US3635768A - Molten salt carburizing process and replenishing salt for same - Google Patents
Molten salt carburizing process and replenishing salt for same Download PDFInfo
- Publication number
- US3635768A US3635768A US820337A US3635768DA US3635768A US 3635768 A US3635768 A US 3635768A US 820337 A US820337 A US 820337A US 3635768D A US3635768D A US 3635768DA US 3635768 A US3635768 A US 3635768A
- Authority
- US
- United States
- Prior art keywords
- salt
- replenishing
- cyanide
- activator
- bath
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 150000003839 salts Chemical class 0.000 title abstract description 34
- 238000005255 carburizing Methods 0.000 title abstract description 13
- 238000000034 method Methods 0.000 title description 14
- 239000012190 activator Substances 0.000 abstract description 45
- XFXPMWWXUTWYJX-UHFFFAOYSA-N Cyanide Chemical compound N#[C-] XFXPMWWXUTWYJX-UHFFFAOYSA-N 0.000 abstract description 25
- 150000002825 nitriles Chemical class 0.000 abstract description 20
- 239000000463 material Substances 0.000 abstract description 9
- 239000000203 mixture Substances 0.000 abstract description 6
- 229910052751 metal Inorganic materials 0.000 abstract description 4
- 239000002184 metal Substances 0.000 abstract description 4
- 150000002739 metals Chemical class 0.000 abstract description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 16
- 229910052799 carbon Inorganic materials 0.000 description 16
- BVKZGUZCCUSVTD-UHFFFAOYSA-L Carbonate Chemical compound [O-]C([O-])=O BVKZGUZCCUSVTD-UHFFFAOYSA-L 0.000 description 12
- WDIHJSXYQDMJHN-UHFFFAOYSA-L barium chloride Chemical compound [Cl-].[Cl-].[Ba+2] WDIHJSXYQDMJHN-UHFFFAOYSA-L 0.000 description 5
- 229910001626 barium chloride Inorganic materials 0.000 description 5
- 238000010791 quenching Methods 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 150000003841 chloride salts Chemical class 0.000 description 4
- 238000000354 decomposition reaction Methods 0.000 description 4
- 230000006866 deterioration Effects 0.000 description 3
- 239000007788 liquid Substances 0.000 description 3
- 230000000171 quenching effect Effects 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 description 2
- KXZJHVJKXJLBKO-UHFFFAOYSA-N chembl1408157 Chemical compound N=1C2=CC=CC=C2C(C(=O)O)=CC=1C1=CC=C(O)C=C1 KXZJHVJKXJLBKO-UHFFFAOYSA-N 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 239000004615 ingredient Substances 0.000 description 2
- 238000003303 reheating Methods 0.000 description 2
- 230000000717 retained effect Effects 0.000 description 2
- 229910000831 Steel Inorganic materials 0.000 description 1
- 229910052784 alkaline earth metal Inorganic materials 0.000 description 1
- 229910052788 barium Inorganic materials 0.000 description 1
- DSAJWYNOEDNPEQ-UHFFFAOYSA-N barium atom Chemical compound [Ba] DSAJWYNOEDNPEQ-UHFFFAOYSA-N 0.000 description 1
- 150000004649 carbonic acid derivatives Chemical class 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 239000003085 diluting agent Substances 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 230000014759 maintenance of location Effects 0.000 description 1
- 235000015598 salt intake Nutrition 0.000 description 1
- 229910000029 sodium carbonate Inorganic materials 0.000 description 1
- 239000010959 steel Substances 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- CIOAGBVUUVVLOB-UHFFFAOYSA-N strontium atom Chemical compound [Sr] CIOAGBVUUVVLOB-UHFFFAOYSA-N 0.000 description 1
- 229910001631 strontium chloride Inorganic materials 0.000 description 1
- AHBGXTDRMVNFER-UHFFFAOYSA-L strontium dichloride Chemical compound [Cl-].[Cl-].[Sr+2] AHBGXTDRMVNFER-UHFFFAOYSA-L 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
- C23C8/42—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions only one element being applied
- C23C8/44—Carburising
- C23C8/46—Carburising of ferrous surfaces
Definitions
- the replenishing material is a mixture of activator and cyanide salt only, wherein the activator cyanide salt proportion is identical to that of the starting salt.
- This invention relates to the process of carburizing metals by means of molten cyanide baths containing activators.
