AMERICAN INSTITUTE OF MINING AND- METALLURGICAL ENGINEERS Technical Publication No. 1553 ---

(CLASS C. IRON AND STEEL DNISION, NO. 330) DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented by the contributor in

person at the New York Meetlng February 1943. when an abstract of the paper will be read. If thls 1s Impos- able. discussion in writing (2 co$es) may be sent to the Secretary. American Institute of Mining and Metal- lurgical Engineers, 29 West 39th Street. New York. N . Y. Unless special arrangement is made, discussion of this Paper will close April I . 1943. Any discussion offered thereafter should preferably be in the form of a new paper.

A Micrographic Study of the Cleavage of Hydrogenized Ferrite

BY CARL A. ZAPFFE* AND GEORGE A. MOORE,* JUNIOR M E ~ E R S A.I.M.E.

(New York Meeting. February 1943)

IN a previous publication from this laboratory1 the conclusion was drawn that the embrittling effect of occluded hydrogen on iron and steel must result from the precipitation of the gas within small openings through the crystal structure. A review of the literature then published indicated the real existence of crystal sub- structures of the mosaic type and gave presumptive evidence that the "rift open- ings" associated with hydrogen occlusion are the same as the "disjunctions" of the mosaic theory.

Inasmuch as no clear conception of the actual nature of the crystal fragments and openings inherent in these theories has been established, it was considered desira- ble to obtain direct micrographic evidence of their true configuration. I n this direc- tion the standard methods of metallographic attack have proved useless. Recourse was had, therefore, to the direct examination of the surfaces upon which failure by cleavage had resulted from the presence of hydrogen. I t was expected that such a surface, un- affected by chemical attack and undis- torted by polishing, would reveal the fine openings penetrated by the gas and thus indicate the real nature of the substructures.

In commercial steels, defects caused by hydrogen take various external forms ac- cording to such factors as structure and purity, and to the extent to which hydrogen

Manuscript received a t t h e office of t h e Institute Oct. 22, 1942.

* Research Metallurgist. Battelle Memorial Institute. Columbus. Ohio.

1 References are a t t h e e n d of t h e paper.

either occludes or produces chemical changes. The present work relates only to the well-known transcrystalline hydrogen embrittlement-"pickling brittleness"-in which elemental hydrogen occluded within the grain develops cleavage characteristics. For example, hydrogen embrittlement re- sulting from hydrogen reaction products is excluded from detailed study. I t was also desirable, as far as possible, to have the study uncomplicated by other elements which themselves cause brittleness.

Three types of iron were used. These included ingot iron, decarburized free- machining steel, and a specially prepared, aluminum-killed, electrolytic iron. This , choice allowed a comprehensive selection of unavoidable minor impurities and in- clusions, which could subsequently be shown to have no specific effects on the structures within the grains. Carbon, and any other impurities free to react with hydrogen, were first removed by a prelim- inary annealing treatment in hyd~ogen. Subsequent treatment in vacuo extracted much of this hydrogen and its gaseous reaction products. The resultant ductile iron was then embrikled either by a second anneal in hydrogen, followed by rapid cooling, or by cathodic electrolysis.

Group I: Prepared jrom Armco Ingot Iron

As shown in Table I, the principal im- purity in this iron was oxygen, which survived the purifying treatment to a large extent. Two series were prepared. The first (Nos. SIO-24) was purified only one day in hydrogen and dehydrogenized one day in nitrogen. Containing both hydrogen and its

Copyright. 1943. by the American Institute of Mining and Metallurgical Engineers. Inc. METALS TEC~INOLOGY. February r g u . Printed in U. S. A.

2 MICROGRAPHIC STUDY OF CLEAVAGE OF HYDROGENIZED FERRITE

TABLE I.-History of Microsamples GROUP I.-INGOT IRON

s18 Cathodic c h i i i i B ~ i i i d t$pii s19 1 Cathodic charge Bz 1 Fibrous - - - - + facets

Bar No.

Sxaa SI 3 S14 SIS S16

Snics 11: In moist hydrogen 392 hours a t IOOOO to ro5o0C.. 4 hours in dry hydrogen. 24 hours in vacuo, cooled in vacuo. Grain size No. I. Microstructure, oxlde veinma.

Granular + facets Granular + facets 2.9 25

Snics I : In moist hydrogen 24 hours a t rooo0C.. in dry hydrogen 2 2 hours, in dry nitrogen 22 hours, cooled in nitrogen. Grain size So. I. Microstructure normal.

-.

GROUP 11.-DECARBURIZED FREE-MACHINING STEEL, S.A.E. X-I I I 2

Treatment

Cathodic charge Ab Cathodic charge A Cathodic charge B Cathodic charge B Cathodic charce Ba

Bar So. I

Impact. F t-lb. Type of Fracture

Granular Granular + facets Granular + facets Fibrous + facets Mixed tvnes F 2

Treatment

Hardness. Rockwell B

I

.Tnies I: Decarburized in moist hydrogen 392 hours a t xooo0 to IOSOOC., 4 hours in dry hydrogen. 24 hours in vacuo, cooled in vacuo. Grain Slze No. 5. Mlcrostructure: many inclusions. no detectable carbon.

