. May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ingots up t o 400 pounds in weight can be cast. This furnace when running a t full capacity can process one ton of cathodes per day. r2t present a limited amount of vacuum-cast copper is being produced and distributed for tests in industrial operations. A market is anticipated in,industrial operations where porosity in ingots cannot be tolerated, where increased ductility is desired, and where copper castings with high electrical conductivity are needed.
Aclmowledgment The authors wish to acknowledge the valuable assistance of E. E. Chadsey, Jr., who provided the analytical data required in this research.
825
Literature Cited (1) Allen, J . Inst. MetaEs, 43,81 (1930). (2) Bever and Floe, Trans. Am. Inst. Mining Met. Engrs., Inst. Metals Div., Tech. Pub. 1802 (1945). (3) Cone, Metals & Alloys, 8 , 3 3 (February 1937). (4) Ellis, Trans. Am. Inst. Mining Met. Engrs., 106,487 (1933). (5) Floe and Chipman, Ibid., 143,287 (1941); 147,28 (1942) (6) Rohn, J. Inst. Metals, 42, 203 (1929); 2. Metallkunde, 21, 12 (1929). (7) Rolle and Brace, Mining and Met., 14,340 (1933). (8) Sieverts and Krumbhaar, 2.physik. Chem., 74,277 (1910). (9) Ibid., p. 295. (10) Webster, Christie, and Pratt, Trans. Am. Inst. Mining Met. Engrs., 104, 166 (1933).
.
RECEWED November 17, 1947
Results of Research in Controlled Vacuum Heat Treating Jack Huebler
SURFACE COMBUSTION CORPORATION, TOLEDO 1, OHIO
’
T h a t numerous previously debatable theories relative to the effects of absorbed or entrapped gases in steel must now give way to concrete facts may now be concluded from the results of the research program presented herein. By a controlled vacuum heat-treating procedure, the effects of hydrogbn, nitrogen, and oxygen in steel are demonstrated for such heat-treating operations as annealing, malleableizing, quenching and drawing, spheroidizing, and carburizing. The influence of these gases on beryllium-copper valve springs is also disclosed. Although no new commercially applicable processes have been developed, the procedures required to obtain improvements in the physical properties of steels while undergoing the various heat-treating operations are indicated. The effects o f these gases during carburizing, spheroidizing annealing, and quenching of steel are shown to be small, but significant improvement is obtained by vacuum heat treatment for malleableizing compared with treatment with the usual prepared atmospheres. Similarly significant improvement in the fatigue life of the berylliumcopper valve springs is also obtainable in vacuum during the age hardening treatment. An incidental but valuable result of the research is development of an equation by which the carbon gradient after any diffusion treatment may be predicted.
T
0 DETERMINE the effect of absorbed or entrapped gases
in steel, the American Gas Association sponsored B research project in the Surface Combustion Research Laboratory. Many theories advanced in recent years hold absorbed gases responsible for a wide variety of physical defects in steel. Generally, these theories have been based on the embrittling effect of hydrogen and nitrogen and the destructive effect of oxygen. Such effects are caused by and do result from excessive quantities of the gases and this natural occurrence leads to speculation as to the effect resulting from small amounts of absorbed gases. Because the major portion of these gases can be extracted from the body of the metal by heating in a vacuum, the research in vacuum heat treating was undertaken to determine accurately the effects of the entrapped gases upon such common operations as anneal-
ing, malleableixing, quenching and drawing, spheriodixing, and carburizing. The gases found absorbed in steel consist almost entirely of hydrogen, nitrogen, and oxygen. Hydrogen constitutes 90% of the total, and is thus the gas of major concern. Because nearly all gaseous atmospheres contain liberal amounts of hydrogen and because the solubility of hydrogen increases rapidly with increasing temperature, the difference between vacuum heat treatment and the conventional methods is accentuated. I n other words, if hydrogen has any pronounced effect upon steel, major improvements could be expected from vacuum heat treating,and, furthermore, need for eliminating hydrogen as a component of controlled atmospheres would be evident. (It will be shown that this is not the case, with the single exception of the mttlleableizing process.) Of the minor constituents, nitrogen and oxygen, nitrogen can be extracted at the higher temperatures, but oxygen is not remqvable a t all in the heat-treating temperaturerange. It has been demonstrated that steel is as easily carburized by a hydrocarbon gas in the absence of oxygen compounds as when such compounds are present-in fact, the Carburizing potential was.slightly higher than would be expected-and that the carbon diffusion rate was identical in vacuum, in nitrogen, and in the conventional hydrogen-containing carburizing atmospheres. I n demonstrating this fact, a valuable equation was devised to predict the carbon gradient after a diffusion treatment. Tests have shown that the spheroidixing rate is not influenced by vacuum. The deep drawing properties of rimmed and killed steels were not improved by vacuum annealing. Quenching steel from vacuum failed to improve either the impact strength developed or the fatigue life.
