220 Prakt. Met. Sonderband 50 (2016) 
rate is known to be very rapid [10], and can therefore be clearly indicated. From the micrographs in with the 
figure 2, it can be seen that the allotropic transition brought about an appreciable grain refinement thermo 
when compared with the fully annealed state. 
0,8 
0,6 
Sos 
= 
cooling 
0,2 
Fig. 3: Ir 
0 WE 
0 100 200 300 400 500 600 700 800 900 1000 
T [°C] 0s 
Fig. 2: Electrical resistance changes during thermal cycling of low carbon steel 
We proceeded our investigation on steel with higher alloying content, for this purpose we chose the . 
commercial medium carbon grade Ck45. The initial microstructure can be seen in figure 3a, it is 
comprised of ferrite grains, fine pearlite, carbides and some martensite. Whereas the microstructure 
after measurements, constituted of grains of ferrite and comparably coarse pearlite colonies which 03 
can be seen in figure 3b. The diagram of electrical resistance measurements performed during the z 
thermal cycling to 850°C can be seen in figure 4. During reheating the martensite present within the E 
microstructure is tempered, this is accompanied by removal of carbon from solid solution in ferrite “02 
and thus decreases the resistivity. The onset of tempering has been determined at 321°C (Cm), and 
the process is thought to be complete at 517°C (Cmy). The volume fraction of martensite is small 
and the carbon content quite low, therefore tempering was detected only as a slightly. Reheating of a 
medium carbon steels with mixed ferrite pearlite microstructures is known to result in separate Ac; 
temperatures for each of the phases [11-12], a similar effect of localized austenite formation is also 
known to occur in the vicinity of cementite particles [13]. Pearlite decomposes into austenite at a 
lower temperature due to its higher carbon content and is marked with Ac;, whereas the ferrite 
grains require further heating and transform into austenite at the Ac;s temperature. On the current 
sample of Ck45, the temperatures obtained for Aci. Acirand Acs are 689°C, 718°C and 789°C Fic. oh 
respectively. B. 0 
Upon cooling ferrite is the first phase to precipitate from austenite. The transformation occurs at The ch: 
lower temperatures when compared with the low carbon grade and within a narrower interval continu 
between the temperatures of 720°C and 680°C. These are indicated by the points Ar; and Ar; on the enriche 
electrical resistance graph. The other distinct region in the cooling curve of Ck45 is in-between the subsequ 
points Ps (perlite start) at 665°C and Ps (perlite finish) at 643°C which indicate the onset and as all th 
completion of the pearlite formation respectively. It is known that the continuous phase exerts the solution 
highest influence on electrical resistivity [9], and this needs to be accounted for when interpreting measur 
the current results. The decrease in resistivity due to the formation of ferrite with a lower resistivity Fe-Fext 
. . . . . oy oe €-r'cs 
is only slight. It was metallographically determined that the volume fraction of ferrite is about 42%. 
~ 
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