- the invention relates more specifically to novel replenishing-salts and novel replenishing steps in the process as a whole.
- the present practice prior to the invention is for the workmen to add to the bath a desired quantity of cyanide salt from a stock of same, and add also a desired or specifiedquantity of the activator, also from a stock of the same.
- the salt for the latter baths contains both cyanide and activator, but also contains diluents such as alkaline chlorides or carbonates.
- the activator content of this premixed salt is not equal to the activator content to be maintained in the bath, so that with such baths it is not possible to produce and maintain over long periods, an exact predetermined surface carbon content.
- this invention airns in the direction of providing a more exact control of the bath and particularly of the replenishing step and materials, with the view of eliminating mistakes and errors as much as possible and to make the replenishing operation as exact and foolproof as possible.
- the first aspect of the improved replenishing method and material is that the replenishing salt is supplied by a single stock of material so that it was not necessary for the workmen to obtain replenishing materials from two stocks and to mix them.
- the first aspect of the novel replenishing salt is that it is a premix of the cyanide salt and the activator in predetermined proportions from which the workman merely draws as much as needed for replenishing.
- the single replenishing premix includes both the activator and the cyanide salt.
- the second aspect is based upon the knowledge that it is only the cyanide salt that is consumed by decomposition during carburizing. We are not speaking here of reduction of liquid level due to drag out losses but rather a change in the proportions of cyanide salt and activator that occurs during the use of the process.
- the replenishing salt of the invention is made up of the activator and cyanide'salt but the novel aspect is that the percentage of activator with respect to the cyanide salt is exactly the same as the percentages of these two ingredients at the start of the carburizing operation and before decomposition of the cyanide salt.
- the novel replenishing salt includes only cyanide salt and activator and these are in the proportions of the same two ingredients at the start of the operation. This novel salt then for replenishing, maintains the bath in such a state that it returns the bath into the state desired to establish the desired carbon potential.
- the bath At the end of the specific period of operation, let us say one day, the bathis found to have its cyanide content reduced to 7 percent and the carbonate content increased to 73 percent but the activator content is the same 20 percent.
- the constancy of carbon potential of a bath can be predetermined and maintained by insuring a con stant prefixed level of the activator
- the constancy of the activator can be maintained by using as a replenishing salt, a premix containing cyanide but whose activator content is exactly the same as the activator content originally desired for the bath.
- replenishing salt which is used to offset chemical deterioration.
- This replenishing salt would be without carbonate and would be 20 percent activator and 80 percent cyanide salt in the example given.
- ADVANTAGES A. Salt consumption was reduced over a long time by the use of a premix formula. There was no tendency to add excess salt either by error or inexperience or guesswork and personal variations and human errors did not affect the replenishing operation and consequently there was no tendency to use more salt than was actually required for replenishing.
- a molten cyanide salt carburizing process employing a bath of the molten cyanide salt and an activator of the group consisting of barium chloride and strontium chloride; and wherein the bath at starting of a first period of operation, has a predetermined ratio of activator to cyanide salt; and,
- the bath at the end of that first period of operation, has the same amount of activator, plus a reduced but satisfactory amount of cyanide and an amount of carbonate formed by decomposition of the cyanide;
- the bath at the end of a second period of operation has the same amount of activator, plus a further reduced and now unsatisfactory amount of cyanide and an increased amount of carbonate; and requires restoration to make up for the loss of cyanide in such second period of operation;
- the step of restoring the bath after the second period of operation by adding a restoration salt premix of the cyanide salt and the activator, wherein the activator-cyanide salt proportion is identical to that at the start of the first period of operation; and without carbonate.
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- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Cosmetics (AREA)
- Crystals, And After-Treatments Of Crystals (AREA)
- Chemically Coating (AREA)
Abstract
Carburizing of metals is effected by using molten cyanide baths containing activators. In the operation of such baths, due to consumption of the cyanide, it is necessary to add replenishing materials. The replenishing material is a mixture of activator and cyanide salt only, wherein the activator cyanide salt proportion is identical to that of the starting salt.
