T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
452
Vol. 5 , No. 6
TABLE111-SHOWINGRELATIVE LOSSESON 2 X 4 INCHTESTPIECES Exposed in rural community December 3, 1911, and taken down November
yet it showed much greater resistance t o the weather in each of the three characters of atmosphere. Steel No. 5 dissolved more than twice as fast as steel No. 2 (Each result is the average of six pieces) RELATIVE (both copper-bearing), while at two of the test stations LOSSES it gives a greater resistance t o the weather. Several ANALYSIS (PERCENTAGES) 100 = other inconsistencies could be pointed out, and i t is .POSI- GREATEST GRADE PANELGA. C Mn S P CUTIONCORROSION the writer's opinion t h a t the accelerated acid test Open Hearth.. 2 27 0.06 0.35 0.036 0.018 0.16 1 51.89 used for the purpose of determining the values of steels Bessemer... .. . 9 27 0.05 0.44 0.075 0.099 0.34 2 52.69 or irons in their resistance t o corrosion is untrustOpen Hearth.. 2 16 0.10 0.34 0.035 0.020 0 . 1 6 3 52.69 worthy and a p t t o be misleading and should be Bessemer ..... . 8 27 0.05 0.44 0.082 0.101 0.23 4 53.88 Bessemer ..... . 9 16 0.07 0.46 0.069 0.095 0.33 5 54.67 abolished, and should copper-bearing steels be desired, OpenHearth.. 3 27 0 . 0 6 0.33 0.035 0.018 0.25 6 55.07 the presence or absence of t h a t element determined by Open Hearth.. 5 27 0.10 0.46 0.035 0.043 0.17 7 55.27 Open Hearth.. 5 16 0.13 0.44 0.035 0.042 0.18 8 55.27 chemical analysis. Open Hearth.. 6 27 0.07 0.47 0.038 0,043 0.25 9 55.67 It is a well-known fact t h a t copper is electronegative Open Hearth.. 3 16 0.10 0.35 0.033 0.019 0.23 10 55.87 t o iron, and when placed in contact with iron it will Bessemer ...... 8 16 0.08 0.45 0.070 0.094 0.21 11 57.26 Low C and low stimulate corrosion in the latter element. That the Mnmaterial. 10 16 0.03 0.03 0.034 0.003 0.06 12 57.46 reverse is true when the copper is alloyed with the iron OpenHearth.. 6 16 0.14 0.46 0.038 0.043 0.27 13 58.25 and in solid solution in the crystal grains, h a y be due Low C and low Mnmaterial. 10 27 0.02 0.03 0.036 0.003 0.07 14 64.42 t o the alloy taking in a measure the non-corrosive Open Hearth.. 4 16 0.13 0.45 0.035 0.042 0.00 15 68.79 properties of the copper. It has also occurred t o the Open Hearth.. 4 27 0.09 0.47 0.037 0.043 0.00 16 71.77 Open Hearth.. 1 16 0.10 0.34 0.034 0.019 0.00 17 76.54 writer t h a t the alloy of copper and iron is less electroBessemer ..... . 7 16 0.08 0.46 0.070 0.098 0.00 18 90.26 positive t o the first film of rust formed than is nonOpenHearth.. 1 27 0.06 0.35 0.033 0.018 0.00 19 90.66 copper steel, and the consequent decrease in difference Bessemer ..... . 7 27 0.05 0.45 0.076 0.100 0.00 20 100.00 of potential lessens the corrosion. solubility of the steel in sulfuric acid. Inasmuch as a It has been suggested by Dr. W. H. Walker that the copper content also increases the resistance t o atmoscopper prevents the oxides of manganese and iron, pheric corrosion, a certain relation is established bewhich may be present, from coming out of solid tween the accelerated acid test and natural corrosion solution as the melt cools, and hence, although the when comparing copper-bearing with non-copperoxides are still present, they are held uniformly disbearing steels or irons. On the other hand, many solved, and not segregated between the iron crystals, instances have been noted and published where the as is normally the case. Sufficient work has not been