Prakt. Met. Sonderband 52 (2018) 101 
0 
gr During annealing at the graphitization temperature the morphology transitions from 
38 Can lenticular bainitic sheaves towards a uniform distribution of very fine grains of ferrite with 
particles of cementite and graphite situated at the prior sheave and grain boundaries 
whereas the prior martensite regions are marked with fine particles of cementite, as can 
be seen in Fig. 3-b. It is predicted that a single sheave of bainite decomposed into two or 
three newly formed ferrite grains in the case of short/thick and long/thin sheaves 
respectively, whereas the martensitic phase experienced a conventional tempering 
reaction. The graphite particles are slightly elongated with a width of between 150-200 
nm and a length up to 500 nm as shown in Fig. 3-b. 
The process of decomposition is depicted schematically in Fig. 4, where the hashed 
region represent sheaves of bainite and the black regions plates of martensite. 
a.) oC 
de free 
NCrostructure. 
[with Vilela), 
FMn it Can be 
xhidit a britfle 
RIK 880°C) 
n Fig. €. The 
ing, d 
Ce Figure 4: Schematic representation of microstructural evolution during 
ar levels even isothermal annealing at 680°C. a.) initial bainitic/martensitic microstructure, The 
en ) b.) Fully annealed microstructure. 
cation density 
ore very low formation of graphite in steels is usually observed steel alloys of very lean compositions 
with respect to carbide formers (Mn, Cr, Mo), but even within such alloys the graphitization 
kinetics are exceedingly sluggish and can easily require in excess of 50 hours or even as 
much as several hundred hours. This compositional constraints can be considered a 
mayor limitation for graphite forming steels as it narrows down the heat treatment 
possibilities in particular the hardenability [8]. The graphitization kinetics have however 
known to be greatly accelerated by prior formation of bainite/martensite [9] and via cold 
rolling [10]. 
During machining of the newly developed steels by standard high speed steel tools, it 
was observed that easy chip breakage occurs in the steel Med C-Al. Somewhat 
surprisingly perhaps the chips formed when machining the graphitized grade have a more 
continuous character, but are much colder compared to those obtained by common 
machining operations of conventional spheroidzed 0.5-0.7 spring steels despite the latter 
having a lower hardness (160-190HBW). The difference in the chip temperature is 
ic/martensitic apparent from their color which exhibits a blue/violet tint, as opposed to a slight brown 
showing 4 color produced with the conventional steels. No build-up or sticking at the tool edge are 
a from initial observable in either of the two grades. 
Nn,