US2042527A

US2042527A - Process for nitriding

Info

Publication number: US2042527A
Application number: US645043A
Authority: US
Inventors: Holt Donald Aubrey
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1932-11-30
Filing date: 1932-11-30
Publication date: 1936-06-02
Anticipated expiration: 1953-06-02

Description

Patented June '2, 1936 UNITED STATES 7 2,042,527 rnooass FOR m'mmme Donald Aubrey Holt, Niagara Falls, N. Y., assig'no'r to E. I. duPont de Nemours & Company, Inc., Wilmington, Del., a corporation of Delaware No Drawing.

Application November. 30, 1932, Serial No. 645,043 v 8 Claims. (01. 148--15i This invention relates to a process for introducing nitrogen into the surface layers of steel articles, and more particularly to the use of fused salt baths for that purpose.

The art of nitriding steel articles consists in treating said articles in contact with a nitrogenous substance, for instance ammonia, whereby a hard outer surface of iron nitride is produced. In commercial practice this process is applied to 10 certain alloy steels at temperatures below 600 C.

In one proposed method, the alloy steel is immersed in a fused salt bath containing substantial quantities of an alkali metal cyanide. Such a cyanide bath, operated at relatively high temperatures, e. g. 800 to 900 0., produces a mixed case; that is, it introduces both carbon and nitrogen into the steel. However, if the cyanidebath is used at temperatures below 600 0., nitriding predominates and only a small amount of carbonis introduced.

The method of nitriding in most common use at the present time consists in= heating the alloy steel in an atmosphere of ammonia. This process gives good results but is disadvantageous in that a relatively long periodsof time is required for 'the nitriding action. The steel is usually heated in contact with the ammonia for a period of from- 15 to 90 hours, in order to produce a satisfactory nitrided case. A further disadvantage of ammonia nitriding is that relatively complicated and expensive equipment is required, and-the vessel in which the nitriding occurs or the walls thereof, must be replaced when the interior becomes coated with a layer of iron nitride. This nitride tends to catalyze the decomposition of ammonia, thus causing waste and decreasing the nitriding effect. Proper control of the operation of the process requires careful.

and constant attention. Moreover, the fact that ammonia must be transported in heavy, pres sure-resistant vessels, adds considerably to the cost of this method.

Nitriding by means of low temperature treatment in fused cyanide baths maybe accomplished in a, shorter time than ammonia nitriding. However, the case produced is neither as hard nor as deep as that produced by thc ammonia process. Moreover, there'is a certain tendency for carburization to occur, with the result that in addi- 60 tionto superficial carburization of the steel, a gray deposit, which may be either carbon or iron carbide, is deposited on the surface of the steel.

When the steel is subsequently polished, this re- An object of this invention is to provide a process for nltriding alloy steels which will give results comparable with thoseobtainable by the ammonia process but in less time and at less expense. A"further object is to provide a fused 5 salt nitriding bath which will not substantially carburize steel nor deposit carbon at nitriding temperatures, and will produce a clean, uniform, nitrided surface.

I have discovered that superior nitriding eflects 10 maybe obtained by treating an alloy steel at a temperature below 600 in a bath of fused salt containing cyanamide. This method of -nitrlding produces nitride cases equal to those obtainable by the ammonia process and in a much shorter 15 time. Furthermore, -I havefound that a fused salt bath containing a metal cyanamide, even in the presence of considerable amounts of cyanide, has substantially no carburizing action at temperatures below 600 C. In fact, under some con- 20 ditions such baths have a .sllght decarburizing action. As a consequence, the cases produced by this improved method are perfectly uniform and present a cleamappearanoe when polished.

I prefer to use an alkali metal cyanamide, e. g. 25 sodium cyanamide, in a bath free from substantial amounts of alkaline earth metal compounds which might react with alkali metal cyanamide to form alkaline earthmetal cyanamides. The presence of substantial amounts of alkaline earth metal 39 cyanamides'fis' objectionable because these compounds have relatively high melting points at the operating temperature, hence unduly increasing the viscosity of the bath. Moreover, as the alkaline earth metal cyanamides decompose during 35 the nitrlding process, they form insoluble oxides and/or carbonates in contradistinction to the alkali metal cyanamides, which decompose to formsoluble oxides and carbonates. The presence of insoluble salts or oxides in the bath 40 causes uneven nitriding and causes overheating of the nitriding vessel because of the heat insulating layer that settles out on the bottom of the vesseL;

Theproportion of alkali metal cyanamide in 45 a fused salt bath capable of being used to carry out my invention, may vary within wide limits.

For example, abath containing around 1% of sodium cyanamide or even less than 1% will have a certain nitriding action. The process may also be carried out with large proportions of cyanamide; for instance, a bathoi' substantially pure sodium 'cyanamide may be used. However, the melting point of'pure sodium cyanamide is'cl'ose to the maximum allowable temperature for point of the mixture.

