I N D U S T R I A L -4. N D E N G I N E E R I N G C H E M I S T R Y
July, 1932
Kling, A . , and Florentin, D., Proc. 2nd Intern. Con/. B i t u m i n o u s Coal, 2, 523 (1928). Levi, M . G., Padovani, C., and Amati, 8.,C. A , , 22,2261 (1928). Levi, Padovani, and Mariotte, Ann. chim. applicata, 20, 361 (1930j.
Lush, E. J., J. SOC.Chem. I n d . , 48, 112 (1929). Oshima and Tashiro, J . Fuel. SOC.J a p a n , 7, 70 (1!328). Research Council of Alberta (Canada), 11th A n n u a l Rept.,
NO.26,
2 i (1930).
Shatwell, H. G., and Graham. J. I., F u e l , 4, 75 (1025).
751
(19) Shatwell, H. G., and Nash, A. W., Colliery Guardian, 128, 1435 (1924). (20) Skinner. H. G.. and Graham. J. I.. Fuel. 7. 543 (1928). (21) Yancey,' H. F.,'Johnston, K.'A., and Selvig, W. A., Bur. Mines, Tech. Paper 512, 68 (1932). RECEIVED February 17. 1932. Presented before the Division of Gas and Fuel Chemistry a t the S2nd Meeting of the American Chemical Society, Buffalo, N. Y., August 31 t o September 4, 1931. C. C. Wright was a du Pant fellow for 1930-31.
The Nitriding of Iron and Its Alloys I. Ammonia Dissociation and Nitrogen Absorption in the Nitriding Process .4.W. COFFMAN,Mellon I n s t i t u t e of Industrial Research, University of Pittsburgh, Pittsburgh, Pa. silica combustion tube. To the end U R I K G t h e past five I n contact with the steels under consideration, of this combustion tube was conyears considerable innected an oil bubbler, the excess ammonia decomposition becomes nearly comterest has been aroused ammonia being absorbed in a water 800" C., while the rate of change of plete at A glass T, one arm of scrubber. in the nitriding process as apwhich was a stopcock, was inserted ammonia concentration with the change of plied in producing very hard and between the quartz tube and oil temperature becomes a m a x i m u m at some s o m e w h a t corrosion-resistant bubbler. The gases ammonia, hysurfaces on steel. T h e r e f o r e , drogen, and nitrogen which issued definite temperature coincident with the temperafrom the furnace were sampled .by at the i n c e p t i o n of the studies ture of m a x i m u m nitrogen absorption. T h e means of this stopcock, using r e p o r t e d in t h i s s e r i e s of two glass sampling bulbs filled temperature of this coincidence is dependent on p a p e r s , it a p p e a r e d w o r t h with heavy liquid p e t r o l a t u m the steel. while to c o n s i d e r quantitasaturated with ammonia. The t e m p e r a t u r e was read by a tively what takes place in the gaseous ammonia phase surrounding steel during the process thermocouple located in a quartz protection tube inside the tube, as shown at 2. The small steel samples of nitrification and to obtain a quantitative indication of the combustion X I/, inch, finished to a smooth commercial grind) (1 x amount of nitrogen absorbed over a range of temperatures. were inserted in the fused silica combustion tube so as to be The present paper describes the research carried out with just a t the end of the thermocouple, thus insuring a coincidence between the temperature of the sample and the indicated temperathese objectives. The decomposition of ammonia accompanied by nitriding ture. action, when reaction takes place between ammonia and steel, The procedure included regulation of the temperature of is considerably more rapid than a decomposition of ammonia the furnace as desired; sweeping the train out with ammonia; b y heat alone or by heat plus a surface such a quartz, glass, regulating the flow until constant at 0.06 cubic foot per hour; glazed porcelain, etc. In this connection it is important to and then, after allowing these conditions to prevail for 30 indicate at the start that true equilibrium is probably never minutes, inserting the weighed samples in the combustion reached in commercial nitriding. The original determination tube and taking gas samples a t intervals through the stopcock. of the ammonia equilibrium was reported by Haber and Van The resultant gas samples were analyzed for ammonia and Ordt ( 2 ) and was followed later by the additional contribu- hydrogen in an Orsat apparatus, removing the ammonia with tions by Haber and his co-workers. Perman and htkinson WATER (4) showed that a t 1100" C. in a porcelain bulb a given d 5CRUBEER FOR dh volume of ammonia gas, under static conditions, was comREMOVAL OF E pletely decomposed in 20 minutes. This fact in itself is evidence that, in nitriding, decomposition is not due to heat effects alone, but also to surface and catalytic effects. Ramsay and Young (5)and also White (7) have shown that ammonia decomposition under conditions of flow through tubes depends to a large extent on the chemical composition of the surfaces with which the ammonia comes in contact, the physical strucTo AMMONIA ture of the surface, and its total area. They have shown in SUPPLY addition that decomposition starts at a little below 500" C. FIGURE 1. DI4GRA.M OF APP.4RATUS and is pract(ica1ly complete when iron is present a t or near 800" C.sulfuric acid, and burning the hydrogen b y passing through a copper oxide furnace, the balance being considered as nitrogen. EXPERIMENTAL PROCEDURE I n this manner errors introduced should be found in the balI n determining the percentage decomposition taking place ance. when gaseous ammonia is passed over a sample of steel, the apparatus shown in Figure 1 was employed. AMMONIADECOMPOSITION
D
and an electric furnace of the-Hoskins type, fitted with a fused
active substance such as steel. Consequently, a series of
I N D U S T R I A L A U D E N G I U E E R I N G C H E \f I S T R Y
752
determinations a t five different temperatures were made ith no steel specimen in the furnace. The results are shown in Figure 2 . The amount of ammonia present starts decreasing a t 500" to 550" C., but a t 775" C. there still remains 83 per cent ammonia in the effluent gases. This, as we shall see later, is not the case when ammonia passes over hot samples of steel instead of hot fused silica walls.
