CA1036976A

CA1036976A - Anodically dissolving group v-a element into molten borate bath

Info

Publication number: CA1036976A
Application number: CA170,304A
Authority: CA
Inventors: Masayoshi Mizutani; Noboru Komatsu; Tohru Arai; Yoshihiko Sugimoto
Original assignee: Toyota Central R&D Labs Inc
Current assignee: Toyota Central R&D Labs Inc
Priority date: 1972-05-04
Filing date: 1973-05-03
Publication date: 1978-08-22
Also published as: FR2183255A1; DE2322159A1; US3930060A; DE2322159C3; DE2322159B2; FR2183255B1; GB1382009A

Abstract

ABSTRACT OF THE DISCLOSURE:

A method for forming a carbide layer of an element of group V-a of the Periodic Table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a boric acid or borate up to its molten state to form a molten bath, dipping a metal block containing a V-a group element into the molten bath, applying an electric current to the molten bath through the metal block being used as anode for anodically dissolving at least part of metal block to supply at least 1% of the V-a group element into the molten bath for preparing the treating molten bath, and immersing the article containing at least 0.05% of carbon in the treating molten bath, thereby forming a very hard carbide layer of the V-a group element on the surface of the article. The method of this inven-tion can form a very smooth carbide layer on the surface of the article.

Description

~36~76 This invention relates to a method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article, and more particularly it relates to the formation of the carbide layer on the surface of the article immersed in a treating molten bath.
The iron, ferrous alloy or cemented carbide article with the carbide layer formed thereon has a greatly improved hardness, wear resistance and machinability.
There have been reported several kinds of methods for coating or forming a metallic carbide layer on the surface of metallic articles. We have developed a method for forming a carbide layer of a V-a group element on the surface of metallic article in a treating molten bath consisting of boric acid or a borate and a metal powder containing a V-a group element (Canadian Patent No. 935 074). The method can form a uniform carbide layer and is highly productive and cheap. The carbide of a V-a group element, such as vanadium carbide (VC), niobium carbide (Nb) and tantalum carbide (TaC) has a very high hardness ranging from Hv 2000 to Hv 3000. Therefore, the carbide layer formed represents a high value of hardness and a superior resistance performance against wear and is thus highly suitable for the surface treatment of moulds such as dies and punches, tools such as pinchers and screwdrivers, parts for many kinds of tooling machines, automobile parts to be subjected to wear.
Further, the carbide of a V-a group element is much harder and less reactive with iron or steel at a high temperature than the tungsten carbide forming cemented carbide is. Therefore, the formation of the carbide layer of a V-a group element on the surface of a cutting tool composed of cemented carbide increases ; 30 greatly the durability of the tool.
The method mentioned above, however, has a drawback.
The method uses a treating molten bath containing metal particles.

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Said metal particles nee~ relati~ely long time to dissolve into the bath, and undissolved metal particles happen to deposite into the carbide layer formed and make the surface of the artlcle treated rough.
Therefore, it is an object of this invention to provide an improved method for forming a carbide layer of a V~a group element on the surface of an iron, ferrous alloy or cemented carbide article, with denseness and uniformity and without any undissolved treating metal particles on the surface of the article.
Broadly, the present invention is directed to an improvement of the method for forming a carbide layer of an element of group V-a of the Periodic Table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath and is characterized in that the treating molten bath is prepared by anodically dissolving a V-a group element into molten boric acid or borate.
- More specifically, the method of the present invention comprises the steps of heating a boric acid or borate up to its 20 molten state to form a molten bath, dipping a metal block con-taining an element of group V-a of the Periodic Table into the molten bath, applying an electric current to the molten bath through the metal block being use as anode for anodically dissolving at least part of the metal block to supply at least 1%
of the V-a group element into the molten bath for preparing the treating molten bath, immersing the article containing at least 0.05% of carbon into the treating molten bath, keeping the article in the treating molten bath for forming the carbide layer of the V-a group element on the surface of the article, and taking the article out of the treating molten bath.

