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T H E J O U R N A L O F I A V D C S T R I A L ALVD ESGISEERI;\TG C H E M I S T R Y
Vol. 7, KO.2
MANGANESE STEEL'
of t h e head a n d a t t h e junction of head a n d web. The average difference between these tu-o prints was I 7 per cent for t h e untreated rails, a n d t h e worst rail showed 40 per cent, while only 29 of these 79 rails showed less t h a n I 2 per cent. For t h e treated rails t h e average difference was 3.1 per cent, a n d t h e \Torst one gave I I . j per cent, none of t h e 3 I showing over this amount.
By
JOHN
COUPOSITIOS AND
H
HALL
CHARACTERISTICS
Manganese steel was discovered in t h e early eighties, and since t h a t time has found a place for itself wherever a steel is required t h a t is highly resistant t o wear. I n t h e original experiments, t h e steel was made b y CHEMICAL ANALYSES UNTREATED RAILS TKEATED RAILS mixing molten ferromanganese a n d carbonless Bessemer Low Low HIGH PERCENTAGES HIGH Carbon.. . . . . . . . . . . . . . . 0 . 5 8 0.82 0.63 0.76 blown metal in such amounts as t o give various proManganese.. . . . . . . . . . . . 0 . 7 1 0.78 0.75 0.79 portions of manganese in t h e finished product; as a Phosphorus.. . . . . . . . . . . . 0 . 0 1 6 0.026 0,018 0,023 0.058 0.031 0.040 Sulfur . . . . . . . . . . . . . . . . . . 0 , 0 3 2 consequence t h e ratio of t h e manganese t o t h e carbon 0,131 0.090 0.099 Silicon . . . . . . . . . . . . . . . . . 0.117 UNTREATED RAILS TREATED RAILS was practically fixed, t h e carbon content decreasing HEAD FLANGE HEAD FLAZGE TENSILE TESTS 60,338 a s t h e manganese was diminished. With less t h a n a Elastic l i m i t . . . . . . . . . . . . . . . ., , 56,071 55,961 j9,738 324.857 125,690 Ultimate strength. . . . . . . . . . .. . 118.138 119,385 certain amount of manganese, t h e metal proved t o be 12.8 15.4 13.1 14.4 Elongation, per c e n t . . . . . . . . . . . Reduction of area, per cent.. . . . 14.3 18.5 15.4 18.2 extremely brittle, a n d in practice t h e manganese was UNTREATED RAILS TREATED RAILS seldom less t h a n 9 per cent, t h e steel as generally Head Web Flange Head Web Flange made containing from I O t o 14 per cent manganese 23,923,628 White-Souther . . . . . . . . 16,550,920 Brinell hardness . . _ ., . , 220 248 216 226 235 227 and from I to 1 . j per cent carbon. The silicon is 1.58 1.21 1.43 I m p a c t resistances.. . . . 1.47 1.08 1 . 2 3 Endurance generally from 0 . 2 t o 0.j per cent, t h e sulfur always Landgraf-Turner.. . . 1312 1001 1324 1280 1084 12i0 very lorn (about 0.001 per c e n t ) , t h e phosphorus The impact resistances were measured m-ith a Fremont averages from 0.08 t o 0.1 per cent. Small variations machine, a n d are expressed as kilogram-meters exin t h e silicon a n d phosphorus have little effect on t h e pended i n breaking a test-piece 7 by I O m m . in crossproperties of t h e steel. T h e sulfur is invariably low, section. because t h e manganese of t h e steel eliminates i t b y T h e railroads have been acquiring d a t a for some time in regard t o t h e wear of titanium-treated rails in track, as compared t o t h a t of ordinary Open-Hearth rails. T h e results have been uniformly favorable t o the treated rails, in some cases very decidedly so. For instance, on a sharp curve on t h e Boston Elevated Railway, titanium-treated rails laid alternately with plain rails of practically t h e same composition showed 41 per cent less wear after 214 days' service. I n a test made b y t h e Rock Island Railroad, titanium-treated rails in 1 7 months had, 0.014 sq. in. abraded from their sections, on t h e average, while electric steel rails showed under t h e same conditions a loss of o . o j 8 sq. i n . , a n d ordinary rails 0.07j sq. in. Other instances of t h e same general t y p e might also be given, b u t enough has surely been said t o show clearly t h e advantages flotation as hInS. B y heating this steel t o between following t h e use of titanium in rail steel. 