Test Equipment and Operation The nature of the vacuum heat-treating experiments that were to be conducted made it necessary to incorporate certain features in the design of the equipment: close temperature cofitrol over a very wide range, rapid attainment of fairly high vacua and high ultimate ‘vacua, means for moving test samples while under vacuum, and means for conducting vacuum and atmosphere tests simultaneously under identical conditions. These con-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Front View of Experimental Vacuum HeatTreating Furnace Developed by Surface Combustion Corporation for Use in the Project
siderations led to the construction of the furnace shown in Figures 1 and 2. The furnace, which is roughly 3 feet cubical, accommodates two 35-15 alloy tubes 4 inches in diameter. The furnace walls are heavily insulated with 5 inches of insulating firebrick and 1 inch of Schundler block. The alloy retorts are sealed into the walls to help extend the hot zone and to prevent the escape of flue gases a t
0
5
10
15
20
25
a0
85
Figure 3. Typical Evacuation Curve, Illustrating Operation of Vacuum Pumps on Furnace
Vol. 40, No. 5
Figure 2. Side and Back View of the Surface Experimental Vacuum Heat-Treating Furnace
any point other than through the flue a t the top. Heat is supplied by six small burners, three on each side, spaced to give uniform heat distribution. An auxiliary air inlet is provided a t each burner for low temperature operation. The introduction of extra air reduces the necessity for a great turndown ratio on the burners, and a t the same time keeps the flue gas volume high, an essential for good heat distribution. As a result, the alloy retorts are heated uniformly over a 12-inch length at any temperature of operation. Unusually close temperature control is obtained by operating the burners on a high-low. firing system in which the high and low settings are made as nearly equal as possible. I n addition, a sensitive temperature control instrument is provided, which operates on not more than * 2 O F. I n Figure 1, the retort to the left is constructed vacuum-tight. At the rear, the tube extends through the furnace wall slightly, and that end is provided with a gas inlet or outlet through a vacuum-tight valve. At the front the tube extends a little over a foot through the wall. This extended length is equipped with a water jacket and is used as a cooling zone. A pipe passing through the water jacket and into the vacuum retort is connected directly to the oil diffusion pump below. The exhaust of the diffusion pump is connected through a vacuum-tight valve to a mechanical vacuum pump. The front end of the retort is constructed to accommodate a removable face plate, which is machined to seal vacuum-tight against a rubber gasket in a flange on the retort end. I n the center of the face plate there is an adapter that holds a short length of heavy-walled rubber tubing through which a closely fitting length of drill rod may be moved without loss of vacuum. The inside end of the drill rod is provided with a hook, n-hich makes it possible t o move the test samples from the cooling zone to the heat zone and back again without interfering with the vacuum, and to carry out vacuum heat treatments without oxidation of the samples. The face plate is also equipped with a sight glass for visual observation of the samples. The vacuum pumps 7%ere purposely oversized to obtain rapid evacuation rates. The pumps will reduce the pressure only to the good vacuum of about 1 micron (10-6 atmosphere), but such a pressure is considered good enough, as no practical equipment could conceivably be built to operate a t lower pressure. Furthermore, such vacua will reduce the amount of absorbed gases to extremely 10s- values. To illustrate the operation of the vacuum pumps a typical evacuation curve is shown in Figure 3. I n this graph it is apparent that, although the evacuation rate is intermittent, owing to the characteristics of the diffusion pump, a working vacuum of 0.01 mm. (10 microns)
May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
827
is obtained in only 15 minutes. The pressure reaches 1 micron in 35 minutes, and an ultimate vacuum of about 0.5 micron is usually obtained in approximately 1 hour. Pressure indications are obtained with the McLeod gage shown in the lower left corner of Figure 1. Because the research to be conducted was concerned with establishing whether or not any significant improvements in physical properties of steel could be obtained by vacuum heat treatment, it was evident that a basis for comparison must be established. It was decided that the most satisfactory way to do this would be to treat identical samples in identical manner, except that one sample would be treated in vacuum and the other in a suitable atmosphere. T o ensure identical treatments the retort on the right in Figure 1 is identical to thevacuum tube except for the vacuum connections. The proximity of the retorts ensures equal temperature and makes i t simple to run duplicate samples. In general, the procedure is to place the samples into the cooling zones, obtain the vacuum and establish the atmosphere, move the samples into the heat zones for the prescribed treatment, pull the samples into the cooling zones until cold, and then remove them.
