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On the industrial importance of metallography
April, r9o3.]
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Mining and Metallurgical Section. Abslraet of a pafler presented November eo, I9oe.
On the Industrial Importance of/gtetallography. By ALBERT S&UVEUR.
T h e lecturer, after expressing his belief in t h e i n d u s t r i a l i m p o r t a n c e of m e t a l l o g r a p h y , said t h a t he was fully aware that many who had only a very superficial and fragmentary knowledge of the subject were inclined to question its practical side. He hoped that he would be able in his short talk to present such arguments as would be, if not conclusive, at least suggestive of its commercial application. As he had been p r e v e n t e d from p r e p a r i n g a paper on the subject, he h o p e d his h e a r e r s would e x t e n d to h i m the i n d u l g e n c e of w h i c h he was in need. It was possibly b e s t to a s s u m e that some of the persons p r e s e n t did not even h a v e a rudiment a r y k n o w l e d g e of m e t a l l o g r a p h y , and to shape his r e m a r k s accordingly. T i m e would not p e r m i t to consider o t h e r m e t a l s t h a n iron a n d steel. T h e l e c t u r e r used a n u m b e r of e n l a r g e d photomicro. g r a p h s to illustrate his remarks. H e first called a t t e n t i o n to the s t r u c t u r e of pure gold, which consisted of irregular polyhedrie grains, s t a t i n g t h a t all pure m e t a l s exhibited a s t r u c t u r e of this character, the size of the grains, however, v a r y i n g w i t h different m e t a l s and also w i t h t h e i r t r e a t m e n t , especially w i t h t h e t e m p e r a t u r e to which t h e y were h e a t e d and w i t h the rate of cooling. P a s s i n g to the s t r u c t u r e of pure or carbonless iron, it was s h o w n t h a t this m e t a l also was m a d e up of irregular polyhedric crystalline grains, its s t r u c t u r e b e i n g v e r y similar to t h a t of gold or to t h a t of a n y o t h e r pure m e t a l It was noticed, however, t h a t some of the grains were dark while others were bright, and this was s h o w n to be due to the fact t h a t each g r a i n is m a d e up of a g r e a t m a n y small cubic crystals which have the s a m e o r i e n t a t i o n in the same grain VOL. CLV. No. 928. 18
274
S a u v e u r ."
[J. F. I.,
b u t are differently oriented in different grains. Those grains which h a v e their small cubic crystals so oriented that t h e y will reflect the light in the t u b e of the microscope will a p p e a r bright, while if the light reflected b y the small crystals fails to e n t e r the t u b e of the microscope, the corresp o n d i n g grains will appear dark. It was seen, then, t h a t iron and other metals were c o m p o s e d of crystalline g r a i n s , each grain being m a d e up of a g r e a t n u m b e r of small true crystals, f r e q u e n t l y cubic. P a s s i n g to the m i e r o s t r u c t u r e of w r o u g h t iron, it was s h o w n that it also consisted of a mass of crystalline grains, b u t that the s t r u c t u r e included beside n u m e r o u s p a r t i c l e s of slag. In a section cut in the direction of the rolling or f o r g i n g of the iron bar, these slag particles ran parallel to that direction i m p a r t i n g a fibrous appearance to the metal. In a cross-section, h o w e v e r , the slag occurred in the shape of irregular small areas c o r r e s p o n d i n g to t h e cross-sections of the fibers e x h i b i t e d in the previous diagram. It w a s t h o u g h t for m a n y years t h a t w r o u g h t iron actually had a fibrous s t r u c t u r e and, indeed, the n u m b e r of persons still holding this view w a s surprisingly large. Many valuable properties were a t t r i b u t e d to p u d d l e d iron on account of its "fibrous s t r u c t u r e , " which were denied to steel b e c a u s e of its " c r y s t a l l i n e structure." T h e microscope h a d s u m m a r i l y disposed of this erroneous belief in s h o w i n g t h a t the carbonless iron c o n s t i t u t i n g the bulk of w r o u g h t iron was j u s t as crystalline as the carbonless iron c o n s t i t u t i n g the bulk o f soft steel. Carbonless iron considered as a microscopical c o n s t i t u e n t had been called f e r r i t e . In i n t r o d u c i n g a small a m o u n t of carbon in the iron, say o'Io per cent., and in p r o d u c i n g it molten so as to obtain it free from slag; in o t h e r words, i n c o n v e r t i n g it into soft steel, the s t r u c t u r e u n d e r w e n t an: i m p o r t a n t modification : the bulk of it was still m a d e up of ferrite b u t it contained besides a n u m b e r of small dark areas which necessarily m u s t contain all of the carbon present in the metal. T h e s e dark areas, however, do not consist of pure carbide of iron, for upon h i g h e r magnification the c o n s t i t u e n t is resolved into two c o m p o n e n t s : it is m a d e
