US2488511A - Nodular cast iron and the manufacture thereof - Google Patents

Description

Nov. 15, 1949 MORROGH 2,488,511

NODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 8 Sheets-Sheet l Fig.1-Typical example of"graph1te flake aggregate" nodule in malle- Fig.2-Spherulitic nodule in malleable iron having sulphur present 'as able cast iron having sulphur pre- Etched in 10 1c acid 35 sent as manganese sulphide. Etched p r X in 5 per cent nital. X500.

ll V v V V .c 1 Fig.5 General structure 0: un- Fig.4-Microstructure of ceriumtr-Eated reznelted pig iron, 1.2-in. treated remelted ig iron, 1.2-in, test bar. Etched in picric test bar. Etched in picric acid. X100. acid. X100.

Fig. 5- Duplex structure of hypereutectic spherulite present in ceriumtreated iron. Unetched. X1500.

INVENTOR HENTON MORROGH fli. ATTORNEY' Nov. 15, 1949 H. MORROGH 2,488,511

MODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25. 1949 8 Sheets-Sheet 2 H Fig.7-Centrifugally cast liner. p segregation of Structure at a point distant from hyper'eutectic Spherulites at inner surface. All granhite is ir inner s rfa of centrifueally form of quasi-flakes. Etched in cast cylinder liner (ceriumpicric acid. oo treated iron). Etched in picric acid. X100.

Fig-.B-Untreated remelted hematite F1g 9- Remeited pig iron with pig iron cast in 6-in. diameter 0.040 per cent Ce, cast in 5-1n. bar. Etched in picric acid. 3/60,

diameter bar. Picric acid etch. X60.

' HENTON oRRoeu ATTORNEY Nov. 15, 1 949 VH. MORROGH 2,488,511

NODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 8 Sheets-Sheet 3 Fig.1l- General structure of l. 2-in. Fig.12- Microstructure of typical bar with Ge addition (No.1 ,Table 5) 71.2-in. bar with cerium and ferro- Etched in picric acid. 100. silicon a iti n (No.2,Table a).

Picric acid etch. 100. Note my spherulitic nodules.

A: LA

Fig.115- Structure of 1.2-in. bar with cerium and ferro-silicon addition (No. 5,1able 5) is elmoat entirely ephem- "m iu-qmotomicm m h of 1.2-in. X100 bar with cerium and ferro-silioon addition (No.4,Table 5) shows small amount of quasi-flake graphite. Picric acid etch. X100.

5 Fig.15- Metal treated simultaneously with misch metal and siliconmanganese-zirconium shows sphe'rulitic nodules with only a trace of quasi-flake graphite in pearlite v and ferrite matrix. Etched in picric acid. X100. lNvE-NTOR Hzmou oRRoqH ZBY #544 ATTORNEY Nov. 15, 1949 H. MORROGH 2,488,511

NODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 r 8 Sheets-Sheet-4 eirofistmc'wre Fig. 17- Structure of chill wedge, apex of chill wedge made from ya-1 from a ex, treated with metal treated with cerium alone cerium alone, hgwg rather more Shows y a trace of yp grephite but still is predominanteutectic eraphite- Picric acid 1y white. Picric acid etch. X60.

etch. X60.

F1g.l9- Chill wedge structure, 1/8-in. from pex, cast from doubler from double-treated metal shows I treateu metal shows presence 0 only a very small amount c af carl/ Small amount of free cement bide 1n the structure. Etched in 1179- Picric etch RQVENTOR picric acid. X60. H -r0 oRROGH BY I We 46, ATTORNEY Nov. 15, 1949 MORROGH 2,488,511

NODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 8 SheetsSheet 5 Fig. 21- Wholly spherulitic strucstructure of bar ture of 1.2-1n. bar (No.2, Table (moi/fable 4) with 0- P 4) with 0.031 pr cent Ge, procent produced by double treat duced by double treatment process. ment process. Some quasi-flake Etched in plane ac1d ,Xloo.

zranhite. Picrin acid etch. X100.

bar- No.2 Table 4 with 0.061 er bar Table 4) with O'o16 per cent Ce, 1 roduced by double trezt cent Ce, double treatment process, went process, shown to be wholly shows some quasi-flake graphite. t emh Picric acid etch. X100. sphemli Merle iNV ENTOR Hemou RROGH IBY Z -10 ATTORNEY Fig. 22 Micro strucfllre of 0.61 n.

NOV. 15, H, MORROGH NODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 8 Sheets-Sheet 6 Fig. 25- Spherulitic nodule at high magnification in 0.6-in.

J V wi -i 1. a a.

Fig.24- Structure of 1.6-1n. bar bar (Table pp (Table 5) in copper-containing ing double-treated iron. Note double-treated iron shows radial Structure- Picric spherulitic nodules in matrix of v etch X pearlite. Etched in pioric acid. X100.

Fig. 26Photomicrograph showing duplex structure of nodule in 1.6-in. bar (Table 5) in Copper- Fig. 27Microstructure of nodular containing double-treated iron. iron with P cent silicon Etched in picric acid. 500. contem- Etched in Picric acid. X100.

INVENTOR HENTON Momzoeu 9} ATTORNEY Nov. 15, 1949 MQRRQGH 2,488,511

MODULAR CAST moms AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 8 Sheets-Sheet 7 A 7' P" 'M. s ,4 Am 1, I r V I 6- za-Photomiclolaph showing F1g.29-M1'crost-ructure of 1.2-in. structure of 1.2-1n. bar('1able 6) bar (No.2,Table "7) produced in i Single r atm nt n dular ir n double-treated nodular iron with with 0.50 per cent phosphorus. 2.92 per cent Mn content. Etched Pier-1o acid etch. X100. in plcric aoid. )(lQO.

F1g.ao-'-3umccure of 0.875-1n. bar in untreated n1ckelcontalning iron (Table 9). Picric acid INVENTOR etch- X100. Heme" oRRofin ATTORNEY Nov. 15, 1949' MQRRQGH 2,488,511

MODULAR CAST IRONS AND THE MANUFACTURE THEREOF Filed Jan. 25, 1949 8 Sheets-Sheet 8 Fig.3l-M1crostructi1re of 0.87541. n I bar in nodular nickel-containing F1g'5 mcrostmcture of bar (Table 11) in cupola-melted iron (Table Picric etch" X double-treated iron, shows spherulitic nodular graphite. Etched in picric acid. X60.

