US3549430A - Bainitic ductile iron having high strength and toughness - Google Patents

Description

Patented Dec. 22, 1970 3,549,430 BAINITIC DUCTILE IRON HAVING HIGH STRENGTH AND TOUGHNESS Frederick K. Kies, Tuxedo, and Robert D. Schelleng, Ramapo, N.Y., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 14, 1967, Ser. No. 682,985 Int. Cl. C22c 37/00, 37/04 U.S. Cl. 14835 3 Claims ABSTRACT OF THE DISCLOSURE Directed to bainitic ductile irons exhibiting high strength and machinability after a simple tempering heat treatment at about 400 F. to about 600 F. containing about 2.9% to about 3.9% carbon, about 1.7% to about 2.6% silicon, about 3.2% to about 7% nickel, with the nickel content being directly related to the section size of castings to be produced, at least about 0.15% up to about 0.4% molybdenum, not more than about 0.2% chromium, up to about 1% manganese, a graphite-spheroidizing amount of magnesium, and the balance essentially iron.

The invention is directed to special nickel-molybdenum bainitic cast irons having controlled composition and having high strength, machinability, and toughness when produced in castings having a wide variety of section sizes.

It is an object of the present invention to provide special nickel-molybdenum bainitic ductile iron castings United States Patent Office characterized by high strength and toughness over a wide variety of casting section thicknesses.

A further object of the invention is to provide heavy sectioned nickel-molybdenum bainitic ductile iron castings having improved properties when provided in the form of chill castings.

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

Generally speaking, the present invention is directed to improved bainitic ductile iron castings having high strength and toughness after a simple heat treatment involving tempering in the range of about 400 F. to about 600 F. and having good machinability. The castings may be produced in a wide variety of section sizes and contain about 2.9% to about 3.9% carbon, about 1.7% to about 2.6% silicon, about 3.2% to about 7% nickel, about 0.15 to about 0.3% or about 0.4% molybdenum, not more than about 0.2% chromium, up to about 1% manganese, magnesium in an amount up to about 0.1% effective to control the occurrence of graphite in the castings to the spheroidal form, and the balance, except for minor elements and impurities, being essentially iron. In order to assure the production of the requisite bainitic structure while avoiding embrittlement due to the production of excessive amounts of lower bainite and martensite in the structure, the nickel content of the casting is related to the section size thereof such that, at a manganese level of about 0.3% to 0.4% and in the absence of chromium, one inch sand castings contain at least about 3.2% up to about 3.6% nickel, three inch sand castings contain at least about 4.2% up to about 4.8% nickel, six inch sand castings contain at least about 4.8% up to about 5.4% nickel, 20 inch chill castings contain at least about 5% up to about 6.5%