I NITROGEN 500
600 TEMPERATURE,T
800
700
FIGURE 2. DECOMPOSITION O F AMNONIA IN FUSEDSILICATUBE
COhTACT MITH
It is interesting to point out in pasing that this difference in ability to decompose ammonia has been utilized recently in a practical manner by Sergeson and Deal (6). I n order to effect an economy in the amount of ammonia used in nitriding, these investigators have resorted to enamel-lined nitriding boxes, thus insuring contact of ammonia with a ceramic material that decomposes ammonia to only a limited extent under the conditions found in commercial nitriding. Using the above described method, steel samples were inserted in the tube furnace and the effluent gases analyzed a t stated intervals of time. Such determinations were carried out over a period of seven hours a t eight different temperatures for steel samples of the compositions shown in Table I. TABLE I. ANALYSIS Carbon ,Manganese Aluminum Molybdenum Phosphorus Sulfur Silicon Chromium
ments until a nearly constant value ib reached, a t which point any decrease becomes much slower. This finding probably mean6 that the original fresh metal surface has a tendency to act as a catalyst, and for a short time forms, as n-ell as decomposes, some ammonia. As time progresses, however, the original surface changes from one of a more or less clean steel to one contaminated with nitrides, and in this manner the decomposition of ammonia is increased. That this drop in concentration is not clue t o the temperature lag present in bringing the specimen up t o temperature was demonstrated by the observation that a t the lower temperatures as many as 3 or 4 hours were necessary to reach the qtage indicating a greatly decreased rate of ammonia decomposition, and a specimen weighing only 13 grams would reach the furnace temperature in a very few minutes. The nature of the sample surface continues t o change upon longer periods of nitriding, as shown by the fact that a t the end of 24 hours the quantity of ammonia present had dropped t o 82.5 per cent a t 500" C., 58.7 per cent at 600" C., and 9.1 per cent at 700" C. This shows that no true equilibrium is reached throughout the nitriding process, but that a marked decrease in the rate of change a t any temperature does take place. TABLE11. DISSOCIATION OF A M M ~ N IIS A COSTACTWITH STEEL1 AT V A R I O ~TEMPERATURES S TEMP.
c.
500 500 '500 500 500 500 500
(Area a n d gas flow constant) T I M EO F SAMPLINQ NHa HZ Minuies % 0
30 60 125 210 270 330 430
9i.7
96.9
96.7 96.5 94 2 94.6 94.2
CoMPoSITIOX O F STEEL SAMPLE5 STEEL1 %
STEEL2
0.19 0.64 2.50 0.85
0.45 0.51 0.93
0,010 0.023 0.29
None
%
None
0.010 0,038 0.26 1.71
STEEI.3 % .. 0.24 0.87 0.81 0.80 0.016 0.018 0.47
Xone
The results obtained in this series of experiments are shown in Table 11. Owing to the difficulty of withdrawing the sampled gas a t a uniform rate, the results obtained a t each sampling interval were not highly accurate, but, when plotted, showed excellent agreement on duplicate experiments. The fact that no hydrogen was found in the samples of gas taken a t 447 " and 500" C. does not mean that there was none present, but may be accounted for by the fact that sampling and analytical methods were not sufficiently sensitive to detect very small amounts of hydrogen. This also may be held accountable for the fact that the relatively small amounts of nitrogen absorbed by the steel did not show up in the hydrogennitrogen analysis. The data given in Table I1 permit several observations to be drawn. At temperatures between 450" and 500" C. the amount of ammonia decomposition is not very different from that found in the case of a fused silica tube (Figure 2). As the temperature is increased, the percentage of ammonia in the effluent gas decreases slowly a t first, but more and more rapidly later until a t 800" C. the decomposition becomes nearly complete. The percentage of ammonia present a t any temperature decreases during the first period of the experi-
VOl. 24, No. 7
600 600 600 600 600 600 600
30 67 120 210 270 330 390
650 650 650 650 650 650
35 95 220 280 340 400
750 750 750 750 750
30 60 130 250 400
75.6 75.6 i2.0
67.7 68,9 67.4
69.0 59.8
57,8 48.5 47.5 47.5 47.5
12.7 11.1 8.6 8.3
5,O
BALANCE %
0 0 0 0
0.9 1.3 1.7 1.4 1.7
0.5 1.4 1.7 2.2 2.1 3.1 4.3
1.9 1.7 1.5 1.2 1.8 2.4 1.6
1.8 1.5 4.1 7.5 7.9 10.5 9.8
1.8 1.5 2.2 3.2 4.1 4.7 4.2
17.6 20.8 24.6 22.3 24.9 23.2
17.8