Of acid tests have been opposite to done to form any definite conclusions, and it is not the results given by the same steels in service. We have intention of this Daper to discuss this phase of the . . + TABLEIV-COMPARATIVETESTSSHOWING SOLUBILITY O F VARIOUS STEEM subject a t length, but rather t o place before you IN 25 PER CENT SULFURIC ACID AT 35' CENTIGRADE results which seem to prove that a small copper Results are the average of four determinations on four different sheets content in steel (approximately 0 . 2 per cent) maof each grade. terially increases the life of steel sheets when subjected Grades 1, 2 and 3 from same heat. Grades 4, 5 and 6 from same heat. Grades 7, 8 and 9 from same heat. t o atmospheric corrosion. 16, 1912
I
PERCENTAGE LOSS AT END
6 GRADE GA. Open Hearth,. 16 1 Open Hearth 16 2 Open Hearth 16 3 Open Hearth 16 4 Open Hearth 16 5 Open Hearth.. 16 6 Bessemer.. . . . . 16 7 Bessemer 16 8 Bessemer . 16 9 Low C and low Mn material.. . 16 10 Open Hearth ..... 27 1 Open Hearth.. 27 2 Open Hearth 27 3 Open Hearth.. 27 4 Open Hearth.. , 27 5 Open Hearth.. . , 27 6 Bessemer 27 7 Bessemer ... 27 8 Bessemer ........ 27 9 Low C and low M n material.. 27 10
... ..... ..... ... .. .... . .. . .. . ........ . ........
... ..... ... .. . ........ . ..... . . .
PITTSBURG, PA.
OF STATED
ANALYSIS
7
Y
-
T
(PERCENTAGES)
-
p
0.10 0 . 3 4 0.034 0.019 Trace 0 . 3 4 0.035 0.020 0.35 0.033 0.019 0.45 0.035 0.042 0 . 4 4 0.035 0.042 0 . 4 6 0.038 0.043 0.46 0.070 0.098 0.45 0.070 0.094 0.46 0.069 0.095
0.03 0.06 0.06 0.06 0.09 0.10 0.07 0.05 0.05 0.05
0.03 0.35 0.35 0.33 0.47 0.46 0.47 0.45 0.44 0.44
0.160 0.230
Trace 0.177 0.265
Trace 0.207 0.327
0.003 0.061 0.018 Trace 0.018 0.018 0.043 0.043 0.043 0.100 0.101 0.099
0.160 0.250
Trace 0.170 0.250
Trace 0.226 0.340
0.02 0.03 0.036 0.003 0.069
INTERGRANULAR CEMENT IN METALS' B y W.
x
0.10 0.10 0.13 0.13 0.14 0.08 0.08 0.07
0.034 0.033 0.036 0.035 0.037 0.035 0.038 0.076 0.082 0.075
PERIODS
16.83 42.26 1.50 2.06 0.87 1.38 31.29 78.15 1.25 2.16 1.22 1.68 99.00 100.00 2.09 4.40 1.22 1.88 1.35 2.64 41.92 100.00 1.28 2.04 0.90 1.54 88.23 100.00 2.02 4.33 1.29 2.06 100.00 1.19 2.40 0.95 1.69
..
4.75
9.08
several instances of this in the tables given with this paper. Steel No. 4 dissolved about twice as fast in acid as steel No. I (both without copper), see Table IV,
E. RUDER
I n the study of metals, the microscope has revealed the fact that all solid metals, even the softest. have a distinct crystalline structure and that this structure has a great influence upon their mechanical properties. This has perhaps been most closely studied in the case of iron and steel, and the general conclusion t o ' be drawn is t h a t for greatest mechanical strength a very fine-grained structure is desirable. Stead" has pointed out that brittleness may be of two kinds, intergranular and cleavage, and that the latter is the most prevalent. The expression grain and granular, herein, refers t o the allotriomorphic crystals of metal in distinction t o the perfect cubic crystallization of the interior. It was, I believe, first pointed out by Rosenhain and E w i n g , ~and later confirmed by Stead and others, that fracture, in most metals, under normal conditions never occurs along the boundaries of the grains but 1 Paper presented at the Annual Meeting of the American Chemical Society, Milwaukee, March, 1913. Metellograghist. 1, 289 (1898); 2. 85. 3 Proc. Roy. Soc., 66, 85 (1899).