INVENTOR 01 ATTORN EY Patented Nov. 15, 1949 NODULAB CAST IRON AND THE MANUFAC- TUBE THEREOF Benton Morrogh, Handsworth, England, assignor to British Cast Iron Research Association, Alvechurch, Birmingham, England Application January 25, 1949, Serial No. 72,575

19 Claims.

This invention relates to nodular cast irons and the manufacture thereof. More particularly, the invention relates to the manufacture of gray cast irons of novel type which contain a variable small proportion of cerium uncombined with sulphur and in which part or all of the graphitic carbon content is in the form of nodules visible under the microscope, which irons have not been subjected to a heat-treatment process for the purpose of producing that graphite structure but, on the contrary, possess their characteristic structure in the as-cast state, and are further characterized by markedly im'- proved mechanical properties as compared with previously known gray cast irons having generally similar analyses, respectively, but not containing cerium.

Ordinary gray cast irons have their graphitic carbon distributed throughout the metallic matrix in the form of flakes, the size and shape of which vary considerably according to composition, cooling rate, method of melting, ladle treatment, etc. These graphite flakes interrupt the continuity of the metallic matrix and so render the material brittle and non-ductile.

For many years, skilled workers in the metallurgy of cast iron have endeavored to improve the mechanical properties of gray cast iron as regards strength and resistance to shock without at the same time rendering it so hard as largely to lose the advantage of good machinability normally characterizing gray cast iron. The past two decades, especially, have witnessed the gradual evolution of high-duty gray cast iron, particularly in the field of castings for the engineering and automobile industries. This evolution began with the realization of the importance of the control of carbon and silicon and the relative aifiustment of these two elements in cast iron. This phase was. accompanied by the development and application of the hot-mold process and the utilization of high-steel mixtures in cupola practice.

controlled low carbon contents; they are usually inoculated and have a combination of alloying elements which, after an appropriate heat-treatment, confer the maximum strengthening eifect on the metallic matrix. However, one frequent objection to present high-duty gray cast irons,

particularly when alloyed with molybdenum, is their poor machinability.

In striking contrast to all gray cast irons previously known, those of the present invention 35 are always characterized and distinguishable by the fact that at least a substantial part of their graphite carbon content exists in nodular form with resultant notable improvement in mechanical properties of the material. It has long been recognized that the mechanical properties of gray cast iron could be considerably improved if the graphite could be obtained in the nodular instead of flake form. Prior to the present invention, however, no way by which to produce gray cast iron having a nodular graphite structure has ever been devised, and the objective appeared to be unattainable. It was known, of course, that by producing castings of white iron.

and then subjecting the white iron castings to a prolonged annealing treatment, the white cast iron would be converted into malleable cast iron having a nodular graphite structure; but mal- Next, the inoculation technique was introduced leable cast iron is a product quite diiierent in and loped a process of ble comvarious respects from gray cast iron, with which the present invention exclusively is concerned, and is readily distinguishable therefrom.

It has now been found that the problem of producing gray cast iron having, in the as-cast state, a nodular graphitic structure and, thereby, improved mechanical properties, can be solved by treating the gray iron melt to be cast with a material, specifically cerium, which will function in the melt both as a desulphurizer and also as a carbide stabilizer.

The process of the present invention involves, in its simplest form, only the addition of an an propriate amount of cerium to a molten cast iron of appropriate composition, and solution of cerium in the iron, shortly before casting.

The principal composition requirements for the production of nodular irons by this process are:

1. The iron to be treated must solidify gray in the untreated condition; the process is not applicable to a white iron. With this reservation, the matrix structure may be pearlitic, ferritic, a mixture of ferrite and pearlite, martensitic, acicular, or austenitic.

2. In irons to be treated which contain less than per cent of nickel (nickel need not be present), a high carbon content is necessary and the iron should be hypereutectlc, that is, it should have a carbon content in excess of that given by the formula: 4.3-% (per cent Si-i-per cent P).

3. Silicon content can have any value, but should most desirably be within the range of 2.3 to '7 per cent. With the special-purpose irons containing 10 to per cent silicon, only the hypereutectic graphite can be obtained with certainty in the nodular form.

4. Any deficiency of silicon may be compensated for by the presence of nickel and/or copper in appropriate amounts. In this respect 3 per cent of nickel and/or copper is to be considered equivalent to 1 per cent of silicon. Alternatively, a deficiency of silicon may be compensated for by the addition of ferrosilicon or similar material to the molten metal.

5. The sulphur content of the molten metal to which the cerium addition is to be made should be low, preferably not more than 0.03 per cent or thereabouts, and it is generally not commercially practicable to treat directly, without preliminary desulphurization, an iron whose sulphur content exceeds 0.06 per cent. One of the functions of the cerium addition is to reduce the sulphur content of the treated metal to a value not exceeding about 0.02 per cent as a maximum and most desirably to below 0.015 per cent, and even as low as 0.005 per cent.

6. The phosphorus content should not exceed about 0.6 per cent, and should preferably be less than 0.1 per cent.

7. Manganese, copper, nickel, chromium and molybdenum may be present in any amounts, singly or in any combination, provided the other conditions listed above are observed.

8. After treatment with cerium, the solidified castings must contain at least a certain minimum abount (0.015 to 0.02 per cent) of the element dissolved in the metallic matrix. However, the amount of cerium dissolved in the matrix must not be so large as to give white cast iron.

Of the foregoing requirements, the most important from the standpoint of practically successful use of the process are those covering the carbon and sulphur contents.

Investigations and discussions concerning the influence of cerium in cast iron appear in the literature, but an examination of this literature 4 shows that previous investigators failed to obtain nodular structures because they added too much and obtained white-iron structures, or the irons were white before treatment, or the sulphur contents of the irons being treated were too high. or the irons were not hypereutectlc, or the cerium yield (amount available for solution as such in the metallic matrix) was poor due to the method of making the addition, several of these factors being combined in some instances. No previous investigators have claimed that nodular structures were or could be produced in cast irons by the addition of cerium.

