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could be detected, probably because t h e differences he found were due t o causes other t h a n bating. CONCLUSION
The mechanism of bating evidently consists of two distinct parts: ( I ) Reducing limed skins t o a condition of minimum swelling; (2) digesting the elastin fibers present in t h e outer layers of t h e skins. INCLUSIONS AND FERRITE CRYSTALLIZATION IN STEEL. 11-SOLUBILITY OF INCLUSIONS' By E. G. Mahin and E.H.Hartwig DEPARTMENT O F CHEMISTRY, PURDUE UNIVERSITY, LAFAYETTE, INDIANA
I n a n earlier paper by one of us2 experimental evidence was offered for t h e widely held theory t h a t nonmetallic inclusions are partly responsible for ferrite segregation in steel, indicating t h a t Stead's view t o t h e contrary3 may be incorrect. Stead had concluded, as a result of his experiments, t h a t phosphorus segregation about inclusions is responsible for this phenomenon, and t h a t the presence of inclusions has no direct bearing upon ferrite formation from slowly cooled hypoeutectoid steels. T h e writer, on t h e other hand, showed t h a t ferrite continues t o segregate about inclusions even after long heating a t temperatures considerably above Ars, although the longitudinal ferrite streaks of rolled steel disappear because of the better dissemination of phosphorus t h a t is brought about by such heating. T o account for this definitely established fact, i t was assumed t h a t either the material of t h e inclusion itself or some reaction product of this with surrounding metal (or both of these) is soluble, t o a slight extent a t least, in iron or in austenite, and t h a t the presence of this dissolved material so alters solubility relations as t o cause the beginning of ferrite separation from austenite in t h e regions so penetrated, before supersaturation has broken down a t other points. The mechanism of the breakdown of t h e solid solution of hypoeutectoid steel is somewhat as follows: As t h e steel cools slowly through t h e transformation range, t h e austenite of higher temperatures becomes saturated with ferrite a t t h e ideal temperature, As. Ferrite does not, in any case, separate at this point, b u t remains in solution until a somewhat lower temperature, Ar3, is reached, when t h e supersaturation then existing can no longer be maintained. When ferrite separation begins, recalescence occurs, and austenite continues t o reject ferrite about t h e crystal nuclei t h v s formed until t h e temperature has fallen t o A n , when austenite of eutectoid composition remains and is changed bodily t o pearlite. However, i t is not t o be supposed t h a t t h e degree of supersaturation of ferrite in austenite is the same in all parts of t h e steel mass, or even in a11 parts of a given austenite grain. Austenite of absolutely uniform concentration is a n ideal substance, probably never existing in a given steel mass. Even if all elements b u t carbon and iron could be excluded, such uniformity could be approached only in steel cooling from t h e li1 Read
at the 59th Meeting of the American Chemical Society St.
Louis, Mo,, April 12 to 16, 1920. a Mahin, THISJOURNAL, 11 (1919). 739. a J . Iron and Steel Inst., 97 (1918), 287, 389.
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quid state, because when a cooled hypoeutectoid steel is reheated, reabsorption of ferrite by austenite, as t h e transformation range is traversed, gives austenite grains containing more iron carbide toward their centers and more iron in t h e outer portions and, while diffusion is a fairly rapid process, i t is not likely t h a t a n y ordinary heating entirely serves t o correct this lack of uniformity in concentration. It is therefore in t h e regions of higher iron concentration t h a t supersaturation during cooling is greatest and, leaving other influences out of account, i t is in these regions t h a t t h e nuclei of ferrite grains are first gerierated. It is a general condition t h a t if a third substance is added t o a binary solution which is already saturated with one of its components, t h e solubility of this component is lowered. T h e opposite is sometimes true, and in certain cases of ordinary solutions t h e presence of t h e third substance has little observable effect, but these cases are exceptional. I n t h e binary solid solution, iron-iron carbide, t h e nonmetallic inclusion or a reaction product may be regarded as t h e third substance. It is not necessary t o assume a large solubility for this third substance. If there is a zone, however narrow, lying about a n inclusion and containing even a trace of some third dissolved substance derived from t h e inclusion itself, the presence of this material should alter t h e solubility of ferrite in t h e austenite there present, and t h u s cause local breakdown of t h e cooling solution first in these regions. This would establish ferrite nuclei, and separation would