Coarsening of Inter- and Intra-granular Proeutectoid Cementite in an Initially Pearlitic 2C-4Cr Ultrahigh Carbon Steel
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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 the last few decades in using UHCS for other applications where traditionally lower carbon content steels have been utilized, such as sheet and automotive steels.[2,3] Eutectoid and proeutectoid cementite in UHCS tend to form a network along austenite grain boundaries, which provides crack initiation sites and propagation pathways. Toughness decreases as a function of network connectivity, plateauing once a critical network MATTHEW D. HECHT, YOOSUF N. PICARD, and BRYAN A. WEBLER are with the Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted December 3, 2016. Article published online February 21, 2017 2320—VOLUME 48A, MAY 2017
connectivity is reached.[4] The network is surrounded by a matrix made up of eutectoid transformation products, such as pearlite (lamellar eutectoid cementite + ferrite), bainite, and/or martensite. The matrix strongly affects steel hardness and may also influence toughness. Studies indicate that low toughness in UHCS can be mitigated through network breakup by microalloying[5–7] or thermomechanical processing.[8] UHCS are typically heat treated after casting to improve mechanical properties. Heating a pearlitic matrix to near or above the eutectoid temperature will cause spheroidization of cementite lamellae into equiaxed particles. Further time at temperature will result in curvature-driven particle coarsening. Carbide particle size distribution influences fracture toughness[9] and yield strength,[10] and potentially also the proeutectoid network volume fraction and connectivity in UHCS. Gurland[11] noted that cracking in tension and torsion in spheroidized pearlite tended to occur in the largest cementite particles. A smaller mean free path between particles, i.e., a smaller particle size, typically improves both toughness and hardness, so processing to produce a finer distribution of particles is usually beneficial. On the other hand, a partially pearlitic microstructure might be desirable for increased wear resistance.[12] Closely spaced particles are more likely to lead to a non-lamellar divorced eutectoid transformation upon cooling from the austenite.[13] Thus, understanding particle coarsening behavior is desirable for improving the tailoring of mechanical
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