46 BULLETIN 184, XHSflTED STATES NATIONAL MLPSEUM 



percent. For a time the iron bad an octahedral structure, Avhile 

 it was within the gamma-alpha range. But this condition was 

 transitory, disappearing when cooling reached about 550°, when 

 the transformation is completed and the iron is wholly alpha. 



B. The saturation ratio of taenite with respect to kamacite is 

 reached with about 6 percent nickel. This can be regarded as the 

 saturation ratio at room temperature; for at about 450° the metal 

 has become so rigid that diffusion virtually ceases and the structure 

 remains unchanged with further cooling. So when the nickel con- 

 tent is above 6 percent, whatever Widmanstatten structure existed 

 at around 450° is preserved. If it only slightly exceeds 6 percent 

 there are only slight traces of the structm-e — in other words, a coarsest 

 octahedrite, which sometimes may be barely distinguishable from 

 hexahedi'ites. It may show no segregation of taenite, consisting 

 entirel}^ of wide, more or less indistinct, kamacite bands. 



C. As nickel increases from 6 percent to around 11 or 12 percent, 

 with the lines AB and AC steadily low^ering, the proportion of kama- 

 cite decreases while that of taenite rises. Kamacite bands grow nar- 

 rower and clearer and are separated more and more widely by plessite 

 fields. This structure corresponds with that of a hypoeutectoid 

 steel in which ferrite predominates. The octahedral structure passes 

 from medium, through fine, to finest, in which the very narrow kama- 

 cite bands appear only sparingly in a groundmass of plessite; this 

 structm-e corresponds with that of a hypereutectoid steel. At 

 around 13 percent nickel (e. g., Cowra) continuous bands give place 

 to a dispersion of minute spindles or rounded particles of kamacite, 

 like the remnants of ferrite in a steel that approaches eutectoid com- 

 position. At around 14 percent kamacite has disappeared, except 

 for scattered particles, the iron now being an ataxite. 



D. At about 17 percent nickel, the hypothetical eutectoid ratio 

 of kamacite and taenite, there is a uniform two-phase structure 

 approaching a eutectoid in character. The iron is now a dense, or 

 paraeutectoid, ataxite — a uniform distribution of kamacite and 

 taenite particles, the taenite predominating. The structure is iden- 

 tical with some forms of dense plessite, and corresponds with pearlite 

 or eutectoid steel m the iron-carbon series. It is not, however, a 

 true eutectoid in which the components have definite compositions, 

 but only a structure of two phases of variable compositions. Vestig- 

 ial kamacite in the form of scattered particles is still found. 



E. As the nickel content further rises, the increasing taenite be- 

 comes still more predominent — an unperfectly transformed ground- 

 mass with an acicular or indistinct structure — and at around 26 to 

 29 percent the iron consists partly of the same groundmass and 

 partly of taenite. 