continue a t these points. T h e cooled steel would t h e n show inclusions embedded in ferrite. One reaction product was suggested in t h e first paper. This was manganese, an equilibrium product resulting from contact of manganese sulfide inclusions with iron. It would appear t h a t t h e steel in contact with such a n inclusion must necessarily contain manganese in slightly higher concentration t h a n in other regions. This would also be t r u e of manganese sulfide itself and of ferrous sulfide existing in equilibrium with t h e other substances involved in t h e reaction. Similar reactions are conceivable in t h e case of oxides, silicates, sulfides, etc., of elements other t h a n iron, a n d t h e reaction products, as well as t h e original inclusions, must dissolve t o a slight extent in t h e surrounding metal. I t was earlier pointed o u t t h a t t h e effort t o produce artificial inclusions by sealing powders into holes in steel could not be expected t o produce a n y very definite results, because air could not be entirely excluded from such cavities, and t h e film of oxide produced on t h e lining of t h e cavity when t h e steel was heated must prevent intimate contact with t h e inclusion, even if t h e latter were fusible. Of nine materials used by Dr. Stead, a t least five (calcium fluoride, calcium oxide, magnesium oxide, silica, a n d manganese sulfide) would be infusible, or practically so, a t 1000' C., the temperat u r e employed. T h e difficulties attending t h e production of intimate contact of artificial nonmetallic inclusions with surrounding metal are not encountered if metallic inclusions are used. Clean holes may be drilled in t h e steel, a n d clean, tightly fitting rods of a different material
-
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T H E J O U R N A L OP I N D U S T R I A L A N D I E N G I N E E R I N G C H E M I S T R Y
Fra. I I - S l a S L
(sr 4.0%).
x 50
FIO.12-Sr3%%
(P 1.06%).
1091
X 50
109 2
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driven into t h e cold, unoxidized steel. I n this way i t is possible t o use nonferrous metals, alloys, or alloy steels t o determine t h e effect of a third metal or combination of metals, thus imitating t h e production of a metal b y reaction of a nonmetallic inclusion with surrounding steel. This was t h e idea which prompted the work described below. From t h e success attained t h e work was extended t o include an investigation of the broader questiox as t o t h e effect of segregation of such substances as silicides, sulfides, and phosphides, and of metallic elements of alloy steels, upon carbon distribution. EXPERIMENTAL PART
A bar of rolled carbon steel, containing 0.j 3 per cent carbon, was cut transversely into plates about one inch in thickness. *Through these were drilled holes one-eighth inch in diameter, no oil or cutting compound being used. Rods of a number of alloys, carefully turned t o 0 . 0 0 2 in. oversize, were driven into these holes. The ends of such as were sufficiently malleable were riveted down. The piece of steel, with its insert, was then placed in a closed muffle furnace and heated t o 850' C. (which was well above t h e transformation range for this steFl) for periods of 7 hrs. After cooling in t h e furnace t h e pieces were sectioned transversely t o t h e axis of t h e insert, polished, and etched in nitric acid. Microscopic examination of t h e polished sections was made before etching, t o determine whether contact of t h e pieces h a d been perfect. I n t h e few cases where t h e insert had not been fitted tightly t h e specimens were rejected. Of t h e others, photomicrographs were made for record. The following alloys and steels were used as inserts in t h e first series of experiments: Aluminium bronze (copper go, aluminium 7, iron 3 ) , stellite, die-casting alloy (zinc and aluminium), high-speed steel, and steel containing 1.74 per cent manganese. The results of t h e treatment are shown in t h e accompanying photomicrographs, in each of which the steel and t h e insert are shown in t h e upper and lower portions, respectively, A well-defined ferrite ring surrounding t h e insert is evident in all cases. ( I n all of t h e photomicrographs there is a narrow, dark line, somewhat indistinct in some cases, a t t h e contact circle separating steel and insert. This is due t o t h e fact t h a t there is usually a difference in hardness of t h e steel and insert, so t h a t polishing produces a rounded shoulder a t the contact surface. With vertical illumination this appears as a dark ring.) ALUYINIUX BRONZE-Fig. I shows t h e appearance of t h e contact line between steel and aluminium bronze insert before thermal treatment. The other experimental pieces were similar t o this in appearance. The contact is excellent, t h e only change in grain structure being a physical distortion of grains adjoining t h e insert, this being produced by driving in t h e cold insert. Fig. 2 shows t h e same piece after 7 hrs. heating a t 850' C. A ferrite ring, clearly defined and uniform in width, entirely surrounds t h e insert. The obvious assumption is t h a t outward diffusion of t h e foreign metal, dissolving in contiguous austenite,
