US1871543A - Malleableizing iron - Google Patents

Description

Patented Aug. 16, 1932 .UNITED STATES PATENT OFFICE RUSSELL H. MCOARROLL AND GOSTA VENNERHOLM, F DEARBORN, MICHIGAN, AS-

SIGNORS TO FORD MOTOR COMPANY, OF DEARBORN, MICHIGAN, A CORPORATION OF DELAWARE MALLEABLEIZING IRON No Drawing.

This invention relates to the manufacture of cast iron and has for its principal object cast iron to form ferrite and temper carbon.

In white cast iron, the carbon content is in the combined form, that is, as iron carbide, or cementite, and as a solid solution of iron carbide and iron. The iron carbide of cast iron is readily decomposed into iron and carbon and as this decomposition can be controlled by such factors as the time and temperature as well as the rate of cooling and the proportion of silicon and manganese contained in the iron it is possible to manufacture white or grey cast iron by varying such factors in accordance with the product desired to be obtained.

The process of manufacturing malleableized iron from white cast iron has long been practiced and such process has consisted in heating the white iron to above the critical range, maintaining the temperature above such range and slow cooling to and below the A1 point of the iron-carbon diagram; the pe-- riod of heat treatment varying from six to fourteen days. Much time and study have been directed to devising ways and means for shortening this long period of heat treatment, but prior investigators have directed their attention mainly to changes in furnace designs or types, and methods of heating and cooling, but, as the laws governing graphitization, i. e., the process of decomposing iron carbide into carbon and iron as heretofore practiced, have been found by competent, skilled, metallurgists to depend upon clearly known chemical fundamentals and fixed natural laws which cannot be changed, but very little h b m d th f t Progress as een a e m 6 way 0 ma e 1 mg combined carbon tends to collect with a rially cutting down the annealing time.

The manufacture of satisfactory malleableized iron depends upon preventing the decomposition of iron carbide during the production of the white cast iron and, thereafter, upon the annealing heat treatment to effect Application filed August 25, 1930. Serial No. 477,796.

graphitization, in such a way as to insure the production of the carbon in the form of temper carbon. It has been known that the presence of silicon will promote decomposition while manganese tends to retard decomposition. Therefore it has been the practice to keep the silicon content of the iron for white cast iron comparatively low and less than 1% so as to avoid as much as possible graphitization or decomposition during the cooling of the casting immediately after the casting thereof. The standard A. S. S. T., specifications for white cast iron is as follows:

The reason for keeping the silicon content low in such standard specifications is that with the relatively slow cooling that characs terizes the usual sand casting operation as heretofore practiced, if appreciable decomposition during such cooling occurs the resulting product will be, not white cast iron, but, mottled or grey cast iron in which the carbon has separated to form graphitic flakes or fissures. This known property of silicon has therefore been extensively'used in the manufacture of soft grey cast iron, it being common practice to employ a rather high silicon content for such iron. The formation of graphitic flakes or fissures in a casting upon freezing of the metal as it cools in the mold precludes the use of such castings for the manufacture of malleableized iron because when such flakes or fissures areformed they cannot be removed, nor their size reduced, by the malleableizing heat treatment, but, on the contrary, during such heat treatment, the graphitic flakes seem to form nuclei about which the carbon of any remainresultant increase in the size of such flakes.

As heretofore pointed out, although many proposals have been made to shorten the annealing period such proposals, at least as far as we are aware, have been directed to changes in the furnaces employed for the annealing, or graphitization, heat treatment, or to changes in the annealing c cle. We have departed from such ,proposa s and, in contradistinction thereto, have directed our attention to changes not in the furnaces or method of annealing,per se, but to changes in the first step of the process, that is, the production of the white cast iron, in order to secure a product that can be graphitized by a. very short annealing heat treatment.

The present invention depends primarily upon a radical departure from the former ractice in the chemical analyses of the iron, in combination with a chill casting thereof.

In determining upon the chemical analyses the following essentials must be borne in mind; first the molten metal must have suflicient fluidity to insure its flowing freely into the mold and to fill thoroughly all recesses, grooves or cavities thereof; second,

the metal must have low shrinkage proper-f: meet the essentials enumerated:

ties; third, the silicon content must be high enough to allow the combined carbon to break down in a short time; and fourth,-the analyses must be such as will insure the" production of white cast iron.

