US2328932A - Salt bath - Google Patents

Description

Patented s i (1943 ,1.

UNITED, STATES PATENT OFFICE a 7 i Y 2,328,982 i SALT BATH Johann S. Streicher, Newark, N. J., assignor to The American Platinum Works, Newark, N. J a corporation of New Jersey N Drawing. Application November 25, 1941, a

, Serial No. 420,341

4 I 4 Claims. -(01. 148-131) v e Thi invention relates to salt bath iurnacesand, salt bath for salt bath furnaces. I Likewise, such 7 is directed particularly to an improvement in salt mas has a low surface tension. "The liquidity ofmetals therein. in the boric acid content.

Salt bath furnaces are used primarily forthe The following table illustrates the melting annealing of metals at elevated temperatures. points of salt'baths' produced from mixtures of Salt baths as heretofore known usually comprise boric acid and potassium fluoride:

mixtures embodying one or more of such ingredi- V baths for salt bath furnaces and the annealing and fluidity gradually decrease with an increase ents' as sodium chloride, potassium-chloride, soda, 1 Melting point ash, potash, sodium nitrates, potassium nitrates, r g g {ggg v cyanides, etc. One well known salt bath comi v t;

prises, for instance, a mixture of 50% potassium i chloride'and 50% sodium carbonate. Such salt 83.3' 16.7 1,425 774 baths produce, however, an alkaline reaction and 3% x23 tend to corrode metals to be annealed. and tol a 6617 3613 11m 130 produce surface conditions on such metals which -3 g5:

are detrimental to'subsequent soldering or weld-' 651 4414 11285 tot ing operations. Salt bathscmbodying cyanides $3 g produce highly poisonous fumes-and salt baths 4716 5214 11200 640 embodying nitrates and'nitrites are highly oxi- 2 g: gg-g g dizingland have only restricted use. 4111 talc 111m 010 It is 'oneobject'of my invention to provide a -9 salt bath for salt'bath furnaces which shall not 1 r corrode metals treated-therein,-which shall not 11- the potassium fluoride content falls below' develop poisonous fumes in use and which shall 25 40%.and the boric acidcontent exceeds 80%, I 7

not be of oxidizing nature. It' is another object the resulting mass is highly amorphousv and via- 01 my invention to. anneal metals in such salt eous on melting and does not hav a definite bath. melting point. Likewise, salt baths produced I have found that an excellent salt bath for from mixtures of 17% to less than 60% boric salt bath furnaces is produced .by an aqueous acid, and 83% to more than 40%-potassium flugmixture of boric acid and potassium fluoride in ride, solidify to a salt cake with a definite crystal the P p r io of 7% to 6 boric acid and structure, whereas masses produced by other 40% to 83% potassium fluoride. A reaction ocmixtures of boric acid and potassium fluoride curs in the'mlxture which isexothermic, evolving solidify as a;transparent glassy mass. considerable heat, and produces ahydrated pol In the salt bath' according to my invention 11 acid c p u P y tetraboric acid there is produced aweak acid reaction, rather and Pentab0r10'a0id, nmew as a 1- than an alkaline reaction as heretofore, and no Stitu n th h p lmd 8180 s to corrosion of metals takes placeduring' the ancontain potassium fluorideas a constituent therehealing.- The liquid salt bath according to my of. When heat d the rea ted m xtur 'qul y 40 invention also has'an excellent cleaning eifect 1 y excess 01 Water, ut e w tetwhi h .1 upon the material annealed therein. Harmful contained in the boric acid compound as a constitalkaline films ar entirely eliminated and no uent th re f is released y slowly and gradually poisonous fumes are developed. In-view of the and only in part. The reaction takes place spOnhigh liquidity and fluidity of my .salt bath metaneously at ro m emp r 1;! e mixture is 4 chanical losses in using such bath are kept at a subjected to a thorough grinding procedure. The minimum. Oxidation or other contamination of reaction also takes place without the-necessity of the objectbeing' annealed is prevented due to'the thorough grinding when the mixture is heated to nature of my salt bath composition.

a temperature of'about 116 C. to 120 C. and Particularly advantageous for the provision of more, whereafter the reacted mixture is cooled. salt baths are mixtures of about 41% to 58% ,The salt bath mixture has a definite melting boric acid and about 59% to 42% potassium fluopoint ranging, in accordance With its composition, ride, in particular mixtures of about'41% to 52% from about 600 C. to 800 C. The molten mass boric acid and about 59% to 48% potassium fluohas a. high liquidity and fluidity, a factor of great ride, for instance a mixture of about 53% potas- 1 importance in producing a successful operative slum fluoride and about 47% boric acid which .proper amount of water.

has a particularly satisfactory combination of low surface tension, low melting point and high cleaning capacity.

My salt baths can be utilized in the annealing of ferrous'as well as non-ferrous metals and metal compositions and is suitable for the anhealing of brass, copper, aluminum, steeLstainof sheet, wire or any other suitable shape. The I potassium fluoride, in the specified proportions,

salt bath furnace, containing the molten salt bath according to this invention, may be operated electrically or may be fuel fired.

The term metal as herein used shall include not only metals as such but also alloys and other metal compositions or combinations, and the term annealing shall also include any other form of heattreatment.

I'he hydrated complex polyboric acid compound salt bath is best produced by first mixing together, preferably in a mortar or grinding machine, the potassium fluoride and boric acid in the specified proportions and then' adding the Thus about 35 to 45 cc. watermay be added to 100 grains mixture according to my invention of potassium fluoride and boric acid to result in the desired hydration. Since boric acid does not readily wet water,

. there may be added to the vwater a trace of wethot water and-then adding boric acid, the hydrated complex polyboric acid compound crystal- The mass may also 800 grams commercial boric acid, 100 grams dehydrated commercial potassium fluoride, and

750 cc. water containing a trace of Lamepon A as wetting agent, is finely ground until the mass is transformed into a thick gray liquid which on standing turns itselfby crystallization into a white mass which is then thoroughly ground and thereby transformed into a smooth paste having a melting point of 704 C.

The salt bath for salt bath furnaces, according' to my invention, may also be produced by fusing a mechanical mixture of boric acid and without the addition of water, as technical boric acid and potassium fluoride contain a suflicient' lizing after cooling. For instance, a mixture of 2. A salt bath for salt bath furnaces, comprising the reaction product of a mixture of 41% to 52% boric acid and 59% to 48% potassium 53% potassium fluoride and about 47% boric 1 acid.

4. The method of annealing metal at a temperaturein excess of 600 C., comprising heat treating said metal in a molten salt bath conslsting of the reaction product of a mixture of 17 to 60%boric acid and to 83% potassium J OHANN S. S'I'REICHER. I