US2887421A - Method of producing castings having high mechanical properties - Google Patents

Description

United States Patent 2,887,421 METHOD OF'PRODUCINGCASTINGS HAVING HIGH MECHANICAL PROPERTIES "Lucien Pras, Billancourt, France, assignor to RegieNationale desUsines Renault, Billancourt, France No Drawing. Application December 27, 1955 .Serial No. 555,304

"Claims priority, application France January 5, 1955 3 Claims. (Cl. 148--21.8)

TThepresent invention relates to a method of producing cast iron castings having high mechanical properties, and constitutes an improvement in or modification of the "method described in the patent application Ser. Number 441,923 of July 7, 1954, now Patent No. 2,791,526, issued May 7, 1957, in tlie name of same applicant.

The aforesaid patent relates to the total primary .graphitisation of sulphur-containing white iron castings then to form the graphite nuclei by epistaxis on the hexagonal pyrites, and finally to permit free development of the nuclei to provide graphite spherules.

The process which permits in the case of cast iron, which is totally white upon the pouring operation, of a graphitization upon annealing without the formation of cementite produced by solidification, resulting in a totally spherulitic graphite, is characterized by the following features:

(1) The composition comprises a proportion of S/Mn which is greater than 1-- and more particularly less than 0.15 manganese and more than 0.2 sulphur, and preferably it is comprised within the following limits:

C=2.10 to 2.70 Si=0.85 to 1.10 Mn=().002 to 0.15 S=0.2O to 0.30 P=0.10

Generally speaking, there are not special components. (2) Solidification in the mold should take place as rapidly as possible, and preferably more rapidly than is strictly necessary to obtain the white structure. This leads to a fine distribution of the iron sulphide, on which there depends the fineness of distribution of the graphite spherules obtained during the course of the subsequent annealing process. Casting in a chill mold is preferable to casting in sand.

(3) The thermal treatment comprises essentially a martensite hardening or, if desired, an inferior hardening before carrying out the graphitization by annealing.

This hardening is performed with advantage in conditions of rapid cooling, preferably for example in oil, or if desired in a bath of salts, which may be a fusible mixture of alkaline nitrate and alkaline nitrite and leads to a martensitic structure.

To avoid the danger of shrinkage upon hardening there has been recognized according to the invention the prime importance of effecting a preliminary annealing, a simple austenization beyond termination of the transformation upon the heating A followed by cooling at a medium speed, such as cooling in still air.

It has also been recognized 'that if the cooling for the purpose of solidification ismoderate (casting in sand),

it may be necessary to carry out two consecutiveaustenization treatments with martensitic hardening.

(4) The thermaltreatment furthermore comprises, followingthe martensitic hardening, a graphitization an nealing beyond the point A i.e., at a temperature sufiiciently high so that the casting will be constituted only by cementite and austenite, such temperature being maintainedfor a suificientlength of time to permit of total disappearance of the cementite not dissolved in the austenite at the annealing temperature. The duration of this annealing process is less than that in the case of usual white iron malleable castings annealed at the same temperature.

The consecutive cooling is carried out in accordance withthe mechanical properties sought to be achieved. There may be obtained a total eutectoidal graphitization in respect to a ferrite and graphite structure, as in malleable castings of'the usual kindvery soft-or preferably thegraphitization may be stopped at such a point that the. matrix has a substantially eutectoidal (lamellar perlite) composition, which leads to increased toughness.

Finally, .it is.possible as a subsequent step to carry out austenization, quenching and tempering in order to obtain a greater toughness.

An example of the above method is given in the following:

Pieces ofmetal having a weight of 700 grams and .a mean thickness of 20 mm. are cast in molds which are partially metal and partially composed of sand.

The castings have the following composition:

Carbon=2.5 Silicon: 1 Manganese: 0.12 Sulphur=0.28 Phosphorus: 0.10

Following stripping and complete cooling of the castings the latter are heated to 820 over a period of 30 minutes and are cooled in still air. There is thereupon carried out a second heating to 825 over a period of 30 minutes, and the pieces of metal are hardened in oil. The pieces are then reheated to 940 for a period of eight hours, after which they are removed from the furnace and cooled in still air.

A structure is obtained comprising fiine graphite spherules on a lamellar perlite base.