always through the grains, i. e . , i t is a cleavage fracture. This greater strength of the boundaries is, then, of considerable importance, and the question of its composition naturally arises. Until quite recently, however, there has been very little investigation of these boundarics and their possible composition.' Rosenhain and Ewing first advanced the view that these boundaries consisted of a eutectic or minute quantity of impurities existing in the mctal, a "fusible and mobile eutectic forming the intercrystalline cement." through which crystal growth was made possible b y electrolysis. Since the eutectic' is of lower melting point, it exists, at the temperature of crystal growth, in a molten or scnii-molten condition. G . T. Beilbys , advances the theory that metal c r y s t a l s when strained become surrounded with a layer vi metal in the amorphous phase, which is tougher and more resistant mechanically than that in the crystalline phase. Recently Rosenhain a n d Ewen3 have elaborated upon this t h e o r y of t h e amorphous phase a n d strengthen their argument b y e x p e r i m e n t s in measuring the relative amount of evaporation from large and small grained samples, fired together in Fie. I a vacuum. They find a constant ratio (1.07 to 2.37 for different metals) between their rates of loss in weight. I n studying the properties of silicon-iron alloys the author has performed several experiments which ought t o throw some light upon this interesting question. Ordinarily, the study of such problems as these is considerably hampered b y the fact that the change in the crystalline grains in metals is too small t o follow closely without the aid of a microscope. If we can, in some way, enlarge these grains the problem would become casier t o handle. The addition of silicon has been found4 t o aid greatly in the production of large grains. D B Y E L O P h i E N T O F L A R G E GR.\IX
The steel used in these Since this
STRUCTURE
had a silicon
there has come t o the author's hands a ~ ~ madmirabk f ~ ~ on~ "The . Intcmristaliine ~ srlcturr xron and Steel.'' carnepie hremoirs, iron o d Sfeel I I L S ~4, .. 8-mi (1012). Phil. Mw..8, 258 (1904): Proc. Roy. Sac.. 79, 463 (1907). a J . inst. M ~ ~[21~8 , 1 I 49 ~ (1912). , M#nflogru*hlsL. 1, 325 (1908). I
article was written
content varying from 2; to j per ccnt and other impurities reduced t o a 1ninirnurn. A typical analysis before any heat treatrnent is here given: Si, 3.80 pcr cent; S , 0.028 per cent; P, 0.010 per cent; Mn, 0 . 0 7 2 per cent; C, 0.050 per cent. Under the proper ccSnditions of annealing, such a steel will produce grains oi enormous size. Fig. I is part of a single grain, natural size. The average size of grain is about 9 5 - 5 0 sq. mm. (cf. Fig. r a ) . Small grains may be produced wherever desired by hammering and subsequent annealing as shown in Figs. 8 and 9. The effect of annealing and mechanical working upon this alloy will be more fully discussed in a later paper. For most of the work no special polishing was r e q u i r e d , merely smoothing up with No. o emery and etching with nitric acid. A rapid and efficient etching was obtained by using concentrated nitric acid and alternately dipping and washing the s a m p 1e s , allowing them to remain in the acid only a fraction of a second a t a time. I n order to prevent oxidation of the steel samples practically all of them were heated in h y d r o g e n in a platinum wound tube iurnace or in an Arsem vacuum furnace. I n investigating t h e growth of grains it was observed that after certain heat t r e a t ments the fracturc of this alloy was granular, instead of crystalline as under normal conditions. I n other words, something had occurred which had weakened the specimen along the lines of the grain boundaries. How then do these boundaries differ from the main mass of metal? Are they merely lines of separation, or are they actual areas, consisting of the metal in a different physical state, or containing some impurity
454
T H E J O U R - V A L OF I N D U S T R I A L A S D E L V G I S E E R I S G C H E M I S T R Y
thrown out by the crystalline grains in of formation and growth?
the process
WTS.:
TIME
3
4
Nos. I and 4 were analyzed as a blank, while 2 and 3 were electrolyzed and the separated grains analyzed ; the results follow: C
Samples 1 and 4 . . . . . . . . . . . . . . . 0 . 0 4 9 Samples 2 and 3 . . . . . . . . . . . . . . . 0.055
Si 3.856 3.856
To determine the loss in weight up to the time the sample crumbled, two sheets were carefully cleaned and weighed. These were then electrolyzed. The loss is here shown.
