US3793160A

US3793160A - Method of forming case-hardened metals by electrolysis

Info

Publication number: US3793160A
Application number: US00098898A
Authority: US
Inventors: O Homan
Original assignee: Triangle Industries Inc
Current assignee: PELTZ NELSON; TRIANGLE WIRE & CABLE Inc A CORP OF DELAWARE
Priority date: 1968-12-09
Filing date: 1970-12-16
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19

Abstract

A process of boronizing a metal article is described which comprises immersing the selected metal article in a fused bath composed of at least one alkali metal halide or alkaline earth metal halide and a boron salt of the empirical formula MxByFz wherein M is an alkali metal, and the ratio of x:y:z is 1:0.4 to 2.0:0.5 to 2.5 with y being preferably above 1 and z is preferably below 1.5, the fused bath being maintained at a temperature between 1200*F to 1750*F for a sufficient duration to impregnate the metal with boron. There is also described novel boron products, their process of manufacture and salt baths containing such boron products. The process produces an extremely hard, uniform, adherent and corrosion resistant boride casing on metals such as carbon and alloy steels.

Description

i United States Patent [191 Human [451 Feb. 19, 1974 METHOD OF FORMING CASE-HARDENED METALS BY ELECTROLYSIS [75] Inventor: Oris T. Human, Las Vegas, Nev.

[73] Assignee: Triangle Industries, Inc., Newark,

1 [22] Filed: Dec. 16, 1970 [21] Appl. No.: 98,898

Related 1. .8. Application Data [62] Division of Ser. No. 782,436, Dec. 9, 1968, Pat. No.

Prinrary Examiner-John l-l. Mack Assistant Examiner-R. L. Andrews Attorney, Agent, or Firm Darby & Darby [57] ABSTRACT A process of boronizing a metal article is described which comprises immersing the selected metal article in a fused bath composed of at least one alkali metal halide or alkaline earth metal halide and a boron salt of the empirical formula M .B,,F 2 wherein M is an alkali metal, and the ratio of x:y:z is 1:0.4 to 20:05 to 2.5 with y being preferably above 1 and z is preferably below 1.5, the fused bath being maintained at a temperature between 1200F to 1750F for a sufficient duration to impregnate the metal with boron. There is also described novel boron products, their process of manufacture and salt baths containing such boron products. The process produces an extremely hard, uniform, adherent and corrosion resistant boride casing on metals such as carbon and alloy steels.

16 Claims, N0 Drawings METHOD OF FORMING CASE-HARDENED METALS BY ELECTROLYSIS This application is a division of US. application Ser. No. 782,436 filed Dec. 9, 1968 and now U.S. Pat. No. 3,634,145.

This invention relates to boron salts, their preparation and use in a fused salt bath to case-harden metals.

The inclusion of boron in carbon and low alloy steels is well known and commonly practiced in improving the hardenability of such steels. One of the processes used for the preparation of a boride coating on a metal comprises exposing the metal article to the vapors of a boron halide at a temperature sufficient to cause the halide to decompose and deposit a coating of boron on the metal. The metal is then heated to a still higher temperature to cause the boron to diffuse into and alloy with the metal. In the absence of a reducing gas, a displacement reaction occurs in which some of the metal replaces the boron in the boron halide. In the presence of hydrogen, the boron halide is reduced to boron and hydrogen halide. In order to obtain satisfactory deposition rates, temperatures in the order of l200-l400C are required. The deposition rate must be carefully controlled since rapid deposition results in the formation of a boride undercoating with a fused boron coating as the outermost layer. In order to obtain adherent coatings, it is usually required to deposit a very thin coating followed by a hydrogen soak to diffuse the coating into the metal. Additional coatings are made in the same manner to produce the desired thickness. This method is not satisfactory for the productionof high precision machined parts which must be accurately machined to very close tolerances before they are borided. The parts warp causing the dimensions to exceed tolerance limits because of the distortion caused by the high temperatures and phase transistions to which the article is subjected in heating and cooling, especially when repeated steps are required to produce the desired thickness of coating. The rate of deposition of the boron is very dependent on the velocity of the boron halide over the surface and the temperature of the article being coated. Since these conditions are difficult to control, especially for large or irregular shaped articles, the coatings are usually not uniform over the entire surface.

