STRUCTURE AND NATURE OF TROOSTITE 103 



Moreover, it was shown that each of the fan-shaped grains is a 

 separate crystalline unit for if a nodule of troostite is revolved about the 

 optical axis of the microscope, these very small fan-shaped grains dis- 

 play orientation phenomena in exactly the same way as a system of 

 polyhedral grains in a pure metal will do if revolved about the optical 

 axis of a microscope while being kept under observation at 100 or 200 

 diameters magnification. The only difference lies in the fact that in 

 nodular troostite the grains are fan-shaped and quite small, making it 

 desirable to carry out the observations with an oil immersion lens 

 which will yield high magnifications. 



Fig. 1 is reproduced from the Franklin Institute paper and illus- 

 trates a typical section on a plane passing through the center of a single 

 nodule. Fig. 2, also from the same paper, is a diagrammatic represen- 

 tation of how the fan-shaped grains develop along axes of crystalliza- 

 tion A, B, C, etc. 



Fig. 1 not only shows crystallization about a nucleus but it also 

 supplies evidence for the conclusion at that time that nodular troostite 

 is either a solid solution of iron carbide in iron or it is a very fine aggre- 

 gate of iron and iron carbide — the carbide so finely dispersed as to lose 

 its identity under the microscope. Failure at that time to resolve the 

 structure of troostite into its ultimate constituents compelled one to 

 recognize the existence of the two possibilities as to structure. 



The improvements in technique previously mentioned have thrown 

 some new light on the structures found in hardened steels and these 

 have been discussed in later papers.^' '*■ '^ 



Certain it is that the structures found in hardened steel are largely 

 mixtures of the things which metallographers call martensite and 

 troostite, the name martensite in this case meaning a needle-like 

 structure. Troostite, generally, is regarded as a lower order of decom- 

 position than martensite. This, however, is not believed to be substan- 

 tiated by the evidence. 



It has been shown ^ that martensite is a decomposition of the austen- 

 ite along the octahedral crystallographic planes. That is, martensite 

 is a structure superimposed by decomposition of the austenite on the 

 old crystallographic system of the austenite. Two changes are in 



' Lucas, "A Resume of the Development and Application of High Power Metallog- 

 raphy and the Ultra Violet Microscope," Vol. I, Proceedings International Congress 

 for Testing Materials, Amsterdam, September 1927. 



* Lucas, "Photomicrography and Its Application to Mechanical Engineering," 

 Mechanical Engineering, Vol. 50, March 1928. 



* Lucas, "Further Obseivations on the Microscructure of Martensite," Trans. 

 American Society for Steel Treating, Vol. XV, February 1929. 



'Lucas, "The Micro-Structure of Austenite and Martensite," Trans. American 

 Society for Steel Treating, Vol. VI, No. 6, December 1924. 