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up of parallel plates a l t e r n a t e l y dark and light. T h e s e plates consist of ferrite and of the carbide, Fe3C. T h i s carbide had been eatled cementite, while this new c o n s t i t u e n t , consisting of a m i x t u r e of ferrite and cementite, was called pearlite, because of its pearly appearance. A n u m b e r of p h o t o g r a p h i c e n l a r g e m e n t s were exhibited i l l u s t r a t i n g the s t r u c t u r e of pearlite. Upon i n c r e a s i n g the a m o u n t of carbon in the steel, the a m o u n t of pearlite increased and t h a t of ferrite decreased, correspondingly. W h e n the iron contained about 0"8 per cent. of carbon, the ferrite h a d entirely disappeared, the metal consisting then 6ntirely of pearlite. Upon a f u r t h e r increase of carbon, the steel still consisted of a mass of pearlite, b u t particles of pure c e m e n t i t e were now p r e s e n t which increased in proportion w i t h the carbon content, while the pearlite decreased correspondingly. T h e rationale of the s t r u c t u r e of steel could e v i d e n t l y be explained as follows: All the carbon p r e s e n t in steel e n t e r e d in c o m b i n a t i o n with some of the iron to form the carbide Fe~C which contains 6'6 7 per cent. of carbon. T h i s carbide or c e m e n t i t e formed a m e c h a n i c a l m i x t u r e with some of the iron, in the proportion, roughly, of I part of c e m e n t i t e to 7 parts of iron (ferrite), a s s u m i n g t h e shape of pearlite. If an excess of iron was left over, it was present in the s t r u c t u r e as ferrite, and this would be the case with low carbon steel : if, on the contrary, an excess of the carbide was p r e s e n t it was f o u n d in the s t r u c t u r e as free particles of cementite. P a s s i n g to the m i c r o s t r u c t u r e of cast iron, the l e c t u r e r showed t h a t perfectly g r a y iron was composed of a mass of ferrite t h r o u g h o u t w h i c h was d i s t r i b u t e d n u m e r o u s particles of graphite, while w h i t e cast iron had the s t r u c t u r e of a very h i g h carbon steel, b e i n g m a d e up of pearlite and a large a m o u n t of cementite. It was, indeed, not possible to draw a sharp line of d e m a r c a t i o n b e t w e e n h i g h carbon steel and w h i t e cast iron. T h e s t r u c t u r a l c h a n g e s occurring in the s t r u c t u r e as the carbon increased was a very g r a d u a l one, from the softest steel to the h a r d e s t grades and finally to w h i t e cast iron. It was t h e n s h o w n t h a t when g r a y cast iron contains a small a m o u n t of combined carbon, the m a t r i x
276
5auveur :
[J. F. I..
of t h e metal contains a small proportion of pearlite in o t h e r words, it consisted of low carbon steel; as the carbon i n c r e a s e d in passing g r a d u a l l y from the perfectly g r a y to t h e w h i t e variety, the m a t r i x or metallic p a r t of the m e t a l u n d e r w e n t exactly the same c h a n g e s which h a d been shown to o c c u r in steel under similar conditions. Cast i r o n cons i s t e d of a mass of steel plus a certain a m o u n t of g r a p h i t i c carbon. T h e lecturer took exception to the oft-repeated s t a t e m e n t of some writers referring to the g r e a t c o m p l e x i t y of cast iron as compared to steel, and expressed his belief t h a t if cast iron was viewed in its true light, it would g r e a t l y help in solving m a n y puzzling problems. T h e close relations between the s t r u c t u r e of steel and its t r e a t m e n t on the one hand, and b e t w e e n the s t r u c t u r e and t h e properties on the other, were t h e n dealt w i t h s o m e w h a t at l e n g t h , the lecturer i l l u s t r a t i n g his r e m a r k s b y photog r a p h s of the same steel which h a d been s u b j e c t e d to a n u m b e r of different t r e a t m e n t s . T h e composition of the m e t a l h a d not been changed, and if given to the c h e m i s t t h e l a t t e r would have h a d to report t h a t t h e various samples, f r o m a chemical standpoint, h a d the same properties. T h e i r properties varied greatly, however, and were closely related to t h e appearance of the structure. T h i s d e m o n s t r a t e d the l i m i t a t i o n s of c h e m i s t r y and the assistance w h i c h m a y be e x p e c t e d from the microscopical e x a m i n a t i o n s in all quest i o n s r e l a t i n g to the t r e a t m e n t to which m e t a l s are s u b j e c t e d ; and it was of as g r e a t i m p o r t a n c e to i m p a r t the r i g h t s t r u c t u r e to a metal as to secure for it a desirable composition. T h e h a r m r e s u l t i n g from a defective s t r u c t u r e was as g r e a t as t h a t r e s u l t i n g from a defective composition. C o n c l u d i n g his remarks the l e c t u r e r stated, t h a t if a n y one, in t a k i n g up metallography, expected t h a t he would be able to solve all his daily troubles, and to solve t h e m at once, he would be d i s a p p o i n t e d ; b u t t h a t if the s t u d y was t a k e n up in a more reasonable f r a m e of m i n d and conducted intelligently, it could h a r d l y fail to prove of very great assistance. T h e lecture was f u r t h e r illustrated by the exhibition of some stereopticon slides. T h e necessary a p p a r a t u s to carry on m e t a l l o g r a p h i e a l work was exhibited, the l e c t u r e r b e i n g
April, 19o3.] industrial Importance o/" MetallograpAy.
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greatly i n d e b t e d to t h e A j a x Metal C o m p a n y and to the Cramp S h i p b u i l d i n g C o m p a n y for the loan of apparatus. DISCUSSION.
