US3375103A - Alloyed cast iron - Google Patents

Description

United States Patent 3,375,103 ALLOYED CAST IRON Albert Collaud, Fribourg, and Jochem Thieme, Balsthal,

Switzerland, assignors to Von Roll 'AG., Gerlafingen, Switzerland, a corporation of Switzerland No Drawing. Filed Nov. 30, 1964, Ser. No. 414,906 Claims priority, application Switzerland, Dec. 4, 1963,

7 Claims. (Cl. 75-423) ABSTRACT OF THE DISCLOSURE An extremely tough cast iron containing 2.5% to 3.8% carbon; less than 0.08%, and preferably less than 0.04% phosphorus; less than 0.08%, and preferably less than 0.04% chromium; less than 3 ml./ 100 g. hydrogen; less than 0.3%, and preferably less than 0.15% manganese; less than 0.03% sulfur; the remainder being iron with trace amounts of disturbing elements such as arsenic, boron, antimony, lead, tellurium, selenium, and a silicon content which is lower than that of conventional gray cast iron. This product is extremely tough and has good casting properties.

The present invention relates to improved alloyed cast iron, preferably alloyed gray cast iron.

The specifications of all countries divide the different types of gray cast iron into specific classes in accordance with their tensile strength for test bars of 30 millimeters diameter. This tensile strength 0 is really, in fact, not characteristic of quality. It is not only dependent upon the quantity of graphitic carbon and its distribution, the effect of which is expressed by the modulus of elasticity E but also from the strength of the matrix which is characterized by the Brinell hardness A. Between these three values there exists the following relationship:

wherein, alpha (a) assumes a value of approximately 1.05 10- for a gray cast iron produced under normal conditions (cf. A. Collaud, lecture given during the International Foundry Congress in Zurich, Switzerland, 1960). By employing different treating processes, for instance, by the addition of inoculants, it is possible to obtain higher value of alpha (or), that is, with given values of E and A to increase the tensile strength, so that alpha (or) can be considered a quality characteristic.

It has also been proposed to determine the quality of a gray cast iron according to the following criteria (W. Patterson, Relative Hardness and Degree of Normality as Concept for the Evaluation of Gray Cast Iron, Foundry (Giesserei) 1958, page 385): presupposing the same analysis (eutectic carbon equivalent, C.E.) a gray cast iron is that much better the greater its strength (degree of normality RG) and the lower its hardness (duret relative, DR). By grouping together both concepts there is obtained a well defined quality index (Q according to the following equation:

Both quality concepts only have reference to static loading, not to dynamic loading however. Furthermore, there is thus overlooked the fact that the gray cast iron is more or less a brittle material, in other words, it possesses a very limited deformability.

It is, therefore, not surprising that the use of gray cast iron is limited and that a mechanical failure of gray cast iron pieces is generally attributable to a sudden impactlike loading. In such cases, the reliability in performance is only capable of improvement by increasing the tensile strength to a value greater than 40 kg./mm.

The observed impact toughness or strength measured at round test bars of 20 millimeters diameter with millimeters spacing between the supports, in the most favorable situations, reaches a maximum modulus .of impact of 10 kg. cm./cm. (impact energy divided by the Volume of the test bar .between the supports according to the proposals of A. de;Sy).

Up to the present one was always inclined to attribute this sensitivity to percussion or impact solelytothe notch effect of the graphite lamellae and attempted to improve the form of graphite separation. Thus, for example, .there appeared castiron with .spherulitic graphite in :which the graphite separated in spherical form. However, with the thereby resulting advantages there is not only brought about an increase in price of the cast iron, rather, there is also impaired the very favorable technical casting properties ofgray castiron with lamellar graphite.

Although it was known from steel production 'thata rather large ductility, as such for example must be present with sheet metal capable of being deep-drawn, could only be guaranteed by a ferrite lattice which is as pure as possible, those skilled in the art .were .of the opinion that with gray cast iron having lamellar graphite dis tribution the notch effect brought. about by the graphite lamellae is so large that an improvement of the ductility i.e. toughness of the matrix would not bring about any success at all. For such reasons, all attempts to improve the properties of gray cast iron always started by eliminating the graphite lamellae and by replacing such with a more favorable form of graphite structure (cast iron with spherical graphite, malleable cast iron). As already mentioned above, a cast iron with spherulitic or nodular graphite, in addition to the advantages which result without doubt, also exhibits certain disadvantages however.

