US2048748A - Method of heat treating metal - Google Patents

Description

Patented July 2d, 1936 STA! ES Tsar METHOD OF RAT TREATING METAL Carl F. Lauensteln, Indianapolis, Ind... amignor to Link-Belt Company, Chicago, 11]., a corporation of Illinois No Drawing. Application May 12, 1934, Serial No. 725,345

11 Claims. (Cl. 148-15) This invention relates to a metal and .to a method of heat treating for producing the metal.

One object of the invention is to provide a method of heat treatment of malleableized iron whereby it is strengthened without appreciabl reducing its malleability.

Other objects will appear from time to time in the specification and claims.

The method of this invention has its main application to the treatment of malleableized cast iron and it is applicable to malleableized cast iron of practically any analysis. Thus it may be used to treat malleableized white cast iron or black heart iron and others which have been malleableized, and it is not limited in its application to the treatment of malleableized iron.

Where malleableized cast iron is to be treated by this method, the castings, whether of black heart malleable or white heart malleable, are treated in substantially the same fashion. The white heart iron, for the purpose of this method, differs mainly from black heart iron in that it is made from an iron having a higher sulphur content. This method makes usable iron castings formed of this higher sulphur iron for certain purposes in connection with which such iron has not heretofore been usable and thus the field of usefulness of this type of iron is expanded when the parts have been subjected to the treatment set out below.

The method may be used in the treatment of irons which contain certain alloying elements.

Thus it might be used in the treatment of iron containing almost any suitable alloy. Among the most common may be mentioned nickel, molybdenum, copper, manganese and vanadium, although many other alloys or combinations of alloys may be used and it is within the contemplation of my invention to treat iron alloyed with any suitable alloy.

The presence of non-ferrous alloying materials in the metal will vary somewhat the'temperatures required in the various steps. Thus the temperature at which the metal will be treated if manganese is alloyed withit will be different from that at which it will be treated if molybdenum is the alloying substance. Consequently the exact temperatures will varywith the difference in analysis of the metal but they will vary within the limits indicated below.

In general, there is a first heating, followed by a quenching, and by a second or drawing heating and afinal quenching. The material to be treated will ordinarily be in the form of a casting and the castings will be poured, cleaned and preferably annealed, to produce malleable iron. It is this malleable iron casting which is ordinarily treated. As generally understood, ordinary malleableized cast iron is granular in its structure, being composed mainly of grains of ferrite and small particles of carbon or graphitic carbon. One step in the present method or process is designed to cause a combination of the carbon and the ferrite in order to form iron carbide.

The first heating is usually carried out in a box or rotary type of furnace with an average charge up to 500 pounds, but it may be carried out in any manner and in any suitable equipment and with a charge of any suitable. size. After the first heating the material is quenched and a second heating step is carried out in a bath of molten cyanide which may be in the form of molten sodium or potassium cyanide salts which may be diluted with other sodium or potassium salts. In one form of the invention the second heating is'carried out in a cyanide bath of approximately 60 per cent cyanide content. Since the second heatingis carried out'in a molten cyanide bath, for this heating there is preferably used a pot into which the cyanide is placed and heated and in which it thus becomes molten and the work to be heat treated is immersed inthe molten cyanide and heated with it. The amount of work which can be treated at a time depends upon the size of the pot. For ordinary commercial purposes an average size pot will hold approximately 100 pounds of work.

While many variations in the method are possible, there are in the main two typical cycles and these will be described below.

In the first cycle the material to be treated, after having been malleableized and cleaned, is

heated. This first heating may be varied between the temperatures of 1375 F. and 1550" F., depending upon thev analysis of the metal and the thickness of the section of the parts being treated. The time of holding at this temperature may also be varied, depending upon the size of the section treated and the physical qualities desired. In ordinary runs it will vary from five to thirty minutes. In this heating, after the material has reached the critical or carbon combining temperature, part of the carbon is combined with the ferrite.

After this heating the parts are quenched.

preferably in oil, but they may be quenched in air or water.

After the quenching a second heating is effected. The parts are reheated to a temperature between 1200" F. and 1375 F. Here again the exact temperature is dependent upon the analysis of the metal, upon the presence of alloying substances in it, upon the section of the parts being treated and the exact physical qualities desired. The most common range of temperatures in the second heating is from 1300 F. to 1350 F., but it may vary between the larger limits above indicated. The second heating'is carried out in a cyanide bath, preferably of approximately 60 per cent cyanide content. The duration of the heating in the cyanide bath depends as before upon the analysis of the metal, and the thickness of section, and will ordinarily be from one to three hours.

