US2369213A - Method of degasifying and decarburizing molten metal baths, and improved agent therefor - Google Patents

Description

Patented Feb. 13, 1945 METHOD OF DEGASIFYING AND DECAR- BURIZING MOLTEN METAL BATHS, AND IIVIPROVED AGENT THEREFOR Hugh S. Cooper,

Cleveland, hio,assignor of onehalf to Frank H. Wilson, Cleveland, Ohio No Drawing. Application March 28, 1944, Serial No. 528,474

6 Claims.

This invention relates to clean-up agents for use in the degasification and decarburization of molten metal baths and more particularly to clean-up agents for use in the degasification and decarburization of molten metal baths consisting predominantly of one of the base metals aluminum and copper,

Heretofore in the art it has been recognized that molten metal baths have a relatively high solubility for gases such as hydrogen, nitrogen, oxygen, carbon monoxide and for carbon and metal carbides which solubility varies with respect to the particular metallic constituents of the bath and that on solidification a considerable portion of the gas in solution in the molten metal is expelled from the metal to the detriment of the soundness of the metal while another considerable portion of the gas reacts with the alloyed constituents of the metal removing the same as oxide and nitride compounds thereby detrimentally efic'ecting the physical properties of the metal. In many alloys the presence of any considerable amount of metal carbide compounds is undesirable.

It has heretofore been proposed to eliminate these dissolved gases and carbon by the use of clean-up agents, some of which act mechanically While others act chemically. It is known, however, that while it is possible by means of an agent to lower the gas and carbon solubility of the molten metal to a relatively small percentage, even this small percentage detrimentally effects the physical properties of the metal upon solidification unless there is provided an alloyed-constituent in such amounts as will be eifective to combine with the residual gases and carbon to form thermally stable compounds therewith that will not subsequently interfere with the normally expected physical properties of the metal during subsequent heat-treating and mechanical deformation operations. The major difficulties involved in the practice of this method resides in lowering the gas and carbon content of the molten metal to a required low residual amount and introducing into the molten bath the precise amount of an alloy constituent capable of chemically reacting with the low residual amount of gas and carbon to fix the same as thermally stable compounds upon solidification of the metal.

The object of this invention is to provide such a gas and carbon clean-up agent.

Another object is to facilitate and improve the degasification and decarburizing agent for use in molten metal baths and particularly in molten metal baths consisting principally of one of the metals aluminum and copper.

A further object is to provide relatively low boron content degasified and decarburized metal and alloys and more particularly low boron content degasified and decarburized aluminum alloys,

Other objects will be apparent as the invention is more fully hereinafter disclosed.

In accordance with these objects I have discovered that by the use of a mixture of gases consisting of boron trichloride and chlorine containing 5 to 20% chlorine, balance boron trichloride, the degasification and decarburization of molten metal baths is greatly facilitated and is controllable to provide the desired excess of boron in the molten metal bath necessary to chemically react with the residual gases and carbon remaining in the molten metal upon solidification.

Heretofore, chlorine and boron trichloride have each individually been proposed as degasification and decarburizing agents. Neither one, acting individually however, are entirely satisfactory. When employed in combination, I have found that results greatly superior to that obtained with either agent when used alone are obtained. The reason for the improved results is believed due to the fact that chlorine when acting alone is effective as a decarburizing agent and degasifying agent only insofar as to those gases and carbon that are in excess of the normal solubility of the molten metal at the temperature of heating; whereas boron trichloride is essentially a chemical reagent that undergoes thermal dissociation with liberation of nascent chlorine and nascent boron, the nascent chlorine acting more effectively than molecular chlorine as a decarburizing agent but not acting as efiectively as molecular chlorine in displacing and removing gases in excess of the normal solubility of the metal or in the mechanical scrubbing of the metal to carry occluded and undissolved non-metallic impurities to the surface of the metal, while the nascent boron entering into solution in the molten metal lowers the solubility of the metal for oxygen, nitrogen and carbon. The difliculty involved in the use of boron trichloride is to limit the amount of boron entering into the molten metal to that precise amount required on solidification to combine, with the residual oxygen, nitrogen, carbon, etc. present in the molten bath in equilibrium with the boron at the melting point but precipitated out on solidification of the alloy as boron compounds, Any excess of boron over this precise amount is usually detrimental to the hardness and workability of the alloy as well as to the tensile strength, elongation and other physical properties of the alloy.