As a result of long and difflcult investigation leading to the present invention, it has become apparent that the first major effect of cerium when added to molten cast iron is to combine with the sulphur to form a cerium-sulphur compound which fioats to the surface of the metal. This accounts for the desulphurizing effect of the element. So long as the sulphur content of the metal is above about 0.015 per cent, or above 0.02 per cent at the most, only this desulphurizing action will take place. Cerium is not free to dissolve in or alloy with the molten iron until the sulphur content has reached this low value. The higher the sulphur content of the metal, the larger is the amount of cerium required to achieve desulphurization, and hence to obtain a given amount of cerium (not combined with sulphur) dissolved in the metal.

After the sulphur content has been reduced to a value of 0.02 per cent, or at any rate to a value of about 0.015 per cent, metallic cerium enters into solution in the molten cast iron, and this cerium, when present in amounts greater than about 0.02 per cent, functions as a powerful carbide stabilizer. It is this dissolved cerium in excess of 0.02 per cent which is the Op ti factor in the process for the production of nodular graphite structures in the as-cast state.

Notwithstanding the distinct novelty of the cast irons of the present invention, it should be clearly understood that nodular graphite structures are by no means unknown, in and of themselves. As has already been pointed out hereinabove, their production in iron castings by heattreatment has long formed the basis of the process for the production of malleable cast iron. For commercial malleable cast iron, the material is cast in the form of hard and brittle white cast iron which contains no graphitlc carbon. These white iron castings are then annealed at elevated temperatures, and during this treatment the hard and brittle iron carbide in the white iron decomposes to give gra hite carbon, commonly termed temper carbon, which exists in roughly spheroidal aggregates or nodules. These temper carbon nodules, by virtue of their approximately spheroidal shape, do not interrupt the continuity of the metallic matrix to the same extent as does flake graphite and so, if the total carbon content is sufllciently low, a material of relatively good shock-resistance and ductility may be obtained. Hitherto this thermal treatment of a solid white cast iron has been the only known way of producing nodular graphite. The present invention obviates the necessity for the lengthy high-temperature annealing treatment of the malleable iron process. However, it should not be misunderstood that the production of nodular gray cast irons by the present novel process in any way constitutes a means of producing malleable cast iron by a short out; for th mechanical properties of the as-cast nodular gray cast irons, as

well as other characteristicaare quite different from those of malleable cast irons as will more fully hereinafter appear. It will accordingly be seen that nodular gray cast iron is a new ferrous to 32 inclusive, which are reproductions of photo- I micrographs of cross sections of various metal specimens, 8. number of which are typical of the novel gray cast irons of the invention.

Before giving further details of the present invention, it may be well to point out that the nodular temper-carbon structure found in malleable cast iron and produced by annealing white iron castings can occur in either of two forms according to the composition of the white iron being annealed. When the white iron has all its sulphur predominantly in the form of manganese sulphide, each nodule in the resulting malleable iron consists of an apparently random clump or aggregate of small graphite flakes, the degree of dispersion of which may vary between wide limits. A typical example of this structure, which may be conveniently termed a graphite-flakeaggregate nodule, is shown in Fig. 1.

However, where the manganese content of the malleable cast iron is inadequate to balance the sulphur, .so that iron sulphide occurs in the material, the temper carbon has a spherulitic structure, a typical example of which is illustrated in Fig. 2. Since spherulitic nodules are obtained also in the nodular gray cast irons of the present invention, it is of some interest and importance to understand and appreciate this graphite structure.

At this point, it should be stated that these structures can be observed only in microspecimens in which thhe graphite itself is polished. A method for accomplishing this has been described by the present applicant in The Journal of The Iron and Steel Institute, vol. 143 (1941), pages 195-205. I

Nodules of graphite having this spherulitic structure each consist of an aggregate of graphite crystallites radiating from a common center or nucleus. The closely packed basal planes of these crystallites are, in general, oriented at right angles to the radii of the spheroid. Graphite spherulites tend to be more perfectly spheroidal Resuming detailed discussion of the invention, it may be pointed out that when cerium is added to hypereutectic cast irons, and when the final sulphur content is below about 0.015 per cent and the cerium content of the-treated metal is above about 0.02 per cent, all the hypereutectic graphite is obtained as well-formed spherulites in the resultant casting. when the silicon content of such an iron is sufficiently high to prevent formation of the white iron eutectic, there is also obtained another form of graphite which originates from the eutectic complex and which, although it sometimes resembles normal flake graphite in general appearancepespecially when the cerium percentage is near the minimum, nevertheless differs radically therefrom. For convenience, this sometimes flakelike graphite is herein termed "quasi-flake graphite. Its manner of formation is quite different from that of normal flake graphite and so, also, is its eflect upon the characteristics of the resultant gray cast iron. Indeed, as will more fully be pointed out hereinafter, its effect upon said characteristics is such and, with increasing amounts of cerium it becomes so increasingly nodular in appearance, that the term nodular, as herein employed, is to be understood as including within its scope such quasiflake graphite as may frequently occur in gray cast irons produced in accordance with the principles of the invention. a

The following example illustrates this effect of cerium on hypereutectic cast irons.

EXAMPLE 1 A 50 lb. charge of hematite pig iron having the following percentage analysis:

Total carbo 3.98 Silicon 3.19 Manganese 0.78 Sulphur 0.028 Phosphorus 0.040

was melted in a crucible furnace and cast, at a temperature of 1380 0., into test bars of four fThe transverse. deflection and tensile ii res given in this application disclosure for 2.1, 1.6, 1.2, 0. and 0.0-in. diameter bars were obtained on bars cast and tested according to British Standard Specification 786/1938; the impact figures were obtained on bars tested according to British Standard Specification 1349/1947 the 3-in. diameter bars were tested in transverse on 18-1n. centers.

