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Changes of the mechanical properties of carburized thoriated tungsten wire
Changes of the MechanicalProperties of Carburized Thoriated Tungsten Wire P. SCHNEIDER Development Prague
and P. MARES
Division, Tesla Roinov n.p. zcivod VrSovice, IO, Vriovice, TJ. SNB 55, Czechoslovakia
(Received 5 June 1962
; accepted 5 September 1962)
The mechanical properties of two different lots of thoriated tungsten wire (0.6 mm dia.) were investigated. A simple method for testing is described. Introduction
For this reason, a specially designed apparatus which fulfilled all requirementsz.
Filaments of thoriated tungsten, generally used for the construction of high-efficiency cathodes, are usually carburized at high temperature in a hydrocarbon vapouri. The carbide layer thus obtained decreases the susceptibility of the filaments to “ poisoning,” and otherwise improves their characteristics. Carburizing is carried out in naphthalene vapour or in vapours of benzene or xylene in a hydrogen atmosphere. In the work to be described in this paper the carburizing was carried out in benzene vapour. The advantage of this procedure over the treatment in naphthalene vapour in vacuum is the prolongation of the life of such a cathode and the shortening of the time of carburization. As different lots of thoriated tungsten wire are not always of the same quality, it is very difficult to avoid trouble during the manufacturing of cathodes. Filamentary cathodes made from some wire lots remained quite ductile after flashing in hydrogen, but became very brittle after carburization, so that the slightest vibration during shipment had a destructive effect. Cathodes manufactured from other consignments, on the other hand, became very brittle after flashing in hydrogen, but their mechanical properties substantially improved after carburizing. In both cases, carburized filaments were generally more brittle than those which were not carburized. After decarburization in hydrogen, the mechanical properties improved in the first case but remained the same in the second case. The first grid of a tube in which the cathode was partly decarburized after initial carburization exhibited high thermionic emission owing to the deposition of evaporated thorium. This could be explained by the lowering of the density or thickness of the carbide layer. The more porous layer enables more rapid diffusion of thorium to the surface. In order to remove grid emission, cathodes were, after decarburization, carburized again, but for a few seconds only, to form a very thin carbide layer on the surface. The grid primary emission was thus decreased, but some consignments of thoriated tungsten became brittle. The first requirement was to find a technique for carburizing reproducibly, in terms of time, temperature, pressure, and amount of benzene vapour in the hydrogen atmosphere.
Experimental
was used,
procedure
(1) Preparation of samples for flashing Experiments were carried out with samples taken from the same consignment of graphitized thoriated tungsten (0.6 mm dia.) which, after flashing, remained ductile but became extremely brittle after carburization. Individual samples were cut to a length of 300 mm and immersed for a few seconds in a hot 25 per cent solution of sodium hydroxide to remove the graphite layer. Then the samples were weighed and clamped between two gripping spring jaws provided with contacts. This arrangement was placed into a carburizing jar. The samples were annealed in pure hydrogen or in benzene vapour in hydrogen. The temperature was monitored by an optical micropyrometer through a little glass window in the jar. (2) Mechanical tests After cooling, the samples were removed from the jaws, weighed again, and mechanically examined in the following way. Each sample was laid between two beams, 200 mm apart, and loaded with shot which was added gradually into a little basket that was attached by a hook at the middle of the filament under test. Simultaneously, the deflection of the samples was measured. The load was increased until the filament broke, and the resulting values were recorded (critical loading in grams, and critical deflection in mm). For the study of the thickness and the structure of layers, a metallurgical section was prepared from every sample. Microsections were etched for 5 set in alkaline ferricyanide; they are generally shown at a magnification of x 650. (3) Schedule of treatment (4 Flashing in hydrogen for 1 min at 2200°K ; (b) carburizing in benzene vapour in a hydrogen atmosphere (0.30 g C6Hs per 10 1. of hydrogen) for 2 min at 2200 “K ; decarburization in dry hydrogen for 2 min at 2200°K ; repeated carburization (after prior decarburization) gradually for 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5 and 25 set at 2200°K. 255
P. SCHNEIDER
256
P. MARES
AND
TABLE I
Di&rent
behaviour of two consignments of thoriated tungsten wire (0.6 mm dia.) treated under the same conditions.
