Prakt. Met. Sonderband 52 (2018) 99
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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.