US1938516A - Process for the manufacture of malleable iron - Google Patents

Description

Patented Dec. 5, 1933 PROCESS FOR THE MANUFACTURE OF MALLEABLE IRON Walter Cotter-ill, Waterbury, Conn.

No Drawing. Application August 25, 1930 v Serial No. 477,804

6 Claims.

My invention consists of a new and useful process in the manufacture of malleable iron.

One object of my invention is to shorten the time required for malleableizing cast iron. Another object is to eliminate the necessity for packing the castings in powdered material.

Still another and important object is to provide a process for manufacturing malleable cast iron which is easy to control and is capable of 10 accurate regulation, whereby it is possible to obtain results which are uniform, and in which it is possible to obtain any desired degree of malleability within generally accepted limits.

Ordinarily, in the production of malleable cast iron, the castings aresubjected to a prolonged heat treatment, requiring several days. The castings are packed during this prolonged heat treatment in powdered iron oxide materials, such as hammer scale, magnetite, hematite, etc., or

inert materials, such as sand. When the heat treatment is completed, the castings are allowed to 0001 very slowly in the furnace. The efiect of this treatment is to convert the combined carbon in the castings to graphite, which takes the form of very minute particles, known as temper carbon, evenly distributed throughout the casting, the amorphous particles reposing in the interstices of the crystalline iron structures, which are opened by the high temperature applications.

The heat treatment also eliminates a portion of the original carbon content.

My invention contemplates a process of converting hard cast iron into malleable cast iron, in which a gaseous but a substantially non-oxi- 36 dizing atmosphere is maintained in the heating receptacle during the range known as the solid solution range, in which the free cementite (Fe C) passes into solution.

It further contemplates the admission of 40 oxygen to the apparatus during any part of the heat treatment, particularly during thev later stages of the heating, known as the carbon precipitation range. The presence of a relatively small quantity of oxygen-not only aids in the formation of the temper carbon, but also eliminates a certain percentage of the carbon, with.

the result that the products obtained are comparable in strength and other properties with the malleable iron produced by prior processes. The time of heating is cut from several days, which were required in the old processes, to a few hours. Since an inert gaseous medium is employed during the solid solution range, it is possible to employ a higherheat without danger 56 of damage to the castings by burning them, for

example, a temperature of 1700 degrees F. to 1800 degrees F. may be employed instead of a temperature between 1500 degrees F. and 1600 degrees F. Furthermore, it is not necessary to use the powdered packing materials heretofore 0o employed, and it has been found that the surface of the castings are not roughened or pitted,

as is the case at times when the iron is treated in contact with the powdered material.

In the practice of my invention the iron-castings to be malleableized are placed in a heating receptacle capable of withstanding the elevated temperatures necessary to carry out the process. The receptacle is equipped with means whereby it may be sealed from the atmosphere and is provided with one or more inlet pipes for the gases and with an exhaust pipe through which the gases may be removed. The pipes are equipped with valves to control the admission and elimination of gas. After the iron castings are placed in the heating receptacle, the receptacle is sealed to exclude the atmospheric air, and the atmosphere in the receptacle is replaced by an inert and substantially non-oxidizing gaseous medium, which is preferably furnished by a separate chamber in which some carbonaceous or hydrocarbon materiaL'such as gas or oil, is burned. For this purpose it is desirable to employ fuel which can be burned as nearly as possible to complete combustion, in which case 8| the products of combustion contain 79% of nitrogen and 21% of,carbon dioxide. While this optimum condition is not always possible, it is desirable to approach it as closely as feasible. Small amounts of carbon monoxide, and even oxygen, are not, however, deleterious.

By means of heat from any suitable source, the receptacle is rapidly raised to a temperature, preferably between 1700 to 1800 degrees F. until substantially all of the graphitic carbon in the I cast iron has been absorbed by the iron. This usually takes approximately from two to four hours, depending upon the temperature used. The receptacle is then allowed to drop gradually in temperature until it reaches .approximate- I 1y 1200 degrees F. This stage is what is known as the carbon precipitation range, and it is during this period that the gaseous medium is modifled to cause precipitation of the carbon. This modification consists of the admission of a deflnite amount of excess oxygen, either in the form -of atmospheric air or pure oxygen which will combine with the inert atmosphere to produce aslightly oxidizing atmosphere. This may be accomplished by changing the conditions in the 1,0

combustion chamber, or by adding oxygen to the products of combustion through a separate pipe, either in the form of atmospheric air, or if desired, pure oxygen. The volume of oxygen admitted to the heat treating chamber is relatively small, for example, from 1% to 3%, although it may be higher at times, depending upon the temperature of the material at the time of admission, it being necessary at all times to guard against the admission of an amount of oxygen sufficient to cause a burning of the material treated. The addition of too much oxygen causes a sudden rise'in the temperature of the material which can be indicated by a pyrometer in contact with the metal. The pyrometric reading serves as a visible warning when an excess of oxygen is being admitted. Preferably, admission of oxygen is continued throughout the entire carbon precipitation range,

causing the precipitation of the carbon as temper carbon and the oxidization of a portion thereof. When the carbon precipitation is sufficiently complete for the particular type of malleable iron desired, the heat is 'shut off, and the container allowed to cool in a sealed condition. Thecooling usually takes from twelve to fourteen hours.