1000 a n d 1100' C., i t can be made, if not of too heavy I n axle steel it has been found similarly advantageous a section, t o consist entirely of t h e metallographic t o use titanium for purifying t h e metal a n d preventing constituent known as austenite. segregation. I n steel castings t h e use of titanium as More recent researches, in some of which t h e author a deoxidizer has usually been successful a n d satishas participated, have shown t h a t if t h e carbon confactory, a n d in soft steel for plates a n d t h i n sheets t e n t of t h e metal is maintained at about I per cent, t h e much titanium is used. This element is preferred t o steel will be austenitic after quenching, even if t h e a n y other deoxidizer because i t does not leave a n y manganese content is as low as j per cent or even a product of its oxidation in t h e steel as do aluminum a n d little less, t h e range of carbon content within which silicon, a n d t h e ingots therefore roll out smoother a n d t h e pure austenite can be obtained being narrow with low finished sheets have a better surface. Small defects manganese content, a n d widening as t h e mangahese on t h e surface of a sheet are very serious in galvanizing, content increases. This can be more readily underso t h a t t h e smoother surface of titanium-treatedsheets, stood by reference t o t h e accompanying diagram due t o cleaner ingots, is much appreciated. showing p a r t of t h e manganese steel series, in which A few photomicrographs a n d sulfur prints are subt h e carbon content is plotted as abscissa and t h e mitted t o illustrate some of t h e points mentioned in manganese content as ordinate. Within t h e area this paper, though of course t h e y are not intended t o TVXYZ t h e steel consists of pure austenite when prove much in themselves, but merely t o serve as quenched. Steels immediately t o t h e left of t h e line illustrations. It is hazardous t o draw conclusions C = r.o;j-o.o4 51n are more or less martensitic from two or three trials or instances, b u t a n average when quenched; steel immediately t o t h e right of t h e from forty or eighty tests should surely be trustworthy. line C = 1.075 0.1/3 M n contain free cementite PHYSICAL TESTING LABORATORY
+
TITANIUM ALLOYMANUFACTURIWG COMPANY NIACARAFALLS, NEW YORIC
Presented before the S e w York Section of t h e Society of Chemical Industry, Chemists' Club, November 20, 1914 1
Feb., 1 9 1 j
T H E JOl*R.V=1L O F I S D 1 7 S T R I d L - 1 S D E S G I S E E R I N G C H E M I S T R Y
when quenched, t h e amount of cementite increasing as t h e carbon content rises. This is also t r u e of t h e prolongations of t h e lines for a certain distance. These lines were plotted as t h e result of a careful research having for its object t h e determination of t h e useful range of manganese content in these steels. T h e line ,4B represents, with no pretense at accuracy, t h e contents of carbon a n d of manganese of t h e steels forming t h e basis of t h e original discovery a n d patent. T h e reason why t h e steel was found t o be martensitic a n d brittle when t h e manganese was still comparatively high, is apparent a t a glance. T h e well-known diagram of Guillet, showing t h e microstructure of t h e steels of t h e iron-carbon-manganese series, is not parallel t o this diagram, as Guillet's figure represents t h e steel in t h e normalized condition, whereas in this figure, t h e structures are those of t h e steel in t h e quenched state. PROPERTIES OF THE STEEL
.
