Title:
Carbonized steel strip and method of making same
United States Patent 2294562


Abstract:
This invention relates to carbonized steel electrodes as used in electron discharge tubes and methods of carbonizing steel- wire and steel ribbon or other steel members wherein the carbonized layer forms an appreciable percentage of the cross-sectional thickness. It is generally recognized...



Inventors:
Kingston, Walter E.
Application Number:
US28458839A
Publication Date:
09/01/1942
Filing Date:
07/15/1939
Assignee:
HYGRADE SYIVANIA CORP
Primary Class:
Other Classes:
148/220, 148/319, 427/228, 428/634, 428/679, 428/680, 428/682, 428/684, 428/686, 428/926, 428/938
International Classes:
C23C16/26; H01J19/00
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Description:

This invention relates to carbonized steel electrodes as used in electron discharge tubes and methods of carbonizing steel- wire and steel ribbon or other steel members wherein the carbonized layer forms an appreciable percentage of the cross-sectional thickness.

It is generally recognized that the efficiency of the elements of an electron discharge tube is increased if the surfaces of these elements are darkened so as to increase the heat radiation 1 from these elements. It is also generally accepted that a carbon coated material will have a smaller secondary electron emission as compared with bright or undarkened material. Accordingly, carbonized metal electrodes are used 1 quite extensively in the art in particular for electron discharge tubes in which a comparatively high heat dissipation takes place during operation.

In the early days, the electrodes were first formed and then covered in one way or another with the desired carbon layer. This method was too expensive, and it was soon understood that the material might be "carbonized" before forming the parts.

Several methods of carbonizing metal strips or wires are known. In one of the known methods the metal strip or wire of which the electrodes are to be made is first oxidized and then passed through a hydrocarbon atmosphere at elevated temperature. This method is superior to the socalled "pasting" method, in which the metal is covered with a paste containing carbon. It yields more nearly a black body condition on the surface than the pasting method. Carbonizing by exposing the metal to a hydro-carbon atmosphere at elevated temperatures, for example 1700-1800° F. has, however, up to the present time not been entirely successful when applied to steel strip and wire. This is one of the reasons why nickel has not yet been completely replaced by iron or steel as a material for electrodes in electron discharge tubes. The ordinary carbonizing method for nickel-wire and strip, when applied to steel, results in a change of ductility of the material due to the formation of a more or less continuous cementite layer of variable thickness on the surface of the material known as "case-hardening." The thickness of the material used for electrodes in the conventional electron discharge tubes, for instance radio receiving tubes, is of the order of a few thousandths of an inch. Slight variations of the thickness of the cementite layer on so thin a material will, therefore, result in large variations of the ductility of the metal strip after carbonizing and it is thus impossible to adjust the tools for forming the electrodes from the carbonized steel.

The shapes and dimensions of the electrodes used in modem electron discharge tubes frequently have small tolerances as the electric characteristics are sensitive to small changes in mechanical dimensions. This is particularly true for radio receiving tubes designed for extremely high 0 gain in which the distances between the electrodes may be only a few thousandths of an inch. Accordingly, the tools for shaping the electrodes must be very accurately adjusted and the ductility of the material must be extremely uniform if L5 the high standard of mechanical precision necessary to meet the electrical specifications be maintained. This uniform ductility cannot be attained by carbonizing steel strip in the conventional way, as explained above.

I have discovered a method according to which it is possible to produce a carbonized steel strip or wire, the ductility of which can be kept within very narrow measurable limits satisfactory for a perfectly uniform mass production.

S It is therefore a principal object of the invention to produce a carbonized steel strip or wire of the thickness of the order of five thousandths of an inch, the ductility of which can be kept constant within a few per cent tolerance. 0 Another principal object is to provide an improved process of manufacture for controlling the ductility of carbonized steel.

Another object of the invention is to coat the Ssteel strip which is used for carbonizing with a nickel layer of very uniform thickness.

Another object of the invention is to use the nickel coating as a catalyser, which helps to increase the speed of carbonizing.

A further object of the invention is to avoid the formation of a continuous cementite layer during the carbonization of the material.