nickel, and 46 inch chill castings contain at least about 6.6% up to about 7.5% nickel. The molybdenum content is an essential feature of the castings and is controlled in the range of about 0.15% or about 0.2% up to about 0.3% or about 0.4% molybdenum in order that, in combination with nickel, the required bainitic structure will be obtained. Preferably, the use of more than about 0.25% or about 0.3% molybdenum is avoided as otherwise there is an increasing tendency for intercellular carbides to form in the castings with undesirable results in terms of machinability and reduced toughness. This tendency becomes more pronounced with larger section sizes and in areas remote from a chill face. In general, chromium is unnecessary in the castings and is avoided since chromium is a strong carbide former and hardens the bainite matrix, effects which can cause embrittlement. Chromium in amounts of up to about 0.2% may be tolerated especially in the larger sectioned chill castings since in some foundries chromium at this level is a constituent of remelt stock. However, the content of chromium plus molybdenum should not exceed 0.5% or harmful carbide with embrittlement and reduced machinability will be encountered even in the chill portion of chilled castings. Chromium at 0.2% can be used to replace about 0.5 nickel in forming a bainitic structure. The manganese content of the castings may be utilized to a limited extent to replace the nickel content for purposes of providing a bainitic structure, e.g., about 0.5% nickel can be replaced with about an equal weight of manganese. However, the toughness and ductility of such irons tends to be lower and the hardness higher than is the case when nickel is relied upon to provide bainite. Manganese can form embrittling carbides when employed in amounts exceeding about 1%, particularly in larger sectioned castings and areas remote from a chill face. Preferably, the manganese content is about 0.3% to about 0.4%. Magnesium is employed in the castings in small amounts up to about 0.1%, e.g., about 0.02% or 0.03% to about 0.06% or 0.07% or 0.08%, to provide spheroidal graphite in the castings. Carbon and silicon generally may be controlled, especially in castings having section sizes up to about six inches, to be present essentially in a eutectic amount. This is highly desirable from the foundry standpoint since improved fluidity and other desirable foundry characteristics are thereby obtained. Control of graphite to the spheroidal form provides essential freedom from the compositional limitations imposed by section size upon carbon and silicon contents which are encountered in flake graphite irons. In the heavier sectioned castings, it is sometimes desirable to control carbon and silicon such that the carbon equivalent of the castings is about 3.6% to about 4.3%. Carbon is controlled in the range of about 2.9% to about 3.7%, or in lighter sectioned castings, to about 3.9%, e.g., about 3.2%, so that carbideforming propensity which may be encountered at lower carbon levels is minimized or prevented but the weakening effect of excessive graphite and possible kish formation at higher carbon contents is avoided. Silicon is controlled in the range of about 1.7% to about 2.6%, e.g., about 2.1%, again to avoid the formation of embrittling carbides which may occur at lower silicon levels and to avoid an increase in the ductile-to-brittle transition temperature which occurs at higher silicon levels. A portion of the silicon present in the casting represents silicon introduced as a late addition to the melt as a siliconcontaining graphitizing inoculant. Common ferrosilicon alloys containing about 50% to about 85% silicon are employed for this purpose to introduce about 0.3% to about 0.7%, e.g., about 0.5%, silicon into the melt. Usual impurities such as sulfur and phosphorus desirably should not exceed about 0.015% and about 0.04%, respectively. Elements known to be subversive to the graphitespheroidizing effect of magnesium should be avoided or be present only in very small, harmless amounts.

It is important for purposes of obtaining the desired high strength, toughness and fatigue resistance in castings provided in accordance with the invention that the microstructure at least in the outer portions thereof, e.g., up to about 4.5 inches from a chilled face when a chill is employed, consist essentially of bainite, i.e., upper bainite, and spheroidal graphite. Provided the compositioned control described hereinbefore is exercised, the presence of undesirable strength-lowering and/or embrittling phases such as retained austenite, pearlite, ferrite, martensite or carbide in the castings will be avoided or controlled to harmless amounts.

In the production of castings having section sizes greater than about six inches, e.g., 20 inches and upwards, it is desirable to employ a chill. Surprisingly, it is found that the structure in the chilled portion of the 4 EXAMPLE I Charges comprising pig iron, Armco iron, electrolytic nickel, molybdenum pellets, ferrosilicon, and ferromanganese were melted in an induction furnace. The melts in each case were superheated to about 2850 F. to dissolve the melt constituents. Magnesium was introduced into the molten irons at a temperature of about 2750 F. as a nickel-magnesium alloy containing about magnesium after which a graphitizing addition of 0.5% silicon as calcium-bearing ferro-85 silicon was introduced as an inoculant. Portions of metal from the melts were cast in sand to provide a variety of castings including one inch double keel blocks, three inch keel blocks and six inch single keel blocks. The castings were cooled in the mold, shaken out, and portions thereof subjected to a four hour tempering treatment at 600 F. The compositions of the three irons produced are set forth in the following Table I (in which the balance of the composition is essentially iron) and the properties obtained thereon in the as-cast and as-tempered conditions are set forth in the following Table II. In each case, the structure of the castings consisted of bainite and spheroidal graphite.