Accordingly, it is avprimary object of the present invention to provide an improved cast iron possessing great toughness and good castability.

Another important object of this invention is to produce a gray cast iron of improved properties.

Thus, in accordance with the invention ithasbeen found that also with cast iron having lamellar graphite formation unsuspected high toughness values can be .obtained, if, with a cast iron having more than 2.5% total carbon, lamellar graphite formation and low content of iron associates, such as phosphorous in a content beneath 0.08%, preferably beneath 0.04%, a chromium, content beneath 0.08%, preferably beneath 0.04%, a hydrogen content beneath 3 cubic centimeters per 100 grams iron,

as Well as .a lowest content of disturbing elements, such as perhaps arsenic, boron, antimony, lead, tellurium, selenium and so forth, there is provided as -,pure. as possible, non-distorted ferrite lattice, a manganese content beneath 0.3%, preferably beneath 0.15%, a sulphur content beneath 0.'06%, preferably beneath 0.03%, and a silicon content lower than for conventional cast iron.

It will be appreciated that percents are given herein as percents by weight, unless otherwise indicated. Furthermore, the term disturbing elements as employed herein is generally intended to signify undesired materials which, due to the'formationof mixed crystals, are responsible for the distortion of the ferrite lattice.

This alloy also possesses the good casting properties of a conventional cast iron.

It is generally known that an increase of the-manganese content has the tendency of impairing the plastic deformability of the gray cast iron. With pearlitic gray cast iron such only comes into appearance due to the fact that a larger portion of the manganese is bound in the cementite as carbide. On the other hand, such influence becomes particularly perceptable if the manganese is com- 3 pletely dissolved in the ferrite, as such occurs with annealed ferritic gray cast iron.

The presence of the manganese in the ferrite lattice causes a distortion, the effect upon, plasticity is much more pronounced than previously thought. From the handbook Hiitte, directed to persons concerned with ironworks, 5th edition (1961), page 3, it can be seen that the lattice constant of a body-centered cubic alphairon amounts to 2.87 A. 9l0 on the other hand 8.91 A. for cubic-centered alpha-manganese 727).

Silicon forms with iron brittle mixed crystals and thereby impairs the plastic deformability. In consideration of the production of cast iron with high toughness it is, therefore, desirable to maintain the silicon contents according to the invention, which are lower than with conventional cast iron, preferably at a value amounting to a fraction thereof.

Due to the considerably lower silicon content the carbon equivalent (C.E.) of a tough gray cast iron provided for a given cast piece is considerably lower than with a conventionally produced gray cast iron. Thus, it could be feared that the favorable technical casting properties are thereby rendered considerably poorer. Experience has, however, shown that this fear is not justified. In both cases, the quantity of graphite separated during solidification is approximately the same.

A correlation exists between the sought-for inventive low manganese and silicon content since, as is known, manganese increases carbide stability, that is, the danger of white solidification throughout the entire cast piece or at least at the edges increases. For this reason, such effect of the manganese content is generally compensated by an appropriate increase of the silicon additive. Constituent with the already required purity or cleanliness of the melt-what is meant are the iron associates, alloying constituents and disturbing elementsa low manganese content only then, however, permits reducing the silicon content considerably below the usual value without impairing the graphitizing tendency and thereby to increase toughness.

From publications there clearly can be seen that a manganese content beneath 0.5% is not suitable for commercial gray cast iron-what is perhaps of even greater importance-the reduction of the manganese content was not recognized as decisive for the improvement of the properties of the gray cast iron, in particular for achieving a tough gray iron. Moreover, the opinion apparently prevailed that a manganese content beneath 0.5 or, in fact, still beneath 0.25% not only did not provide any advantages, rather, in fact, brought with it disadvantages for the quality of the structure. The surmounting of this technical prejudice is one of the most essential aspects of the present invention.