After the completion of the last or cyanide heating the parts are quenched, preferably in water.

One of the purposes of the last or cyanide heating is to cause the formation upon the parts under treatment of a hardened case and the depth of case required on the finished product is a factor in determining the length of time of this heating. The depth of case will under normal operating conditions vary from .005 inch to .020 inch, dependent upon the duration of the last or cyanide heating. The case so formed is ex- I tremely hard, the hardness being due to the combined carbon and nitrogen absorbed from the cyanide.

A second cycle by means of which the present method may be can'ied out is substantially like the first, except for a variation in the manner of arriving at the full or maximum temperature of the first-heating step.

In the second cycle, as in the case of the first cycle above described, the material treated will ordinarily be in the form of malleableized iron castings, which will be cast, malleableized, cleaned and thus prepared for treatment in any desired manner. With the castings so made and prepared the first heat treatment comprises raising the metal to a temperature which varies between 1375" F. and 1550 F., but instead of being raised directly to the maximum temperature which it is to attain, it is initially raised to a temperature approximating or slightly above the effective critical temperature, that is to say, the parts under treatment are initially raised to a temperature at or slightly above that actually necessary in commercial practice to initiate but not to complete the combination of the carbon with the iron. In practice, this initial temperature is frequently higher than the theoretical The temperature is thus held at a point where only very small quantities of carbon can recombine and at this temperature the actual effective practical recombination of the carbon does not occur completely, since the temperature is carefully kept below the temperature at which this general recombination can occur completely in practice. The temperature of this initial heating will vary somewhat, depending upon the analysis of the metal under treatment and depending also upon the quantity of metal under treatment at a given time, upon the size of the pieces and upon other factors. Thus the castings or other parts which are -to be treated are raised slowly to this initial temperature, which, in one example, is approximately 1400 F. and held at this temperature, being allowed to soak until all of the material in the furnace has evenly reached this temperature. In one typical cycle, this may take from an hour to an hour and three-quarters.

Whatever the quantity of material being treated and whatever the length of time during which it is subjected to this initial heating, care is taken that the initial heating is held at or very slightly above a temperature at which carbon combining commences, in practice. This initial temperature to which the material is first heated in the first step of this method, although definitely below that at which satisfactory general recombination of carbon can occur, is relatively close to the final temperature at which this recombination will occur, and, therefore, after the soaking, the material may be rapidly raised to a suitable temperature to cause efiective general carbon recombination.

After the completion of the initial heating and the soaking the material is raised more rapidly to the next temperature; ordinarily this next temperature is approximately 1475 F. and the material may be raised to this temperature quite rapidly. It is then held at that temperature for a short period-in one cycle it is held at that temperature for five minutes, and it is raised from 1400 F. to 1475 F. in from twenty to twenty-five minutes.

After the material has been suitably held at the second heating temperature, it is quenched, preferably in oil. After this quenching the material is reheated in the second or cyanide heating step as described above, thus being reheated in a .cyanide bath to a temperature between 1200" F. and 1375" F., 'at which-temperature it is held from one to three hours, depending on the analysis of the metal, the thickness of section and the thickness of case desired. Upon the completion of this cyanide heating it is quenched, preferably in water. The two cycles thus differ from each other only by inclusion in the second cycle of the soaking step as a part of the first or carbon combining heating.

between 1400 F. and 1500 F. may be carried out in a carburizing atmosphere or in the presence of a carburizing agent of any desired nature.

I have spoken above of the use of alloys in the metal. The process of this invention may be applied to ferrous metal associated with non-ferrous alloys. Among these are nickel, molybdenum, copper, manganese, vanadium and many other alloys one or more of which may be present in the material being treated in varying quantities, depending upon the physical properties desired.

Where in the specification and claims the expression effective practical critical temperature" occurs, there is meant that temperature at which in practice efiective carbon recombining takes place,-in distinction to the theoretical critical temperature,

While I have spoken of a cyanide bath and while this is entirely effective to produce the result desired, the second heating can also be 6 the proportions within which they will preferably be used:

. Per cent Copper .35 to 1.00 Manganese .50 to 1.00 Molybdenum .20 to .60 Nickel .50 to 1.50 .Chromium .10 to .60 Vanadium .10 to .60 Aluminum .10 to 1.00

These alloying substances may be used singly or in any combination and thus there maybe one, two, three or more in a single casting which is treated. The efiect of the alloys is generally to increase the strength of the base or core material and may also be to increase the hardness of the case.

