Prakt. Met. Sonderband 50 (2016) 59 
NdB4) (Abb. 13, ; i ; 
a Quantitative metallography of multiphase steels 
Peter Kirbis 2, Mihael Bruntko?, Ivan Anzel?, Tatjana Ve&ko Pirtovsek' 
1. Metal Ravne d.d. Korogka cesta 14, 2390 Ravne na Koroskem, Slovenia 
2. University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia 
Abstract 
Metallographic methods of qualitative and quantitative phase identification have long been 
known for their superior accuracy. The contrast obtained via color etching techniques is often 
advantageous, compared to a greyscale contrast. In the current work phase fractions of 
martensite, bainite and austenite in newly developed experimental nanostructured bainitic 
steels, have been determined from image analysis of micrographs taken using dark field 
Ie imaging and color etching. The interpretation of the results was compared to results of X-ray 
diffraction (XRD) measurements of the different steels. 
Introduction 
SND tes Color etching is an established method used in the characterization of steel microstructures 0, 
it has been routinely applied to multiphase TRIP assisted steels delineating the phases of 
bainite, martensite and retained austenite !?l, Its reliability was also confirmed using different 
methods, like for instance Nano indentation measurements. Common multiphase TRIP steels 
are usually isothermally heat treated at temperatures between 400°C and 500°C, where the 
anomal und ent- formation of bainite is completed in about 10 minutes often combined with a hot galvanizing 
it. Mit Hilfe ei- step. Unlike upper or lower bainite, which are comprised of laths/plates of ferrite and 
ter genau gezielt precipitated carbides, the bainite formed within TRIP steels is of the carbide free morphology 
er aktuellen For- [3], where all the carbon is partitioned from the bainitic ferrite into the adjacent retained 
man eutektische austenite, thereby stabilizing the latter to low temperatures. As the temperatures of bainite 
r in Magnetlegie- formation decrease the required isothermal holding times scale exponentially 41, Is therefore 
| der Forschungs- common for the reaction to last for several days at temperatures less than 200°C, which are 
sometimes considered “abnormally low]. At such low temperatures the ability of carbon to 
fully partition is severely limited. Investigations at atomic scale resolution by means of the 
APT method have shown that martensite is the first phase to form in such steels and that the 
same region can be identified as bainite only after substantial annealing times [l, Despite the 
Anwendungen“. carbon atoms being mobile at the annealing temperatures, a substantial amount is still 
observed to be present in solid solution within the bainitic ferrite, due to the tetragonal 
distortion of the lattice !”! 
A new type of nanostructured bainitic steels named kinetically activated bainite (KAB) have 
os > been recently developed at the Institute for materials technology, associated with the Faculty 
1 of Fe-Nd-B and of mechanical engineering in Maribor, and have entered their trial period in Metal Ravne d.d. 
he 5th Int. Symp. What makes these alloys unique is that the bainite reaction is completed at temperatures 
Bod Soden. FRG below 200°C in a matter of seconds. The resulting microstructure is similar to other 
nanostructured bainitic steels characterized by slender plates of bainitic ferrite with a 
ns in the System thickness of about 40 nm in a matrix of retained austenite. This is accomplished by 
introducing the nuclei for bainite formation at a higher temperature, which will then activate 
as the steel is cooled below Bs resulting in a rapid growth, henceforth the name. Unlike