Flaking in Alloy Steels - library.aimehq.orglibrary.aimehq.org/library/books/Open Hearth 1944/Open Hearth 1944... · of hydrogen in the metal bath during nickel and molybdenum increases

Flaking in Alloy Steels BY E. R. JOHNSON, S. W. POOLE A N D J. A. ROSA

THE problem of flaking is& condition ever established for large sections such a s present with the producer of alloy,steels. blboms, biilets and heavy forgings, and The susceptibility ,of nickel-chromium, these cycles must be carefully followed for nickel-chromium-molybdenum, chromium- best results. molybdenum,steels, etc., to the formation of flakes in the cooling of large sections like PREVIOUS INVESTIGATIONS ON FLAKING - - blooms, billets and forgings is a well known fact. The inherent susceptibility to this particular type of internal defect imposes a responsibility upon. the producer of allov steels, which is reflected in control measures for minimizing or eliminating this condition that starts with the raw materials used in melting a heat of alloy steel.

I t is generally believed that hydrogen- which may come from any source, such as moisture, rust, oil-is t h e fundamental cause of flaking. For th i s reason scrap and raw materials used m u s t be as free as possible from moisture or material containing chemically co,mbined water as .rust or p a r t l y hydrated burnt lime. Once hydrogen has diffused into :the molten metal it can be removed best by oxidation, as in ore boiling. The condition of the atmosphere may also b e a factor in hydrogen pickup. Acid open-hearth alloy steel, basic open-hearth and basic electric steel are all flake susceptible; the flake susceptibility, however, being largely controlled by type of analysis melted by these various methods.

Control measures for prevention of flaking after the steel has been made are concerned largely with establishing cooling cycles that have as their objective the twofold purpose of diffusing hydrogen out of solution in the solid metal and producing a relatively soft and ductile steel matrix.

Any survey of the technical literature with regard to flaking and the role of hydrogen as a cause for flaking shows that a large number of investigators in this country and abroad have been concerned with .this problem. A considerable amount of work on flake prevention was conducted as early as 1923 a t the Central Steel Company's works a t Massillon, Ohio, now a unit of Republic Steel Corporation, by Morris and Dittmar, and the methods developed have been successful since tha t date. Some of the earliest reported work, given by .Hultgrenl in 1925, dealt with carbon-chromium steel. .

Hultgren definitely showed that flakes occur during. cooling - after hotrworking and also that if a forging or .bar is cooled down to . ordinary temperature between ,two operations of hot-working flakes may form. His investigations were confinkd to high-carbon, high-chromium steel of acid open-hearth steel manufacture. The analysis of the steel used was: C, 1.0 per cent; Mn, 0.25 to 0.35; Si, 0.25 to 0.35; P, 0.012 to 0.017; S, 0.010 to 0.015; Cr, 1.50.

Some special large ring forgings that developed flakes were of the following analysis: C, 0.70 per cent; Mn, 1.00; Cr, 1.00.

Some factors of considerable interest were discussed including the following:

Hence special cooling cycles must be 1 References are on page 368.

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1944 OPEN HEARTH CONFERENCE 359

I. No relation could be established buried, annealing results became normal between variation in melting practice and and flake formation was also eliminated. variation in the number of flakes present Hultgrenl also stated that the influence in bar stock. of variations in forging practice, such as

Pressure of H2 in Atmspberes 10 100 lo00 4000

I

2400 \ / / 1- So/ubi/if y Curve of

2200 Hydrqen in Ir on I t

2. Reheating alone without mechanical work does not remove flakes.

3. Disks hot-sawed from a bar after leaving the rolls did not contain flakes, although flakes might be present in the bar.

4. Flakes in 434 by 436-in. billets could be removed by rolling to 3 1 x 6 by 3l416 inches.

According to Hultgren,' the effect of a slow cool starting directly off the forging press was found to give difficulty in annealing because of the formation of heavy carbide boundaries, difficult to take into solution in annealing operations. Investiga- tions on cooling to prevent flaking showed that if the forged product was allowed to air-cool to 750°C. (1380°F.) and then

amount of reduction, and rapidity of blows on the formation of flakes during cooling seems to be slight or nil.

With respect to the susceptibility to flaking of alloy steels and the relation between flake susceptibility and electric- steel melting practice, i t has been shown by M i r t s i r n o ~ , ~ in a survey of some two thousand high-carbon high-chromium-bearing steel heats that a correlation exists between flake susceptibility and: (I) ore- boil period and extent of carbon reduction; (2) Length of time under carbide-type or neutral-type lime-silicate slag in the refining period.

The longer boiling period produces less flake susceptibility and the effect of a

360 1944 OPEN HEARTH CONFERENCE

prolonged treatment under the second or pressure. This 'relationship is also reputed refining slag increases the tendency toward to hold good for commercial steels in flake formation due to progressive hydro- which the solubility of hydrogen is in- gen absorption under reducing conditions. fluenced by alloying elements. I t is further

As early as 1911 Sieverts3 showed the maintained, by B~denstein,~ that hydrogen solubility relations for hydrogen in iron prevails in iron in atomic solution. Chuikos and the fact that the solubility of hydrogen has discussed in some detail the physical is proportional to the square root of the chemistry involved with respect to solution


of hydrogen in the metal bath during nickel and molybdenum increases the melting and refining in the electric furnace, solubility of hydrogen in metal, although As far as electric-furnace steel melting is the effect is not very pronounced with low- concerned, the hydrogen content of the alloy steels.