Manganese steel has a v e r y low conductivity f o r both heat a n d electric current, a low melting point (some 1,360' C.), a very high coefficient of expansion (making necessary a shrinkage allowance of "16 inch t o t h e foot i n foundry work), a n d is practically nonmagnetic i n all ordinary conditions. I n t h e cast s t a t e i t is very brittle. T h e cast material after heating t o 1oo0-11oo' C. a n d quenching has only a moderately high tensile strength, about 60,000 t o 80,000 lbs. per sq. in., a rather low elastic limit in both tension a n d compression, a n d a rather high ductility; i t s shearing strength is remarkably high. When rolled or forged a n d treated, t h e tensile strength is increased very greatly, sometimes reaching 150,ooo lbs. per sq. i n . , a n d t h e ductility much improved; t h e other properties are not much altered. T h e rolled material. if unt r e a t e d , is quite brittle. T h e chief characteristic t o which t h e metal owes i t s usefulness is its hardness. I n t h e treated s t a t e in w h i c h ' t h e steel has t o be used, this hardness is of a peculiar kind, inasmuch as hardness tests m-hich depend upon indenting t h e material do not give a high figure for this steel-in fact i t can be made t o peen or flow in t h e cold under t h e blows of a h a m m e r , t o a considerable extent. I n a way, therefore, t h e steel is soft; this is due t o its low elastic limit. B u t , unless specially heat-treated, it can not be cut with tools t o a sufficient extent t o make machining practicable, a n d its resistance t o most kinds of \Tear is extraordinary.
As t h e manganese content is reduced, if t h e carbon is k e p t a t t h e proper figures, t h e properties change much less t h a n was originally supposed. The strength falls off a little, t h e toughness diminishes progressively, a n d t h e magnetism increases a little. T h e resistance t o \Year, however, is very little altered as long as t h e manganese is k e p t above about j or 6 per cent, a n d t h e tendency t o peen or flow decreases as t h e manganese content diminishes. T h e usefulness of these low manganese steels is, however, limited t o a rather narrow field b y their comparative lack of toughness; t h e y are n o t extremely brittle, like t h e martensitic steels of t h e same manganese content, b u t t h e y are
not a t all as tough as t h e steels containing over per cent of manganese.
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CSES
F r o m this brief description of t h e properties of manganese steel, i t will be plain t h a t t h e chief use of t h e metal is t o resist wear. Accordingly, i t is very widely used for t h e wearing parts of stone crushers a n d rolls working on hard rock, for t h e lips a n d teeth of steam shovels a n d ladder dredgcs handling rock or gravel, for centrifugal p u m p cases a n d fliers, handling gritty water, for stone screens, coal cracking rolls, drive chain exposed t o grit. a n d other service of a similar nature. For railroad a n d trolley frogs, switch points, crossings, etc., t h e cast material is very widely used, a n d for rails on sharp curves manganese steel is almost indispensable. These rails were formerly all of cast material, b u t are now very generally rolled from ingots. I t was a t first thought t h a t t h e steel would be very useful for car wheels, b u t its tendency t o flow or peen under heavy cars prevented its being used for railroad work. Under light loads in mine a n d quarry cars, blast furnace charging barrows, etc., i t is widely used. T h e tendency t o flow or peen has also limited t h e usefulness of t h e metal for tires in such grinding mills as t h e Fuller a n d Griffin, a n d for t h e lining plates of t h e various forms of ball mills. I n most cases this tendency has been taken care of in t h e design of t h e casting, a n d t h e steel has long been widely used for lining plates in t h e various types of ball mills used in t h e manufacture of cement. Certain of these plates, however, are in m a n y cases made of other steels, as manganese steel has proved t o peen entirely t o o much t o make its use possible. I n other cases, as for instance certain sorts of grinding mill tires, t h e use of a steel of lower manganese content, which peens less, has proved a successful means of overcoming t h e difficulty; t h e general