Lot designation
Critical loading (9)
Treatment
Critical deflection (mm)
Entirely elastic
Weight increase (%)
Average
Metallurgical section
layer thickness (p)
0
0
A
17
0
0
B
20
14
0.355
Carbide
layer 26~
C
Same as I above
28
19
0.363
Carbide
layer 8.5~
D
I
Annealed 1 min in hydrogen, carburized 2 min and decarburized 2 min
30
18
0.121
The first decarburized layer 9p The second carbide layer 13~
E
II
Same as I above
28
19
0.143
The first layer caused by the shifting of the surface carbide layer toward the centre of the filament (9p). The second carbide layer 10.5~
F
I
Annealed
1 min in hydrogen
II
Same as I above
24
I
Annealed 1 min in hydrogen and carburized 2 min
II
30
0
0
0
A (g)
0
25 /
x
+-x-x--x-x--x-x
B (mm)
x
x
i5 -
IO -
I I
j-
0
5 Time
of
I
I
IO additional
I
I
15 carburization,
1
I
20
I
set
FIG. 1. The dependence of the critical loading and critical deflection on the time of the additional carburization for filaments from consignment I. Line A represents the values of critical loading (g) ; line B represents the values of critical deflection (mm).
The resulting values, comparing the samples taken from two consignments of thoriated tungsten (0.6 mm dia.) (further written WTh) and treated under the same conditions are given in Table I.
I
5
25 Time
of
I1 10
additional
I 15
/
carburirotion.
I
I
20
11 25
set
FIG. 2. The dependence of the critical loading and critical deflection on the time of the additional carburization for filaments from consignment II. Lines A and B as in Fig. 1.
(4) Evaluation Table II and Fig. 1 show the dependence of the critical loading and critical deflection on the time of repeated carburization which was carried out after the decarburizing
Changes of the Mechanical
Properties
of Carburized
Thoriated
Tungsten Wire
(d)
(b)
FIG.
3(a)-(f).
Metallurgical
sections.
257
258
P. SCHNFJDER&m
procedure. This dependence was plotted for 11 samples from both consignments. The following differences of the two wire lots result from Table I. 1 exhibits a (a) Carburized WTh from consignment strength 30 per cent lower than that of WTh from consignment Il. (b) The strength of WTh from consignment I was substantially improved after subsequent decarburization, whereas the WTh from the consignment II remained unchanged. (c) The strength of the decarburized WTh from consignment I corresponds to the strength of the carburized WTh from the consignment I. (d) The strength of the decarburized WTh from the consignment I corresponds to the strength of the decarburized WTh from the consignment II. (e) Annealed WTh from consignment I exhibits a higher strength than that of consignment II. The difference in the structures of the carbide layers can be understood from the microsections and from the differences in weight after treatment of the samples taken from the two consignments. (a) After carburization, the carbide layer of samples from the consignment I was 3 times thicker than that of samples from consignment II. TABLE
II
Dependence of the critical loading and critical deflection on the time of additional carburization of two diflerent consignments of thoriated tungsten wire.
