US2796373A

US2796373A - Method of forming malleableized iron castings

Info

Publication number: US2796373A
Application number: US408548A
Authority: US
Inventors: Berg Claes
Original assignee: Oeverums Bruk Ab
Current assignee: Overums Bruk AB
Priority date: 1954-02-05
Filing date: 1954-02-05
Publication date: 1957-06-18
Anticipated expiration: 1974-06-18

Description

June 18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 l4 Sheets-Sheet 1 9 P" 51; mm P51 Kg per sq. mm I P 100 0oo' HB (S /3 600 60 20 5aooo HB Q5 200 20 I I I I I 0 100 500. v 600 100 800 900 1000 F 0 I 100 000 500 'c IT g1 INVENTOR.

June 18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet 5 K9 er sq.mm P51 150.ooo 100 -IOQOOO 2 3 H- 5 5 7 8 910 15 20 Charge No.

INVENTOR;

c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS June 18, 1957 14 Sheets-Sheet 4 Filed Feb. 5, 1954 O 0 0 m w w w o w M w w M 2 2 H w W. M w m w o B 9 1, a 5 0 EM M E l 6, \c) I i 7 t I I x x u M I I I I m .17 m, IL 5 V w l V r F I I c 81 0 mm .l I, 6 4 IN- l l li O R Mf/ 07 WW ma m, M 0 0 O 0 O 0 O O 0 0 O m m m m w w m w w June 18', 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Slieets-Sheet 5 Fig. 10. White cast iron with decomposed austenice and s. cementite net Fig. 11. Same as annealed. 650-200 temper carbon particles pro mm lizzventor June 18, 1957 c. BERG 25796373 METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb 5, 1954 '14 Sheets-Sheet e;

Fig. 12. Hardened 8 min/900 C/quenched. in

water 500 x Fig. 15. Same tem aged 1+0 min/1400 0 12 kg m 1.7% elong., 565 Brinell Inc: enter June 18, 1957 c. BERG METHOD OF FORMING MALLEABLEIZED IRON CASTINGS 14 Sheets-Sheet 7 Filed Feb Fig. Hardened l8 min/9OO C/quenched in water nell "0 ;0% 810118., 588 Bra].

Fig. 15. Same, tempered 11.0 min/L 00 126.5 kg/mm June 18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet 8 Fig 16. Hardened 25 min/900C/quenched in water 500 X Fig. 17. Same, tempered lg.0 min/14.00 6

12L- .5 kg/mm LM e 0ng., 588 Brinell 500 x C. fiery June 18, 1957 c. BERG METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet 9 Fig. 18. Hardened 115 min/9OO C/quenched in water 500 x Fig. 19. Same, tempeI 'e d 110 min/1100 0 117.6 kg/mm 1.5% 610113., 588 Brinell 500 x .Zizzv elztaz" June18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet l0 Fig. 20. Hardened 120 min/900 C/quenched in water Fig. 21. Same, tempered ho min/)+0OC 118.9 kg/mm 1.5% elong., L 15 Brinell 500 x June 18, 1957 CQBERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet l1 Fig. 22. Austempring. 1' min/900C 1o h/250c 110 kg/mm 11 .71 elong., 14.15 Brinell Fig. 25. Axistempring. l7 min/900C 50 min/510C 112.9 kg/mm 1.5% along. 565 Brinell 500 x Ire/v avatar C .15 81 J1me 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet 12 Fig. 2 Austempring. 17 min/900C. 5 min/11.00 0

99.5 kg/mm 1.7% along. 521 Brinell Fig. 25. 0.126% P. Intercrystalline fissures 61.6 kg/mm 1.0% along. 588 Brinell 500 x liz'zvezataz June 18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS- Filed Feb. 5, 1954 I 14 Sheets-Sheet 13 Fig. 26. 0.126% P. Severe cracks.

22 kg/mm o,9% elong., 588 Brineg Fig. 27. 0.28% Mn. Coarse structure with crack 95.2 kg/mm 1.0% elong., 588 Brinell' June 18, 1957 c. BERG 2, 3

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14 Sheets-Sheet 14 Fig. 28-Spher0ida1 structure, annealed 55.5 kg/mm 16.3% el0ng., 125 Brinell 300 x Fig. 29. Same heat treated. Spheroid and temper carbon. 18 min/900C L 0 min/14.00 0

125.9 kg/mm 2.0% e10ng., 592 Brine'll 500 x United States atentQ METHOD OF FDRMING MALLEABLEIZED IRON CASTINGS Claes Berg, Overum, Sweden, assignor to Aktiebolaget Overnms Bruk, Over-um, Sweden Application February 5, 1954, Serial No. 408,548

Claims. (Cl. 148--21.7)

The present invention broadly relates to the art of metal treatment.

The invention particularly relates to a method of producing ferrous castings and the casting so produced.

This application is a continuation in part of my applications Serial No. 23,858, filed April 28, 1948, now abandoned, entitled Manufacture of Malleableized Iron Castings Particularly Wearing Parts of Agricultural Implements, and Serial No. 384,313, filed October 5, 1953, and entitled, Heat Treated Malleable Cast Iron and Methods of Producing the Same.

More particularly the invention relates to the production of ferrous castings having markedly increased physical properties as compared with known castings, with particular reference to cast parts that are subjected to wear under various abrasive conditions.

The invention therefore is particularly related to the manufacture of ferrous castings including the heat treatment thereof and to the casting thus produced having considerably increased utility, wearability and toughness.