Another principal object of the invention is the use in radio tubes, of carbonized steel electrodes which are made of a material processed in accordance with the objects of the invention and described below in connection with the attached drawings in which, Fig. 1 is a schematic Iron-carbon phase diagram near the eutectold region of .86% carbon.

Fig. 2 is a characteristic stiffness curve in terms of carbonizing temperature explanatory of the invention.

Fig. 3 is a schematic sketch of the oxidizing and carbonizing furnaces.

Figs. 4a, 4b and 4c are explanatory photomicrographs.

Figs. 5 to 8 show typical arrangements for testing the bending-yielding characteristics of a strip processed according,to the invention.

According to the invention, the ductility of the steel strip is regulated in carbonizing by providing a number of controls which will be described.

Generally speaking, in the carbonizing of steel pearlite plays a particular part. Pearlite is the I eutectoid of the iron-carbon system in which about 12.25% by weight of cementite crystals are imbedded into about 87.75% by weight of ferrite.

In the so-called case hardening, it is generally desired to avoid the formation of free carbon on 1 the outer surface, which should consist of a con-' tinuous layer of the very hard hyper-eutectoid.

Between this hard hyper-eutectoid and the inner unaffected part of the steel body, a layer of the eutectoid, and below this, a layer of hypo-eutec- 2 toid are formed. This may be easily understood from Fig. 1 in which S is the eutectoid point characterized by a temperature of about 720° C. and a carbon content of .86%. This varying carbon content at different depths formed in case 2 hardening by temperature treatment in a hydrocarbon atmosphere is due to the slowing down of the speed of penetration of the cracked hydrocarbons through the surface into the depth of the steel body. Therefore, hypo-eutectoid and 3( pearlite will be formed only in the deeper regions, while the carbon content at the surface becomes high enough for the production of hyper-eutectoid.

In the carbonizing of steel, as distinguished 31 from case hardening, particularly of a thin strip which later is to be formed into electrodes the dimensions of which have to be kept within very close tolerances, it is necessary to obtain a tenacious black carbon coating on the surface, to 4( prevent the formation of the hyper-eutectoid and to keep the thickness of the eutectoid within reasonable limits, as explained above. With this aim in mind, and after long experimentation, careful comparison of micro-photographs with proc- 40 essing records and ductility (or stiffness) measurements, for which special gauges had to be devised, I have been able to specify a process according to which the ductility of the material before and after carbonizing remains either unchanged or may be changed slightly to any predetermined value within limits.

According to one theory, in the carbonizing of nickel, when the surface is sandblasted prior to carbonization, two different layers of carbon must be discriminated. The primary layer, which consists of nickel carbide, and the secondary layer, which consists of free carbon adhering tightly to the surface. Mutatis mutandis, this discrimination has to be made for nickel plated 6 steel and for steel. A thin coating of nickel is very helpful in carbonizing steel-strip or wire.

It protects the iron from undesirable oxidation and increases the speed of cracking the hydrocarbon gases, by acting in the nature of a catalyser.

The new method of carbonizing is characterized by the following requirements: The temperature during carbonizing must be kept as low as possible, in order to slow down the speed of reaction of the cracked hydro-carbons with the steel underneath the thin and porous nickel layer. The lower temperature limit is determined by the lowest temperature at which the hydro-carbon will be cracked in the presence of nickel. The pearlite layer should not be greater than approximately 10 to 20% of the thickness of the steel strip in order to avoid any undesirable decrease of ductility. Running speed and temperatures should, whenever possible, be so controlled that the decrease of ductility caused by the pearlite layer is partly or completely compensated by a further annealing of the steel incidental to the carbonizing process.

.0 Referring to Fig. 3, the steel strip 1, which has been previously provided with a thin coating of nickel, and approximately 0.005 inch thickness, is paid off from a supply reel 2 and is passed through any well-known type of oxidizing fur5 nace indicated diagrammatically by numeral 3.

The oxidizing furnace may for example include a heat resistant muffle 4 which may be heated electrically or by gas and containing either dry or humidified air. The oxidation is preferably 0 effected at a temperature of approximately 1475° F. to approximately 1625° F., and for this purpose a thermocouple 5 and associated pyrometer 6 are provided.