Influence of Vacuum on Malleableizing Process Of the experiments that have been conducted, probably the most interesting are the malleableizing tests. The atmosphere surrounding malleable iron during its annealing has a profound effect upon the resulting iron. The object of the
Edge
Figure 5.
Edge
Figure 4.
Core
Samples Malleableized in Cracked Ammonia, 24-hour Cycle (~70)
Core
Samples Vacuum-Malleableized, 12-Hour Cycle (X70)
experiments described is to find how the malleableizing reactions progress when no atmosphere and no dissolved gases are present and at the same time to determine the effect of various types of atmospheres. T o be assured of as uniform results as possible, a set of unannealed malleable test bars were obtained, all poured from a single heat. Chemical analysis of this iron showed 2.54% carbon, 1.26% silicon, and 0.47% manganese. Proper anneal of this iron under the conventional treatment develops 53,000 to 54,000 pounds per square inch tensile strength and 12 to 19% elongation. Anneals were conducted in vacuum, in DX gas (5% carbon dioxide, 1% carbon monoxide, 12% hydrogen, 3% water, and 7% nitrogen), in RX gas (20% carbon monoxide, 40% hydrogen, and 40% nitrogen), in cracked ammonia (75% hydrogcn and 25% nitrogen), and in substantially pure nitrogen (96% nitrogen, 2y0 hydrogen, and 294 carbon monoxide) using the normal cycle, a cycle requiring about three quarters the time, and a cycle requiring about one half the time. The exact cycles are shown in Table I. The physical properties developed by the various anneals are shown in Table 11. To complete the information on these tests, photomicrographs ( X 70) of yarious samples are shown (Figures 4 to 9, inclusive). From these data some important generalizations may be drawn. Table I1 is arranged so that the hydrogen content of the atmosphere increases from top to bottom and the annealing time
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40, No. 5
plex. This rim is possibly formed by decarburization which lowers the carbon content enough to prevent first stage malleableization (high temperature) in the affected part of the sample. Consequently, second stage malleableization is greatly inhibited by the absence of graphite nuclei in that zone. Thus a pearlitic area \+illresult. The work that has been done shows that vacuum malleableizing gives superior results from the standpoint of consistency, speed, and the elimination of rims. However, the cheapness of malleable iron makes it difficult to. visualize a vacuum furnace competing with the present type of equipment. These results lead to the conclusion that high hydrogen atmospheres are to be avoided in malleableizing.