r~6 Upon intensive investigation of the mechanism for forming a layer on the surface of an article by diffusing a V-a group element from the treating molten bath composed of a boric acid or borate and a metal powder containing the V-a group element, it was found that the main source of the V-a group element forming the layer is the dissolved element in the treating molten bath rather than directly the solid metal particle undissolved. Namely, the V-a group element contained within the metal powder is dissolved into the treating molten both, and then said dissolved V-a group element reaches the surface of the article and diffuses into the article for forming the carbide layer with the carbon contained within the article. It is not necessary for the treating molten bath to contain the metal powder, and it is enough for the bath to contain a V-a group element dissolved therein.
To dissolve the V-a group element into the treating molten bath for forming the carbide layer having a smooth surface, it is proposed to dip a block of metal, without contacting the article to be treated, instead of the metal powder mentioned above.
However, the use of a block of metal instead of the metal powder 20 reduces greatly the whole surface area of the metal so that the ~-veloclty of the dissolution of the V-a group element decreases remarkably. The metal block may deteriorate by reacting with oxygen in the air before dissolving enough V-a group element into the treating molten bath. By experiments, we found that said deterioration of the metal occurs with a metal block having a diameter of 2 mm or larger than 2 mm. In order to prevent ` the deterioration of the metal, it is proposed to use a thin film of the metal instead of the metal block or to employ an inert gas atmosphere for covering the treating molten-bath and thereby preventing oxygen from being absorbed in the treating molten bath. However, as to the former case, said thin film of the metal is not easily obtained and the latter requires a compli-~3ti~76 cated and costly equipment and takes a long time to dlssolveenough V-a group element for the treatment.
We have overcome the shortcomings mentioned above by anodlcally dissolving a V-a group element from a large block of the metal containing the V-a group element. Namely, using said large metal block as an anode in the molten bath, sald boron oxide is electrolyzed by using the vessel of the bath as its cathode.
By the anodic dissolution of a V-a group element, the treating molten bath of the present invention is prepared. The carbide layer of a V-a group element can be formed by immersing an article in the treating molten bath. The formation of the carbide layer on the surface of an article may be carried out during the anodic dissolution of a V-a group element into the treating molten bath.
As the substances consisting the bath, boric acid (B2O3) or borate, such as sodium borate (borax) (Na2B4O7), potassium borate (K2B4O7) and the like and the mixture thereof can be used. The boric acid and borate have a function to dissolve a metallic oxide and to keep clean the surface of the article to be treated, and also the boric acid and borate are not poisonous and hardly vaporize. Therefore, the method of the present inven-tion can be carried out in the open air.
As the V-a group elements dissolved in the treating molten bath, one or more elements of vanadium (V), niobium (Nb) and tantalum (Ta) can be used, 1% by weight (hereinafter % means % by weight) of V-a group element in the treating molten bath being sufficient. In practice, however, the V-a group element may be dissolved into the treating molten bath in a quantity between 2 and 20%.
In order to dissolve the V-a group elements, a block of the metal of a V-a group element or a block of the alloy containing a V-a group element can be used. As the alloy the ~$ - 4 -1(~3~97~
ferrous alloy of a V-a group element may be practical because it is relatively cheap and easily obtained. It is not desirable for said alloys to contain more than 10~ of Ti, Zr, Hf, Mn, Si, Al, Mg, Ca or the rare earth element which reduce the boric acid or borate to metallic boron. In order to lower the viscosity of the treating molten bath, said such as chloride and fluoride of alkali metal can be added into the treating molten bath.
During the anodic dissolution of the V-a group elements, the practical current density may be selected within a range from 0.2 to 5A/cm2, the increase of the current density makes the necessary time short for dissolving a certain amount of the V-a group element in the treating molten bath. The anodic dissolution of the V-a group elements can be carried out at a relatively low voltage at which the actual electrolysis of the boric acid or borate does not occur. Although the necessary time for the anodic dissolution of the V-a group elements depends upon the current density, the volume of the treating molten bath, the size of the block as the anode and the compounds included in said block, the -practical time of the anodic dissolution may be from 30 minutes to 5 hours.
The iron, ferrous alloy or cemented carbide to be treated must contain at least 0.05% of carbon, preferably contain 0.1% of carbon or higher. The carbon contained in the article forms a component of the composition of the carbide during the treatment. Namely it is supposed that the carbon in the article diffuses to the surface thereof and reacts with the V-a group element from the treating molten bath to form the carbide on the surface of the article. The higher content of the carbon in the article is more preferable for forming the carbide layer. The iron, ferrous alloy or cemented carbide article containing less than 0.05% of carbon may not be formed with a uniform and thick carbide layer by the treatment. Also, the article containing at ;~