The oxidized strip is then passed in the direc5 tion of the arrow, through a carbonizing furnace 7 containing a heat resistant muffle 8 heated by electricity or gas. The ends of muffle 8 may be closed by alloy covers 9 and 10, each cover having a narrow vertical slot to allow the strip I to pass ) without preventing escape of the carbonizing gas.

This carbonizing gas may enter the muffle through the inlet opening II, and escapes and is burned at the exit opening I la. The oxidized surface of the strip as it passes through the fur5 nace 7 is simultaneously reduced and the ferrous base or matrix which may be for example ferrite or alpha iron is carbonized. For this purpose, the muffle 8 is supplied with a hydro-carbon gas such as methane, propane, butane or mixture » thereof with other saturated hydro-carbons.

Preferably, and in accordance with the invention, the carbonizing is closely controlled as to temperature and duration so that the pearlite or pearlite-cementite layer which is formed between the nickel coating and the ferrite matrix extends on an average only approximately 0.001 inch below the surface of the ferrite and in any event is not greater than 20% the strip thickness.

Thus the temperature of carbonization should be from approximately 14000 F. to approximately 1600" F. and the strip should be fed at a speed of from about 2 feet per minute to 10 feet per minute. A thermocouple 12 and associated thermometer 13 may be provided for maintaining the above-mentioned temperature limits.

The emerging carbonized strip may then be passed adjacent rotating bristle brushes 14 whereby excess or loose carbon is removed. The strip is then wound on the reel 15 which is preferably driven at a uniform speed by a suitable motor, and a suitable friction drag may be provided for reel 2 in order to maintain the strip at the desired tautness.

Referring to Figs. 4a, 4b and 4c, there are shown photo-micrographs of cross-sections of a metal strip after three different stages of carbonizing. Fig. 4a shows .the strip as processed in accordance with the invention at the right temperature and timing as above described. It will be noted that the small pearlite lumps represented by the vertical cross-lining, have started to form under the nickel coating (represented by the 45° cross-lining), and the outer continuous carbon layer is indicated by the horizontal cross-lining. The inner or core section of the * unaffected iron which may be alpha iron or ferrite or low carbon steel, is shown unshaded. It will be noted that the pearlite extends very little below the surface of the ferrite matrix preferbly about 0.001 inch. Figs. 4b and 4c show the strip after too high a carbonizing temperature or too long a carbonizing period or both. In Fig. 4b the pearlite lumps have grown together and extend an appreciable distance into the ferrite core. In Fig. 4c, the pearlite lumps form the major part of the core so that only a small proportion of the ferrite core is left unaffected.

While in the foregoing description the intermediate layer between the ferrous core and the nickel coating has been referred to as pearlite, it will be understood that this intermediate layer may consist of pearlite or cementite or a combination of both, the important feature being that this intermediate steel layer, which has materially lower ductility than the core is confined to a thickness such that the ductility of the carbonized body is not materially decreased. As pointed out above, I find that by limiting this intermediate layer to a thickness approximately 0.001 inch, satisfactory carbonized iron electrodes for radio tubes such as the usual plate or anode electrode can be manufactured economically with the desired dimensional accuracy and the anodes or electrodes may be shaped to any desired form without destroying the desirable electrical and heat radiating properties thereof. Furthermore, while in the particular furnace shown in Fig. 3, the carbonizing gas is described as traveling in a path opposite to the direction of the strip movement, it will be understood that the gas may enter at the opening II a and may exit at the opening II, or if desired, the carbonizing gas may enter at both ends of the muffle 8 and may be vented at the middle thereof. Furthermore, if desired, the metal strip I prior to being carbonized may be sandblasted or otherwise roughened to improve the mechanical bond between the carbon coating and the strip. While the invention is not limited to any particular kind of iron strip, preferably there is used a strip of a low carbon type of steel of high purity.

The extent of the investigation made for obtaining the specifications neoe:sary for a process leading to a satisfactory mass production of carbonized steel strip can be inferred from the fact g( that there is no permanent standard for measuring ductility of materials. The temporary A. S. T. M. standard E 16-38 T measures ductility of rods between 1/4" and 1/2" diameter by a bending test, in which the elongation of the 51 longer outside fiber is measured at the moment of breaking.