H o u r s FIG. 4.-A&%%ALIWG-FURNACE CYCLE ON S.A.E. 4340.

1 2 by 12 ingot; furnace hours, 72.

1600

1500

1400

1300

4 1200 d L l I00 3 +

1000 0

900

800

700

600 500

0 5 1' '5 20 25 30 35 40 45 50 55 Hours

FIG. ~ . - A N N E A L ~ C - F U R N A ~ CYCLE ox S.A.E. 2515. 7%-inch ingot; furnace hours, 53.

metal is a function of the water-vapor The work of the Hair-Line Crack Sub- pressure and hydrogen in the furnace Cdmmittee, as reported in the Jourpzal of atmosphere. The area of the electric arc the British Iron and Steel Institute, con- affects the decomposition of molecular tains some of the most comprehensive hydrogen (Hz) to atomic hydrogen (H), work performed to date on the fundamental the latter being capable of dissolving in causes and mechanism of flakit!. Andrew, the molten metal more readily. The addi- Lee and Quarrel1,s in their first experiments tion of alloying elements as chromium, on flake formation, investigated the loss of


hydrogen from solid specimens on cooling require an aging period. The authors with particular reference to thc changes in suggest that these facts imply that the solubility occurring a t the gamma-alpha cracks are not due entirely to the stresses transformation. resulting from the drastic heat-treatment

Andrew7 and his co-workers were able to show that internal defects similar to flakes were formed in solid steels by soaking a representative series of steels in hydrogen for go hr. a t a high temperature (2190°F.) followed by rapid cooling through the Arl point. The cracks developed are not formed immediately after quenching but

ND INGOT OF S.A.E. 3312 MODIFIED.

undergone by their specimens. Similar treatments, in which the nickel, nickel- chromium, molybdenum and carbon steels employed were treated in a vacuum or a nitrogen atmosphere before quenching did not produce internal rupturing.

The conclusion followed that hydrogen must be considered as the major tactor in

I944 OPEN HEARTH CONFERENCE 363

producing flakcs. No cracks were produced cooling produced no cracks and thus, in in cast specimens under the conditions the authors' estimation, proved conclu- described. Whether flakes are due to the sively that hydrogen is the fundamental difiusion of hydrogen as such into micro- cause of hair-line cracks or flakes.

scopic voids in the steel or the breakdown of a hydrogen-rich constituent, which forms hydrogen in the steel, has not been decided as yet by these investigators.

I n their second paper AndrewR and his co-workers conducted experiments on commercial sections sufficiently large to dcvelop flakes on air cooling. The flakes so produced by their hydrogen treatment were characteristic in appearance, deep seated and in random distribution. Treatment of similar sections in nitrogen followed by air

I t is statcd that the hydrogen that subse- quently causes cracks is not in solid solution but, for a time, is held in thc form of a hydrogen-rich constituent, which is formed on rapid cooling through the gamma-alpha transformation. It is claimed that the breakdown of this constituent releases hydrogen, some of which diffuses into voids and builds up to a disruptive pressure. The influence of composition upon thc susceptibility of a given steel to flake formation is explained by the effect of


alloying elements upon the stability of discontinuity in the steel matrix, molecular the resultant hydrogen-rich constituent. hydrogen (H2) then forms by a combination The breakdown of the hydrogen-rich con- of two atoms of atomic hydrogen, 2H + HI. stituent a t low temperatures must lead to Molecular hydrogen is insoluble in steel

FIG. 8.-FLAKE IN S..i.E. 4340 OCCURRING I N ACICULAR PRODUCT OF TRANSFORMATION FORMED BELOW ('NOSE" OF S-CURVE IN SEGREGATED A ~ A IN SLOWLY COOLED BILLET. X 100.

flake formation. It is claimed that flakes will not result if breakdown of this constituent and diffusion of hydrogen from the steel can be brought about a t a sufficiently high temperature.

The fundamental reason for flaking has been the cause of extensive discussion in the technical literature and has led to two schools of thought on this controversial subject. A great deal of evidence has been submitted to show that hydrogen is the fundamental cause, and the mechanism whereby hydrogen produces flakes is thought to be as follows. The hydrogen present in steel is present in the atomic form and is contained in the iron lattice. If it leaves this lattice, within which it may move freely, by entering a small

and cannot escape by diffusion. If this process continues and more atomic hydrogen diffuses into a small discontinuity, forming molecular hydrogen-whether it is atomic hydrogen as such or atomic hydrogen resulting from the decomposition of a hydrogen-rich constituent as advanced by Andrew-pressures amounting to many thousands of atmospheres may within a short time be set up in these microscopic discontinuities. According to Zapffe,¶ a tenfold decrease in hydrogen solubility on cooling is equivalent to a hundredfold increase in molecular hydrogen pressure. I t is these very high, extremely localized internal pressures that exert the disruptive stresses causing flakes (Fig. I).

