Prakt. Met. Sonderband 50 (2016) 61 
teels The specimens were then tint etched with 7% aqueous metabysulfite. This type of etchant 
colors martensite brown and the bainite blue, whereas retained austenite is colored in a violet 
lei purple shade or observed lighter due to a lesser deposition of the sulfide film. No pre etching 
a A was used, the samples were held vertically into the solution until a darkened tint developed on 
1ahlON the surface, then they were rinsed immediately with a flow of alcohol, followed by drying 
using hot air. Generally speaking the volume fraction obtained via image analysis differs 
> depending on the magnification at which the images were taken, therefore it is important to 
tres choose the latter, where the view of the microstructure is most representative. For the coarse 
grained KAB Hi-Mn a magnification of 200x was deemed the most suitable, whereas the fine 
Wes grained samples of KAB 1, and KAB Hi-Mn were observed at a higher magnification of 
500x. A base of 20 images of each individual state were then evaluated using image analysis, 
ysis whereby the given results represent the average values. 
gs, For X-ray diffraction measurements the samples were electrolytically polished to ensure an 
undeformed surface and analyzed using a Bruker D8 Advance, the measured peaks were 
interpreted using the Rietveld method. 
Ti Results and discussion 
d ih Alloying with Mn decreases both the Ms and Bs temperatures of steels, whereby the latter is 
mo affected more strongly with increasing Mn alloying. Accepted theory would therefore suggest 
| . . . 
on that at a certain Mn content the Bs temperature should in fact be below Ms, this was observed 
in the current steel KAB Hi-Mn. As can be seen in Fig. 1 martensite was the first phase to 
form, as is evident from its morphology, whereby the coarse brown-yellow colored plates 
ZZ grew unhindered across the whole grain, whereas the blue bainitic sheaves are oriented 
| perpendicularly to the plates. As a reliable brown coloration is known to be obtained when 
yz etching martensite formed in medium carbon steels, the carbon super-saturation is thought to 
Enel have a minor influence on the etching response of the current reagent. A possible explanation 
2 for the color contras might be found in the different properties of the a/y interphase as bainite 
g prop y ınterp 
— is thought to be comparably more coherent then martensite 1! 
1 
ith 
inal 
1€ 
ites] 
100 pm 100 um 
Figure 1: Microstructure of the steel KAB Hi-Mn after air cooling from the homogenization temperature of 1200°C, 
determination of martensite phase fraction based on color contrast and indicated as red (etched with 7% Na 25205)