Boride coatings have also been deposited on ferrous alloys by electrolyzing a fused bath of a boron compound such as boron oxide, boric acid, borax, etc., using the ferrous alloy article as the cathode and graphite as the anode. Voltages of 4-40 volts and a current density of 50-100 amperes per square decimeter are required. Lower current densities permit the ferrous alloy to be dissolved in the bath at a rate faster than the boron is deposited on the alloy, so that there is a net weight loss. This effect is very noticeable with the boron compounds which are acidic such as boric acid or boron compounds containing boric acid as an impurity.

In more recent years boride coatings have been produced electrolytically using a fused bath composed of at least one alkali metal fluoride and at least one alkali metal fluoborate (NaBF This process is carried out in an electric cell in which an electric current is generated when an external electrical connection is made between a metal cathode and a boron anode. Preferably,

additional voltage is impressed upon the circuit from an external power source. This process must be carried out in the substantial absence of oxygen, e.g., in an inert gas atmosphere or in a vacuum. Impurities have been reported to interfere with the electrode reactions and affect the quality of the boride coating. The foregoing process is costly in that it requires special equipment (e.g., an electric cell, electrodes) and carefully controlled processing conditions such as oxygen-free atmosphere, and temperatures which cannot exceed the decomposition point of the alkali metal fluoborate.

Case hardening of metals by impregnation of metal surfaces with boron has also been carried out using a boronizing bath composed of one or more borates. In such a system the boron is either introduced into the bath as elemental boron or a metal is introduced into the bath e.g., calcium, which will in situ liberate boron from the borate base material in the bath. In the latter case the boron liberating metal is introduced into the bath in ignot form under an inert atmosphere.

It has now been discovered that a uniform, adherent, corrosion resistant boride coating can be produced by a simple process using a fused bath containing a boron salt of the empirical formula M,,B,,F wherein M is an alkali metal, B stands for boron, and F is the fluoride ion, which does not use any externally supplied electric current and is carried out under normal atmospheric conditions and temperatures at least as low as those previously employed in the prior art.

Therefore, one aspect of the present invention is to provide a relatively inexpensive, simple and effective diffusion process for boronizing a metal article using a fused salt bath containing a boron salt of the empirical formula M,B,,F, which does not require externally supplied electric current or special equipment or process conditions such as a boron anode, an inert atmosphere, highly purified chemicals and a specially designed electric cell.

Another aspect of the present invention relates to novel boron salts of the empirical formula M B F in which the ratio ofx:y:z is 1:04 to 220.5 to 2.5 which are employed in a fused salt bath containing at least one alkali metal halide and/or alkaline-earth halide to produce a boronized metal article.

Yet another aspect of the present invention relates to boronized metal articles such as carbon and alloy steels having a uniform, adherent, tough, corrosion resistant coating and a hardness comparable to tungsten carbide.

A further aspect of the present invention relates to novel fused salt bath compositions for use in casehardening metal articles.

An additional aspect of the invention relates to processes of producing the novel boron salts of the empiri-' cal formula M,,,B,,F' wherein the ratio of x:y: z is 110.4 to 2:0.5 to 2.5. a 7

These and other aspects of the present invention will become apparent from the following description:

Unexpectedly, it has now been discovered that a uniform, non-porous, adherent, corrosion resistant coating can be formed on specific metals by a diffusion process employing a fused salt bath wherein an electrical circuit is formed through an electrical connection, which is external to the salt bath, between the pot containing the salt bath and the metal sample holder which holds the sample to be borided. This process produces boronized metal articles of extremely high hardness which is at least equal to and in many cases substantially greater than the hardness heretofore attainable in borided metals obtained in accordance with prior art processes.