MR, P. KREUZPOINTNER.--Mr. C h a i r m a n and Gentlemen: It is a g r e a t p l e a s u r e for me to corroborate all that P r o f e s s o r S a u v e u r has said a b o u t the use of the microscope in examining metals, so far as its practical v a l u e is concerned to the expert, in a v a r i e t y of w a y s w h e r e the chemical analysis and physical test fail to give us a satisfactory explanation of certain p h e n o m e n a . I h a v e used the microscope for the p a s t e i g h t e e n years, and I consider its use as v a l u a b l e and i n d i s p e n s a b l e an addition to a physical l a b o r a t o r y as the plane is to t h e carpenter, or the m i c r o m e t e r is to the machinist. I do not share the o p t i m i s t i c view, t h o u g h , of some, t h a t in the near future we m a y be able to use the microscope as a s u b s t i t u t e for the chemical analysis or the physical test. T h e r e is such an endless v a r i e t y of conditions that affect t h e structure of, say, a piece of steel, w h i c h m i g h t lead t h e microscopist to j u d g e that, w i t h the c h a n g e of structure, a c h a n g e in the physical quality, or ability of a given steel to do a g i v e n piece of work, has taken place. S u c h an assumption, however, does not hold good, as t w o pieces of steel of different s t r u c t u r e m a y and do give the s a m e tensile strength, and the one w h i c h is inferior often gives a b e t t e r elongation even, d u e to g r e a t e r stretch at the p o i n t of r u p t u r e and l e s s ' s t r e t c h along the section. Thus, in m y opinion, and j u d g i n g from experience, it will b e some time y e t b e f o r e we can tell the w o r k i n g qualities of steel w i t h o u t o t h e r m e a n s than the m i q r o s c o p e . . M o r e o v e r , even then, w h e n e v e r we should reach t h a t desirable p o i n t of perfection and sire. plicity in m e t h o d of d e t e r m i n i n g the qualities of metals, for a long time to come there w o u l d be b u t a few experts w h o w o u l d be able to do so w i t h reliability, since the determination of the q u a l i t y of iron and steel for their physical ability to resist t h e forces of destruction, requires long experience and the o p p o r t u n i t y of e v e r l a s t i n g c o m p a r i s o n of metals of all kinds and of all makes, n e w and old, m a d e b y t h e s a m e
278
Kreuapointner :
J-J. F. I.,
m a n u f a c t u r e r at different times, or different m a n u f a c t u r e r s at the same time F o r these reasons t h e microscope, v a l u a b l e and i n d i s p e n s a b l e as it is, as an a u x i l i a r y to the physicist, will not become a s u b s t i t u t e for the tensile test for a long time to come, a l t h o u g h the t e n s i l e t e s t is n o t by far so reliable as m a n y engineers a s s u m e it to be. T h e best and m o s t reliable service the microscope has done t h u s far, and is destined to do still more so in the future, is in the d e t e r m i n a t i o n of the h e a t - t r e a t m e n t of steel and the compo~,~,~:*'~- of ~.~oy~ . . . . . . . WhiCh h a v e not yet u n d e r g o n e m e c h a n i c a l treatment. W i t h the h a m m e r i n g or rolling of metals a c o m p l e x i t y of conditions arise, to d e t e r m i n e w h i c h requires a g r e a t e r v a r i e t y of means and personal practical experience. Conc e r n i n g the question, s u b s e q u e n t l y raised, w h e t h e r the appearance of the coarse f r a c t u r e of a piece of steel, or r a t h e r metal, t h a t is of the fracture as it originates by n i c k i n g and s u b s e q u e n t breaking, or b r e a k i n g in service, bears a n y relation to the m i c r o g r a i n or m i c r o s t r u c t u r e u n d e r n e a t h the fracture, I do not believe there is a n y relation b e t w e e n t h e respective layers or planes of the m e t a l sufficiently well defined t h a t we could draw conclusions from the appearance of the fracture to the m i c r o s t r u c t u r e below, or the succeedi n g plane of the metal. In order to allow us to draw conclusions from the fracture to the m i e r o s t r u c t u r e u n d e r n e a t h , we m u s t a s s u m e t h a t the distortion of the particles or crystals of the metal, caused by the rupture, would c o n t i n u e deep into the metal, so t h a t succeeding transverse planes of the metal e x h i b i t the same a m o u n t and n a t u r e of distortion, which then will be the micrograin in question. If we a s s u m e t h a t no distortion of the particles takes place d u r i n g rupture, which, however, is inconceivable, since in t h a t case we would h a v e to a s s u m e t h a t there was no resistance offered by the m e t a l to the force of r u p t u r e , then still the appearance of the rup. t u r e d surface would be so m u c h different to t h e n a k e d eye t h a n the micrograin u n d e r n e a t h , t h a t we are not w a r r a n t e d to draw a n y conclusions more t h a n to say t h a t the metal u n d e r n e a t h is fine or coarse-grained.