It is therefore an object of this invention to provide an improved method of producing ferrous castings and an improved casting of the type having a white fracture and to subject the casting to such heat treatment steps that the resultant product has a markedly increased tensile strength as compared with known cast iron parts.

A further object is to so control the composition of the casting and the heat treatment steps that the resultant product has an exceptionally high Wear resistance, particularly when subjected to conditions such as abrasion in soil, particularly of a rocky nature.

A further object is to provide a cast iron part broadly constituting a heat treated malleable iron that can be forged and rolled in a manner similar to ordinary steel parts and which, contrary to expectation, can be welded electrically.

An additional object is to correlate the ingredients or component compositions of the cast iron with the various heat treating steps so that production of the finished product can be controlled as to uniformity and desired physical properties.

It is therefore a specific object of this invention to provide a method of producing ferrous castings and the resultant cast product in which a particular range of certain ingredients is incorporated in the castings with the view of permitting the development of exceptionally high tensile strength and in which the heat treating steps include an initial annealing or malleabilization controlled as to the structure produced from this step, followed by a further heat treatment or hardening that is controlled as to temperature and time after which a liquid quenching is employed of sufiicient rapidity that the matrix of the structure is, as far as possible, free from fissures and micro cracks and is free of such disadvantages that would affect the properties of the finished casting.

As a further specific aspect following the hardening or reheating of the casting and the quenching thereof, this invention has an additional object to provide a tempering step that results in a cast product having the desired physical properties.

The prior art is replete with disclosures of the production of malleable iron cast products in which an iron casting that is termed to be more or less a white iron casting is annealed and then heat treated to obtain carbon in the combined form. The tensile strength obtained in the prior art methods, .as far as is known, has a general maximum of 100,000 p. s. i. Consistent with this factor, the ASTM Standards Specification 1952 Ferrous Metals designation A220-50 indicates that the minimum tensile requirements for pearlitic malleable iron castings are between from 60,000 to 90,000 p. s. i. tensile.

In accordance with the method fully set forth hereinafter, the present invention results in the production of a casting that is of such improved tensile strength that this factor has been increased to 200,000 p. s. i. and somewhat above. Therefore the method of the present invention results in the production of castings, the tensile strength of which is uniformly within the range of 120,000 to 200,000. The method is of such nature that the tensile strength of the finished product can be well controlled within these ranges. It has been found that in castings produced according to the invention elongation and wear resistance usually increases, if the heat treatment is performed in such a way that a maximum tensile strength is obtained.

Therefore in accomplishing the present invention, a casting is produced which has a substantially completely white structure as revealed by microscopic examination of a polished surface and which structure, insofar as is possible, is marked by an absence of flake graphite. In this connection, the composition of the iron to be cast is controlled within specified limits and the metal contains controlled amounts of certain specific elements, the quantities of which, when the high tensile strength is produced, are critical.

Secondly, the annealing or initial heat treatment which includes a cooling cycle must be effected in such fashion that upon a subsequent hardening treatment the fissures and micro cracks, if any are formed, are so minute so as not to adversely affect the properties of the hardened casting. In this connection, the cast product after annealing is practically free of free cementite.

Further objects and advantages of my invention will be readily apparent from the following description of the invention, which is more clearly understood by reference to the accompanying drawings, in which:

Figure 1 is a diagram illustrating as an example the tensile strength as a function of the phosphorus content,

Figures 2 and 3 are diagrams illustrating preferred annealing cycles,

Figures 4 and 5 are diagrams illustrating reheating or hardening cycles,

Figure 6 is a diagram illustrating times and heat factors in a tempering step,

Figure 7 is a diagram illustrating the tensile strength, hardness and elongation as a function of the heat applied during the tempering step,

Figure 8 is a diagram illustrating the uniformity in tensile strength of finished castings in plural and successive production cycles in the continuous production of ferrous parts,

Figure 9 is a diagram illustrating the stable and metastable systems and the formation of and into austenitic structure,

Figure 10 is a microphotograph illustrating the structure of an iron A as cast to a white iron,

Figure 11 illustrates the structure of iron A as annealed in accordance with the 'cycle of Figure 3,

Figures 12, 14, 16, 18 and 20 illustrate the structure of iron A after reheating for hardening to 900 C. hold- 3 ing at this temperature respectively 8, 18, 25, 45 and 120 minutes, and quenching in water,

Figures 13, 15, 17, 19 and 21 correspond each to the preceding figure and illustrate the structures after tempering 40 minutes at 400 C,

Figures 22, 23 and 24 illustrate the structure of the iron A annealed in accordance with Figure 11, reheated for hardening at 900 C. during 17 minutes, and quenched in a bath at 250 C. during hours, 310 C. during 50 minutes and 400 C. during 3 minutes, respectively,

Figures 25 and 26 illustrate cracks in the structure of an iron with 0.126% phosphorus hardened and tempered as in Figures 14 and 15,

Figure 27 illustrates the structure of a similar iron as Figures 25 and 26 but with a normal amount of phosphorus and an amount of manganese increased to 0.28%,

Figure 28 illustrates the structure of an iron as annealed and containing magnesium, and Figure 29 the same iron after hardening and tempering' In performing the present invention, 1 have established that a pre-requisite for obtaining the desired and markedly improved physical properties, particularly tensile strength and resistance to wear and for accomplishing the other objects of the invention, the material as hardened shall be as free as possible from flake graphite, free cementite, and micro cracks. Further, that the material be such that when hardened and quenched in water the matrix will consist substantially only of martensite. Therefore the cast iron composition as a starting material consists preferably of pig iron or other iron base materials that contain only small amounts of elements that affect the properties of the finished product. Good results in casting have been obtained by utilizing a rotary or stationary air or electric furnace although the type of melting unit is not considered to be a restriction of the present invention. Casting conditions should be such, that coupled with the composition of the melt, a casting is obtained that is as free as possible from graphite. Thus when casting in dry-sand moulds, it will be advisable to take the castings out of the moulds immediately after solidification in order to promote a rapid cooling.