If it is desired to increase the surface ductility and resistance to corrosion of the material treated, a substantially carburizing gaseous medium, such as carbon monoxide, may be introduced into the heating receptacle. This may be produced by adding carbonaceous gas liquid or solid material in the combustion chamber used to produce a substantially inert gaseous medium and the carbon monoxide gas thus generated is introduced into the heating receptacle through the pipes provided, or carbon monoxide may be otherwise introduced into the heating receptacle. This. carburizing ,medium is introduced subsequent to the carbon precipitation range, and it may be desirable to raise the temperature in the heating receptacle to ap,- proximately 1600 degrees F., or more. The heat should be maintained at this high temperature from one to eight hours, depending upon the thickness of the skin or the degree of penetration desired.

, It will be understood that the cast iron referred to throughout this specification will be of a chemical analysis usually considered good practice in the production of malleable iron castings.

Considerable modification is possible in the lengths of time of treatment, the temperatures employed and the amount of oxygen admitted, without departing from the essential features of my invention. Obviously, the thickness of the casting will have a bearing upon the time necessary to accomplish the process and will govern the amount of oxygen or oxidizing element used.

I claim:

1. The process of producing malleable iron which comprises placing cast iron in a heating receptacle, displacing the, normal atmosphere therefrom by the passage of a substantially inert non-oxidizing gaseous medium therethrough,

heating the cast iron in this inert non-oxidizing medium during the preliminary heating and at an elevated temperature until substantially all the graphitic carbon in the cast iron has been absorbed by the iron, and thereafter reducing the temperature of the cast iron to allow carbon precipitation to take place and admitting a relatively small controllable percentage of an oxidizing gaseous medium to the heating receptacle during at least a portion of the carbon precipitation range to cause precipitation of the carbon as temper carbon and the oxidation of at least a portion thereof.

2. The process of producing malleable iron which comprises placing cast iron in a heating receptacle, displacing the normal atmosphere therefrom by the passage of a substantially inert non-oxidizing gaseous medium therethrough, heating the cast iron in this inert non-oxidizing gaseous medium during the preliminary heating and at'a temperature between approximately 1700 F. and 1800 F. until substantially all the graphitic carbon in the cast iron has been absorbed by the iron, and thereafter gradually reducing the temperature to approximately 1200 F. to allow carbon precipitation to take place and admitting a relatively small percentage of an oxidizing gaseous medium to the heating receptacle during at least a portion of the carbon precipitation range to cause precipitation of the carbon as temper carbon and the oxidation of at least a portion thereof.

3. The process of producing malleable iron which comprises placing cast iron in a heating receptacle, displacing the normal atmosphere therefrom by the passage of a substantially inert non-oxidizing gaseous medium therethrough, heating the cast iron in this inert non-oxidizing medium during the preliminary heating and until substantially all the graphitic carbon in the cast iron has been absorbed by the iron, thereafter reducing the temperature of the cast iron to allow carbon precipitation to take place and admitting a relatively small controllable percentage of an oxidizing gaseous medium to the heating receptacle during at least a portion of the carbon precipitation range to cause precipitation of the carbon as temper carbon and the oxidization of at least a portion thereof, and thereafter bringing a carburizing gaseous medium into contact with the cast iron at a carburizing temperature to increase the carbon content of the surface of the cast iron.

4. The process of producing malleable iron which comprises placing cast iron in a heating receptacle, displacing the normal atmosphere therefrom by the passage of a substantially inert non-oxidizing gaseous medium therethrough,

heating the cast iron in this inert non-oxidizing gaseous medium during the preliminary heating and at a temperature between approximately 1700" F. and 1800%F. until substantially all the graphitic carbon the cast iron has been absorbed by the iron, thereafter gradually reducing the temperature to approximately 1200" F. to allow carbon precipitation to take place and admitting a relatively small percentage of an oxidizing gaseous medium to the heating receptacle during at least a portion of the carbon precipitation range to cause precipitation of the carbon as temper carbon and the oxidation of at least a portion thereof, and thereafter re-heating the cast iron to a carburizing temperature and bringing a' carburizing gaseous medium into contact with the cast iron at this temperature to increase the carbon content of the surface of the cast iron.

5. The process of producing malleable iron which comprises placing cast iron in a closed heating receptacle, displacing the normal atmosphere therefrom by the passage of a substantially inert non-oxidizing gaseous medium tberetbrough, continuously passing an inert non-oxidizing gaseous medium through said heating receptacle during the preliminary heating and at an elevated temperature until substant'i'ally all the graphitic carbon in the cast iron has been absorbed by the iron, and thereafter reducing the temperature of the cast iron to allow carbon precipitation to take place while continuously passing a gaseous medium through said heating receptacle, said gaseous medium containing a relatively small controllable amount of oxygen during at least a portion of the carbon precipitation range to cause precipitation of the carbon as temper carbon and the oxidation of at least a portion thereof.

5. The process of producing malleable iron which comprises placing cast iron inaclosedheating receptacle, displacing the normal atmosphere therefrom by the passage of a substantially inert non-oxidizing gaseous medium therethrough, continuously passing an inert non-oxidizing least a portion thereof, and thereafter passing a carburizing gaseous medium through said receptacle in contact with said iron maintained at a carburizing temperature to increase the carbon content of the surface of the cast iron.

WALTER CO'ITERILL.