On the other hand, some authorities maintain that hydrogen is a contributing factor only in flake formation and that


transformation stresses occurring on cooling, particularly with the alloy construc- tional steels, are potent factors in flake formation. R u ~ s e l l ' ~ states that he does not know of an experimental result that throws any light on the relative potency of hydrogen as a flake former when it is present in such quantities as are found in commercial steels. He also feels that experimental conditions imposed upon a flake-susceptible steel, wherein hydrogen can apparently be made to flake the steel, are far removed from actual steel-mill operations.

TABLE I.-Republic Steel Data on Hydrogen i n Steel

A. Hydrogen in Steel (Vacuum Fusion)

Stresses may be set up in a cooling mass of steel by temperature gradients and phase changes, of which two factors the latter is by far the most important, especially if the transformation occurs below goo°F., when the steel has lost much of its plasticity. Woolman" reports that calcula- tion of stresses in a homogeneous cooling mass shows that the tensile stresses resulting from the transformation change as it passes from the outside to the center but are always greatest a t the axis or near the outer surface and smallest in the inter-

mediate regions where flakes are usually formed. The stresses produced in a homogeneous mass during cooling cannot, therefore, be responsible for flake formation, and evidently some superimposed stress must be prescnt to account for this phenomenon. The presence of highly segregated areas that transform a t a later stage than the normal composition can produce these locally superimposed stresses. Any added alloy element or combination thereof that can segregate during freezing, and thus retard transformation, should be effective in producing flake formation. As hydrogen is soluble in austenite and capable of segregation, i t is thus capable of stabilizing this phase and thereby contributing to flake formation.

From the point of view of the alloy-steel producer, the present authors feel that it would seem wise to consider hydrogen togethcr with superimposed transformation stresses in segregated areas within the cooling mass of steel as the cause for flake formation. I t would also appear that a sensitive balance between transformation and hydrogen stresses may be maintained. I t is felt that the magnitude of the hydrogen stresses developed in cooling are related primarily to steel analysis and in addition to steel-melting practice. Segregation effects, which influence internal stresses in the cooling mass of steel, can be related to such factors as analysis, steel pouring temperatures, ingot size and ingot-mold design.

The attempt to eliminate flaking by means of controlled slow cooling from forging or rolling temperatures dates back to 1923, and the main thought has always been to increase ductility to take care of stresses prcsent. The earliest method of slo~v cooling to prevent flaking consisted simply of placing the section to be cooled


on the ground and covering it with sand, Although the bury pit and its modifica- dirt or ashes. Later, crushed slag, cinders, tions was and still is successful in generally fuller's earth and other insulzting media controlling flaking, the long time cycle were used. As tonnages increased and the involved and the increasing demands for

FIG. 9.-FLAKE I N ACICULAR TRANSPORMATION PRODUCT I N 4340 SLOWLY COOLED BILLET. X 500.

need for closer control in cooling became apparent, this crude method became inadequate. Out of necessity, the bury pit was evolved.

The bury pit is nothing more than a cavity in the ground lined with brick or concrete. I t may terminate a t the ground level or may extend several feet above ground. Some are equipped with burners for greater control in cooling. Essentially the pits are alike a t all plants, in that they all serve as insulated chambers for the slow cooling of a charge of hot steel. Modifications of the bury pit have been adapted to meet specific needs and one can find today simple "tote" boxes of a few hundred pounds capacity up to spe- cially built railroad cars of. 50 tons capacity. After the pits-are loaded they may be covered with any of the previously mentioned insulating media or with elabo- rate insulated steel covers, or both.

greater tonnages made i t imperative that a faster method be devised. When it is considered that the cooling period for a charge of steel in a bury pit runs from 2 to ro days, the need for a faster method becomes self-evident.

Early experimental work indicated that flaking occurred a t fairly low temperatures in most alloy type steels. With this in mind many experiments were conducted to determine the possibility of using furnaces to reduce the total time required and to increase the factor of safety. Considerable experimental work was done in our Canton plant on high-carbon chromium-bearing steel. Some of this work was reported by 0. A. Bamberger.12 This experimental work resulted in a heating and cooling cycle sometimes known as the recrystallization cycle, which is substantially as follows:

I . Charge into furnace from hot working, level off.

1944 OPEN HEARTH CONFERENCR 367

2. Cool with furnace or a t controlled of the smaller amount of re-solution of rate to below Arl, level OR, soak. microstructural constituents.

3. Reheat to just above Ac3, soak. I n a third modification of the original 4. Cool to room temperature a t the cycle the reheat temperature was ldwered

rate necessary to give the desired cold sawing structure and hardness.

In the light of present knowledge the fair success of this cycle can be ascribed chiefly to the increased strength resulting from grain refinement and partial diffusion of hydrogen while soaking, heating and cooling in the alpha-phase region.