S S ~ Residual hydrogen only 1 (Fibrous skin. faceted core.) S59 1 Cathodic charge B

.Series I: Short deoxidation in hydrogen. degassed in vacuo 16 hours a t 85oqC.. rzo hours a t 1o~o"C.. and 24 hours a t 700°C.. cooled in vacuo. Grain size much larger than No. I. Mlcrostructure clean except for

traces of Alto#.

Series 11: Specimens from Series I deoxidized 18 hours in dry hydrogen a t ro5o0C.. degassed 72 hours in vacuo. cooled in vacuo. Microstructure unchanged.

S61d S62d S63d S66d S67 S69 s70

" S ecimen numbers listed in figure captions throughout paper give first this number then A or B to desig- nate t i e two halves of the fractured specimen, finally a number designating the particdlar grain among those photogra hed.

b ~ a t f o d i c electrolysis from platinum anodes in 5 per cent HcSO.: Charge A: 30 min. a t I amp. per sq. decimeter. Charge B: 2 hr. a t 7.5 amp er s . decimeter. Charge Bz: Same, followed gy 2 t o 10 min. with polarity reversed, designed to remove hydrogen and

return ductility. but completely ineffective. Charge C: 6 hours a t 4.8 amp. per sq. decimeter. believed to be the current density of maximum efficiency. Prepared by melting e ectrolyt~c iron in a magnesia crucible under a calcium aluminate slag and deoxidizing

with excess aluminum metal (0.60 per cent added). The ingot was hot-rolled, bars scalped and cut to size. cleaned and stacked in the furnace and given the treatment listed.

d Samples not included in photomicrographs shown, but listed for comparison of physical properties. ' After "series" treatments and before hydrogen charging. J 0 by vacuum fusion.

Snies 111: Specimens from Series I1 soaked in dry hydrogen 64 hours a t 850°C. and normalized.

S75 / Residual hydrogen only I Facets 3 I 48

As annealed AS annealed As annealed Cathodic charge A Cathodic charge B

. atenal Group I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Group 11.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Group 111. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.082 0.034 0.01a

No fracture No fracture Fibrous partial fracture Facets Facets

Mn

0 .76

A 0.004 0.004

- -.

5 2

Cathodic charge B Facets Cathodic charge C I Facets 65 1 51

0.011 0.084

<o.oos

S 0.044 0 .25 0.010

Si I A1 (metal)

0.02 <O.OI 0 .14

CARL A. ZAPFFE AND GEORGE A. MOORE 3

reaction products, this material yielded fractures of all types from intergranular to the most detailed transgranular facets, further hydrogen additions having but little effect. Notched-bar impact values ranging from 50 to 0.75 ft-lb. show the degree of embrittlement produced in each sample, but are not considered to show any direct relation to the hydrogen content.

A second series of bars in this group (S25-49) was treated for 16 days in hydro- gen, without, however, completely remov- ing the oxides and similar impurities. Extensive gaseous reaction was evident, especially a t the grain boundaries, which appear to have been greatly weakened by the partial removal of the boundary films and by the gases that must have collected there. The fractures passed both through these films and through some of the grains, where the usual facets were formed. All samples were brittle, showing only about 3 ft-lb. notched-bar impact strength.

The oxide-hydrogen reaction causes intergranular embrittlement that is char- acteristic of this type of material. The transgranular facets formed, however, were always identical with those in the other materials.

Group 11: Prepared by Decarburizing Commercial Free-machining Steel

The material of group 11, having less oxygen, is better suited for studying trans- crystalline hydrogen embrittlement. The steel was decarburized 16 days and vacuum- treated to remove hydrogen. In the first series (Sgo-60), the hydrogen was incom- pletely removed and the samples remained brittle a t the center. Further vacuum treatment of series I1 (Nos. S~I-99) pro- duced a ductile material having a notched- bar impact strength of 120 to 130 ft-lb. Hence it is evident that the large amounts of sulphur and phosphorus still remaining do not themselves lead directly to brittle- ness. Cathodic hydrogenizing of this

material lowered the impact value to 33 and 65 ft-lb., producing transgranular cleavage only. The residual hydrogen of the first anneal left the material with about equal brittleness. Resaturation at 850°C. in a hydrogen atmosphere gave complete em- brittlement, with the notched-bar impact value falling to 3 foot-pounds.

Important to observe is that the impuri- ties remaining in this iron do not react with hydrogen sufficiently to impose any frac- ture structures not caused by hydrogen alone. The facets in this material are identical with those in the first group where the impurities were of completely different nature.

Croup 111: Prepared from Electrolyti~ Iron, Remelted and Deoxidized with Excess

Aluminum

The only appreciable impurities in the iron of group I11 were aluminum metal and Al2o3. A shorter hydrogen anneal and longer vacuum treatment than ordinary were given. The gas-free material was very ductile, with notched-bar impact values between 152 and 186 ft-lb. As might be expe~ ted ,~ diffusion seemed slow in such pure metal and complete embrittlement was difficult to obtain. Many samples showed only a mild drop in impact value, while others were more thoroughly affected. A cathodically charged piece broken with 17 ft-lb., and a gas-saturated piece broken with only 4 ft-lb., were subjected to detailed study when found to give the most complete representation of all observed structures.