casting contains little or no harmful carbide. In addi- TABLE I tion, the portion of the casting extending inwards away Percent from the chill is devoid of carbide for a substantial dis- 0 Si Mn Mo Mg tance, e.g., about two inches when referred to a 20 inch chill casting. Accordingly, the chill-casting procedure 356 214 Q39 322 M 007 provides in at least the outer portion of a heavy casting 3.78 2.12 0. 41 4.38 0.25 0.077 3. 07 2.00 0. 4.9 0. 24 0. 07 a tough, strong, fatigue resistant structure whlch con- TABLE II E1, R.A., Casting Y.S I.S perpersrzc Condition K 5.1 K s.1 ccnt cent BIIN Alloy No.:

. 137.0 3.5 2.0 358 7 135. 5 4. 0 4. 5 7 1 keel 145. 2 5. 0 5. 0 321 145.3 4.5 5.0 321 7 .0 122. 9 2. 0 1. 5 324 2 17 do 79.0 120.0 2.5 3.0 321 Tempered..- 111.3 137.5 2.5 3.5 332 0 110.1 130.5 2.5 Tern ere 110.0 1 5.7 1.5 3 wheel "hun 108.6 135.0 2.0 2.5

Y.S. Yield strength. T.S. =Tensile strength.

El=Elongation.

tributes improved properties to the casting as a whole. It was thus discovered that chill-casting of the special iron compositions would improve the properties of heavy section castings. Castings provided in accordance with the invention are characterized by high strength. Thus, sand cast sections on the order of about three inches and even up to about six inches possess, in the as-tempered condition, yield strengths of 100,000 pounds per square inch (p.s.i.) and more together with useful ductility and toughness. It is a feature of the invention that the high strength which characterizes castings produced in accordance therewith is achieved with an exceptionally simple heat treatment involving merely tempering castings in the temperature range of about 400 F. to about 600 F. for relatively short periods of time such as about 2 to about 14 hours, e.g., 4 hours. It is unnecessary to employ high temperature heat treatments such as normalizing, i.e., a heating up to about 1600 F. followed by air cooling. The fact that high strength can be achieved merely with a simple tempering treatment provides in the special castings an important technical advantage. Thus, equipment such as a core oven, which is available in most foundries, can be employed for the purpose of carrying out the requisite heat treatment.

In order to give those skilled in the art a better understanding and/or appreciation of. the advantages of the invention, the following illustrative examples are given:

As an illustration of the importance of maintaining the nickel content of castings provided in accordance with the invention so as to provide a bainitic matrix structure having regard for the section size to be cast, another alloy containing about 3.76% carbon, about 2.12% silicon, about 0.37% manganese, about 3.82% nickel, about 0.25% molybdenum, about 0.06% magnesium, and the balance essentially iron, was produced as a three inch keel block casting. After tempering four hours at 600 F., the casting had (on the basis of an average of two tests) a yield strength of 96.7 K s.i., a tensile strength of 125.5 K s.i., an elongation of 3.5%, a reduction in area of 3.5% and a hardness of 300 BHN. Miscroexamination of the tempered structure demonstrated that the casting contained ferrite which accounted for the reduced strength. The keel block castings referred to herein had area to volume ratios of 1.43 per inch for the one inch keel, 1.09 per inch for the three inch keel and 0.65 per inch for the six inch keel.

EXAMPLE II Further bainitic castings were produced using the procedure described in Example I and were cast into one inch keel bars which were program cooled to simulate the cooling rate of a chill-cast 20 inch cylindrical section. The cooling rate was such that at one hour the casting temperature was 1800 F., at 4.4 hours the casting temperature was 1200 F., at 6.4 hours the casting temperature was 1000 F., at 9.4 hours the casting temperature was 800 F. and at 14.5 hours the casting temperature was 600 F. Portions of the bars thus obtained, which had structures consisting of upper bainite and spheroidal graphite, were subjected to a bend test after two tempering treatments at 400 F. or 600 F., each for seven hours, followed by furnace cooling. The' compositions of the irons produced (balance essentially iron) are set forth in the following Table III and the properties obtained thereon after the bend test are set forth in the following Table IV. The bend test was conducted upon a three point loaded flat bar having dimensions of about 4 inch by inch by 5 inches with supports 3%; inches apart, and with the load being applied at the mid-point on the opposite side. The ultimate bend strength was calculated from the standard elastic flexure formula for beams having a concentrated load at the mid-point. Toughness was evaluated by integrating the area under the load-deflection curve. The