The need for a more foolproof cycle eventually led to the following modification of this original cycle: Instead of reheating to above A c ~ , the charge was reheated to just above Acl and soaked there; subsequent cooling to room temperature was generally controlled a t a slow rate. The increased effectiveness of this cycle may lie in the fact that a relatively smaller portion of hydrogen is redissolved and transformation stresses are lower because

to just below Ac,. The effect of lowering the reheat temperature to below Acl has been to eliminate entirely the secondary transformation stresses. The apparent effectiveness of this cycle may lie in the fact that the soak a t just below Acl relieves the prior transformation stresses and simultaneously puts the steel into a ductile condition. While this tempering action is going on hydrogen in excess of equilibrium is free to diffuse out of the section a t a fairly high rate. The sum effect of this cycle is to minimize to a harmless degree the two factors generally accepted as being the principal agents in causing flaking-transformation stresses and hydrogen pressure.

Tha t the cycle is effective is evidenced by the fact that more than 3000 heats of


all types of alloy steels have been treated in the authors' plant with less than 0.5 per cent loss due to flakes. I n addition to reducing flaking loss to a minimum, the cycle has increased production because of the relatively short treating time required. The average charge of rolled blooms or billets requires 40 to 7 2 hr. total treating time.

I n the search for a cycle that would eliminate flakes, the practical operator has kept in mind a treatment that would simultaneously achieve two objectives in a minimum time: (I) complete elimination of flakes and (2) maximum softening of the steel.

Some laboratory investigation under consideration a t present will be concerned with the application of isothermal annealing of nickel-chrome-molybdenum analyses t o investigate the ductility that may he developed by such treatment.

While the isothermal annealing tech- nique is believed to be effective in softening the steel, its relationship to the elimination of flakes is not too well understood. I t is known that s precipitation or rejection of hydrogen occurs on gamma-alpha transformation. I t is also known that the hydrogen diffusion rate a t elevated temperatures in the alpha phase is fairly high. On the basis of this rapid gamma-alpha transformation and high diffusity of hydrogen, i t may be assumed that the twin evils-transformation stresses and hydrogen pressure-may be rapidly brought under control by isothermal annealing in the plastic range.

I n line with some of our other laboratory studies on flakes occurring in nickel- chromium-molybdenum steel, Figs. 7 to 11 illustrate a few interesting features with respect to occurrence of flakes and their formation in structurally segregated areas.

The present situation with regard to control of flaking in flake-sensitive steels resolves itself into two main considerations:

I. Control of raw materials used in steel melting is necessary in order to

eliminate as much moisture and moisture- containing agents as possible, and also to minimize the amount of contaminating oil on oily scrap charged.

2. The use of properly constructed bury pits for cooling flake-susceptible steels has proved to be a satisfactory procedure for minimizing flake formation. I n order, however, to shorten the time required to prevent flaking on cooling, suitable annealing cycles are now established for preventing flaking, which represent a considerable saving in time required a s compared with the older bury-pit cooling methods.

I n addition, it may be said that the cycle annealing of large billets and blooms conditions the steel more adequately for subsequent operations as chipping, cold sawing or machining.

I t is felt that the net result of experimental work both in the laboratory and the steel mill over the past 2 0 years has eliminated flaking as a serious problem in the production of alloy steels.

I . Hultgren: Jnl . Iron and Steel Inst. (1925) 33. ,113-167.

2. M~rtslmov: Melallurg (Russian) (1938) 13 (1) 39-54.

3. Sieverts: Ztsch. Physik. Chem. (1911) 77, 591-613.

4. Bodenstein: Zjsch. Elekwochemie (1922) 28, 517-526.

5. Chuiko: Teoriya I Praktika. Melallurgri (1938) 9 ( 5 ) . 31-37.

6. Andrew. Lee and Quarrell: Jnl . Iron and Steel Inst. (1942) 2, 181-192.

7. Andrew. Bose, Geach and Lee: Jnl . Iron and Steel Inst. (1942) 146, 193-202.

8. Andrew, Bose, Lee and Quarrell: Jnl . Iron and Steel Inst. (1942) 146,203-243.

9. Zappfe: Metal Progress (Aug. 1942) 201- 206.

10. Russell: Discussion on papers of Andrews e t al. Jnl . Iron and Steel Inst. (1942) 146, 251-253.

I I. Woolman: Discussion on papers of Andrews et al. Jnl . Iron and Steel Inst. (1942) 146,253-254.

12. Bamberger: Iron and Steel Engineer (Nov. 1943) 2 0 , 68-73.

DISCUSSION

F. B. Foley presidilzg

THE CHAIRMAN.-IS there discussion of this paper?

1944 OPEN HEAR

C. A. ZAPFFE.-At the risk of becoming known as a bore, I want to take just a couple of minutes to see if I can possibly help clarify some of these points. Most of you have been too busy doing more important things to be able to take six years of your life and study hydrogen, as I have.

First, I want to point out that flaking is very definitely a combination of two things, hydrogen embrittlement and superimposed stress. I n all of the work that stands on hydrogen, there is not a case where hydrogen alone has definitely caused a crack. No matter how much hydrogen a steel may contain, that gas will not in itself cause cracking. I t will only cause low ductility.

That is very fortunate, for it allows us two factors to con t ro l s t ress and hydrogen content. For example, we know that a hydrogen-containing steel may be slow- cooled to avoid flaking even when the ingot is so large that very little of the hydrogen is lost. A brittle glass, of course, can also be cooled in this manner without cracking. Such a n ingot, however, if reheated and then cooled too rapidly, may be subject to flaking. If it had lost sufficient hydrogen on the first cool, it never again could flake.