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lowers ferrite solubility and causes separation first in t h e zone penetrated. zIm-ALuJr[IxIum--The die-casting alloy was heated above its fusing point, a n d t h e line representing t h e contact surface of the cavity and melted metal has become irregular. However, t h e ferrite ring, not so wide in this case, 'follows this outline, as shown i n Fig. 3. STELLITE-In Fig. 4 is shown t h e very striking effect of a stellite insert. Either cobalt or chromium appears t o diffuse quite rapidly into steel a n d t o exert a considerable effect upon ferrite solubility. Possibly both metals are responsible for this effect. T h e ferrite ring here produced is t h e widest of all t h a t were obtained in t h e experiments described in this paper, with t h e exception of t h a t produced b y manganese and nickel, t o be discussed later. CHROMIUM-TUNGSTEN-The action of a high-speed steel insert is illustrated i n Fig. 5 . The ferrite ring i s narrow, b u t regular in width, and very distinct. MANGANESE-The action shown in Fig. 6 is interesting. Here t h e insert and t h e steel body are of practically identical composition except with regard t o manganese, t h e insert containing 1.74per cent of this element as against 0.67 per cent in t h e outer body. Above t h e transformation range, manganese has diffused outward from t h e region of greater concentration into t h a t of lower concentration. Ferrite solubility has apparently been diminished in t h e outer piece and, as might have been predicted, somewhat increased in t h e outer border of t h e insert. consequently, a ring of increased pearlite concentration is observed in t h e last-mentioned zone. (A still wider ferrite ring, produced b y manganese diffusion, is shown in Fig. 1 7 . ) ILLIuw-Illium metal (chromium-nickel-copper) produces a narrow ferrite ring when t h e specimen is heated for 9 hrs. a t 850' C. This is shown in Fig. 7. CHROMIUX A N D NICKEL-A chrome steel (chromium 1.0per cent) shows a well-characterized effect (Fig. 8), and a nickel steel (nickel 3.5 per cent) produces a wide ferrite band, as shown in Fig. 9. I n t h e experiments just described the materials used as pseudo-inclusions consisted of alloys and steels t h a t happened t o be available, and they were used in this way t o test t h e general effect of localizing metallic impurities. I n t h e further attempt t o use inserts made f r o m miscellaneous steels, each high in some single element, i t was soon found t h a t t h e results of t h e heat treatment were complicated b y differences existing between t h e insert and body, with respect t o t h e per cents of carbon, as well as of t h e common elements, silicon, phosphorus, sulfur, and manganese. The National Malleable Castings Co., through t h e courtesy of Mr. H. A. Schwartz, chief metallurgist, kindly undertook t o prepare a set of special steels t h a t would be better adapted t o this use. One stock steel'was made, containing 0 . 5 per cent of carbon a n d normal per cents of t h e four other common elements. T o various other portions of this same steel abnormally high amounts of special elements were added in t h e crucible, These special steels were used as in-
F m 13-S.m~ SIBELAS i;w Fro. I2
COPPER
R B A ~ E N T .X 50
serts in the stock for heat treatments similar t o those described in the first part of this paper. The following per cents of the special elements were found by analysis: Phosphorus 1.06, sulfur 1.36, silicon 4.0, manganese 1.46, chromium 0.84, and copper 1.34. Aspecial titanium steel also was available, hut the experiments upon this are still incomplete. Some further work upon all of these specimens is still in progress, hut it may be of interest tu dwell briefly upon the effects of some of the special elements upon carbon distribution. The method used fur making the special steels was, as noted above, the addition of ferromanganese, ferrosilicon, and ferrotitanium, as well as of steels containing high proportions of iron phosphide, iron sulfide or chromium, t o a crucible containing fused carbon steel. The carbon was thus maintained practically c.onstant, but under the conditions it could scarcely be expected t h a t the special steels would be uniform in composition, with respect t o the special element added. This lack of uniformity in the special steel insert gives two interesting results: ( I ) The ferrite ring produced in the carbon steel surrounding the insert varies greatly in width, presumably according to the variation in conccntratiai of the special element along the cylindrical surface of the insert. (2) The special steel itself shows pronounced carbon segregation.