In the production of ordinary malleable iron castings the composition of the metal as cast may vary within rather wide limits with respect to carbon, silicon, manganese, phosphorus and other alloy elements, without essentially impairing the properties desired in the malleable iron as such. In connection with my invention, however, and to obtain a finished product having the properties in accordance therewith it is necessary that the composition of the metal as cast be controlled within narrow limits as to the specific amounts of several elements present.

The total amount of carbon and silicon should be sufficiently low to allow freezing of the melt to a white iron. It is not suificient to produce a semi-white iron or partly mottled iron. Such condition can be indicated by the fracture or by observation of a section under the microscope. The casting must be entirely white and free from any form of flake graphite. Therefore the carbon content and the silicon content can principally vary within the limits shown in known diagrams for white iron, such as the diagram of Maurer, but the carbon content is usually limited to between 1.83.2% and the silicon content adversely to between l.60.5%. The best results have been obtained with carbon between 2.22.4% and silicon about 1%. The total amount of carbon and silicon is preferably between 3.4 and 3.8%.

In the hardened product according to the invention, sulphur, especially combined as iron sulphide, is an impairment in the material since iron carbide appears to dissolve some iron sulphide which is thereby stabilized and more difiicult to decompose by annealing. If the sulphur content is not sufficiently low, cementite will remain in the hardened structure thereby inducing brittle characteristics reducing the ductility as measured by elongation percent and causing the promotion of strains which can result in the formation of micro cracks which are readily revealed in the polished sections etched and submitted to microscopic examinations. The sulphur in the castings should therefore be very low and definitely less than 0.1% combined with iron. It is preferred that the total sulphur content be less than 0.05%. Irons are now available having a sulphur content as low as 0.01%. Therefore it is preferred that the sulphur content be between 0.01 and 0.09% with the best results following the reduced sulphur content within this range.

While manganese to a certain extent is necessary to hold the sulphur in chemical combination, the manganese also stabilizes the austenite and cementite by forming complex carbides. In the successful performance of the present invention, the presence of a high manganese content is to be avoided since this element impairs the properties of the hardened product in a manner similar to sulphur. It has especially been observed that a high manganese content causes intercrystalline hardening cracks if the iron is heated for hardening above about 925 C., and in case of a relatively high manganese content a temperature of about 900 C. is therefore preferred. However, it should be pointed out that a high manganese content also reduces the possibility of welding the material. Tests have been conducted on materials manufactured and processed according to the present invention with variations in the manganese content in instances where the other ingredients of the casting are Within the ranges set forth on the table hereinafter disclosed and reveal that an increase of the manganese content lowered the tensile strength of the finished product. Therefore manganese content should be kept low. However, the content of the metal should not be too low since manganese is required to assure the complete solution of ferrite during the reheating of the casting. Therefore, there should be a minimum of 0.05% and a maximum of 0.45% manganese, and in any case the manganese dissolved in iron should be below 0.20%. The total manganese content usually varies between 0.1 to 0.3%, and the preferred range of manganese is between 0.18 and 0.20%.

The phosphorus content is of a critical nature when the end product must have an extremely high tensile strength. Phosphorus has previously been considered to have no influence on the graphitization, and in ordinary malleable iron, as revealed in handbooks, a relatively high content of between 0.10 to 0.20% and more is often used with the object in view of increasing the fluidity of the material as cast. I have found, however, that a high amount of phosphorus content causes the graphitization during annealing to follow more closely the curve corresponding to the metastable system rather than that of the stable system and thus some of the cementite content to resist decomposition. Thus a high phosphorus content will result in the formation of ferrite-graphite-cementite eutectoid with grain boundary cementite around the austenite crystals instead of a ferrite-graphite eutectoid which is aimed at according to the invention. When a material of a high phosphorus content is hardened, particularly by rapid quenching, the undecomposed cementite distributed in the form indicated is undesirable. Intercrystalline cracks are formed along the boundary cementite, and the material becomes brittle, as a low tensile strength, and cannot be welded or forged. Tests have been conducted on material prepared in accordance with the table of ranges set forth hereinafter with varying phosphorus content and, as shown in Figure 1, the tensile strength is definitely a function of the phosphorus content. In this figure it is revealed that tensile strength of 200,000 p. s. i. is attained with a phosphorus content of less than 0.05%, usually 0.04%. When the phosphorus content was increased to 0.10% and 0.15% respectively, the tensile strength decreased to 120,000 p.- s. i. and approximately 82,000 p. s. i. respectively. The tests have also revealed that the increase of phosphorus retards the formation of austenite when the material is heated for hardening, and in order to avoid undissolved ferrite it is necessary, either to extend the holding time with the disadvantage that some austenite will be too saturated and the hardening cannot be controlled, or to raise the hardening temperature. In both cases the hardened iron will be further impaired.