Table 1.-Mechanical properties of untreated 1 and cerium-treated 2 cast irons 1 Analysis oi remelted pig iron: total carbon 3.77; silicon, 3.05; manganese, 0.73; sulphur, uwilfihosphorus, 0.039 Remelted pig iron with cerium additions: total carbon, 3.72; silicon, 3.13; manganese, 0.74; s ph Transverse Impact Com ression Test Rupture Stress, f f ffi BHN Siren Stre ugth, i Bar p. s. i. it.-l p. s. i. Diameter,

in. W/O With W/O With W/O With (70 With W/O With W/O, With W/O With Ce Ce Ce Ce Ce Ce 0 Ce Ce Ce (-0 Co Ce Ce 1. 6 51, 500 99, 000 0. 20 0. 32 25.100 55, 050 154 186 71,000 127. 1.2 64, 500 101, 500 1% 0. 38 32, 050 55, 500 1% ...a 36,500 49, 700 0. 875 68, 870 105, 400 0. 18 0. 23 37, 200 59, 550 102 199 0. 6 68, 900 128, 500 0. 11' 0. 22 41. 650 69, 090 ms 239 115, 750 153,01)

er cent.

phur, 0.007 crus, 0.038; cerium, 0.040

Microstructures of all the untreated bars show mixtures of undercooled and normal flake graphite in matrices of ferrite with some pearlitethe amount of flake graphite increasing and the amount of pearlite decreasing with increasing section size. The microstructure of the untreated 1.2-in. bar is shown in Fig. 3. The treated bars all showed hypereutectic spherulites, together with quasi-flake graphite in a matrix of ferrite with some pearlite. The structure of the treated 1.2-in. bar is shown in Fig. 4.

The structure shown in Fig. 4 is, judged by previous standards, completely novel and, bearing in mind the distribution of the graphite, it is easily understandable that the mechanical properties of the treated material are at a distinctly higher level than those of the untreated material. It is to be observed in Table 1 that, even with this simple cerium addition to a high carbon and relatively high silicon iron, 9. material is obtained which immediately falls into the category of a high-duty cast iron.

Although it is to be understood that the invention is not dependent upon any theory of operation, the available evidence indicates that the mechanism of solidification of the type or structure illustrated in Fig. 4 is probably about as is assumed in the following suggested explanation.

The hypereutectic spherulitic nodules form in the melt before solidification of the metallic phases begins. It is not known for certain whether these graphite spherulites are deposited directly from the melt or whether they form by the decomposition of a carbide phase. While the possibility of the latter has .to beconisdered, it

is not easy to imagine and it involves a case oi! extremely rapid carbide decomposition.

The remainder of the melt solidifies as a "eutectic or, more correctly, 9. binary complex of austenite and cementite which decomposes shortly after solidification to give the quasi-flake graphite. spherulites appears to initiate the rapid decomposition of the "eutectic cementite after solidification of the iron.

It will be observed in Fig. 4 that the hypereutectic nodules are surrounded in each case by a volume of metal entirely free from the quasiflake graphite. The reason for this becomes apparent when these nodules are examined at a high magnification, when each is found to have, in addition to the characteristic radial spherulitic structure, a duplex structure consisting of a central spherulite of nodular graphite surrounded by a peripheral layer of graphite.

Such a duplex structure is seen in Fig. 5, which shows a. hypereutectic spherulite in a ceriumtreated iron at a fairly high magnification. This micrograph has been taken under plane polarized light. The central core of this duplex nodule represents .the actual hypereutectic graphite, and the peripheral layer represents the graphite arising from the decomposition of the surrounding "eutectic cementite which has crystallized onto the hypereutectic spherulite. This explains the absence of quasi-flake graphite in the vicinity of each nodule in Fig. 4.

Each hypereutectic nodule appears to have a "sphere of influence within which all further graphite is made to crystallize on the exist n hypereutectic nucleus. This provides a clue leading to a refinement of the simple method of cerium addition to the melt, whereby a larger proportion, or even the whole, of the graphite can be obtained in the spherulitic nodular form. This further refinement or more complete and optimum embodiment of the invention will be hereinafter described in detail but, from what has already been said, it will be seen that ii, the number of hypereutectic spherulites in a given volume of The presence of the hypereutectic metal could be so increased that all their spheres of influence" overlapped, then all of the graphite should occur in the form of duplex spherulitic nodules with a further improvement in mechanical properties. A preliminary indication of the validity of this latter idea is found by studying the structures of centrifugally cast cerium-treated cast irons.

When cerium-treated hypereutectic cast irons are centrifugally cast, the hypereutectic spherulites forming in the liquid are forced to points most distant from the mold face, by virtue of their low relative density. In the centriiugal casting of cylinder liners, for instance, a segregation of these nodules is obtained along the inner surface of the casting. By this segregation the number of hypereutectic nodules is thus artificially increased so that their s heres of influence overlap.

Figure 6 shows a typical segregation of these hypereutectic spherulites near the inner surface of a centrifugally cast cylinder liner made in a cerium-treated hypereutectic cast iron. It will be seen that no quasi-flake graphite occurs in the vicinity of this segregation. At points nearer the mold face containing very few hypereutectic nodules, all of the graphite was in the form of quasifiakes, as illustrated in Fig. 7.

These two micrographs clearly indicate that entirely spherulitic nodular graphite structures can be achieved by increasing the number of hypereutectic spherulites. This ideal structure of spherulitic nodular graphite has been obtained by the use of what will be referred to later as the double-treatment process.

In the example already given the cerium was added to the molten cast iron in the form of the pure element. In this form cerium is a very expensive addition, but it can be added in a variety of cheaper forms, among which misch metal (mischmetall") is perhaps the most convenient. Misch metal usually contains between 43 and 50 per cent cerium, together with the other rare earths and a little iron and manganese. The

' presence of these other elements does not influence the efllcacy of cerium in producing nodular structures in cast irons, and for this purpose they can be ignored.

Misch metal, in pieces of appropriate size, dissolves readily in cast irons at all temperatures above 1200 C. Its solution in cast iron is not explosive or violent in any way. Misch metal has been used as a cerium addition in all the remaining examples to be given hereinafter. However, the addition can be made, if desired, in any other suitable form such as cerium carbide, for example, or as any of the various available alloys of cerium, among which may be mentioned ferrocerium, copper-cerium, nickel-cerium, silicon-cerium, etc.

Cerium not combined with sulphur in cast iron is a powerful carbide stabilizer, and so, for any given section size, there is an upper limit of cerium which must not be exceeded if white iron structures are to be avoided. This remark applies only to the cerium content as found by analysis of the solidified casting when the sulphur content is not in excess of about 0.015 per cent. Because of the carbide stabilizing influence oi cerium its effect can best be studied in relatively large sections.