application of these low manganese steels t o this field is prevented b y their lack of toughness, which makes i t necessary t o exercise very good judgment in applying t h e m . T h e mandrels used for welding iron a n d steel pipe ha\-e for years been made of cast iron, a n d have b u t a very short life. I n t h e smaller sizes, a mandrel seldom makes over three lengths of pipe before i t becomes so cut a n d scored as t o be useless for further service. Ordinary manganese steel proved unsuccessful for these mandrels, as its lack of t h e right sort of hardness resulted in t h e scoring of t h e balls before t h e y h a d made enough pipes t o compensate for their high cost. Special compositions a n d special treatments b y t h e score have been t r i e d ; b y t h e use of additions of chromium, t h e steel can be made harder, a n d t h e t r e a t m e n t can be varied so t h a t t h e hardness is somewhat increased without too great a sacrifice of toughness. T h e high coefficient of expansion of t h e steel, however, makes a certain a m o u n t of toughness necessary, as t h e balls grow hot in use a n d hence expand very considerably, so t h a t if brittle t h e y crack badly. Moreover, for reasons which will be given later, t h e application of heat greatly decreases t h e toughness of manganese steel, so t h a t t h e mandrels grow brittle
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with use. I t must, therefore, be somewhat tough a s first p u t in service. Hence, t h e successful application of manganese steel t o pipe welding mandrels is still a n unsolved problem. MAKCFACTURE O F THE STEEL
A simple calculation shows t h a t t h e simplest a n d cheapest way t o produce manganese steel is t o mix together molten 80 per cent ferromanganese a n d molten soft steel containing from 0.10t o 0.2 j per cent carbon. The carbon content of t h e ferromanganese is such t h a t t h e steel resulting from this mixture is of t h e correct composition for t h e majority of uses. T h e steel therefore is seldom made b y t h e crucible process, on account of t h e difficulty of obtaining low carbon steel b y t h a t method. This difficulty is increa$ed b y t h e f a c t t h a t if t h e scrap is remelted in crucibles, t h e a m o u n t of carbon absorbed from a graphite p o t will be excessive, because high manganese metal has a much greater tendency t o absorb carbon from t h e graphite t h a n metal of low manganese cont e n t . I n order therefore t o remelt t h e scrap, which for reasons of economy is essential in foundry work, i t would be necessary t o use clay pots, which are both troublesome a n d expensive. E v e n if t h e scrap were n o t remelted, i t would be necessary t o melt t h e low carbon steel a n d t h e ferromanganese in separate pots, as t h e absorption of carbon would be excessive if t h e y were melted together i n a graphite pot. Moreover, t h e expense of crucible melting is t o o great t o justify t h e use of t h e process b y t h e makers of manganese steel, except i n very special cases. T h e open hearth furnace can be used t o produce t h e soft steel required, b u t i t h a s i t s drawback. In t h e first place, t h e metal is not generally as hot as Bessemer metal, a n d for t h e majority of manganese steel foundries hot metal is essential. I n t h e second place, most manganese steel castings are comparatively light. H-ence, i t is desirable t h a t t h e metal be received in the foundry, not in large h e a t s a t infrequent intervals, b u t in small heats a t short intervals. I n order t o make t h e open hearth furnace applicable under these conditions, i t is necessary t o use a tilting furnace a n d t a k e t h e metal o u t in small lots; even t h e n t h e metal generally has t o be disposed of in a number of successive ladelfuls, one after t h e other. I n most cases this is n o t as convenient as t o have t w o or three heats a n hour of small size f o r six or seven hours of t h e working d a y . Moreover, t h e small foundries generally find it more convenient n o t t o pour at night, a n d as t h e open hearth furnace m u s t be r u n continuously, it does n o t suit their purposes a s well as t h e small Bessemer converter. T h e small Bessemer plant has a number of a d vantages in t h e manufacture of manganese steel a n d isusedin more shops t h a n i s t h e open hearth furnace. It makes cheap metal, though not quite as cheap as t h e basic open hearth furnace; it provides small lots of metal distributed over t h e d a y , i t does n o t have t o be r u n double t u r n , a n d i t does not cost anything like as much t o s h u t t h e plant down several days in t h e week, as it would t o s h u t down t h e open hearth furnace. Even if we grant t h a t Bessemer metal is