{ which are not incompatible with the cyanamide, may be added for this purpose. Because of their cheapness and availability, I prefer to use for this purpose sodium or potassium cyanide or a mixture of the two. In place of cyanides. other low-melting salts or salt mixtures may be used; for example, a mixture of lithium, sodium and potassium chlorides has been found to be satisfactory. Whether cyanide or other salts are used, I preferto maintain the concentration of alkali metal cyanamide in the'bath at 10-40% by weight.

Myprocess may be used to nitride any type of steel which has a surface composition adapted for nitriding. By this, I mean a steel whose composition is such that nitrogen will be absorbed in substantial amounts when the steel is heated to a temperature of not more than 600 C. in contact with'a nitrogenous substance, with the resuit that a hard, durable, nitrided surface is formed. Steels especially adapted for nitriding by my process are those which contain, either throughout oronly in surface layers, one or more of the following alloy constituents: aluminum, molybdenum, chromium, vanadium, manganese or s'ilicon. As in the case of other nitriding processes, the degree of nitriding and the character of the case will depend to a large extent upon the amount and kind of alloy constituents present in the steel. i v

Obviously, by process may be used to increase the hardness of alloy steels, which have been partially hardened by other casehardenin'g or nitriding processes.

In general, my process may be carried out by immersing. the steel in a fused salt bath containing a metal cyanamide at a temperature between 400 to 600 C. for a period of time which will vary, depending upon the depth of case and degree of hardness desired. In most cases it will not be necessary to continue the treatment longer than 15 hours; for instance,'by a 15 hour treat- 'ment, I have obtained nitrided cases having a depth of 0.012 inch, while cases 0.007 inch deep were obtained in a 5 hour treatment, and as later described, increased hardness was attained in certain steels with a one-half hour treatment The surface hardness of such cases will be greater than 1000 Vickers-Brinell units with certain alloy steels. During thenitriding process the cyanamide content of the bath slowly decreases and must occasionally be replenished. However, as briefly indicated above, the amount of cyanamide in the bath is not important, providing there isaround one or more per cent present at all times.

I have found that the rate of cyanamide decomposition may be greatly decreased without impairing the nitridingeffect of the bath, by partially excluding air 'from the surface of the molten bath by covering the surface with a floating layer 01' finely divided graphite or charcoal. I prefer to use flaked graphite for this purpose, since charcoal has a certain tendency to react with the cyanamide, whereas graphite is substantially inert. Partial exclusion of air may also be effected by placing over the bath container and almost air-tight cover.

In one method of carrying out my invention, alloy steel is treated at 400-600 C. in a fused salt 75 bath containing an alkali metal cyanamide and content of the bath remains substantially constant, and is substantially independent of the amount of cyanamide present in the bath. Moreover, in the presence of the alkali metal cyanamide, the cyanide present in the bath has substantially no carburizing action upon the steel, nor does it tend to depositout any appreciable amount of carbon on the steel, or elsewhere. One demonstration of this is the fact that during operation, the cyanide content of the bath re- 'ma.ins substantially constant, except in case where a reactive form of carbon (e. g.- charcoal) is present. Furthermore, when my invention is carried out with a fused salt bath containing no cyanide; for example,'a bath composed of sodium chloride, potassium chloride, lithium chloride, and sodium cyanamide, operated at temperatures below 600 C., the nitriding effect is substantially the same as that obtained with baths containing cyanide and cyanamide.

My improved nitriding bath may be advantageously employed for drawing or tempering certain steels, e. g. high speed tool steels, which may be tempered at temperatures below 600 C. In

such cases, the exterior portions of the steel will be increased in hardness because of the nitrid ing effect of the bath, while the steel is simultaneously tempered. The degree of surface hardness obtained by this method will depend upon the time the steel remains in the bath; an appreciable hardening effect can be obtained in as short a time as 30 minutes.

lowing examples:

Example I Percent Carbon 0.33 Manganese 0.51 Silicon 0.20 Aluminum 1.24 Chromium 1.58 MolybdenunL 0.20

After removing the treated steel articles from the bath they were cooled in the air and tested for hardness by the Vickers-Brinell method. The articles were found to have a Vickers-Brinell hardness of 1200 units. The case depth obtained, as determined by grinding to expose the core, and etching with a solution of nitric acid in methanol, was found to be 0.007 to 0.008 inch.

' Example II Samples of the alloy steel described in. Example I were treated by inunersion in a fused bath composed of about equal parts by weight of potassium cyanide, sodium cyanide, and sodium cyanamide at about 538 C. Samples treated in this bath for 5 hours were found to have a nitrided case 0.007 inch in depth and had a Vickers- Brinell hardness of- 1100 units. Other samples, which remained in the bath for 15 hours, were My invention is further illustrated by the fol 35 found to have a nitrided'case of 0.012 inch in depth and a Vickers-Brinell hardness of 1200 units. Analyses of the bath during the operation showed that the average rate of decomposition of the cyanamide was about 3 per hour.