EXAMPLE 2 To illustrate the eil'ect of increasing additions of cerium, 270 lb. of pig iron of the following composition was melted in an Five test bars, each of 2l-in. length and 3-in. diameter, were cast in green sand molds. For this, five lots of metal each weighing 50 lb. were taken from the furnace. No addition was made to the metal in the first ladle, but increasing amounts of misch metal were added to the metal in the remaining four ladies. The actual additions were: Tap l-no addition; Tap 2-50 grams misch metal; Tap 3-65 grams misch metal; Tap 4-90 grams misch metal; Tap 5-100 grams misch metal. The mechanical properties and cerium analyses of these bars are given in Table 2.

Table 2.Mechanical properties and cerium 1 Tested on l8-in. centers.

Transverse rupture stress, deflection and tensi e strength show a progressive improvement with increasing cerium content. The microstructures of these bars also showed a progressive change with increasing cerium content. The untreated bar No. 1 had coarse flake graphite in a matrix of ferrite with some pearlite, as is shown in Fig. 8; bar No. 2 had a few hypereutectic spherulites with quasi-flake graphite, as fllus- 5 nuclei could be increased ,sufliciently, all of the graphite would occur in sperulitic nodular form and the quasi-flake graphite would be absent.

Production of this ideal structure can be realized in practice through addition of a graphitizing inoculant simultaneously with, or preferably after, the cerium addition. This treatment of the molten metal will be referred to as the "double treatment process. Suitable graphitizing inoculants have been found to be 80 per cent ferro-silicon, silicon-manganese-ziroconium, and

, calcium silicide, the best response having been obtained with the first and second. It is important that the inoculant be not added before thecerium addition as the success of the opera-- 1) tion depends upon obtaining the solution of the metal are added simultaneously. For the inoculant to have the required eifect it is necessary to add sufilcient, in the case of siliconmanganese-zirconium or ferro-silicon, to give a definite increase in silicon of not less than about 0.2 per cent, but no useful purpose is served by using amounts giving silicon yields in excess of 0.5 ner cent.

Results given in Table 3 illustrate the effect of adding varying amounts of 80 per cent ferrosilicon for the same addition of misch metal in each case. These figure are average values, obtained from four 1.2-in. bars cast in each case from lb. of metal treated as follows: Tap 1-70 grams misch metal; Tap 2-70 grams misch metal 2 oz. 80 per cent ferro-sillcon: Tap 3-70 grams misch metal 4 oz. 80 per cent ferro-silicon; Tap 4-70 grams misch metal 8 oz. 80 per cent ferro-silicon.

Table 3.Eflect 0] 80 per cent term-silicon additions Composition, pucont Transverse M Tensile Tap No gm? tion, Slieligth, BHN n??? no s1 Mn 8 r 00 nu.

am 2.07 0.86 0.005 0051 0.010 121.200 0.00 53.200 221 54 3.52 213 0.051 138.000 000 11.000 ms 01 3.40 239 0.050 138.000 0m 74.500 2:03 84 3.50 3.00 0001 142.000. 071 75.100 31 10 trated in' Fig. 9: with increasing amounts of cerium in bars Nos. 3, 4 and 5 the quasi-flake graph te became more nodular in appearance until bar No. 5 had the structure shown in Fig. 10.

That quasi-flake graphite differs from normal flake graphite is made clear when Figs. 8 and 9 are compared, and when the enormous difference in mechanical properties between bars Nos. 1 and 2 is considered. The relative amounts of ferrite and pearlite did not change appreciably with increasing cerium content, but in spite of this there was a sharp increase in hardness when the structure changed from the normal to the quasi-flake graphite.

Double-treatment technique The influence of relatively large amounts of cerium in causing the aggregation of the quasi- Transverse rupture stress, tensile strength and impact strength show a marked increase with the first addition of inoculant (compare bars 1 and 2). The impact results are particularly striking-these figures are obtained on a 0.798-in. diameter unnotched test piece which rarely gives a value in excess of 30 ft.-'lb. for high-duty gray cast iron. and only occasionally a value as high flake rap te c be tilized only in relatively We does not differ greatly through this series.

amen

11 EXAMPLES Similar results can be'obtained when the metal is treated simultaneously with the cerium addition and the inoculant, as is shown by the following example. A mixture of 70 grams of misch metal and 6 oz. of silicon-manganese-zirconium was placed in the bottom of a ladle and 60 lb. of metal run onto it. Four bottom-run 1.2-in. bars were cast and their average mechanical properties and the analysis of one bar are:

Chemical analysis, per cent The microstructure of one of these bars is shown in Fig. 15, and is seen to have spherulitic nodules with only a trace of quasi-flake graphite in a matrix of pearlite and ferrite.

Application the double treatment process tends to offset the carbide stabilizing influence of the cerium and hence to reduce the danger of chilling. This effect automatically permits the use of higher percentages of cerium in the solidifled casting, a factor which in addition helps to improve the structure of the material.

EXAMPLE4 To illustrate the reduction in chilling tendency with the double treatment technique, the following example may be cited. A wedge test piece having the dimensions of one inch at base. 2 in. from apex to base, and 6 in. long was cast in a dry sand mold. The wedge was fractured transversely and gave a chilled zone approximately %-in. deep. The percentage analysis of this wedge was:

' Total carb n 3.77 Silicon 2.69 Manganese 0.58 Sulphur 0.011 Phosphorus 0.024 Cerium 0.061

In this case no double treatment was applied, and the white iron structure of the apex of the wedge is shown in Fig. 16. Only a trace of hypereutectic graphite can be seen. The structure %-in. from the apex is shown in Fig. 17 and, while rather more graphite can be seen, the structure still is predominantly white.

On the other hand, when a similar wedge of the composition Total carb n 3.70 Silicon 2.73 Manganese 0.59 Sulphur 0.008 Phosphorus 0.055 Cerium 0.061

was cast after the application of the double treatment process, it showed no visible chill when fractured. The structure of the apex of this wedge is shown in Fig. 18, where only a ,very small amount of carbide can be seen. At a point %-in.

from the apex only-traces of free cementite ex isted,asisshownlnI'l8. 19.

It appears that an optimum amount of cerium is necessary, even with the double treatment technique. to cause the structure to be entirely spherulitic. When the cerium content is below this value quasi-flake graphite may persist in the structure. This is shown by the results given in Table 4, and the structures shown in Figs. 20, 21,

22 and 23.

