US2400511A - Heat-treatment baths - Google Patents

Description

Patented May 21; 1946 HEAT-TREATMENT BA'IHS Artemas F. Holden, New Haven, Conn.

No Drawing. Application May 18, 1944, Serial No. 536,195

7 Claims.

The present invention relates to a harmonious group of heat treatment baths adapted to be operated at such temperatures as will produce the metallurgical effects desired in ferrous materials which are passed consecutively through these' baths.

The present baths are intended for increasing certain desirable characteristics such as hardness, elongation, elasticity and other physical properties required for certain ultimate uses of the steel I articles treated.

The present baths are useful with most commonly used steels, alloy steels and other ferrous alloys in common use.

The present invention comprises more specifically a high temperature bath and quenching bath of such chemical constituents as to permit the high temperature bath being carried over into the lower temperature bath; without undesirable changes taking place in the low temperature bath and which cannot be easily corrected. The invention further contemplates the use of more than two successive baths, all of which are of harmonious constituents so that undesirable changes do not exist through the several baths.

The constituents (hereinafter specified) of the No. 1 bath may be operated at 1300 F. to 1350 F. to comprise a pre-heat bath which allows the relief of stresses set up in the work during fabrication of the work and thereby lowers the tendency of distortion. The work, when heated to the temperature of the pre-heat bath, is transferred to the high temperature No. 1 bath which is operated at temperatures that are above the critical of steel. Since the constituents of the preheat bath and the No. 1 bath are identical, no problem is presented by carrying of some of the pre-heat bath over into the No. 1 bath.

In accordance with the present invention a bath No. l is a high temperature bath adapted to operate at from 1350 F. to 1700 R, which temperatures may be maintained within desired limits by use of a pyrometer control. The baths that may be heated in any conventional way, preferably electrically. -This high temperature bath No, 1, in accordance with the present invention, comprises potassium chloride 39%-50%; sodium chloride 40%60%; and sodium fluoride 1%-10%.

The preferred bath, however, being potassium chloride 42%; sodium chloride 55%; and sodium fluoride 3%. These constituents in the percentages specified form a stable liquid bath which does not fume extensively or sublime or disintegrate within the temperature ranges specified and is quite liquid at the lower temperature of 1350.

After the work has been suspended or submerged in the liquid bath No. 1 at the temperature specified, until the work is completely heated through and is stable at the temperature of the bath, the work is moved immediately from high temperature bath No. 1 to a quenchingbath No. 2. This quenching bath No. '2 is adapted to be run at temperatures from 500 F. to 700 F. During the operation of the two baths, the work being moved from bath No. 1 to bath No. 2, carries with it some of the-bath No. 1. Therefore, the percentages in bath No, 2 of the various constituents may, upon analysis, be found to have changed during the operation of the baths, and suitable corrections may be made whenever necessary. Therefore, the percentages of the No. 2 bath about to be given represent the percentages of the materials at the start of the operations and with this in mind the bath No. 2 comprises potassium chloride 8%18%; sodium chloride 10%18%: sodium fluoride Ai%-4%; and zinc chloride %-80%. The preferred percentages of the constituents at the starting of the No. 2 bath are potassium chloride 12%; sodium chloride 14%; sodium fluoride /2%; and zinc chloride l3 /2%. As previously stated, the No. 2 bath is enriched or changed by the No. 1 bath being carried over when the work is moved from the No. 1 bath to the No. 2 bath. This raises the amounts of sodium chloride, potassium chloride and sodium fluoriderelative to the preferred percentage of zinc chloride. When this has been carried to a point where the potassium chloride plus sodium chloride plus sodium fluoride is about 40% of the bath, the bath starts to thicken at this relatively low temperature. This thickening is undesirable and may be overcome .by the addition of zinc chloride to cause the zinc chloride content to be raised relative to the other constituents specified. One of the factors in the thickening of this low temperature No. 2 bath is the matter of temperature as well as the constituents. In other words, where the sodium, potassium chlorides are increased, the temperature should also be increased to maintain fluidity but the use of this bath is in this lower temperature range and therefore the temperature cannot be raised to control the situation; therefore the fluidity must be maintained by chemical corrections. It is to be understood that the work, that is the steel parts being treated,'shall be retained in the No. 2 bath a sumcient length of time to permit molecular changesto take place in the steel composition and this period of time is dependent upon the size of the steel parts being treated as well as the character of the steel, and usually is within a. period of one minute up to thirty minutes, depending upon the type of steel being heat treated and the size of the parts.