6
No. 1, 6.2800 GRAMS;h-0. 2, 6.5226 GRAMS LOSS CURRENT
No. 1
ETCHING W I T H POTASSIUM DICHROMATE
Examination under the microscope gave no indication of the existence of any area of measurable width. Various etching agents were then tried in the hope of obtaining some discriminating agent which would dissolve or reject the material between the grains. Such a result was finally obtained by making the sample the anode in a solution of some oxidizing agent a s potassium permanganate, chromic acid, or potassium dichromate. The last named gave the best results. I t was found t h a t when a sheet of this silicon-iron alloy was treated in this way for about I j minutes, the grains on the end began to drop off, and the whole sheet could be crumbled in the fingers, the grains being perfectly separated a t their boundaries. I t was soon noted t h a t certain sheets fired in hydrogen did not give this effect, a t least not so perfectly, but t h a t sheets fired in nitrogen, air or vacuum seldom failed t o give this effect. I say seldom failed, but as a matter of fact there was only one lot of metal that failed. What the difference is between this and the others is difficult to say. Indications are that it is composition, for, if we take a single grain of this kind of metal and form in its center a series of new grains by hammering and heating ( c f . Figs 8 and 9 ) , these new grains also are not effected and the metal is as strong as ever, while those made in a like manner in the metal whose grain bou*ndaries were soluble are, like the original ones, causedko fall apart. The question of the quality and quantity of material removed b y the electrolysis naturally followed. Ana1ysis"of the solution after electrolysis showed a large amount of Fe present (6.0-7.6 per cent), b u t this is more-than could possibly be accounted for as coming from,, the boundaries alone. Attempts a t analysis of the'gases given off, on the assumption t h a t carbon present as carbide in the boundaries would be oxidized in the electrolysis, were unsuccessful. A further attempt at analysis was made on four samples cut from a single sheet a s shown in the sketch.
j, N O .
Vol.
0.045 0.046 0.094 0.085
gone
No. 2 0.0226 0.032 0.0365 0.041 0.0165 0.058 0.035
No. 1 0 . 2 1 8 Amp.
so. 2 0 155 Amp.
Total loss till softening ( a ) , S o . 1, 0.25 gram (4 per cent) Total loss till softening, No. 2, 0.26 gram (4 per cent).
(01 B Y "softening" is meant the point where the last trace of cohesion between the grains disappears, and the sample begins to fall apart from its own weight.
If these figures are plotted on a curve ( c j . Fig. 2 ) we find that the loss in weight increases with the 1.3 power of t h e time. This is probably due t o a uniform increase in the dissolving of the grains themselves, or t o the weakening of the solution by the ironchromium compound which separates out as a precipitate.
Aelation
0~ LOSSia Wts t o &me
e
E/ecfio/ysis of Si-Steel /jl /rt Cr20, EG,2. "
June 14 / w z .
The concentration of the electrolyte was varied from I saturation and i t was found that the rate of solution increased with the decrease in concentration. I t was desirable t o ascertain just what width the intergranular lines would assume on continued etching, and two samples were prepared with a backing of soft solder which, i t was found, was not attacked by the acid. This solder prevented the piece from falling apart. The increase in the width of lines was measured with a micrometer eye-piece. The lines a t the beginning (during the first, 24 hours or so) were very uniform in width. The measurements are here given:
'is to
WIDTHO F LINE TIME 48 hrs. 6 0 '' 72 I ' 9 6 '( 120 ( 1 144 " 168 '' 192
No. 1 0 . 0 5 mm. 0.07 '1 0.086 I 0.130 " 0 . 1 4 7 (' 0.184 " 0.190 "
...