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F r o m w h a t experience I h a v e in r e g a r d to this question, I would say t h a t t h e distortion, caused by r u p t u r e , does not go b e y o n d t h a t a m o u n t of m e t a l w h i c h we h a v e to file and grind off in order to get to the plane of m e t a l below the r u p t u r e plane, and, therefore, w h a t we see in t h a t plane w i t h the microscope is not w h a t we see in t h e g r a i n of the r u p t u r e d surface of the metal. T h e u n r e l i a b i l i t y of conclusions d r a w n would, of course, be the g r e a t e r t h e f a r t h e r we go a w a y from t h e point of rupture, because of the v a r i a t i o n s of m i c r o s t r u c t u r e we are apt to find in a n y piece of commercial m e t a l in its various portions or sections. MR. W. R. WEBSTER i n q u i r e d w h e t h e r the appearance of the f r a c t u r e of a piece of steel, as seen by the eye, corresponds w i t h t h a t u n d e r t h e microscope ? MR. ROBERT JOB : - - I n a n s w e r to Mr. W e b s t e r ' s q u e s t i o n w h e t h e r the appearance of the fractu*e of a piece of steel, as seen by the eye, corresponds with t h a t u n d e r the microscope, I am satisfied t h a t in the large m a j o r i t y of cases it does d o so. In other words, w h e n the f r a c t u r e appears to the eye coarse-grained, the steel has a coarse microstructure. It has been our experience t h a t the converse is not i n v a r i a b l y true. A case in point came u n d e r our observation several years ago in m a k i n g drop-tests upon a lot of rail-butts. All h a d been rolled u n d e r the same conditions t h r o u g h o u t , yet, w h e n f r a c t u r e d u n d e r the drop, some s h o w e d a coarse g r a n u l a r f r a c t u r e while others appeared to be relatively fine. Sections were r e m o v e d from some of the latter, a n d t h e y were f o u n d to h a v e the coarse, open, micros t r u c t u r e characteristic of t h a t rolling. Therefore, I feel t h a t d e d u c t i o n s from an a p p a r e n t l y fine-grained f r a c t u r e m a y be t o t a l l y at variance from the actual structure, and t h a t the fine g r a n u l a r form m a y be m e r e l y i n c i d e n t to some peculiarity in the conditions of fracture. W h e n the f r a c t u r e d surface is filed s m o o t h and etched, the g r a n u l a r appearance, as viewed b y the u n a i d e d eye, is relatively the same as w h e n seen u n d e r the microscope, and a g e n e r a l e s t i m a t e m a y often be m a d e w i t h o u t the aid of the glass.
280
Clamer :
[J. F. I.,
MR. G. H. CLAMER: - - T h e structure of alloys presents m a n y of the characteristics exhibited by cast iron and steel. In fact, we may regard these as,alloys of iron with varying amounts of carbon, silicon, manganese, etc. One of the most simple examples by way of comparison is the lead and antimony alloy, which has by experiment and microscopic examination been shown to consist of dendrites of pure lead embodied in a eutectic of lead and antimony, if the content of antimony be below 13 per cent.--the saturationpoint of these two metals. As in steel, free ferrite is contained in diminishing proportions from pure carbonless iron up to "89 per cent. carbon. Just as from pure lead to the alloy of I3 per cent. antimony, or thereabouts, is the free lead present in diminishing proportions. A t 13 per cent., so-called eutectic alloy, lead is saturated with antimony, antimony is saturated with lead. No free lead is present, no free antimony. As the proportion passes the eutectic composition, free crystals of antimony make their appearance. Here again we see the analogy to steel, which segregates above the saturation-point of' 89 per cent. carbon-free eementite. Another interesting example is the alloy of copper and tin, which has, perhaps, a little more complex composition. Alloys rich in copper (above 73 per cent.) consist of dendrites of pure copper euteetic and chemically combined copper and tin, generally conceded to be of the formula SnCua. The above alloy, 27 per cent. tin and 73 per cent. copper, is the first eutectic, the high tin-content alloy formi n g another. The formation of these eutectics exhibiting themselves constantly in the everyday manipulation of alloys has a very practical bearing on foundry practice. I once met with a very peculiar demonstration of the presence of copper and tin eutectic in casting phosphor. bronze, a composition which contains 79 per cent. copper, io per cent. tin, io per cent. lead and i per cent. phos. phorus. This metal was cast into connecting-rod bushings for locomotive construction. The castings showed on the outside a thin skin of metal about the thickness of paper, w h i c h could readily be detached from the casting, showing
April, x9o3.]
Notes and Comments.
28I
it had been forced to the surface t h r o u g h Pores after the b o d y of the casting h a d solidified. T h e composition of this was a b o u t as f o l l o w s - - I cannot locate the exact figures : Copper . . . Lead . . . . Tin . . . . . Phosphorus .
. . . .
. . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . .
. . . .
. . . .
. . . .
. . . . . . . . . . . . . . . . ....
70 per 3 22 3
cent. " " "
thus s h o w i n g p e r h a p s a q u a d r u p l e eutectic, or, at a n y rate, a tendency t o w a r d the copper-tin ev.tectic--27 per cent. tin and 73 per cent. copper. A n o t h e r curious t h i n g in connection with this w a s the presence of 3 per cent. phosphorus, s h o w i n g the fact discovered b y Charpy, that p h o s p h o r u s is localized in the eutectie. A n o t h e r curious p h e n o m e n o n c o m i n g daily u n d e r m y observation is the f o r m a t i o n of euteetic on top of compositions, such as the above, in e a s t i n g ingots. A n alloy of metal, w h e n apparently solidified, will often have a p p e a r on its surface w h a t we term eutectic sweats, b e c a u s e it s w e a t s or oozes o u t in thin s t r e a m s a p p a r e n t l y t h r o u g h pores, and forms globules on the surface. T h e s e g l o b u l e s are hard and brittle, and entirely dissimilar to the m a i n portion of the ingot, the analysis always s h o w i n g a h i g h tin alloy, a l t h o u g h the original m a y contain b u t 5 to io per cent. I can heartily endorse this a p p a r e n t l y n e w m e t h o d of investigation, for which P r o f e s s o r S a u v e u r has done so much in this country, and can s a y t h a t its field of usefulness is not confined solely to the province of iron and steel, b u t can be e x t e n d e d also in a praetical w a y to the m a n u f a c t u r e of metallic alloys.
P L A T I N U M E X T R A C T I O N IN T H E U R A L REGION. Professor Demaret-Freson has lately published some interesting facts in regard to the production of platinum. W i t h i n the last few years t h e applieations of this metal, especially in t h e eleetrieal industries, have rapidly increased, while the production remains about stationary, and it is estimated that the entire world's production is not over 7 t o n s annually. It is owing to this cause t h a t there has been a considerable rise in t h e price of platinum.