Vol. 7 , NO. z
n o t as good as open hearth because i t is more highly oxidized, we easily see t h a t t h e enormous addition of manganese will deoxidize the metal almost completely, so t h a t this objection is not a t all as valid a one as i n t h e case of carbon steel. Moreover, t h e majority of manganese steel castings d o not require metal a n y better t h a n can be easily made in t h e Bessemer converter. The steel-making problems presented in t h e manufacture of manganese steel are not greatly different f r o m those encountered i n a n y foundry, with t h e exception t h a t t h e scrap produced, which amounts in most small foundries t o some 40 or 5 0 per cent of t h e fluid metal, must for complete economy be remelted. This can be done i n t h e open hearth furnace, although if b u t a single furnace is available a n d t h e chief product is manganese steel, t h e problem becomes a difficult one because metal containing a great deal of manganese produces a slag which c u t s into t h e b o t t o m of t h e furnace severely. In t h e small bottom-blown Bessemer vessel, much of t h e scrap produced can be used u p b y melting i t with t h e pig iron in t h e cupola. T h e resulting high manganese metal can be successfully blown b y taking proper precautions. I n t h e side blown converter i t has so far been practically impossible t o blow high manganese mixtures, so t h a t t h e users of these vessels who make a specialty of manganese steel h a v e been quite generally forced t o sell their scrap at a considerable loss. Within t h e last year or two, t h e a u t h o r has conducted experiments bearing on this problem with some success. If t h e scrap is remelted in t h e open hearth furnace, or remelted in t h e cupola a n d blown in t h e Bessemer converter, all t h e manganese contained in i t is oxidized a n d lost in t h e slag. T h e yearly loss of money represented b y this oxidized manganese a m o u n t s t o a very t i d y s u m a n d m a n y efforts have been made to remelt t h e scrap without loss of t h e manganese. This problem has been attacked in m a n y different ways, with varying success. T h e most obvious solution of i t is t o melt t h e scrap in t h e electric furnace, which will melt i t almost without loss of manganese. This is all very well from a metallurgical point of view, b u t when we come t o figure t h e expense, we find t h a t t h e saving, as compared for instance t o remelting a n d blowing, is not very great. This is especially t h e case in shops already having a Bessemer equipment sufficient for their needs, because if t h e y a d d a n electric furnace t o remelt their scrap, t h e y cut down t h e o u t p u t of t h e Bessemer equipment a n d hence raise t h e cost of t h e steel made in i t . For this reason t h e electric furnace has not so far proved at all as useful in t h e manufacture of manganese steel as i t s inventors hoped i t would. I n order t o make electric steel cheaply, moreover, i t is necessary t o use a furnace of considerable size a n d t o keep i t running continuously. This, as we h a v e said i n discussing t h e open hearth furnace, is a condition n o t well suited t o t h e average manufacturer of manganese steel castings. T h e electric furnace of course produces a very high grade of metal, b u t as we have already said,
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T H E JOURNAL OF INDITSTRIAL A N D ENGINEERING CHEMISTRY
we do not need for manganese steel a n y better metal t h a n can be easily made in t h e Bessemer converter. MELTIXG THE F E R R O X A K G A S E S E