Diffusion Rate of Carbon in Steel At the time the vacuum research was contemplated, it seemed plausible to expect that gases dissolved in steel might play an important role in the diffusion of carbon through steel. On the one hand, such gases could increase the diffusion rate very appreciably by the formation of mobile gas carbides and, on the other hand, could decrease the diffusion rate by some sort of blocking action. Tests were, therefore, designed to find the truth of the matter. Carbon concentration gradients were estabEdge Core lished in test bars in two ways. I n one case plain carbon steel, containing 1.06% carbon, Figure 6. Samples Malleableized in Surface RX Gas, l%-HourCycle IX70) was decarburized for 2 hours a t 1700" F., .. -, and in the other case plain carbon steel, containing 0.207, carbon, was carburized for decreases from left to light. I t is immediately apparent that the 2 hours a t 1700" F. The gradients that resulted from completeness of anneal decreases rapidly with high hydrogen these treatments were found by chemical analysis and are content of the atmosphere as in RX gas and in cracked ammonia, shown in Figurp 10. These steels were then subjected to a and that malleableizing in a vacuum gives a more complete diffusion treatment of 4 hours at 1620 O F. in vacuum and in dry anneal than when the low hydrogen DX and nitrogen atmosnitrogen. When the gradients were again obtained by chemical pheres 'are used, although here the difference is rather slight. analysis, the vacuum samples and the dry nitrogen samples were found to be in excellent agreement. The gradients after diffusion An explanation of this phenomenon must depend upon dissolved hydrogen and possibly, t o a lesser extent, upon dissolved nitrogen are also plotted in Figure 10. No change in the amount of carbon acting t o slow down the dissociation of cementite into graphite. Accelerated annealing cycles were used t o emphasize the effect of the atmosphere and to find the shortest possible cycle under vacuum. In vacuuin, repeated tests using the 12-hour Table I. Annealing of Test Bars cycle (one halt the normal cycle) gave elongations from 13 to 1 j n Cycle Normal Cycle 3/4 Cycle 14% and microstructures substantially free of pearlite. Repeated heat t o 1 5 7 5 ~F. Rapid heat to 15750 F. Rapid heat to 15750 F, Heat 1575 t o 1750 in 5 Heat 1575 to 1750 in Heat 1575 to 1750 in 2.5 tests Tvith DX gas gave a much greater spread of results, the hours 3 7 5 houra hours elongation going from 9 to 1570 and the microstructure a h a p Hold a t 1760 for 7 Hold a t 1750 for 5.25 Hold a t 1750 for 3.5 hours hours F&$~ cool t o 1400 Furnace t o 1400 Furnace cool to 1400 showing an appreciable amount of pearlite. In general, the Cool 1400 t o 1300 in 12 Cool 1400 to 1300 1n 8 Cool 1400 to 1300 in 6 microstructures are in agreement with the physical data. hours hours hours Total time, 12 hours An effect of the atmosphere, which is not reflected in the physTotal time, 18 hours Total time, 26 hours ical properties, occurs a t the edge oi the samples. It may be seen in the photomicrographs that only when the samples are malleableized in vacuum or in nitrogen is the resulting structure Table 11. Physical Properties of Anneals uniform out to the edge of the piece. When the remaining Atmosphere 24-Hour Cycle 18-Hour Cycle 12-Hour Cycle gaseous atmospheres were used there invariably resulted a pearlitic rim, with or without an additional ferritic rim. When va+$ :e lb.jsq. in. 50,500 53,300 50,500 16 15 13 9 the atmosphere was decarburizing, as i t was with DX gas, cracked Elongaklon, % Xitrogen 54,000 51,600 ammonia, and in one case with RX gas, the outer rim consisted of Tensile, lb./sq. in. .... 13.5 12 D ~ l ~ ~ ~ t76l o n v ferrite followed by a pearlitic rim, When the atmosphere was Tensile, Ib./sq. in. 54,400 56,000 58,000 carburizing only the pearlitic rim resulted. In either case these 16.5 14 12.5 Elongation. % rims are to be avoided, because the pearlite is too hard and the RX gas 60,200 62,600 Tensile, lb./sq. in. 54,700 6 ferrite too soft for good machinability. The pearlitic rim, Elongation, yo 12,s 7 which occurs when a carburizing gas is used, is simply a carCracked Tensile,S Ib./sq. Hs in. 60,700 .... .... Elongation, yo 10 buriaed case. The formation of the pearlitic rim inside of the ferritic rim when a decarburizing atmosphere is used is more com-
May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
in the steel was made during the diffusion cycle and nitrogen, at least, does not influence the diffusion of carbon in steel. There remains the possibility that hydrogen and oxygen do have some effect. However, because neither of these gases is neutral to carbon, no direct test could be devised. The answer to this problem was, however, available through a mathematical analysis. The diffusion rate or, more-properly, the diffusivity of carbon in s t e d has been the subject of much research (3, 4). As a result, the diffusivity is very well established under the normal atmosphere conditions. I n addition, a recent article by Harris (2) gave a method whereby the result of a carburizing-diffusion cycle or a decarburizing-diffusion cycle could be calculated. This method was based upon the assumption that the case depth increases according to the square root of the time during a diffusion cycle just as it does during a carburizing or decarburizing cycle. However, the results shown in Figure 10 were immediately seen to be very different from the result obtained by this method of calculation. This difference may be explained if one considers the atmosphere used by Harris during the diffusion cycle. His gas contained a high percentage of carbon monoxide and a low percentage of water vapor and was, therefore, mildly carburizing. The steel contained appreciably more carbon after diffusion than before; consequently, he did not accomplish true diff uFigure 7. sion but rather continued to carburize at a slow rate. The assumption concerning case depth holds reasonably well for this type of process, but it does not hold a t all when the atmosphere (or lack of atmosphere) is inert to carbon. I n order t o check the results obtained after a true diffusion cycle, the equation below was derived. This solution differs from numerous similar ones only' in so far as it satisfies the necessary condition that carbon is neither added to nor removed from the steel during the diffusion cycle.