As used herein a boride case means any solid solution or alloy of boron and metal regardless of whether the metal forms an intermetallic compound with boron (e.g., FeB or Fe B in a boronized ferrous base alloy) in stoichiometric proportions which can be represented by a chemical formula. The expression boronized metal means a metal article in which boron has been diffused into the core of the metal without forming a substantial overlying boron coating on the surface of the metal article.

The boronizing process of my invention may be carried out in a stainless steel or silicon carbide pot. An electric circuit is formed external to the fused salt bath by using a conductor to join the pot containing the salt bath to the metal holder from which the sample is suspended into the bath. The salt bath contains at least one alkali metal halide and/or alkaline earth halide in addition to the selected novel boron salt. The boron products produced by the present invention have not been definitely proven to be chemical compounds as contrasted with mixtures and therefore will be referred to as chemical products. The novel boron products of this invention conform to the empirical formula M B F in which M is an alkali metal e.g., sodium, lithium, potassium, etc., B is boron, F is a fluoride ion and the ratio of x:y:z is 120.4 to 2:05 to 2.5. The preferred products used in the fused salt bath have the empirical formula M B F wherein M is an alkali metal and the ratio of x:y:z is 1:1 to 210.5 to 1.5.

These novel boron salts may be produced by reducing an alkali metal fluoborate with a reducing agent such as amorphous boron. Other reducing agents such as oxalic acid, sodium borohydride, metallic alkali metals, e.g., sodium, potassium, etc., mixtures of amorphous boron with metallic sodium, etc., can be employed suitably for reducing the alkali metal fluoborate. The foregoing process is carried out just above the melting point of the alkali metal fluoborate that is being used.

While the alkali metal boron salts of the empirical formula M B F are the more desirable boron salts, it is alsopossible to use the corresponding alkaline earth salts in a fused salt bath to boronize a metal. The drawback to using alkaline earth boron salts in place of al-' kali metal boron salts, particularly when treating ferrous alloys is that the alkaline earth boron salts have substantially higher melting and decomposition points than the corresponding alkali metals. Hence, in boronizing steel the required operating temperatures would damage the ferrous alloy causing warpage, distortion, etc. which is obviously undesirable. However, in boronizing metals such as tungsten, molybdenum and other metals which will not be damaged at thehigh temperature required for melting the alkaline earth boron salts in the fused bath such salts can be employed to good advantage.

The aforedescribed boron salts are employed in accordance with the invention in a fused salt bath composed of the selected boron salt and atleast one alkali metal halide and/or alkaline earth halide. Since it is desirable to use as low a temperature as practical to avoid damaging or distorting the metal article to be borided,

a mixture of at least two alkali metal halides is preferred to provide a salt bath having lower fusion temperatures than the individual components. As used herein alkali metals and alkaline earth metals mean those metals set forth in the periodic chart of elements which fall within these two classifications.

It has been found that a good, adherent and tough borided metal article is obtained if the salt bath contains from about 10 to 40 percent by weight of the selected boron salt M B F preferably from 20 to 35 percent by weight. The predominant portion of the bath is made up of alkali metal and/or alkaline earth metal halides. The selected boron salt comprises about 10 to about 35 percent by weight of the alkali metal halide and/or alkaline earth halide salts present in the bath. Preferably, alkali metal halides are used in the bath because they provide salt baths with lower fusion temperatures. As previously indicated, at least two alkali metal halide salts are preferably used, with one of these salts comprising preferably at least about percent by weight of the salt bath. However, satisfactory results may also be obtained where no one alkali metal halide salt comprises at least 50 percent by weight of the bath composition.

