US3488233A - Surface treatment of steels - Google Patents

Description

Jan. e, 1970 l. L. NEwE-LL l 3,488,233

SURFACE TREATMENT OF STEELS Filed July 27; 1967 PM .fl m um W my .W XW -W as mg Q u Q Q Q United States Patent O M 3,488,233 SURFACE TREATMENT OF STEELS Isaac L. Newell, Wethersfield, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed July 27, 1967, Ser. No. 656,531 Int. Cl. C21d 1/46 U.S. Cl. 14S- 15.5 8 Claims ABSTRACT OF THE DISCLOSURE A process for carburizing materials, such as the ferrous metals, with or without the formation of a protective coating, by simple immersion in a molten salt bath containing lithium carbonate as the active ingredient.

BACKGROUND OF THE INVENTION It is known that the carburization of various materials, particularly the ferrous base alloys, may be conveniently effected by simple immersion in molten salt baths. Exemplary of some of the various techniques previously employed in the art for this purpose are the processes set forth in the patents to Leininger 2,492,803 and 2,568,680, and Freudenberg 1,796,248. In most processes of this type wherein the desired carburization is performed by simple immersion and without the application of an external electromotive force, the usual carburizing ingredients utilized include the various poisonous cyanides or suspended free carbon. A further characteristic of these prior art techniques is the use of an elevated treatment temperature, typically 15001`800 F.

SUMMARY OF THE INVENTION The present invention relates to the surface treatment of materials, particularly the ferrous metals, in molten salt baths. It contemplates the use of lithium carbonate as the active carburizing ingredients in such baths, with or without the inclusion of additional salts or other ingredients to adjust the melting point of the composition and control the rate and character of the carburization, including such ingredients which are commonly identified as catalysts, activators and inhibitors. It further contemplates a technique whereby carburization of the underlying substrate may be effected without the buildup of a surface layer on the part, or carburization with the concurrent formation of a corrosion resistant coating on the part, depending upon the particular requirements sought in connection with the finished article.

BRIEF DESCRIPTION OF THE DRAWING The drawing comprises a graph of hardness versus depth of penetration for several samples of low carbon steel carburized by the process of the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As previously indicated, molten lithium carbonate is the active carburizing ingredient present in the treatment bath of the present invention. In various tests conducted utilizing this process, parts have been carburized in melt composition which have been varied from 100 percent lithium carbonate to those in which the lithium carbonate was present in amounts as low as 20 mol percent, with equally good results in all cases. Several additives have been included in the bath compositions for various reasons and, in a more preferred composition, lithium fluoride was added and, in fact, comprised the predominant element in the bath on a molar basis. This addition was made solely to lower the melting point of the bath and experience has demonstrated that the lithium fluoride appears to provide no other function as far as the carburization process is concerned. The preferred composition incorporating lithium fluoride was found to provide effective carburization at a temperature of about 1418 F. An example of such a bath would be one containing about 20 to 99 mol percent lithium carbonate with the balance a lithium halide.

In another preferred bath composition boron oxide was also added to the bath, primarily to prevent the formation of a coating buildup on the surface of the part being treated in those instances where the coating was considered undesirable. The boron oxide was found to be effective in this regard without significantly interfering with the basic substrate carburization. In some embodiments of the present invention, therefore, the boron oxide addition will be preferred. Other activating agents or coating inhibitors which also appear satisfactory include sodium tetraborate, silicate and the various alkali metal silicates. It is apparent, on the other hand, that if the formation of a corrosion-resistant coating is desired on the finished product, boron oxide or the other comparable substances should be substantially eliminated from the melt. Although the boron oxide concentration does not appear particularly critical and substantial `quantities appear tolerable in the melt, the concentration utilized should not be so high as to interfere with the basic carburization process. Accordingly, concentration of about 3 mol percent boron oxide will normally be preferred when carburization alone is desired. A suitable bath would consist essentially of about 20 to 99 mol percent lithium carbonate, up to mol percent lithium iiuoride and 1 to 5 mol percent of a coating inhibitor. A further example of a suitable bath would be one containing 20 to 50 mol percent lithium carbonate, 50 to 80 mol percent lithium liuoride and 1 to 5 mol percent boron oxide.

A fourth ingredient included in a more preferred composition is nickel chloride which was added to increase the rate of reaction and, hence, exhibits a catalytic function. It appears to perform this function although present in a very minor amount, a concentration of approximately 200 parts per million having been demonstrated as satisfactory in this regard.

The present invention is described in further detail in the specific examples which follow. It will be understood that these examples are included for the purposes of illustration only and no limitation is intended or should be implied therefrom.

EXAMPLE I A carbon steel rod, formed from SAE 1010 material, having a nominal composition, by weight, of

Percent Carbon .08. 13 Manganese .30-.60 Phosphorous max-- .04 Sulfur max .05 Silicon max .10 Iron remainder was immersed in a molten bath of lithium carbonate at 1400 F. for a period of 21 hours. At the end of this time the rod was quenched in water. The rod was sectioned and examined microscopically, examination revealing a uniform coating approximately 0.001 inch in thickness. Analysis by X-ray dilfraction showed this coating to consist of lithium ferrate (-LiFeOz), and chemical analysis of the steel showed a carbon content increase in the surface layer to 0.25 percent by weight.

