.: foot pounds Room 24 Average of two tests except for value at 423 F. which represents average of four tests. In addition, castings made of the alloy were unbroken in the Naval Research Laboratory drop weight test at 320 F., thus indicating that the nil ductility temperature (NDT) for the alloy was below 320 F. The aforementioned drop Weight test is described in the literature, for example, in The Welding Journal, volume 33, No. 9, Research Supplement, page 481s et seq. (1954), and in The Welding Journal, volume 38, No. 5, Research Supplement, page 209s et seq. (1959).

It will be seen from the foregoing that the special austenitic ductile iron composition produced in accordance with the present invention provides very high ductility at room temperature together with high tensile strength at room temperature. Thus, the alloy will exhibit a room temperature tensile elongation in the normalized condition of at least about 35%, and usually at least about 40%, together with a tensile strength of at least about 65,000 psi. The alloy will have a very low nil ductility temperature which will usually be below 423 F. In addition, the alloy has high impact resistance at room temperature and this high impact resistance is retained to a marked degree even at temperatures as low as 420 F. For example, the alloy is characteriZed by an impact resistance as measured by the Charpy V-notch impact test of at least about 15 foot pounds at 320 F. When the nickel content of the alloy is at least about 21%, the manganese content is at least about 3.7% and the carbon content is not more than about 2.6%, the alloy will display a Charpy V-notch impact value of at least about 20 foot pounds at 320 F. and of at least 15 foot pounds at -423 F. Furthermore, the alloy does not exhibit thermal martensite even down to temperatures as low as -423 F. Thus, metal from Alloy No. 1 was cycled between room temperature and 320 F. for 20 cycles and this treatment did not produce any martensite in the alloy.

The special and distinctive combination of properties characterizing alloys within the invention enable the use of the alloy in castings at the very low temperatures encountered in cryogenic service. Thus, pump castings, pump impellers, valves, fittings, compressor components, etc., may be produced as castings from the alloy provided in accordance with the present invention.

The alloy produced in accordance with the present invention may be welded using any of the standard techniques employed for Welding cast iron. Thus, the stickelectrode arc, oxyacetylene, inert-arc and heli-arc methods may be employed in welding the alloy.

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.

We claim:

1. An austenitic ductile iron consisting essentially of about 2% to 3% carbon, about 1% to 3% silicon, about 20% to 24% nickel, with the carbon, silicon and nickel contents being so related that the sum of the carbon content plus 0.2 times the silicon content plus 0.06 times the nickel content does not exceed 4.4, about 3.25% to 5% manganese, a small amount up to about 0.12% of magnesium effective to induce the occurrence of spheroidal graphite in the cast iron and the balance essen tially iron.

2. An austenitic ductile iron consisting essentially of about 2% to about 2.6% carbon, about 1% to about 3% silicon, about 21% to about 24% nickel, with the carbon, silicon and nickel contents being so related that the sum of the carbon content plus 0.2 times the silicon content plus 0.06 times the nickel content does not exceed 4.4, about 3.7% to about 5% manganese, a small amount up to about 0.12% magnesium effective to induce the occurrence of spheroidal graphite in cast iron and the balance essentially iron.

3. An austenitic ductile iron consisting essentially of about 20% nickel, about 3.55% manganese, about 2.61% carbon, about 2.04% silicon, about 0.07% magnesium and the balance essentially iron.

4. An austenitic ductile iron consisting essentially of about 21.2% nickel, about 3.75% manganese, about 2.48% carbon, about 1.75% silicon, about 0.032% magnesium and the balance essentially iron.

5. A11 austenitic ductile iron consisting essentially of about 21.7% nickel, about 3.86% manganese, about 2.58% carbon, about 1.9% silicon, about 0.031% magnesium and the balance essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,842,437 Guenzi July 8, 1958

Claims (1)

1. AN AUSTENITIC DUCTILE IRON CONSISTING ESSENTIALLY OF ABOUT 2% TO 3% CARBON, ABOUT 1% TO 3% SILICON, ABOUT 20% TO 24% NICKEL WITH THE CARBON, SILICON AND NICKEL CONTENTS BEING SO RELATED THAT THE SUM OF THE CARBON CONTENT PLUS 0.2 TIMES THE SILICON CONTENT PLUS 0.06 TIMES THE NICKEL CONTENT DOES NOT EXCEED 4,4 ABOUT 3.25% TO 5% MANGANESE, A SMALL AMOUNT UP TO ABOUT 0.12% OF MAGNESIUM EFFECTIVE TO INDUCE THE OCCURRENCE OF SPHEROIDAL GRAPHITE IN THE CAST IRON AND THE BALANCE ESSENTIALLY IRON.