It is therefore of great importance to assure a low phosphorus content with less than about 0.10% or 0.09% and preferably less than 0.05%. As stated, the best results have been obtained with less than 0.04%, such as by the use of a charcoal iron having a phosphorus content of about 0.03%.

I further want to point out that the improved properties of the product resulting from following the teachings of my invention do not require the use of additional alloying elements such as chromium, nickel, molybdenum and the like. In certain instances such additions have deleterious effects. Thus chromium will retard graphitization and nickel will reduce the solubility of carbon in iron. However, within the framework of the present invention I do not exclude for certain special purposes the addition of small amounts of alloying elements such as copper to improve corrosion resistance or aluminum or titanium which favorablyinfluence elongation and can be used with advantage up to about 0.1%.

Therefore within the teachings of the present invention the chemical compositions of the casting after hardening will include in addition to iron the following elements in the stated ranges: carbon 1.8-3.2% and preferably between 2.0-2.6%, silicon 0.51.6% and preferably about 1%, manganese ODS-0.45% and less than 0.20% combined with iron, preferably a total conw been added, and the properties of I the finished product tent of 0.18-0.20%, sulphur less than 0.10% combined with iron, preferably less than 0.05%, phosphorus less than 0.10% and preferably about 0.04% and the remainder substantially iron.

The iron alloy described above can be cast to a completely white iron only if the maximum section size of the casting is less than about or mm. In heavier castings more or less flake graphite will appear, which would make the material unusable for further heat treatment according to the invention. This disadvantage can be avoided, if a metal of the group magnesium, cerium, calcium and lithium is added to the melt, as these metals have the capacity of preventing the segregation of flake graphite. Of these metals magnesium is preferred. It melts at 657 C. and evaporates at 1102 C., which is below the melting point of iron, and it is therefore recommended to add the magnesium alloyed with other metals, such as Si and'Fe. In order to increase the specific gravity above that of iron to cause sinking of the alloy in the iron melt a further and heavy metal may be used in the alloy, such as copper. A preferred alloy is thus 12-22% Mg, 25-35% Si, 40-50% Fe and 10-15% Cu. Some of the magnesium added to the melt will react with the air and be consumed by burning, and an other portion thereof will react with S to MgS. The remaining amount of Mg should be more than 0.02%, preferably 0.03-0.09%, and particularly about 0.05% is recommended. Similar amounts of cerium, calcium or lithium can be used instead of magnesium and in the form of a suitable alloy. Such alloys can be added either as a melt or as small pieces to the melted iron shortly before casting, and in order to secure that as great part as possible of the metal remains in the iron the metal can be introduced into the melt at a low level by being blown into the same by means of compressed air. An example of such an iron alloy is: 2.5% C, 1% Si, 0.15% Mn, 0.04% S, 0.04% P, net 0.05% Mg or Ce, and the rest substantially iron.

A further advantage of the addition of Mg or one of the other special metals is that a great number of inoculation points are obtained for the subsequent formation of graphite in the form of rounded, dense particles, sometimes called nodules, and more fluffy temper carbon nests. Most of these carbonparticles are formed during the subsequent annealing operation, but under certain conditions and without inconvenience a small amount thereof may appear in the material already as cast.

The more compact and substantially spheroidal nodules are in this way obtained instead of flake graphite and do not detrimentally affect the properties of the finished material, as flake graphite would do. In contrary, the properties are improved compared with the case that only temper carbon had been obtained as they have a less volume and more desired form. On the other hand this compact carbon can only with great diificulty be dissolved and diffuse into the ferrite to form austenite during the hardening operation. For this reason a sufficient portion of the free carbon should be present as temper carbon, which dissolves very easily into austenite. It is usually sufiicient that 50% or more of the free carbon occurs as temper carbon, but 60-80% is preferred. The desired proportion between the two kinds of carbon can be attained by varying the amounts of Mg, Ce, Mn and other elements in the iron alloy, and thereby also the desired matrix of the finished product may be varied. Thus it is possible by increasing the proportion between temper carbon and the compact particles to get a material with a higher content of combined carbon in the hardened castings for such articles as wear parts, which should have a great tensile strength and a high wear resistance. By lowering the said proportion the content of combined carbon is reduced and the elongation is increased, and such a material is of special advantage for construction parts. This iron alloy is to be heat treated in the same way as if magnesium, or the like had not are also substantially unchanged.

For the purpose of ascertaining the dilferent steps of the invention, illustrating the structures by photographs and testing the finished product with respect to its mechanical properties a great number of test bars were cast which after heat treatment had the following chemica composition:

2.38% C, 0.89% Si, 0.19% Mn, 0.037% P, 0.031% S and the rest substantially iron. This composition is hereinafter called Iron A.

The Iron A was cast, and immediately after solidification of the castings the sand was removed and the castings were allowed to cool in air. The structure of the casting is shown in Figure 10 and comprises decomposed austenite crystals in a net of cementite without any flake graphite. The magnitude of this and all other photographs is 300. The tensile strength was 46,000 p. s. i. and the hardness 415 Brinell.

The iron was then annealed according to the cycle shown in Figure 3, in which the curve illustrates the temperature within the oven. The casting was loosely placed in closed pots containing some charcoal to bind the oxygen in the air and prevent decarbonization of the iron. To obtain a distribution of temper carbon favorable to the subsequent hardening and to efiect decomposition of practically all cementite it is of importance to anneal the material at a temperature considerably higher than 800 C. Preferably the annealing temperature should lie between 850-1050 C. or about 950 C. The temperature of annealing must be held sufliciently long to convert all cementite into austenite and temper carbon but not held to such an extent asto materially reduce the carbon content by surface decarbonization. On the other hand since elements such as sulphur and manganese retarding conversion are present only in small amounts, the required holding time for complete conversion will be relatively short.