The selection of the particular combination of alkali metal halide salts for use in the salt bath is not critical. However, it has been found that optimum results are obtained when the cation portion of the predominant alkali metal halide or alkaline earth halide salt in the bath is the same as the cation portion of the boron salt in the bath. Results are further optimized if the cation portion of one of the alkali metal halides in the bath is of the next lower period, in the periodic table than the other alkali metal halide. Thus, if the one alkali metal halide is KCl, the other alkali metal halide is preferably NaCl. l

The anion portion of the boron salt in the fused bath may be the same as or different from the anion portion of the alkali metal halide salts in the bath. Best results are obtained if the anion portion of the alkali metal or alkaline earth metal salts are chlorides, because this reduces the corrosive effects of the salt bath.

The operating temperature of the fused salt bath is determined by numerous factors including the nature and characteristics of the metalto be borided and the melting temperature of the bath. In general, a bath formed in accordance with the present invention is useful for boronizing at any temperature between its melting point and its boiling point provided that the selected metal to be boronized or the case obtained, is not damaged at the selected temperature. In order'to, produce a reasonably fast boronizing rate and achieve adequate penetration of the boron into the selected metal immersed in the fused salt bath, it is desirable to maintain the salt bath at a temperature no lower than about 1250F. While lower temperatures may be used, it has been found that the degree of penetration of the boride case and the thickness of the case are adversely afiected because the fluidity of the bath is substantially reduced. Best results are obtained when the fused salt bath is highly fluid. For carbon and alloy steels the bath temperature is maintained between 1250 andl700F during the boronizing process, the higher end of the temperature range being used for stabilized steels and the lower end of the temperature range for mild steels.

Temperatures should be avoided which will result in annealing of the product and which will damage or distort the metal article to be borided. With other metals to be case hardened such as tungsten, tantalum, niobium, molybdenum, etc., bath temperatures up to 2000F could be used.

It has been found that the thickness of the boride case and the degree of penetration is a function of immersion time in the bath, temperature of the salt bath, boron content of the bath, and the properties of the metal article to be treated. A good boride case has been obtained after about two hours immersion of a low carbon steel in the fused salt bath. Obviously the rate of boron diffusion is not the same for every metal. Hence, the treatment conditions necessary to obtain the desired case thickness and penetration may vary from one metal to another. With regard to ferrous alloys it has been found that the degree of penetration is higher in low carbon steels than in high alloy steels. The preferred time of immersion in the boronizing bath is variable and depends upon a number of factors including the thickness of the case desired, the bath temperature and the characteristics of the metal article being boronized.

The current density generated by the external electrical connection between the pot and the specimen holder from which the specimen is immersed in the salt bath varies with the temperature of the bath, the specimen size and the percent boron in the product of the empirical formula M B F Usually the current density lies between about 50 microamperes at l200F and 300 microamperes at about l600F using a rod three inches long and having a diameter of 0.5 inch.

The process of this invention has produced boride cases having a thickness of at least 2 mils and thicknesses over mils have been obtained. It has been observed that the original dimensions of the treated metal article does not change substantially.

The boronizing process of my invention is applicable to a large number of metals. Excellent case hardened products have been obtained with ferrous alloys such as low carbon steels and high alloy steels. With steels having a high carbon content, such as in 1080 steel, the penetration is somewhat less due to the higher carbon content. Other metals which may be borided by my process are those into which boron will diffuse such as metals having atomic numbers 22-28 inclusive, 41-46 inclusive, and 73-78 inclusive. This range of atomic numbers includes those metals included inthe periodic chart of the elements shown on pages 56 and 57 of Langes Handbook of Chemistry, 9th edition, l-landbook Publishers, Inc, Sandusky, Ohio, 1956, such as the group VB metals, which are'vanadium, niobium and tantalum, group VlB metals, which are chromium, molybdenum and tungsten, group VIIB metals such as manganese and group Vlll metals such as iron, cobalt and nickel. Alloys-of these metals with each other, or alloys containing these metals as the major constituent, i.e., over 50 mole percent but usually over 75 mole percent and preferably at least 90 mole percent, alloyed with other metals as a minor constituent, i.e., less than 50 mole percent but usually less than 25 mole percent and preferably less than 10 mole percent, can also be borided by my process, providing the melting point of the resulting alloy is not less than 1700F. The fact that other metals may be the minor constituents of an alloy with the metals with which this invention is concerned does not prevent the formation of the desired boride case. These minor constituents may be any of the other metals of the periodic system, i.e., the metals of groups lA, "A, "B, lIlA, lllB, IVA, IVB, VA and VIA. These metals have atomic numbers 3-4 inclusive, 11-13 inclusive, 19-21 inclusive, 30-33 inclusive, 37-40 inclusive, 48-51 inclusive, 55-72 inclusive, -84 inclusive and 87-98 inclusive.