specimens were finish ground and the long edges were beveled to minimize corner effects. Experience has indicated that the calculated ultimate bend strength is approximately twice the ultimate strength measured in the conventional tensile tests.

TABLE III Percent C Si Mn Ni Cr Mo M g Alloy No.:

TABLE IV Hardness, Bend Bend Rockwell toughness, strength, Condition C fL/lbs. K s.i

Alloy No.1

Tempered 400 F. 35. 5 70 301 4 .do 36. 70. 296 Tempered 600 F. 30. 0 61. 5 288 5 Tempered 400 F. 3 .7 60.4 304 do 64.2 304 6 Tempered 400 F 53. 0 293 do 40. 0 277 7 Tempered 400 F 74.2 303 .do 73.4 205 8 Tempered 400 F 77. 2 319 do 61. 8 307 position but containing 0.2% chromium had a hardness 9 of 37 Rockwell C, a reduced bend toughness of 39.2 foot-pounds, and an ultimate bend strength of 293 K s.i.

EXAMPLE III Two further melts were prepared in accordance with the procedure described in Example I. Castings from these melts included a chill-cast block having dimensions of five inches by five inches by ten inches with a carboncoated chiller being held against a five inch by five inch face of the casting and with the remainder of the mold being dry sand. The chiller was five inches thick. These castings were devised to provide a substantial sized casting for test purposes and had a cooling rate comparable to that encountered in a chilled round about inches in diameter. The compositions of the irons produced are set forth in the following Table V (in which the balance of the composition is essentially iron) 6 ample II were cut from the block such that the tension face of the bend specimen represented the metal structure at distances of 0, 0.5, 1.0, 3.0, and 4.5 inches from the chill face of the casting. The specimens were given two tempering treatments, with each treatment being for seven hours at 400 F., followed by furnace cooling. The results of the bend tests are set forth in the following Table VI:

TABLE VI Distance Bend Bend Hardness. from chill, strength, toughness, Rockwell in. K s.i. ft.-lbs. C

Alloy N 0.:

In addition to the foregoing bend tests, tensile test specimens and Charpy V notch specimens were cut from a five inch by five inch by ten inch chill-cast block at distances of 1% inches and 2 /2 inches from the chill face. The results of the testing on these specimens, after two tempering treatments for seven hours at 400 F., furnace cool, are set forth in the following Table VII:

TABLE VII Distauee from 131., R.A., chill, Y.S., I.S., perpor- OVN,

in. K.s.1 K.si cent cent it.-lbs

Alloy N 0.:

CVN=Cl1arpy V notch.

EXAMPLE IV Further irons were produced in the form of one inch keel blocks cast in sand and program cooled at a rate simulating the cooling rate of a chill-cast 46 inch cylindrical section. The cooling rate was such that at 1.5 hours the casting temperature was 1800 F., at 7 hours the casting temperature was 1200 F., at 13 hours the casting temperature was 1000 F., at 21 hours the casting temperature was 800 F., and at 38 hours the casting temperature was 600 F. Bars thus obtained were double tempered, with each treatment being conducted for seven hours at 400 F. and furnace cooled. The structure of the castings consisted of upper bainite and spheroidal graphite. The compositions of the castings (balance essentially iron) are given in the following Table VIII and the results obtained upon bend tests conducted in the manner described in Example 11 are set forth in the following Table IX:

TABLE v Percent TABLE VIII 0 Si Mn Ni Cr Mo Mg Percent 0 Si Mn Ni Cr Mo Mg 3.11 2.30 0.33 5.0 0.21 0.10 0.00 3.20 2.31 0.31 5.7 0.21 0.10 0.00 1 O;

Bend specimens having the dimensions described in Ex TABLE 1X Hardness, Bend Bend Rockwell toughness, strength, C ft.-lbs. K.s.i.

