US1979214A

US1979214A - Case hardening

Info

Publication number: US1979214A
Application number: US638175A
Authority: US
Inventors: Harding Earle Atherton; Holt Donald Aubrey
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1932-10-17
Filing date: 1932-10-17
Publication date: 1934-10-30
Anticipated expiration: 1951-10-30

Description

Patented f0... 30, 1934;

UNITED STATES PATENT OFFICE CASE HARDENING No Drawing.

Application October 17, 1932, Serial No. 638,175

12 Claims. (01. 148-15) This invention relates to the case-hardening of various metal articles and more particularly to the case-hardening of iron or steel articles by treatment in a bath of fused salts containing cyanogen compounds.

A commonmethod of case-hardening various metal articles consists in heating them in' a bath of fused salts containing cyanides, for instance sodium cyanide, or other cyanogen compounds. In one case-hardening bath of this sort large amounts of calcium salt, for instance calcium chloride, are employed and case-hardening properties are imparted to the bath by adding thereto 1% or less of alkali metal cyanide'or other cyanogen compound. These baths are subject to a number of disadvantages, chief among which are the corrosive action of the calcium salt, precipitation of calcium oxide, and the relatively short life of the bath necessitating frequent addition of the cyanogen compound. Furthermore, if substantially more than 1% of cyanogen compound is added to this bath, it tends to foam and boil with the formation of a heavy scum to such extent that normal operation becomes impossible. This abnormal condition persists until the'cyanogen compound content has decreased to around 1%. In our co-pending application, Serial Number 559,140, we have described a calcium-free case-hardening bath containing considerable amounts of barium salt and upwards of of alkali metal cyanide since lo'wer amounts of cy anide do not give rapid enough penetration in the presence of barium salts which are relatively more inactive than the calcium or strontium salts.

The object of this invention is to provide a cal- .cium and/or barium free, non-scumming cyanide bath which may be operated for relatively long periods of time with a cyanogen compound content not greater than by weight. A further object is to provide such a bath which has a relatively low fusion point and relatively low viscosity at operating temperatures.

We have discovered that iron or steel articles may be case-hardened with excellent results by treating them in a bath of fused salts containing considerable quantities of a strontium salt, for instance 25-30%or more, and upwards of 1% of alkali metal cyanide. The case produced is of excellent depth and hardness and is characterized by containing relatively large amounts of both carbon and nitrogen.

In making up a bath in which to carry out our invention we prefer to use an eutectic mixture of strontium and sodium chlorides. This mixture which may be made by fusing together substantially equi-molecular weights of the salts, melts at about 560 C. This low melting point makes this composition especially suitable for case-hardening operations, since it allows operation over a wide range of temperature and insures a low viscosity at the lower temperatures. To this eutectic mixture we add 1% by weight or more of cyanogen compound, preferably 2-15% by weight of alkali metal cyanide. The proper amount of cyanide may vary considerably and is determined by the desired case-hardening result and the desired temperature of operation. For instance, a bath containing 5% by weight of sodium cyanide operated at about 900 C. will produce a hard and deep case, and the bath is quiescent and substantially non-scumming. If a bath containing the above mentioned eutectic and around 15% by weight of sodium cyanide is operated at 900 C. it also produces a good case but the bath tends to froth and scum.

The amount of strontium salt used in our casehardening bath may vary between wide In general, an increase of strontium 'salt content increases the case-hardening properties of the bath, cyanide content remaining constant. Likewise, the case-hardening properties of a bath containing a given amount of strontium salt may be increased by increasing the amount of cyanogen compound. The operating temperature may be varied between about 600 C. and 950 C. Increase of temperature usually increases the activity of the bath and the operating temperature is adjusted according to the desired results. Wehave obtained excellent results by operating our bath at around 900 C. with a cyanide content of 2-15% by weight.

The tendency to froth and scum in a bath made up according to our invention depends on the activity of the bath. Thus, as indicated above, there is some tendency to froth and scum if a bath containing around strontium chloride and 15% sodium cyanide is operated at around 900 C., while a bath containing around of strontium chloride and 5% of sodium cyanide operated at 900 C. is quiescent and non-scumming. By varying the three factors: strontium salt content, cyanide content and temperature, the activity of the bath obviously may be adjusted to any desired degree, and the bath may be maintained quiescent or not, as the operator desires. In any case, the tendency to froth and scum in the highly active baths is not great and does not seriously interfere with the case-hardening operation, provided the activity is not substantially greater than that of a bath containing 60% of strontium chloride and 15% of sodium cyanide in Example I. The results obtained are given in operated at 900 C. the following table:

In place of. sodium chloride as used in the above described eutectic mixture, other inert salts, for example potassium chloride, may be used with the strontium salt. By inert salt we mean one which has no substantial effect on the case-hardening properties of the bath but acts merely as a.

1st batch 2nd batch 3rd batch 0. s9 0. o. 69 0. s9 0. 79 diluent. We prefer to use sodium chloride as in- 2nd M4 M1 032 58 Q32 ert salt because it is readily available, inexpensive, 43 39 an 0.34 0.27 0.08 0. 25 0.06 and gives satisfactory results. While a carbon- 5th (105 M0 ate, for example sodium carbonate, may be used as inert salt, it is not as satisfactory as a chloride because it tends to cause formation of strontium carbonate, the presence of which increases the viscosity of the bath. It is well recognized in case-hardening practice that a bath of low viscosity is desirable to prevent undue amounts of salt being removed along with the treated articles leaving the bath.