The facets of this relatively pure material were similar to those in the other specimens. I n Figs. gc and gd the detailed structures shown cover the entire range of types found in any other cases. This effectively shows that all structures found within the grain must be due primarily to the ferrite structure as developed, or revealed, by hydrogen, and not by any other impurity.

4 MICROGRAPHIC STUDY OF CLEAVAGE OF HYDROGENIZED FERRITE

Comparative Properties of the Three Groups

The response of these materials to hydro- gen treatment varied widely, which might be expected, since published studies on the rate of hydrogen d i f f u s i ~ n ~ - ~ show large variations both with the state of purity of the iron and with its structural condition. Thus material 11, allowing free diffusion, responds quickly both to charging and to annealing. Material I , because of the films of impurity, is slow to surrender its gas content and does not readily regain ductility. Material 111, as is common with pure and well-annealed metals, seems slow to absorb the gas,8 hence slow to pass from its natural ductile state to that of complete embrittlement .

The gross differences in rate of diffusion between materials, and the smaller differ- ences between samples, cause the various samples to attain varying degrees of em- brittlement after similar treatment. There- fore, physical properties, such as the impact resistance, cannot be related directly to the treatment given, but do give some indication of the result obtained. In spite of these differences in rate, once transcrystalline embrittlement is attained, the structure always appears the same.

Although hydrogenizing has been shown to accelerate etching or chemical attack of iron on a macroscopic scale,' attempts to reveal a h e system of rifting within the grain by chemical etching have been inde- cisive. The present work takes advantage of a more specific method.

When hydrogen-embrittled iron is frac- tured, the fracture surface is brilliant and silvery. In the stereoscopic view* in Fig. 14,

such a surface shows many flat, mirrorlike cleavage surfaces, or "facets," which pass directly through the grains-a single facet often traversing an entire grain. An indi- vidual facet, oriented correctly on a micro- scope, proves quite suitable for examination a t high magnification and is free from the effects of the ordinary metallographic preparation.

Facets are found in all simple hydrogen breaks.' The structure of a "snowflake," or a "fisheye," is typical. In tensile testing, the longer period for fracturing may allow the surfaces to be distorted by secondary deformation, and the fracture may cause inconvenient orientation of the facets. The choice for this study of fracturing by im- pact insured undamaged cleavage surfaces lying in positions where they could be con- veniently examined by the microscope. The impact values also are useful as a qualitative indication of the degree to which hydrogen embrittlement has de- veloped in each specimen.

By macroscopic examination, the effec- tiveness of the annealing operations could be followed in preparing the specimens. Fig. ~b shows the result of the incomplete vacuum treatment first given irons of group 11. A ductile, degassed surface layer encloses a central core, which remains embrittled. An area of facets is thus seen surrounded by a dark layer of deformed metal.

Macroscopic examination also shows the nature of the intergranular fracture found in many specimens of the group I irons made from Armco ingot iron. As shown in Fig. IC, the surfaces of failure are rounded and irregular, to be distinguished from the flat surfaces formed by hydrogen in the absence of oxide-film types of impurities.

These stereographs may be examined with The various specimens were Cut to any viewer made to fit the standard inter- pupillary distance. Such viewers are available standard for bars for about one dollar. Two cheap magnifying (1 X I X 5.5 cm.) and then loaded with lenses of about 3 X power will serve the same purpose. Most people can obtain fusion by hydrogen either by a cathodic treatment holding the stereograph directly before the or by soaking in the gas (Table I). Immedi- unaided eyes and staring through it at an imaginary distant object. ately after charging, a 2-mm. V-notch was

FIG. I .-STEREOAIACROGRAPHS 01: CLEAVAGE FRACTURES CAUSED BY HYDROGEN. x 10. a. Ingot iron (Sr3A) purified in hydrogen and vacuum and embrittled by cathodic electrolysis. b. Small facets in hydrogen-treated iron (S53A) after treating 2 4 hours in vacuo a t IOSO~C.,

showing removal of hydrogen from the surface layer only. c. Intergranular fracture in ingot iron (SIZA) probably caused by grain-boundary accumulation

of reaction products of hydrogen and nonmetallic impurities. 5

6 MICROGRAPHIC STUDY OF CLEAVAGE' OF HYDROGENIZED FERRITE

cut, and the bar was broken. One half of the bar was mounted with the broken end projecting from a I-in. ring, which could then be mounted in a special orienting mechanism on the microscope. Adjustment of three leveling screws allowed a desired facet to be oriented exactly perpendicular to the axis of the microscope. When the specimen was properly set, examination was possible well into the oil-immersion range, long-nose objectives generally being neces- sary to prevent contact with promontories on the rough surface.

MICROSCOPIC EXAMINATION AND TYPES OF

STRUCTURE

The only tenable explanation for "pick- ling brittleness" and the occlusion of hydrogen a t room temperature requires the accumulation of the gas in fluid form with- in openings through the metal structure. Brittle failure is held to occur through or along these openings, which lack cohesion when filled with gas. Microscopic examina- tion, therefore, was directed toward finding these openings, describing, and classifying them.

