214 Prakt. Met. Sonderband 52 (2018) 
2. MATERIAL AND METHODS 
21 MATERIAL AND MICROSTRUCTURE 
The hypoeutectoid fine-grained ferritic-bainitic steel 22NiMoCr3-7 with segregations was 
chosen for this study. The average chemical composition of this material is 0.22% C, 
0.8% Ni, 0.53% Mo and 0.4% Cr. The microstructure after hot rolling and cooling down to 
ambient temperature is very fine and complex, containing bainite, pearlite, ferrite with 
carbides, FesC and martensite. The crystallographic phases are not always clearly 
distinguishable; the transition between them is often blurred. The regions enriched during 
crystallization with readily soluble elements like chromium (Cr), nickel (Ni), molybdenum 
(Mo) are prone to transform locally to hard martensite during cooling down. 
In Fig. 1 optical images of two types of microstructure of 22NiMoCr3-7 — without (a) and 
with martensite in the segregation area (b) are presented. The color etching of Beraha II 
visualized martensite in blue colour (c). ” 
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Fig. 1: Local microstructure of 22NiMoCr3-7 (a) homogeneous ferritic-bainitic 
microstructure, (b) segregation region with martensite, (c) colour etching of Beraha Il in the 
segregation region from (b) 
2.2 EXPERIMENT 
2.2.1 SEM, EBSD AND LOCAL CHEMISTRY (EDX) nas 
The nontrivial problem of this microstructure is to distinguish between ferrite and aan 
martensite. They have the same bcc crystallographic structure but different mechanical oo 
properties and hardness. For the electron microscopic and EBSD investigations, a a 
scanning electron microscope (FEG-SEM) of the type AURIGA from Carl Zeiss was used. * 
In Fig. 2 SEM images are presented: (a) shows the grey value contrast between mn 
martensitic phase and surroundings using back scattered electrons. The light-dark contrast ne 
depends on the atomic number and is a result of the enrichment of Mo and Cr in the 
martensitic phase (see Fig. 3). The ROI that was studied here in detail is selected by a 
yellow frame (Fig. 2a). In Fig. 2b the crystallographic orientation of all phases in the ROI is ue 
depicted giving a good overview about the microstructural fineness and complexity. Using ... 
an Image Quality (IQ, Fig. 2c) the differentiation between ferrite (white, high IC) and be 
martensite (dark, low IC) was done. be 
EX meg 
MS az 
SER Mm