On the Spheroidized Carbide Dissolution and Elemental Partitioning in High Carbon Bearing Steel 100Cr6

Abstract

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) 3 C 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) 3 C, 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.

Effects of Annealing on Carbides Size and Distribution and Cold Formability of 1.0C-1.5Cr Bearing Steel

“…The average volume fraction of carbides is 15.9 pct, which is slightly larger compared with the measured value by Song et al [25] That is mainly due to the larger carbon content of the experimental steel in this work. [26] In addition, there may exist statistical error.…”

Carbon Redistribution in Martensite in High-C Steel: Atomic-Scale Characterization and Modelling

“…By using metallographic methods (i.e., grinding, polishing, Klemm etching and analysis of the following image by contrast), the volume fraction of carbides is estimated from the area fraction in the SEM images by the measuring of 2000 particles. This yielded 14 vol% and 3 vol% before and after austenitization, respectively [4]. During austenitization, the average carbide size decreases from 0.63 ± 0.02 µm to 0.49 ± 0.02 µm.…”

“…Lath martensite consists of fine hierarchically arranged laths with a thickness of 100-300 nm. Laths are characterized by containing a high dislocation density with carbon enrichment at the dislocations via Cottrell atmospheres [4]. The dislocation density as a function of carbon content has been predicted in a previous study and it will be used to estimate the strain energy accommodated by laths [14].…”

“…As shown in Figure 2b,d, the spheroidized particles become smaller and more spherical after quenching. The partially dissolved spheroidized carbides have been identified in a previous study [4] to be (Fe, Cr)3C by employing TEM for crystallographic analysis with the zone direction of [011] using the method of SAD (Selected Area Diffraction) and APT for the identification of chemical composition.…”

“…The atom probe tomography investigations of the lath-structured martensite revealed localized carbon concentrations of 12-14 at%. Clustering was attributed to the segregation of carbon to dislocations and the formation of Cottrell atmospheres despite significant higher carbon concentrations in comparison to the reported values of 6-8 at% [4,5]. Li et al [6] studied the influence of rolling contact loading on the plate-like martensitic microstructure and the resulting distributions of the major alloying elements C and Cr using atom probe tomography.…”