Large openings such as cracks, blowholes and blisters have been discussed in other places.1a9 I t has been shown that these usually do not account for more than a small portion of the hydrogen involved, and that they do not explain general em- brittlement. The openings of interest here areexpected to be less than the size of the grain. Some small openings may form in the boundary, or along oxide veins. Smaller openings may, in theory, take the form of minute holes or '.'canalsH through the grain,'" or of "rifts." A "rift" may be defined as a flat opening, within a grain, ranging in size down to any space just large enough to admit a hydrogen molecule. Originally they were proposed as lying along slip planes after deformation."J2 More generally, they may be considered intimately associated with the structure of the crystal. The major cleavage forming a

flat facet is considered to pass through a rift that has grown until- i t completely traverses the grain. I t has been <hown13 that the thinner an opening of this type, the greater the forces acting to retain the gas molecules, and' hence the greater the occlusive effect. Most of the openings in the present samples may be considered as rifts.

Since rifts derive from the structure of the crystal, they should be discussed and classified in terms of this structure. Grenin- ger14 has simplified the many possible intragranular structures of metals into two main types, lineages and mosaics. Accord- ing to Buerger,l6 a "lineage" is any structure that is basically continuous, but formed in branches. The branches are separated by discontinuities a t which there is a change of orientation. Reconciling Darwin16 with Webster, one may define a "mosaic" as any structure built up of small units or fragments. No restriction is placed on the: dimension, shape, or order of the arrangement. Mosaic fragments are sepa- rated by "disjunctions" from which part, or all, of the possible metal atoms are missing. A change of orientation is possible, but not required, a t the disjunction. By definition alone, a rift and a mosaic dis- junction, therefore, are essentially the same. In general, a lineage pattern is expected to be larger than a mosaic, which may therefore compose one of the branches of the lineage. There is, however, no pro- hibition of the separate existence of the two types in overlapping size scales.

Examination of the detailed structure of a large number of facets of hydrogenized ferrite reveals that all of these structures can be arranged in a progressive series from the grain itself through lineage types and then mosaic types to the limit of optical resolution. The crystallographic perfection increases with decreasing size of the structural unit and does not depend on the material. As already noted, all of the intra- granular structures can be realized on any

CARL A. ,ZAPFFE AND GEORGE A. MOORE 7

FIG. 2.-FRACTURE TYPES. Original magnifications given. Reduced f g in reproduction.

a. Grain boundary in high-oxygen iron (S25A2). Xzoo. b. Grain boundary, showing one family of planes sprung by hydrogen ( S ~ Z A I ) . X zoo. G. Lineage, showing interlineage discontinuity and wavy surface (S26Az). Xzoo. d. Sharp lineage centering on impurity (Sr9B3). X350.

8 MICROGRAPHIC- STUDS OF CLEAV AGE OF : HYDROGENIZED FERRITE

of the materials studied, regardless of purity. This will be illustrated by interlac- ing pictures from the different groups of material - throughout the progression of types to be shown. Without reference to the identifying numbers and the table, it will be impossible to determine that a picture came from any particular quality of iron. Thus, the embrittling mechanism is fundamentally the same in all types of ferrite, and relates to the same basic structures.

Grain Boundary or Intergravzular Embrittlement

This type of embrittlement can be found only in the first group of samples where the presence of films of iron oxide and similar substances allows their reaction with the hydrogen to form gases, such as steam, which create and open voids between the grains. Two unusual views of the rough, outer surfaces of grains are shown. In Fig. 2a there is no sign of fracturing except through the grain boundary, the hydrogen having so far failed to show any effect on the body of the grain. In Fig. ab, the grain itself shows some response, one set of parallel rifts having been opened, intersect- ing the surface. Apparently the fracture was on the verge of passing down one of these rifts to form a facet.

Lineage Structures

All pictures to follow are made directly on the untouched, flat cleavage facets without mechanical or chemical modifica- tion. Branched, diffuse structures showing poor crystallographic development are found intersecting this facet plane in all types of iron. A typical example in Fig. 2

shows two major branches, which redivide on smaller scales. The facet surface itself is seen to be somewhat wavy, indicating variations in orientation in this direction as well. Also, it is evident that only a few of the sharper and darker lines may with any

certainty be identified as disjunctions, or openings, while most of. the discontinuities fail to show evidence that they involved voids large enough to take part in the embrittling process. Thus the rift volume resulting from lineage irregularities appears to be comparatively small.

A sharper lineage pattern appears in Fig. nd. What appears to be an inclusion occurs a t the center of tGs structure. This raises the possibility that the production of the lineage could be controlled by the action of some impurity. A coarie lineage structure is shown in Figs. ga and b, first in ordinary and then in polarized light. The major discontinuities are plainly aniso- tropic, indicating the presence of impurities, or representing severe distortion in the lattice arrangement. These structures are all from the high-oxygen iron, group I. Many similar structures, not reproduced, were found in the free-machining iron. The supposition that impurities cause the lineage is opposed by the presence of this structure in the relatively pure iron of group 111. A well-developed structure from this material appears in Fig. gc. Thus, if an impurity is active, it must operate in very small concentration to be present in all three materials. ~nclusions, comprising the usual impurities, are often carefully avoided by the lineage discontinuities (see Fig. 2 6 ) .

Transition Structures

The diffuse and noncrystallographic arrangement of the lineage passes gradu- ally into the sharper crystallographic structures associated with the mosaic type of substructure. A puzzling example appears in Fig. 3d. Lines falling close to two differ- ent crystallographic directions appear in Fig. 4a.

