STRUCTURE AND NATURE OF TROOSTITE 105 



per cent and by quenching small pieces from very high temperatures, 

 polyhedral grains of austenite containing martensitic needles and 

 troostitic nodules were found to occur. Both constituents were found 

 to occur in the same grain and both seemed to be entirely surrounded 

 by austenite. Had the needles formed first and the nodules developed 

 from the needles, one might expect to find some nodules with untrans- 

 formed needles sticking out around the boundaries of the nodules. 

 This was found not to be the case. The boundaries of the troostitic 

 nodules are always sharply defined. 



In some specimens of commercial plain carbon steels in which some 

 tempering had taken place troostitic nodules were found in which it 

 appeared that the nodule had grown at the expense of some martensitic 

 needles. The needles seemed to be dimly visible in outline in the back- 

 ground of the nodule. Cases of this kind appear very infrequently. 

 Microscopic evidence does not support the conclusion that one type of 

 structure replaces the other. 



If a specimen of commercial tool steel heat treated to produce some 

 troostite in a martensitic matrix is tempered, one might expect the 

 troostite nodules to grow in size if the nodular form of structure re- 

 places the needle structure. As a matter of fact the nodules remain the 

 same size and the carbide which they contain tends to coalesce into 

 small globular particles, marking not only the border outline of the 

 nodule but also the outlines of the fan-shaped grains. 



The needle and nodular patterns are structures which result from 

 quenching and not from tempering. The excess constituent in the 

 case of hypo- or hyper-eutectoid steels appears to be eliminated or 

 cleared by means of the constituent troostite. The constituent mar- 

 tensite (needles) appears not to be involved in this phenomena in 

 quenched specimens when both troostite and martensite are present. 



If one examines a normalized specimen of plain carbon tool steel of 

 about 0.90 per cent carbon content he will find a large polyhedral struc- 

 ture marked by a carbide network, but within these grains will be found 

 a great many smaller grains of pearlite, usually fan-shaped. In many 

 cases the outlines of the old troostitic nodules can be traced without 

 difiiculty. From the configuration of the pattern it seems likely that 

 these small grains within the larger (old austenitic) grain must differ in 

 their inner crystalline symmetry, i.e., it is probable that the ferrite is not 

 everywhere oriented the same throughout the old austenitic grain. 

 Under some circumstances controlled by heat treatment, it appears that 

 grain growth does occur among these small fan-shaped grains and the 

 old austenitic grain may be uniformly oriented ferrite containing sphe- 

 roidized particles of cementite which by their positions mark the old 

 structure and tell the history of the transformations. 