No. 1 2 3 4
i 6 I 8 9 10 11
Time of carburization (set)
Critical deflection (mm)
Consignment I of WTh (0.6 mm dia.) 0 30 2.5 28 5.0 7.5 10.0 12.5 15.0 20 17.5 18 20.0 17 22.5 16 25.0 . 15 Consignment
I
2 3 4 5 6 7 8 9 10 11
Critical loading (g)
i.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0
19 18 17 16 15 15 14 12 11 9 9
P. MARE&
09 The two carbide layers differ from each other not only in thickness but also in structure. In the case of consignment I, the carbide layer causes brittleness ; in the case of consignment II, on the contrary, it improves the mechanical properties. The carbide layer of samples of consignment II is by far more dense, and the weight increase after carburization is also slightly higher than that of samples of consignment I in spite of a 1 : 3 ratio of the thicknesses. (4 Decarburization of samples of consignment I makes the carbide layer thinner. In the case of consignment II, decarburization does not cause reduction of the thickness, but shifts the layer from the surface toward the centre of the filament. After decarburization, the resulting layers have practically the same density as the layer in the case of consignment I. (The increase in weight is practically the same in samples of both consignments). The difference in behaviour of both consignments after repeated carburization is apparent from Table II and Fig. 1 and 2. The high sensitivity of the samples from consignment I to carburization in contrast to samples from consignment II is illustrated in Fig. 1. From these experiments it follows that the density of the thoriated tungsten of consignment I is substantially lower than that of consignment II. During the manufacture of tubes it was found that cathodes constructed from thoriated tungsten from consignment I are less sensitive to poisoning and are more rapidly activated than those constructed from consignment II.
Conclusion
The difference of sensitivity to carburization of thoriated tungsten can be detected by a simple mechanical test. Two samples are taken from the consignments in question. The hrst is carburized, and the second is carburized and then decarburized. When the values obtained from the above described loading measurement differ much from each other, the consignment belongs to the first group (of lower quality). In the- opposite case, the consignment has the properties of the better, second group. By adjustment of the distance between the two beams, the proposed method can be applied to the testing of thoriated tungsten of any diameter.
II of WTh (0.6 mm dia.) 33 29 28 28 29 29 29 29 28 28 29
J9 19 18 19 19 18 19 19 20 19 19
Acknowledgment
The authors are indebted to Dr. W. H. Kohl of LOS Altos, California, U.S.A., for editorial assistance in the preparation of this paper.
References
1 W. H. Kohl, Materials and Techniques for Elecrron Tubes, Reinhold Publishing Corp., New York (1960). 2 P. Schneider, J. Chem. Phys. 28 (1958), 675-682.
and P. MARES
Division, Tesla Roinov n.p. zcivod VrSovice, IO, Vriovice, TJ. SNB 55, Czechoslovakia
(Received 5 June 1962
; accepted 5 September 1962)
The mechanical properties of two different lots of thoriated tungsten wire (0.6 mm dia.) were investigated. A simple method for testing is described. Introduction
For this reason, a specially designed apparatus which fulfilled all requirementsz.
Filaments of thoriated tungsten, generally used for the construction of high-efficiency cathodes, are usually carburized at high temperature in a hydrocarbon vapouri. The carbide layer thus obtained decreases the susceptibility of the filaments to “ poisoning,” and otherwise improves their characteristics. Carburizing is carried out in naphthalene vapour or in vapours of benzene or xylene in a hydrogen atmosphere. In the work to be described in this paper the carburizing was carried out in benzene vapour. The advantage of this procedure over the treatment in naphthalene vapour in vacuum is the prolongation of the life of such a cathode and the shortening of the time of carburization. As different lots of thoriated tungsten wire are not always of the same quality, it is very difficult to avoid trouble during the manufacturing of cathodes. Filamentary cathodes made from some wire lots remained quite ductile after flashing in hydrogen, but became very brittle after carburization, so that the slightest vibration during shipment had a destructive effect. Cathodes manufactured from other consignments, on the other hand, became very brittle after flashing in hydrogen, but their mechanical properties substantially improved after carburizing. In both cases, carburized filaments were generally more brittle than those which were not carburized. After decarburization in hydrogen, the mechanical properties improved in the first case but remained the same in the second case. The first grid of a tube in which the cathode was partly decarburized after initial carburization exhibited high thermionic emission owing to the deposition of evaporated thorium. This could be explained by the lowering of the density or thickness of the carbide layer. The more porous layer enables more rapid diffusion of thorium to the surface. In order to remove grid emission, cathodes were, after decarburization, carburized again, but for a few seconds only, to form a very thin carbide layer on the surface. The grid primary emission was thus decreased, but some consignments of thoriated tungsten became brittle. The first requirement was to find a technique for carburizing reproducibly, in terms of time, temperature, pressure, and amount of benzene vapour in the hydrogen atmosphere.