In accordance with the present invention I have discovered that when chlorine and boron trichloride in combination are employed as a degasifying and decarburizing agent with the chlorine limited to about to the free chlorine functions to limit the amount of boron entering into solution in the molten metal bath, and that by varying the percentage of chlorine within the range of 5 to the final boron content of the molten metal bath may be widely varied and limited to that found desirable with any given alloy.

In addition I have found that by the use of a liquid mixture of the gases BClz and chlorine a highly beneficial economical result may be obtained. This beneficial result mainly is that at normal atmospheric temperatures (-30 C.) the pressure of liquid B01: in a liquid gas storage cylinder approximates 8 pounds per square inch which is insufiicient to force the gas out of the cylinder into a molten metal bath against any great hydrostatic head of pressure. This limits the depth to which BCl: gas normally may be introduced into a molten metal bath and tends to increase the time during which the BC13 must be introduced and requires the use of mechanical stirring to bring the bottom metal in contact with the gas.

I have found that as the amount of liquid chlorine in admixture with liquid BCls increases, the pressure of the gas in the cylinder increases to the extent that at 10% chlorine the pressure at 25 to C. is about double that of pure BCIJ while at 20% chlorine the pressure is about 3 times that of pure BCla.

Accordingly, by the use of 10 to 20% Cl in combination with B01: in the liquid phase I am able to facilitate the introduction of the B013 into the molten metal bath to a depth greater than heretofore obtainable and thereby greatly facilitate the degasification and decarburization reaction irrespective of the advantages obtained through the use of the two agents in combination.

In the production of the agent of the present invention I prefer to mix BCla and Cl together in the gaseous phase and to liquify the mixture before placing the mixture in the storage pressure cylinder. The particular manner in which liquidation of the gas mixture is obtained forms no part of the present invention.

The specific manner in which the gasified mixture of BCls and C12 is introduced into a molten metal bath also forms no part of the present invention. Inasmuch as one skilled in the art will recognize that the amount of the gas mixture of the present invention that may be employed may vary Widely depending upon the specific metal bath and the amount of gaseous and carbon impurities present therein, no specific example of the practice of the present invention appears essential to the proper understanding of the same by one skilled in the art.

As an example, however. the application of the present invention to the cleaning-up of gases and carbon contained in secondary aluminum alloys may be described. Secondary aluminum alloys consist primarily of scrap aluminum which has been remelted. Such alloys usually have a large gas content and a high carbon content which must be removed therefrom before casting and solidification. Heretofore in the art it has been known that chlorine and chlorine liberatin agents are effective for this general purpose and both chlorine and boron trichloride have been heretofore proposed for separate use in this manner in these alloys. It is known, however, that chlorine when used alone is ineffective as to those gases and carbon retained in solution in the molten metal after the treatment and that boron trichloride when used in amounts necessary to obtain equivalent scouring results to that obtained with gaseous chlorine as to the undissolved entrapped impurities normally introduces an excessive amount of boron over that needed to combine with the gases and carbon precipitated from solution on solidification, this excess being detrimental to the physical properties of the metal.

In accordance with the present invention the use of a gas and carbon clean-up agent consisting of a mixture of the two gases B01: and Cl and containing Cl 10 to 20% is effective to gain the full advantages of each agent with none of the attendant disadvantages of either.

This gas mixture is passed into the molten metal identically as heretofore practiced with the advantage that as a result of the higher cylinder pressure available the BCls may be introduced at a much lower level than heretofore possible. Otherwise the degasification and decarburizing reaction proceeds substantially as heretofore experienced with the end-point being reached within a time interval considerably shorter than with either agent when used alone. The endpoint, namely, the formation of a substantially pure white powdery dross on the surface of the melt, is substantially identical to that heretofore obtained. The time at which this end-point is reached depends entirely on the amount of gas and carbon present in the melt and varies widely from melt to melt as between seemingly identical composition metal.

