US2201111A - Heat treatment of metals - Google Patents

Description

Patented May 14, 1940 UNITED STATES PATENT OFFICE 2,201,111 HEAT TREATRIENT 0F METALS Josef Martin Michel, Bitterfeld, Germany, assignor to I. G. Farbenindustrie Aktlengesellschaft, Frankfort-on-the-Main, Germany No Drawing. Application August 30, 1939, Serial No. 292,607. In Germany September 2, 1938 5 Claims.

This invention relates to improvements in the heat treatment of metals, especially of aluminium base alloys.

When improving aluminium base alloys byheating in a salt bath followed by quenching in water, it hasvbeen observed that the workpieces,

particularly sheets, treated in this manner ex-.

hibit local stains or discolorations. In order to avoid such staining or discoloration it has been suggested to add chromates to the bathof fused saltpetre employed for the heating, in the assumption'that the staining is due to a corrosion of the workpieces caused by the decomposition products of the saltpetre which are produced in the course of use of the heating bath over a long period. Although this leads to a substantial lessening of the staining, the latter to some extent still persists.

It has now been found that the staining is in part also due to an action of the quenching bath upon the metal. When workpieces, forexample sheets, with a completely clean surface are quenched in pure water, no staining takes place; in practical operation, however, appreciable quantitles of the fused salts constituting the bath in whichthe metal is heated prior to quenching adhere to the sheets and become dissolved in the quenching water when the sheets are placed therein. As successive batches of work-pieces are heat-treated'and quenched, the quenching water thus takes up progressively increasing quantities of the latter in course of time and becomes correspondingly more alkaline. It has been observed that this alkalization of the quenching 'water occurs not only when the fused salts employed in the heating bath are itself weakly alkaline, but also when any" alkalinity of the fused salts em ployed as the heating bath has been compensated by the addition, to the heating bath, of salts having an acid reaction, for example dichromates, until the reaction was neutral. This latter phenomenon is possibly due to a decomposition, occurring on contact with water vapor, of the saltpetre (which is generally the main constituent of the heating bath), accompanied by the formation of oxides of nitrogen and freealkali. The degree of staining of the metal has been found to be directly dependent on the alkalinity of the quenching water and to increase with increasing alkalinity of the latter. o

Based on the foregoing observations, the invention provides a process for the heat treatment of metals, especially aluminium base alloys, by '5 heating in a fused salt bath containing a substantial proportion of saltpetre and subsequently quenching in an aqueous medium, in which the pH-value of the quenching medium is preferably by the addition of suitable quantities of acids kept constantly numerically below 7 and preferably at about 6.

Acids having such, anions as are also present, in the form of their salts, in the fused salt bath employed for the heating are preferably used for this purpose. Thus, for example, when using heating baths consisting of saltpetre with an addition of chromates, the acidification of the quenching bath can be effected with the aid of chromic acid, while it is found convenient to regulate the addition of acid in such a manner that the pH-va lue of the quenching water is numerically closely below 7, i. e. in such a manner that a certain proportion of monochromate remains present in the quenching water together with the dichromate.

Since the amount of salts dissolving in the quenching water in the course of time is not inconsiderable and, when the salts used in the heating bath are expensive,'represents a source of appreciable loss, it may sometimes be advantageous to recover the salts from the quenching water. To this end, in order to ensure a sumcient concentration of these salts in the quenching bath, the volume of the latter is kept as constant as possible, the evaporation losses merely being made up with fresh water as required; the resulting progressive enrichment of the quenching water with dissolved, salts has no adverse effect on the quenching efiiciency thereof. By suitably selecting and proportioning the acids to be added to the quenching water for the purpose of adjusting the pH-value thereof in accordance with the invention, it is then possible to make the concentration ratio of the salts dissolved in the quenching bath approximately correspond to that prevailing inthe salt mixture to be supplied to the heating bath; to this end it is naturally necessary that the latter mixture should also have a neutral or weakly acid reaction. After having attained a sufficient concentration, the quenching solution is then evaporated to dryness, whereby a saltmixture is recovered which, by virtue of its composition, is directly utilizable for making up the heating baths.

Ezampla -For soaking-sheets of an aluminium alloy at a temperature of 500 C., a fused salt bath is used which is composed of:

r Per cent NaNOa 65 KNO: 30 moi The quenching'bath, which has a capacity oi. 20 cubic metres and is initially charged only with fresh water, contains, after treatment of 18,000

square meters of sheets Grams/litre Na+ 61.6

K+ 34.8 NOa- 201.2 Nor 6.7 CrOr- 10.7

and has a pH-value of 8.2. In order to adjust the pH-value to a value in the proximity of 6.75 kgs. of nitric acid (40 per cent HNOa) or kgs. of chromatic anhydride are added to-the bath, whereby at the same time the concentration ratio of the salts contained in the quenching water is made to correspond approximately with the relative portions of the salts constituting the soaking bath, so that after evaporation of the water content of the quenching bath, the salt recovered (6300 kgs.) can be used directly for making up 1. An improvement in the heat treatment of metals, especially of aluminium base alloys, by a process involving heating successive batches of metal in a fused salt bath substantially consisting of saltpetre and subsequently quenching such batches in an aqueous medium, which improvement consists in constantly maintaining the pH- value of the quenching medium numerically below about 7. v l

2. An improvement in the heat treatment of metals, especially of aluminium base alloys, by a process involving heating successive batches of metal in a fused salt bath substantially consisting of saltpetre and subsequently quenching such batches in an aqueous medium, which improvement consists in constantly maintaining the-pH- value of the quenching medium numerically at about 7.

,3. An improvement in the heat treatment of metals, especially of aluminium base alloys, by a process involving heating successive batches of metal in a fused salt bath substantially consisting of saltpetre and subsequently quenching such batches in an aqueous medium, which improvement consists in constantly maintaining the pH- value of the quenching medium numerically below 7 by the addition of acids. v

4. An improvement in the heat treatment of metals, especially of aluminium base alloys, by a process involving heating successive batches of metal in a fused salt bath substantially consisting of saltpetre and subsequently quenching such batches in an aqueous medium, which improvement consists in constantly maintaining the pH- value of the quenching medium numerically below about '7 by the addition of acids, having such anions as are alsopresent, in the form of their salts in the fused heating bath, recovering the salts dissolved in the quenching bath from time to time, and supplying such recovered salts to.

value of the quenching medium numerically below about. 7 by the addition of acids, having such anions as are also present, in the form of their salts in the'fused heating bath, recovering the salts dissolved insthe quenching bath from time to time, and supplying such recovered salts to the heating bath, thev acids to be added to the quenching medium being so selected and proportioned that the concentration "ratio of the salts dissolved in the quenching medium is made to correspond approximately to that prevailing in the salt, mixture constituting the heating bath.

JOSEF MARTIN MICHEL. 6