7 shown in Figure 3, in which the curve illustrates the ternperature within the oven. The temperature was raised to 350 C. in 2 /2 hours, maintained at this temperature for 10 hours, raised to 960 C. in 2 /2 hours, maintained constant for 25 hours, lowered slowly to 750 C. in 4 hours and thereafter more slowly to 650 C. in 25 hours, whereafter the oven is allowed to cool.

In order to make clear the transformation steps during the annealing process reference is made to Figure 9. The point E represents the melting point 1145 C. (1.7% C) of the iron in the iron-carbon or metastable system and the point So the corresponding eutectoid point 721 C. with 0.86% C. If a steel containing maximum 1.7% C completely solidifies in a point on the curve represented by point E0 austenite is formed, and if the temperature is lowered, the carbon content of the austcnite will dccrease according to the line E050 and the precipitated carbon will form cementite until the eutectoid or pearlite point So is reached. When this point is passed all austenite will be transformed into ferrite and cementite forming pearlite.

ES represents the corresponding transformation line in the metastable system of an iron alloy containing carbon and 1% silicon which line owing to the content of Si is displaced to the left. In point B the temperature is 1160 C. and carbon content 1.6%, and the point S, where the temperature is 760 C. and carbon content 0.64%, represents the upper limit of the pearlite range, about 20 C., within which at decreasing temperature the austenite is transformed into pearlite. If at a certain temperature the carbon content is higher than that shown by line ES, the metastable system can be transformed into the stable system E 8 It should be observed, however, that the exact form of these three transformation lines is not fully known and that they may be slightly curved, but in the present case it is thought to be sulficient to use straight lines in the diagram.

If now the Iron A containing 2.38% C is heated to a temperature which may be 950 C., austenite is formed which dissolves cementite and precipitates temper carbon, until the point U is reached, in which all cementite is dissolved. By further heating the austenite containing 1.1% C may yield further temper carbon, until point V in the stable system is attained and the combined carbon has decreased to 0.93%. On lowering the temperature to 770 C. in point S the combined carbon content is 0.57% and thereafter in the pearlite range the austenite will be transformed either to pearlite or to ferrite and temper carbon in dependence of the rate of cooling. If this rate is as slow as shown in Figure 3 the structure will contain substantially only ferrite and temper carbon. If, on the other hand, the iron should contain too much sulphur, phosphorus, manganese and other metals or if the holding time has been too short, some cementite will remain undecomposed and cause cracks on hardening.

In-order to establish the absence of. cementite, an examination of the tensile strength and elongation after the annealing treatment will be indicative of the absence of the cementite. A low tensile strength of up to about 50,000 p. s. i. in association with a high elongation of more than about 16% will indicate a malleable iron which may be suitable for the manufacture of a material of the invention. Usually such properties are not found in malleable iron manufactured for use in annealed condition, or for further heat treatment as embodied in the prior art.

As shown in the diagram Figure 3 the temperature has been maintained constant at 350 C. for hours. The iron will thereby emit hydrogen to the effect that the time of holding at 960 C. may be shortened at least l0 hours and a more even distribution of a greater number of small temper carbon is obtained. The anneal process, however, can be varied in different ways. According to the .diagramlshown in Figure 2 the temperature of the casting is heated continuously to 960 C. in 20 hours,

maintained for 30 hours, lowered to 760 C. in v5 hours and to 670 C. in 30 hours, whereafter the casting is removed from the oven and is cooled in the air. Also in this way a similar ferritic matrix is obtained. This complete anneal cycle can be interrupted in a point B between the anneal temperature 960 C. and the upper limit 770 C. of the pear-lite or eutectoid transformation range, preferably at about 850 C., by removing the casting from the oven and allow it to cool in the air as indicated by a dotted line. By Figure 9 it is revealed that the iron at 850 C. contains about 0.72% C combined in austenite and the rest of the total carbon content 2.38%, i. e. 1.66% C, as temper carbon. The austenite will thereby be transformed into pearlite, and the proportion of the amounts of pearlite an'd ferrite can be regulated by the choice of a suitable temperature A.

The microstructure of Iron A annealed as shown in Figure 3 appears from Figure 11. No cemcntite remains, and the temper carbon is well distributed to a number of about 600 to 700 pro square mm.

