US3702269A

US3702269A - Ultra high strength ductile iron

Info

Publication number: US3702269A
Application number: US108702A
Authority: US
Inventors: Nathan Lewis Church
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1971-01-22
Filing date: 1971-01-22
Publication date: 1972-11-07
Anticipated expiration: 1989-11-07
Also published as: CA936720A

Abstract

DIRECTED TO ALLOYED DUCTILE IRONS HAVING IN THE ASTEMPERED CONDITION A YIELD STRENGTH (0.2% OFFSET) OF AT LEAST ABOUT 150,000 P.S.I., AND EVEN AT LEAST ABOUT 170,000 P.S.I., WHICH CONTAIN ABOUT 2.6% TO ABOUT 4.0% CARBON, ABOUT 1.5% TO 4% SILICON, ABOUT 6% TO 11% NICKEL, UP TO ABOUT 7% COBALT, AND EFFECTIVE AMOUNT OF A GRAPHITE SPHERIODIZING AGENT, UP TO ABOUT 1% MANGENESE, UP TO ABOUT 0.4% MOLYBDENUM AND THE BALANCE ESSENTIALLY IRON.

Description

NOV. 7; 1972 c -lu c 3,702,269

ULTRA HIGH STRENGTH DUCTILE IRON Filed Jan. 22, 1971 .INVENTOR. NR THHN L. CHURCH United States Patent Ofice Patented Nov. 7, 1972 3,702,269 ULTRA HIGH STRENGTH DUCTILE IRON Nathan Lewis Church, Warwick, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y. Continuation-impart of abandoned application Ser. No.

878,938, Nov. 21, 1969. This application Jan. 22, 1971,

Ser. No. 108,702

Int. Cl. C22c 37/00, 37/04 U.S. Cl. 148-35 4 Claims ABSTRACT OF THE DISCLOSURE Directed to alloyed ductile irons having in the astempered condition a yield strength (0.2% ofiset) of at least about 150,000 p.s.i., and even at least about 170,000 p.s.i., which contain about 2.6% to about 4.0% carbon, about 1.5% to 4% silicon, about 6% to 11% nickel, up to about 7% cobalt, an effective amount of a graphite spheroidizing agent, up to about 1% manganese, up to about 0.4% molybdenum and the balance essentially lron.

The present application is a continuation-in-part of U.S. application Ser. No. 878,938, filed Nov. 21, 1969, now abandoned.

Industry is demanding in many areas metallic materials having higher and higher strength together with freedom from production difficulties. This is particularly true with respect to the demands being placed on foundries which produce steel and cast iron castings. It is possible to provide very high strengths in many castings made of alloy steels. However, in order to produce high strengths, e.g., yield strengths (0.2% offset) on the order of at least 150,000 pounds per square inch to 180,000 pounds per square inch in steel castings a relatively complex and expensive heat treatment is required. Such heat treatments usually involve a quench from an elevated temperature followed by a tempering treatment. The quenching treatment causes severe thermal stresses in steel castings With the result that a substantial number of apparently satisfactory castings are scrapped due to quench cracking and distortion encountered during heat treatment. The resulting scrap losses together with the high cost due to heat treatment has limited the application of steel castings in many areas, particularly those in which castings of complex section are involved.

It would be desirable to employ castings made of ductile iron instead of steel castings, if the requisite strength could be developed in ductile iron. Thus, the castability of alloyed ductile iron is substantially better than that of alloy steels and this is of advantage in producing castings of complex section. Presently available alloyed ductile iron castings are capable of providing yield strengths of up to about 100,000 pounds per square inch and it would be very desirable to provide ductile iron castings capable of developing a markedly higher strength level.

I have now discovered alloy ductile iron castings capable of providing yield strengths after a simple tempering treatment on the order of at least about 150,000 pounds per square inch to about 180,000 pounds per square inch.

It is an object of the present invention to provide alloyed ductile iron castings having a high yield strength on the order of at least about 150,000 pounds per square inch (p.s.i.) after a simple tempering treatment.

Other objects and advantages of the invention will become apparent from the following description taken in conjunction with the drawing which is a reproduction of a photomicrograph taken at 250 diameters depicting the fine lower bainite microstructure obtained in spheroidal graphite castings of the invention.