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Mining and Metallurgical Section. Abslraet of a pafler presented November eo, I9oe.
On the Industrial Importance of/gtetallography. By ALBERT S&UVEUR.
T h e lecturer, after expressing his belief in t h e i n d u s t r i a l i m p o r t a n c e of m e t a l l o g r a p h y , said t h a t he was fully aware that many who had only a very superficial and fragmentary knowledge of the subject were inclined to question its practical side. He hoped that he would be able in his short talk to present such arguments as would be, if not conclusive, at least suggestive of its commercial application. As he had been p r e v e n t e d from p r e p a r i n g a paper on the subject, he h o p e d his h e a r e r s would e x t e n d to h i m the i n d u l g e n c e of w h i c h he was in need. It was possibly b e s t to a s s u m e that some of the persons p r e s e n t did not even h a v e a rudiment a r y k n o w l e d g e of m e t a l l o g r a p h y , and to shape his r e m a r k s accordingly. T i m e would not p e r m i t to consider o t h e r m e t a l s t h a n iron a n d steel. T h e l e c t u r e r used a n u m b e r of e n l a r g e d photomicro. g r a p h s to illustrate his remarks. H e first called a t t e n t i o n to the s t r u c t u r e of pure gold, which consisted of irregular polyhedrie grains, s t a t i n g t h a t all pure m e t a l s exhibited a s t r u c t u r e of this character, the size of the grains, however, v a r y i n g w i t h different m e t a l s and also w i t h t h e i r t r e a t m e n t , especially w i t h t h e t e m p e r a t u r e to which t h e y were h e a t e d and w i t h the rate of cooling. P a s s i n g to the s t r u c t u r e of pure or carbonless iron, it was s h o w n t h a t this m e t a l also was m a d e up of irregular polyhedric crystalline grains, its s t r u c t u r e b e i n g v e r y similar to t h a t of gold or to t h a t of a n y o t h e r pure m e t a l It was noticed, however, t h a t some of the grains were dark while others were bright, and this was s h o w n to be due to the fact t h a t each g r a i n is m a d e up of a g r e a t m a n y small cubic crystals which have the s a m e o r i e n t a t i o n in the same grain VOL. CLV. No. 928. 18
274
S a u v e u r ."
[J. F. I.,
b u t are differently oriented in different grains. Those grains which h a v e their small cubic crystals so oriented that t h e y will reflect the light in the t u b e of the microscope will a p p e a r bright, while if the light reflected b y the small crystals fails to e n t e r the t u b e of the microscope, the corresp o n d i n g grains will appear dark. It was seen, then, t h a t iron and other metals were c o m p o s e d of crystalline g r a i n s , each grain being m a d e up of a g r e a t n u m b e r of small true crystals, f r e q u e n t l y cubic. P a s s i n g to the m i e r o s t r u c t u r e of w r o u g h t iron, it was s h o w n that it also consisted of a mass of crystalline grains, b u t that the s t r u c t u r e included beside n u m e r o u s p a r t i c l e s of slag. In a section cut in the direction of the rolling or f o r g i n g of the iron bar, these slag particles ran parallel to that direction i m p a r t i n g a fibrous appearance to the metal. In a cross-section, h o w e v e r , the slag occurred in the shape of irregular small areas c o r r e s p o n d i n g to t h e cross-sections of the fibers e x h i b i t e d in the previous diagram. It w a s t h o u g h t for m a n y years t h a t w r o u g h t iron actually had a fibrous s t r u c t u r e and, indeed, the n u m b e r of persons still holding this view w a s surprisingly large. Many valuable properties were a t t r i b u t e d to p u d d l e d iron on account of its "fibrous s t r u c t u r e , " which were denied to steel b e c a u s e of its " c r y s t a l l i n e structure." T h e microscope h a d s u m m a r i l y disposed of this erroneous belief in s h o w i n g t h a t the carbonless iron c o n s t i t u t i n g the bulk of w r o u g h t iron was j u s t as crystalline as the carbonless iron c o n s t i t u t i n g the bulk o f soft steel. Carbonless iron considered as a microscopical c o n s t i t u e n t had been called f e r r i t e . In i n t r o d u c i n g a small a m o u n t of carbon in the iron, say o'Io per cent., and in p r o d u c i n g it molten so as to obtain it free from slag; in o t h e r words, i n c o n v e r t i n g it into soft steel, the s t r u c t u r e u n d e r w e n t an: i m p o r t a n t modification : the bulk of it was still m a d e up of ferrite b u t it contained besides a n u m b e r of small dark areas which necessarily m u s t contain all of the carbon present in the metal. T h e s e dark areas, however, do not consist of pure carbide of iron, for upon h i g h e r magnification the c o n s t i t u e n t is resolved into two c o m p o n e n t s : it is m a d e