6.6 6.7 7.1 7.6 8.7 7.7 7.8
29.2 33.2 39.1 39.6 39.7 41.1
10.9 9.3 12.4 12.8 12.8 13.2
59.5 60.1 59.8 63.3 68.2
13.8 17.5 22.2 21.1 21.7
65.5 67.5 69.4 69.5 71.5
21.7 21.4 22.0 22.2 23.5
70.8 70.2 70.5 74.0 73.7
24.7 25.6 25.9 24.1 23.9
I n Figure 3 the compositions of the effluent gases a t the end of 7 hours of treatment are plotted against the temperatures of nitriding. From this set of curves it may readily be seen that the amounts of hydrogen and nitrogen present in the reaction gases increase with the temperature, while the ammonia concentration falls off.
July, 1932
I N DU STR IA L AN D EN GIN EEK IY G CHEM ISTR Y
753
as varying from 25 per cent a t 510" C. to 90 per cent a t 650" C. It is well t o bear in mind when considering the discrepancies which may seem to be present in the work of various investigators on the subject of ammonia dissociation a t various temperatures during nitriding that varying the area of the surface as well as the gas flow has a large effect on the amount of decomposition. Sets of data have been included to emphasize these points. Working with steel 1, the area was varied and the gas flow held constant a t 0.06 cubic foot per hour a t 500", 600", and 700" C. The results are tabulated as follows, all decomposition values being taken a t the end of 7 hours : TEMPERATURE,T
FIGURE 3. DECOMPOSITION OF AMMONIA I N CONTACT STEEL1 AFTER NITRIDISG FOR 7 H O ~ J R S
WITH
Similar data have been obtained on two other nitriding steels a t 500", 600°, 700", and 800" C. These data are presented in Figure 4, where the ammonia concentration has been plotted against temperatures, omitting the nitrogen and hydrogen curves. A replot of the data given on steel 1 is also included here for comparison. These two steels had the chemical composition shown in Table I. The same conclusions may be drawn from these results as were drawn from the data obtained on steel I . It is also apparent that the steels may be arranged in a definite order, this order depending upon the amount of ammonia decomposition apparent a t the end of 7 hours. This is more readily seen by examining Figure 4. Here the curves for these steels are ranged one above the other in the order 1, 2, 3. This is in the order of decreasing aluminum content of the steels and indicates the well-known fact that aluminum is very active in decomposing ammonia. I n addition, it is to be noted that in passing from one temperature to another (from 500" to 800" C., for example) the rate of change of the rate of ammonia decomposition, as indicated by the points of inflection of these curves, moves toward a lower temperature in the same order. "
100,
NITRIDIVG SHa NITRIDING K-HZ TEMP. DECOMPOSED . h E A TEMP. DECOMPOSED Sq.cm. C % Sq rm. C. % h E . 4
500 500 500
11.11 41.61 81.45 9.94 39.50 79.56
600 600
600
GAB N I T R I D I N G NHs F L O W T E M P . DECOMPOfiED Cu.ft / h r . C. % 0.06 600 30.0
1.00 2.75
600 600
Harder, Gow, and Willey (3) recently published results on a steel of nearly the same composition as steel 2. Unfortunately the exact rate of ammonia flow was not recorded in their paper, so it is rather difficult to compare the two sets of data. Harder and his co-workers found in general, however, that ammonia decomposition increases with rise in temperature and that, dependent on the gas flow maintained, the percentage of ammonia decomposition occurring a t 63.5" C. varied from 74 to 84 per cent. Similar results are reported for two other steels, although little difference is shown from steel to steel. Grossman (1) gives the ammonia dissociation occurring during the nitriding of Nitralloy (0.31 per cent carbon)
85 96 98
9.0 4.0
G\s N I T R I D I N G NHJ F L O W TEMP. DECOMPOSED Cu. ft./hr. * C. % 0.06 700 85.0
1.00 2 75
700 700
27.0 8.0
The nitrogen absorption occurring during the nitriding cycles described above was determined by weighing each sample before and after nitriding. The gain in weight was considered as the nitrogen absorbed. At high nitriding temperatures this measure of nitrogen absorbed is not strictly correct, as some decarburization occurs owing to hydrogen reduction. As an indication of the amount of decarburization to be expected over the temperature range 500" to 800" C., standard samples of steel 1 showed the following losses in weight due to the removal of carbon after heating for 24 hours a t the indicated temperatures in a stream of hydrogen: LOSS IN WEIGHT
c.