Mechanical tests carried out on test pieces taken from these metal parts have produced the following results:

Tensile strength 55 to kg./mm. Breaking strength to kg./mm. Elongation limit 5 to 6% Properties of this nature have not been obtained collectively heretofore in conjunction with castings devoid of all alloyed substances.

The limit of resistance to rotative bending has been measured in conjunction with Moore test pieces.

The resistance limit has been found to be 38 to 40 kg./mm. which is greater than that of the usual spheroidal castings.

The method referred to has the following advantages:

It is applicable to readily produced castings which make use of ores containing sulphur, and are of low commercial value, and it does not resort to any special substance alloyed therewith.

It provides, upon annealing, a very even distribution of graphite in the form of spherules, this structure being associated itself with very high mechanical properties, the breaking strength being between 95 and 1.05 kg./mm. with an elongation of between 5 and 6.5% if the matrix is lamellar perlitic.

According to the present invention, a modified method of treatment consists in heating white iron castings to a temperature of between 880 and 950 C. over a period of time varying between 4 and l'h'ours. For example, an ordinary heating to 900 C. over a period of 9 hours followed by a cooling in still air brings about complete graphitisation of the primary cementite. Micrographic examination shows that the graphite is produced in the form of fine spherules on a lamellar pearlite base.

, According to a modification of the present invention graphitisation may be effected by carrying out successive operations as follows: a heating followed by a quenching operation, which imparts to the metal a martensitic structure, a germination treatment eifected at temperatures located between 300 and 700 C. over a period of from 1 to 4 hours, and finally a graphitisation heating opera tion, which may be effected at a lower temperature and over a shorter period of time than in those cases in which there is carried out merely a preliminary quenching operation, or those cases in which graphitisation is performed directly with neither quenching nor germination. Total graphitisation of the primary cementite is obtained, for example, by a heating for 3 to 8 hours to a temperature of between 850 and 900 C.

Heating, 850 C., 30 minutes; oil hardening, Reheating, 470 C., 4 hours; cooling in still air, Reheating, 900 C., 6 hours; cooling in still air,

may be quoted as an example of this latter method of carrying out the process. A casting thus treated comprises fine, well-formed spherules on a lamellar pearlite base.

I claim:

1. In a process of thermally treating white castings to produce malleable iron with annealed spheroidal graphite therefrom, said castings containing 0.85 to 1.10 silicon, 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratio S/Mn being greater than 1, the steps which comprise heating said white castings at a temperature between 880 and 950 C. for 4 to 10 hours, and cooling in still air.

2. In a process of thermally treating white castings to produce malleable iron with annealed spheroidal graphite therefrom, said castings containing 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratio S/Mn being greater than 1, said process comprising the steps of heating said castings and directly quenching to impart a martensitic structure to the metal, heating at 300 to 700 C. for 1 to 4 hours and grhaphitizing by heating for 3 to 8 hours at a temperature of 850 to900 C.

3. In a process of thermally treating white castings to produce malleable iron with annealed spheroidal graphite therefrom, said castings containing 0.85 to 1.10 silicon 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratio S/Mn being greater than 1, the step which comprises heating said white castings at 900 C. for 9 hours, and cooling in still air.

References Cited in the file of this patent UNITED STATES PATENTS Drysdale June 16, 1925 Hultgren Ian. 2, 1940 OTHER REFERENCES

Claims (1)

1. 2. IN A PROCESS OF THERMALLY TREATING WHITE CASTINGS TO PRODUCE MALLEABLE IRON WITH ANNEALED SPHEROIDAL GRAPHITE THEREFROM, SAID CASTINGS CONTAINING 0.2 TO 0.3% SULFUR AND 0.002 TO 0.15 MANGANESE WITH THE RATIO S/MN BEING GREATER THAN 1, SAID PROCESS COMPRISING THE STEPS OF HEATING SAID CASTINGS AND DIRECTLY QUENCHING TO IMPART A MARTENSITIC STRUCTURE TO THE METAL, HEATING AT 300* TO 700* C. FOR 1 TO 4 HOURS AND GRHAPHITIZING BY HEATING FOR 3 TO 8 HOURS AT A TEMPERATURE OF 850* TO 900* C.