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Coarsening of Inter- and Intra-granular Proeutectoid Cementite in an Initially Pearlitic 2C-4Cr Ultrahigh Carbon Steel
http://crossmark.crossref.org/dialog/?doi=10.1007/s11661-017-4012-2&domain=pdf
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Web End = MATTHEW D. HECHT, YOOSUF N. PICARD, and BRYAN A. WEBLER
We have examined spheroidization and coarsening of cementite in an initially pearlitic 2C-4Cr ultrahigh carbon steel containing a cementite network. Coarsening kinetics of spheroidized cementite and growth of denuded zones adjacent to the cementite network were investigated by analyzing particle sizes from digital micrographs of water-quenched steel etched with Nital. Denuded zones grew at a rate proportional to t1/4t1/5. Spheroidization of pearlite was completed within 90 minutes at 1073 K and 1173 K (800 C and 900 C), and within 5 minutes at 1243 K (970 C). Bimodal particle size distributions were identied in most of the samples and were more pronounced at higher temperatures and hold times. Peaks in the distributions were attributed to the coarsening of intragranular and grain boundary particles at dierent rates. A third, non-coarsening peak of particles was present at 1073 K (800 C) only and was attributed to particles existing prior to the heat treatment. Particle sizes were plotted vs time to investigate possible coarsening mechanisms. The coarsening exponent for the growth of grain boundary carbides was closest to 4, indicating grain boundary diusion control. The coarsening exponent was closest to 5 for intragranular carbides, indicating suppression of volumetric diusion (possibly due to reduced eective diusivity because of Cr alloying) and control by dislocation diusion.
DOI: 10.1007/s11661-017-4012-2 The Minerals, Metals & Materials Society and ASM International 2017
I. INTRODUCTION
A. Ultrahigh Carbon Steels
ULTRAHIGH carbon steels (UHCS) have been used for many years in various applications demanding high strength and excellent wear resistance. UHCS are hypereutectoid steels with carbon contents in the range of 1.0 to 2.1 wt pct. As a result of this high C content, there is precipitation of proeutectoid cementite (Fe3C) during cooling from casting and/or during heat treatment processes. The hard and brittle cementite contributes to the high hardness and wear resistance of UHCS, which is desirable for applications involving the cutting or shaping of other metals. UHCS have been used in rolling mills as far back as 1913[1] and are also commonly used in tool steels. There has been interest over...