Mosaic Structures with Line Dislocations

. Regular crystallographic arrangements of dislocations associated with mosaic pat-

CARL A; ZAPFFE AND GEORGE A. NOORE 9

FIG. 3.-LINEAGE AND TRANSITION PATTERNS. Original magnifications given. Reduced 45 in reproduction.

a. Lineages and facets on three planes ( S I ~ A ~ ) . Xsoo. b. Same area in polarized light, showing anisotropy of discontinuities. G. Lineage patterns in purified iron ( S Z S ~ A I ) . X250. d. Transition stage between lineage and crystallographic structure ( S ~ ~ B I ) . X3jo.

I 0 MICROGRAPHIC STUDY O F CLEAVAGE OF HYDROGENIZED FERRITE

FIG. 4.-~'~OSAIC LINE PATTERNS. Original magnifications given. Reduced 45 in reproduction.

a. Transition from lineage; lines in two crystal directions ( S I ~ B I ) . X3jo . b. Crystallographic rifts on single set of planes (SrSA-2). X35o. c . Mosaic lines lightly developed (Sz6A1b). X35o. d . Sharp crystallographic lines with steps around mosaic blocks ( S Z ~ A I ) . X ~ O O .

CARL A; ZAPFFE AND GEORGE A. MOORE I I

FIG. S.-~'[OSAIC BLOCK PATTERNS. Original magnifications given. Reduced >$ in reproduction.

a. Lineage, line, and block structures with hole and crack (S ISA~) . X350. b. Mosaic blocks in high-oxygen iron (s1641). X 500, c. Mosaic blocks i n impure iron (S69A2). X ~ o o . d. Mosaic blocks in purified electrolytic iron (Sz~7Ara) . X 2 jo.

A11 samples cathodically charged with hydrogen.

I 2 MICROGRAPHIC STUDY OF CLEAVAGE OF WDROGENIZED FERRITE

terns may occur within, or may be associ- ated with, lineage structures, as shown, or may descend directly from the grain with- out the complication of branching arrange- ments. The development of a single parallel set of rift openings traversing the whole grain is shown in Fig. qb. This is to be compared with Fig. ~ b , where a similar set of rifts appears on the outer grain surface.

The line dislocations are seldom confined to a single set of planes, but usually run in several directions as the opened plane changes from one crystal position to another (see Fig. 4c). Many of the lines of intersection of these rift openings with the facet appear a t first to be curved. These curved traces, however, may usually be resolved a t sufficient magnification, show- ing themselves to proceed stepwise around small blocks of regular crystallographic form. These steps may be clearly seen in Fig. qd, and in other photo:raphs. Some of these lines are found preserved in all patterns of finer detail.

Mosaic Block Structures

The crystallographic blocks bounded by two rift planes in the stepped line patterns may often border instead on three or more well-developed disjunctions. In this case the blocks are loosely held together, and pull away during the cleavage. A complete progression to this stage is seen in Fig. ga. A small blowhole and a fine crack are in- cluded among the openings in this grain.

Three views of grains running largely to the block type of fracture are shown in Fig. 5. The removed fragments vary from single, small particles about I micron in true diameter up to large and irregular pieces, probably composed of many of the smaller units. Fig. gb is from the high- oxygen iron, group I , whereas Fig. gc is from the group I1 iron made from free- machining stock. The two structures are for all practical purposes identical. The purified iron of group 111, having a larger

grain size, also shows a somewhat larger block size, so is shown a t lower magnifica- tion in Fig. gd.

Laminated Structures

A reasonable supposition, confirmed by direct observation, is that the fragmenta- tion exposed on the cleavage face of these crystals is in no way unique on that plane. Each of the last three pictures in Fig. 5 shows the edge of another extensive plane, which can be only a little less completely opened than the facet rift itself. Pure chance has thus determined that the facet fell as observed, rather than on a similar plane parallel to this surface. The fine structures that are evident may thus be supposed to extend through the entire body of the grain.

By varying the focus of the microscope during visual observation, it is possible to see that the original cleavage has often traveled on a series of parallel planes of the order of one micron in separation. The exposure of a series of these rift planes leads to a stepped or terraced structure, which is much more common than would appear from the photomicrographs. An identical situation has been observed previously in other hydrogenized metals.l1,l2 By special focusing and lighting, the terraces may be photographed as a series of parallel lines. In Fig. 6a the lines marking the edges of these laminations are shown bordering the two largest rifts, and also along the junc- tion of the two dissimilar areas. In Fig. 6b the terraces may be seen along several large rifts. At higher magnification, the holes of the larger block structure may also be seen to take the form of steps, as in Fig. 6c. Finally, in Fig. 6d, the terraces are shown a t the outer edges of fractured grains, especially between the two facets.

The exposure of these laminations indi- cates that the fine structures observed in these metals are essentially the same in all three principal directions.

CARL A., ZAPFFE AND GEORGE A. MOORE ' 3

FIG. 6.-TERRACED AND LAVINATED STRUCTURES. Original magnifications given. Reduced % in reproduction.

a. Terraces a t large cleavages and junction of grains (Sz6oB1). X350 6 . General terracing of surface (Sz6oBz). X350. c. Terraced edges of mosaic block vacancies (S67Br). X rooo. d. Terraces a t blocks and between grains ( S ~ ~ A I ) . X 1000.

