Effects of Cr Concentration on Cementite Coarsening in Ultrahigh Carbon Steel
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INTRODUCTION
ULTRAHIGH carbon steels (UHCS) are a class of steels containing 1.0 to 2.1 wt pct C. The high carbon content causes precipitation of hard, brittle carbide phases during casting and/or heat-treatment processes. These carbide phases usually form as a network along the prior austenite grain boundaries in the UHCS. The type of carbide depends on the composition; M3C (cementite), M7C3, and M23C6 are the most common carbide phases. The carbide phases increase the surface hardness and wear resistance, properties necessary for forming applications such as shaping or rolling other metals. Thus, UHCS are commonly used as tool steels and in rolling mills. UHCS are typically heat treated to improve mechanical properties or machinability. In the 1970s, researchers found that UHCS could be made superplastic at elevated temperatures after thermomechanical processing.[1] The thermomechanical processing also improved the room temperature toughness of the UHCS,[2] leading researchers to explore UHCS for applications traditionally utilizing lower carbon steels, such as sheet and automotive steels.[3,4] The heat-treatment processing relevant to the present study is spheroidization, in which the steel is heated near or above the eutectoid temperature. During spheroidization, cementite lamellae in pearlite (an alternating layered structure of ferrite and cementite) transform into equiaxed particles. If heating is continued after
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 October 24, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
spheroidization, carbide particles will undergo curvature-driven coarsening. The rate of coarsening at a given temperature depends on the diffusion mechanism which is dominant during coarsening,[5–9] and the diffusion coefficient of the diffusing species. Both fracture toughness[10] and yield strength[11] are affected by carbide particle sizes; finer particle sizes are usually more beneficial to mechanical properties. However, finer particle sizes are also more likely to cause a divorced eutectoid transformation inhibiting pearlite reformation during cooling.[12] Reforming the pearlite might help increase wear resistance,[13] so a heat treatment that produces larger particles with larger spaces between them might ultimately be more suitable. Understanding the particle coarsening behavior is thus very desirable when designing heat-treatment processes. This study investigates effects of Cr content on cementite coarsening by administering a series of different heat treatments to an UHCS containing 2 wt pct C and 1 wt pct Cr. This study builds on a previous study in which the authors administered the same series of heat treatments to a 2C-4Cr UHCS.[14] Two phenomena related to particle coarsening were noted during heat treatment of the 2C-4Cr UHCS: (1) multimodal coarsening particles on grain boundaries (GB particles
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