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November 18, 1892.IRON.447MICROSTRUCTURE OF CHROMIUM-STEEL.1176 B. /16 7. C. \\ 29Cr./1176 0./■W7.C. \ V48 7. Cr./1176 F./ -27 TC. \ Vl-18 V.Cr.J1176 J./ ‘77 7. C. \ \5‘19 7. Cr./1176 L./ -717. c. y \9 18 7*0./\o g§g.dS.d32oOQ§ ) I0-2|o * pcells of iron cemented together by carbide ; but I have not been able to see any trace of the- alternate layers of iron and carbide first observed and described by Professor Sorby in his admirable researches. The absence of the lamellar structure is general in all the chromium-steels whether forged or annealed. The amount of carbon present may be estimated at 0’35 per cent.Annealed at 1030° C.—The effect of the annealing is the same as in the case of E. The grains of iron are a little larger, and reach a diameter of 0‘OL millimetre. The passage of the granules into the matrix is less distinct than in the case of ordinary steels.1176 J.—Forged into bars of J inch x ,1 inch.—Etched by nitric acid of 20 per cent. vol. for ten to fifteen seconds. The tint assumed was that of black hardening carbon. This steel is very hard (self-hardening). This metal is characterised by the presence of a number of small white points of * great hardness and brilliancy, which are scarcely at all attacked by the acid (see photograph). These white points, which we find to be much more numerous in sample L, appear to be composed of iron, chromium, and carbon. The matrix has the structure of a tempered steel, and is entirely composed of simple cells of about 0‘001 millimetre in diameter. The photograph shows the white points well, and also the fact that composite structure is absent.Tempered at 1030° 0.—No notable change of structure metal softer, hut still hard.Tempered at 1020° C.—Complete change of structure. Etching with acid, which must be continued thirty seconds, shows large brilliant grainB of irregular shape and size.und sharply distinct from the dark matrix. These brilliant portions are very hard, although they do not show the reaction of gardening carbon when treated with nitric acid ; they appear amorphous, and, in accordance with the result of my experiments, I am inclined to attribute them to solution of the hard compound which we found isolated in the metal when less heated. The dark matrix is of ordinary steel.1176 L.—Forged into bars of\ inch x J inch. -^Etched with nitric acid of 20 per cent. vol. for ten to fifteen seconds. The etched surface is not so black as that of J- The brilliant white points which we have discovered in J are infinitely more numerous, although preserving the same specific characteristics ; their form is spheroidal and more or less regular, and their diameter O’OOl millimetre to O’Ol millimetre. The photograph with thenlargement of 300 diameters shows the general appearance in this sample, and reminds one to a great extent of a starry sky. The dark matrix, which the photographdoes not resolve, is entirely composed of simple cells.Annealed at 1320° C. —The effeot of this high temperature is similar to that noticed in J ; but, as the amount of chromium which it contains is very much greater, the transformation of the matrix into hard metal, little capable of being reacted upon by acids and no longer showing the reaction of hardening carbon is almost complete. The steel is composed of comparatively large polyhedrons (0‘08 millimetre to 0‘2 millimetre), but they are much smaller than they would be in the case of ordinary steel heated to the same temperature. The etching with nitric acid may be continued as long as two minutes without sensibly altering the appearance of the sample. The polyhedrons continue brilliant and highly polished. The photograph shows the polyhedrons separated by black joints presenting a bright middle line ; these joints are possibly simple fissures, or perhaps a residue of unconverted steel. The few traces of polish which the great hardness of the metal prevented from disappearing are easily discernible.Tempered, at 1320° C. and annealed in the air at a cherry-red.-—Etched by nitric acid 20 per cent. vol. for two minutes. The state of the metal is the same as above, but, as might he foreseen, when the chilling is more rapid the size of the polyhedral grains diminish. These latter are not more than 0‘05 millimetre average diameter, and are more coherent, The joints are extremely fine, and can be well seen by a microscope, but are not easily seen in the photograph.Annealed at 1320° 0., again, heated to a cherry-red and cooled in the air.—The structure is not altered.Annealed at 1320° C., then reheated and forged into clierry-red and cooled m the air.—The forging causes the special structure due to the tempering at 1300° to disappear, and it reverts to that of the original bar.Conclusions.In conclusion, it appears that the chromium interferes with and impedes the crystallisation of the iron to a considerable extent, in this way reacting upon the structure :—1. By the absence of Sorby’s alternating lamellae. 2. By the absence or diminution of crystallised structure, as shown by Mons. Brustlein in his publications on cbromium-steels. This action of chromium persists at all temperatures, but it is, generally speaking, much more’ distinctly marked when the temperature of 1000° or thereabouts is not exceeded,than in the case when we reach 1200° or 1300° C. The effect produced by the chromium is not proportional to the amount of chromium which the metal contains. The small amounts (B. and C.) are relatively more active than the medium amounts (E. and F.). In the first case the chromium appears to be dissolved ; the acid dissolves the steel without leaving any residue. Accordingly, as the amount of chromium increases, the compound of iron, chromium, and carbon appears to be formed, which is only partially attacked by acid, and possesses great hardness. (This is what takes place in silicon-iron.) This hard compound may even be isolated in the form of globules, as shown in the case of theforged samples J. and L. ; or it may be dissolved at high temperatures (1200° O. and higher) in the metallic matrix, bestowing upon it a high degree of hardness (J. and L. annealed at 1300°). The absence or diminution of the crystallisation probably plays an important r61e in the modifications which the chromium effects in the mechanical properties of steels.(To be concluded.)THE GREENWICH VEHICULAR FERRY.About four years since a vehicular ferry of special construction was completed and opened at Greenwich, thus affording the densely-populated district on either side of the river at that point a ready and useful means of inter-communication. Circumstances , however,mainly financial, caused the ferry to be closed after a short spell of successful working, which closing was a misfortune to all interested, particularly to those having heavy goods to transport from one side to the other. The plant and machinery were, however, put in thorough working order, and the ferry was opened about four months since. With the exception of a short stoppage, caused by an accident to the machinery due to carelessness, the working has been perfectly successful. The ferry and plant were inspected on Monday, November 14, by a large party of visitors, the company including Admiral Sir E. A. Inglefield, Mr. Foster, M.P., and representatives of the London County Council, of the Greenwich Vestry, and of the Greenwich Board of Works.The Greenwich ferry is a good example of.the manner in which heavy goods traffic may be conveyed across a tidal river with the least inconvenience to the users,
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Iron
London, Middlesex, GB
Fri, Nov 18, 1892
Page 27
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