The invention may be said to be based I upon a recognition of the known fact that si icon promotes or hastens the decomposition of iron carbide; therefore the annealing time may be markedly decreased by the addition of a. much larger amount of silicon than heretofore employed or considered feasible, provided however that the cooling of the casting immediately after the pouring thereof be effected very quickly so as to prevent any appreciable graphitization during the freezing of the metal. 1 The invention however cannot be carried out merely by the addition of the larger amount of silicon and quick cooling, because, due to the property of silicon to promote graphitization, difliculty would be encountered in securing a white cast iron, it being the practice in the art as hereinbefore statedto increase the silicon content in order to insure the production of soft grey castings. Therefore it becomes necessary to counteract the tend; ency of silicon 'to form a soft grey iron;

This is efiected by increasing the proportion of manganese which element has t 1e property of retarding graphitization. The carbon content is also an important factor. As 1ncrease in carbon increases fluidity and also, depending upon the amount of manganese and silicon present, decreases the annealingtime, while increase in manganese increases annealing time, but furthers the formation of white iron, increases shrinkage and does not affect fluidity; and as silicon increases fluidity, decreases annealing time and decreases shrinkage; the determination of the chemical analysis to be employed, in order that the objectof the invention be attained,

with it nice proportioning and balancing of v carbon, silicon and manganese contents so as to insure that the iron will have the requisite properties. It is also known that high phosphorous content causes cold shortness or brittleness of the iron, but also increases fluidity more than any other element except carbon. Therefore in order to reduce objectionable ell'ects due to high phosphorus we have cut down the phosphorus content much below standard practice and compensate for the loss of fluidity, which in the case of die casting would be a serious objection, by increasing the silicon and carbon contents.

As a result of a thorough and extensive series of experimental beats and laboratory tests we have determined that iron having an analysis within the following range will fully Carbon from 2.80 to 2.00 per cent.

Manganese from .40 to 1.20 per cent. Silicon from 1.80 to 2.80 per cent. Phosphorus .05 maximum. Sulphur .10 maximum.

We have employed with highly successful results on a commercial production scale of malleableized cast iron the following analysis:

Carbon 2.50 to 2.60% Manganese .15 to .50% Silicon 2.15 to 2.25% Phosphorus .05 maximum. Sulphur .10 maximum.

The molten iron prepared preferably in an electric furnacelis poured into molds having high. heat conductivity so that'a quick cooling of the metal results. After the removal of the castings from the molds the castings are transferred to an annealing furnace, heated by any means capable of maintaining the desired temperature, in which the temperature of the castings is raised to a maximum, above the critical range, or from 1650 to 1825 F. The time required for bringing the castings to such maximum does notafi'ect the graphitization, hence this oper ation may be performed as quickly as possible. The castings are then-held at such maximum for a period of time depending upon their cross sectional area, after which the temperature is lowered, preferably by an air cooling effected by transferring them to a secfor the heat treatment for shock absorber housings having relatively thin walls (about inch thick) was as follows: Holding at 1825 F., one and one-half hours; cooling from 1825 to 13? 5, one-half hour; and cooling from 1375 to 1200, one hour; making a total elapsed time of three hours for the entire annealing treatment.

For castings of greater cross so -tioual area, the heat treatment consisted in holding at the maximum heat above the critical range for five hours; cooling from maximum to 1400", one and one-half hours; and two hours for cooling from 1400 to 1200; making atotal elapsed time of eight and one-half hours.

In both instances above set forth, as well as in many other intermediate cases, the castings after fracture, were found to be perfectly malleableized and of high physical properties. We have found as a result of our work that the time of heat treatment for malleableizing may, by employing the principles of the present mvention, be out down to approximately from 2 to 15 hours dependtreatment comprising heating the iron quick- Ar critical point, such cooling being effected in approximately 15% of the time for the entire annealing heat treatment, and the slow cooling through and slightly below such point, such slow cooling being effected in approximately 25% of the time for the entire annealing heat treatment.

In testimony whereof we aflix our signatures hereto. v

.RUSSELL H. MOCARROLL. GOSTA VENNERI-IOLM.

ing upon the size of the castings, with the! heating cycle approximatel divided as follows: 60% of the time for olding at maxi mum heat, 15% for cooling to lower critical point, and 25% for cooling through and below lower critical point.

As will be seen from the foregoing We employ a much larger proportion of silicon than heretofore used in the manufacture of, malleableized iron and prevent any appreciable graphitization during the freezing of the casting, by a quick cooling thereof, the manganese and quick cooling insuring the production of a white cast iron. When the castings are subjected to the annealing heat treatment, the silicon promotes the decomposition of the iron carbide to such an extent that complete graphitization is accomplished in but a small'fractional part of the time heretofore considered to be absolutely essential and irreducible.

It will be understoodthat the proportions of silicon, manganese and carbon contents may be varied, and that the annealing time may also be varied, from the limits given in the typical examples described, all in accordance with the particular requirements or exigencies of the work.

We claim:

The method of manufacturing malleableized iron which consists in preparing molten iron having a carbon content between 2.00% and 2.70%, a manganese content between 1.20% and 40% and a silicon content between 2.80% and 1.80%, casting the molten iron in molds, cooling the molten metal quickly in said molds to prevent decomposition of iron carbide during freezing of the metal, there- .by producing white cast iron of high silicon content, and subjecting such white cast iron to an annealing heat treatment of from two to fifteen hours duration, such annealing heat