ils t h e t o t a l weight of ferromanganese needed for t h e production of manganese steel is roughly one-fifth of t h e weight of t h e blown metal, i t is absolutely essential t h a t t h e ferromanganese be melted. It would n o t be possible t o a d d i t solid t o t h e blown metal, as t h e latter naturally would n o t be able t o melt i t a n d absorb t h e manganese. F o r m a n y years crucibles have been used for melting t h e ferromanganese, i n , spite of t h e fact t h a t crucible melting is more expensive t h a n a n y other method. T h e expense is further increased by t h e fact t h a t either h a r d coal or oil is commonly used for fuel, t h e gas burning regenerative furnaces never having been used for this purpose because of t h e fact t h a t t h e y must be r u n d a y a n d night. Melting in coal holes is comparatively easy, a n d there is n o danger of t h e absorption of carbon from t h e pot, as t h e ferromanganese is already s a t u r a t e d with t h a t metalloid. T h e pots are seldom covered, b u t this does not result in t h e absorption of sulfur, because i t is almost impossible t o make t h e metal absorb sulfur. T h e great advantage of crucible melting is t h a t t h e melting losses are relatively slight, which is a most i m p o r t a n t point with so expensive a metal. Efforts have been m a d e from t i m e t o time t o melt ferromanganese in t h e cupola furnace. T h e use of this very cheap melting method, however, has never become general because t h e melting losses are very high even when special precautions are t a k e n to reduce t h e m , a n d these precautions necessarily complicate t h e operation of t h e cupola. T h e air furnace a n d even t h e small open hearth furnace have been used t o a certain extent for melting ferromanganese, b u t have n o t been generally a d o p t e d because t h e melting losses are quite high, a n d i n t h e case of t h e open h e a r t h furnace because night t u r n work is not commonly desirable. T h e electric furnace has often been suggested as t h e best method of melting ferromanganese, a n d is being used t o a n increasing extent i n foreign steel works making ordinary steels. It has certain disadvantages for t h e maker of manganese steel in large quantity. T h e first of these is t h a t t h e t o t a l melting capacity of the furnace m u s t be high, a n d this necessitates t h e installation of one or t w o large furnaces. T h e first cost of t h e furnaces is high, a n d t o keep t h e m idle a t night results in a very considerable loss of money. T h e second disadvantage is t h a t quite a large b a t h of metal has t o be melted down a n d drawn off a little at a time, since t h e furnace will n o t melt t h e metal as fast as i t is needed. Hence a considerable expense has t o be incurred for t h e current necessary t o keep t h e molten b a t h h o t . I n m a n y cases, therefore, i t will be f o u n d t h a t it will not p a y t h e maker of m a n ganese steel t o instal a n electric furnace for melting his ferromanganese, unless he keeps t h e furnace busy at night melting manganese steel scrap. This necessitates pouring a t night, which does n o t suit t h e ordinary small f o u n d r y ; a n d in some cases t h e electric furnace
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will be found profitable i n a n existing shop only when t h e existing melting capacity must be increased, as i t would not p a y t o r u n i t if t h e o u t p u t of t h e original reduced’ equipment were 31 0 U L D I N G 11E T H 0 D S
T h e differences between manganese steel foundry practice a n d ordinary steel practice are due t o t h e greater shrinkage of manganese steel a n d i t s low melting point a n d great fluidity. T h e latter properties enable t h e maker of manganese steel t o obtain sound castings in m a n y cases with less waste of steel in sink-heads t h a n would be possible with ordinary steel. Chills are very extensively used t o assist t h e action of t h e sink-heads, a n d t h e heads themselves are often almost completely drained, leaving a mere shell of metal. T h e increased shrinkage of t h e metal necessitates, of course, t h e use of a somewhat larger p a t t e r n t h a n t h a t used for a n ordinary steel casting. ‘This great shrinkage. a n d t h e brittleness of t h e steel in t h e cast condition, moreover, greatly increase t h e tendency of t h e castings t o crack in cooling down in t h e moulds. T h e makers of manganese steel castings, therefore, are obliged t o t a k e more pains with their castings t h a n d o t h e ordinary steel foundrymen. The moulds a n d cores often have t o be broken up while t h e casting is still h o t , a n d