No. 2
... 0 . 0 9 mm. 0.12 " 0.122 " 0.165 " 0.183 " 0 . 1 8 4 '' 0.190 "
Thc action did not. cease. however, although it became much slower, as the figures show The exppri. ment was discontinued because certain of the grains where measurements were taken, dropped out. Photographs 3 and 4 show the clean cut way in which thc
surface is pitted with small cavities about 0.08 mm. in diamctcr on the average, and about o.xj mm. apart. 'The surface of the metal is also discolored. Around the grain cdges, however. there is a bright band. varying from 0.25 t o 0 . 5 0 mm. in width, which is almost entirely free from any marking whatever. From these grain edges, in practically every grain. start tiny fissures which run back into the grain for ahout 0 . 2 ~ 0 . 5 mm. These fissures are evidently along crystal edges, for they run in parallel straight lines, varying in dixection in the different grains. j B is a contiguous section of thc same casting treated in the same may, hut having first hcen annealed. In this samplr the grain surfaces were uiitouched, and only the boundary lincs had been attacked. These experiments seem to indicate that there is some chemical change in the grains themselves during anneal, that is to say, beforc annealing, the material .xx>lcii so quickly that the impurities had no time to [migrate to the grain boundaries except those vcry near ir,f 5.4). Aftcr this piece is annealed, however, we
1:xi. .I
solution works. The black areas are the places where grains have become loosened and dropped out. I t was found in the above experiments t h a t the crosssection of the grains after electrolysis was elliptical, so that there must have been a dissolving away of the edges, thereby widening the spaces more than the "space" material would justify. Mechanical cracks of various kinds were made in the material t o see if the mere junction of two surfaces would cause similar electrolytic attack, but in no case were these cracks affected in the least. The polished surfaces of the grains remained after lengthy treatment, and there was no sign of attack except at the boundaries. The chromic acid appeared t o hold them entirely passive. Assuming then that the attack was caused by the foreign material present between grains, it w d s deFro. 4 cided to try the effect upon discs of iast metal. Here we have large grains but the sudden cooling might not have no evidence of action on the surface of the grains, allow the impurities time to diffuse out. of the grain. only a t the boundaries, The photographs j.A and gH show the results obHYDROGEN TREATMENT tained. Fig. SA shows an unannealed cast disc treatcd with By treating a bar of this material in dichromate the KzCr,O, by electrolysis. In this sample the whole grains could be readily separated, as shown in photo-
4.76
T r m J O U R ~ V A LOF I N D U S T R I A L A N D E N G I N E E R I N G C I I E M I S T R Y
graph 6 . In this way large masses of material making up a single grain, could be isolated. Following up the results noted in the beginning of this work, namely, that heating t o a high temperature in hydrogen near the melting point seems to cause a fusion between grains, and a decided weakness, I tried a large number of bars in different atmospheres and at different temperatures, t o ascertain what the conditions were that caused this weakening of the boundary lines between grains. It was found that when a bar or sheet is heated t o 1 4 0 0 ~C. in hydrogen for about 8 hours it becomes very brittle, and the grains may be separated by hammering the specimen. A ‘/,-inch rod so treated, broke into several pieces just by dropping 3 feet t o a wooden floor, and when hammered lightly the grains were readily separated.
vel. j, N
~6 .
I n answer to the Srst two questions, cight bars were heated at different times t o the same temperature (above r400° C.) as those treated in hydrogen. Some of these were heated in air, and others in air freed from oxygen, but in no case was the fracture changed from the original cleavage fracture. I then fired a bar in hydrogen. to obtain this weak intergranular condition, and then, assuming that t h e
A
Pro. 6
PIC. 5
It was also ohse.rvcd that this intergranular hrittleness was not obtained under 8-10 hours on large bars I ’ / ~ x x/z inch cross-section, while on the ,/,inch rods i t was obtained in about a n hour. Photograph 7 shows a granular (I) and a cleavage (11) fracture. The time factor together with the KSCraO, experiments led us to helicve that the action of the hydrogen must be of a chemical nature, and several other experiments suggest themselves whereby this theory might be proved or disproved: First, Iloes any other gas give thc same result? i. e . , is it a temperature effect? Second, If something is removed can i t be restored again, and if so, will this restore the toughness of the metal? Third, If something is removed, then the composition of the “cement” is changed, and i t is a auestion whether the chromate electrolvsis will then fail to separate the grains.
“cement” was carbide which had been destroyed on heating. refired in hydrogen containing a hydrocarbon vapor. This refiring was done at the same temperature and time as in the original treatment. When
. . .
.