As in the case under consideration the thickness of the material is only a few thousandths of an-inch, it was necessary to devise new means 6 of measuring the ductility. One of the methods applied is similar to a method used in the art for specifying the ductility or rather the stiffness of nickel strip. This method comprises the bending of a piece of the metal strip to be tested over 6 an angle of 80° to 90° of an arc and observing the angle to which the strip, under certain specified conditions will recoil. The material is here bent far above the yield point, but not to the breaking point. It does therefore, not measure 7 the ductility in a strict sense, but it gives some valuable information for the adjustment of the tools when the strip is to be formed into any desired, say cylindrical or channeled shape.

The particular material used during the devel- 7 opment of the present process was specified to recoil between 34 and 38 degrees after the bendLng over an angle of 80 degrees.

Another method to measure the ductility of the strip which I prefer consists in the determination of the weight needed for bending a short piece of the strip which is supporter at one section, by applying a load at two sections, which are located symmetrically to the support until the material "yields." In this method the stiffness can be measured directly in grams per inch.

The larger the weight needed for making the material yield the smaller will be its ductility.

This method is preferable because it comes nearer to an absolute measurement of the yield point for bending, and according to my experience allows a closer adjustment of the tools.

Figs. 5, 6, 7 and 8 illustrate the bending-yielding test. The metal strip I is placed between a support I s which has the cross section of a wedge, and a loading block 11 from which an opening has been cut out into which the wedge shaped support IS fits snugly. Support 16 and loading block 1 are connected by a guide not shown in the drawings, which insures a vertical motion of IT downward when table 18 is loaded. Fig. shows the shape of the metal strip before any weights are placed on table 18. Figs. 6 and 7 show the increasing bend of the metal strip I for increasing load before yielding takes place.

In Fig. 8, two consecutive stages are shown of the motion caused by adding a small weight 19, just sufficient to cause yielding. The stiffness is expressed in terms of the number of grams formed by the weight of the table, the block, and the three weights needed for yielding, divided by the width of the strip in inches.

Comparative tests made with the described stiffness gauge and measurements of elongation at the yield point for tension, the Brinell and Rockwell-hardness and of other physical characteristics show, that the gauge as illustrated in Figs. 5 to 8 gives the best and most reliable information for adjusting the tools for the shaping Sof electrodes from the material.

It has been found that a carbonized iron strip without the nickel coating but processed according to the invention, has approximately 200% of the stiffness of a similar nickel strip carbonized 0 in the usual way. Likewise the carbonized iron strip with the nickel coating processed according to the invention has approximately 150% of the stiffness of a similar nickel strip carbonized in the usual way, the stiffness being measured with 5 the gauge shown in Figs. 5, 6, 7 and 8. Various changes and modifications may be made without departing from the spirit and scope of the invention. Thus, while reference has been made to a matrix of low carbon ferrous alloy, any iron of a carbon content between electrolytic iron and 0 medium carbon steel may be employed, and the running speed of the strip during carbonization may vary with the thickness in order to limit the thickness of the pearlite layer as abovementioned. Where reference is made in the claims to a pearlite layer it is understood that this layer may consist of pearlite or cementite or a combination of both.

What I claim is: 0 1. The process of producing an iron-alloy matrix with a carbonized surface which comprises, nickel plating the alloy, oxidizing the plated alloy, then simultaneously reducing the oxide and carbonizing the iron at a temperature 5S sufficient to form a pearlite layer in the matrix, the thickness of the said plating and carbonizing temperature.being correlated so that the depth of the pearlite layer between the matrix and its carbonized surface is not in excess of approximately 20% of the matrix thickness and the ductility of the matrix is not materially decreased.

2. The process according to claim 1, in which the thickness of the pearlite layer is limited to approximately not more than 20 percent of the 1 alloy thickness by limiting the carbonizing temperature between predetermined limits.

3. The process according to claim 1, in which the thickness of the pearllte layer is limited to approximately not more than 20 percent of the I alloy thickness by limiting the duration of the carbonizing time.

