On the Spheroidized Carbide Dissolution and Elemental Partitioning in High Carbon Bearing Steel 100Cr6
WENWEN SONG
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PYUCK-PA CHOI
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GERHARD INDEN
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ULRICH PRAHL
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DIERK RAABE
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WOLFGANG BLECK
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WENWEN SONG, Scientific Staff, ULRICH PRAHL, Material Simulation Group Leader, and WOLFGANG BLECK, Head,
are with the Department of Ferrous Metallurgy, RWTH Aachen University
, Aachen,
Germany
. Contact
We report on the characterization of high carbon bearing steel 100Cr6 using electron microscopy and atom probe tomography in combination with multi-component diffusion simulations. Scanning electron micrographs show that around 14 vol pct spheroidized carbides are formed during soft annealing and only 3 vol pct remain after dissolution into the austenitic matrix through austenitization at 1123 K (850 C) for 300 seconds. The spheroidized particles are identified as (Fe, Cr)3C by transmission electron microscopy. Atom probe analysis reveals the redistribution and partitioning of the elements involved, i.e., C, Si, Mn, Cr, Fe, in both, the spheroidized carbides and the bainitic matrix in the sample isothermally heat-treated at 773 K (500 C) after austenitization. Homogeneous distribution of C and a Cr gradient were detected within the spheroidized carbides. Due to its limited diffusivity in (Fe, Cr)3C, Cr exhibits a maximum concentration at the surface of spheroidized carbides (16 at. pct) and decreases gradually from the surface towards the core down to about 2 at. pct. The atom probe results also indicate that the partially dissolved spheroidized carbides during austenitization may serve as nucleation sites for intermediate temperature cementite within bainite, which results in a relatively softer surface and harder core in spheroidized particles. This microstructure may contribute to the good wear resistance and fatigue properties of the steel. Good agreement between DICTRA simulations and experimental composition profiles is obtained by an increase of mobility of the substitutional elements in cementite by a factor of five, compared to the mobility in the database MOBFE2.
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A special soft annealing treatment, hereafter also
referred to as spheroidization heat treatment, produces a
mixed microstructure of relatively coarse spheroidized
cementite particles embedded in ferrite, which facilitates
machining, as well as warm and cold forming of the steel.
This microstructure can be subjected to further heat
treatment to achieve a final martensitic or bainitic
microstructure. Spheroidization kinetics has been long
known to be influenced both, by carbon and chromium
diffusion and by their respective concentration values.
Higher carbon concentration promotes the
spheroidization process, because it provides a higher number density
of nucleation sites. Chromium reduces the inter-lamellar
spacing of pearlite, which is often the starting structure
for spheroidization.[15] Spheroidization in 100Cr6
bearing steel has great influence on the subsequent bainitic
and pearlitic transformation. By varying the
spheroidization process parameters, namely the holding time and
temperature, the dissolution kinetics can be controlled.
In this way, the desired content of spheroidized carbides
and the distribution of carbon content in both
spheroidized carbides and ferrite can be achieved.
In the present work, we characterized the spheroidized
carbides with respect to the morphology,
crystallography, phase fraction, size distribution, transformation
kinetics, and near atomic-scale chemical gradients of
different elements using electron microscopy and atom
probe tomography (APT) in combination with
multicomponent diffusion simulations (DICTRA). APT was
employed to study the elemental distributions in
spheroidized carbides and bainite.[16] The partitioning behavior
of carbon and other alloy elements across the phase
boundaries are discussed, with an emphasis on the effect
of Cr, Mn, and Si on the growth kinetics of cementite.
II. EXPERIMENTAL
The chemical composition of the 100Cr6 steel studied
here is given in Table I. The steel is mainly alloyed with
Cr and microalloyed with Mo. Si and Mn contents are
at a low level, while the Al content is almost negligible.
The N content in the steel is 75 ppm.
The heat treatment cycle (HTC) and the conditions
investigated are illustrated in Table II and Figure 1.
After hot forging, the material was soft annealed
industrially and cooled down to form a spheroidized
microstructure (HTC1). Starting with the spheroidized
microstructure, the samples were heated up at a rate of
3.3 K/s and austenitized at 1123 K (850 C) for 300 sec
onds. After austenitization, two heat treatment routes
were performed. One was quenching to room
temperature in Ar (HTC2) and the other was rapidly cooling to
773 K (500 C) at a rate of 55 K/s. At 773 K (500 C),
the samples were isothermally held for 1200 seconds in
order to form a bainitic microstructure (HTC3) with
subsequent air cooling. Austenitization and
bainitization were performed in a Ba hr 805A dilatometer, where
the dimension of the specimen was F3 mm 9 10 mm.
Micro (...truncated)