.-
Pro. 7
tested after this treatment it was found that the sample was as strong as it had been originally, and the fracture was now across the grains instead of along t h e boundaries
June,
1973
T H E J O U R N A L OF I N D U S T R i A L AI\-D
Ten or more repetitions of the experiment were made at different times, varying the conditions and samples, etc., but always with the same result, uiz., the "cement" material weakened b y the hydrogen was always returned t o its original toughness by the hydrocarbon treatment, the piece then always splitting across the grains and never a t the grain junctions. Moreover, the same sample could be reverted from one condition to the other at will and as often as desired. Another cxperirnent points very strongly to the theory t h a t something is removed b y the hydrogen treatment. If a bar which has bcen treated in this manner so t h a t the grains are readily separated on hammering, is soaked in molten copper at about izoo0 C. for an hour or so, i t is found that the copper has diffused into the bar between the grains, and t h a t every grain is covered with a very thin coating of copper. This could not be done with material having a cleavage fracture. In answer t o the third question, we had already observed that s o m e hydrogenfired s a m p l e s (possibly t h o s e heated a t l o w temperatures) did not separate on the grain boundaries whenelectrolyzed in chromate solutions. Numerous experiments were subsequently tried in the light of these other experiments and they all showed the same results; namely, that samples, w h i c h h a d a cleavage fracture and would readily fall apart at the grain junctions if electrolyzed in chromate, would not, I after having been Fio 8 so treated a s t o give a granular fracture. show any effecta t all when electrolyzed in K&r20v The reverse, however, is not always true, i. e . , all samples which do not separate by the chromate treatment have not, of necessity, intergranular brittleness (only one or two samples). It will be seen then that all of these experiments seem t o indicate that the chemical theory is the correct one. One other experiment, however, seems to make this conclusion still somewhat doubtful. When a sheet has been annealed t o 1350" C. or so, and the large grains form, according to the above assumption the imnurities have been thrown out of the grains. Now, if inside one of these large grains
EXGIiVkERISG CHEWISTRY
457
smaller ones can be produced ( c f , Figs. 8 and 9 ) . these should show no separation when electrolyzed in the chromate solution. This was tried on three samples, and in every case the sccondary grains
PIG. 9
droppcd out as wcll as the original ones. This argues rather against the theory that thcre is a chemical difference between the grains and the boundary lines, unless we assume that there has been some impurity absorbed from the gases during annealing. This is quite possible, confiidcring that the strained portion must have contained quite a little of thc more active amorphous phase. RESISTIVITY XBASUREMENTS
By drawing this iron-silicon alloy out into a wire ( 0 . 0 2 inchj it was possible to obtain pieces containing comparatively few grains along its length. Most of the grain junctions extended right across the wire, so that compared with the fine-grained material, a n electric current has much fewer junctions t o pass through. If the crment has any different resistance than the grain material then it should show up in resistivity measurements. A series of ten measurements was made and resistances taken: (I) as drawn, ( 2 ) annealed at 800' C., (3) annealed at 13350~C. The results showed an average of 3 per cent increase in resistance for the 8 o o o C. anneal, and a subsequent 1 5 per cent decrease, from the original value, for the large-grained samples. An attempt to calculate the resistivity of each boundary by counting the number proved fruit-
458
T H E JOCRi‘\7A4L OF I,YDC-STRIAL A K D E S G I S E E R I - Y G CHELVISTRY.