t h e castings often have t o be cooled slowly, either b y burying t h e m in s a n d or b y placing t h e m when hot in a hot furnace, where t h e y are allowed t o cool slowly a n d evenly. RIany of t h e castings also have t o be cleaned of sand a n d placed in t h e treating furnace before t h e y have grown really cold, a s t h e y would assuredly crack were this precaution not t a k e n . Finally this great tendency t o crack necessitates t h e utmost care in t h e design of t h e casting, a n d in general t h e manganese steel foundryman has t o avoid great variations in section between different p a r t s of t h e piece t o be cast. Frequently it is necessary t o make very considerable alterations in t h e original design of t h e casting before undertaking t o manufacture i t . HEAT T R E A T M E K T
Manganese steel is heat-treated, as we have said, by heating i t t o a temperature between I O O ~ I I O OC., ~ a n d cooling i t rapidly in cold water. Naturally this very drastic t r e a t m e n t frequently results in t h e cracking of t h e casting, so t h a t for this reason also unevenness of section must be avoided as far as possible. T h e cooling in t h e foundry can be made slow in order t o minimize t h e danger of cracks, b u t in t r e a t m e n t there is no way of avoiding rapid cooling. Moreover, t h e low heat conductivity of t h e metal a n d its high coefficient of expansion make t h e heating u p of t h e castings in t h e treating furnace a delicate m a t t e r . If they are heated u p t o o rapidly, t h e outside of t h e heavy portions a n d t h e whole of t h e lighter p a r t s outstrip t h e interior of t h e heavy sections very greatly, much more so t h a n in t h e case of ordinary steel. T h e great expansion of t h e steel gives rise under these conditions t o very heavy stress, a n d as t h e steel in t h e cast condition is extremely brittle, these stresses are almost certain t o result in t h e cracking of t h e
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casting. Hence heavy a n d complicated castings of this steel have t o be heated u p extremely slowly. ROLLING A N D F O R G I N G
For m a n y years t h e rolling a n d forging of manganese steel was a problem only partly solved, a n d m a n y rolled shapes were made, n o t from a n ingot, b u t f r o m a cast blank which approximated t h e shape of t h e finished product. T o d a y , however, large ingots are successfully rolled into rails a n d other shapes. As a general rule i t is essential t h a t t h e ingot be rolled or hammered at first very lightly all over a n d returned t o t h e heating furnace again before much reduction of size is a t t e m p t e d . T h e ingots t o d a y a r e successfully brought t o rolling temperatures which it was formerly t h o u g h t could n o t be attained without risk of having t h e ingots crumble under t h e hammer or in t h e mill. There is n o t a very wide range of temperature within which t h e steel m a y be readily rolled, since if i t is heated t o o hot i t will crumble a n d if i t is t o o cool i t will burst t h e housings of t h e rolling mill. T h e heat t r e a t m e n t of t h e rolled steel is t h e same as t h a t of t h e cast material. Frequently, however, t h e shape is finished slightly above t h e proper temperature for quenching, a n d is slid i n t o t h e t a n k s of water without reheating. Very t h i n sections, whether cast or rolled, can be made quite tough b y cooling i n t h e air, since t h e toughness is roughly a function of t h e r a t e of cooling. MET A L L 0 G R A P H Y
In t h e cast condition manganese steel consists of a ground-mass of austenite, containing manganiferous cementite in a network, a n d in m a n y needles a n d small lakes within t h e austenite grains. These cementite lakes a n d networks are bounded b y a zone in which t h e austenite is more or less transformed t o troostite or sorbite. There are also places where t h e cementite a n d partly transformed austenite form a eutectic. T h e higher t h e carbon t h e greater t h e q u a n t i t y of this eutectic. On heating t o t h e t r e a t m e n t temperatures, t h e cementite is absorbed i n t h e austenite, a n d is n o t liberated in cooling if t h e cooling is sufficiently rapid. I n a t h i n b a r cooled in t h e air there will be b u t little cementite liberated. I