Example III 'sten were treated in the bath for various lengths of time. The average rate of decomposition of the sodium cyanamide during the run was 0.28%

per hour. The results obtained are given in the following table:

Age of both T- I Vickers- Sample No. during Case depth Brinell treatment treatment hardness Hours Hours Inch 1 5 s I 0.007 1112 2 7-12 5 0.007 1112 a 12-18 0 0.008 1200 4 l228 0. 014 1200 5 30-30 0 0.007 1112 0 l. 3545 0 1200 7 55-01 0 0.004 1112 Samp1e No. 6 was the high 5 eed steel;

the other samples were the steel described in xample I. 7

At the end of the 61st hour the bath contained 5.8% of sodium cyanamide.

Example IV A mixture containing 37.6% by weight of lithium chloride, 46.9% of potassium chloride, and 15.5% of sodium chloride was fused, and about 10% by weight of sodium cyanamide was added to the melt. A sample of the steel described in Example I was'immersed in the molten bath for a period of 2}/ hours at 530 C. After cooling, the sample had a hardness of 1112 Vickers-Brlnell units. Another sample of this steel acquired a Vickers-Brinell hardness of 1200 after a 4 hour treatment in this bath at 530 C. A sample of a chrome-vanadium steel, treated in the bath at 530 C. for-4 hours, acquired a hardness of 734 Vickers-Brinell units. This steel had the following alloying constituents:

Per cent Carbon 0.30-0.40 Chromium 1.25

Vanadium 0.45

Example V and the hardness at various depths below the surface were determined in the case of a sample of the same steel which had been treated for 8 hours in a bath consisting of an eutectic mixture of sodium cyanide and potassium cyanide at 538 C. The following results were obtained:

Vickers-Briaell Hardness Depth below Steel treated steel t t d original by bath of suflm Example In NaCN KON bath Inch Example VI -Approximately equal parts of sodium cyan- A sample of alloy steel having the following constituents also attained a hardness of 1000 Vickers-Brinell units after a 4-hour treatment in this bath:

Percent Carbon--- 0.36 Manganese 0.51 Silicon 0.27 Aluminum 0123 Chromium. 1.49 Molybdenum 0.18

A sample of a high speed steel of unlmown composition, which had been treated one-half hour below 600 C. in a bath composed of approximately equal parts of sodium cyanide and potassium cyanide, had a surface hardness of 950 Vickers-Brinell units. After a one-half. hour treatment in the above described cyanamide bath, the hardness of this sample was increased to 1200 Vickers-Brinell units.

During a period of 5 hours, the average rate of decomposition of the sodium cyanamide in the above-described bath was 1.95% per hour. Samples of the bath, removed at various intervals of time were in each case substantially white, showing that the charcoal did not penetrate into the bath to any appreciable extent.

The rate of cyanamide decomposition in Example VI is higher than in Example III, probably because of the tendency of the charcoal to react with the cyanamide to form cyanide, whereas graphite does not so react.

My improved process for nitriding steel is capable of producing a nitrided case of depth and hardness equal to that produced by the ammonia process, and has the advantage that it is simpler in-operation and control, requires less expensive and less elaborate equipment, and requires less expenditure for treating materials. A further advantage is that it produces a greater degree oi. hardness and a deeper nitrided case than is possible with nitriding baths heretofore used, and does not produce an undesirable appearance in the nitrided article.

' ,I claim:

1. A process comprising heating a steel having a surface adapted for nitriding, in a fused salt bath comprising as the active nitriding agent an alkali metal cyanamide at a temperature below 600 C. while partially excluding air from the surface of said bath.

2. A process comprising heating a steel having a surface adapted for nitriding, in a fused salt bath comprising as the active nitriding agent an alkali metal cyanamide at atemperature below 600 0., the surface of said bath being covered with a layer of flake graphite.

3. A process comprising heating a steel having a surface adapted for nitriding, in a fused salt bath containing sodium cyanide and as the active nitriding agent 1-40% by weight of sodium cyanamide at atemperature below 600 C. for 2 to 15 hours, while partially excluding'air from the surface of said bath.

4. A process comprising heating a steel having a surface adapted for nitriding, in a fused salt bath containing sodium cyanide and as the activenitriding agent 1-40% by weight of sodium cyanamide at a temperature below 600 C. for 2 to 15 hours, the surface of said bathbeing covered with a layer of flake graphite.

5. A process comprising heatinga steel having a .surfaceiadapted for nitridin'g, in a fused salt bath containing 1-40% by weight of as the active nitriding agent an alkali metal cyanamide' at a temperature below 600 C., while providing means for allowing restricted amounts of air to come in contact with the surface thereof.

6. A process comprising heating a steel having a surface adapted for nitriding, in a cyanide-free fused saltbath containing as the active nitriding agent-an alkali metal cyanamide at a temperature below 600 C.

7. A process comprising heating a steel having a surface adapted for nitriding in a cyanidefree fused salt bath comprising alkali metal chlorides and as the active nitriding agent 1 to 40% by'weight of analkali metal cyanamlde at a temperature below 600 C. I

8. A process comprising heating a steel having a surface adapted for nitrlding' in' a fused salt bath consisting of lithium chloride, potassium chloride, sodium chloride and 1 to 40% by weight of sodium cyanamide at a. temperature below 600 C.

\ x DONALD AUBREY HOLT.