Description
United States Patent Mueller [4 Jan. 18, 1972 [54] MOLTEN SALT CARBURIZING PROCESS AND REPLENISHING SALT 211 App], No.: 820,337
[52] US. Cl ..148/15.5,148/15,148/20 [51] Int.Cl ..C23c 9/12 [58] Field oiSearch l48/l5, 15.5,20, 27, 28, 3O
[56] References Cited UNITED STATES PATENTS 1,970,852 8/1934 Hardingetal, ..l48/15.5
2,819,992 1/1958 Albrecht ..148/30 X 3,117,038 1/1964 Hewson et al. ..148/15.5
OTHER PUBLICATIONS Nletals Handbook, 1948 Ed. pages 692- 696 Molten Salt Baths for Heat Treatment and Case Hardening of Steel," published by E. 1. du Pont de Nemours &' Co. May 7,1943,pgs.6 8, 25,26, 31 & 33.
Primary Examiner-Char1es N. Lovell AttorneyCullen, Sloman & Cantor 57 1 ABSTRACT Carburizing of metals is efiected by using molten cyanide baths containing activators. 1n the operation of such baths, due to consumption of the cyanide, it is necessary to add replenishing materials. The replenishing material is a mixture of activator and cyanide salt only, wherein the activator cyanide salt proportion is identical to that of the starting salt.
1 Claims, N0 Drawings MOLTEN SALT CARBURIZING PROCESS AND REPLENISHING SALT FOR SAME BRIEF DESCRIPTION OF THE INVENTION This invention relates to the process of carburizing metals by means of molten cyanide baths containing activators. The invention relates more specifically to novel replenishing-salts and novel replenishing steps in the process as a whole.
THE BACKGROUND OF THE ART It is known to carburize metal workpieces by the use of molten cyanide baths containing activators. Known activators are alkali earth metal chlorides and chlorides known for such purposes are chlorides of barium and strontium.
It is also known to design a bath specification according to the desired carbon potential which is the point on the carbon content curve where the carbon content in relation to the depth of the carbon content from the surface is at a maximum.
It is also known that if a higher carbon potential is desired, a greater proportion of activator salt in the bath should be maintained.
During the operation of a bath, not only does the liquid level of the bath decrease due to drag out losses and other losses but also the cyanide proportion decreases as the cyanide breaks down in the carburizing process. Hence, it becomes necessary for the workmen practicing the process to add replenishing materials.
The present practice prior to the invention is for the workmen to add to the bath a desired quantity of cyanide salt from a stock of same, and add also a desired or specifiedquantity of the activator, also from a stock of the same. Great possibilities for variation have been observed and have plagued the industry and particularly the segment of the industry which was anxious to provide an exact control of the carbon potential and for this purpose to provide an exact control of the bath and particularly its replenishing steps.
Besides the method of replenishing two different types of salt, there are also known baths that are replenished with just one salt. The salt for the latter baths contains both cyanide and activator, but also contains diluents such as alkaline chlorides or carbonates. The activator content of this premixed salt is not equal to the activator content to be maintained in the bath, so that with such baths it is not possible to produce and maintain over long periods, an exact predetermined surface carbon content.
Therefore, this invention airns in the direction of providing a more exact control of the bath and particularly of the replenishing step and materials, with the view of eliminating mistakes and errors as much as possible and to make the replenishing operation as exact and foolproof as possible.
THE INVENTION First, in the investigation of the operation of the bath with a view to providing a better control for same came the discovery that the carbon potential of the bath is exclusively dependent on the amount of activator contained in the bath. The activator is not subject to decomposition during the operation of the bath. It is essential for this discovery that the activator content be controlled within extremely close limits to produce and consistently maintain over long periods a specified carbon potential.
Therefore, an effort was made to design a practical method of replenishing.
The first aspect of the improved replenishing method and material is that the replenishing salt is supplied by a single stock of material so that it was not necessary for the workmen to obtain replenishing materials from two stocks and to mix them. Hence, the first aspect of the novel replenishing salt is that it is a premix of the cyanide salt and the activator in predetermined proportions from which the workman merely draws as much as needed for replenishing.
The single replenishing premix includes both the activator and the cyanide salt.
The second aspect is based upon the knowledge that it is only the cyanide salt that is consumed by decomposition during carburizing. We are not speaking here of reduction of liquid level due to drag out losses but rather a change in the proportions of cyanide salt and activator that occurs during the use of the process. The replenishing salt of the invention is made up of the activator and cyanide'salt but the novel aspect is that the percentage of activator with respect to the cyanide salt is exactly the same as the percentages of these two ingredients at the start of the carburizing operation and before decomposition of the cyanide salt.