My second point has to do with the use of the word " supersaturation " and the general mechanism of embrittlcment and flaking. The chemistry of hydrogen in steel is very similar to that of C 0 2 in a carbonated beverage. There, the beverage is not "supersaturated" unless the pressure of COZ a t a surface of the liquid is less than that escaping pressure of the gas in the liquid. While capped, the brew is saturated; uncapped, it is supersaturated and liable to gassing; but if a greater pressure of COz is applied, the same brew is then unsaturated and further absorption occurs.

For hydrogen in steel, the same principles hold; and we must remember to use the word "saturate" with extreme care. For example, if Hz is bubbled through liquid steel, we saturate the steel; but as soon as

.TII CONFERENCE 369

the gas supply is cut off the steel is supersaturated; and if that steel is then poured into a damp mold, it may behave as an unsaturated steel-in all cases the hydrogen content being exactly the same, only the surface conditions changing.

Consequently, the important factor in determining saturation in this particular system is the surface condition. At ordinary temperatures, all stecls are probably supersaturatcd with hydrogen, unless they are being pickled or cathodized, for the surface conditions ncvcr carry the tremendous pressure of Hz necessary for a n absorption of hydrogen equivalent to the quantity that is easily absorbed by liquid steel from much lower pressures. As we know, steel containers store HZ under 2,000 Ib. per sq. in. without measurable absorption by the cold steel. Tens of thousands of pounds per square inch of Hz would be necessary to drive the amount of hydrogen into cold steel that is dissolved from one atmosphere of the gas a t steel- making temperatures. Conversely, without that surface condition, the gas inherited from melting will attempt to restore that equilibrium and will bchave just a s the gas in a beverage, escaping from the steel a t every brcak in the latticework of iron atoms until such surface conditions are obtained.

Corresponding to "putting the cork back into the bottle," one might build a close- fitting jacket around a piece of hydrogen- containing steel, in which case the gas would escape under the jacket until the saturation pressure was reached. Hydrogen- caused defects in vitreous enamel and electroplate are evidences of just that phenomenon.

Some of you have probably observed that it is quite unusual to find the thing we call a "flake" in a casting. A casting, of course, is more porous than a forging, corresponding to a looser " jacket "; and the hydrogen content of the steel will deplete itself more before reaching the high saturation pressure possible in the denser structure of a forging.

3 7 O 1944 OPEN HEARTH CONFERENCE

Cause of Cooling Cracks not occur in fine austenitic grain size (Fig. 3 ) .

THE CHAIRNAN (F. B. FOLEY).-~~~LY I I t is known that cooling cracks occur in call attention to a possible explanation of ingots (Fig. 4) and that they are found to be

the occurrence of cooling cracks without resorting necessarily to hydrogen. Cooling cracks became notorious during the last war. I had occasion then to examine many of them. It is rare that the investigator of cooling cracks has the opportunity, which came to me, to examine them in the forged steel prior to any subsequent reheating. I t was my observation that cooling cracks were intercrystalline in the forged metal: that is to say, they were found in the ferrite network of the "as-forged" structure. This is illustrated in Fig. I. Another observation, which I find by looking over my corre- spondence during the last war on the subject, was that cooling cracks accom- panied a coarse, very coarse, austenitic grain size in the forging (Fig. 2) and did

intercrystalline. The order of susceptibility places ingots first, forgings next and normalized steel last. I n fact, it has yet to be demonstrated that cooling cracks are ever produced by a normalizing treatment. This order of susceptibility coincides with a decreasing order of grain size.

Let us look a t another fact. Cooling cracks are not found in the outer inch or two of the section through a forging. This is so in spite of the fact that the outer layers of the forging cool most rapidly, although slow cooling is known to prevent the occurrence of cooling cracks. I t is in the slow-cooling interior of the forging that cooling cracks actually occur.

It is characteristic of cooling cracks that they are present in forgings of large sections

1944 OPEN HEARTH CONFERENCE 37l

-in spite of the fact that these cool more continual decrystallization and recrystal- slowly than small sections of, say, less than lization incident to severe hot deformation. 246 to 3 in., in which cooling cracks are I n smaller scctions and rolled bars, dcfor- rarely if ever found. mation sufficient to alter completely the

I ; - S I ; , \ - F O R 1 1 A \ - FIG. 3.- FIN^< AS-FORGED G R . \ I S SIZE F R E E OF

1st; COOLING CRACKS. X 7. COOI.ISG CR.\CKS. X 7.

One would suppose, without any other evidence to go on, that the way to prevent cooling cracks would be to cool rapidly. Their absence in the outer fast-cooling region of forfiings and in small rapidly cooling sections would dictate such procedure.