The latter point is illustrated by Figs. IO, 2 1 , and photomicrographs of the annealed titanium, silicon a n d phosphorus steels, respectively. The segreI2,
gated masses of ferrite grains proba higher proportio parts of the met d t o be the case, a t a comparison of least with the ph Figs. I Z and 1 3 , tter being a representation of s copper reagent. this special steel The bright high-phosphorus areas, free from copper, correspond well with the ferrite grains of Fig. 12. PmsPXoXUs-It has already been noted by Stead, and confirmed by the work reported in a previous paper by one of the present writers,' t h a t iron phosphide migrates very slowly in austenite. It is interesting t o notice that the insert of steel containing 1.06 per cent of phosphorus, when heated t o 850' C. for 4 hrs., produces only a very narrow (and somewhat irregular) ferrite ring in the steel surrounding t h e insert (Fig 1 4 ) . It is planned t o give this specimen a protracted heat treatment a t a higher temperature in order t o observe whether the effect will be more pronounced smPux-The effect of sulfur, through migration of ferrous sulfide, is shown in Fig. 15. The contact line is somewhat obscured, in this view, on account of the smearing t h a t occurs during the polishing action upon the somewhat brittle material. corPcR-Copper migrates readily and affects carbon distribution, as do the other elements noted (see Fig. 16) !,xmc,ANEsE-Manganese shows a more pronounced effect than does any other of the common elements I
IOC
Clt
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dispose of this question. Fig, 1 4 0 evidence on this point,
uce .inch a regular ' band as was
vi,
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T ~ A,= O P~,FRBIS ~
IN L * l s " N
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nianmnese steel, where the contact was exceptionally good. 50 4-The actual amount of carbon that could be oxidized by the minute amount of air enclosed in an occasional poorly fitting insert, is nee of iron carbide would qui by oxidation. Pig. 20 shows a pronounced air pocket een a plain carbon steel and a brass insert, while 21 illustrates a similar condition with the special phosphorus steel insert already discussed. In both cases any carbon that has been removed by oxidation has been replaced by carbide migration, so that pearlite grains extend t o the very border of the cavity. This ability of iron carbide t o migrate readily is, of course, well known, and it i s the basis for such commercial processes as case carburizing. That iron carbide can easily and continuously cross the border joining these metallic inserts with surrounding steel is, finally, illustrated by using a n insert of Armco ingot iron. After giving this specimen the same treatment as was given the others, pearlite was found over the entire cross section of the insert. This is shown in Fig. 22. This is sufficient proof of perfect metallic contact. 5-Actual diffusion ot iron phosphide from the special phosphorus steel is shown by use of Stead's copper reagent. Fig. 23 is from a photomicrograph taken t o include two regions where there was no contact, separated by a short line of good contact. It is only into the region immediately outside the contact surface that phosphide has migrated, as is shown by B
~
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of ordinary steel.
Figs. 1 7 and 18,at 100 and 12 diameters, respectively, illustrate this very marked effect. A wide ferrite ring, uniform in width, figures. Fig. 1 8 was made without of the microscope and the definition the etching effect is easily seen. sIircoh;--In Fig. 19 the effect of The ferrite patches about the insert are very wide in places, and vcry irregular. The irregularity is, no n doubt, due to lack of uniformity in distribution of silicide in t h e special steel. OXIDATION
Throughout this discussion it has been assumed that from the diffusion of the special or insert into the adjacent steel has been the of ferrite segregation nCar the contactsurface. It is luortll while to inqluire whether oxidation may have ilad some influence, through leakage of air into minute by badly fitting inserts, with resulting cavities oxidation of carbon. pive lines of evidence lie against this possibility: I.-hficrOscopic esainination tile easily discloses points of poor contact, and ferrite segregation is almost invariably inost marked where contact is ciosest. 2-Oxidation should act upon steel insert and body alike. If thc inner lining of the hoie in the body is t o be decarburized by oxidation, the outer layers of the insert also should be decarburized, where the insert is of steel. Figs. j, 6,8, 9, 16, 17, and 18 very strikingly
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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
t h e absence of a copper deposit. It may be noted also t h a t t h e bright surface here shown is of approximately t h e same width as t h e ferrite ring shown for this same specimen in Fig. 14. I n addition t o t h e experiments above described, a few metals a n d alloys used as inserts in heat-treated steels have failed t o give any striking results. Whether this has been due t o ( a ) failure t o heat t o t h e necessary temperature, ( b ) extremely low migration velocities of t h e special elements or compounds, or (c) absence of a n y effect of diffused material, is not now clear. These materials and others will be used for additional experiments. SUMMARY