It is known that in carburizing, the cyanide breaks down and there is formed within the bath a quantity of carbonate. The activator does not break down and remains constant. The novel replenishing salt includes only cyanide salt and activator and these are in the proportions of the same two ingredients at the start of the operation. This novel salt then for replenishing, maintains the bath in such a state that it returns the bath into the state desired to establish the desired carbon potential.
For example, let us consider a bath which at a specific period of its operation, and long after a previous period of operation is of the content of 10 percent cyanide salt, 20 percent activator salt, and 70 percent carbonate. This bath is considered to have been designed as providing a carbon potential of 0.5 and for this purpose was set up as a bath of 20 percent activator and percent cyanide. During the long period of operation and just prior to the specific period of operation, the bath has changed to, as stated, 10 percent cyanide, 20 percent activator, and 70 percent carbonate.
At the end of the specific period of operation, let us say one day, the bathis found to have its cyanide content reduced to 7 percent and the carbonate content increased to 73 percent but the activator content is the same 20 percent.
Accordingly, for replenishing, I propose by this invention to use a replenishing salt of 20 parts activator and 80 percent cyanide and this returns the bath to its original condition but at all times maintains the bath at a not over 20 percent activator content.
While it is known that the constancy of carbon potential of a bath can be predetermined and maintained by insuring a con stant prefixed level of the activator, I consider as new the disclosure that despite the need for replenishing, the constancy of the activator can be maintained by using as a replenishing salt, a premix containing cyanide but whose activator content is exactly the same as the activator content originally desired for the bath.
REPLENISHING FOR CHEMICAL DETERIORATION If the cyanide content has dropped, I use as a replenishing mix a mixture of activator and cyanide salt only, with the activator percentage being the same as that of the starting salt.
REPLENISHING FOR VOLUME LOSS Considering a bath whose original content is such that the activator level was predetermined according to the desired carbon potential with the rest of the bath being cyanide and carbonate, when the liquid level of the bath has dropped below the desired level, replenishing is done here by a premix whose percentages and content are the same as that of the starting salt, namely activator plus cyanide plus carbonate. Thus, in the example given, replenishing for volume loss would be done by a premix of 20 percent activator, 10 percent cyanide salt, and 70 percent carbonate.
Contrast the foregoing with the replenishing salt which is used to offset chemical deterioration. This replenishing salt would be without carbonate and would be 20 percent activator and 80 percent cyanide salt in the example given.
FURTHER EXAMPLES I. In carburizing chains made up of links, I have used as a starting salt a mixture of 20 percent barium chloride plus 10 percent cyanide salt and 70 percent carbonate to insure a carbon potential of 0.5. The salt I use for replenishing then is a premix of 20 percent barium chloride and 80 percent cyanide.
2. In carburizing certain front wheel automobile drive components, such as drive shafts and universal joints, 1 have operated to'insure a carbon potential of 0.8. The starting mixture at the specific period of operation was set up as 35 parts barium chloride plus parts sodium cyanide and 55 parts sodium carbonate. The replenishing salt for chemical deterioration is a premix of 35 parts barium chloride and 65 parts sodium cyanide.
ADVANTAGES A. Salt consumption was reduced over a long time by the use of a premix formula. There was no tendency to add excess salt either by error or inexperience or guesswork and personal variations and human errors did not affect the replenishing operation and consequently there was no tendency to use more salt than was actually required for replenishing.
B. This replenishing program provided a highly satisfactory control for the level of retained Austenite. By using a controlled carbon potential, I enable the control of the amount of retained Austenite. Hence, using the process hereof it did not become necessary to harden by repeated quenching and heating as was formerly the case to insure a predetermined level of Austenite retention. Previously, because of uncertainties of the Austenite level, it was the practice to carburize and quench and thereafter to harden by reheating and quenching again. Because of the certainty provided by my invention, it is necessary now only to carburize and quench and there is no need for the additional steps of hardening by reheating and quenching.