EXAMPLE Il A sample identical to that of Example I was treated under substantially identical conditions, except that a blanket of argon was 'maintained over the bath during the treatment-period. -v The-rodez was subsequently-foundto have a coating similar to that found in Example l and, in addition, the surface carbon content of the steel was increased to 0.36 percent by weight.'y r H f EXAMPLE 111 a A rod of SAE 1010 low carbon steel was immersed in Samples formed of SAE 1010 steeltubing in 14 inch l lengths, cut from the same stock, were thoroughly cleaned utilizing emery'cloth abrasion followed by immersion'for 5 minutes in a bath of 30 percent hydrochloric acid.

The samples were immersed in a salt bath prepared to the following composition:

Mols Weight, gms.

Ingredient:

Lithium fluoride 60 445 Lithium carbonate- 422 Boron oxide 5. 100 Nickel chloride l 1 37 l The nickel chloride was held in a hard vacuum for 3 days at 160 F. before addition to the melt.

A variety of treatment conditions were utilized for the samples as summarized in the following table:

Carbon Average Sample test condition VHN content Sample:

A Annealed, untreated 103 11 D Immersed in molten salt at 1,400 F. 708 .36

for 22 hours, argon blanket, water 4 quenched. l 13--... Heated to 1,400F., argon blanket, 192 .10

` water quenched. n p 0...-. Immersed in l-week'old molten salt at 405 .14

. 1,400 F.,tor 20 hours, argon blanket, n v y water quenched. y E....- immersed in 2-week'old molten salt at 374 1.-16

1,400" F. for 20 hours, @5% C02 in argon blanket,.water quenched. 11...-- Same as (E), section 1 inch above im- 93 02 v mersed portion.

The results of the `various treatments as analyzed metallurgically are illustrated in the drawing, the sample number referred to in the preceding table corresponding t'o that utilized in the drawing.

It is evident from the tests conducted that the hardness, depth of hardening, and extent of the corrosionresistant coating are functions of the process parameters primarily time, temperature and meltcomposition although the bath condition and article pretreatment are of importance. Treatment temperatures as low as 12`O0l F. appear to provide the desired carburization. l l

The corrosion-resistant coating, when formed, appears to comprise lithium ferrate. While it is not certain how the reactions occur in the formation of this coating, it is believed that decomposition ofthe lithium carbonate takes place with a subsequent, formation of va conversion coating withA` the iron and at the same time reduction "to l.at-carbide, -according to the following equation:V

It may readily be seen that substantial regeneration of the bath can be effected by treatment with carbon dioxide to reform the lithium carbonate.

Samples from the product of Examples I and 'VII were tested in a salt spray box for a period` of 24 hours. While no corrosion was noted on the coatedareas, those regions wherein the base metal was exposed by cutting away the coating were badly corroded. Similarly, a humidity test at 120 F. for 24 hours and atotal immersion test for 72 hours at 70 F.A showed no corrosion in the coated areas. I# 1', v v v .From the foregoing descriptionvit w ill readily be seen that there has been'provided by this invention means for carburizing materials in asimple immersion process lcharacterized by the absence of toxic salts. Further, the process is inherently flexible in that the car'burization may be effected with or without the formation of a protective coating on the finished article bythe, simple adjustment of thebath'compo'sition. Alternatively, therefore, the process in one form 'may be characterized as a coating process, the coated article produced displaying excellent corrosionresistance characteristics.

What is claimed is:

1. A process for carburizing the surface of the ferrous metals which comprises immersing the metal in a molten salt Ibath containing' from about 20-100Y percent lithium carbonate as the active carburizing ingredient, maintaining the bath at a temperature of at least 1200 F., and continuing the' immersion until the desired carburizng treatment is elected.

2. The process according to claim 1 wherein the molten salt bath consists essentially of about 100 percent lithium carbonate. v

3. The process -according to claim 1 wherein the molten salt bath consists essentially of about 20-99 mol percent lithium carbonate, balance a lithium halide.

I 4. The process according to claim 3 wherein the lithium halide is lithium iiuoride.

y 5. The process according `to claim 1 wherein the molten salt bath consists essentially of about 20-99 mol percent lithium carbonate, up to mol percent lithium iiuoridc, and 1-5 mol percent of a coating inhibitor. l "6. VThe process of claim 5 vwherein the coating inhibito is boron oxide.' I v v 7.A process for carburizing the ferrous'metals withoutthe formation of a coatingfthereon which comprises immersing'the metal in a molten salt bath consisting essentially of 20,-50 mol percent lithium carbonate, 50-80'mol percent vlithium uoride, andA 1'-5 mol percent boronoxide,

1 maintaining the bath ata temperature of at least 'l200 F.,

References Cited UNITED STATESv PATENTS 2,049,806 8/1936 Holt l48-15.5 y2,568,860 9/1951 Leininger et al 14S-15.5I

CHARLES LOVELL, Primary Examiner y ,I Us. C1. X.R. 14s-6.11, 2o, 31.5, 39

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