Aluminum alloys treated with the agent of the present invention consistently show the desired fine grain size, low free boron content within the range .03.05% and the non-porous structure of good metal.

As one typical example of the practice of the present invention on a specific alloy the following may be given.

A molten metal bath of an aluminum alloy known in the trade as the T6 alloy containing Cu 4-5%; Si 1.5% max; Zn 03% max; Fe 1.0% max.; Mn 30% max.; Mg. .03% max; and Ti 20% max., balance Al, heated to temperatures within the range 1200-1300 C., and treated with a mixture of BC13 and chlorine containing about 12% Cl until the typical end-point is obtained, was found on solidification to contain between .03 to 05% metallic boron.

The standard specifications for this alloy call for a tensile strength of 32,000 pounds per square inch at 3% elongation in the sand cast alloy. Metal processed as above consistently showed a tensile strength of 43,000 to 44,000 pounds per square inch at an elongation of 5.5%. Repeated tests indicate that the boron content in the same alloy treated with boron trichloride only varies widely and is generally considerably higher than that usually resulting from the use of the BCls-Cl (10-12%) mixture of the present invention, and in fact may rise as high as 50% with resultant detrimental effect on the tensile strength and elongation of the sand cast alloy. On the other hand repeated tests with the gas mixture of the present invention indicate that rather wide variations in time of application of the agent to the molten metal result in very small variations in the boron content of the alloy and that the equilibrium percentage of boron in the alloy at 1200-1300 C. using the to 12% chlorine balance BCl: mixture approximates .03 to .05%. In addition all of the advantages of fine grain structure heretofore obtainable only with BCla in this type alloy are retained.

Substantially the same beneficial result has been obtained in all of the more common aluminum alloys, in most of which the equilibrium percentage of boron remaining in the molten metal appears to consistently provide on solidification an excess of boron within the range .03 to .05%.

As further examples, an alloy known in the trade as the 355'16 alloy which contains Cu 1.3%; Si 5.0%; and Mg .50%, when treated as above described consistently shows a tensile strength of 40,000-42,000 pounds per square inch with an elongation of 2.5% as contrasted to the established specification of 32,000 T. S. at 2% elongation.

An alloy known under the trade name 356'I'6 which generally shows the following analysis:

Per cent Cu .20 Si 6.5-7.5 Zn .03 max. Fe .60 max. Mn .10 max. Mg .20-.40 Ti .20 max.

.03 to .05% may be given. The above examples, however, are typical of the results obtainable.

From the above disclosure it is believed apparent that the present invention may be widely varied without essential departure therefrom and all such modifications are contemplated as may fall within the scope of the following claims.

What I claim is:

1. A degasifying and decarburizing agent for molten metal baths, said agent consisting of a mixture of the gases BCla and chlorine, the chlorine content of the mixture being within the range 5 to 20%.

2. A degasifying and decarburizing agent for molten metal baths, said agent consisting of a mixture of the gases BCla and chlorine, the chlorine content of the mixture being within the range 10 to 20%.

3. A degasifying and decarburizing agent for molten metal baths, said agent consisting of about 12% chlorine and the balance B013.

4. The method of introducing BCls into a molten metal bath which comprises forming a mixture consistin of 5 to 20% chlorine, balance BCls, in the liquid phase, enclosing the said liquid in a pressure cylinder provided with a pressure release valve, and introducing the mixture of gases passing through the said valve at atmospheric temperatures into the said molten metal bath.

5. The method of treating molten aluminum and aluminum base alloys to degasify and decarburize the same which comprises introducing a mixture of gases comprising 5 to 20% chlorine, 95 to B013 into the bath, limiting the amount of said mixture of gases to that required to obtain a substantially pure white powdery dross on the surface of the molten metal.

6. The method of claim 5, wherein the temperature of the metal is maintained at 1200- 1300 C. and the Cl content of the gas is maintained at about 10 to 12% thereby to limit the free boron content of the metal to the range .03 to .05% in the solid state.

HUGH S. COOPER.