Table 4.-A11alyses and mechanical propertiescerium and silicon-manaanese-zirconium additions Composition, per cent 'r.o. s1 Mn s P 00 e011. 3.00 200 0.31 0.000 0.024 0.010 Bet2 3.00 3.10 0.52 0.007 0.02; 0.001

Transverse Defleo- Impact Tensile Set Bar Rupture she Sm ion, BHN Strength Strength,

p a. L in lt.-lb. p. s. i.

Fullcapacity oi the Izod impactmachineusedislwlb.

Table 4 gives the analyses and mechanical properties of two sets of test bars prepared by treating two 50-lb. taps of metal poured from the same melt with the following additions: Set 1-40 gra'ms misch metal followed by 5 oz. siliconmanganese-zirconium; Set 2-60 grams misch metal followed by 5 oz. silicon-manganese-zirconium.

Microstructures of the 1.2-in. bars are shown in Figs. 20 and 21, and those of the 0.6-in. bars in Figs. 22 and 23. The 1.2-in. and 0.6-in. diameter bars of set 1 clearly have some quasi-flake graphite, whereas this form of graphite is entirely absent and the structure is wholly spherulitic in the bars from set 2.

EXAIVIPLE5 With silicon contents below about 2.3 per cent and in the absence of other graphitizing elements, the chilling tendency of cerium-treated nodular irons, even when the double treatment process is applied, is so great that very careful control is necessary in order to avoid chilling in sections of less than %-in.

However, when any deficiency of silicon is compensated for by the presence of appropriate amounts of copper and/or nickel a material of very good mechanical properties can be obtained, even in relatively thin sections. An example of this type is illustrated by the results given in Table 5, which are taken from a double treated nodular iron of the following percentage composition:

Table 5.''Mechanical properties of double treated nodular iron scum and the test bars gave the following figures on subsequent analysis:

The general structure of the 1.6-in. bar from this set is shown in Fig. 24 to consist of spherulitic nodules associated with ferrite in a matrix of pearlite. Figure 25 shows a spherulitic nodule in the 0.6-in. bar at a high magnification-the radial structure can be clearly seen. Occasionally in such samples the duplex structure of the nodules is revealed in a quite interesting manner. A typical instance of this is shown in Fig. 26, which shows a duplex nodule in the 1.6-in. bar. The central hypereutectic nucleus can be clearly seen with the remainder of the graphite, still in the spherulitic form, attached to the nucleus at only a few points.

EXAMPLE 6 The double treatment can be applied and useful mechanical properties obtained with silicon contents as high as 6 per cent. With increasing silicon up toabout per cent the transverse rupture strength, tensile strength and Brinell hard-' ness number do not change appreciably, but the shock-resistance drops progressively. The following flgures, obtained on a fairly high silicon iron, illustrate the type of results which can be obtained:

Chemical analysis, per cent- Total carbon 3.14 Silicon 4.13

Manganese 0.85 Sulphur 0.004 Phosphorus 0.046 Cerium 0.051

Mechanical properties (1.2-in. diameter bar)- Transverse rupture stress, p. s. i 141,500

Deflection, in 0.38 Tensile strength, p. s. i 77,500 Brinell hardness 234 Impact strength, ft.-lb 29 The microstructure of this sample consisted of spherulitic nodules in a matrix of ferrite with a little pearlite, as is shown in the photomicrograph of Fig. 27 at a magnification of 100.

. EXAMPLE! For instance, 50 lb. of a phosphoric pig iron was melted in a crucible furnace and treated vwith 100 grams of misch metal when at a temface of the metal. This was skimmed oil and the rest of the metal poured into test bars. The

Transverse Composition, per cent Bar Tensile Impact V Size, gf g ff Strength, BHN Strength, 5

p. s. i. Scum Metal Poured .44 Not detected Obviously, there is a considerable segregation of cerium and sulphur in the material skimmed from the surface of the liquid metal.

Although nodular structures can be obtained with up to 0.6 per cent phosphorus, the mechanical properties fall of! progressively with increasing amounts of this element. The results shown in Table 6 give the mechanical properties of a fairly high phosphorus iron of the following percentage composition:

This metal was treated with misch metal alone-the double treatment was not applied. The results obtained may therefore be compared with those in Table 1. The microstructure of the 1.2-in. bar from this series had large hypereutectic spherulites and quasi-flake graphite in a matrix of pearlite with phosphide eutectic. This structure is shown in Fig. 28.

EXAIWPLEB the element is present, to give martensitic structures. The results given in Table 7 show the tensile strength and hardness of two relatively high manganese nodular irons produced by the double treatment process. These high mananese irons were cast in the form of 1.2-in. diameter bars.

Table 7.--High manganese nodular irons Composition per cent Tensile N 0. Strength, BHN

'r. 0. s1 Mn s Ce Bar No. 1, containing 2.01 per cent manganese, had a microstructure consisting of uniformly dis- Ce, 0.023 per cent 18 tributed spherulitic nodules in a matrix of about 70 per cent pearlite and 30 per cent ferrite. The graphite structure of bar No. 2 was very similar tothatofbarNo.1,butthematrixinthiscase was almost entirely pearlitic with a small amount of martensite and a little carbide. The structure of bar No. 2 is illustrated in Fig. 29. The tensile strength and hardness values of the irons are seen to increase as the structure becomes pearlitic.

EXAMPLE 9 High manganese nickel-containing austenitic irons may be treated with cerium to give nodular structures with a considerable improvement in mechanical properties. This is illustrated by theresults given in Table 8, which show the mechanical properties of an untreated and a ceriumtreated nickel-manganese austenitic iron. The double treatment was not applied in this case. The untreated material had a structure of coarse flake graphite in a matrix of austenite, and the treated material had a nodular structure in a smiliar matrix with a little carbide.

Table 8.- Nickel-manganese austenitic iron Transverse Bar Tensile Impact Size, ggy fif Strength, BHN Strength, in. p. s. i. it.-lb.

p. s. i.

Untreated: T. 0., 3.23; 01, 3.0a; Mn, 5.8; 8, 0.022; P, 0.012; Ni, 11.87 per cent Transverse Bar Tensile pact Size, gfg fg Strength, BEN Strength, PM: p. s. i. .-l

Treated: '1. 0., 3.03; s1, 3.18; Mn, 5.0; 8, 0.018; P, 0.053; Ni, 12.00

EXAMPLE When the nickel content of nodular cast iron exceeds about 10 per cent it is not necessary for the iron to be hypereutectic according to the formula: per cent 0/ (for eutectic) 4.3 (per cent Si per cent P) Therefore the treatment can be readily applied to ordinary nickel-containing austenitic irons. In Table 9 the chemical analyses and mechanical properties of treated and untreated nickel-containing iron are given for material cast in the form of 0.8'75-in. diameter test bars.

