On the Spheroidized Carbide Dissolution and Elemental Partitioning in High Carbon Bearing Steel 100Cr6
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CONTROLLING precipitation in steels for enhancing their wear resistance and fatigue properties has been a subject of intense and long-lasting research employing a variety of advanced experimental and modeling approaches.[1–14] 100Cr6 steels with the basic composition of 1 wt pct C and 1.5 wt pct Cr are among the most extensively used materials for mechanically highly stressed bearings in multiple critical applications in the fields of mobility, safety, manufacturing, mining, and energy infrastructures. As these materials encounter substantial thermomechanical loading in service, the material and heat treatment design should meet the requirements of high fatigue and wear resistance as well as an outstanding combination of strength and toughness.
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 e-mail: [email protected] PYUCK-PA CHOI, Atom Probe Tomography Research Group Leader, GERHARD INDEN, Professor, and DIERK RAABE, Chief Executive, are with the Max-Planck-Institut fu¨r Eisenforschung GmbH, Du¨sseldorf, Germany. Manuscript submitted March 6, 2013. Article published online October 30, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
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 VOLUME 45A, FEBRUARY 2014—595
Table I. Element wt pct at. pct
Chemical Composition of the Investigated Steel 100Cr6
C
Si
Mn
P
S
Cr
Mo
Ni
Cu
Al
0.967 4.325
0.30 0.58
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