Gram
500 600
HOURS
700 700 700
NITROGEN ABSORPTIOX
0
TEMPERATURC~C.
10.30 41.87 80.64
Holding the temperature and area constant but varying the gas flow also shows quite marked differences in ammonia decomposition at the end of 7 hours as follows:
TEMP.
FIGURE 4. DECOMPOSITION OF AMMONIA IN CONTACT WITH STEELS1, 2, AND 3 AFTER NITRIDIVG FOR 7
5 9 10 31 55 63
0.0004 0.0006
Loss I N WEIGHT
TEMP
c.
Gram
700 800
0.0042 0.0118
These losses in weight, when considered with the relatively large observed gains resulting from nitriding, may be neglected when drawing conclusions concerning the relationships existing between temperature and nitrogen absorption. I n Table I11 are listed the amounts of nitrogen absorbed by samples of the three steels nitrided for 7 hours under the above described conditions. Figure 5 shows a plot of these data. It is evident therefrom that there is in the case of each steel a temperature at which nitrogen absorption is greater than at either higher or lower temperatures. This temperature of maximum nitrogen absorption is not the same for all steels but is variable, and in these particular cases falls in the neighborhood of 700" C. for steel 1, and 650" C. for steels 2 and 3. TABLE 111.
NITROQEN ABSORBED BY STEEL SAMPLES IN 7
NITRIDING N I T R O G E N STEEL TEMP. ABSORBED c. Cram .. Q
447 500 600 650 700 750 800
0.0077 0.0219 0.0458 0.0650 0.0715 0.0641 0.0460
HOURS
NITRIDINQ NITROQEN STEEL TEMP. ABSORBED O .C. Cram
500 600 700 800 500 600 700 800
I K D U S T R I A 1. A N D E N G 1 N P; I.: I< I N G C I t t.: \II S T I< Y
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Tlieje gh-in-weight curves, iriiiicating the relative iiitrogcn sbsorbine ability of the steel, demonstrate that aluminum
gained by a spi!eimen on nitriding increases with iircrease in temperature ovcr the temperature range 468" to 635' C. If tire data given in Figure 4 are reglotted as in Figure 6, it is readily observable that the temperature at xhich tlre change in ammonia composition with temperature is most rapid corresponds to tlre temperature at which the maximum ;&sorption of nitrogen occurs. For the steels under consideration, the coincidental temperatiires are 700" for steel 1 and 650" C. for steels 2 arid 3. It m:iy perhaps he that such n coirreidaocc of t h e maxiina is attributable to a turning point in the reactions taking pl:tce iir iiitride furmation, and that at the tcmperatures of tliese maxiiira a dccided thernial ~ decomposition of the nitrides~ udiieb have hcen formed is initiated. Or perhaps it may he that a t t h e temperatures the metal atoms have a themmi vibration whioli is peculiarly well suited to animoiiia (I?i:omponitiiin ~ i i dnitrogm absorption.
ACIXOWLEDGIIEST This series of two papers prcsents tile results of :in irii-cstigation of tire nitriding of steel coiidirctcd at Melion Institute of Iiiilustrial Tiesearch during the 1x:riod 1927-28. The fcllowshipmurk, u41ich ivas sustaiiied by the 13. II. Kobcrtsoii Company, Pittsburgh, Pa., as carried on co6peratir-ely by that company and tiit: Molybdenum Corporation of Smerica. Acknodcdgment is here made of the assistance rendered by tlie I\lolybdonunr Corporation in making d l ;rnalysrs of steels and alloys and in tlie maeliining of 811 saniplvs. Tlin author wishes bo take this opportunity to express Iris appreciation of the helpful niivice and criticism of J. 11. YorrnK, under whose direct supervision the research was performeii.
j
the receptiveness of the metals toward nitrogcrr as ncll as the ability of the steel to decompose ammonia. Harder, Gow, and Willey have sliown in this coImecti()n that with a given set of wnditions the amount of weight
Vol. a, No. 7
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(1980h (7) White, J . Am. Chern. Sue., 27,373 (19061. Febiueiy 19, 19x2. TI,^ a u t ~ t o r is ~ ~ ~ t i Induatriai t ~ t , ~ Research. ~ t Kl;cE,YEn
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