co -1 c -- l - 2 R - -
m
Cl-$(1 m
2R da2
-cos%)
e
--n42Dt R=
n=l
= carbon concentration at any point CO = surface carbon concentration at the beginning of the
diffusion
CI = base carbon concentration at the beginning of the diffusion
= one half the thickness of the test bar
D
= the diffusivity constant
d
= the equivalent initial case depth
T = time of diffusion X = distance into bar from the surface n = the integers from 1 to infinity *
Core
Samples Malleableized in Surface R X Gas, 24-Hour Cycle (XW of dissolved gases present in normal amounts.
Furthermore, as a result of this work, an exact formula is presented by means of which the carbon distribution may be calculated after any diffusion period when the initial carbon gradient is known. The formula shows that the rate of increase of the pseudo case depth is much greater during diffusion than during carburizing or decarburizing.
Carburizing in a Deoxidized Atmosphere nirx
cos -
where C
R
Edge
829
The equivaleht initial case depth, d, is determined (after Harris, 1) by replacing the actual initial carbon gradient by a pseudo straight-line gradient between the points (0, CO)and (d, Cl), so that the straight line represents the same amount of added or missing carbon that the actual gradient does. This replacement introduces a very small error. When the diffusivity, which is known t o hold for the diffusion of carbon with all the gQses present, is used in this equation, the calculated dibtribution of carbon is found to be in excellent agreement with the vacuum and nitrogen results. I n Figure 10, the circles represent the calculated results. Thus, i t must be concluded that the diffusion rate of carbon in steel is effectively independent of the atmosphere or
R
Much space in the literature has been occupied by discussions of the role of oxygen in carburizing. It has been the contention of some writers that hydrocarbons in the complete absence of oxygen, water, carbon dioxide, and carbon monoxide have no carburizing potential whatever. These conclusions are based, apparently, on the recognized fact that the hydrocarbons tend to decompose less readily when not accompanied by oxygen. The construction of the vacuum equipment offered an opportunity to obtain a t least an indicative check on these theories.
A freshly machined test bar was washed in absolute alcohol, dried, and inserted into the vacuum retort. The sample was then alternately degassed and treated with very dry, pure hydrogen until the entire system was completely deoxidized. A mixture of 96.4% nitrogen, deoxidized and pure, and 3.6% propane, chemically pure, was then passed over the sample for 1.75 hours. The dew point entering and leaving the retort was a t least -50' F. throughout the entire period, which indicated that the oxygen content of the atmosphere was extremely low. At the completion of the carburizing cycle the vacuum was again obtained and the sample cooled and removed for examination. Examination of the sample showed a surface carbon concentration of about 1.5% and a case depth of 0.027 inch. A strip of low carbon steel, 0.008 inch thick, which was put in with the sample, showed a carbon content of 2.13%. The case depth was in agreement with regular carburizing experiment, whereas the surface carbon of the test bar and the carbon content of the thin strip were both somewhat higher than usual. The carburizing potential of the hydrocarbon, deoxidized to the degree indicated here,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 40, No. 5
quantities of hydrogen are introduced into the steel by severe pickling, very marked loss of ductility and increase of hardness result. When this hydrogen is removed by vacuum or allowed t o escape by itself, the normal properties of the steel are restored.