14 MICROGRAPHIC STUDY OF CLEAV 'AGE OF HYDROGENIZED FERRITE

Origin of Structures The evidence of the present experiments

appears to show that intragranular struc- tures are inherent in the ferrite crystal and are delineated and made visible on a fracture by the action of hydrogen. The hydrogen may be regarded as playing the part of an etching agent, but an agent which, by being noncorroding and much more fluid than a liquid, is capable of revealing finer detail. Impurities may be discounted as the cause of the internal structures because similar structures are found regardless of the degree of purity. If some secondary element does control the production of these structures, it must act in amounts not shown by chemical analysis.

In the present case, the brittleness, and consequently the exposure of these struc- tures on the fracture, is the direct result of the action of hydrogen. All of the materials when freed of hydrogen are very ductile and tough, in spite of the presence of their various impurities.

Possible intergranular structures of the ductile state cannot be detected by the present technique, since any facets that may be formed are too small for observa- tion. The actual amount of hydrogen present, although probably critical in determining the transition from the duc- tile to the brittle state, does not appear to control closely the type of structure ob- tained. There is no analytical method adequate to determine the true concen- tration a t the exact areas studied.

There does, however, appear to be a definite trend toward finer detail as the hydrogen content increases. Facets are observed to be more numerous, hence smaller, in the most thoroughly hydro: genized samples; and in the long run more of the complicated block structures are found in the most completely embrittled pieces. A comparison of the pictures with

the brittleness of the bars from which they are taken shows this trend.

Of course, structures of both the mosaic and lineage types may be developed by other methods than hydrogen embrittle- ment. The specific advantage of this treat- ment lies in the fact that hydrogen is known in advance to be occluded in openings. Consequently, while the diffuse markings of the lineage structures are of uncertain meaning, the sharp lines in the mosaic line and block structures give every evidence of being the thin, sheetlike voids, or rifts, which were postulated.

Similar brittle facets produced without benefit of hydrogen show similar structures, modified by their different lattice arrange- ments. Bismuth, zinc, magnetic iron oxide (mill scale), and other materials may be mentioned. The faceted fracture may be induced in ferrite by the addition of silicon-below the supposed solubility limit, incidentally. A similar fracture was even produced by cooling the pure gas-free iron to the temperature of liquid air, under which conditions a notched-bar impact value of 3 ft-lb. was obtained.

These similarities of cold ferrite or silicon ferrite to the hydrogenized struc- tures indicate that the same type of embrittlement may be developed in differ- ent ways, thereby lending credence to the belief that the lineage and mosaic patterns are inherently present in a t least incipient form in pure ferrite. Hydrogen embrittle- ment, then, simply utilizes this existing vacuous structure, as will now be explained.

Action of H ydrogelt

In other p l a ~ e s , l ~ ~ ~ l ~ the voluminous literature on the iron-hydrogen system has been considered. Pertinent to present understanding are measurements in the Sieverts' l a b o r a t ~ r y ~ ~ ~ ~ ~ which thoroughly establish the values of true lattice solution of hydrogen a t higher temperatures, and the experiments of Smith and others,a.g-"JZ.21 which demonstrate that the gas is occluded

in small openings a t lower temperature. In blocks, since one of the accepted modes of experiments on other r n e t a l ~ , ~ ~ J ~ these formation of such blocks is the fragmenta- openings were established as rifts, which tion occurring during cold-working.

PH2- ATMOSPHERES I I0 I00 1,000 10 om

pH,-LB:PER SO. IN.

SOLUBILITY CURVES

PY CURVES FOR ALLOYS -.- MARKED 0

. , I . . 0 I i!

.('E') WEIGHT PER CENT HYDROGEN XIO'

FIG. 7.-FULL CURVE: SOLUBIEITY OF HYDROGEN IN HYDROGEN-IRON ALLOYS AT P = ONE ATMOS- PHERE, SHOWING STRONG PRECIPITATIONAL CHARACTERISTICS.

BROKEN CURVES: CALCULATED PRECIPITATION PRESSURES FOR THE COOLING OF THREE ALLOYS DESIGNATED BY CIRCLES.

Curves are sections of solid C-P-T model. , . .

lay along slip planes when the metal was Previously, published diagrams of the studied in the worked condition. No con- solubility of hydrogen in iron have used flict with this is raised by the present study reversed coordinates, thereby obscuring placing the rifts a t the faces of mosaic consideration of the system in a metal-

. .

16 M~CROGRAPHIC STUDY OF CLEAC 'AGE O F HYDROGENIZED FERRITE

lurgical sense as a series of hydrogen-iron alloys. In Fig. 7, the best available solu- bility data are repl0ttedl7-20,2~,23 to simu- late the iron-rich side of a typical binary constitutional diagram. Actually, however, the behavior of hydrogen-iron alloys re- quires consideration of both pressure and temperature as variables. The diagram represents a section taken a t P = I

atmosphere through a solid model. The variation of P with the square of the concentration is lost in the third dimension. Thus, there is no true "solubility limit" for hydrogen in iron unless both P and T are fixed.

