US2906653A - Die-casting of iron in chill-moulds - Google Patents

Description

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1 2,906,653 DIE-CASTING OF IRON IN CHILL-MOULDS Lucien Peras, Billanconrt, France, assignor to Regie Natiouale des Usines Renault, Billancourt, France No Drawing. Application December 27, 1955 Serial No. 555,285 Claims priority, application France February 1, 1955 13 Claims. (Cl. 148-213) In the patent application Ser. No. 336,188 of February 10, 1953, a process was described for manufacturing parts in ferrous metals having high mechanical properties and very accurate dimensions, according to which the white iron castings are made by die-casting in chillmoulds, said castings then being subjected to a heat treatment in three phases: quenching, nucleation, graphitization.

Thus, as stated in said application, the combination of the processes of chill-casting, die-casting, and graphitization with pre-treatment for the nuclear formation, leads industrially to new products of high quality, high output of mass-produced parts, precision and good surface qualities on the castings, high strength of the cast iron and regularity of production.

In fact, the die-casting of iron in chill-molds is greatly facilitated, compared with steel, by the great stability of the metal bath in the storage furnace, by the lowering of the casting temperature, and by the considerable reduction in the effects of hot erosion on the chill-mold.

Thus, white iron can be cast without micro-shrinkagecracks or blow-holes, thanks to the pressure exerted during the solidification. Cracks are avoided by stripping immediately after the solidification is finished.

Due to the chill-casting, the iron can have a relatively high silicon content which lowers the tapping temperature while preserving a white structure in the rough-cast pieces. Thus the normal content in malleable cast irons can be considerably exceeded, reaching values of between 1.5 and 2%. Moreover, higher carbon contents of 2.6 to 3% can be allowed for the same reasons.

In order to obtain the optimum effect, it is preferable, according to the invention, to use irons containing copper though without that being an absolute necessity. Copper acts as a constituent to improve the casting qualities, as an agent for the nuclear formation of the graphite, and as an additive with a view to suitability for the hardening and tempering heat-treatment.

In order for the process to have its full effect, it is necessary, according to the invention, to carry out the triple treatment of hardening, nuclear formation and graphitization. The hardening treatment can be carried out, starting from the casting heat, by stripping fairly hot, above 810 and quenching directly in a salt-bath, for example at 180 for 1 minute, then cooling in still air, which makes it possible to obtain chilled castings without the risk of cracks or shrinkage cracks. If the casting comprises narrow parts which have been overcooled, it can, after being stripped, be immersed in a stabilizing bath at 810 one minute, after being salthardened at 180. The casting is then subjected to nuclear formation treatment, for example 48 hours at 450", then, with or without intermediate cooling, the casting is subjected to graphitization of the primary cementite; in order for the graphitization to be absolutely complete. it is necessary to maintain it at 875 for 2 to 6 hours, but this time can be reduced to between 40 minutes and two hours by annealing at 900.

The following table gives the number N per mm. of fine spherules of graphite having an average diameter of 2 to 6 microns obtained by this process. In these tests, the chill-casting was 14 mm. thick; stripping was done at more than 820 and the piece was hardened directly ice in salt at The annealing comprised a first cycle of 48 hours at 450, and a second: 3 hours at 875.

In order for the process to attain its full effectiveness, it is preferable to allow the piece to cool on leaving the matrix and to reheat it for austenization at 810 for 30 minutes, for example, and hardening in stages, at 180 for 1 minute for example. The castings are then subjected to nuclear formation treatment, for example 48 hours at 450, cooled, then reheated to 875 for example, just long enough for the graphitization of the primary cementite, then cooled in still air, failing which, if the time taken is too long, the graphite undergoes a coalescence with a reduction in the number of spherules and lowering of the mechanical properties. This is shown by the following table which gives the number of spherules of graphite per mm. having a diameter of less than 2 microns:

In each case, the quantity of cementite is nil, so that the annealing for 1 hour at 875 is suflicient. It will be seen that here the best result is obtained with the cast iron containing 1.2% copper, which has 40,000 spherules of graphite per mm.

These chill-cast irons, reheated 810/ 30 minutes, hardenecl in salt at 180 for 1 minute, cooled in still air, reheated to 450 for 48 hours, cooled in still air then reheated at 875 for 1 hour, and cooled in still air, are lamellar pearlitic and have the following characteristics in test-pieces 4 mm. in diameter, machined from pieces 20 mm. thick:

After this treatment, a piece of casting 2162 was reheated to 835, hardened in oil, tempered at 700; it then showed: 12:80, 12:82, A=2.5% in a machined test-piece 4 mm. in diameter.

it is also possible, after this last tempering at 700, to stop this by oil-hardening and temper it for 2 hours at 500 to induce the structural hardening of the copper.

Finally, it is possible to modify the compositions and properties of these cast irons by alloys such as: Ni, Mo, Ti, Al, Zr and the like. The machinability, after graphitization, with or without hardening and tempering, is particularly easy, due to the graphite.

The copper content of the casting is suitably 0.4 to 3% and is preferably 1 to 2%, as stated in said copending application.

Further research has shown that when the castings obtained according to the aforesaid patent application by die-casting in chill-moulds are subjected to a heat treatment in a simple cycle, of the usual type in the standard manufacture of malleable cast iron, diffuse distribution of graphite is effected and mechanical properties are obtained, which, without reaching the exceptional values permittedby the three-phase treatment, are greatly superior to those obtained in the normal manufacture of malleable cast iron, and suflicient for certain applications.