Influence of Vacuum on Spheroidizing The spheroidizing heat treatment is mechani-. cally similal t o second stage malleahleizing. The reaction in the metal, however, is different. I n spheroidizing the iron carhide does not deeompose, but separates from the pearlitic structures and collects into small spheroids. The question arises as t o nhether the rate of this procebs is influenced by dissolved gases. T o answer this question, two steels, ShE 1095 and 52100, nere spheroidixcd in vacuum and in RX gas using a normal cycle, a 25% accelerated cycle, and a 50TGaccelerated cyclc. The results shoved no differences between the vacuum and the conventional treatments.
Quenching and Drawing in Vacuum
Core
Edge
Figure 8.
Samples 3Ialleableized in Surface DX Gas, 24-Hour Cycle (X70)
The occuirence of quench cracks in steel may well be attributed to entrapped gases when other causes are not apparent. When a steel containing gas-filled voids is heated to hardening temperature, the relativply large expansion of the gas causes a pressure of several atmospheres in the void. At the high temperatule the metal is sufficiently plastic probably not to be injured by the pressure. When the sample is qupnched,
showed no tendency t o become l o m r but rather a slight tendency to become higher. I t is readily admitted that this experimental system was not devoid of oxygen, as no real system could ever be. However, the oxygen content of the system was sufficiently low to give a rather positive indication that the reaction of hydrocarbon with steel to form iron carbide is not dependent upon the catalytic influence of oxygen.
Vacuum Treatment of Deep Drawing Steels The adverse effcct of hydrogen upon the drawability of deep drawing steels is well known. The question then arises as to whether or not the drawability may be increased by complete extraction of the hydrogen. Simultaneously, it appeared possible to gain an indication of the rate at which hydrogen may bc withdrawn from steel. To this end a rimmed steel (analysis 0.087, carbon, 0.38ye manganese, 0.006’% silicon, 0.0307, sulfur, and 0.0127e phosphorus) and a killed steel (analysis 125’34carbon, 0.4170manganese,0.028a/o silicon, O.l2Ye aluminum, 0.030Ye sulfur, and 0.00670 phosphorus) were obtained in the cold reduced condition and given a variety of treatments. Immediately following each treatment the samples were tested for hardness, drawability, tensile strength, and elongation. It was evident from the results that, while hydrogen is rapidly removed, no significant improvements in the physical properties of drep drawing steels are produced by vacuum treatment. Vacuum-treated rimmed steel proved slightly softer and slightly more ductile, but the differences were not significant. When large
Edge
Figure 9.
Core
Samples Malleableized in Surface DX Gas, 12-Hour Cycle ( ~ 7 0 )
INDUSTRIAL AND ENGINEERING CHEMISTRY
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831
treated in vacuum or in R X gas. The 4140 steel samples heated in RX gas exhibited lower impact strengths than the samples heated in vacuum. This difference resulted from the fact that the RXtreated samples were badly cracked whereas the vacuum samples were perfectly sound. Microscopic examination of the original metal and the vacuum samples gave no indication of a special reason for the cracking. As all the samples came fromthesame barofmeta1,itmust be concludedthat the degassing action was instrumental in preventing quench cracking in vacuum-treated samples.
1.2
1.0
0.8
0.6
Fatigue Life of Springs
At the conclusion of the tests embracing the common heat treatments it was evident that the gross physical properties such as hardness, tensile strength, and ductility were not to be influenced by vacuum heat treating. Attention was directed to fatigue life, because it was known to be greatly affected by rather delicate changes in the material.
