Effects of Nb Modification and Cooling Rate on the Microstructure in an Ultrahigh Carbon Steel
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ULTRAHIGH carbon steels (UHCS) are hypereutectoid steels in the range of 1.0 to 2.1 wt pct C, typically used for cutting or shaping other metals. The high carbon content results in the precipitation of hard and brittle carbide phases during casting and/or heat treatment processes. 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. There has been interest over the last few decades in using UHCS for other applications where traditionally lower carbon content steels have been utilized. Lesuer et al.[1] argued that the high strength and ductility of spheroidized UHCS were appropriate for applications in sheet and automotive steels. In unprocessed as-cast UHCS, allotriomorphic carbide phases tend to form a network on austenite grain boundaries, providing undesired sites for crack initiation and propagation. Hence, the network is very deleterious for toughness and reducing network connectivity is of interest for improving mechanical properties. The authors[2] quantify network connectivity in terms of a connectivity index which is related to the
MATTHEW D. HECHT, BRYAN A. WEBLER, and YOOSUF N. PICARD are with the Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15232. Contact e-mail: [email protected] Manuscript submitted August 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
probability of a connection existing between points in the network. Other parameters of interest regarding the network are volume fraction and branch spacing. The carbide network in UHCS is surrounded by a matrix, which could be pearlite (lamellar eutectoid cementite + ferrite), bainite, martensite, and/or retained austenite depending on the processing conditions. While the matrix always strongly affects steel hardness, the authors[2] found that its effects on toughness may be limited once the network exceeds a critical connectivity index. Two methods of network break-up have achieved success in UHCS: thermomechanical processing and chemistry modification (addition of high temperature carbide/oxide-forming elements to the melt). Thermomechanical processing aims to break connections within an existing network through heavy mechanical deformation, while chemistry modification seeks to prevent the network from forming as a highly interconnected structure in the first place. Walser and Sherby[3] demonstrated that the thermomechanical working greatly increased ductility of processed UHCS over the as-cast UHCS in tensile elongation at 923 K (650 C). Hamidzadeh et al.[4] found that additions of about 0.03 wt pct of Ce and La caused significant network break-up and toughness enhancement of about 75 pct in a 1.5C-12Cr UHCS by providing additional nucleation sites for austenite and eutectic carbides. Hamidzadeh et al.[5] also found that Nb additions caused a similar network break-up and toughness enhancement. In this work we have opted to study Nb modification of t
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