The following table gives, by way of example, a comparison of the number of spherules of graphite per mm. and of the mechanical properties of castings having the same composition as follows: C=2.50%, Si=1.50%, Mn=0.50%, O1=0.40%, some cast in sand as is the usual practice in the manufacture of malleable cast iron and treated in known manner by heating for 18 hours at 940 C followed by cooling in the furnace to 740 C,, which temperature is maintained for 24 hours, followed finally by cooling in air; the others die-cast in chill-moulds and treated either by the usual treatment which has just been described or by the three-phase treatment: quenching, nucleation, graphitization.

In this table, E represents the elastic limit, R the breaking strength, and A the elongation, according to the French standards for steels.

The subject of the present invention is therefore a process which consists, as a modification of that described in the aforesaid patent application, in proceeding successively with die-casting in chill-moulds to obtain white iron, and with a graphitization treatment in a single heat cycle comprising annealing to a temperature above 850 C., for a period comprised between 30 minutes and 6 hours, which may be followed by a holding at a temperature not very different from A for a period comprised .between and 8 hours, finally followed by cooling in air. This last holding at a temperature in the vicinity of A is useless if a tough product with a pearlitic structure is desired. It is used to obtain a malleable ferritic structure, the period of holding being selected between 0 and 8 hours depending on the desired proportion of pearlite and ferrite constituents, in other words depending on the balance which has to be obtained between toughness .andmalleability, for a given application.

For concerns which have furnaces for the conventional treatment of malleable cast iron, at their disposal, the new process has the advantage of enabling castings to be obtained having precise dimensions and high mechanical properties, without investing in neaw heat-treatment apparatus.

Another advantage results from the fact that these mechanical properties, which are superior to those of ordinary malleable cast iron, are obtained, in the treatment according to the invention, by a shorter annealing time. For example, for the complete graphitization of Lthe primary cementite, the minimum period at 950 C. would be 18 hours for ordinary malleable cast iron, whereas a period of two hours at the same temperature is sufficient for the new process.

Cooling from the annealing temperature may be reg- 11lated in .such a manner that the structure of the matrix is wholly or partially pearlitic. The hardness and toughness of the metal can thus be regulated to any desired value.

1. A method of obtaining castings of pearlitic structure containing diifused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of white iron, stripping the casting from the mold while hot, and heat-treating the casting in a single cycle comprising the steps of annealing to a temperature above 850 C. for A2 to 6'hours, and cooling in air.

2. A method of obtaining castings of ferritic structure containing diffused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of white iron, stripping the casting from the mold while hot, and heat-treating the casting in a single cycle comprising the steps of annealing to a temperature above 850 C. for /2 to 6 hours, and cooling in air.

3. A method of obtaining iron castings of pearlitic and ferritic structure containing diffused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of white iron, stripping the casting from the mold while hot, and heat-treating the castingin a single cycle comprising the steps of annealing to a temperature above 850 C. for /2 to 6 hours, holding the casting at a temperature approximating the A transformation point for a period up to 8 hours, and cooling in air, the formation of the ferritic structure being greater the longer the holding of said casting at said last-named temperature.

4. A process as defined in claim 1, treated contains 0.4% to 3% copper.

5. A process as defined in claim 1, treated contains 1% to 2% copper.

6. A process as defined in claim 2, treated contains 0.4% to 3% copper.

7. A process as defined in claim 2, treated contains 1% to 2% copper.

8. A process as defined in claim 3, treated contains 0.4% to 3% copper.

9. A process as defined in claim 3, treated contains 1% to 2% copper.

10. A method of obtaining castings containing diffused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of white iron, stripping the casting from the mold while hot, and heat-treating the casting in a single cycle comprising the steps of annealing to a temperature above 850 C. for A2 to 6 hours, and cooling in air.

11. A method of obtaining iron castings containing diffused fine particles of graphite which comprises die casting in a chill mold an iron suitable for the formation of white iron, stripping the casting from the mold while hot, and heat-treating the casting in a single cycle comprising the steps of annealing to a temperature above 850 C. for /2 to 6 hours, holding the casting at a temperature approximating the A, transformation point for a period up to 8hours, and cooling in air.

12. A process as defined in claim 10, wherein the iron treated contains 0.4% to 3% copper.

13. A process as defined in claim 10, wherein the iron treated contains 1% to 2% copper.

wherein the iron wherein the iron wherein the iron wherein the iron wherein the iron wherein the iron References Cited in the file of this patent UNITED STATES PATENTS 1,498,128 Sowers June 17, 1924 1,747,728 Morris et al. Feb. 18, 1930 1,815,361 Morris et al. July 21, 1931 2,185,894 Hultgren Jan. 2, 1940 2,331,886 Boegehold Oct. 19, 1943 FOREIGN PATENTS 999,242 France Oct. 3, 1951 OTHER REFERENCES Die Casting Practice, page 4. Edited by Stern. 'Published in 1930 by McGraw-Hill Book Co., NY.

Die Casting, Machines, Dies, Alloys, pages 289 to 292. Copyright 1936. Edited by Herb. Published by the Industrial Press, 148 La Fayette Street, New York, NY.

Metals Handbook, 1948 edition, page 5. Published by the American Society for Metals, Cleveland, Ohio.

Claims (1)

1. A METHOD OF OBTAINING CASTINGS OF PEARLITIC STRUCTURE CONTAINING DIFFUSED FINE PARTICLES OF GRAPHITE WHICH COMPRISES DIE CASTING IN A CHILL MOLD AN IRON SUITABLE FOR THE FORMATION OF WHITE IRON, STRIPPING THE CASTING FROM THE MOLD WHILE HOT, AND HEAT-TREATING THE CASTING IN A SINGLE CYCLE COMPRISING THE STEPS OF ANNEALING TO A TEMPERATURE ABOVE 850* C. FOR 1/2 TO 6 HOURS, AND COOLING IN AIR.