Especially impressive are the precipita- tion-hardening characteristics of the solu- bility curve, showing two unusual iso- thermal precipitations. This system is again unusual because the precipitate is a gas, which fortunately allows the precipitation pressure to be calculated23 from the solu- bility law.18 The three dotted curves super- imposed on the diagram show the pressures in equilibrium with three typical alloys during cooling. Of course, the loss of gas from the lattice to discontinuities modifies these pressures slightly.

For iron to become embrittled in prac- tice, from two tenths to one relative volume of hydrogen is required, a quantity fre- quently present, but often regarded as negligible.

Measurepents of the density of ideal and real iron crystalsz4 show a divergence of 0.01 to 0.1 per cent, which may be partly interpreted as indicating the volume of the voids under consideration. If a ratio of the order of I:IO,OOO for space-to-solid occurs in ordinary metal, one relative volume of hydrogen forced into that vacuity by a declining solubility relationship would suffer compression to ~o,ooo atmospheres, or 150,ooo lb. per sq. in. Naturally, residual solution, or reaction, can modify these values, and the introduction of larger openings, such as blowholes, can greatly decrease the pressures realized. I t is never-

theless evident that on cooling a solution of hydrogen in iron there is ample oppor- tunity for the gas to precipitate under pressures that may exceed some critical value probably lying near the elastic limit, or the yield point.

The analogy of this precipitation to the crystallographic Widmansttitten precipi- tation of solid phases is marked, except for two important differences. First, the pre- cipitation of the gas appears to require the pre-existence of openings, or internal sur- faces, on which the dissolved atoms can combine to form as molecules. The pres- sure developed is sufficient to spread or "spring" these surfaces, enlarging existing openings, yet apparently seldom sufficient to create entirely new openings. While such openings could probably be utilized by solid precipitation, there is yet no evidence to indicate that Widmannstatten reactions are unable to force whatever crys- tallographic adjustments may be necessary.

Secondly, in the Widmanstatten action, the precipitate, being solid, has a strength and ductility of its own and may add to the strength of the base metal. In hydrogen, however, the pools of gas can have no ductility, and, lacking cohesion, can add nothing to the total strength. The gas-filled rifts will then fail very easily in cleavage. whereas this result does not necessarily follow from a solid precipitation.

Absence of Slip

Tensile bars of hydrogenized iron break without elongation or yield point,lalJ hence without operation of the slip mechanism. The photomicrographs also indicate little evidence of deformation by slip. The presence in the structure of easy planes of cleavage need not in itself preclude slip when the breaking load is above the normal yield point, hence an explanation is neces- sary. Two lines of reasoning are possible.

The gas pressure existing within the rifts or mosaic disjunctions compresses only the portions of ,the structure that are not

CARL A. ZXPFFE AND GEORGE A. MOORE = 7

coherent with adjoining metal. This pres- sure is balanced by a state of tension on all parts that are still coherent and are capable of supporting external stresses. Deriving from a fluid pressure, the tension is triaxial. Varying importance may be attached to such a state, but it is generally agreed that it always results in a preference of cleavage over slip.25.26 On some proposed models this triaxial tension would make slip more difficult.

An alternate idea has been developed by Bragg,=' who shows that in order that a small crystal, or mosaic block, may undergo internal slip it is necessary for two opposite faces to be displaced in shear a distance greater than one half an interatomic distance. As the size of such a block falls to a fraction of one micron, the force necessary to establish the initial displace- ment against the elastic modulus rises rapidly until it passes the ultimate strength of the structure, making slip before failure impossible. Although in the present photo- micrographs blocks are not resolved quite as small as those in Bragg's calculations, there is reason to suppose that smaller units are present, but unresolved. In ordinary iron of normal, small grain size, better agreement with the calculated block size might be expected. In the presence of easy cleavage, slip in these structures appears likely to be subjected to Bragg's restriction.

Both the triaxial tension resulting from the gas pressure and the separation into small mosaic blocks probably cooperate to suppress the activity of the slip mechanism in this system.

Previously published conclusions regard- 1 ing the occluding and embrittling processes

1 are confirmed and extended, as follows: 1 I. The occlusion of hydrogen by iron

a t ordinary temperatures occurs as the retention of fluid, compressed gas within microscopic openings, or rifts, into which

the gas precipitates from supercooled or supersaturated solid solutions.

2. The failure of hydrogen-embrittled iron occurs by direct cleavage through these gas-filled rifts, which have no ductility.

3. The action of slip is restricted in this structure either by the action of triaxial tensile forces reacting against the pressure of the occluded gas, or by the fact that the rifts cut the structure into fragments too small to experience internal slip under the forces available, or by the cooperation of both mechanisms.

4. The rifts constitute internal surfaces within the grain and may be broadly classi- fied in two types:

a. The Lineage, a continuous structure composed of branches in angular disarray. Some rifts form on or through this struc- ture. Failure is by major cleavage. Within the structure only a small number of true openings can be observed. The lineage dis- continuity thus appears as a change of orientation without visible disjunction, and the total rift volume of this structure appears comparatively small.

b. The Mosaic, a structure obtained by fitting together small particles or blocks of more or less regular crystallographic form. These blocks are separated by disjunctions, which probably compose the largest propor- tion of the total rift volume.

5. Some mosaic and lineage structures apparently are inherent in the nature of the ferrite crystals themselves, and are present, even though not observable, in all ferrite grains. Hydrogen may be regarded as a tool, or a noncorroding agent, which by mechanical action develops the dis- junctions of these structures to the stage where the material may be parted on some disjunctions and the intersections with others made visible.