Experimental
was used,
procedure
(1) Preparation of samples for flashing Experiments were carried out with samples taken from the same consignment of graphitized thoriated tungsten (0.6 mm dia.) which, after flashing, remained ductile but became extremely brittle after carburization. Individual samples were cut to a length of 300 mm and immersed for a few seconds in a hot 25 per cent solution of sodium hydroxide to remove the graphite layer. Then the samples were weighed and clamped between two gripping spring jaws provided with contacts. This arrangement was placed into a carburizing jar. The samples were annealed in pure hydrogen or in benzene vapour in hydrogen. The temperature was monitored by an optical micropyrometer through a little glass window in the jar. (2) Mechanical tests After cooling, the samples were removed from the jaws, weighed again, and mechanically examined in the following way. Each sample was laid between two beams, 200 mm apart, and loaded with shot which was added gradually into a little basket that was attached by a hook at the middle of the filament under test. Simultaneously, the deflection of the samples was measured. The load was increased until the filament broke, and the resulting values were recorded (critical loading in grams, and critical deflection in mm). For the study of the thickness and the structure of layers, a metallurgical section was prepared from every sample. Microsections were etched for 5 set in alkaline ferricyanide; they are generally shown at a magnification of x 650. (3) Schedule of treatment (4 Flashing in hydrogen for 1 min at 2200°K ; (b) carburizing in benzene vapour in a hydrogen atmosphere (0.30 g C6Hs per 10 1. of hydrogen) for 2 min at 2200 “K ; decarburization in dry hydrogen for 2 min at 2200°K ; repeated carburization (after prior decarburization) gradually for 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5 and 25 set at 2200°K. 255
P. SCHNEIDER
256
P. MARES
AND
TABLE I
Di&rent
behaviour of two consignments of thoriated tungsten wire (0.6 mm dia.) treated under the same conditions.
Lot designation
Critical loading (9)
Treatment
Critical deflection (mm)
Entirely elastic
Weight increase (%)
Average
Metallurgical section
layer thickness (p)
0
0
A
17
0
0
B
20
14
0.355
Carbide
layer 26~
C
Same as I above
28
19
0.363
Carbide
layer 8.5~
D
I
Annealed 1 min in hydrogen, carburized 2 min and decarburized 2 min
30
18
0.121
The first decarburized layer 9p The second carbide layer 13~
E
II
Same as I above
28
19
0.143
The first layer caused by the shifting of the surface carbide layer toward the centre of the filament (9p). The second carbide layer 10.5~
F
I
Annealed
1 min in hydrogen
II
Same as I above
24
I
Annealed 1 min in hydrogen and carburized 2 min
II
30
0
0
0
A (g)
0
25 /
x
+-x-x--x-x--x-x
B (mm)
x
x
i5 -
IO -
I I
j-
0
5 Time
of
I
I
IO additional
I
I
15 carburization,
1
I
20
I
set
FIG. 1. The dependence of the critical loading and critical deflection on the time of the additional carburization for filaments from consignment I. Line A represents the values of critical loading (g) ; line B represents the values of critical deflection (mm).
The resulting values, comparing the samples taken from two consignments of thoriated tungsten (0.6 mm dia.) (further written WTh) and treated under the same conditions are given in Table I.
I
5
25 Time
of
I1 10
additional
I 15
/
carburirotion.
I
I
20
11 25
set
FIG. 2. The dependence of the critical loading and critical deflection on the time of the additional carburization for filaments from consignment II. Lines A and B as in Fig. 1.