least 0.05% of carbon only in the surface portion thereof can be treated to form a carbide layer on the surface of the article.
For example, a pure iron article, which is case-hardened to increase the carbon content in the surface portion thereof, can be used as the article of the present invention.
Here, iron means iron containing carbon and case hardened iron, ferrous alloy means carbon steel and alloy steel, and cemented carbide means a sintered tungsten carbide containing cobalt. Said cemented carbide may include a small amount of titanium carbide, niobium carbide, tantalum carbide and the like.
In some cases, the carbon contained in the treating molten bath can be used as the source of the carbon for forming the carbide layer on the surface of the article. However, the formation of the carbide layer is not steady and the use of the carbon in the treating molten bath is not practical.
Before the treatment, it is important to purify the surface of the article for forming a good carbide layer by washing out the rust and oil from the surface of the article with acidic aqueous solution or another liquid.
The treating temperature may be selected within the wide range from the melting point of boric acid or borate to the melting point of the article to be treated. Preferably, the treating temperature may be selected within the range from 800 to 1100C. With lowering of the treating temperature, the vis-cosity of the treating molten bath increases gradually and the thickness of the carbide layer formed decreases. However, at a relatively high treating temperature, the treating molten bath is worsened rapidly. Also the quality of the material forming the article is worsened by increasing the crystal grain sizes of said material.
The treating time depends upon the thickness of the carbide layer to be formed. Heating shorter than 10 minutes will, however, provide no practically accepted formation of said layer, although the final determination of the treating time depends on the treating temperature. With the increase of the treating time, the thickness of the carbide layer will be increased correspondingly. In practice, an acceptable thickness of the layer can be realized within 30 hours or shorter time. The preferred range of the treating time will be from 1 to 30 hours.
The vessel for keeping the treating molten bath of the present invention can be made of graphite or heat resistant steel.
It is not necessary to carry out the method of the present invention in the atmosphere of non-oxidation gas, but the method can be carried out into effect either under the air atmosphere or the inert gas atmosphere.
Preferred embodiments of the invention will now be described with reference to the following working examples and the accompanying drawings, wherein:
Fig. 1 is a photomicrograph showing a vanadium carbide layer formed on the surface of a carbon tool steel according to Example l;
Fig. 2 is a photomicrograph showing a niobium carbide layer formed on the surface of a carbon tool steel according to Example 2;
Fig. 3 is an X-ray diffraction chart of the layer formed on the surface of a cemented carbide according to Example 3.

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1()~0 ~ S of bora~ w~s introduced into a graphite crucible ancl h~ated up to 900C ~or mel-ting the borax in an elec-tric furnace under the air, and then a me-tallic plate, 6 x 40 60 mm, made of ferro-vanadiu3n (containing 53.7~ of vanadium) was dipped in the middle of the molten borax. With use of the metallic plate ~3d the cr~cible as an anode and cathode respect-ively, said metallic plate was anodically dissolved into the mol-ten borax by applying a direct current for 2 hours a-t an elec-tric current deDsity of 2A/cm2 of the anode. Thus a treating molten bath containing 9~8a/o o~ said ferro-vanadium was prepared.
Next, a polished specimen of 7 m3m diameter and made of carbon -tool steel (JIS SK4, containing 1.0% of carbon) was im-mersed into the treating molten bath and kept therein for 2 hours, taken out therefrom and air cooled. ~he treating material -adhered to the surface of the specimen was removed by washing with ho-t water and then the specimen trea-ted was investigated.
The sur~ace of the specimen was very smooth. After cutting and polishing -the speci~en, the specimen was micrographically ob-serbed, and it was found that a uniform layer sho~m in Fig. ; was formed. ~he thic~ness of the layer was abou-t 7 microns. And the layer was identi~ied to be vanadium carbide (VC) by X ray dif~raction method and by an ~-ray microanalyzer.
~XAMP~E 2:
I~ thc same manner as described in ~xample 1, a metal-lic plate, ~ x 40 x 50 m~, made of ferro-niobium (containing 58.9% of niobium and 3.6,b of tantalum) was anodically dissolved, thus a treating molten bath was prepared.
~ext, a polished specimen made o~ carbon tool steel (JIS SX~) was immersed into the treating molten bath and kep therei~3 for 2 hour~3. ~y -the treatment, a uniIorm layer of 9 mi-cron thick, which is ~imilar with tna-t ~ormed in ~xample 1 was ~V3f~9~