After the boronized metal is removed from the salt bath it is cooled. The selected method of cooling is not critical. However, care should be exercised to select a method of cooling that does not adversely affect previously established characteristics of the boronized metal. Air cooling or using a quenching oil have been found most satisfactory, but other cooling cycles may be used as necessary to obtain the desired substrate qualities. After the boronized sample is cooled it is further treated by conventional techniques to remove residual salts that may be adhering to the metal surface.

The following examples are given by way of illustration and not by way of limitation. It is readily apparent that variations from the specific reaction conditions and reactants may be made without departing from the scope of the invention.

EXAMPLE 1 To 330 grams of sodium fluoborate placed in a steel crucible there was added 22 grams of amorphous boron, the mixture was heated just above the melting point of the sodium fluoborate which is 384C for approximately two hours. The crucible and its contents were cooled and the recovered product was foundto weigh 248 grams. The product removed was a cake-like solid mass which was pulverized into a powder-like material.

A suitable sample of the solid material was analyzed by analytical chemical techniques and identified as having the empirical formula Na B F wherein the ratio of x:y:z is l:1.22:1.33.

EXAMPLE 2 To 330 grams sodium fluoborate placed in a steel crucible there was added 11 grams amorphous boron, the mixture was heated just above the melting point of the sodium fluoborate (i.e., 384) for approximately two hours. The recovered product was found to weigh 262 grams and upon analysis was indicated to have the empirical formula Na,,B,,F wherein the ratio of x:y:z is 1:0.83: 1.1.

EXAMPLE 3 The process of the Example 1 was repeated except that 44 grams of amorphous boron was used. The resulting product had the empirical formula Na B F wherein the ratio of x:y:z is l:l.7:0.95.

EXAMPLE 4 To 330 grams sodium fluoborate in a steel crucible were added 46 grams metallic sodium beads and 22 grams of amorphous boron. The mixture was heated for two hours at a temperature above the melting point of sodium fluoborate, after which the crucible was cooled and the material removed. Upon analysis this material had the empirical formula Na B F, wherein the ratio of x:y:z is 1:0.8:l.8.

EXAMPLE To 378 grams potassium fluoborate was added 22 grams amorphous boron, the materials being well mixed. The mixture was heated to a temperature above 530C, the melting point of potassium fluoborate, for approximately two hours. The recovered product was found to weigh 312 grams. This product was analyzed chemically and found to have the empirical formula K B,,F, wherein the ratio of x:y:z is l:l.16:2.42.

EXAMPLE 6 To 281 grams lithium fluoborate was added 43.6 grams amorphous boron, these materials being well mixed. The mixture was heated to a temperature above 300C for approximately two hours. The crucible and its contents were cooled and the recovered product found to weigh 170 grams. This product was analyzed chemically and found to have the empirical formula Li B F wherein the ratio of x:y:z is l:l.9:0.5.

EXAMPLE 7 A product having the empirical formula Na,,B,,F in which the ratio of x:y.'z is l:0.4:l.3 was produced by heating one mole sodium fluoborate with one mole of oxalic acid, in a steel crucible just above the melting point of the oxalic acid (i.e., 101C) for six hours driving off carbon dioxide, water and hydrofluoric acid.

EXAMPLE 8 A product having the empirical formula Na B F in which the ratio of x:y:z is about l:0.9:1.2 was produced as described in Example 4 by mixing 330 grams sodium fluoborate, 115 grams metallic sodium and 66 grams amorphous boron.