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up of parallel plates a l t e r n a t e l y dark and light. T h e s e plates consist of ferrite and of the carbide, Fe3C. T h i s carbide had been eatled cementite, while this new c o n s t i t u e n t , consisting of a m i x t u r e of ferrite and cementite, was called pearlite, because of its pearly appearance. A n u m b e r of p h o t o g r a p h i c e n l a r g e m e n t s were exhibited i l l u s t r a t i n g the s t r u c t u r e of pearlite. Upon i n c r e a s i n g the a m o u n t of carbon in the steel, the a m o u n t of pearlite increased and t h a t of ferrite decreased, correspondingly. W h e n the iron contained about 0"8 per cent. of carbon, the ferrite h a d entirely disappeared, the metal consisting then 6ntirely of pearlite. Upon a f u r t h e r increase of carbon, the steel still consisted of a mass of pearlite, b u t particles of pure c e m e n t i t e were now p r e s e n t which increased in proportion w i t h the carbon content, while the pearlite decreased correspondingly. T h e rationale of the s t r u c t u r e of steel could e v i d e n t l y be explained as follows: All the carbon p r e s e n t in steel e n t e r e d in c o m b i n a t i o n with some of the iron to form the carbide Fe~C which contains 6'6 7 per cent. of carbon. T h i s carbide or c e m e n t i t e formed a m e c h a n i c a l m i x t u r e with some of the iron, in the proportion, roughly, of I part of c e m e n t i t e to 7 parts of iron (ferrite), a s s u m i n g t h e shape of pearlite. If an excess of iron was left over, it was present in the s t r u c t u r e as ferrite, and this would be the case with low carbon steel : if, on the contrary, an excess of the carbide was p r e s e n t it was f o u n d in the s t r u c t u r e as free particles of cementite. P a s s i n g to the m i c r o s t r u c t u r e of cast iron, the l e c t u r e r showed t h a t perfectly g r a y iron was composed of a mass of ferrite t h r o u g h o u t w h i c h was d i s t r i b u t e d n u m e r o u s particles of graphite, while w h i t e cast iron had the s t r u c t u r e of a very h i g h carbon steel, b e i n g m a d e up of pearlite and a large a m o u n t of cementite. It was, indeed, not possible to draw a sharp line of d e m a r c a t i o n b e t w e e n h i g h carbon steel and w h i t e cast iron. T h e s t r u c t u r a l c h a n g e s occurring in the s t r u c t u r e as the carbon increased was a very g r a d u a l one, from the softest steel to the h a r d e s t grades and finally to w h i t e cast iron. It was t h e n s h o w n t h a t when g r a y cast iron contains a small a m o u n t of combined carbon, the m a t r i x
276
5auveur :
[J. F. I..
of t h e metal contains a small proportion of pearlite in o t h e r words, it consisted of low carbon steel; as the carbon i n c r e a s e d in passing g r a d u a l l y from the perfectly g r a y to t h e w h i t e variety, the m a t r i x or metallic p a r t of the m e t a l u n d e r w e n t exactly the same c h a n g e s which h a d been shown to o c c u r in steel under similar conditions. Cast i r o n cons i s t e d of a mass of steel plus a certain a m o u n t of g r a p h i t i c carbon. T h e lecturer took exception to the oft-repeated s t a t e m e n t of some writers referring to the g r e a t c o m p l e x i t y of cast iron as compared to steel, and expressed his belief t h a t if cast iron was viewed in its true light, it would g r e a t l y help in solving m a n y puzzling problems. T h e close relations between the s t r u c t u r e of steel and its t r e a t m e n t on the one hand, and b e t w e e n the s t r u c t u r e and t h e properties on the other, were t h e n dealt w i t h s o m e w h a t at l e n g t h , the lecturer i l l u s t r a t i n g his r e m a r k s b y photog r a p h s of the same steel which h a d been s u b j e c t e d to a n u m b e r of different t r e a t m e n t s . T h e composition of the m e t a l h a d not been changed, and if given to the c h e m i s t t h e l a t t e r would have h a d to report t h a t t h e various samples, f r o m a chemical standpoint, h a d the same properties. T h e i r properties varied greatly, however, and were closely related to t h e appearance of the structure. T h i s d e m o n s t r a t e d the l i m i t a t i o n s of c h e m i s t r y and the assistance w h i c h m a y be e x p e c t e d from the microscopical e x a m i n a t i o n s in all quest i o n s r e l a t i n g to the t r e a t m e n t to which m e t a l s are s u b j e c t e d ; and it was of as g r e a t i m p o r t a n c e to i m p a r t the r i g h t s t r u c t u r e to a metal as to secure for it a desirable composition. T h e h a r m r e s u l t i n g from a defective s t r u c t u r e was as g r e a t as t h a t r e s u l t i n g from a defective composition. C o n c l u d i n g his remarks the l e c t u r e r stated, t h a t if a n y one, in t a k i n g up metallography, expected t h a t he would be able to solve all his daily troubles, and to solve t h e m at once, he would be d i s a p p o i n t e d ; b u t t h a t if the s t u d y was t a k e n up in a more reasonable f r a m e of m i n d and conducted intelligently, it could h a r d l y fail to prove of very great assistance. T h e lecture was f u r t h e r illustrated by the exhibition of some stereopticon slides. T h e necessary a p p a r a t u s to carry on m e t a l l o g r a p h i e a l work was exhibited, the l e c t u r e r b e i n g
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greatly i n d e b t e d to t h e A j a x Metal C o m p a n y and to the Cramp S h i p b u i l d i n g C o m p a n y for the loan of apparatus. DISCUSSION.