When an ingot is soaked for many hours a t the high temperatures used in forging, a very large grain size is produced. The section shown in Fig. 2 is only 7 diameters magnification. During forging a certain amount of movement of the metal is necessary in order to obliterate this large grain size. This is accomplished for some distance beneath the point a t which the force of the hammer or press is applied. Immediately the force of deformation is removed, the metal crystaIlizes in a grain size characteristic of the esisting temperature of the metal. Deep-seated metal that is not deformed sufficiently to change the crystal structure established in heating for forging remains coarse grained. Cooling cracks are not produced in the fine-grained outer zone of the forging produced by the

crystal structure set up in heating the billet or ingot for hot-working penetrates to the center of the section. There is left no coarse-grained metal in which cooling cracks may develop.

I t may be, therefore, that not hydrogen alone, but coarse austenitic grain, is the prerequisite of cooling cracks. ?his itlea gains support from the fact that forgings reheated to above the critical temperature for normalizing and then air cooled do not develop cooling cracks. Such heating recrystallizes the center and replaces its coarse, austenitic grain size with a fine one. The same explanation applies to the work a t Timken, which showed that a forging cooled to ~oooOF., after completion of hot-work, held there 4 hr., reheated through the critical temperature to r500°F. and held 4 hr. could be cooled in air without developing cooling cracks, whereas the same steel cooled directly to room temperature after forging, without being subjected to recrystallization by reheating through Ac, invariably developed cooling cracks.


S. W. POOLE.-That is right. That cycle cracks are so much greater. The chances are represents an experimental procedure on less with a fine grain size. high-carbon chrome-bearing steel. A forging properly cooled, so that no

FIG. 4.-COOLING CRACKS I N 19-INCH CORRUGATED INGOT.

THE CHAIRYAN.--T~~ other operation that Mr. Poole described is one in which all of the transformation is permitted to occur completely in a range of temperature where the metal is quite plastic, so that the part of this cooling-crack trouble that may be due to a low-temperature transformation of the material is avoided.

Low-temperature transformation is caused partly by large austenitic crystal size. The larger the crystal size, the lower the temperature of transformation and the deeper hardening steel becomes, and if we have, in addition, sufficient amounts of hydrogen, the chances of getting cooling

cracks are present, may be heated to 1600' and cooled in air without danger of producing cooling cracks, because the reheating through Ac has permitted recrystallization with a fine austenitic grain size.

This hypothesis offers another point of view in which hydrogen is not considered as the essential element in the formation of cooling cracks. I should have said that all of these cooling-crack questions can be answered by the hydrogen theory just as well as by the scheme I have proposed. Is there any further discussion?


C. A. ZAPFFE.-Mr. Foley's observations are consistent with our argument, since thermal stresses are only one half -of the picture. The best way of growing large grains in steel is to treat the steel with hydrogen. Hydrogen is a very strong deoxidizer. I n fact, i t is a desulphurizer and a decarburizer, as well, reacting with almost all of the known metallic constituents to form insoluble products, which tend to escape from the steel. A principal factor in grain growth, of course, is the presence of foreign matter, or interferring phases, which limit grain size. A steel with a high hydrogen content, then, is likely to contain less of the other nonmetallics. Hydrogen therefore tends both to purify the steel and to grow large grains; and that factor should be kept in mind when discussing the relationship of grain characteristics to phenomena that may concern hydrogen.

A common observation you all recognize is the so-called " picture-frame " effect produced by shatter cracks or flakes, especially in large forgings. One of the outstanding characteristics of flakes is that they usually occur in a zone that is intermediate between surface and core. The distance of the zone from the surface depends upon the size of the forging and its past treatment. Less frequently do flakes occur a t the center of large forgings.

Let me review the two points that I brought out in my previous discussion, to show how simply this fits into the general picture. As is well known, the core of a forging is less dense than the rim. Conse- quently, given a uniform distribution of hydrogen, the precipitation in the core will develop less pressure than it will in the denser rim. The requisite embrittlement, then, is less likely to obtain in the core, and flakes will be unlikely there with or without the presence of critical stress.

However, we know that hydrogen is not uniformly distributed across an ingot or forging, since it is always escaping from

the surface. Hot-working removes hydrogen; so does cold-working. The removal is effected as explained before, by the hydrogen precipitating both a t the surface and a t internal surfaces. Working the steel breaks up the structure sufficiently to open the rifts that have accumulated this gas and permits more of it to reach the surface.

At the surface, then, the hydrogen content approaches zero; and toward the center it will become a maximum. But for a certain depth there will not be enough hydrogen to cause embrittlement, regardless of the density of the metal. We are

,

then left with only a n intermediate zone, which is deep enough to retain sufficient hydrogen and yet near enough to the surface to contain dense, worked metal. Tha t zone contains the "picture frame."

With increasing degrees of forging, the outer flake-free rim becomes thicker as hydrogen is gradually lost. A hydrogen- removing anneal 'similarly increases this flake-free rim; which moves inward until i t meets the flake-free center, and the forging is free of flakes. In industry, that is first accomplished generally after forging to a section 2 to 4 in. in size.

In experimental tests, steel may be loaded wi th hydrogen and cooled so quickly that the hydrogen content is almost uniform across the entire section. I n just a skin layer, will it drop to sub- embrittling vilues. Consequently; the "picture frame" moves right out toward the edge, perhaps within 3i6 in. of the surface. The more hydrogen that is absorbed, and the less chance that is given it to escape, the nearer to the edge will be the flakes.