The material can be reheated for hardening either from room temperature or from any temperature below the pearlite range or below about 750 C. As the material has to be transferred from the anneal oven to the hardening oven, however, the temperature during this operation usually will decrease considerably below 750 C. On reheating, austenite will be reformed in point S from the ferrite and some temper carbon or pearlite. During the long anneal period the austenite grains have had sulficient time to grow to a coarse structure, and hardening from a temperature above the pearlite range would lead to a coarse and brittle structure. The reformed austenite, in contrary, has very fine grains, and as the short heating period for hardening is insufiicient to allow the crystals to grow a very fine and desired structure after hardening is obtained. When the casting is heated up to a temperature of about 900 C. the carbon content of the austenite varies principally as indicated by line R in Figure 9, according to which the austenite will first slowly and thereafter more rapidly dissolve temper carbon and thereby increase its original carbon content 0.57%. On the temperature line 900 C. the point representing the carbon content moves to the right towards point V1 on the line E 5 which represents the stable system. Now tests with the material to be hardened according to the invention have indicated that the austenite in this material can dissolve considerable amounts of temper carbon up to substantially all temper carbon present and that the carbon content probably may pass beyond the stable system. These tests will be discussed hereinbelow with reference to the photographs. The carbon content can thus pass the point V1 towards or beyond point U1 on the metastable line ES. If the austenite is allowed to be fully saturated or saturated to a carbon content above a certain value, quenching in a liquid will result in the formation of free cementite, which would make the material brittle, cause cracks and considerably reduce the tensile strength and elongation. A maximum carbon content of about 0.8% has thus been found suitable, and preferably 0.65% to 0.70% is used. A simple method of controlling that the best carbon content is obtained is to control the heating and holding time. This control is easier, if the time for heating the casting to the desired temperature is as short as possible. The hardening oven is, therefore, preferably preheated to the holding temperature or to 850-950 C. Since the annealed casting following the complete anneal cycles of Figures 2 and 3 consists of substantially only ferrite and temper carbon, and the casting following the interrupted anneal cycle according to Figure 2 consists in addition to ferrite and temper carbon of some formed pear-lite, rapid heating does not appreciably deform the casting as in case of ordinary steel. If the teachings of this invention have been followed up to the completion of the annealing cycle the casting as reheated is initially substantially stress free. As indicated the temperature range is well above the eutectoid or critical transformation range, and I have ascertained that the best results have been obtained by heating at a temperature about 900 C. I have further discovered that if the temperature is raised above the range mentioned herein, namely 950 C., the carbon content of the austenite increases so rapidly that this content cannot be controlled. Castings of different section sizes heated together will reach the holding temperature only successively and be subjected to such different holding times that a very uneven product is obtained with too much combined carbori in the small sized castings. Furthermore, when the material is thereafter quenched so as to promote the formation of a martensitic structure, the martensite thus formed will exhibit a coarse configuration of needle-like structure. The large needles thus formed will result in a final product constituting a brittle and unsatisfactory casting. If, on the other hand, the holding temperature is below 850 C., then it will be diflicult to ensure that all ferrite present in the annealed matrix is transformed into austenite.

If the annealing cycle is interrupted in a point B in Figure 2, pearlite is formed, and upon reheating for hardening austenite will be reformed more rapidly from the pearlite than from ferrite and temper carbon or spherolites. The holding time must in this case be still shorter than if the austenite is to be formed only by ferrite and free carbon, and the control of the holding time is in this case of still greater importance. Also if the hardening is performed in combination with an isothermal transformation it is of importance to control the holding time and especially to obviate too long time.

Therefore, the material or casting should be kept at the hardening temperature, preferably between 875-925 C. or about 900 C., substantially only for such period of time as will ensure that all ferrite present has been transformed into reformed austenite. Therefore, I wish to point out that if the reheating period, particularly the holding time, is too long or such as the time customarily used in connection with hardening steel, then the product will be unsuitable for this invention.

The hardening heating cycles shown as an example in Figure 4 indicate the temperatures of the castings placed in an oven preheated to about the hardening temperature 900 C. The cycle C corresponds to castings having a maximum section size of up to about 25 mm., D between 25-50 mm. and E more than 50 mm. The heating times are shown to be about 6, l5 and 35 minutes and the

subsequent holding times

15, 20 and 30 minutes respectively. The holding times may vary within certain limits in dependence of the composition and of the properties desired, but preferably only within to 25 minutes in cycle C, to 30 minutes in cycle D and to 50 minutes in cycle E.

After the material or casting has been reheated, liquid quenching can be effected in different ways. According to Figure 4 the casting is hardened by water quenching to a martensitic structure sufficiently rapidly to avoid the formation of any essential amount of pearlite at least in the outer layer of the casting, but if the casting is of a heavy size it is obvious that the interior thereof is cooled somewhat slower and may contain more or less pearlite which, however, is not of any great disadvantage. The matrix will also be substantially free from free residual cementite assuming that the teachings of the invention pertaining to the ingredients of the composition, the annealing cycle and the reheating cycle have been followed. Consequently, any cracks having a deleterious effect on the end product are avoided. If some rest austenite should remain, the casting can be placed in cold water or cooled down to 85 C. for transformation into martensite. The structure of the material as quenched in water is shown in Figure 14.