According to these embodiments or examples it could be concluded that the manganese content should be reduced as much as possible, or, better still, completely avoided; however, such as not possible becouse the manganese has the function of completely or partially binding the present sulphur because it is an exceptionally strong carbide stabilizer. The following rule of thumb is known:

Manganese percent=0.25 +1.7 sulphur percent However, it has been found that for the production of tough gray cast iron a different correlation exists between the manganeseand sulphur content and that, in particular, the production of tough gray cast iron is not bound to this formula. The toughness of a casting according to the invention is then at its greatest if the following relationship is fulfilled:

Manganese percentZS sulphur percent,

preferablyZLlO sulphur percent Additionally it has been found that also a gray cast iron with 4 Manganese percent5 sulphur percent,

preferably 3 sulphur percent still possesses a toughness which lies above that of conventional gray cast iron, in addition thereto, however, possesses an increased wearibility and hardness with concurrent good workability.

During the extensive tests and experiments of applicants it was determined that a portion of all cast test pieces of tough gray cast iron contained a content of bound carbon of more than 1%, generally up to 1.4%. Surprisingly, the appearance in fracture and photomicrographs of the test pieces was not speckled or mottled, rather gray, corresponding to a maximum content of bound carbon of 0.8%. This increased content of bound carbon can be attributed to the mentioned unexpected properties. The applicants have examined the interrelations or correlations and found that only the Mn/S-ratio is decisive therefore. Naturally, there also exists the possibility of rendering the sulphur content harmless by binding with other elements, whereby a further reduction of the manganese content is again possible. In this case, that is, as soon as the sulphur is bound to other elements, the determined analysis values no longer give the actual present Mn/S-ratio. In this situation, alloyed cast irons, the analysis of which indicates a Mn/S-ratio beneath 5:1 still correspond to those with a ratio of above 5:1, they are, therefore, also alloyed cast irons with maximum toughness.

As process for the metallurgical production of the inventive alloyed gray cast iron, processes serving this purpose during the production of steel for example can be employed, whereby, depending upon the properties of the cast iron, the carbon and silicon must not be considered as disturbing elements.

it is possible, for example, to start from a pig iron suitable especially for the purpose of the present invention and to correct such to the desired terminal analysis in a suitable, preferably electrically-heated furnace having a slag blanket or a furnace cover in order to completely enclose the furnace proper.

It is possible to melt a gas-poor and sulphur-poor pig iron which is as free as much as possible from adhering rust and possessing low content of manganese and chromium in a suitable acid-lined, better still, basic-lined furnace, whereby care must be taken to ensure that the melt cannot come into contact with steam in order to prevent the taking-up of hydrogen (covering with a compact, highly-fluid, slag blanket; employing a tightly sealing hood, and so forth). If necessary, the hydrogen content must be reduced by suitable processes (gas cleansing or flushing treatment, refining or oxidation process, vacuum treatment, and so forth).

It .is also possible to start with a steel poor in gas and manganese with subsequent recarburization and siliconizing, such steel preferably being regenerated in a basic-lined furnace or crucible, whereby the taking-up of hydrogen must be prevented.

It is likewise possible to start from a conventionally produced liquid gray cast iron with the content of phosphorous, chromium and sulphur (de-sulphurization if necessary) as low as possible, and to blow such in a preferably acid converter or crucible with air or oxygen which is as dry as possible. After a blowing time of several minutes there is not only observed a considerable carbonization or degasing, rather, also, a burning-out of the manganese till reaching the desired value. Carburizing agents (preferably fragments of graphite electrodes) can thus, if necessary, be added as much as possible until the carbon content has reached the desired value after the provided blowing time. The same holds true for silicon, by means of whose consumption or melting loss there results the quantity of heat necessary for maintaining the temperature constant.

A tough gray cast iron produced according to the present invention can be employed in the as-cast condition as well as also after a heat-treatment. A heat=treatment permits further increasing the toughness. It is possible to ferritize tough gray cast iron quicker and -essentially more completely than normal or conventional gray cast iron by a suitable heat-treatment. 5

It is possible to further increase the properties of a tough gray cast iron by alloying elements. In the-selection of such elements, in addition to economical considerations, the following metallurigical requirements are also applicable however:

(a) The element should not possess any or only a weak carbide stabilizing effect, since otherwise the silicon content must be increased.