EXAMPLE 9 Into a steel vessel there was placed 1000 grams sodium chloride, 500 grams lithium chloride and 400 grams of the product of the empirical formula Na B F produced in Example 1. These materials were heated to a temperature of about 1400F to produce a clear fluid fused salt bath. In this bath there was suspended from stainless steel wire a rod (0.5 inch diameter X 3 inches long) of 1018 steel having a carbon content of 0.17 percent. The metal wire holder from which the rod is suspended was joined by a conductor to the pot handle thereby forming an electric circuit externalto the fused salt bath. This circuit operates as a selfgenerating cell. The rod remained immersed in the bath for about six hours, and was then removed from the bath and permitted to air cool.

The sample had a hard, uniform, smooth, adherent boride layer which proved to be 3 mil thick upon photomicrograph examination. The microhardness was measured in accordance with the Knoop hardness test using a 100 gram load and found to be about 2000. This boride case is as hard as tungsten carbide and much harder than typical nitrided or carburized cases (300m 1000 Knoop value). Dimensional analysis showed that after immersion in the bath the original thickness of the rod changed less than 1 mil. 7

Some of the boride case was removed and examined by X-ray diffraction analysis. Substantial amounts of Fe B and FeB were detected.

The boron content of the case of this example was EXAMPLE 10 Into a stainless steel vessel there was placed 750,

grams sodium chloride, 750 grams lithium chloride and 400 grams of the product of the empirical formula Na,,B,,lproduced in Example 8. These materials were heated to a temperature of about 1600F In this bath there was immersed a rod (0.5 inch diameter X 3 inches long) of 1018 steel having a carbon content of 0.17 percent. An external electrical connection was made as previously described. The rod remained in the bath for 8 hours and was removed and oil quenched.

The sample had a hard, uniform, smooth adherent boride layer which proved to be 6 mils thick upon photomicrograph examination. The microhardness was measured in accordance with the Knoop hardness test using a gram load and found to be about 2200. Dimensional analysis showed that after immersion in the bath the original thickness of the rod changed less than 2 mils.

EXAMPLE 1 1 Into a stainless steel vessel there was placed 720 grams of calcium chloride, 465 grams of barium chloride, and 400 grams of the product having the empirical formula NaxByFz produced in Example 1. The bath was heated to about l600F at which point it was molten but carried a soft opaque crust of insoluble constituents. In this bath was immersed a rod one-half inch diameter and three inches long of 1018 steel. An external electrical connection was made as previously de-' scribed. The rod was allowed to remain immersed for about 6 hours, was then removed from the bath and oil quenched. The specimen had a hard borided layer which proved to be over 3 mils thick upon photomicrograph examination and the Knoop valuewas found to be about 2100 using a 100 gram load. The case was not as uniform as that obtained in Example 9 or Example 10.

Samples of 1018 steel immersed for longer periods of time, and for various periods of time at higher temperatures, up to 1700F in the same bath, exhibited deeper cases but no improvement in uniformity.

EXAMPLE 12 at 1600F for 8 hours. The nut was then removed from the salt bath, air cooled, split, mounted and examined metallographically. A hard, uniform hard borided case had been formed on the surface of the nut, including the roots, faces and crowns of the threads. Variations in the case thickness between the thread crowns and thread roots was less than 10 percent, a substantial improvement over electroplating of similar surfaces. The

thickness of the case was measured to be 3.5 mils average and the hardness varied from 2270 to 2470 Knoop, taken with a l gram load.

EXAMPLE l3 In the salt bath described above in Example 9, heated to 1500F, there was placed a one-half inch by three inch long bar of l 137 steel. After 8 hours treatment in the salt bath, te specimen was removed, air cooled, sectioned and examined metallographically. A hard, smooth uniform borided case was formed on the surface of the specimen, averaging 4 mils thick and having a hardness of over 1900 Knoop, taken with a 100 gram load.