CONCLUSION Having described and stated all that I know of the process and salts hereof at the present time, I wish it understood that all of the foregoing is to be interpreted as illustrative only rather than limited and accordingly, I refer to the claims which follow for a determination of the scope of the invention;
I claim: 1. In a molten cyanide salt carburizing process employing a bath of the molten cyanide salt and an activator of the group consisting of barium chloride and strontium chloride; and wherein the bath at starting of a first period of operation, has a predetermined ratio of activator to cyanide salt; and,
wherein the bath, at the end of that first period of operation, has the same amount of activator, plus a reduced but satisfactory amount of cyanide and an amount of carbonate formed by decomposition of the cyanide; and,
wherein the bath, at the end of a second period of operation has the same amount of activator, plus a further reduced and now unsatisfactory amount of cyanide and an increased amount of carbonate; and requires restoration to make up for the loss of cyanide in such second period of operation;
the step of restoring the bath after the second period of operation by adding a restoration salt premix of the cyanide salt and the activator, wherein the activator-cyanide salt proportion is identical to that at the start of the first period of operation; and without carbonate.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US82033769A | 1969-04-29 | 1969-04-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| US3635768A true US3635768A (en) | 1972-01-18 |
Family
ID=25230518
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| US820337A Expired - Lifetime US3635768A (en) | 1969-04-29 | 1969-04-29 | Molten salt carburizing process and replenishing salt for same |
Country Status (10)
| Country | Link |
|---|---|
| US (1) | US3635768A (en) |
| JP (1) | JPS5440493B1 (en) |
| AT (1) | AT300874B (en) |
| CA (1) | CA928621A (en) |
| CH (1) | CH534219A (en) |
| DE (1) | DE2018711C3 (en) |
| ES (1) | ES379136A1 (en) |
| FR (1) | FR2040405A1 (en) |
| GB (1) | GB1312982A (en) |
| SE (1) | SE358421B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3876512A (en) * | 1973-09-10 | 1975-04-08 | Nippon Furnace Koga Kaisha Ltd | Electrolytic carburizing process using a carbonate electrolyte |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US1970852A (en) * | 1931-08-24 | 1934-08-21 | Du Pont | Process and composition for treating metals |
| US2819992A (en) * | 1955-04-27 | 1958-01-14 | Degussa | Carburization of iron and steel |
| US3117038A (en) * | 1960-11-14 | 1964-01-07 | Ici Ltd | Fused salt baths-composition and method for carburising |
-
1969
- 1969-04-29 US US820337A patent/US3635768A/en not_active Expired - Lifetime
-
1970
- 1970-04-17 AT AT350670A patent/AT300874B/en active
- 1970-04-18 DE DE2018711A patent/DE2018711C3/en not_active Expired
- 1970-04-22 CH CH598670A patent/CH534219A/en not_active IP Right Cessation
- 1970-04-28 ES ES379136A patent/ES379136A1/en not_active Expired
- 1970-04-28 SE SE05889/70A patent/SE358421B/xx unknown
- 1970-04-29 GB GB2053170A patent/GB1312982A/en not_active Expired
- 1970-04-29 CA CA081411A patent/CA928621A/en not_active Expired
- 1970-04-29 FR FR7015745A patent/FR2040405A1/fr active Pending
- 1970-04-30 JP JP3713570A patent/JPS5440493B1/ja active Pending
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US1970852A (en) * | 1931-08-24 | 1934-08-21 | Du Pont | Process and composition for treating metals |
| US2819992A (en) * | 1955-04-27 | 1958-01-14 | Degussa | Carburization of iron and steel |
| US3117038A (en) * | 1960-11-14 | 1964-01-07 | Ici Ltd | Fused salt baths-composition and method for carburising |
Non-Patent Citations (2)
| Title |
|---|
| Metals Handbook, 1948 Ed. pages 692 696 * |
| Molten Salt Baths for Heat Treatment and Case Hardening of Steel, published by E. I. du Pont de Nemours & Co. May 7, 1943, pgs. 6 8, 25, 26, 31 & 33. * |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3876512A (en) * | 1973-09-10 | 1975-04-08 | Nippon Furnace Koga Kaisha Ltd | Electrolytic carburizing process using a carbonate electrolyte |
Also Published As
| Publication number | Publication date |
|---|---|
| CH534219A (en) | 1973-02-28 |
| DE2018711B2 (en) | 1973-10-25 |
| DE2018711A1 (en) | 1970-11-26 |
| ES379136A1 (en) | 1972-09-01 |
| JPS5440493B1 (en) | 1979-12-04 |
| SE358421B (en) | 1973-07-30 |
| CA928621A (en) | 1973-06-19 |
| DE2018711C3 (en) | 1974-06-06 |
| GB1312982A (en) | 1973-04-11 |
| AT300874B (en) | 1972-08-10 |
| FR2040405A1 (en) | 1971-01-22 |
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