Example I Our improved case-hardening bath requires a smaller amount of cyanide than is necessary in the ordinary cyanide case-hardening baths which contain no alkaline earth metal salt. On the other hand, the amount of cyanide added to the bath may be large enough to insure case-harden= ing action over an extended period of time with: out the necessity of too frequent additions of cyanide to the bath. The activity of the bath is not too great; that is, emcient case-hardening P Portions, of Strontium P P i action may be obtained without excessive frothchlonde euttectlc mlxtureg contammg approxl" ing or scumming. The bath constituents are not matelY eqm'molecular Welghts of the salts were corrosive to metals. In its preferred form, the fused in separate vessels. To one was added 2% bath has a low melting point which adapts it for by weight of.sodium cyamdeand to the other 5% use over a wide temperature range. The visa by welght of Sodmm cyamde' F two fused cosity of the bath is sumciently low to prevent baths were heated to about 900 and after excessive amounts of salt being removed when they had been held at that temperature for about treated articles are withdrawn therefrom. The

' one hour, rods of 1020 S. A. E. steel were treated case produced has excellent depth and contains by immersion in the baths for a period of one relativel y igh portions of both carbon and nihour. After treatment the rods were cooled and trogen An iron or Steel article which has been Washed free of ajdhermg Salt- The P were case-hardened according to our invention may be then each placed in a lathe and successive layers quenched or otherwise heat treated by known 0.004" thick were cut therefrom. Four succesmeans to obtain a case whose hardness is equal 7 sive layers were cut from each rod and each layer s was separately analyzed to determine its carbon and nitrogen contents. The results obtained with representative rods are given below:

2 percent NaCN 5 percent NaCN Cut (0.004") Percent 0 Percent N Percent 0 Percent N During the operation of the above described baths, samples were removed periodically o ver a period of several hours and analyzed to determine 'the sodium cyanide content. These analyses showed that the cyanide was decomposing at the approximate rate of 1% per hour.

\ Example -I I The eutectic mixture described in Example I was prepared and 15% by weight of sodium cyanide was added thereto. The bath temperature was at about 900 C. when the cyanide was added and this temperature was maintained throughout the entire operation described below. One hour after the cyanide addition, rods of 1020 S. A. E. steel were treated in the bath by immersing therein for one hour. At the end of this time another batch of rods was treated in the bath for one hour. Four and one-half hours after the expiration of the second treatment, a third batch of rods was treated in the bath for one hour. The depth and composition of the cases obtained were determined by cutting five 0.004" layers to or better than 50 Rockwell units.

The eutectic mixture of strontium and sodium chloride referred to in the appended claims is substantially that which has been described above.

We claim:

l. A method of case-hardening ferrous metal articles comprising immersing said articles in a fused salt bath containing a strontium salt, alkali 55 metal chloride and alkali metal cyanide, said bath being substantially free from calcium or barium compounds.

2. A method of case-hardening ferrous metal articles comprising immersing said articles in a fused salt bath containing strontium chloride, sodium chloride and sodium cyanide, said bath being substantially free from calcium or barium compounds. l

3. A method of case-hardening ferrous metal articles comprising immersing said articles in a fused salt bath containing strontium chloride, sodium chloride and 2-15% of sodium cyanide, said bath being substantially free from calcium or barium compounds.

4. A method of case-hardening ferrous metal articles comprising immersing said articles in a fused salt bath containing 25-60% of strontium chloride, 73-25% of sodium chloride and 215% of sodium cyanide, said bath being substantially free from calcium or barium compounds.

5. A method of case-hardening ferrous metal articles comprising immersing said articles in a fused salt bath containing sodium cyanide and an eutectic mixture of strontium chloride and sodium chloride, said bath being substantially free from calcium or barium compounds.

6. A method of case-hardening ferrous metal from the rods and analyzing them as described articles comprising immersing said articles in a v fused salt bath containing 245% of sodium cyanide and an eutectic mixture or strontium chloride and sodium chloride, said bath being substantially free from calcium or barium compounds.

'I. A salt mixture for case-hardening comprising a strontium salt, alkali metal chloride, alkali metal cyanide and said mixture being substantially free from calcium or barium compounds.

8. A salt mixture for case-hardening comprising strontium chloride, alkali metal chloride, alkali metal cyanide and said mixture being substantially free from calcium or barium com- Pounds.

9. A salt mixture for case-hardening comprising strontium chloride, sodium chloride, sodium cyanide and said mixture being substantially free from calcium or barium compounds.

10. A salt mixture for case-hardening comprising 25-60% of strontium chloride, sodium chloride, sodium cyanide and said mixture being substantially free from calcium or barium compounds.

11. A salt mixture for case-hardening comprising an eutectic mixture of strontium chloride and sodium chloride, 2-15% of sodium cyanide and said mixture being substantially free from calcium or barium compounds.

12. A salt mixture for case-hardening comprising 25-60% of strontium chloride, 73-25% of sodium chloride, 2-15% of sodium cyanide and said mixture being substantially free from calcium or barium compounds.

EARLE ATHERTON HARDING. DONALD AUBREY HOLT.