If a greater elongation and toughness are desired, the casting may be hardened by quenching rapidly in a bath to a temperature substantially above the upper limit of the martensite range and below the pearlite range, preferably to between ZOO-450 C. If an efiective cooling is secured it is possible, owing to the special chemical composition, to pass to the left of the S-curve, also known as the TTT-curve (Time-Temperature-Transformation), without the use of special metal alloy additions practiced heretofore, which would give rise to not desired complex carbides, and it is thus possible to avoid any essential amounts of pearlite. A lead-tin bath or a salt bath may be used, and the casting is maintained therein for a certain time dependent on the temperature to secure an isothermal transformation of the supercooled austenite. This 'bainite reaction is not fully known, but a needle-like or acicular structure is obtained, which is to some extent similar to martensite it formed at a relatively low temperature and which is an intermediate stage between pearlite and martensite. In order to obviate as far as possible the formation of cementite during this slow transformation it is advisable to hold the casting at the hardening temperature a period of time which is 5 to 10 minutes shorter than'that indicated above for quenching in water so that the austenite has a very low content of combined carbon. A holding time of 6-10 minutes in case of small-sized castings has thus been found sufficient. The structure may contain bainite, troostite and some ferrite depending on the nature of the heat treat ing. If the material is quenched down to about 200-250 C. some martensite may be formed, and this can in a separate step be transformed into sorbite by tempering. The time of holding the material in the bath, which holding time determines the effectiveness of the isothermal transformation, is dependent on the temperature. Figure 5 shows examples of the heat cycles. The curve F differs from curve C in Figure 4 in that the casting is liquid quenched only to 250 C. and held at this temperature for about 30 minutes, after which the material is quenched in water to transform into martensite such residual austenite, which may remain. According to curve G the material is liquid quenched to 400 C., held at this temperature for about 35 minutes and thereafter cooled in air. The time for holding the casting in the bath depends on the temperature thereof and the holding time for hardening. Testing rods of Iron A were thus reheated for hardening to 960 C. and held at this temperature for 17 minutes, whereafter the castings were quenched to different temperatures and held for 10 hours at 250 C., 4 hours at 275 C., 50 minutes at 310 C., 6 minutes at 350 C. and 3 minutes at 400 C. Figure 22 shows the structure of the material quenched at 250 C., Figure 23 quenched at 310 C. and Figure 24 quenched at 400 C. A highest tensile strength of 163,000 p. s. i. and an elongation of 1.5% were attained by quenching to 310 C., and other tests have given as a result an elongation of 6% in association with 'a tensile strength of 125,000 p. s. i.

After the material has been water quenched according to Figure 4 it is tempered or drawn at a period of time dependent on the sectional size of the casting to obtain tempered martensite. The diagram in Figure 6 shows some preferred heating cycles. The castings are placed in an oven preheated to 400 C. and removed after a certain time to be cooled in the air. The curves H, K and L indicate as examples the cycles for castings having a section size up to 25 mm., between 25-50 mm., and between 50-100 mm. respectively, in which cases the total heating times are 40, 60 and minutes respectively. The temperature should be above 250 C. and below 550 C., preferably between 350-450 C., whereby a hardness of between 240 and 500 Brinell is obtained, but the preferred hardness is between 350-450 Brinell. In dependence of the temperature more or less sorbite, troostite and similar tempering structures are formed. The best combination of tensile strength, hardness, elongation and wear resistance has been obtained by drawing at about 400 C. Figure 7 reveals that the tensile strength sharply varies in dependence of the tempering temperature. The

curves shown by full lines relate to the Iron A, and it is remarkable that the curve TS for the tensile strength has a sharp maximum point at 400 C. corresponding to 183,000 p. s. i., whereas 350 C. corresponds to 121,000 p. s. i. and 450 C. to 126,000 p. s. i. The dotted curve shows the result of other similar tests with a maximum at 400 C. of 200,000 p. s. i. The curve S representing the elongation shows a maximum of 2.0% at the same temperature 400 C. and 1.3% at 350 C. and 450 C. The hardness varies as shown by the curve HB and is at 400 C. about 400 Brinell. The microphotogrnph in Figure 15 shows the mainly sorbitic structure of the Iron A hardened from 900 C., tempered at 400 C. and representing the maximum point of curve TS. In commercial production it has been found that the elongation as an overage for the products obtained from 82 consecutive charges was 1.85%.

It has been pointed out above that the holding time at the hardening temperature is of a vital importance for the properties of the finished product, and a series of tests have been made for illustrating by photographs the structure of the iron at varying holding times. For this purpose test bars of the Iron A were used, and the structure of this iron as cast is shown in Figure and as annealed in Figure 11. Figures 12, 14, 16, 18 and 20 show the microstructure of the rods held at 900 C. for 8, 18, 25, 45 and 120 minutes respectively and thereafter quenched in water. Figures 13, 15, 17, 19 and 21 correspond each to the preceding figure and show the structure after tempering at 400 C. for 40 minutes. The composition of the iron and the heat treatment with a holding time of 8, 18 and 25 minutes are thus in accordance with the invention. Figures 12-15 illustrate a material with temper carbon of normal form, size and distribution and without any cracks. Figure 16 reveals, however, that a fine fissure extends between difierent temper carbon particles, although this does not reduce the tensile strength essentially. In Figures 18 and 19 fissures pass across the whole photograph, and the holding time 45 minutes has thus been too long so that the carbon content of the austenite has exceeded allowable values and caused free cementite and cracks. Figures 20 and 21 reveal that most temper carbon has been dissolved under formation of cementite, and from the residues of the temper carbon severe fissures extend towards other carbon centres. This solution of the temper carbon as shown in Figures 20 and 21 and several other photographs from similar tests may serve as a support of my assumption that the point representing the carbon content of the austenite in Figure 9 can pass along the temperature line 900 C. into the field between the points V1 and U1 and possibly beyond U1.

The test rods of Iron A as annealed in accordance with Figures 3 and 11 had a tensile strength of 46,500 p. s. i., an elongation of 15.4% and a hardness of 103 Brinell. The rods were then put into an oven preheated to 900 C., and 18 minutes thereafter the rods were removed and dropped into pouring water of C. They were tempered at 400 C. during 40 minutes and then tested. The result appears in Figure 7 which shows a tensile strength of 183,000 p. s. i., an elongation of 2.0% and a hardness of about 400 Brinell. This material represents a normal product which is made according to the invention and which is to be compared with the materials produced and described below in which certain composition elements have been increased.

