US2004774A - Short cycle anneal of malleable iron - Google Patents

Description

June 11, 1935. i. R. VALENTINE 2,004,774

SHORT CYCLE ANI IEAL OF MALLEABLE IRON Filed March 21, 1930 M0023; E 600 c g R 600 c 3 ii 400 c 200": 0 5 m u z; a;

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7 Irving Rvalentine, by MW His Attorneg.

Patented June 11, 1935 UNITED STATES SHORT CYCLE ANNEAL F MALLEABLE IRON Irving R. Valentine, Erie, Pa., assignor to General Electric Company, a corporation of New York Application March 21, 1930, Serial No. 437,823

4 Claims.

The present invention relates to a short cycle annealing process for producing malleable cast iron and particularly to an annealing process adapted to be employed either when the white cast iron, from which malleable cast iron is made, has become hard during the usual annealing process or when the white cast iron contains less than the normal total content of carbon and silicon.

Ordinarily, white cast iron from which malleable cast iron is made has a combined carbon and silicon content which varies from about 3.2% to about 3.6% of the total content of the casting. For example, if the carbon content is 2.50% the silicon content will be from about .70% to 1.10%, or, if the carbon content is 2.30% the silicon content may vary from about .90% to 1.30% making a total of 3.2% to 3.6%. It sometimes happens that the total silicon and carbon content of the white cast iron is not within this range. If the total content of silicon and carbon exceeds about 3.6% the casting easily may be annealed in accordance with the process disclosed in my copending application Serial No. 293,276, filed July 16, 1928, now Patent 1,830,630. However, if the white iron casting has a combined silicon and carbon content less than 3.2%, it is difllcult to anneal the casting and produce satisfactory malleable cast iron and this difilculty increases as the total content of silicon and carbon decreases from 3.2%.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. The invention itself however, will best be understood from reference to the following specification when considered in connection with the accompanying drawing in which Fig. 1 shows diagrammatically an annealing cycle which may 40 be employed in carrying my invention into effeet, while Figs. 2 and 3 illustrate modifications which may be introduced into the annealing cycle.

In my copending application, Serial No. 293,276, I have disclosed a short cycle annealing process for converting normal composition white cast iron into malleable cast iron. My prior process consists broadly in heating white cast iron of normal composition to a tempera- 50 ture of about 1000 C., holding the casting at that temperature for about 4 hours or long enough to cause the cementite or carbon in the casting to go into solid solution, cooling the casting to a lower temperature zone having an upper limit of about750 C. and a lower limit of about 675 C. and maintaining the casting in this lower temperature zone until it has normal malleable characteristics. In order to completethe malleabilizing process in as short a time as possible the temperature of the casting is while in the 675-750 temperature zone is given one or more changes. For example, the casting is held at about I40" C. for about 4 hours. The temperature is then dropped to 730 C., held at that point for about 4 hours and again dropped 10 to about 720 C. and held at the latter point for about 4 hours, at the end of which time the casting is completely'malleabilized, i. e. it has the physical characteristics of normal malleable cast iron. Instead of stepping the temperature 15 downwardly in the 675-750 C. zone, it may be stepped upwardly. For example, the casting may be cooled from 1000 C. to 710 C. held at the latter temperature for 4 hours, then raised to 720 C. held at that temperature for 4 hours, then'raised to 730 C. and held at that temperature about 4 hours at the end of which time the iron is completely malleabilized.

If my prior process is applied to white cast iron having a combined carbon and silicon con- 25 tent less than 3.2% or if the cast iron is chilled in the annealing process, while reducing the temperature from about 1000 C. to the lower temperature zone, in such a manner as to produce a form of pearlite which cannot be broken 3 up readily at temperatures below 760 C., it may be found that the castings are not completely malleabilized but are hard at the end of the annealing process. However, if such castings are again subjected at the end of the cycle to sub- 5 stantially the same or an equivalent annealing cycle, it will be found thatthe castings are completely malleabilized at the end of the second cycle. 1

In carrying out my invention, it is desirable 40 to employ at least two furnaces, preferably of the electric type and of about 5 or 6 tons capacity, one being employed to heat the castings to about 1000 C. and the other to heat the castings to temperatures in a zone having limits 4 between 675 C. and 750 C. In operation, white iron castings are heated in the high temperature furnace to a temperature of about 860 C. and held at that temperature for about 3 to 6 hours and then raised in about 1 hour to a temperature in the neighborhood of 1000 C. and held at the latter temperature for about4 to 6 hours. The temperature of the castings is then lowered to a temperature zone having an upper 56 limit of about 750 C. and a lower limit of about The reduction in temperature may be effected in any one of several ways. For example, the castings may be transferred directly from the furnace in which they have been heated to 1000 C. to a furnace having a temperature of about 740 C., the latter furnace being provided with rapid cooling features such as an auxiliary water cooled chamber fitted with flues through which the hot furnace gases are drawn and cooled, as disclosed in my copending application Serial No. 442,921, filed April 9, 1930. The castings when introduced into the 740 C. furnace should however have a temperature of at least 860 C. or higher. Instead of employing a furnace provided with rapid cooling features the castings may be transferred from the furnace in which they have been heated to 1000 C., to a cooling hood and allowed to remain in the hood until they reach a temperature of about 860 C. In transferring the castings to the cooling hood the outer portion of the castings may be cooled to a temperature below 860 C. while the inner portion remains at a somewhat higher temperature. When castings have remained in the cooling hood for about an hour they generally attain a uniform temperature of about 860 C. and may then be transferred to a furnace having a temperature of about 740 C. It is desirable to cool the castings from 860 C. to 740 C. as quickly, as possible, without quenching. To accomplish this result the castings may be lowered at intervals from the furnace into the air and then returned to the furnace. Ordinarily, the time required to drop a load from the furnace will vary from about ten to sixty seconds, while about one and one-half minutes will be required to return it to the furnace. After the load is lowered it may be returned immediately to the furnace or, if desired, allowed to stand in the air for a period of time which may vary from about two minutes at the start to about ten seconds at the end of the cooling process. The intervals during which the load is lowered from the furnace may be from about fifteen minutes to one hour apart, preferably one-half hour.