THE CHAIRMAN.-Of course, we all know that these flakes or cooling' cracks can all be closed up by forging and proper handling afterward.

C. E. SIMS.-I don't know whether I can clear up anything but I want to mention some pertinent facts.

374 1944 OPEN HEAR TIT CONFERENCE

I t has been shown that castings are not nearly as prone to flaking as are forgings and rolled billets. I t has also been shown that when a forging or billet has becn very slowly cooled and then reheated above the critical temperature and cooled normally, it is very ap t to be free of shatter craclrs.

We find that if billets are very slowly cooled they can be dcliveretl to the cus- tomer free of cracks. That is fine. That lets the steelmaker out. He has delivered an acceptable product. However, the fabri- cator has the cracking problem all over again if he 'is going to use that billet as forging stock. On the next heating for forging he must cool very slowly to prevent shatter cracks from forming.

I t has been shown that castings are not entirely free of shatter cracks under certain unusual conditions. For example, the English work described here showed definite cracks being produced in castings, and shatter cracks may be developed in small sections if those small sections have been drastically cooled. That was the case of the small castings described in this English work. There, much higher stresses were set up by the fast cooling, and pre- sumably this condition requires a smaller amount of hydrogen to generate the same troubles.

There is another manner in which hydrogen manifests its presence other than by shatter cracks, and that is'in small brittle areas in the steel, which show up when the steel is fractured by external stresses. You find them in fractures of test bars, and they show as small bright spots. These areas are extremely brittle and cause the steel to fail with ductilities far below what might normally be expected.

I t has also been shown quite definitely that these small brittle areas can be removed by a sufficiently long aging treatment. The time of the aging treatment is dccreased very markedly by increases in temperature up to about 800' or goo°F.

In a square billet that has becn air

cooletl, it is well known that no shatter cracks arc fount1 in the surface layers ant1 few, ifany, in thc center. Therc is, however,. an intermediate concentric zone in which they are abundant. In this zone, the cracks tend to be circumferential in dircction, indicating that radial stresses are involved.

'This position and direction appear logical when one considers thc manncr in which a billet cools. The surface cools faster, of course, and becomes rigid while the center cools later and tends to shrink away from the surface This sets up radial stresses but, because the differential in cooling rate decreases with approach to the center, the greatest stress is set up next to the surface layers. The surface, naturally, is in compression.

The manner in which hydrogen precipitates may be important in determining whether shatter cracks will be formed. If there are discontinuities in the steel structure, such as microshrinkage cavities or space between steel and inclusions, these areas will act as nuclei for the precipitation of hydrogen. Undelr such conditions the hydrogen precipitates locally under high pressure, but the pressure is partly relieved by opening rifts in the slip planes of the steel in ' a concentric zone around the nucleus. This produces embrittled arcas, as in castings, but no cracks.

When all discontinuities are closed by hot-working, as in forging or rolling, however, the hydrogen has no natural place to precipitate and develops a tremendous precipitation pressure during cooling. Then, if the stresses set up in cooling open up submicroscopic rifts in the space lattice, so small that they would readily heal during stress relieving, the hydrogen will precipitate headlong and rend them asunder. This may explain the difference between castings and wrought steel.

But it is important to note that under ordinary conditions hot-working is a prerequisite to shatter cracks. I t has often becn noted that steels just heated and

1944 OPEN HEAR T H CONFERENCE 375

cooled do not tend to crack. The treatment ju'st described which includes cooling below the critical temperature, reheating above and then cooling to room temperature to prevent shatter cracks, has been tricd too often to doubt that i t works. I t probably could be shortened even more than was described.

A similar treatment was tricd on a heavy section casting that had an annoying prevalence of brittle areas, but the brittle areas persisted through the treatment. I t is obvious that hydrogen is not eliminated even though shatter cracks are prevented. This correlates with the earlier statement that avoidance of shatter cracks does not indicate elimination of hydrogen or danger from cracks in later processing. Thus, by putting together seemingly anomalous observations, one can often obtain a more concise picture of the \vhole problem. .

Q u ~ s ~ 1 0 ~ . - I s n ' t the word "precipitate" rather loosely used in connection with hydrogen? Hydrogen is supposed to be present in atomic form in steel.

C. E. SIMS.-Hydrogen dissolves and forms a solid solution in austenite and has a lower solubility in the ferrite, particu-. larly a t the higher temperatures in the ferrite, so that when it comes out of the lattice to form molecular hydrogen it is ,a true precipitation, I think.

QUESTION.-I read .the statement some- where that it was possible for the hydrogen to precipitate itself; that it had to have a point of action, you might say, a point of origin for the precipitation to begin.

C. E. SIMS.-~~S, ordinarily that is so. I t has to have a surface on which to precipitate; that is, it cannot initiate bubble formation in a liquid or precipitate in a solid except a t a surface.

QUESTION.-Ilie realize that hydrogen can diffuse. T o me diffusion and precipitation are two different things.. Diffusion

indicates that the atoms of hydrogen are steadily marching out of the steel while precipitating indicates they are hitting a block and building pressure up forming a crevice or hair-line crack.