MR, P. KREUZPOINTNER.--Mr. C h a i r m a n and Gentlemen: It is a g r e a t p l e a s u r e for me to corroborate all that P r o f e s s o r S a u v e u r has said a b o u t the use of the microscope in examining metals, so far as its practical v a l u e is concerned to the expert, in a v a r i e t y of w a y s w h e r e the chemical analysis and physical test fail to give us a satisfactory explanation of certain p h e n o m e n a . I h a v e used the microscope for the p a s t e i g h t e e n years, and I consider its use as v a l u a b l e and i n d i s p e n s a b l e an addition to a physical l a b o r a t o r y as the plane is to t h e carpenter, or the m i c r o m e t e r is to the machinist. I do not share the o p t i m i s t i c view, t h o u g h , of some, t h a t in the near future we m a y be able to use the microscope as a s u b s t i t u t e for the chemical analysis or the physical test. T h e r e is such an endless v a r i e t y of conditions that affect t h e structure of, say, a piece of steel, w h i c h m i g h t lead t h e microscopist to j u d g e that, w i t h the c h a n g e of structure, a c h a n g e in the physical quality, or ability of a given steel to do a g i v e n piece of work, has taken place. S u c h an assumption, however, does not hold good, as t w o pieces of steel of different s t r u c t u r e m a y and do give the s a m e tensile strength, and the one w h i c h is inferior often gives a b e t t e r elongation even, d u e to g r e a t e r stretch at the p o i n t of r u p t u r e and l e s s ' s t r e t c h along the section. Thus, in m y opinion, and j u d g i n g from experience, it will b e some time y e t b e f o r e we can tell the w o r k i n g qualities of steel w i t h o u t o t h e r m e a n s than the m i q r o s c o p e . . M o r e o v e r , even then, w h e n e v e r we should reach t h a t desirable p o i n t of perfection and sire. plicity in m e t h o d of d e t e r m i n i n g the qualities of metals, for a long time to come there w o u l d be b u t a few experts w h o w o u l d be able to do so w i t h reliability, since the determination of the q u a l i t y of iron and steel for their physical ability to resist t h e forces of destruction, requires long experience and the o p p o r t u n i t y of e v e r l a s t i n g c o m p a r i s o n of metals of all kinds and of all makes, n e w and old, m a d e b y t h e s a m e
278
Kreuapointner :
J-J. F. I.,
m a n u f a c t u r e r at different times, or different m a n u f a c t u r e r s at the same time F o r these reasons t h e microscope, v a l u a b l e and i n d i s p e n s a b l e as it is, as an a u x i l i a r y to the physicist, will not become a s u b s t i t u t e for the tensile test for a long time to come, a l t h o u g h the t e n s i l e t e s t is n o t by far so reliable as m a n y engineers a s s u m e it to be. T h e best and m o s t reliable service the microscope has done t h u s far, and is destined to do still more so in the future, is in the d e t e r m i n a t i o n of the h e a t - t r e a t m e n t of steel and the compo~,~,~:*'~- of ~.~oy~ . . . . . . . WhiCh h a v e not yet u n d e r g o n e m e c h a n i c a l treatment. W i t h the h a m m e r i n g or rolling of metals a c o m p l e x i t y of conditions arise, to d e t e r m i n e w h i c h requires a g r e a t e r v a r i e t y of means and personal practical experience. Conc e r n i n g the question, s u b s e q u e n t l y raised, w h e t h e r the appearance of the coarse f r a c t u r e of a piece of steel, or r a t h e r metal, t h a t is of the fracture as it originates by n i c k i n g and s u b s e q u e n t breaking, or b r e a k i n g in service, bears a n y relation to the m i c r o g r a i n or m i c r o s t r u c t u r e u n d e r n e a t h the fracture, I do not believe there is a n y relation b e t w e e n t h e respective layers or planes of the m e t a l sufficiently well defined t h a t we could draw conclusions from the appearance of the fracture to the m i c r o s t r u c t u r e below, or the succeedi n g plane of the metal. In order to allow us to draw conclusions from the fracture to the m i e r o s t r u c t u r e u n d e r n e a t h , we m u s t a s s u m e t h a t the distortion of the particles or crystals of the metal, caused by the rupture, would c o n t i n u e deep into the metal, so t h a t succeeding transverse planes of the metal e x h i b i t the same a m o u n t and n a t u r e of distortion, which then will be the micrograin in question. If we a s s u m e t h a t no distortion of the particles takes place d u r i n g rupture, which, however, is inconceivable, since in t h a t case we would h a v e to a s s u m e t h a t there was no resistance offered by the m e t a l to the force of r u p t u r e , then still the appearance of the rup. t u r e d surface would be so m u c h different to t h e n a k e d eye t h a n the micrograin u n d e r n e a t h , t h a t we are not w a r r a n t e d to draw a n y conclusions more t h a n to say t h a t the metal u n d e r n e a t h is fine or coarse-grained.
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F r o m w h a t experience I h a v e in r e g a r d to this question, I would say t h a t t h e distortion, caused by r u p t u r e , does not go b e y o n d t h a t a m o u n t of m e t a l w h i c h we h a v e to file and grind off in order to get to the plane of m e t a l below the r u p t u r e plane, and, therefore, w h a t we see in t h a t plane w i t h the microscope is not w h a t we see in t h e g r a i n of the r u p t u r e d surface of the metal. T h e u n r e l i a b i l i t y of conclusions d r a w n would, of course, be the g r e a t e r t h e f a r t h e r we go a w a y from t h e point of rupture, because of the v a r i a t i o n s of m i c r o s t r u c t u r e we are apt to find in a n y piece of commercial m e t a l in its various portions or sections. MR. W. R. WEBSTER i n q u i r e d w h e t h e r the appearance of the f r a c t u r e of a piece of steel, as seen by the eye, corresponds w i t h t h a t u n d e r t h e microscope ? MR. ROBERT JOB : - - I n a n s w e r to Mr. W e b s t e r ' s q u e s t i o n w h e t h e r the appearance of the fractu*e of a piece of steel, as seen by the eye, corresponds with t h a t u n d e r the microscope, I am satisfied t h a t in the large m a j o r i t y of cases it does d o so. In other words, w h e n the f r a c t u r e appears to the eye coarse-grained, the steel has a coarse microstructure. It has been our experience t h a t the converse is not i n v a r i a b l y true. A case in point came u n d e r our observation several years ago in m a k i n g drop-tests upon a lot of rail-butts. All h a d been rolled u n d e r the same conditions t h r o u g h o u t , yet, w h e n f r a c t u r e d u n d e r the drop, some s h o w e d a coarse g r a n u l a r f r a c t u r e while others appeared to be relatively fine. Sections were r e m o v e d from some of the latter, a n d t h e y were f o u n d to h a v e the coarse, open, micros t r u c t u r e characteristic of t h a t rolling. Therefore, I feel t h a t d e d u c t i o n s from an a p p a r e n t l y fine-grained f r a c t u r e m a y be t o t a l l y at variance from the actual structure, and t h a t the fine g r a n u l a r form m a y be m e r e l y i n c i d e n t to some peculiarity in the conditions of fracture. W h e n the f r a c t u r e d surface is filed s m o o t h and etched, the g r a n u l a r appearance, as viewed b y the u n a i d e d eye, is relatively the same as w h e n seen u n d e r the microscope, and a g e n e r a l e s t i m a t e m a y often be m a d e w i t h o u t the aid of the glass.