4. The process according to claim 1, in which the carbonization is effected at approximately 14000 F. to 1600° F. and for a period of approxi- 2i mately one-half to five minutes.

5. The process of carbonizing an iron body which comprises, providing the body with a layer of a metal which acts as a catalyzer in cracking a hydrocarbon atmosphere heating the iron in 2: said carbonizing atmosphere at a temperature and for a duration such that a pearlite layer is formed between the carbon layer and the iron matrix but with the pearlite penetrating on an average to a depth not materially greater than 3( twenty percent of the body thickness.

6. The process of carbonizing an alloy of the alpha iron type .which comprises, providing the alloy with a metal coating which acts as a catalyser in increasing the carbonization speed while allowing carburization of the surface of the iron, then carbonizing the metal coated alloy and terminating the carbonization when the pearlite layer is approximately 10 to 20 percent the thickness of the alloy. 7. The process of carbonizing an alloy of the alpha iron type which comprises, providing the alloy with a thin nickel coating, and then carbonizing the nickel coated alloy at a temperature of about 1400° F. to 1600° F., and terminating the carbonization when the pearlite layer is of the order of 10-20 percent the thickness of the alloy.

8. The process of carbonizing a ferrous alloy matrix of the alpha iron type which comprises, 5O providing the alloy with a thin nickel coating, oxidizing the coated alloy, and then simultaneously reducing the oxide and carbonizing the alloy at a temperature at which a pearlite layer is formed at the surface of the matrix, the said nickel coating being proportioned in thinness to permit the desired carburization of the matrix whereby the thickness of the pearlite layer is approximately 10 to 20% of the alloy thickness.

9. The process according to claim 8, in which 60o carbonization is effected at a temperature of about 14000 F. to 1600" F.

10. The process according to claim 8, in which the oxidation is effected at a temperature of about 14750 F. to 16250 F., the carbonization is effected at a carburized temperature of approximately 14000 F. to 16000 F. and a layer of a metal between the said intermediate layer and the outer carbon layer, said metal layer being of sufficient thinness to permit the carburization of the matrix therethrough and acting as a cat0 alyzer when the matrix is heated in a carbonizing atmosphere.

11. A carbonized iron strip comprising a matrix of iron, an outer carbon layer, an intermediate thin porous layer of a metal which acts 5 as a catalyser in increasing the carbonization speed while allowing carburization of the surface of the matrix, and a carburized eutectold steel layer beneath the said intermediate layer of insufficient thickness to decrease materially ) the ductility strip as a whole as compared with the uncarbonized strip.

12. A carbonized iron strip of the order of 0.005 inch thickness comprising a matrix of the alpha iron type, an outer carbon layer, an intermediate o pearlite layer of the order of 0.0010 inch average thickness, and a layer of a metal between said intermediate layer and the outer carbon layer, said metal layer being of sufficient thinness to permit the carburization of the matrix thereI through and acting as a catalyzer when the matrix is heated in a carbonizing atmosphere.

13. A carbonized iron strip comprising an iron matrix, a layer of nickel of sufficient thinness to permit the carburization of the iron therethrough, an outer layer of carbon, and a layer of carburized eutectoid steel between the nickel and matrix in the form of an iron-carbon alloy but of insufficient thickness to decrease materially the ductility of the strip as a whole.

14. A thin, ductile, carbonized strip of nickel coated ferrous alloy comprising a matrix of said ferrous alloy, a layer of nickel of sufficient thinness to permit the carburization of the matrix therethrough, a layer of carbon, and between the nickel layer and the matrix of said ferrous alloy, a thin layer of pearlite.

15. A thin, ductile, carbonized matrix of ferrous alloy in which there is a pearlite layer which does not penetrate to a depth greater than .001 inch from the surface of the matrix, and in which there is an intermnediate thin layer of a metal which acts as a catalyser during carbonization and which is sufficiently thin to permit formation of the said pearlite in the matrix.

16. A thin, ductile, carbonized strip of nickel coated ferrous alloy in which there is a pearlite which does not penetrate to a depth greater than .001 inch from the surface of the ferrous alloy.

WALTER E. KINGSTON.