6. Intercrystalline embrittlement by gas iilms in openings between the grains appears to be limited to cases where the openings can be created by the reduction of non-

18 MICROGRAPHIC STUDY OF CLEAVAGE OF HYDROGEN~ZED FERRITE

metallic films by hydrogen, and is to be dis- fects in Steel. Trans. A.I.M.E. (1941) 145, 225-261; discussion. 261-271.

tinguished sharply from the transcrystal- 2. W. Baukloh and W. Retzlaff: Hydrogen line brit tleness from elemental hydrogen Permeability of Steel in Electrolytic

Pickling. Archiv Eisenhiittenwesen (1937). considered in the present experiments. 11, 97-99.

3. T. S. Fuller: The Penetration of Iron by Hydrogen. Trans. Amer. Electrochem.

S U B ~ A R Y Soc. (1919) 36, 113-129; discussion.

The rift openings within the iron struc- 130-138. Gen. Elec. Rev. (1920) 23, 702-711.

ture in which hydrogen is occluded a t 4. W. Baukloh and H. Guthmann: Hydrogen Diffusibility and Hydrogen Decarburi-

ordinary temperatures are studied micro- zation of Steel. Armco Iron. Copper, graphically. Ferrites of three different Nickel and Aluminum a t High Pressures.

Ztsch. Metallkunde (1936) a8. 34-40. origins were first rendered ductile by puri- 5. T. C. Poulter and L. Uffelman: The fication in hydrogen with a subsequent Penetration of Hydrogen through Steel

a t Four Thousand Atmospheres. Physics treatment in oacuo and were then embrit- (1932) 3, 147-148. tied by the addition of hydrogen alone. 6. V. Lombard: Permeability of Iron and

Platinum to Hydrogen. Compt. Rend. The ferrites were so chosen that their indi- (1927) 184, 1557-155 . vidual impurities did not lead to brittleness, 7. W. R. Ham and J. D. Zauter: Diffusion of

Hydrogen through Iron and Palladium. and there was no common impurity to Phys. Rev. (1935) [21 47. 337.

8. D. P. Smlth and G. J. Derge: Role of interact with the hydrogen subsequently Intragranular Fissures in the Occlusion added. and Evolution of Hydrogen by Palla-

dium. Jnl. Amer. Chem. Soc. (1934) Micrographic examination is made 56. 2513-2525.

directly upon the flat cleavage facets 9 . G. A. Moore and D. P. Smith: Occlusion and Evolution of Hydrogen by Pure

formed when the hydrogenized ferrite Iron. Trans. A.I.M.E. (1939) 135, 255- 292.

suffered brittle fracture. Without etching G. T~~~~~~ and H. ~ ~ ~ d ~ ~ ~ i ~ ~ : than- or polishing, these surfaces are unaffected nels in Metal which Communicate with

the Surface. Zlsch. anorg. allge. Chem. by either chemical attack or physical dis- (192.5) 142, 54-40. tortion. On such surfaces the disjunctions 11. D. P. Sm~th and G. J. D e w : The Occlu-

sion and Diffusion of Hydrogen in utilized and affected by the gas are found Metals: A Metallographic Study of to be revealed in great detail. Examples Palladium-Hydrogen. Trans. Electro-

chem. Soc. (1934) 66; 263-270. are shown of structures ranging in size 12. G. A. Moore and D. P. Smith: Occlusion

from the grain itself down to the limits of and Diffusion of Hydrogen in Metals: A Metallographic Study of Nickel-Hydro-

resolution of the microscope. These struc- gen. Trans. Electrochem. Soc. (1937) 71, 545-564. tures are considered in relation to the 13. C. A. Zapffe and C. E. Sims: Hydrogen.

phenomenon of hydrogen embrittlement Flakes and Shatter Cracks. Metals and Alloys (1940) I I (5) . 145-151; 11(6).

and to the theory of metallic crystals. The 177-184; I ~ ( I ) , 44-51; 12(2), 145-151. crystal fragments lying between the rift 14. A. B. Greninger: Crystallographic Uni-

formity of Lineage Structure in Copper openings are presented as direct evidence Single Crystals. Trans. A.I.M.E. (1935) for the reality of "mosaic structure." 117, 75-88.

IS. M. J. Buerger: The Lineage Structure of Crystals. Ztsch. Krist. (1934) 89, 195-

ACKNOWLEDGMENT 220.

Acknowledgment is made to Battelle Memorial rnstitute for the support of this work as part of its program for fundamental research, and to Mr. C. E. Sims, under whose valuable supervision the research was conducted.

C. G. Darwin: Theory of X-ray Reflection. Phil. Mag. (1914) 27, 315-333. 675-690.

A. Sieverts: On the Knowledge of the Occlusion and Diffusion of Cases by Metals. Ztsch. physik. Chem. (1907) 60, 125-201.

P. Beckman: Dissertation. Leipzig, 1907. A. Sieverts: The Absorption of Gases by

Metals. Ztsch. Metallkunde (1929) a1 37-46. Abst.. Metallurgist (1930) 5, 1681

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CARL A. ZAPFPE AND GEORGE A. MOORE I9

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