Alloy N 0.:

The properties obtained in the special nickel-molybdenum bainitic ductile iron provided in accordance with the invention are much superior in terms of strength, toughness and fatigue resistance than nickel alloyed pearlitic irons or of alloyed bainitic ductile irons which contain carbide. The fact that excellent properties are provided in castings produced in accordance with the invention over a wide variety of casting section sizes makes the castings useful in many applications, including crankshafts, diesel blocks and heads, cams, brakedrums, clutch plates, track shoes for crawler-type tractor treads, centrifugally cast sleeves, and particularly with regard to heavy section castings, rolling mill rolls, forming dies, wear plates, 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.

We claim: 1. A sand cast ductile iron casting having a microstructure consisting of bainite and spheroidal graphite,

having high strength, toughness and good machinability after a tempering heat treatment in the range of about 400 F. to about 600 F. having about 2.9% to about 3.9% carbon, about 1.7% to about 2.6% silicon, about 3.2% to about 5.4% nickel, about 0.15% to about 0.4% molybdenum, not more than about 0.2% chromium, with the total content of molybdenum and chromium not exceeding 0.5%, up to about 1% manganese, magnesium in an amount up to about 0.1% effective to control the occurrence of graphite in the spheroidal form,

and the balance, except for minor amounts of incidental elements and impurities, being iron, with the nickel content of said castings, at a manganese content of about 0.3% to 0.4% and in the absence of chromium, being related to section size such that one inch sand castings contain at least about 3.2% nickel, three inch sand castings contain at least about 4.2% nickel, six inch sand castings contain at least about 4.8% nickel, 20 inch chill castings contain at least about 5% nickel, and 46 inch chill castings contain at least about 6.6% nickel.

2. A casting in accordance with claim 1 wherein the molybdenum content does not exceed about 0.3%.

3. A chill-cast ductile iron casting characterized by a microstructure consisting essentially of bainite and spheroidal graphite and being further characterized in that the chilled portion and the portion of the casting extending inwards away from the chill for a substantial distance is devoid of carbides and having high strength, toughness and machinability after a tempering heat treatment in the temperature range of about 400 F. to about 600 F. having about 2.9% to about 3.7% carbon, about 1.7% to about 2.6% silicon, about 5% to about 7% nickel, about 0.15% to about 0.3% molybdenum, not more than about 0.2% chromium, up to about 1% manganese, magnesium in an amount up to about 0.1% effective to control the occurrence of graphite in the spheroidal form and the balance, except for minor amounts of incidental elements and impurities, being iron.

References Cited UNITED STATES PATENTS 1,900,125 3/1933 vMeric'a et a1. 128 1,910,034 5/1933 Mitchell et a]. 75128 1,948,246 2/1934 Seaman 75128X 2,324,322 7/1943 Flinn et al. 2,485,760 10/1949 Millis et al. 2,771,358 11/1956 Spear 75-128 3,273,998 9/1966 Knoth et al.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

Po-ww UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION p g 3,549,430 Dated December 22,;970

Inventor) FREDERICK K. KIES and ROBERT D. SCHELLENG It is certified that error appears in the above-identified patent; and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 13, after "inch", first occurrence, insert single-.

Col. 4, line 61, for Miscroexamination" read Microexamination-.

Col. 5, line 24, for "tests", read --test-.

Col. 5, line 48, for "White", read --While-.

Col. 6, Table VI, under fourth column entitled"Bend toughne ft.lbs. eighth number under Alloy No. 9, for "40.4",

read -40.3--.

Col. 6, Table VII, under fourth column entitled "El. perce seventh number (third number under Alloy No. 10) for "5.0"

Signed and sealed this 10th day of August 1971.

(SEAL) Attest:

EDWARD M.FLETCIIER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents