US2927875A - Process for carburizing and carboni-triding iron and steel - Google Patents

Description

PROCESS FOR CAR'BURIZING AND CARBONI- TRIDINGIRONANDS TE EL I Johannes Miiller, Frankfurt am MaihQGiiimahy, assignor to Deutsche Goldund Silber-Scheideanstalt vormals Roessler, Frankfurt (Main), Germany 1 No Drawing. Application June 20, 1958 Serial No. 743,467

Claims priority, application; Germany June 26, 1957 8 Claims. (Cl. 14815. 5)

The present invention relates to an improved process 7 for carburizing and carbonitriding iron-and steels in,

cyanide and cyanate containing salt baths whereby espeoially increased case depths can be obtained. It is known that iron and steel articles can be cemented, that is, carburized to a certain depth below their surface by treatment in fused salt'baths containing cyanides. The salt baths which are known for this purpose contain one or more cyanides as the carbon source and substantial proportions of alkali metal and/or alkaline earth metal haldies. The decrease in carbon content with increased distance from the surface of the articles and therefore also the time required for the carburization depends essentially upon the cyanide-content of the salt. bath, the presence or absence of accelerators or activators, such as, for example, certain alkaline earth metal halides, and particularly the carburizing temperature employed.

Baths without an activator produce a low carbon content in the edge zone but the nitrogen which is-simultaneously split'ofl from the cyanide produces a high'surface hardness. However, deep case depths cannot be achieved with such baths and the cases cannot be ground off without causing the hardness to decrease strongly. On the other hand, such baths are suited for the production of very thin carburization cases at temperatures below the pearlite point, above 600 C. and preferably about 650-700" C. The cases thus obtained are very hard and in view of the low temperatures employed the distortions produced by such treatment are only very slight. It was, however, previously not possible to increase the case depth obtained sufficiently so that this technique is only seldom used.

Baths without an activator in general contain more than 40% of cyanide. Because of high cyanide losses at high temperatures such baths are only operated at temperatures below 900 C. In salt baths which contain additions which promote the liberation of carbon, the cyanide content is considerably lower and usually is about 10% and barium chloride in a quantity of about 50 to 55% is present as the main constituent of the bath. The usual working temperature of such baths is about 930 C. and the use of substantially higher temperatures which would accelerate the carburization is generally not possible, as otherwise impractically high cyanide losses are engendered. I

It has also been proposed to carburize iron and steels with cyanide containing baths predominantly composed of 2, As in general cyanide containing baths a small quantity of cyanate is automatically produced, an oxygen content in the baths is automatically provided so thatfit is only necessary to add at least'one of the other elements; tellurium or selenium to such baths. In general,.0.05 .to

5% of such added element or elements snflicesand pref erably a 'quantity between 0.2% and 3% is employed:

According to the process of the invention the bathscontain 5 to 40% KCN or equivalent amounts of eg NaCNO, LiCNO, RbCNO and the like as well as 0.1' to 60% NaCN or equivalent amounts of e.g. KCN;-

The remainder consists essentially of carbonates and halogenides espe-' cially chlorides and fluorides of alkaline metals and LiCN, RbCN, Ca(CN) and the like.

eventually of earth alkaline metals. Small quantities-of oxides are nearly always present. Mixtures of Na/KCO Also ood yields are" obtained with fluorides. The fused salt 'bathtemperatures and Na/KCl arevery suitable;

employed according to the invention lie between 600 and 1100 C.,'preferably-between 650 and (3. Expediently, temperatures between 650 and 930 C. are

used. Y

The following examples will serve to illustrate the unexpectedly improved results obtained with the process according to the invention.

. Example 1 To a bath of 47% ture of 700 C. for 2 hours. Thereafter thethickness of the so-called transition zones and .the total thickness of the case obtained on the treated samples were deter-' mined microscopically. The total thickness was 0.14

mm. and the thickness of the transition zone was 0.041

Example 3 To a bath of 50% NaCN, 15% KCNO and the remainder potassium carbonate, 0.7% sodium telluride are added. Samples of a low carbon steel (containing 0.15 of carbon) were treated in this bath at a temperature of 700 C. for 2 hours. Thereafter the thickness of the so-called transition zones and the total thickness of the case obtained on the treated samples were determined microscopically. The total thickness was 0.17 mm. and the thickness of the transition zone was 0.02 mm.

Example 4 A bath of 50% sodium cyanide, 10% sodium cyanate and the remainder sodium carbonate was compared with a bath of the same composition having 1% of selenium added thereto in the form of sodium selenite. Samples of a low carbon steel (containing 0.15% of carbon) were treated in such baths at a temperature of 700 C.

' for 1, 2, 5 and 10 hours. Thereafter the thickness of the 7 addition oxygen at least contain one of the elements selenium and tellurium.

so-called transition zones and the total thickness of the case obtained on the treated samples were determined microscopically. The following table compares the thicknesses attained.

p 7 Patented Mar. 8, 1960,,

NaCN, 35% KCNO and the re mainder K/NaCO 0.5% sodium selenide are added. Samples of nearly carbon free steel (Armco-iron with; 0.05% carbon) were treated in this bath at a 'tempera-- Without Se With Se Hours Total Trans. Total Trans. Thickness, Zone, Thickness, Zone,

mm. mm. mm. mm.

The, portion of the case not forming a part of the transition zone was essentially martensitic. seen that,,even only after 1 hours treatment, the total thickness of the case was substantially greater when the V selenium containing hath was employed. The thickness of; the transition zone is even approximately twice that when thebath. in which they selenium was omitted was employed. The ratio becomes worse with longer treating periods but is noticeably still to the detriment of the bath notcontaining selenium. It is particularly remarle.

It will be.

the group consisting of selenium and tellurium has been added, said salt bath being maintained at a temperature between 600 and 1100 C;

2. The process of claim 1 in which the temperature of said salt bath is maintained between 650 and 930 C. 3. The process of claim l in'which said cyanides and cyanatesare alkali metal cyanides and alkali metal cyanates.

4. The process lof claim 1 in which said cyanides and cyanates are alkalineearth metal cyanides and alkaline earth metal cyanates.

5. The process of claim 1 in which said added element is selenium.

6. The process of claim 1 inwhich said added element is tellurium. 7 V

7.' A process for carburizing and carbonitriding iron and steel which comprises treating such metals in a fused 7 salt bath containing alkali metal cyanides equivalent to able, however, that in selenium free baths the sum of thicknesses of the martensitic zone and the'transition 7 ,zonedoes not increase upon increase of the, treating period from 1 to 10 hours, whereas in selenium containing baths the thickness increases over five fold.

Analogousresults were obtained with tests carried out at; higher temperatures. Also, replacement of the selenium by tellurium provides an increase of the same order in.

ofv NaCN as well as cyanates equivalentto. 5-40% of' KCNO and the remainder essentially consisting of carbonates and halides of alkali and alkaline earth metals, to which 0.05-5% of at least one element selected from 01-60% of NaCN as well as alkali metal cyanates equivalent to 5-40% KCNO and the remainder essentially consisting of carbonates and halides of alkali and alkalineearth metals, to which 0.05-5% of selenium has beenadded, saidsalt'bathbeing maintained at a temperature between 600 and 1100 C.

8..A process for carburizing and carbonitriding iron and steel which comprises treating'such metals in a'fused salt bath containing. alkali metal cyanides equivalent to.-

7 01-60% of NaCN-aswell as. alkali metal cyanates equivalent to 5-40% KCNO andthe remainder essentially con-.- sistingrof carbonates and halides of'alkali and alkaline: earth metals, to which 0.055% of tellurium has been: added; said salt bath beingmaintained at a temperature:

between 600. and 1100 C.

References Cited in the tile of this patent Troup: Materials and Methods, vol. 44, September 1956, pages 110-113 (TA 401 M5).

Johnson: Metal Working andHeat Treatment Manual, v0l.-III, pages -64, page 55 relied upon (TS 205 16- V3).

Materials and Methods, vol. 25, March 1947, page 108 (TA 401 M5).

Claims (1)

1. A PROCESS FOR CARBURIZING AND CARBONITRIDING IRON AND STEEL WHICH COMPRISES TREATING SUCH METALS IN A FUSED SALT BATH CONTAINING CYANIDES EQUIVALENT TO 0.1-60% OF NACN AS WELL AS CYANATES EQUIVALENT CONSISTING OF CARKCNO AND THE REMAINDER ESSENTIALLY CONSISTING OF CARBONATES AND HALIDES OF ALKALI AND ALKALINE EARTH METALS, TO WHICH 0.05-5% OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF SELENIUM AND TELLURIUM HAS BEEN ADDED, SAID SALT BATH BEING MAINTAINED AT A TEMPERATURE BETWEEN 600* AND 1100*C.