Characterization of the carbon and retained austenite distributions in martensitic medium carbon, high silicon steel

Donald H. Sherman, Steven M. Cross, Sangho Kim, Fernande Grandjean, Gary J. Long, Michael K. Miller

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Abstract

The retained austenite content and carbon distribution in martensite were determined as a function of cooling rate and temper temperature in steel that contained 1.31 at. pct C, 3.2 at. pct Si, and 3.2 at. pct noniron metallic elements. Mössbauer spectroscopy, transmission electron microscopy (TEM), transmission synchrotron X-ray diffraction (XRD), and atom probe tomography were used for the microstructural analyses. The retained austenite content was an inverse, linear function of cooling rate between 25 and 560 K/s. The elevated Si content of 3.2 at. pct did not shift the start of austenite decomposition to higher tempering temperatures relative to SAE 4130 steel. The minimum tempering temperature for complete austenite decomposition was significantly higher (<650 °C) than for SAE 4130 steel (∼300 °C). The tempering temperatures for the precipitation of transition carbides and cementite were significantly higher (<400 °C) than for carbon steels (100 °C to 200 °C and 200 °C to 350 °C), respectively. Approximately 90 pct of the carbon atoms were trapped in Cottrell atmospheres in the vicinity of the dislocation cores in dislocation tangles in the martensite matrix after cooling at 560 K/s and aging at 22 °C. The 3.2 at. pct Si content increased the upper temperature limit for stable carbon clusters to above 215 °C. Significant autotempering occurred during cooling at 25 K/s. The proportion of total carbon that segregated to the interlath austenite films decreased from 34 to 8 pct as the cooling rate increased from 25 to 560 K/s. Developing a model for the transfer of carbon from martensite to austenite during quenching should provide a means for calculating the retained austenite. The maximum carbon content in the austenite films was 6 to 7 at. pct, both in specimens cooled at 560 K/s and at 25 K/s. Approximately 6 to 7 at. pct carbon was sufficient to arrest the transformation of austenite to martensite. The chemical potential of carbon is the same in martensite that contains 0.5 to 1.0 at. pct carbon and in austenite that contains 6 to 7 at. pct carbon. There was no segregation of any substitutional elements.

Original languageEnglish
Pages (from-to)1698-1711
Number of pages14
JournalMetallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
Volume38
Issue number8
DOIs
StatePublished - Aug 2007
Externally publishedYes

Funding

The authors thank Drs. R.P. Hermann and C. Piquer and Ms. L. Rebbouh, for help in obtaining the Möss-bauer spectra, and Professor C.L. Briant, the late Professor H.P. Leighly, Dr. P. Zhao, and Mr. D. Akers, for many helpful discussions over the years. One of the authors (FG) acknowledges the ‘‘Fonds National de la Recherche Scientifique,’’ for Grant No. 9.456595. Research with the local electrode atom probe at the ShaRE user facility at Oak Ridge National Laboratory was sponsored by the Division of Materials Sciences and Engineering, United States Department of Energy, under Contract No. DE-AC05-00OR22725, with UT—Battelle, LLC. Transmission synchrotron X-ray diffraction was done at Argonne National Laboratory under General User Proposal 2679 on the Sector 1 Insertion Device beamline at the Advanced Photon Source. The use of the Advanced Photon Source was supported by the United States Department of Energy, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38. Support for the TEM work was from the Materials Research Science and Engineering Center on Micro-and Nano-Mechanics of Electronic and Structural Materials, Brown University (NSF Grant No. DMR-0079964). Finally, the authors acknowledge the support for this work from Caterpillar Inc. and the Department of Energy, Office of Heavy Vehicle and Transportation Applications, WR10648, Program Manager–Sid Diamond (late).

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