Broadly stated, the present invention is directed to alloyed ductile iron compositions containing about 2.6% to about 4.0% carbon, about 1.5% to about 4% silicon, at least about 6% up to about 11% nickel, up to about 7% cobalt, up to about 1% manganese, up to about 0.3%, or even 0.4%, molybdenum, a small amount up to about 0.1% magnesium eliective to control graphite in the cast iron to the spheroidal form and the balance essentially iron. Castings produced within the aforedescribed compositional range develop high yield strengths, e.g., 150,000 pounds per square inch, after a simple tempering treatment in the range of about 400 F. to about 700 F. e.g., about 500 F. to about 600 F., conducted for times of at least about 2 hours up to about 8 hours, and are characterized by a fine lower bainite microstructure.

Preferred alloys within the invention, which provide exceptionally high yield strengths in the as-ternpered condition in section sizes up to four inches and even greater contain about 2.9% to 3.6% carbon, about 0.1% to 0.4% manganese, about 2% to about 2.5% silicon, about 7% to 9% nickel, about 0.1% to about 0.3% molybdenum, about 0.04% to about 0.07% magnesium and the balance essentially iron. Such alloys are characterized by spheroidal graphite, a fine matrix structure of lower bainite, a high yield strength usually exceeding about kilo pounds per square inch (k.s.i.) with the accompanying useful ductility and impact values.

A compositional range for other alloys within the invention which will generally provide irons having yield strengths of at least about 160,000 p.s.i., in the as-tempered condition comprises about 3% to about 3.6% carbon, about 0.15% to about 0.25% manganese, about 2.2% to about 2.5 silicon, about 8% to about 10% nickel, about 2% to about 4% cobalt, up to about 0.24% molybdenum, about 0.045% to about 0.06% magnesium and the balance essentially iron. Such castings in the as-tempered condition are characterized by a microstructure comprising essentially lower bainite, with the graphite being present in the spheroidal form.

The most significant alloy constituent in the irons of the invention is nickel. Thus, nickel within the ranges employed, promotes the formation of fine lower bainite in the microstructure accompanied by a yield strength of at least about 150,000 p.s.i. and toughens the matrix. If nickel is reduced below about 6%, undesirable 'weaker upper bainite or pearlite will form thereby decreasing the attainable strength, whereas nickel contents exceeding about 10% or 11% increase the amount of martensite in the structure thereby limiting ductility. In addition, too high nickel contents can result in the presence of retained austenite in the structure thereby decreasing strength. Cobalt, when used in conjunction with nickel, acts to limit retained austenite formation. If cobalt is too high, the strength will be reduced. As the casting section size increases above about one inch, more nickel is employed to preserve the fine lower bainite microstructure and to overcome the tendency for the coarser upper bainite structure to form, a result which would markedly reduce yield strength.

Carbon is essential in the iron to provide graphite. Carbon contents below 2.4% yield brittle semi-carbidic irons with an undesirable intercellular carbide structure with reduced castability. On the other hand, carbon contents exceeding about 3.6% result in lowered strength. Silicon is also an important constituent of the iron, since it promotes graphitization of the alloy and helps to decrease eutectoid carbon content. Hence, the silicon content is at least about 1.5% to aid in preventing carbide formation, but does not exceed about 4% as otherwise the matrix tends to become embrittled. Increasing silicon raises impact transition temperatures. Advantageously,

silicon is about 1.9% to about 2.5% to provide high yield strengths in conjunction with nickel and the other constituents of the alloy. Magnesium is present in the iron for the purpose of spheroidizing graphite. Other known graphite spheroidizers such as cerium, lanthanum and other rare earth metals of the Lanthanide Series may be employed by themselves or in conjunction with magnesium in amounts up to about 0.1% for this purpose. Manganese is not required in the alloys, but may be employed in amounts up to 1% in the higher strength irons to promote the formation of lower bainite. Manganese promotes an undesirable stable austenite and can cause carbide problems in heavier sections when present in excessive amounts. Molybdenum may also be employed in amounts up to about 0.3%, or, in some cases, even up to 0.4%, e.g., about 0.2%, as a strengthening element in conjunction with nickel, particularly in the special cobaltfree irons and in heavier-section castings to preserve high strength. Chromium is not employed in the alloys since this powerful carbide former exerts an undesirable embrittling effect which becomes particularly aggravated in heavier sections and which in heavier sections may be accompanied by segregation. Accordingly, the chromium content does not exceed 0.2%.