EXAMPLE 14 In the salt bath described above in Example 9 there were placed 6 rods of4l40 steel, each having a diameter of one-half inch and length of three inches. The salt bath was maintained at 1600F throughout the length of the experiment. The 6 rods were singly withdrawn from the salt bath and allowed to air cool at two-hour intervals; e.g., the first was withdrawn after two hours in the salt bath, the second after four hours, etc., and the sixth after twelve hours immersion in the bath. Each sample was then sectioned and examined metallo graphically. A hard smooth uniform borided case had formed on each rod, of nearly equal hardness but with thickness of case varying according to the following table:

The hardness of the cases on these samples varied from 1900 to 2100 Knoop, taken with a 100 gram load EXAMPLE l Into the salt bath described above in Example 9, there was placed a rod of 1045 steel, having a diameter of one-half inch and length of three inches. The salt bath was maintained at a temperature of l400F for 8 hours, after which the 1045 rod was removed and air cooled. The bath was then heated to l500F and a second rod of equal size and shape of 1045 steel was immersed in the salt bath for 8 hours, removed and air cooled. This was repeated, using one-half inch by three inch rods of 1045 steel for 8 hour treatments, at l550F, l600F, l650F and 1700F.- All rods were then sectioned and examined metallographically. Each had a hard, smooth, uniform borided case of nearly equal hardness 1900 to 2100 Knoop, as taken with a lOO gram load. The thickness of the cases on the rods varied according to the following table:

Tempeature "F. Case Thickness (Mils) EXAMPLE l6 Into the bath described above in Example 9, there was placed a 0.040 inch diameter molybdenum wire 6 inches long. The salt bath was held at l700F for 12 hours. The wire was then withdrawn from the bath, air cooled, sectioned into several one-quarter inch lengths and examined metallographically. A very hard case had formed on the wire and was diffused 2 mils into the wire substrate. This case was 3 mils thick, had a hardness of 2940 Knoop, taken with a 100 gram load.

EXAMPLE 17 Into the bath described above in Example 10, there was placed a Vs inch diameter tungsten rod. The salt bath was held at l700F for 4 hours. The rod was then withdrawn from the bath, air cooled, sectioned into several lengths and examined metallographically. A very hard case had formed on the rod which was 1.5 mils thick and had a hardness value of 4200 Knoop, taken with a 100 gram load.

EXAMPLE 18 I obtained having a thickness of 1 mi] and a Knoop hardness value above 4200 using a gram load.

The above examplesare illustrative of the preferred embodiments of the invention. However, other modifications can be made without departing from the scope of the present invention. It is therefore to be understood that changes may be made in the embodiment of the invention which are within the full intended scope of the invention as defined by the appended claims.

' What is claimed is:

l. A method of boriding a metal composition, said method comprising immersing said metal composition in a fused salt bath composition maintained at a temperature above about 1200F and below the melting point of said metal composition, and forming an electrical circuit which comprises said metal composition, said fused salt bath and the container for said salt bath by making an electrical connection external to said salt bath between said container and said metal composition, said metal composition being immersed in said salt bath composition until the desired thickness of boride coating has formed said salt bath composition comprising (a) at least one halide salt selected from the class consisting of alkali metal halides and alkaline earth halides and (b) the product of the reduction of an alkali metal fluoborate with a reducing agent.

2. A method according to claim 1 wherein said reducing agent comprises at least amorphous boron.

3. A method according to claim 2 wherein all of the electrical energy required to boride said metal composition is self-generated in said electrical circuit.

4. A method according to claim 3 wherein said halide salt comprises two different alkali metal chlorides.

5. A method according to claim 4 wherein said metal composition is a member selected from the class consisting of a carbon steel, an alloy steel, molybdenum, tungsten, tantalum and titanium.

6. A method according to claim 5 wherein said alkali metal chlorides comprise at least 50 percent by weight of said fused salt bath.