0.4
0.2
.Ol
0
.02
.03
.04
.05
.06
.07
For experimental purposes, steel and berylliumcopper valve springs were obtained and processed. The steel springs were 2.25 inches long, 1.25 inches in diameter, and made of 5.5 coils of 0.164-inch diameter wire. The beryllium-copper springs were 1.875 inches long 1.25 inches in diameter,' and made of 5.75 coils of 0.148-inch diameter wire. The steel springs were heated to 1550 O F. in vacuum and immediately oil quenched. The springs were given the regular shot blasting treatment and air draw and then tested fqr fatigue life. The results were exactly normal, the springs breaking a t 90,000 pounds per square inch 0 0 ~ 0 0 ~ torsional stress in ~ ~ ~ ~ flexures. The beryllium-copper springs were age hardened for 1 hour in at 625 F. After this treatme+ part of the springs were tested for fatigue life. The breaking point was 45,000 pounds per square inch in 10,000,000 flexures. The normal result is given as 40,000 ounds. The remainder of the springs were shot blasted for 0.5 &our and then blued for 0.5 hour a t 450" F. in air. After this treatment the fatigue limit was 65,000 pounds, whereas the normal result is reported to be 60,000 pounds. Over 200 springs were processed in two separate runs; this gain in fatigue life is a definite fact. However, there is some doubt that the improvement obtained is commercial in this instance.
.OS
Figure 10. Carbon Gradients after 4 Hours' Diffusion i n Carburized and in Decarburized Steel
however, the metal cools much more rapidly than the gas, and the pressure is accentuated. The gas pressure and the normal stress set UP in the metal by the rapid Cooling both tend to ruPture the void. At this instant the metal becomes extremely brittle; a quench crack may be developed. If, then, the gas were t o be extracted from the void before quenching, the resultant 1oWering of the stress might be sufficient to avoid the cracking. Absorbed hydrogen could possibly cause quenching damage by a similar mechanism. The solubility of hydrogen in steel is much higher a t hardening temperature than a t room temperature. Therefore, when the steel is quenched the hydrogen, in trying to come out of solution, may cause submicroscopic cracks or voids that could materially lower the strength - of the sample. To test these theories several different steels were heated t o 1550" F., held for 1 hour, quenched in brine, and then drawn to a common hardness. The samples were tested for hardness after quenching and for impact strength after drawing. Duplicate samples were treated in identical manner in vacuum and in R X gas. The average physical results are given in Table 111. In Table I11 it is evident that the effect of vacuum was no different from that of R X gas upon the 5145, 3115, 8640, and 8620 steels. The hardness after quenching, the necessary drawing temperature, and the impact strength were identical whether
Table 111. Hardness and Impact Strength of Steels Sample
4140
Vacuum RX
5145
VPcuum
RX 3115
Vacuum
RX 8640
Vacuum
RX
8620 Vacuum
RX
Rockwell C after Quench
Drawing
TfoT.
Rockwell C after Draw
Impact Value
59 58
935 935
36 37
39 46
59 60
950 950
36 36
34 34
48 48
800 800
36 35
48 48
53 54
875 875
36 36
44 42
53 52
825 825
36 35
66 76
1
Conclusions Theresultsof vacuum heat-treating experiments, although positive in nature, do not apparently lead t o new commercial processes. In general, small quantities of absorbed gases have deleterious, but rather small, effects upon the pro erties of the metal. Comparatively coarse measurements sue as hardness and tensile strength are for the most part unaffected, whereas malleableizing or fatigue life exhibits reco nizable. differences. Absorbed gases can probabl best be thougft of a s inadvertent minor alloying additions. Wgen these additions are allowed t o become large through pickling or faulty melting practice, for example, pronounced effects can result. When the gases are present in amounts corresponding to their solubility values, which are small, their effects are correspondingly small. Vacuum malleableizing is superior in consistency, physical properties, and speed to common prepared atmospheres. The improvement over low hydrogen atmospheres is not very pronounced. Tests show that hydrogen interferes with the malleableizing process, making use of high hydrogen atmospheres inadvisable. I n one case evacuation prior to quenching prevented the samples from cracking, whereas the same steel did crack when quenched from R X gas. Definite improvement in the fatigue of beryllium-copper valve springs was developed by carrying out the age hardening treatment in vacuum rather than in air.
K
Literature Cited (1) Harris, F.E.,&letaZProgress, 44,265-72(August 1943). (2) Harris, F. E.,Metals Technol., 14,No.5,Tech. Pub. 2216 (August 1947). (3) M e h l , Trans. Am. Inst. Mining Met. Engrs., 122 (1936). (4)Wells and M e h l , Ibid., 140 (1940).
RXCEIVED Deaember 16, 1947.
