. Transactions. onsible for this name,he wishes to repeat the statement that in so far as our present knowledgeof their actual chemical constitution goes, there is no evidence to justifythe calling of these micrographic constituents iron nitride. The name ^Chem. tfr Met. Eng. (March 3. 1920) 22, 403. DISCUSSION 307 was used for convenience, mainly, and as it is truly descriptive anddistinctive its continued use is justified until such time as a better namecan be given. The presence of carbon is not necessary for the formation of nitrideneedles and certainly they are not necessarily associated
. Transactions. onsible for this name,he wishes to repeat the statement that in so far as our present knowledgeof their actual chemical constitution goes, there is no evidence to justifythe calling of these micrographic constituents iron nitride. The name ^Chem. tfr Met. Eng. (March 3. 1920) 22, 403. DISCUSSION 307 was used for convenience, mainly, and as it is truly descriptive anddistinctive its continued use is justified until such time as a better namecan be given. The presence of carbon is not necessary for the formation of nitrideneedles and certainly they are not necessarily associated with anycritical carbon content, as suggested by the author, rather they arecaused by a certain nitrogen content, and then only on comparativelyslow cooling. Carbon may be associated with the pearlite-appearing areas,but micrographic evidence points to the assumption that the lines andneedles are a true iron-nitrogen compound, while the patches are abinary eutectic (Feo-N-Fe?). Its appearance resembles Fig. 60. Fig. 61. Fig. 60.—Electrolytic iron, NH3, 700° C, 14 hr. Etched with nitricACID. X 75. Fig. 61.—Same as Fig. 60, etched with Steads reagent. The authors statement (p. 301) that the outer nitride layer diffusesinto the ferrite on heating agrees with the experiments covered by thefirst set of micrographs given herewith. Due to the similarity between nitrogen-bearing areas and pearliteon nitric acid etching, it became desirable to find a method for distinguish-ing between them. Heat tinting gave some promise, as nitride needlesand areas were colored differently from ferrite or pearlite. Ferritecontaining small amounts of dissolved nitrogen colored more rapidlythan pure ferrite. The distinctions were slight, however, between thenitride areas and pearlite, so a more certain method was sought. Dilute HCl colored the nitride a deep black, while affecting pearliteto a less degree. As the distinction was not definite enough, copper-bearing solutions were experimen
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