Structure and Nature of Troostite ^ 



By FRANCIS F. LUCAS 



In this paper the structure and nature of the constituent troostite (found 

 in hardened steels) is discussed. High power metallography was first ap- 

 plied to this problem about six years ago and the early results were presented 

 in an address before the Franklin Institute. 



Since that time many improvements in technique have been developed 

 which have resulted in better resolution and definition. The subject has 

 been reviewed in the past two years and with the aid of the improvements 

 in technique, hardened steels are found to be largely mixtures of the things 

 which metallographers call martensite and troostite. 



In small specimens of 0.90 per cent carbon tool steel hardened to C-65 on 

 the Rockwell scale, innumerable particles of troostite are found. When 

 these particles of troostite are examined by present high power methods the 

 structure is clearly resolved into laminated pearlite. In certain stages of 

 development of a troostitic nodule its structure borders on the verge of 

 present methods of resolution. 



Nodular troostite develops under favorable conditions as a globular mass. 

 At the center is a nucleus about which the growth occurred. Radial, fan- 

 shaped grains extend outward from the nucleus and these grains show orien- 

 tation phenomena when revolved about the optical axis of the microscope. 



It is believed that when martensite forms, the structure develops on the 

 old austenitic crystallographic planes. Troostite appears not to follow the 

 old austenitic system but seems to be a reorientation of the freshly trans- 

 formed alpha iron about a nucleus which usually is an inclusion, a void, a 

 sharp corner in a grain boundary or some other detail of structure. 



The structure of troostite in various stages of its formation is illustrated by 

 means of high power photomicrographs, many of which are shown at this 

 Congress for the first time. 



The following conclusions were reached: 



Nodular troostite appears to be an aggregate of ferrite and carbide and in 

 the very early stages of formation its structure is on the border of present 

 methods of resolution. The condition of the ferrite and carbide in relation 

 to each other is not stable — they tend to stratify, forming pearlite. 



Troostitic nodules grow about a nucleus which may be an inclusion, a 

 void, a corner in a grain boundary or some other detail of structure. 

 The nodules contain fan-shaped radial grains. 



The development of troostite results in a reorientation of the ferrite — 

 seemingly without particular reference to the old austenitic crystallographic 

 planes. Martensite does follow the old system of austenitic planes. 



The small fan-shaped grains in nodular troostite may persist as small 

 grains or they may undergo grain growth by union. It is a matter seem- 

 ingly dependent upon the thermal treatment of the specimen. 



IN a paper^ presented before the Franklin Institute in the year 1924 

 some observations on the structure and probable nature of the 

 constituent troostite were given. Two types of troostite were shown 

 to occur in hardened steels depending on the mode of heat treatment. 

 If a bar of 0.50 per cent carbon steel is given a taper heat treatment, 



1 Presented by the author before World Engineering Congress, Tokio, Japan, 

 October 30, 1929. 



^ Lucas, "High Power Metallography — Some Recent Developments in Photo- 

 micrography and Metallurgical Research," Journal of the Franklin Institute, Vol. 201, 

 February 1926. 



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