Prakt. Met. Sonderband 52 (2018) 99 
«Ma 
i RESULTS AND DISCUSSION 
0 form 
tig 1 The initial microstructure obtained after forging consists consist predominantly of carbide 
dA bu is free lower bainite and a small fraction of martensite and retained austenite. It can be seen 
3 the ry y in the optical micrograph in Fig. 1, where the microstructure was revealed by etching 
using 7% aqueous Naz2S20s, which was determined as the optimal concentration of this 
reagent for the coloration of bainite and martensite, with blue and brown colors 
COS Of Si ang respectively. 
8 Such seglg 
Procedures of 
NS are shown 
hich result jn 
ed HIGH C-A| 
nder Argon in 
It 180°C with 
Figure 1: OM micrographs: a.) Initial carbide free bainitic/martensitic microstructure 
(etched with 7%Na2S20s). b.) Microstructure after annealing at 953°K (680°C) for 6 
au Te hours, 
S TiHZr 
00 00 Analysis using XRD of steel High C-Al have determined that the initial microstructure is 
— comprised of bainititic ferrite (62%), martensite (15%) and retained austenite (23%), the 
10 00 martensite content is also confirmed via image analysis (estimated at 12%). Whereas the 
— Med C-Al steel develops a microstructure of bainite with 8% retained austenite. To our 
observations only high Al containing KAB steels, form graphite when heat treated from 
abrasive disk an initially bainitic/martensitic microstructure. Our observations have shown that the main 
VORGE U factor determining the final microstructure of either spheroidite or graphite after the same 
nding using a heat treatment is applied is the ratio between carbon and carbide forming elements. This 
JEDEN can be seen from the chemical compositions of the steels High C-Al modified with a lower 
Mn content and the steel Med C-Al, which form graphite and spheroidite respectively. In 
steels with a higher content of carbide forming elements very fine spheroidite is 
ned to ensure developed, with carbides located predominantly at prior sheave boundaries, as seen in 
asured peaks Fig. 2. The average size of the newly formed grains and carbides is 300nm and about 
1um respectively, which is comparable to data from, where the spheroidized steels begun 
exhibiting brittle behavior [6]. The fine size and high strain rate during cutting warrant that 
the steel won't work harden when machined, proving a theoretically high machinability 
index according to the criteria of specific absorbed energy.