(b) The element should be completely soluble n the range of use in the iron also in solid condition, without forming a bond and should possess a lattice constant similar to iron (2.87 A.), so that there is considerably avoided a distortion of the ferrite lattice.

(c) The element should not undertake any bond with carbon.

Due to these requirements, as a practical matter there only come under consideration elements suchas nickel (3.52 A.), molybdenum (3.15 A.), copper (3.62 A.) and cobalt (2.51 A.).

It has been found that there can be achieved very high values of strength (30-35 kg./mm. With concurrent maximum toughness (modulus of impact kg. cm./cm. in annealed condition with an alloy of 0.5 1 Ni and/or 0.5%1% Mo. Furthermore, an addition of 0.2%-0.6% M0 is recommendable for cast members subjected to heat, where it is desired to utilize the good heat conductivity and toughness and additionally an increased heat resistance is sought.

The addition of nickel is also-of advantage due to the following considerations:

A certain quantity of silicon is required for the gray solidification of tough gray cast iron, even if such quantity is smaller than in comparison with conventional gray cast iron. However, even in smaller quantities silicon embrittles the iron; it has a lattice ,constant of 5.43 A. However, gray solidification is also enhanced by nickel, even if only approximately one-third as intensive as with silicon. It is possible, therefore, to produce a tough gray cast iron which instead of, for example, -1.0% Si contains approximately 2.4% Ni and only 0.2% Si.

In the as-cast condition and after a ferritizing heat treatment, in both cases, the following properties were measured at test bars of 30 millimeters'diarneter:

[Transverse-bending test (test bar of 20 millimeters diameter with 400 millimeters spacing between supports)] Bending strength, kg./mm. 70. 9 54. 9 71.0 55. 5 Total deflection; mm. 11.3 16. 3 18. 0 25. 8 Plastic or permanent deflection,

mm 10. 9 ll. 1 20. 2 Total energy of fracture, kgm./

cm. 48. 8 57. 1 89. 0 107. 2 Plastlc or permanent energy of fracture, kgmjemfi 23. 6 42. 4 64. 5 91. 7

From this comparison it will be observed that tough gray cast iron according-to the invention is not characterized by tensile strength, hardness and modulus of elasticity, rather by its dynamic and static energy of fracture as *Well as its impact modulus, the values of which are 2 to 3 times higher thanthose of a commercial gray iron of good quality.

EXAMPLE 2 With the following alloy it is possible 'to cast completely gray solidified round bars of 30 millimeters diam- 'eter with considerably reduced silicon content due to inoculation, which would otherwise solidify white without inoculation:

Brinell O, S1, Mn, P, S, Hardness Percent Percent Percent Percent Percent (H13) If such type 'rodsarecast from conventional cast iron then they solidify white, even if such is inoculated.

EXAMPLE 3 A comparison of the analyses for casting boiler members with '6-8 millimeters wall thickness results in the following data:

Type 01 Casting C, Si, Mn, P, S, GE.

Percent Percent Percent Percent Percent Conventional Gray Cast Iron 3. 62 1.80 0.70 '0. 26 0.07 1.00 Tough Gray Cast Iron 1.03 0.11 0.02 0.01 0.84

EXAMPLE 1 The boiler members of tough gray cast iron solidified completely. gray and dense. An examination of the boiler members showed that the plastic deformability and rupture limit amounted to a multiple of that of boiler members formed of conventional gray cast iron.

EXAMPLE 4 Cooking utensils possessing 2-4 millimeters wall thickness were cast from tough gray cast iron and later enameled. A comparison of the analyses resulted in the following data:

Type of Casting C Si 'Mn P S 7 Percent Percent I Perceht Percent Percent C'E.