Under certain conditions, physical characteris-- tics desired may not be obtained by use of the two baths alone. For example, the steel articles, when I taken from the low temperature No. Zbath and permitted to cool to room temperature, a certain Rockwell hardness of the material. will be produced. This may be higher than is desired for the ultimate use of the steel products. In order to retain the advantages Obtained by the No. 2 l5 bath and at the same time obtain certain other desirable characteristics, for example, the lowering of the Rockwell hardness, it becomes desirable to subject the steel articles to a third bath which is run at a temperature between' the high temperature bath No. l and the quenching bath No. 2. Preferably the No. 3 bath retains the constituents of the No. 2 bath but these constituents are run at a higher temperature than the No. 2 quenching bath. The No. 3 bath preferably comprises the same chemical constituents as the No. 2 bath but in difierent proportions sincethe No. 3 bath is operated at a higher temperature and therefore will remain liquid with the proportions of the materials different from those in the No. 2 bath.

Preferably the proportions of materials in the No. 3 bath, when this bath is first started, are as follows: potassium chloride 9%-20%; sodium chloride 12%-25%; sodium fluoride A .%--3%; and zinc chloride %80%. The preferred proportions at the start of this No. 3 bath being potassium chloride 17%; sodium chloride 22%; sodium fluoride 1%; and zinc chloride This No. 3 bath remains quite liquid at the temperatures .0 from 700 F. to 1100" F. at which temperatures I this bath is intended to be operated.

It will be noted that as the work progresses from the No. 1 bath, which is high temperature, to the N0. 2 bath, which is low temperature, to 45 the No. 3 bath, which is intermediate temperature, and each time the work is carried from one bath to another, a part of the preceding bath is carried over to the succeeding bath. In operaoccur between the No. 2 bath and the No. 3 bath whereby the percentage of the ingredients in the No. 3 bath will be very close to the percentages in the No. 2 bath. The percentage of zinc chloride in the No. 3 bath should be substantially 55 lower than the ratio of zinc chloride to the other constituents in the No. 2 bath. Where the No. 2 bath has been substantially changed, asto ratio of the zinc chloride to the other constituents, due

to the bringing over of the No. 1 bath to the No. 2 00 bath, a condition may arise where it is desirable to bail out a portion of the No. 2 bath and replace the bailed out material with zinc chloride, thereby again restoring the balance which is desirable in this bath. It will be noted however, that where 5 the proper balance is maintained in the low temperature No. 2 bath, the balance in bath No. 3

is automatically maintained because the correct ratio of materials is coming over with the work when it is moved from low temperature bath No. 2, to the intermediate temperature bath No. 3.

It will be observed that the constituents of the present baths include no chemicals containing oxygen molecules and that when the work is submerged in the high temperature bath No. 1 it is covered with bath salts thereby excluding atmosphere from the work from that time on to the final cooling down to room temperature. Therefore the work is protected against atmospheric oxygen and as a cons quence there is no oxidizing of the surface of the work and therefore the work leaves the baths with what is known in the art as a "bright finish. It therefore will be observed that no oxidation from the atmosphere is possible because the work is covered by non-oxidizing salts and no oxidizing is possible from the salts themselves. Therefore the work leaves the baths in a bright clean condition.

The final quenching is usually done in cold water which so quickly reduces the temperature of the steel parts below the oxidizing point that oxidizing does not take place in the water, since the parts are relatively cold before all of the neutral chemicals have been dissolvedfrom their surfaces.