7. A method according to claim 6 wherein said alkali metal chlorides are sodium chloride and lithium chloride.

8. A method of boriding a metal composition, said method comprising immersing said metal composition in a fused salt bath composition maintained at a temperature about l200F and below the melting point of said metal composition, and forming an electrical circuit which comprises said metal composition, said fused salt bath and the container for said salt bath by electrically connecting said container and said metal composition external to said salt bath, said metal composition being immersed in said salt bath composition until the desired thickness of boride coating has formed, said fused salt composition comprising (a) a product of the empirical formula M B F, wherein M is an alkali metal, B is boron, F is the fluoride ion and wherein the ratio ofx:y:z is 120.4 to 2:05 to 2.5 and (b) at least one halide salt selected from the class consisting of alkali metal halides and alkaline earth metal halides, said halide salts comprising at least 50 percent by weight of said fused salt bath composition.

9. A method according to claim 8 wherein said halide salts comprise two different alkali metal chlorides.

10. A method according to claim 9 wherein said product of the empirical formula M B F comprises between 20 and about 35 percent by weight of said salt bath.

11. A method according to claim 10 wherein said alkali metal chlorides are sodium chloride and lithium chloride.

12. A method according to claim 11 wherein M is sodium.

13. A method according to claim 10 wherein the a ratio of xzyzz in said empirical formula is 1:1 to 2:0.5 to

14. A method according to claim 10 wherein said metal composition is a material selected from the class consisting of carbon steels, alloy steels, tungsten, tantalum, titanium and molybdenum.

15. A method according to claim 14 wherein all of the electrical energy required to boride said metal composition is self-generated in said electrical circuit.

16. A process according to claim 8 wherein said salt bath is maintained at a temperature between about

Claims

2. A method according to claim 1 wherein said reducing agent comprises at least amorphous boron.
3. A method according to claim 2 wherein all of the electrical energy required to boride said metal composition is self-generated in said electrical circuit.
4. A method according to claim 3 wherein said halide salt comprises two different alkali metal chlorides.
5. A method according to claim 4 wherein said metal composition is a member selected from the class consisting of a carbon steel, an alloy steel, molybdenum, tungsten, tantalum and titanium.
6. A method according to claim 5 wherein said alkali metal chlorides comprise at least 50 percent by weight of said fused salt bath.
7. A method according to claim 6 wherein said alkali metal chlorides are sodium chloride and lithium chloride.
8. A method of boriding a metal composition, said method comprising immersing said metal composition in a fused salt bath composition maintained at a temperature about 1200*F and below the melting point of said metal composition, and forming an electrical circuit which comprises said metal composition, said fused salt bath and the container for said salt bath by electrically connecting said container and said metal composition external to said salt bath, said metal composition being immersed in said salt bath composition until the desired thickness of boride coating has formed, said fused salt composition comprising (a) a product of the empirical formula MxByFz wherein M is an alkali metal, B is boron, F is the fluoride ion and wherein the ratio of x:y:z is 1:0.4 to 2:0.5 to 2.5 and (b) at least one halide salt selected from the class consisting of alkali metal halides and alkaline earth metal halides, said halide salts comprising at least 50 percent by weight of said fused salt bath composition.
9. A method according to claim 8 wherein said halide salts comprise two different alkali metal chlorides.
10. A method according to claim 9 wherein said product of the empirical formula MxByFz comprises between 20 and about 35 percent by weight of said salt bath.
11. A method according to claim 10 wherein said alkali metal chlorides are sodium chloride and lithium chloride.
12. A method according to claim 11 wherein M is sodium.
13. A method according to claim 10 wherein the ratio of x:y:z in said empirical formula is 1:1 to 2:0.5 to 1.5.
14. A method according to claim 10 wherein said metal composition is a material selected from the class consisting of carbon steels, alloy steels, tungsten, tantalum, titanium and molybdenum.
15. A method according to claim 14 wherein all of the electrical energy required to boride said metal composition is self-generated in said electrical circuit.
16. A process according to claim 8 wherein said salt bath is maintained at a temperature between about 1200*F and 1700*F.