Ctfilventional Gray Cast 3 l on .4 3.5 2.6/2.8 0.4/0.5 0.4/0.6 0.-1 1.02 1.07 Tough Gray Cast Iron 3.5/3.6 1.6/1.8 Approx. 015 Approx. 0.03 Approx. 0.02 0.94/098 parison there will be considered a lowcarburized, alloyed gray cast iron having particularly good mechanical properties, and possessing the following analysis:

C,.2.98%;Si, 1.92%; Mn, 1.03%;P, 0.18%; S, 0.07%;

-Cr, 0.09%; Ni, 0.38%;Mo, 0.59%; C.E.,:O.834..

It is completely impossible to work with such a low Si-content with conventional cast iron. The utensils :formed of tough .gray cast iron could "be excellently :enameled. Similarresults were also obtained during tests with pipeproduction. I

7 EXAMPLE Examples for the bonding of sulphur to other elements with special inoculating agents:

(a) There was undertaken comparative inoculation with CaSi (conventional inoculation) and with an alloy containing (special inoculation) approximately 55% Si, approximately 21% Ca and approximately 3% Mg. The result was as follows:

Inoculation C, Si, Mn, P, S. Bound 0 Percent Percent Percent Percent Percent Conventional 3.30 1. 0. 145 0.03 0. 015 0.67 D0 3. 23 1. 07 O. 067 0.03 0.016 1. 24 Special 3.18 1. 25 0.059 0.03 O. 014 0. 51

(b) Again in comparison with CaSi (conventional inoculation) there was worked with an alloy containing (special inoculation) approximately 68% Si, approximately 23% Ca and approximately 4% rare earth. The

conventional cast iron which is possible in consequence of the lower carbide stability, that is, due to the much greater tendency to gray solidification and due to the inoculation.

re ult wa as follows; 20 Thus, values of heat conductivity of 0.145/ 0.165 cal./

Inoculation C, Si, Mn, P, S, Bound 0 Percent Percent Percent Percent Percent The content of bound carbon, in this instance, was determinative as a measure for the Mn/S-infiuence.

EXAMPLE 6 With the use of cast iron as the material for members subjected to heat its heat conductivity is an important factor. This property is of even greater importance the quicker and more disuniformly the cast pieces are heated or cooled down. Under these circumstances, there always results the formation of stresses which can lead to a destruction, at least to a deformation of the workpiece however. Hence, the smaller the heat conductivity of the material the greater will be the temperature differences within the workpiece and that much greater will be the occurring stresses. Accordingly, a heat conductivity which is as large as possible is desirable for such members.

The maximum heat conductivity obtainable with ironworks materials is that of pure iron which is given as 0174/0190 caL/cm. sec. C. With gray cast iron this value experiences a certain reduction, even if it is a small one, due to the forced presence ofgraphite. This statement is, however, only of theoretical importance, since it presupposes pure iron-carbon-alloys, which never come into use in actual practice. Technical cast iron always contains changing amounts of silicon, manganese, phosphorous, sulphur, and so forth.

However, all of these elements lead to a further reduction of the heat conductivity. Of these elements, silicon and manganese are of particular interest. If the heat conductivity of pure iron is made equal to 100, then with the presence of silicon and manganese, depending upon cm. sec. C. were measured at tough gray cast iron which previously were thought to be impossible for cast iron.

Striking examples for the useful application of the improved heat conductivity in combination with the actual exceptional toughness of the tough gray cast iron in comparison to conventional cast iron are the production of iron molds, rollers, special steel mill cast iron molds and rolling mill rollers. It was already desirable for some time to replace the previously employed hematite casting by a material possessing greater toughness, in order that such would be in a position to reduce thermal stresses by plastic deformation.

The high heat conductivity of the tough gray cast iron prevents the appearance of thermal stresses and the good toughness is sufficient to reduce stresses still appearing without, however, being so great that it is necessary to fear a change of form.

Whereas, the cast iron molds of steel mills generally possess approximately the following analysis: C, 3.2/ 3.8%; Si, 1.5/2.2%; Mn, O.5/1.2%; P, 0.15%; S, 0.10%; they would be cast from a tough gray cast iron with the following composition:

C, 3.2/3.8%; Si, O.3/O.-8%; Mn, 0.3% (preferably O.l5%); P, 0.08% (preferably 0.04%); S, 0.06% (preferably 0.03%).