Thus the deleterious effect of phosphorus has been pointed out above with reference to Figure 1, and in order to show the structure of an iron with too high amount of phosphorus test rods were cast from the same charge as the rods Iron A but with a small addition of phosphorus. The composition as analyzed was 2.36% C, 0.89% Si, 0.19% Mn, 0.035% S and 0.126% P. The castings as annealed had an average tensile strength of 49,000 p. s. i., an elongation of 18.3%, a hardness of 88 Brinell and thus very desired properties. Five rods were a 12 after annealing hardened and tempered in the same way as the Iron A, and the tensile strengths obtained were (1) 83,000, (2) 105,000, (3) 89,000, (4) 15,800 and (5) 31,500 p. s. i. The hardness was 388 Brinell. The samples (4) and (5) had cracks extending almost through the whole rod, whereas samples (1), (2) and (3) had only intercrystalline fissures or microcracks of a considerable length. Although the rods were cast and heat treated in quite the same way the material was very uneven and unreliable. Figure 25 illustrates the microcracks in the structure of sample (3) and Figure 26 the cracks in sample (5). It is obvious that such a material cannot be commercially used, and that a phosphorus content as high as 0.126% considerably impairs the finished product.

In order to further show the eifect of a high content of manganese test rods were cast and heat treated as in the preceding case with the exception that the phosphorus content was the same as in Iron A and the manganese content was raised from 0.19% to 0.28%. The material as annealed showed a normal average tensile strength of 48,000 p. s. i., a hardness of 88 Brinell and an elongation reduced to 12.7% as a result of the higher manganese content. Five rods were hardened and tempered in the same way as the Iron A and tested. The hardness was 388 Brinell, the average elongation was reduced to 1.1% and the tensile strength varied between 137,000 p. s. i. and 149,000 p. s. i. with an average of 145,000 p. s. i. The slight increase of Mn from 0.19% in Iron A to 0.28% results thus in a lowering of the tensile strength from 183,000 p. s. i. to 145,000 p. s. i. and the elongation from 2.0% to 1.1% probably depending on the formtion of complex carbides of Fe, Si and Mn. Although these properties represent an improve ment compared with known products of malleable iron the tests show the impairment of the material by increasing the manganese content. Figure 27 is a microphotograph of the rod having the lowest tensile strength 137,000 p. s. i., and it appears that the structure is rather coarse and a crack is formed and partly visible at the periphery.

Tests have also been conducted to show the properties and structure of the material containing spheroidal carbon particles or nodules obtained by addition of Mg, Ce, Ca or Li. For casting test rods a melt was prepared, but as the material is particularly intended for use in heavy castings the contents of C and Si were slightly increased above the normal values as in Iron A. The analysis showed 2.62% C, 1.14% Si, 0.19% Mn, 0.045% P and 0.024% S. The content of S is of special importance in this material because it has been found that less than about 0.02% S will result in a material having only spheroids or nodules and no temper carbon. The proportion between these carbon particles can thus be regulated by the content of sulphur. In connection with casting the rods 0.64% of a mixture of alloys was added which alloys contained Mg, Cu, Si, Fe and Ca. A completely white casting was obtained, which was subjected to the same anneal process as Iron A. This material had a tensile strength of 50,000 p. s. i., an elongation of 16.1% and a hardness of Brinell. The structure thereof is shown in Figure 28 in which a dense or spheroidal carbon particle and two temper carbon nests are visible. After hardening and tempering of five rods in the same way as Iron A the material had a hardness of 392 Brinell, an average tensile strength of 176,000 p. s. i. and an average elongation of 1.9%. Figure 29 illustrates the structure and shows a spheroid and a temper carbon nest. As a consequence of the high contents of C and Si the tensile strength has been somewhat reduced, but by restoring them to normal values in connection with casting heavier sizes the tensile strength will be improved. It is obvious that also this material can be subjected to isothermal transformation.

When following the above teachings it is possible to vary the composition and the heat treatment so as to obtain a product corresponding to the properties as desired. The different steps can be well controlled and a

Claims

1. THE METHOD OF FORMING AN IRON BODY, COMPRISING ANNEALING A WHITE IRON CASTING FREE OF FLAKE GRAPHITE HAVING A COMPOSITION OF 1.8 TO 3.2% CARBON, 0.5 TO 1.6% SILICON, THE TOTAL AMOUNT OF CARBON AND SILICON BEING LESS THAN ABOUT 3.8%, LESS THAN 0.1% SULFUR, 0.05 TO 0.45% MANGANESE, OF WHICH LESS THAN ABOUT 0.20% IS DISSOLVED MANGANESE, LESS THAN 0.1% PHOSPHORUS AND THE REMAINDER SUBSTANTIALLY IRON, BY RAISING THE TEMPERATURE TO A RANGE BETWEEN 875%C. AND 1050*C. AND HOLDING THE CASTING AT THIS TEMPERATURE UNTIL SUBSTANTIALLY ALL OF THE CEMENTITE IS DISSOLVED INTO AUSTENITE AND FREE CARBON PARTICLES OF SUBSTANTIALLY ROUNDED FORM AND SLOWLY LOWERING THE TEMPERATURE TO AT LEAST BELOW 750*C. TO OBTAIN A MATRIX SUBSTANTIALLY FREE OF CEMENTITE AND COMPRISING FERRITE AND TEMPER CARBON, AND THEN HARDENING THE CASTING BY