The ability of the irons to develop high strength after a simple tempering treatment is an outstanding advantage. Thus, the tempering treatment may readily be performed in equipment available at most foundries. Since no quenching is required, the possibility of quench cracking and distortion is eliminated. Particularly with reference to the higher strength alloys, as described hereinbefore, it is found that essentially no dimensional changes occur during tempering. This is an important advantage particularly in applications requiring castings of complex configuration. It has been found that heat treatments conducted at austenitizing temperatures, e.g., 1450 F., followed by isothermal transformation at temperatures on the order of 450 F. to 500 F. followed by tempering, gave poorer combinations of strength and ductility than were obtained by simply tempering mold cooled castings.

In order to give those skilled in the art a better understanding of the invention a series of heats was prepared by induction melting charges of high grade pig iron, electrolytic nickel, electrolytic cobalt (when cobalt was employed) and standard ferroalloys. In instances where ferromanganese was added, it was added after melt-down at a bath temperature of 2750 F. and the bath was then heated to 2850 F., held five minutes and cooled to 2750 F. In each instance, after melt-down the iron was treated at a temperature of about 2750 F. with a nickel magnesium alloy to introduce magnesium and was then given a graphitizing inoculation comprising an addition of 0.5% silicon as a calcium-bearing ferrosilicon alloy containing 85% silicon and metal from the treated melt was then cast into sand molds. The castings produced were keel blocks. The compositions of alloys produced within the invention are set forth in the following Tables I and V for nickel and for nickel-cobalt irons, respec tively, and the results of tensile testing performed upon the tempered castings are set forth in the following Tables II, III, IV and VI. The casting section size and tempering heat treatment conditions are given in each of Tables II, III, and IV, and the casting section size was one inch for the results reported in Table VI.

TABLE II 1" Section (4 hr.600 FJAC) YS at CVN at 0.2 Percent room Hardofiset U TS el. in R.A., temp. ness (K s.i.) (K SJ.) 1 percent (it-lbs.) (R5) 150.3 105. 3 4 0 5.0 44.4 152. 4 102. 3 3 5 5. 0 44. 0 1 170.4 210.4 2 3 4.0 40.0 I 177.3 210.5 2 2 4.0 40.7 1 100.3 203.4 2 3 4.0 40.2 1 105.7 100.0 1 2 4.0 40.4 1 175.2 215.7 3 3. 5 4.5 47.2 1 174.7 200.4 2 3.5 4.0 40.0 5 172.0 213.0 2 4.5 4.0 47.0 170.0 200.2 2 3.5 4.0 45.4 0 105.3 171.3 1 4 22:; 7 1 176.8 207.5 1 4 4.0 47.7 "1 175.1 202.2 1 Nil 4.0 40.0

TABLE III 2" Section (4 hr./600 FJAC) YS at CVN at 0.2% Percent room Hard- I ofiset U'IS el. in R.A., temp. 11855 Alloy No. (K s.i.) (K 5.1.) 1 percent (it-lbs.) (R

5 130.3 103.7 5 0 1 "1 122.3 ig 0 3.2 i

p103 102.1 3 i M 107.7 202.; i i 140. 0 17 6 "1 140. 7 .2 12.2 i

2" Section (4 hr./500 FJAC) YS at CVN at 0.2% Percent room Hardofiset UT e1. in R.A., temp. ness (K s.i.) (K s.i 1 percent (ft-lbs.) (R

i g 1 2 7.0 37.1 00.0 151.0 100.2 2; 1 g i 100.7 104. 12 -7 200 a 0 i 0. 157. z; t 105. 7. 102.4 200.4 2 i M 140.2 100.7 140.2 ns 1.2 i 157.0 0 .0 1. 150.0 102.2 1 1 i l Broke in specimen neck.

