DE924453C - Salt bath to prevent decarburization during the heat treatment of steel - Google Patents

Description

Salt bath to prevent decarburization during heat treatment of Steel violin stand of the invention is the use of a salt bath for prevention de.koh.lung in the heat treatment of steel. It is known to work pieces too harden by bringing them to the required hardening temperature in such baths heated and then in a suitable manner, if necessary by quenching., cooled.

To avoid the decarbonization of the workpieces that would -ren füh to poor hardness due to the formation of a soft skin, is it necessary to add the heating baths are substances that prevent the decarbonization.

Cyanides of alkalis and alkaline earths are used as such substances at hardening temperatures up to about 95o ° C as well as borax, acidic oxides, for example des Titanium and silicon, with hardening temperatures of up to 130o ° C and above. While it At temperatures of up to 95o ° C, decarburization largely succeeds by adding cyanides to prevent ice is still associated with difficulties and uncertainties, the decarburization at higher. Temperatures by the means mentioned, especially boric acid or silica, to prevent. Too little additions result. Decarburization, too high Corrosion, the annoying formation of tough slag almost always occurs.

Surprisingly, it has now been found that the Entkoh Jung with certainty can be prevented without the above. Disadvantages occur when, that Salt bath contains semi-volatile halides of such metals, their oxides in divalent Form a shape with iron oxide spinels. can. Very good results have been obtained with fluorides the divalent oxide form for spinel formation prone metals achieved; According to a preferred. An embodiment of the method is magnesium fluoride used. The contents of the salt baths to be added according to the invention. Metal halides can: depending on the composition of the molten salt bath to a large extent. Limits, e.g. B: vary between i and 50%, based on the weight of the molten salt. in the in general, contents of up to 10 0/0 are sufficient.

The technical effect achieved by using a salt bath according to the present invention is illustrated by the following examples: A 0.05 mm thick plate made of steel with a carbon content of 430 / a was placed in a conventional bath at 1300 ° C. for 5 minutes , consisting of a melt. Barium chloride, which contained 1 0/0 of an acidic barium silicate to prevent decarburization. After 5 minutes the carbon content of the sheet had sunk to 0.7%. Such decarburization can no longer be described as acceptable in all cases, but based on experience it must always be warned that it will occur in the specified strength . For comparison, an identical sheet was immersed in a bath for 5 minutes, which. consisted of barium chloride with the addition of 4 0/0 Magnesiu.mfluorid according to the invention. After 5 minutes the carbon content of the sheet was still 1.3%. The decarburization was completely prevented.

That sodium fluoride, the use of which is known, the decarburization not prevented, results from the following experiment: A sheet, as above indicated, was brought into a melt made of barium chloride with an additive consisted of 5.6% nabrium fluoride. To. 5 minutes pointed. the sheet metal has a carbon content from 0.8% up. It has thus been proven that the use of sodium fluoride, which is known per se the decarburization, as opposed to using one. Salt bath according to the invention, not prevented.

As experiments by the inventor have shown, decarburization in Chlorbarium baths are caused. through from the crucible or from the electrodes originating iron oxide, which by giving off oxygen to the steel in iron oxide passes over and decarburizes the steel. Barium oxide, too, by hydrolysis of the barium chloride through moisture and crystal water during the melting may play a detrimental role in decarburization. The mode of action of the halides of the bi-, vertigenous metals added according to the invention, in particular Magnesia fluoride can be traced back to the fact that initially by ion exchange from the formed, decarburizing barium oxide and magnesium fluoride magnesium oxide and barium fluoride arise :. This removes the harmful barium oxide, while magnesium oxide does not have an initial carbonic effect. From the educated Magnesium oxide and the existing iron oxide can now form a spinel which in the bathroom on the floor. of the melting vessel settles. To; this finding the iron is bound in a trivalent form in the spinel Deliver oxygen to the steel and decarbonize it.

In contrast to the use of boric acid, acidic silicates and the like Substances are formed during this reaction, apart from the one that is excreted Spinel, no viscous or difficult to melt compounds. The bathroom. Stays always thin and does not form a blanket on the surface of the ball, which experience has shown the temperature measurement, which in these baths is preferably done optically, with special needs.

The effect of magnesium fluoride is particularly noticeable when the bath is operated for a long time. The table reproduced below shows the decarburization effect of a bath made of pure chlorine barium and one with 4% magnesium fluoride on the one hand and the equivalent amount of. On the other hand, 5.6% sodium fluoride. Service life of the bath hours 0 I 1 1 1 6 1 24 1 32 1 40 - I 4 8 1 56 64 1 72 Ball composition C-content of the test sheets in 0/0 - BaC12 ..................... 1.3 o, 85 o, 84 0.87 0.57 0.87 0.74 0.49 067 0.23 BaC12 + 4% M9F2. . . . . . . . 1.3 1,20 1, 32 1.29 1.21 L33 i, 22 1,20 1, 23 1.21 BaC12 -j- 5.6% NaF ........ 1.3 1.00 i, io 0.92 0.87 0.47 0.36 o; 39 0.14 o, 15 The table shows stich. the clear superiority of the bathroom. with an addition of. Magnesium fluoride according to the invention.

The bath was operated for 7-2 hours and, as in the practical, Usual operation, for 8 hours 1 / 10th of the: Ball content by fresh mixture of the same Composition replaced.

Instead of magnesium fluoride can also be used, albeit with not so good result, the halides of other divalent ones that tend to form spinel Metals can be used, but care must be taken that they are in the salt bath do not decompose too quickly and with the; Working temperatures essentially not are volatile. So has z. B. also the zinc fluoride as: proven suitable ..

The use of a salt bath according to the invention is not only done with advantage at high temperatures, e.g. B: from 120o to. 130o ° C, rather this also performs well at lower hardening temperatures below 100 ° C. So were z. B. with a bath consisting of 50 parts of barium chloride, 4 parts of potassium chloride and io parts magnesium fluoride, achieved very good results.

Claims (3)

PATENT CLAIMS: i. The use of a salt bath containing low-volatility halides of those metals, the oxides of which can form spinels in divalent form with iron oxide, in such an amount that a decarburizing effect of the bath on steel is prevented, as a heating bath in the heat treatment of steel. 2. The use of a salt bath according to claim i, the Halo @ genide in quantities. from i to 5o, / o contains, for the purpose according to claim i. 3. The use of a salt bath according to claim i or 2, which contains magnesium fluoride in amounts of i to 5o%, for the purpose according to claim i. Referred publications: German patent specification No. 622 8o2; Chem. Zentralblatt No. 26 / 1942I, 3o. 6.1942, A. F. Ho 1 d ei n and H. S o 1 a k i a u, heat treatment bath; U.S. Patents No. 2,400 5 i i, 2o46822, 2,002 180; Van Nostraud "Scientific Encyclopedia", p. 1354; P, ö m p p, Chemie L.e! Xicon, article: Spinell.