EXAMPLE 7 There will subsequently be given an example of an alloy before and after inoculation in which there is provided a high nickel content and instead a lower silicon content. As the table clearly points out, the mechanical properties in the as-cast condition are excellent.

C, Si, M11, P, S, Ni, Test Piece Per- Per- Per- Per- Pcr- Percent cent cent cent cent cent Starting Melt 3. 26 0.04 0.105 0. 03 0.011 2. 24 After Inoculation 3.25 0,19 0.219 0.03 0.011 2.22

the concentration, it amounts to only the following frac- Mechanical properties in the as-cast condition tio n Tensilestrength, kg./rnm. 33.0 Concentration Silicon Manganese Bending strength, kgjmm? 55.7 0 100 100 Total deflection, mm 19.6 0.5 65 85 Plastic or permanent deflection, mm. 13.4 {2 2g Impact modulus, kg. cm./cm. 13. 3 2.0 37

From the foregoing examples, it will be seen that the carbon content of the cast iron hereof can generally range from about 2.5 to about 3. 8 percent.

The term conventional gray cast iron as used herein and in the appended claims is well recognized in the art and is defined in the eighth edition, 1961, of Metals Hand- 9 book, published by A. S. M., pages 349 365 and more particularly, Table 9 on page 355.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. An alloyed gray cast iron for use as a workpiece for cast pieces with lamellar graphite formation, great toughness and high heat conductivity in the cast condition, consisting essentially of:

(a) 2.5% to 3.8% carbon;

(b) less than 0.08%, preferably less than 0.04% phosphorus;

(c) less than 0.08%, preferably less than 0.04% chromium;

(d) less than 3 ml./ 100 g. hydrogen;

(e) less than 0.3%, preferably less than 0.15% manganese;

(f) less than 0.03% sulphur;

(g) the balance being iron as Well as a silicon content which is reduced by about 0.8% up to /2 of the silicon content required for conventional gray cast iron of cast pieces with the same wall thickness.

2. An alloyed cast iron according to claim 1, wherein the ratio of manganese to sulphur is greater than 5:1, preferably is greater than 7:1 whereby maximum toughness is obtained.

3. An alloyed cast iron according to claim 1, wherein the ratio of manganese to sulphur is less than 5:1, preferably is less than 3:1 whereby high wear resistance is obtained.

4. An alloyed cast iron according to claim 1, wherein an alloying element selected from the group consisting of nickel in an amount of from about 0.5% to 1 percent and molybdenum in an amount of from about 0.5 to 1% and mixtures thereof is added to the alloyed cast iron in order to further increase the strength of said alloyed cast iron.

5. An alloyed cast iron according to claim 4, wherein at least a portion of the silicon content of the alloyed cast iron is replaced by nickel in a ratio of approximately 3 parts nickel to 1 part silicon replaced.

6. An alloyed cast iron according to claim 1, wherein the alloy consists essentially of ferrite.

7. An alloyed cast iron according to claim 1, wherein an alloying element selected from the group consisting of nickel in an amount of from about 0.5% to about 1% and molybdenum in an amount of from about 0.2% to about 0.6%, and mixtures thereof is added to the alloyed cast iron in order to further increase the strength of said alloyed cast iron as well as the heat resistance thereof.

References Cited UNITED STATES PATENTS 1,948,246 2/1934 Seaman 148-35 2,245,876 6/1941 Schwartz 43 2,324,322 7/1943 Reese 14835 2,796,373 6/1'957 Berg 14835 3,055,756 9/1962 Kanter 75l23 FOREIGN PATENTS 721,717 1/1955 Great Britain.

OTHER REFERENCES Open Hearth Proceeding, vol. 45, 1962, published by the A.I.M.E., pages 416-421.

Metals Handbook, 8th edition, 1961, published by A.S.M., pages 349-365.

The Alloys of Iron and Carbon, vol. I, Alloys of Iron Research Monograph Series, 1936, pages -124.

Physical and Engineering Properties of Cast Iron, 1960', published by B.C.I.R.A., England, pages 208211.

CHARLES N. LOVELL, Primary Examiner.

DAVID L. RECK, Examiner.

P. WEINSTEIN, Assistant Examiner.