Prakt. Met. Sonderband 46 (2014) 125
ch CHARACTERIZATION OF ACICULAR FERRITE
“schnittliche MICROSTRUCTURES USING ETCHING METHODS,
ngtiefe der OPTICAL MICROSCOPY AND HT-LSCM
an zwischen
schen Zone D. Loder, S.K. Michelic, C. Bernhard
vesentlichen
e Korner zu Chair of Ferrous Metallurgy, Montanuniversitaet Leoben, Leoben, Austria
erte je Korn
ABSTRACT
Acicular ferrite is a needle shaped modification of ferrite, which nucleates intergranularly at
lentation zur non-metallic inclusions. Due to the fine grained structure of acicular ferrite, it offers
geeignet ist. excellent toughness. By increasing the amount of this component within the
> gelegt und microstructure, the properties of HSLA steels can be optimized significantly. The formation
ng konnten of acicular ferrite is influenced by four main parameters: Steel composition, cooling rate,
im Gefüge austenite grain size and non-metallic inclusions. These parameters are interacting
ung und die strongly, making a systematic study essential. By using a Laser Scanning Confocal
wurde eine Microscope combined with a High Temperature furnace (HT-LSCM) for the in situ
observation of acicular ferrite formation in HSLA steels, fundamental information about the
formation mechanism can be gained. Due to the inert furnace atmosphere, the accurate
adjustment of austenitizing temperature and the well controllable cooling conditions, the
interactions between steel composition, austenite grain size, cooling rate and the acicular
Cp . ferrite amount can be analyzed in detail. Up to now no automated quantification of the
© a acicular ferrite amount has been described in literature. The present study focuses on the
> characterization of acicular ferrite microstructures by a combination of metallographical
methods. Conventional sample preparation is combined with the in situ observation of the
acicular ferrite formation in a HT-LSCM. Special attention is paid to the determination of
austenite grain size by optical microscopy and its influence on the acicular ferrite amount.
Finally, various etching methods for the illustration of acicular ferrite are presented,
focusing on their applicability for an automated quantification of the acicular ferrite amount.
ed Surfaces
cience
. | 1. INTRODUCTION
)ompression
en In the 1990s a new scientific field called “Oxide Metallurgy” developed which aims on the
je limit”, specific control of steel microstructure by an appropriate tailoring of non-metallic inclusions
q d [1-2]. Up to now there have been considerable efforts to develop a method for grain
nethods an refinement during steel production by means of oxide metallurgy. Acting as heterogeneous
3-819 nucleation sites during the y — a transition, non-metallic inclusions can refine the
| Bi microstructure by promoting the formation of acicular ferrite (AF) within the austenite grain.
indentation’, Beside non-metallic inclusions the nucleation of AF is mainly affected by steel
5 composition, cooling rate and austenite grain size [3-4]. Conventional methods for the
analysis of AF, most notable optical microscopy, manual and automated SEM/EDS and
TEM, only allow the analysis of the microstructure and the inclusion landscape after heat
treatment. Hence, during the last decade a new analytical method for the investigation of
AF microstructures emerged. Using a Laser Scanning Confocal Microscope attached to a