I claim:

1. The process of treating malleableized cast iron which includes the following steps: Heating the metal to a temperature sufiiciently above the practical carbon combining temperature to effect substantial carbon recombination, quenching it, reheating it in a fused salt case hardening bath to a temperature slightly below the effective carbon combining temperature to form a hardened case, the second heating being held sufliciently below the carbon combining temperature to pre vent substantial internal carbon recombination, and suiiiciently close to the carbon combining temperature to cause active case hardening.

2. The process of treating malleableized cast iron containing ferrite and graphitic carbon, which includes the following steps: Heating it to a temperature sufiiciently above the carbon combining temperature to cause a substantial carbon recombination, and forming martensite and troostite, by quenching it, reheating it in a fused salt case hardening bath to a temperature sufficient to cause the martensite and troostite to break down to form sorbite or pearlite and to form a hardened case, and then cooling the metal.

3. The process of heat treating malleableized cast iron which includes heating the iron to a temperature between 1375 F. and 1550" F., quenching and reheating it in a cyanide bath to a temperature between 1200" F. and 1375 F.

4. The process of heat treating malleableized cast iron which includes heating the iron to a temperature between 1400 F. and 1550" F., holding it at that temperature from five to thirty minutes, quenching it and thereafter reheating it in a cyanide bath to a temperature between 1200" F. and 1375" F. and holding it at that temperature from one to hours and thereafter quenching.

5. The process of heat treating malleableized cast iron which includes heating the iron to a temperature between 1400" F. and 1550" R, holding it at that temperature from five to thirty minutes, quenching it in oil and thereafter reheating it in a cyanide bath to a temperature between 1200" F. and 1375 F. and holding it at that temperature from one to three hours and thereafter quenching it in water.

6. The method of treating malleableized cast iron which includes the following steps: Heating it initially to approximately, but slightly below, the effective practical carbon combining temperature, holding it at that temperature a sufli- 5 cient time to attain a uniform temperature throughout the mass of metal, subsequently raising it to a temperature sufliciently above the practical carbon combining temperature to effect substantial carbon recombination, thereafter quenching it and reheating it in a fused salt case hardening bath to a temperature somewhat below the effective carbon combining temperature, to form a hardened case.

7. The method of treating malleableized cast iron which includes the following steps: initially heating to a temperature approximately 1400 R, holding at that temperature a period sumcient to raise all of the material to a uniform temperature, thereafter raising the material to a temperature approximating 1475 F., quenching, and after the quenching reheating the material to approximately 1300" F. in a cyanide bath to form a hardened case, and quenching.

8. The method of treating malleableized cast iron which includes the following steps: initially slowly heating to a temperature approximately 1400" F., holding at that temperature a period suflicient to raise all of the material to a uniform temperature, thereafter raising the material more rapidly to a temperature approximately 1475" F" quenching, and after the quenching reheating the material to approximately 1300 F. in a cyanide bath to form a hardened case, and quenching.

9. The method of treating malleableized cast iron which includes the following steps: initially heating to a temperature approximately 1400" R, holding at that temperature a period sumcient to raise all of the material being treated to a uniform temperature, thereafter raising the material to a temperature approximating 1475" R, holding at that temperature to attain uniformity of temperature in all the material under treatment, quenching, and after thequenching reheating the material to approximately 1300" F. in a cyanide bath to form a hardened case, and quenching.

10. The process of heat treating malleableized cast iron which includes heating the iron to a temperature between 1375" F. and 1550: E, quenching and reheating it in a molten salt case hardening bath to add carbon and nitrogen to the surface, to a temperature between 1200" F. and 1375 F.

11. The process of heat treating malleableized cast iron which includes heating the iron to a temperature between 1400 F. and .1550 R, holding it at that temperature from five to thirty minutes, quenching it and thereafter reheating it in a molten salt bath case hardening to add car- 60 bon and nitrogen to the'surface, to a temperature between 1200" F. and 1375" F. and holding it at that temperature from one to three hours and thereafter quenching.