less, because of the existence of a large number of grains that did not run all the way through. The increase a t 800’ C. is unusual, because in most metals the resistivity is lowered by annealing. If we consider the results entirely from a point of view of the number of grain boundaries along the length of the wire, we would expect a slight rise in resistivity, owing t o the fact that in the unannealed state the grains are drawn out, and so present fewer boundary lines along the length of the wire than when subsequently annealed. If the change in resistivity were due wholly to the presence or absence of the amorphous phase then we would expect the usual decrease in resistivity after the first anneal. When heated in a high vacuum the cement apbarently volatilizes more rapidly than the metal grains, and leaves deep fissures between the grains. Such a n experiment, using pure silver, is given by Rosenhain and Ewen a s proof that the cement was amorphous silver, but it is readily conceivable that any foreign substance present may also have a lower vapor pressure and give the same result. We also obtained like results with wrought tungsten. Although the facts seemingly indicate that carbon, either free or combined, is the cementing material, all attempts to etch out the Fe,C from a cementitic steel b y electrolyzing in chromate have failed. A piece of “burned” steel containing a smaller proportion of cementite, similarly treated, separated nicely on the grain boundaries. Commercially pure ingot iron is not attacked, while iron containing a high percentage of free carbon is rapidly dissolved. Analyses were made of samples from a bar: ( I ) as rolled, ( 2 ) fired in hydrogen to 1 3 2 j O C. to a partially granular fracture, (3) fired t o 1425’ t o a complete granular fracture, and ( 4 ) refired in hydrogen over hydrocarbon till it reverted to a cleavage fracture. The results gave no indication that any of the impurities (S, P, C or Mn) had been removed or reduced b y any of these treatments, with the exception of carbon, which was reduced from 0.065 per cent to 0.013 per cent on the first heating in hydrogen. The carbon content, however, did not go up again on heating with dilute hydrocarbon vapor. The proof that this material is carbide or any other of these impurities is still indirect, but on the other hand the assumption that the cement is only the amorphous phase of either metal or of the solid solution is in view of these experiments insufficient. If we assume that this is the case, we would expect this amorphous material to exist in all samples, and moreover, once destroyed by heat we would hardly expect i t t o be restored on reheating t o the same temperature in a slightly different atmosphere, and this change reversible indefinitely without the least change in size or shape of the grains. On the other hand, the assumption that these boundary lines hold certain impurities explains most of the facts observed, for then the action of the hydrogen partakes of a chemical nature, and the reversing of the physical state is due t o the addition or subtraction of some specific element whose presence or absence determines
Vol. j, NO. 6
the strength of the junction. The experiments with samples of cast metals also strengthen this view, and when the metal is fused in pure hydrogen the old grain boundaries vanish, while the new ones are not attacked by the chromic acid, owing to the removal of the impurity. These experiments, i t will be remembered, have all been performed upon a product having only a commercial degree of purity, and the existence of sufficient impurity in highly purified metals may be questioned, but the amount required is so small t h a t it would be difficult t o obtain a metal sufficiently pure. S U M M 4 RY
It has been shown that the intergranular cement in a 4 per cent silicon-iron alloy may be completely removed by making the alloy the anode in a solution of potassium dichromate, and also by firing in hydrogen to a temperature just below the melting point. I t is again replaced in the latter case b y firing in a dilute hydrocarbon vapor. I t is shown t h a t in the unannealed cast metal impurities exist throughout the mass of the grain, and not only a t the boundaries, while after annealing these impurities have passed t o the boundaries. Resistance measurements on large and small grain wires show that the “cement” has a higher resistance than the crystalline metal. . I n view of the facts it does not seem sufficient t o say that the intergranular cement is composed entirely of the grain substance in the amorphous phase, b u t rather that it consists of the metal in combination with certain impurities which have been rejected by the crystalline grains. I t is not the purpose of this paper to deny any existing theory regarding the composition of the intergranular cement in metals, for the author realizes its limited scope, but it is hoped that the facts observed may aid somewhat in the general investigation of this subject. RESEARCH
LABORATORY
GENERALELECTRIC COMPANY SCHENECTADY
INFLUENCE OF VARIOUS ELEMENTS ON THE CORRODIBILITY OF IRON’ B y CHARLESF.
BURGESS
AND JAMES
ASTON
Incidental t o a n extensive investigation on electrolytic iron and alloys produced therefrom, as carried out under a grant made a number of years ago by the Carnegie Institution, tests were made upon the corrodibility of a number of alloys. Acknowledgment is made t o Mr. B. F. Bennett for his assistance on this particular phase of the work. The electrolytic iron which has been used as a basis for the alloys, when compared t o materials commercially available, may be taken as essentially pure and in making up the alloys care was taken to exclude impurities as far as possible. The method of preparing the test samples consisted, first in melting the electrolytic iron or the electrolytic iron alloy in closed graphite magnesia-lined crucibles heated in an elec1 Paper presented at the Eighth International Congress of Applied Chemistry, New York, September, 1912.
.
