. Transactions. Fig. 21.—Cementite and Pearlite. Fig. 24.—Lementite and 60 Diameters. Magnified 250 Diameters. THE MICROSTRUCTUKE OF IRON AND STEEL. 503 nothing but fine-grained austenite. On cooling down again,the ferrite separates out as before; but now it is confined tovery small grains of austenite, and hence the resulting structureis fine. Fig. 5 shows the same steel heated to 850° C. and thusrefined. It is not uncommon to find that such a treatment has notcompletely refined a specimen. The reason lies in the pres-ence of globules and films of MnS or oxides, which oft
. Transactions. Fig. 21.—Cementite and Pearlite. Fig. 24.—Lementite and 60 Diameters. Magnified 250 Diameters. THE MICROSTRUCTUKE OF IRON AND STEEL. 503 nothing but fine-grained austenite. On cooling down again,the ferrite separates out as before; but now it is confined tovery small grains of austenite, and hence the resulting structureis fine. Fig. 5 shows the same steel heated to 850° C. and thusrefined. It is not uncommon to find that such a treatment has notcompletely refined a specimen. The reason lies in the pres-ence of globules and films of MnS or oxides, which often assume ai/f ?. CARBON,PER CENTFig. 25.—Austenite Diagram. a dendritic form, due to their being deposited in the dendritesof the original casting. Heating to above truly refinesthis steel, but on cooling down again the ferrite tends to pre-cipitate on these films and globules of MnS, and lines of weak-ness result. However, the heat-treatment has materially im-proved such material, as the tensile tests will show. Fig;. 6 isa piece of steel with C, , and Mn, per cent., after refin-ing, and shows the dendritic structure due to manganese sul-phide, etc. 504 THE MICROSTRUCTURE OF IRON AND STEEL. 2. Axle-Steel.—The structure of axle-steel is similar to thatof structural steel, allowing for the increase in pearlite with in-crease in carbon. Grain-size increases with section. A greatmany cases of failure have been ascribed to fatigue or recrys-tallization due to vibration. We see a fracture the outer partof which is verj fine and the inner core comparatively coarselycrystalline, as
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