C. A. ZAPFFE.-May 1 answer that, since it is my paper that is referred to?

You are confusing precipitation in the liquid with precipitation in the solid. I11

either case, however, the gas can precipitate only in the manner we have already described.

I n the solid, microsurfaces, or rifts, are available throughout every grain; but in a liquid, obviously, there are no rifts. Therefore the point to be made is that hydrogen-caused porosity cannot be ascribed to hydrogen alone because without another agent there is no place within the liquid in which it can evaporate. You never saw a bubble in a capped bottle of beer, regardless of the gas pressure. Once in solution, a gas is no longer a gas, and has no such property as pressure except a t a break in the matrix, where its escaping tendency has a pressure relationship.

I n other words, the popular conception of the degassing of carbonated beverages is wrong. Most people think that when the pressure in the bottle is released the gas precipitates throughout the body of the liquid because of a supersaturation phenomenon. However, if champagne or beer is decapped very carefully, allowing no cavitation from jarring the bottle, no dust or foreign particles to drop in it, and if the wall of the container is perfectly wet and clean, no bubbles will form. As a matter of fact, I have often accused bars and restaurants of using dirty glasses because the imperfectly wetted surface allows the gas to evaporate there under pressure sufficient to form a bubble, and a nice tantalizing stream of bubbles arises.

iipparently hydrogen comes out of ,steel according to similar principles. The gas cannot precipitate within the metal unless


there is a preexistent surface. Consequently, we may assume that there is first a new surface created, such as from the formation of a new phase. The new phase may be a reaction product, such as a metal oxide or sulphide inclusion, or, more simply, a reaction product of oxygen or sulphur with hydrogen itself. The bubble will then contain acombination of free hydrogen and its reaction product in some approxima& equilibrium ratio depending upon tempera- . ture and the composition of the'steel.

J. A. ROSA.-I should like to make a comment on isothermal annealing of steels. The primary object is to eliminate.flaking and soinetimes to soften the steel, to make i t ductile. We know we can (from a laboratory standpoint) perform beautiful work with isothermal annealing. We can take, for instance, a Krupp analysis or modified S.A.E. 3310 or 3312 in I-in. round bars, and in 6 hr. get a Brinnell hardness of 162 to 163. We know we can do that with I-in. round-bars, but don't let anybody ever tell you that you can do the same thing with a 9 by 9-in. billet.

Isothermal annealing is going rather rapidly from the laboratory to the mill and i t i s going to simplify things considerably if you implicitly believe the S-curves.

We .know from experience that although we can reach complete transformation in 10 hr. in I-in. samples in the laboratory, it may take 2 0 to 30, and more likely40 hr., to get complete transformation in large sections and large loads. The reasons for that, of course, are obvious. First of all are the temperature gradients existing between the surface and the center of the large sections; next we run into structural segregation.

I don't believe anybody in this couniry, or in the world for that matter, is making today' a perfectly homogeneous ingot of any size. There is structural segregation throughout the section; i t is more pronounced, of course, toward thecore, and i t

varies from top to bottom of the ingot. All of these things must be'taken into account and suitable allowance must be made in holding times. The S.A.E. 4340 steel containing a little vanadium has an optimum isotherm of 1225OF. Ten hours is sufficient to give complete transformation with maximum softness. This does not hold true of course in a 9 by 9-in. billet. With that, as I pointed out, temperature gradients, structural segregation and other factors such as temperature control, affect the results.

So if anybody comes to you and shows you an S-curve, let us say for 2340, showing that 3 hr. is sufficient time to completely transform, don't believe it. I n the laboratory that is true, but not in the mill.

THE CHAIRMAN.-Does anyone think , that isothermal transformation can be carried out with a 9 by 9-in. billet?

J. A. ROSA.-I do, because we have done it.

THE CHAIRMAN.-YOU mean you can cool down to a temperature of 11,ooO without any transformation happening in the steel?.

J. A. ROSA.-Oh, no.,

THE CHAIRMAN.-If transformation occurs during cooling, i t is not taking place isothermally. If ferrite precipitation takes place on the way down to II~oOF., the concentration of carbon has changed and the whole S-curve is thro!vn out. T h a t steel is not the same steel. D o you see what I mean? In a hypoeutectoid steel some ferrite forms on the way down to 1i50°. Then. the austenite a t 1150~ is n o t the austenite that appears on the S-curve for that material.

Using very thin specimens, i t is possible to prevent transformation during cooling to a predetermined temperature. Tha t is what the S-curve is built on. You cannot build the S-curve by the other method. You must build i t on true isothermal trans-

1944 OPEN HEARTH ' CONFERENCE 377

formation, which cannot be produced in the whole curve down and somewhat to the large masses. right. 1

J. A. ROSA.-The surprising thing is that THE CHAIRMAN.-IS there any more the S-curves are substantially correct also discussion? If not, I will declare the for large sections. However, cooling rates meeting adjourned. Thank you all for your do have an effect on them and tend to shift attention.

(The session was adjourned al 12.20 noon)

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Flaking in Alloy Steels - library.aimehq.orglibrary.aimehq.org/library/books/Open Hearth 1944/Open Hearth 1944... · of hydrogen in the metal bath during nickel and molybdenum increases