Table 9..Mechanical properties of nickel- The structure of the untreated bar is shown in 1'18. 80 to consist of undercooled graphite in a terial was, in this case, produced by treatment of the melt with misch metal alone the double treatment technique was not employed.' It is apparent that the nodular nickel-containing iron has mechanical properties considerably in excess of those in the untreated material.

EXAMPLE 11 With relatively high sulphur contents the yield of cerium depends upon the extent of the desulphurization produced by the addition. In such cases, if desulphurization by the cerium is allowed to take place, the yield of cerium will be low, but it is imperative for the success of the process that this desulphurization should occur.

Table 10.-Desulphurizing influence of cerium 1111??? or (0 22111 e as 0 me p y r tructure per cent per mm sis), per ogit 0. l 0. 099 0. 066 Flake graphite.

0.3 0. 014 0.003 Noduiar graphite.

If desulphurization by the cerium addition is not effected, the apparent yield of cerium will be high, but this cerium, as found by chemical analysis, will be largely combined with the sulphur and hence will not operate to produce nodular structures. As an instance of the desulphurizing influence of cerium, the figures in Table 10 may be quoted for the treatment of cupola-melted nickel-containing iron. These figures clearly indicate the importance of the cerium not combined with the sulphur in determining the graphite structure.

EXAMPLE 12 The production of nodular cast irons by the process described herein presents no special metallurgicai' difliculties with batch-type melting units if high-carbon and low-sulphur charges are used. The material has been successfully pro-' duced in directand indirect-arc furnaces, highfrequency induction furnaces, crucible furnaces and oil-fired rotary furnaces by melting low sulphur (of the order of 0.03 per cent sulphur) hematite pig iron and making the appropriate additions of cerium in the ladle.

With the cupola melting furnace the production of molten iron siutable for the application of this process becomes more difficult and, in general, special precautions need to be taken to ensure that the metal is of the correct composition. The production of high-carbon or hypereutectic cast irons from the cupola is not a difficult matter provided the average carbon content of the charge is reasonably high.

Production of satisfactory low-sulphur contents presents much more difficulty. Even when lowsulphur charges are used there generally is a sulphur pick-up from the coke and the sulphur content of the metal tapped will be too high to allow immediate treatment with misch metal, and it therefore becomes necessary to interpose a preliminary desulphurizing treatment of the metal in the ladle.

If the sulphur content of the metal tapped can;

from the cupola 'is within the range of 0.05-0.08 per sent, it is possible to desulphurize the metal to a satisfactory low-sulphur content by one treatment with sodium carbonate, preferably using a ladle lined with basic refractory. With sulphur contents in excess of 0.08 per cent the "double ladle technique of desulphurizlng may have to be applied.

Nodular' cast irons can be produced from the cupola, using the sodium carbonate desulphurizer prior to the addition of the misch metal. The mechanical properties given in Table 11 illustrate the results which have been obtained from a eupola operating under industrial conditions.

Table 11. Mechanical properties of canolamelted cast iron desulphurized with sodium carbonate lcomposition; 'l. 0., 3.69; Si, 2.77; Mihuos; 8, 0.005; P, 0.046; Ce,

0.057 per Transverse Test Bar Tensile Diameter, gggg' g Strength, BHN

in. p. s .i. p. s. i.

3. 106, 000 0. 51 85, 000 211 2.1 142,700 2.14 1.6 152,000 1.30 u. 1. 2 137, 500 0. 05 so. 400 218 0. 875 174, 300 0. 83 76, 500 214 0.6 179,500 0.53

These results were obtained from test bars cast from a cupola run in which the metal was tapped directly onto sodium carbonate placed in the bottom of the ladle, the slag thickened with limestone and skimmed oil, and then misch metal and silicon-manganese-zirconium added. The micro- 18 limits, may materially enhance the improvement in mechanical properties.

Where the described double treatment technique is employed, the cerium content of the novel gray cast irons is no longer limited to the maxima given in Table 12, and the amount required to yield maximum mechanical properties can be employed without risk of producing white iron. Generally speaking, however, about 0.50 per cent cerium (uncombined with sulphur) in the casting can be regarded as the practical maximum, and

. it is seldom necessary to exceed 0.25 per cent. In

, may be added thereto.

structure of all bars of this set had spherulitic nodular graphite. The structure of the 1.2-in. bar is shown in Fig. 32.

For a casting of any given size, there is a. maximum proportion of cerium which must. not be exceeded if the formation of white iron is to be avoided. By section size is. meant the thickness of the casting at its smallest cross-section. In Table 12 below are given the approximate maximum proportions 'of cerium that are permissible in castings of various sizes, where cerium additions alone are concerned; but it is to be understood that this is illustrative only and that the invention is not limited in its applicability to castings of any particular order of magnitude.

Table 12 Maximum Per- Section Size in Inches outage 01 erium White Iron Less than u". M- /i er rs-iis H-U for Avoidance of the great majority of cases, gray cast irons produced in accordance with the invention and containing not more than about 0.10 per cent cerium are eminently satisfactory.

It may be pointed out that, for accurate determination of small quantities of cerium in cast iron, no reliable analytical methods were available until comparatively recently. In connection with work leading to the present invention, it was therefore necessary to arrange for the development of special analytical procedure (The analyst, vol. 73, pages 2'l5282, May 1948). This was employed in obtaining the analytical figures for cerium given herein.

The requirement that "cerium-treated cast irons within the scope of the present invention must solidify gray. that is, must be gray. when cast, imposes a practical limitation on the amounts of other metals which the untreated cast iron starting material may contain or which Typical practical maxima are: nickel, 40%; copper, 7%; chromium, 4% molybdenum, 2% vanadium, 2%. The gray cast iron starting material, and also the cerium-treated gray cast iron produced. may contain as little as 0.5 per cent silicon when the deficiency below 2.3 per cent is compensated for by the presence of nickel and/or copper in appropriate amounts on the equivalency basis mentioned hereinabove.