(4) Evaluation Table II and Fig. 1 show the dependence of the critical loading and critical deflection on the time of repeated carburization which was carried out after the decarburizing
Changes of the Mechanical
Properties
of Carburized
Thoriated
Tungsten Wire
(d)
(b)
FIG.
3(a)-(f).
Metallurgical
sections.
257
258
P. SCHNFJDER&m
procedure. This dependence was plotted for 11 samples from both consignments. The following differences of the two wire lots result from Table I. 1 exhibits a (a) Carburized WTh from consignment strength 30 per cent lower than that of WTh from consignment Il. (b) The strength of WTh from consignment I was substantially improved after subsequent decarburization, whereas the WTh from the consignment II remained unchanged. (c) The strength of the decarburized WTh from consignment I corresponds to the strength of the carburized WTh from the consignment I. (d) The strength of the decarburized WTh from the consignment I corresponds to the strength of the decarburized WTh from the consignment II. (e) Annealed WTh from consignment I exhibits a higher strength than that of consignment II. The difference in the structures of the carbide layers can be understood from the microsections and from the differences in weight after treatment of the samples taken from the two consignments. (a) After carburization, the carbide layer of samples from the consignment I was 3 times thicker than that of samples from consignment II. TABLE
II
Dependence of the critical loading and critical deflection on the time of additional carburization of two diflerent consignments of thoriated tungsten wire.
No. 1 2 3 4
i 6 I 8 9 10 11
Time of carburization (set)
Critical deflection (mm)
Consignment I of WTh (0.6 mm dia.) 0 30 2.5 28 5.0 7.5 10.0 12.5 15.0 20 17.5 18 20.0 17 22.5 16 25.0 . 15 Consignment
I
2 3 4 5 6 7 8 9 10 11
Critical loading (g)
i.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0
19 18 17 16 15 15 14 12 11 9 9
P. MARE&
09 The two carbide layers differ from each other not only in thickness but also in structure. In the case of consignment I, the carbide layer causes brittleness ; in the case of consignment II, on the contrary, it improves the mechanical properties. The carbide layer of samples of consignment II is by far more dense, and the weight increase after carburization is also slightly higher than that of samples of consignment I in spite of a 1 : 3 ratio of the thicknesses. (4 Decarburization of samples of consignment I makes the carbide layer thinner. In the case of consignment II, decarburization does not cause reduction of the thickness, but shifts the layer from the surface toward the centre of the filament. After decarburization, the resulting layers have practically the same density as the layer in the case of consignment I. (The increase in weight is practically the same in samples of both consignments). The difference in behaviour of both consignments after repeated carburization is apparent from Table II and Fig. 1 and 2. The high sensitivity of the samples from consignment I to carburization in contrast to samples from consignment II is illustrated in Fig. 1. From these experiments it follows that the density of the thoriated tungsten of consignment I is substantially lower than that of consignment II. During the manufacture of tubes it was found that cathodes constructed from thoriated tungsten from consignment I are less sensitive to poisoning and are more rapidly activated than those constructed from consignment II.
Conclusion
The difference of sensitivity to carburization of thoriated tungsten can be detected by a simple mechanical test. Two samples are taken from the consignments in question. The hrst is carburized, and the second is carburized and then decarburized. When the values obtained from the above described loading measurement differ much from each other, the consignment belongs to the first group (of lower quality). In the- opposite case, the consignment has the properties of the better, second group. By adjustment of the distance between the two beams, the proposed method can be applied to the testing of thoriated tungsten of any diameter.
II of WTh (0.6 mm dia.) 33 29 28 28 29 29 29 29 28 28 29
J9 19 18 19 19 18 19 19 20 19 19
Acknowledgment
The authors are indebted to Dr. W. H. Kohl of LOS Altos, California, U.S.A., for editorial assistance in the preparation of this paper.
References
1 W. H. Kohl, Materials and Techniques for Elecrron Tubes, Reinhold Publishing Corp., New York (1960). 2 P. Schneider, J. Chem. Phys. 28 (1958), 675-682.