280
Clamer :
[J. F. I.,
MR. G. H. CLAMER: - - T h e structure of alloys presents m a n y of the characteristics exhibited by cast iron and steel. In fact, we may regard these as,alloys of iron with varying amounts of carbon, silicon, manganese, etc. One of the most simple examples by way of comparison is the lead and antimony alloy, which has by experiment and microscopic examination been shown to consist of dendrites of pure lead embodied in a eutectic of lead and antimony, if the content of antimony be below 13 per cent.--the saturationpoint of these two metals. As in steel, free ferrite is contained in diminishing proportions from pure carbonless iron up to "89 per cent. carbon. Just as from pure lead to the alloy of I3 per cent. antimony, or thereabouts, is the free lead present in diminishing proportions. A t 13 per cent., so-called eutectic alloy, lead is saturated with antimony, antimony is saturated with lead. No free lead is present, no free antimony. As the proportion passes the eutectic composition, free crystals of antimony make their appearance. Here again we see the analogy to steel, which segregates above the saturation-point of' 89 per cent. carbon-free eementite. Another interesting example is the alloy of copper and tin, which has, perhaps, a little more complex composition. Alloys rich in copper (above 73 per cent.) consist of dendrites of pure copper euteetic and chemically combined copper and tin, generally conceded to be of the formula SnCua. The above alloy, 27 per cent. tin and 73 per cent. copper, is the first eutectic, the high tin-content alloy formi n g another. The formation of these eutectics exhibiting themselves constantly in the everyday manipulation of alloys has a very practical bearing on foundry practice. I once met with a very peculiar demonstration of the presence of copper and tin eutectic in casting phosphor. bronze, a composition which contains 79 per cent. copper, io per cent. tin, io per cent. lead and i per cent. phos. phorus. This metal was cast into connecting-rod bushings for locomotive construction. The castings showed on the outside a thin skin of metal about the thickness of paper, w h i c h could readily be detached from the casting, showing
April, x9o3.]
Notes and Comments.
28I
it had been forced to the surface t h r o u g h Pores after the b o d y of the casting h a d solidified. T h e composition of this was a b o u t as f o l l o w s - - I cannot locate the exact figures : Copper . . . Lead . . . . Tin . . . . . Phosphorus .
. . . .
. . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . .
. . . .
. . . .
. . . .
. . . . . . . . . . . . . . . . ....
70 per 3 22 3
cent. " " "
thus s h o w i n g p e r h a p s a q u a d r u p l e eutectic, or, at a n y rate, a tendency t o w a r d the copper-tin ev.tectic--27 per cent. tin and 73 per cent. copper. A n o t h e r curious t h i n g in connection with this w a s the presence of 3 per cent. phosphorus, s h o w i n g the fact discovered b y Charpy, that p h o s p h o r u s is localized in the eutectie. A n o t h e r curious p h e n o m e n o n c o m i n g daily u n d e r m y observation is the f o r m a t i o n of euteetic on top of compositions, such as the above, in e a s t i n g ingots. A n alloy of metal, w h e n apparently solidified, will often have a p p e a r on its surface w h a t we term eutectic sweats, b e c a u s e it s w e a t s or oozes o u t in thin s t r e a m s a p p a r e n t l y t h r o u g h pores, and forms globules on the surface. T h e s e g l o b u l e s are hard and brittle, and entirely dissimilar to the m a i n portion of the ingot, the analysis always s h o w i n g a h i g h tin alloy, a l t h o u g h the original m a y contain b u t 5 to io per cent. I can heartily endorse this a p p a r e n t l y n e w m e t h o d of investigation, for which P r o f e s s o r S a u v e u r has done so much in this country, and can s a y t h a t its field of usefulness is not confined solely to the province of iron and steel, b u t can be e x t e n d e d also in a praetical w a y to the m a n u f a c t u r e of metallic alloys.
P L A T I N U M E X T R A C T I O N IN T H E U R A L REGION. Professor Demaret-Freson has lately published some interesting facts in regard to the production of platinum. W i t h i n the last few years t h e applieations of this metal, especially in t h e eleetrieal industries, have rapidly increased, while the production remains about stationary, and it is estimated that the entire world's production is not over 7 t o n s annually. It is owing to this cause t h a t there has been a considerable rise in t h e price of platinum.