In practicing the new process, the ceriumsupplying material, whether it be the element itself or an alloy or compound thereof such as misch metal, is best added to the molten cast iron starting material in the ladle. Certain losses of cerium occur which vary with different operating conditions 'and for which due allowance must be made in determining the amount of cerium-supplying material to be added. A good procedure is to run the molten iron on to the addition placed, for instance, in the bottom of the ladle.

It is to be understood that all percentages stated herein are by weight.

The cerium-containing nodular gray cast irons of the invention are not ductile and are still relatively brittle. They differ from malleable cast iron additionally in that their carbon and silicon contents are normally appreciably higher. These two elements, as well as pearlite which most as-cast gray irons of the invention contain, tend to reduce ductility and increase brittleness. In the as-cast condition the novel gray cast irons commonly. have a shock resistance considerably better than that of ordinary high-duty gray cast iron, and tensile strengths considerably higher than those of either ordinary malleable cast iron or high-duty gray cast iron.

Moreover, ahigh level of mechanical properties can be achieved in nodular gray cast irons at a fairly low level of hardness.

This application is a continuation-in-part of prior applications of this applicant, Ser. No.

19 a 760,118, filed July 10, 1947, now abandoned, and Ser. No. 23,859, filed April 28, 1948, and co-pendin herewith. All subject matter contained in said applications not inconsistent with the present disclosure, is to be regarded as included herein by reference.

What is claimed is:

1. Gray cast iron which contains a small proportion of cerium uncombined with sulphur and in which, in its as-cast state, at least the greater part of its graphite content has a nodular microstructure. said iron containing no sulphur in excess of about 0.02 per cent.

2. Gray cast iron which contains a small proportion of cerium uncombined with sulphur and in which, in its as-cast state, at least the greater part of its graphite content has a nodular microstructure, said iron containing no sulphur in excess of about 0.02 per cent, said iron exhibiting by standard strength tests mechanical properties superior to those of comparable gray cast iron not containing cerium.

3. Gray cast iron according to claim 2, the cerium content of which is within the range of about 0.02 to 0.50 per cent.

4. Gray cast iron according to claim 3, the cerium content of which is within the range of about 0.02 to 0.25 per cent.

5. Gray cast iron according to claim 4, the cerium content of which is within the range of about 0.02 to 0.10 per cent.

6. Gray cast iron according claim 2, which contains from about 0.02 to 0.50 per cent cerium, is of hypereutectic carbon content, and has a nodular graphite structure that is in substantial part spherulitic.

7. Gray cast iron according to claim 6, in which said nodular graphite structure is partly spherulitic and partly quasi-flake, said iron containing at least about 0.02 per cent and not more than 0.25 per cent cerium.

8. Gray cast iron according to claim 2, which contains from about 0.02 to 0.10 per cent cerium and no sulphur in excess of 0.015 percent, is of hypereutectic carbon content, and has a nodular graphite structure ,that is substantially wholly spherulitic.

9. Gray cast iron according to claim 2, which contains from about 0.02 to about 0.05 per cent cerium and no sulphur in excess of 0.015 per cent,

is of hypereutectic carbon content, and has a nodular graphite structure that is in substantial part. quasi-flake.

10. Gray cast iron which. in the as-cast state, has its graphitic carbon content largely in nodular form; said cast iron containing from about 0.02 to 0.50 per cent cerium, from about 2 to 4.4 per cent total carbon, not more than 0.02 per cent sulphur, not more than 0.6 per cent phosphorus, from 0.5 to 7.0 per cent silicon and, when its silicon content is less than 2.3 per cent, another graphitizing agent comprising at least one metal selected from. the group consisting of nickel and copper, the total amount of such other graphitizing agent employed being at least three times the deficit of the silicon prcentage from 2.3,. but the nickel content of said iron never exceeding 40 per cent and the copper contentnever exceeding 7 per cent.

11. The process of producing an improved gray cast iron wherein the graphitic carbon is largely in nodular form, which comprises melting an iron which on casting gives a gray cast iron containing not more than 0.6 per cent of phosphorus, and adding to the melt, before casting, an amount of cerium-supplying material which will result in a gray cast iron containing not less than about 0.02 nor more than 0.5 per cent cerium uncombined with sulphur, and not more than 0.02 per cent sulphur.

12. The process according to claim 11, wherein the amount of cerium added to the melt is such as will result in a gray cast iron containing from about 0.02 to 0.25 per cent cerium.

13. The process according to claim 11, wherein the amount of cerium added to the melt is such as will result in a gray cast iron containing from about 0.02 to 0.10 per cent cerium.

14. The process according to claim 11, which further includes adding a graphitlzing inoculant to the melt before casting, but not prior to adding the cerium-supplying material.

15. The process according to claim 14, wherein the graphitizing inoculant is selected from the group consisting of ferro-silicon, silicon-manganese-zirconium and calcium silicide.

16. The process of producing improved gray cast iron which comprises providing a melt or low-sulphur gray iron of hypereutectic carbon content which on casting gives a gray or :t iron, adding to said melt while its temperature is within the range 1200 to 1600 C. an amount of a cerium-supplying material sufllcient t reduce the sulphur content of the metal to below 0.02 per cent and also to provide cerium free to dissolve in said metal in amount equal to at least 0.02 per cent thereof but not exceeding 0.50 per cent and not sufficient to produce white iron, and casting the cerium-treated melt.

17. The process according to claim 16, which further includes adding a graphitizing inoculant to the melt before casting, but not prior to add- :ing the cerium-supplying material.

18. The process of producing improved gray cast iron which comprises providing a melt of low-sulphur gray iron which on casting gives a gray cast iron, and which contains at least one metal from the group consisting of nickel and copper in amount at least suilicient to compensate for any deficiency there may be in silicon content of said iron below 2.3 per cent, adding to said melt while its temperature is within the range 1200 to 1600 C. an amount of a ceriumsupplying material suflicient to reduce the sulphur content of the metal to below 0.02 per cent and also to provide cerium free to dissolve in said metal in amount equal to at least 0.02 per cent thereof but not exceeding 0.50 per cent and not 'sufllcient to produce white iron, and casting the cerium-treated melt.

19. The process according to claim 18, which further includes adding a graphitizlng inoculant to the melt before casting, but not prior-to adding the cerium-supplying material.

, HENTQN MORROGH.

No references cited.

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