The higher temperature No. 1 bath may be used with either the low temperature No. 2 bath or the intermediate temperature No. 3 bath, thus making the operation a two-bath operation instead of pearlitic state with a bright, clean finish which may be continuously limed previously to the wire being drawn.

drawing of steel parts that have been quenched in water or oil.

When such work leaves the No. 2 bath at 550 F. to 700 F., or leaves the No. 3 bath at 700 F. to 1100 F. and is quenched in water or oil, the work will be bright finish. Therefore, these baths, No. 2 or No. 3, while very useful as a bath in a series of balanced baths, are also useful as a single bath.

I claim:

1. In a series of bright finish salt baths a No. 1 bath adapted to operate at temperatures of 1350 F. to 1700 F. and comprising potassium chloride 39% to 50%, sodium chloride 40% to 60%, and sodium fluoride 1% to 10%; a N0. 2 bath adapted to operate at temperatures from 500 F. to 700 F. and comprising potassium chloride 8% to 18%, sodium chloride 10% to 18%, sodium fluoride to 4%, and zinc chloride 60% to and a No. 3 bath adapted to operate at temperatures from 700 F. to 1100 F. and comprising potassium chloride 9% to 20%, sodium chloride 12 to 25%, sodium fluoride /z'% to 3%, and zinc chloride 50% to 80%, the work being treated being transferred from the No. 1 bath to the No. 2 bath, and from the No. 2 bath to the No.. 3 bath without being cleaned, so that a portion of the No. 1 bath is being continually carried into the No. 2 bath and a portion of the No. 2 bath is being continually carried into the No. 3 bath.

2. A series of bright finish liquid salt baths comprising a No. 1 bath of potassium chloride 42%, sodium chloride 55%, and sodium fluoride 3%; a No. 2 bath of potassium chloride 12%. sodium chloride 14%, sodium fluoride /2%, and zinc chloride B /2%; and a No. 3 bath of potassium chloride 17%, sodium chloride 22%. sodium fluoride 1%, and zinc chloride 60%.

3. A'series of bright finish salt baths comprising materials adapted to be carried from one bath to another by the work, and whereby the materials of a preceding bath are harmonious with the materials of a succeeding bath in such manner that the materials of the preceding bath do not contaminate the materials of the succeedin bath; and wherein the bath No. 1 consisting essentially of alkali metal chloride and alkali metal fluoride, the latter ranging from approximately to is adapted to operate at temperatures from 1350 F. to 1700" F., bath No. 2 operating at from 500 F. to 700 F., and bath No 3 operating at from 700 F. to 1100 F., and wherein baths No. 2 and No. 3 consist essentially of alkali metal chloride and alkali metal fluoride, the latter ranging from approximately 1% to 10% have zinc chloride as an ingredient 60% to 80% of the composition of bath No. 2 and 50% to 80% of bath No. 3. I

4. A bright finish quenching bath comprisin potassium chloride 9% to sodium chloride 12% to sodium fluoride /2% to .3, and zinc chloride to 80%.

5'. A bright finish quenching ,bath consisting essentially of alkali metal chlorides, alkali metal fluoride ranging from approximately 1% to 10% and zinc chloride ranging from 50% to 80% of the bath.

6. A series of bright finish salt baths comprising a high temperature bath having potassium chloride 39% to 50%, sodium chloride 40% to sodium fluoride 1% to 10%, and a succeeding quenching bath comprising potassium chloride 9% to'20%, sodium chloride 12% to 25%,

sodium fluoride /2% to 3%, and zinc chloride 7. A series of brightflnish salt baths comprising a high temperature bath of alkali metal chlorides, plus an alkali metal fluoride 1% to 10%, and a quenching bath consisting essentially of alkali metal chlorides, an alkali metal fluoride ranging from approximately 1% to 10%, and zinc chloride ranging from 50% to 80% of th quench ing bath.

ARTEMAS F. HOLDEN.