formed (shown in Fig. 2). The layer was identified to be niobium (NbC) containing a small amount of tantalum by X-ray diffraction method and by an X-ray microanalyzer.
EXAMPLE 3:
500 grams of borax was introduced into a graphite crucible and heated up to 1000C for melting the borax in an elec-tric furnace, and then an electrolytic niobium plate, 40 x 35 x 4 mm, was anodically dissolved into -the molten borax by applying a direct current for 2 hours at an electric current density of lA/cm2 of the surface of the anode. By calculating the loss of the weight of the plate, a treating molten bath containing about 9.4% of niobium dissolved was prepared.
Next, a specimen, 1 mm thick, 5.5 mm wide and 30 mm long, made of cemented carbide composed of 91% of tungsten car-bide and 9~ of cobalt was immersed into the treating molten bath and kept therein for 14 hours, taken out there from and air-cooled. Treating material adhering to the surface of the specimen was removed by dipping the specimen into hot water. The surface of the specimen treated was smooth. After cutting and polishing the specimen, the cross section of the specimen was micrographi-cally observedand tested by X-ray diffraction method and by X-ray microanalyzer. By the observation, a layer having a uniform and dense structure and a 26 micron thickness was found. By X-ray diffraction method, strong niobium carbide (VC) diffraction lines were detected in the layer. In Fig. 3, the chart of the X-ray diffraction is shown. By X-ray microanalyzer, the layer was found to contain a large amount of niobium. The hardness of the layer measured from the surface of the specimen was about Hv (Micro Vickers Hardness): 2888. Also the hardness of the mother material of the specimen was measured to be about Hv 1525.
EXAMPLE 4:

In the same manner as described in Example 3, an elec-~?~ g ,.~ .

trolytic tantalum plate, 50 x ~0 x 4 mm, was annodically dis-solved for 1 hour at1000C at an electric current density of 1A/cm2. Thus, a treating molten bath containing about 11,2 of tantalum dis~olved was preparedJ
~ ext, a specimen having the same size and made of the same material as the specimen used in Example ~ was immersed into the treating molten bath and kept -therein for 16 hours. ~y the treatment, a uniform and dense layer of 15 micron thick was formed. The layer wa~ identified to be tantalum carbide (TaC) by X-ray diffraction method. ~he hardness of the layer was about Hv 1720.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for forming a carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising the steps of heating a boric acid or borate up to its molten state to form a molten bath, dipping a metal block containing an element of group V-a of the Periodic Table into said molten bath, applying an electric current to the molten bath through said metal block being used as anode for anodically dissolving at least part of said metal block to supply at least 1% of the V-a group element into the molten bath for preparing the treating molten bath, immersing the article containing at least 0.05% of carbon into said treating molten bath, keeping said article in said treating molten bath for forming the carbide layer of the V-a group element on the surface of said article, and taking said article out of said treating molten bath.

2. A method according to claim 1, wherein said borate is selected from the group consisting of sodium borate and potassium borate.

3. A method according to claim 1, wherein said metal block is one selected from the group consisting of metallic vanadium, niobium and tantalum.

4. A method according to claim 1, wherein said metal block is a metallic alloy.

5. A method according to claim 4, wherein said metallic alloy is a ferrous alloy.

6. A method according to claim 1, wherein said ferrous alloy article is made of one selected from the group consisting of carbon steel and alloy steel containing at least 0.05% of carbon.

7. A method according to claim 1, wherein said cemented carbide article is made of a sintered tungsten carbide containing cobalt.

8. A method according to claim 1, wherein said treating molten bath contains between 2 and 20% by weight of the V-a group element dissolved.

9. A method according to claim 1, wherein the electric current density of the anode is selected in a range from 0.2 to 5 A/cm2 and said metal block dipped into the molten bath is ano-dically dissolved for a time ranging from 30 minutes to 5 hours.

10. A method according to claim 1, wherein a vessel containing the molten bath is used as the cathode during the step of applying the electric current.

11. A method according to claim 1, wherein during the step of keeping the article in the treating molten bath, the undissolved part of said metal block previously dipped into the treating molten bath is again subjected to anodic dissolution for supplying the V-a group elements into the treating molten bath.

12. A method according to claim 1, wherein said article is kept in the treating molten bath for 1 to 30 hours at a temperature ranging from 800 to 1100°C.

13. A method according to claim 1, wherein said treating molten bath contains chloride or fluoride of alkali metal for lowering the viscosity of the treating molten bath.