. Transactions. Fig 9 —Microstructure of low-carbon steel above the Acz transformation. a. The polished specimen was heat-etched by heating 30 min at 950° C Two patterns corresponding to the a and the y forms (compare Fig. 1) are to ^^ rthe pooshbd specimen was etched with 2 per cent, nitric acid beforeheating at 1000° FOR 30 MiN. The original pearlite pattern shown by the FIRST ETCHING HAS PERSISTED THROUGHOUT THE HEATING; UPON THIS TWO OTHERPATTERNS SIMILAR TO THOSE IN a, HAVE BEEN SUPERIMPOSED. X500. 426 MICROSTRUCTURE OF IRON AND MILD STEEL AT HIGH TEMPERATURES ^^^^^^^^^^Bssimwv^m^^^v^- ■
. Transactions. Fig 9 —Microstructure of low-carbon steel above the Acz transformation. a. The polished specimen was heat-etched by heating 30 min at 950° C Two patterns corresponding to the a and the y forms (compare Fig. 1) are to ^^ rthe pooshbd specimen was etched with 2 per cent, nitric acid beforeheating at 1000° FOR 30 MiN. The original pearlite pattern shown by the FIRST ETCHING HAS PERSISTED THROUGHOUT THE HEATING; UPON THIS TWO OTHERPATTERNS SIMILAR TO THOSE IN a, HAVE BEEN SUPERIMPOSED. X500. 426 MICROSTRUCTURE OF IRON AND MILD STEEL AT HIGH TEMPERATURES ^^^^^^^^^^Bssimwv^m^^^v^- ■ i l^^^^ff^^ ^^^A» ^ ^. ^. Fig. 10.—Surface changes produced by heating in vacuo. Etching, 2 per cent. ALCOHOLIC nitric ACID. a. Heated for 30 min. above the Aci transformation (760° C).h. Heated just below the Ad transformation (820° C).c. Heated above the Ad transformation (890° C). HENRY S. RAWDON AND HOWARD SCOTT 427 Act on heating, and below Ac^ on cooling. The twin crystal (Fig. 96),which had its origin when the steel was in the 7 state, is seen to cut throughthe pre-existing islands of pearlite and to bear no relation to them. Itshould be borne in mind, however, that the carbide has been removedfrom the surface material, probably before the change to the 7 state wasbrought about, as has been described above. Nature and Extent of the Surface Changes upon Heating The magnitude of the change (apparently decarburization) whichoccurs in some samples (Fig. 8) makes the explanation offered byHowe^^appear inadequate. In order to show clearly the nature of the change, andto test his explanation, a series of
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