When the castings reach a temperature of about 740 C. they are held at that temperature fora period of time varying from about 2 to -4 hours. The temperature of the castings is then lowered somewhat abruptly to 730 C. The latter temperature is maintained for about 4 hours and then dropped again somewhat abruptly to 720 C. and maintained at that temperature for about 4 hours.

At the end of the latter period, if the castings which are being annealed have a total content of silicon and carbon which is less than about 3.2%, or if the castings have a total content of silicon and carbon which is greater than 3.2% but have become brittle due to the formation of a certain form of pearlite while cooling from 1000 C. to the lower temperature zone, the temperature of the castings is again raised to a temperature in a zone having a lower limit of about 860 C. and an upper limit of about 1000 C. and maintained in that zone for about 2 to 4 hours. The castings are then cooled in a few hours to a temperature zone having an upper limit of about 750 C. and a lower limit of about 675 C. and maintained in this zone until the castings have acquired normal malleable cast iron characteristics.

In the latter zone the temperature may be varied in different ways. For example, if the temperature of the castings is lowered from the 760-1000 C. zone to 720 C. as indicated in Fig. 2 of the drawing, the temperature of the castings may then be raised in about 2 hours to 730 C. maintained at the latter temperature about 2 hours and then lowered somewhat abruptly to 720 C. and maintained at the latter temperature for about 2 to 4 hours. On the other hand, if the temperature of the castings is lowered from the 760-l000 C. zone to 740 C. as indicated in Fig. 3 of the drawing, it may be reduced in about 2 hours from 740 C. to 730 C. and held at the latter temperature for about 2 hours, and, if desired, then lowered to 720 C. and held at the latter temperature from about 2 to 4 hours. Also, if desired, when the temperature is reduced from the 760-1000 C. zone to 740 C. the castings may be held at the latter temperature from about 2 to 4 hours, lowered somewhat abruptly to 730 C. held at the latter temperature from 2 to 4 hours and then lowered to 720 C. and held at the latter temperature for a period of from 2 to 4 hours, as indicated in Fig. 1. At the end of the heating period in the 750 to' 675 C. zone the castings are completely malleabilized, i. e. they have the characteristics of normal malleable cast iron, by which I mean that the castings have-a tensile strength of at least 50,000 pounds per square inch and an elongation of at least 10% in two inches.

At the start of my process, the castings may be heated directly to 1000 C. from room temperature instead of holding the castings at an intermediate temperature of 860 C. for an appreciable time and then heating to 1000 C. as illustrated on the drawing. advantages are obtained by heating the castings to an intermediate temperature and holding them' 1 at that temperature for a few hours.

ple, if heated too quickly to 1000 C. there is a For examstrong tendency to burn the outside portion of the castings. This is completely avoided if they are held for an appreciable time at an elevated intermediate temperature such as 860 C. Although I prefer to hold the castings at a temperature of about 860 C. for an appreciable period of time, that temperature is not critical. Other elevated temperatures in the neighborhood of 800 or 900 C. may be employed if desired. Furthermore', the temperature of the castings may be held substantially constant at a plurality of intermediate points instead of at a single point. For example, they might be heated for about 2 hours at 860 C., about 2 hours at 920 C. and 1 hour at 960 C. and then raised to 1000 C.

In cooling from the upper temperature zone to the 750-675 C. zone, it is economical to reduce the temperature of the castings as rapidly as possible. The temperature however should not be reduced so quickly as to give the effect of a quench. A quick reduction in .the temperature may be effected by. removing the casting from the 1000 C. furnace either to a lower temperature furnace equipped with rapid cooling features or to a cooling hood as hereinbefore indicated.

The first portion of the annealing cycle requires about 24 hours for its completion, as indicated in Fig. 1. The second portion of the cycle is substantially a repetition of the first portion. I have indicated several ways in which the temperature may be varied in the temperature zone having limits of 675 C. and 750 C. It should be understood however that any of these However, certain.

is I

reached substantially the temperature oi the Iur-.

variations may be employed either in the first or second portion or the cycle or in both portions of the cycle it desired.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. The method of making malleable cast iron which comprises the following steps: heating white cast iron at a temperature materially higher than 750 C. but not materially higher than 1000" C., cooling the casting gradually and in a few hours to-a temperature lower than 750 C. but higher than 675 0., maintaining the casting at said last mentioned temperature for about twelve hours, again heating the casting to a temperature materially higher than 750 C. but not materially higher than 1000 C. and then cooling the casting to a temperature zone having an upper limit of 750 C. and a lower limit of 675 C., and maintaining the casting in said temperature zone for several hours, the duration 01' said steps being approximately hours.

2. The method of cooling a casting from an elevated temperature to a lower temperature which comprises placing the casting in a furnace heated to said lower temperature and alternately removing the casting from the furnace and reinserting it in the furnace until the casting has nace.

3. The method of cooling a casting from an elevated temperature to a lower temperature which comprises placing the casting in a furnace heated to said lower temperature, and removing the casting from the furnace for a relatively short period of time and reinserting it in the furnace for an appreciably longer period or time and re-' mentioned furnace and reinserting them in said furnace until the castings have substantially reached the temperature of said furnace, maintaining the castings at the temperature 01' said furnace for several hours and then repeating the above steps.

IRVING R. VALENTINE.

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