n a heavy section, there will be considerable cementite i n t h e interior portions even after quenching, because t h e interior necessarily cools t o o slowly t o restrain t h e separation of cementite. Hence heavy sections cannot be made as tough as light sections a n d there is a distinct limit t o t h e thickness of manganese steel castings which it is practicable t o manufacture. If t h e steel is cooled slowly from t h e t r e a t m e n t temperature. t h e cementite, as we have said, is liberated in a structure more or less resembling t h a t of t h e cast material. T h e temperature a t which cementite begins t o be liberated, however, is several hundred degrees lower t h a n t h a t at which t h e last traces of i t are absorbed in heating. Hence t h e casting m a y be allowed t o cool considerably i n t h e furnace or in t h e air on its way t o t h e t a n k , a n d yet be quite tough after quenching; b u t if t h e castings are
Vol. 7 , NO. 2
very heavy, manifestly more cementite will be liberated in their interior portions t h a n if t h e y h a d struck t h e water at t h e maximum temperature. Hence t h e larger t h e casting t h e more care must be t a k e n t o prevent i t s cooling off before quenching. If t h e quenched steel be reheated t o a comparatively low temperature, its toughness is almost completely removed. This is largely due t o t h e liberation of cementite f r o m t h e austenite a t a temperature of about j O O o C . , in a very fine netviork, a n d in countless little needles distributed broadcast through t h e austenite. These needles break u p t h e continuity of t h e austenite, which n o d o u b t also partially transforms, SO t h a t t h e steel becomes very brittle. This brittleness, in fact, is greater t h a n t h a t due t o cooling in t h e air from t h e t r e a t m e n t temperature, so t h a t if a bar of t h e toughened steel be reheated a t one e n d t o a white h e a t a n d cooled i n t h e air, i t will generally break when struck with a h a m m e r a t t h e point where i t was a t or slightly below red heat. T h e maker of manganese steel is much troubled by claims for broken castings which have been broken because t h e users heated t h e metal i n order t o bend or work it. Frequently, t h e casting breaks quite a distance from t h e spot where i t mas heated, a n d i t is t h e n difficult t o convince t h e customer t h a t it was t h e heating which caused t h e damage. B u t microscopic examination will prove t h e case beyond t h e shadow of a d o u b t . If heated for 24 hours or more t o a temperature between 500 a n d 600' C., t h e steel becomes very brittle, strongly magnetic a n d very much softer a s tested b y t h e drill t h a n t h e steel i n t h e cast or treated condition, although i t s Brinell hardness figure is raised. This t r e a t m e n t results in t h e transformation of t h e austenite t o sorbite, a n d if t h e heating were long enough t h e sorbite would no d o u b t transform t o pearlite. If t h e steel after this t r e a t m e n t is immersed in liquid air, i t s magnetism is increased. On reheating this magnetic material, a critical point is found a t about 730' C., accompanied b y loss of magnetism. If reheated t o t h e quenching t e m p e r a t u r e a n d quenched t h e metal is austenitic a n d t o u g h ; if reheated t o t h e quenching temperature a n d cooled slowly i t will be brittle a n d will show t h e same microstructure as if it h a d never been made magnetic. T h e controversy which is now raging as t o t h e existence of Beta iron has involved manganese steel, a n d much new d a t a have been published on t h e behavior of manganese steel in heat t r e a t m e n t . I t has long been claimed b y some of those who t h i n k t h e y cannot accept t h e allotropic theory t h a t there a r e features i n t h e behavior of manganese steel which t h a t theory will not explain. I personally have not yet found a feature of t h e metallography of manganese steel which t h e allotropic theory fails t o explain. I n fact, when I first investigated t h e metallography of manganese steel I was able t o save myself a great deal of labor b y assuming t h a t t h e carbon-iron diagram a n d t h e allotropic theory would be a n absolute guide i n m y work, a n d this proved t o be t h e case. 2 RECTORSTREET, N E W
Y O R K CITY