TABLE IV 4 Section (4 Ina/600" FJAC) Ys at CVN at:

0.2% Percent room Hardofis'et U S 01. in R.A., temp. ness Alloy No. (K 5.1.) (K 5.1 1" percent (it.-lbs.) (R.,)

ps2 2 101; E 5

gag 3313 i i 5 103.0 182.5 1 Nil 1 Broke in specimen neck.

The data of Tables II through IV demonstrate that the higher nickel irons retain high strength (and the desired lower bainite microstructure) as section size is increased above one inch. Alloy 6 serves to illustrate the desirability of maintaining silicon above 1.9% to provide high yield strength, although, as shown in Table III, Alloy 6 is indeed strong.

The photomicrograph depicted in the drawing was derived from the casting of Alloy No. 3, the properties for which are given in Table II. The very fine lower bainite microstructure is clearly evident therefrom.

TABLE V Composition (wt. percent) Mn Si Ni M0 Mg TABLE VI.MECHANICAL PROPERTIES Heat Y.S. (p.s.i.) Hardtreat- 0.2% U.T.S. El., R.A., uess, Alloy No. ment 1 ofiset (p.s.i.) percent percent B11 Heat treatment code: A=4 hrs/500 FJAC; B=4 hrs/500 FJAO plus 4 hrs/600 FJAC; C=4 hrs/600 FJAO.

Standard Charpy V-Notch impact specimens were prepared from Alloys 22, 23 and 27 and were tested at room temperature with impact values in the range of 3 to 3.5 foot-pounds being determined.

While the data in Table VI was obtained upon one inch castings, the results of other tests have indicated that strengths of a high order, together with substantial retention of hardness and impact values, are obtained in heavier castings, e.g., castings having sections up to four inches thick. The fact that Charpy V-notch values were obtained in irons at the high strength levels demonstrated hereinbefore illustrates the remarkable combinations of properties possessed by alloys within the invention. It is to be remembered in this connection that conventional pearlitic ductile irons provide essentially zero Charpy V-notch values at strengths on the order of 80,000 p.s.i.

Alloys provided in accordance with the invention are machinable and castings made therefrom can be employed in the production of complex parts such as high pressure centrifugal gas compressor impellers, pump parts, 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.

I claim:

1. A ductile iron sand casting characterized in the tempered condition by a fine lower bainite structure consisting essentially of 2.6% to about 4.0% carbon, about 1.5% to about 4% silicon, about 6% to about 11% nickel, up to about 7% cobalt, up to about 1% manganese, up to about 0.4% molybdenum, not more than about 0.2% chromium, a small amount up to about 0.1% of a graphite spheroidizing agent effective to promote the occurrence of spheroidal graphite in said ductile iron and the balance essentially ll'OIl.

2. A ductile iron casting in accordance with claim 1 containing about 2.9% to about 3.6% carbon, about 0.1% to 0.4% manganese, about 2% to about 2.5% silicon, about 7% to about 9% nickel, about 0.1% to about 0.3% molybdenum, and about 0.04% to about 0.07% magnesium.

3. A bainitic ductile iron casting in accordance with claim 1 wherein the graphite spheroidizing agent is selected from the group consisting of magnesium and a metal of the Lanthanide Series.

4. A bainitic ductile iron casting in accordance with claim 1 containing about 3% to about 3.4% carbon, about 2.2% to about 2.5% silicon, about 8% to about 10% nickel, about 2% to about 4% cobalt, about 0.15% to about 0.25% manganese, up to about 0.24% molybdenum, and about 0.045% to about 0.06% magnesium and having a matrix microstructure comprising essentially lower bainite.

References Cited UNITED STATES PATENTS 1,496,979 6/1924 Corning 148-35 2,046,913 7/1936 Kormann l23 CB 3,273,998 9/1966 Knoth 148-35 X 3,549,430 12/1970 Kies 148-35 2,485,760 10/1949 Millis et al. 75l23 CB 2,516,524 7/1950 Millis et al 75l23 CB 2,970,902 2/1961 Alexander et al. 75-123 CB 3,125,442 3/1964 Alexander 75l23 CB L. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner US. Cl. X.R. 75--123 CB, 128 C