A considerable number of alloys and special steels have been turned into small rods a n d driven i n t o holes in carbon steels, t h e whole assembled specimen then being heated t o temperatures above t h e transformation range for t h e steel. The section of t h e slowly cooled piece then shows, in nearly all cases, ferrite segregation around t h e insert. T h e hypothesis t h a t has been advanced t o explain this phenomenon is t o the effect t h a t the special element, elements, or compounds have diffused into t h e surrounding steel, and t h a t they have there exerted a n influence toward throwing ferrite o u t of t h e aiwtenitic solution when t h e latter cools i n t o t h e transformation range. These experiments are cited as possibly throwing some light upon t h e character of the influence of nonmetallic inclusions upon ferrite segregation. T h e inclusion must have a certain slight solubility in austenite, a n d t h e concentration of t h e dissolved matter is therefore greatest in t h e immediate vicinity of t h e inclusion, There must also be equilibrium products of reactions occurring between t h e material of t h e inclusion and t h e steel, These products also must have a slight solubility, and they also are localized about t h e inclusion. Such a condition of localized dissolved impurities might have t h e effect of starting ferrite crystallization first about t h e inclusion, t h u s breaking down t h e state of ferrite supersaturation t h a t always occurs in hypoeutectoid steels between t h e temperatures As and Ar3. If these hypotheses are correct i t is easy t o see t h e importance of uniform distribution of all elements of carbon steel, as well as of alloy steels, as otherwise i t is not possible t o have uniform carbon distribution, and the finished piece cannot be brought into its best condition by any ordinary heat treatment. The effect of phosphorus upon carbon distribution is then b u t one illustration of t h e general law. Practically all of t h e elements t h a t can enter steel can exert a similar influence. T h e importance of this fact in eonnection with t h e subject of nonmetallic inclusions comes from t h e fact t h a t the inclusion furnishes a continuous supply of t h e dissolved impurity, a n d t h a t therefore no amount of heat treatment can cure its evil effects, while t h e opposite is t r u e with the dissolved element or compound t h a t is not associated with discrete particles of inclusions.
kb95
AN ELECTROMETRIC METHOD FOR DETECTING SEGREGATION OF DISSOLVED IMPURITIES IN STEEL' By E. G. Mahin and R. E. Brewer DEPARTMGNT OF CHEWISTRY, PURDUE UNIVERSITY,LAPAYETTE, INDIANA Received July 23, 1920
As a result of work described in two earlier papers% i t was concluded t h a t segregation of dissolved material derived from nonmetallic inclusions, as well as dissolved phosphides, silicides, sulfides, a n d elements of alloy steels, causes carbon segregation in steel, iron carbide usually leaving t h e region of higher concentration of impurity as t h e steel cools from above Ara. I n such a case t h e ferrite grains of t h e regions so contaminated should possess a solution tension dif ferent from t h a t of purer grains in other parts of t h e steel body. An electrolytic cell in which two of such grains form t h e electrodes against a common electrolyte, as for example impure ferrite-KC1-pure ferrite, should generate a n electromotive force which could be measured b y use of a suitable potentiometer system. It is manifestly impossible t o isolate two ferrite grains of a given piece of steel in such a way as t o make a n e. m. f . measurement of such a cell practicable. On t h e other hand, i t should be possible, a t least in principle, t o make successive measurements of t h e e. m. f . of each of two systems, in each of which a standard calomel or hydrogen electrode is connected against t h e grain in question, t h e same electrolyte being used in t h e two cases. I n this way a comparison could be made of the electrode potentials of various grains of apparently pure ferrite a n d differences in degree of purity detected.
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PRECAUTIONS TO B E OBSERVED
Many precautions must be observed in t h e a t t e m p t t o apply this method in practice. Some of these m a y be enumerated briefly as follows: I-Immersion of t h e specimen in t h e electrolyte is, obviously, not practicable. I t then becomes necessary t o pick out certain areas in t h e polished a n d etched section, making contact of t h e electrolyte with these alone. The microscope must be used for observation of t h e specimen, a n d a n extremely fine point of some sort must be used for touching t h e grain under investigation, this point carrying t h e solution of t h e electrolyte and establishing t h e necessary connection with t h e remainder of t h e circuit. 2-In any electrode-potential measurement, contact of electrode with electrolyte must be maintained for a certain period of time, in order t o establish equilibrium conditions. Using the method here discussed, i t then becomes necessary t o maintain a microscopic point or tube in contact with t h e grain under observation for some time. This presents considerable mechanical difficulties. 3-Evaporation of water from t h e solution a t t h e point changes t h e concentration of t h e electrolyte at. t h e contact surface. Using a molar solution of potassium chloride, t h e relative change in concentration is 1 Presented a t the 60th hfeeting of the American Chemical Society. Chicago, Ill.,September 6 t o 10, 1920. *THISJOURNAL, 11 (1919),739, 12 (1920),1090.
